93. Folding Graphene Film Yields High Areal Energy Storage in Lithium-Ion Batteries

ACS Nano, Accepted.

Bin Wang, Jaegeon Ryu, Sungho Choi, Gyujin Song, Dongki Hong, Chihyun Hwang, Xiong Chen, Bo Wang, Wei Li, Hyun-Kon Song, Soojin Park* and Rodney S. Ruoff*

We show that a high energy density can be achieved in a practical manner with freestanding electrodes without using conductive carbon, binders, and current collectors. We made and used a folded graphene composite electrode designed for a high areal capacity anode. The traditional thick graphene composite electrode, such as made by filtering graphene oxide to create a thin film and reducing it such as through chemical or thermal methods, has sluggish reaction kinetics. Instead, we have made and tested a thin composite film electrode that was folded several times using a water-assisted method; it provides a continuous electron transport path in the fold regions and introduces more channels between the folded layers, which significantly enhances the electron/ion transport kinetics. A fold electrode consisting of SnO2/graphene with high areal loading of 5 mg cm-2 has a high areal capacity of 4.15 mAh cm-2, well above commercial graphite anodes (2.50–3.50 mAh cm-2), while the thickness is maintained as low as ∼20 μm. The fold electrode shows stable cycling over 500 cycles at 1.70 mA cm-2 and improved rate capability compared to thick electrodes with the same mass loading but without folds. A full cell of fold electrode coupled with LiCoO2 cathode was assembled and delivered an areal capacity of 2.84 mAh cm-2 after 300 cycles. This folding strategy can be extended to other electrode materials and rechargeable batteries.

92. Foldable Electrode Architectures Based on Silver-Nanowire-Wound or Carbon-Nanotube-Webbed Micrometer-Scale Fibers of Polyethylene Terephthalate Mats for Flexible Lithium-Ion Batteriesy

Advanced Materials, Accepted.

Chihyun Hwang, Woo-Jin Song, Jung-Gu Han, Sohyun Bae, Gyujin Song, Nam-Soon Choi, Soojin Park*, Hyun-Kon Song*

A crumply and highly flexible lithium-ion battery is realized by using microfiber mat electrodes in which the microfibers are wound or webbed with conductive nanowires. This electrode architecture guarantees extraordinary mechanical durability without any increase in resistance after folding 1000 times. Its areal energy density is easily controllable by the number of folded stacks of a piece of the electrode mat. Deformable lithium-ion batteries of lithium iron phosphate as cathode and lithium titanium oxide as anode at high areal capacity (3.2 mAh cm−2) are successfully operated without structural failure and performance loss, even after repeated crumpling and folding during charging and discharging.

91. Superoxide stability for reversible Na-O2 electrochemistry

Scientific Reports 7, 17635 (2017).

V. S. Dilimon, Chihyun Hwang, Yoon-Gyo Cho, Juchan Yang, Hee-Dae Lim, Kisuk Kang, Seok Ju Kang, and Hyun-Kon Song*

Stabilizing superoxide (O2) is one of the key issues of sodium-air batteries because the superoxide-based discharge product (NaO2) is more reversibly oxidized to oxygen when compared with peroxide (O22−) and oxide (O2). Reversibly outstanding performances of sodium-oxygen batteries have been realized with the superoxide discharge product (NaO2) even if sodium peroxide (Na2O2) have been also known as the discharge products. Here we report that the Lewis basicity of anions of sodium salts as well as solvent molecules, both quantitatively represented by donor numbers (DNs), determines the superoxide stability and resultantly the reversibility of sodium-oxygen batteries. A DN map of superoxide stability was presented as a selection guide of salt/solvent pair. Based on sodium triflate (CF3SO3)/dimethyl sulfoxide (DMSO) as a high-DN-pair electrolyte system, sodium ion oxygen batteries were constructed. Pre-sodiated antimony (Sb) was used as an anode during discharge instead of sodium metal because DMSO is reacted with the metal. The superoxide stability supported by the high DN anion/solvent pair (CF3SO3/DMSO) allowed more reversible operation of the sodium ion oxygen batteries.

90. A Lithium-ion Battery Using Partially Lithiated Graphite Anode and Amphi-redox LiMn2O4 Cathode

Scientific Reports 7, 14879 (2017).

Yuju Jeon, Hyun-Kuk Noh* and Hyun-Kon Song*

Delithiation followed by lithiation of Li+-occupied (n-type) tetrahedral sites of cubic LiMn2O4 spinel (LMO) at ~4 VLi/Li+ (delivering ~100 mAh gLMO−1) has been used for energy storage by lithium ion batteries (LIBs). In this work, we utilized unoccupied (p-type) octahedral sites of LMO available for lithiation at ~3 VLi/Li+ (delivering additional ~100 mAh gLMO−1) that have never been used for LIBs in full-cell configuration. The whole capacity of amphi-redox LMO, including both oxidizable n-type and reducible p-type redox sites, at ~200 mAh gLMO−1 was realized by using the reactions both at 4 VLi/Li+ and 3 VLi/Li+. Durable reversibility of the 3 V reaction was achieved by graphene-wrapping LMO nanoparticles (LMO@Gn). Prelithiated graphite (LinC6, n < 1) was used as anodes to lithiate the unoccupied octahedral sites of LMO for the 3 V reaction.

89. Graphene-wrapped Porous Sb Anodes for Sodium-Ion Batteries by Mechanochemical Compositing and Metallomechanical Reduction of Sb2O3

Electrochimica Acta 252, 25-32 (2017).

Chihyun Hwang, Sinho Choi, Gwan Yeong Jung, Juchan Yang, Sang Kyu Kwak, Soojin Park* and Hyun-Kon Song*

Antimony metal nanoparticles wrapped with a-few-layer graphene coat (Sb@Gn) were fabricated from their oxide form (Sb2O3) in a micrometer dimension using a novel two-step ball-milling process. The first mechanochemical process was designed to decrease the particle size of Sb2O3 microparticles for ensuring advantages of nano size and to subsequently coat the Sb2O3 nanoparticles with a-few-layer graphene (Sb2O3@Gn). The second metallomechanical ball-milling process reduced the oxide to its metal form (Sb@Gn) by the help of Zn as a metallic reductant. The graphene layer (@Gn) blocked the alloying reaction between Sb and Zn, limiting the size of Sb particles during the metallomechanical reduction step. During reduction, oxygen species were transferred from of Sb2O3 through @Gn to Zn along redox transfer pathways rather than direct mass transfer via unsaturated vacancies in the @Gn. the redox transfer involving oxidation of @Gn by O2− is plausible routes for O2− transfer in the metallomechanical reduction. The Sb@Gn anode exhibited outstanding capacity retention along charge/discharge cycles and improved rate capability in sodium-ion batteries. The @Gn provided conductive pathways to the Sb core and limited size expansion during sodium-lithium alloying.

88. Bifunctional hydrous RuO2 nanocluster electrocatalyst embedded in carbon matrix for efficient and durable operation of rechargeable zinc–air batteries

Scientific Reports 7, 7150 (2017).

Han-Saem Park, Eunyong Seo, Juchan Yang, Yeongdae Lee, Byeong-Su Kim* and Hyun-Kon Song*

Ruthenium oxide (RuO2) is the best oxygen evolution reaction (OER) electrocatalyst. Herein, we demonstrated that RuO2 can be also efficiently used as an oxygen reduction reaction (ORR) electrocatalyst, thereby serving as a bifunctional material for rechargeable Zn–air batteries. We found two forms of RuO2 (i.e. hydrous and anhydrous, respectively h-RuO2 and ah-RuO2) to show different ORR and OER electrocatalytic characteristics. Thus, h-RuO2 required large ORR overpotentials, although it completed the ORR via a 4e process. In contrast, h-RuO2 triggered the OER at lower overpotentials at the expense of showing very unstable electrocatalytic activity. To capitalize on the advantages of h-RuO2 while improving its drawbacks, we designed a unique structure (RuO2@C) where h-RuO2 nanoparticles were embedded in a carbon matrix. A double hydrophilic block copolymer-templated ruthenium precursor was transformed into RuO2 nanoparticles upon formation of the carbon matrix via annealing. The carbon matrix allowed overcoming the limitations of h-RuO2 by improving its poor conductivity and protecting the catalyst from dissolution during OER. The bifunctional RuO2@C catalyst demonstrated a very low potential gap (ΔEOER-ORR = ca. 1.0 V) at 20 mA cm−2. The Zn||RuO2@C cell showed an excellent stability (i.e. no overpotential was observed after more than 40 h).

87. Curvature-Induced Metal–Support Interaction of an Islands-by-Islands Composite of Platinum Catalyst and Carbon Nano-onion for Durable Oxygen Reduction

ACS Applied Materials & Interfaces 9, 23302-23308 (2017).

Juchan Yang, Su Hwan Kim, Sang Kyu Kwak* and Hyun-Kon Song*

Geometry of carbon supports significantly affected electrochemical durability of Pt/C (platinum electrocatalyst supported by carbon) for oxygen reduction reaction (ORR). Carbon nano-onion (CNO) was used as the support, which is characterized by its nanosize (similar to Pt size) and high curvature. Superior ORR durability was guaranteed by Pt/CNO due to (1) its islands-by-islands configuration to isolate each Pt nanoparticle from its neighbors by CNO particles; (2) highly tortuous void structure of the configuration to suppress Ostwald ripening; and (3) the curvature-induced strong interaction between CNO and Pt. The finding that highly curved carbon surface encourages electron donation to catalysts was first reported.

86. Conductive and Porous Silicon Nanowire Anodes for Lithium
Ion Batteries

Journal of the electrochemical society 164, A1564-A1568 (2017).

Chihyun Hwang, Kangmin Lee, Han-Don Um, Yeongdae Lee, Kwanyong Seo*, and Hyun-Kon Song*

Silicon nanowires (SiNWs) were prepared by chemically etching silicon wafers with silver nanoparticles. Their electrical conductivities and porosities were tuned by adjusting the doping concentration of silicon wafers from which the SiNWs were prepared. Porosity of the SiNWs were proportional to doping concentrations of the mother wafer because the dopant population provides nucleation sites for etching. The electrical conductivities of the doped SiNWs were 100 times higher than those of the intrinsic SiNW. However, there was no difference in the conductivity between two different doping level SiNWs (Na = 2.7 × 1015 and 5.7 × 1019) due to the trade-off between porosity and the intrinsic conductivity of the solid backbone. The doping-dependent properties of SiNWs determined the capacity, stability and kinetics of the lithium alloying reaction of the SiNWs. The medium-level doping SiNWs, characterized by a mechanically obust porous structure, showed the most improved electrochemical performances in a full cell of a lithium manganese oxide || SiNW battery as a result of the balanced trade-off between coulombic efficiency and capacity retention.

85. ZnO decorated germanium nanoparticles as anode materials in Li-ion batteries

Nanotechnology 28, 095402 (2017)

Tae-Hee Kim, Song Yi Park, Tack Ho Lee, Jaeki Jeong, Dong Suk Kim, Mark T Swihart, Hyun-Kon Song*, Jin Young Kim* and Seongbeom Kim*

Germanium exhibits high charge capacity and high lithium diffusivity, both are the key requirements for electrode materials in high performance lithium ion batteries (LIBs). However, high volume expansion and segregation from the electrode during charge–discharge cycling have limited use of germanium in LIBs. Here, we demonstrate that ZnO decorated Ge nanoparticles (Ge@ZnO NPs) can overcome these limitations of Ge as an LIB anode material. We produced Ge NPs at high rates by laser pyrolysis of GeH4, then coated them with solution phase synthesized ZnO NPs. Half-cell tests revealed dramatically enhanced cycling stability and higher rate capability of Ge@ZnO NPs compared to Ge NPs. Enhancements arise from the core–shell structure of Ge@ZnO NPs as well as production of metallic Zn from the ZnO layer. These findings not only demonstrate a new surface treatment for Ge NPs, but also provide a new opportunity for development of high-rate LIBs.

84. Coffee-Driven Green Activation of Cellulose and Its Use for All-Paper Flexible Supercapacitors

ACS Applied Materials & Interfaces 9, 22568-22577 (2017).

Donggue Lee, Yoon-Gyo Cho, Hyun-Kon Song, Sang-Jin Chun, Sang-Bum Park, Don-Ha Choi, Sun-Young Lee*, JongTae Yoo*, and Sang-Young Lee*

Cellulose, which is one of the most abundant and renewable natural resources, has been extensively explored as an alternative substance for electrode materials such as activated carbons. Here, we demonstrate a new class of coffee-mediated green activation of cellulose as a new environmentally benign chemical activation strategy and its potential use for all-paper flexible supercapacitors. A piece of paper towel is soaked in espresso coffee (acting as a natural activating agent) and then pyrolyzed to yield paper-derived activated carbons (denoted as “EK-ACs”). Potassium ions (K+), a core ingredient of espresso, play a viable role in facilitating pyrolysis kinetics and also achieving a well-developed microporous structure in the EK-ACs. As a result, the EK-ACs show significant improvement in specific capacitance (= 131 F g−1 at a scan rate of 1.0 mV s−1) over control ACs (= 64 F g−1) obtained from the carbonization of a pristine paper towel. All-paper flexible supercapacitors are fabricated by assembling EK-ACs/carbon nanotube mixture-embedded paper towels (as electrodes), polyvinyl alcohol/KOH mixture-impregnated paper towels (as electrolytes), and polydimethylsiloxane-infiltrated paper towels (as packaging substances). The introduction of the EK-ACs (as an electrode material) and the paper towel (as a deformable/compliant substrate) enables the resulting all-paper supercapacitor to provide reliable/sustainable cell performance and also exceptional mechanical flexibility. Notably, no appreciable loss in the cell capacitance is observed after repeated bending (over 5,000 cycles) or multiple folding. The coffee-mediated green activation of cellulose and the resultant all-paper flexible supercapacitors open new material and system opportunities for eco-friendly high-performance flexible power sources.

83. Mesoporous Germanium Anode Materials for Lithium-Ion Battery with Exceptional Cycling Stability in Wide Temperature Range

Small 13, 1603045 (2017)

Sinho Choi, Yoon-Gyo Cho, Jieun Kim, Nam-Soon Choi, Hyun-Kon Song, Guoxiu Wang*, Soojin Park*

Porous structured materials have unique architectures and are promising for lithium-ion batteries to enhance performances. In particular, mesoporous materials have many advantages including a high surface area and large void spaces which can increase reactivity and accessibility of lithium ions. This study reports a synthesis of newly developed mesoporous germanium (Ge) particles prepared by a zincothermic reduction at a mild temperature for high performance lithium-ion batteries which can operate in a wide temperature range. The optimized Ge battery anodes with the mesoporous structure exhibit outstanding electrochemical properties in a wide temperature ranging from −20 to 60 °C. Ge anodes exhibit a stable cycling retention at various temperatures (capacity retention of 99% after 100 cycles at 25 °C, 84% after 300 cycles at 60 °C, and 50% after 50 cycles at −20 °C). Furthermore, full cells consisting of the mesoporous Ge anode and an LiFePO4 cathode show an excellent cyclability at −20 and 25 °C. Mesoporous Ge materials synthesized by the zincothermic reduction can be potentially applied as high performance anode materials for practical lithium-ion batteries.

82. Significance of Ferroelectric Polarization in Poly(vinylidene difluoride) Binder for High-Rate Li-ion Diffusion

Nano Energy 32, 255–262 (2017)

Woo-Jin Song, Se Hun Joo, Do Hyeong Kim, Chihyun Hwang, Gwan Yeong Jung, Sohyeon Bae, Yeonguk Son, Jaephil Cho, Hyun-Kon Song, Sang Kyu Kwak*, Soojin Park*, Seok Ju Kang*

An interesting and effective route for improving battery performance using ferroelectric poly(vinylidene difluloride) (PVDF) polymer as a binder material is demonstrated in this work. A ferroelectric PVDF phase developed under the appropriate thermal annealing process enables generation of suitable polarization on active materials during the discharge and charge process, giving rise to longer capacity with lower overpotential at a high current rate. Electrochemical analysis including in situ galvanostatic electrochemical impedance spectroscopy and a galvanostatic intermittent titration measurement revealed that the ferroelectric binder effectively reduced Li-ion diffusion resistance and supported fast migration in the vicinity of active electrodes. Computational results further support that the binding affinity of the ferroelectric PVDF surface is much higher than that of the paraelectric PVDF, confirmed by ideally formed ferroelectric and paraelectric PVDF conformations with Li-ions. Furthermore, we consistently achieved high Li-ion battery (LIB) performance in full cell architecture consisting of a LTO/separator/LFP with a ferroelectric PVDF binder in the anode and cathode materials, revealing that the polarization field is important for fabricating high-performance LIBs, potentially opening a new design concept for binder materials.

81. Polypyrrole-assistant Oxygen Electrocatalysis on Perovskite Oxides

Energy & Environmental Science 10, 523-527 (2017)

Dong-Gyu Lee, Su Hwan Kim, Se Hun Joo, Ho-Il Ji, Hadi Tavassol, Yuju Jeon, Sihyuk Choi, Myeong-Hee Lee, Chanseok Kim, Sang Kyu Kwak, Guntae Kim and Hyun-Kon Song*  

Nitrogen-containing electrocatalysts such as metal-nitrogen-carbon (M-N-C) composites and nitrogen-doped carbons are known to exhibit high activities for oxygen reduction reaction (ORR). Even if the mechanism by which nitrogen improve the activities is not completely understood, strong electronic interaction between nitrogen and active sites has been found in these composites. Herein, we demonstrate a case in which nitrogen improves electroactivity, but in the absence of strong interaction with other components. The overpotentials of ORR and oxygen evolution reaction (OER) on perovskite oxide catalysts were significantly reduced simply by mixing the catalyst particles with polypyrrole/carbon composites (pPy/C). Any strong interactions between pPy (a nitrogen-containing compound) and active sites of the catalysts were not confirmed. A scenario based on the sequential role allocation between pPy and the oxide catalysts for ORR was proposed: (1) molecular oxygen is incorporated into pPy as a form of superoxide (pPy+O2-); (2) the superoxide is transferred to the active sites of perovskite catalysts; and (3) the superoxide is completely reduced along 4e ORR process.

80. Carambola Shaped VO2 Nanostructures: A Binder–Free Air Electrode for Aqueous Na–Air Battery

Journal of Materials Chemistry A 5, 2037-2044 (2017)

Ziyauddin Khan, Basker SenthilKumar, Sung O Park, Seungyoung Park, Juchan Yang, Jeong Hyeon Lee, Hyun-Kon Song, Youngsik Kim, Sang Kyu Kwak* and Hyunhyub Ko*

Binder free and bi–functional electrocatalyst plays a vital role in the development of high performance metal–air batteries. Herein, we have synthesized a vanadium oxide (VO2) nanostructure as a novel binder free and bi–functional electrocatalyst for rechargeable aqueous sodium–air (Na–air) battery. VO2 nanostructures have been grown on reduced graphene oxide coated on carbon paper which have carambola morphology. We have confirmed bi–functional nature of VO2 nanostructure by analyzing its electrocatalytic activity associated with oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The reaction pathway associated with electrocatalytic activity was also affirmed by computational modeling & simulation studies. Thereafter, aqueous Na–air cell has been built using novel binder free VO2 nanostructures as air electrode. The fabricated cell displays 0.64 V overpotential gap, 104 mW g-1 power density at 80 mA g–1 current density, 81% round trip efficiency and good cyclic stability up to 50 cycles, which are comparable to the previous best known Na-air batteries.

79. A surface-reactive high-modulus binder for the reversible conversion reaction of nanoparticular cobalt oxide

Electrochimica Acta 225, 78–85 (2017)

Myeong-Hee Lee, Tae-Hee Kim, Chihyun Hwang, Jieun Kim, Hyun-Kon Song*

Conversion-reaction-based anode materials for lithium ion batteries (LIBs) such as transition metal oxides have been considered as high-capacity alternatives to graphite. In the conversion reactions, interestingly, microparticles have been known to be superior to nanoparticles in terms of capacity retention along repeated cycles. In this work, a cross-linked two-component binder system of poly(acrylic acid) and carboxymethyl cellulose (PAA/CMC) was used for nanoparticular Co3O4. The binder was characterized by high modulus and strong bonding to the surface oxide of Co3O4. Even without carbon coating, the composite electrodes of nanoparticular Co3O4 in the presence of PAA/CMC showed significantly enhanced cycle retention with improved reversibility of the conversion reaction.

78. A Metal–Organic Framework Derived Porous Cobalt Manganese Oxide Bifunctional Electrocatalyst for Hybrid Na–Air/Seawater Batteries

ACS Applied Materials & Interfaces, 8, 32778-32787 (2016)

Mari Abirami, Soo Min Hwang*, Juchan Yang, Sirugaloor Thangavel Senthilkumar, Junsoo Kim, Woo-Seok Go, Baskar Senthilkumar, Hyun-Kon Song and Youngsik Kim*

Spinel-structured transition metal oxides are promising non-precious-metal electrocatalysts for oxygen electrocatalysis in rechargeable metal–air batteries. We applied porous cobalt manganese oxide (CMO) nanocubes as the cathode electrocatalyst in rechargeable seawater batteries, which are a hybrid-type Na–air battery with an open-structured cathode and a seawater catholyte. The porous CMO nanocubes were synthesized by the pyrolysis of a Prussian blue analogue, Mn3[Co(CN)6]2·nH2O, during air-annealing, which generated numerous pores between the final spinel-type CMO nanoparticles. The porous CMO electrocatalyst improved the redox reactions, such as the oxygen evolution/reduction reactions, at the cathode in the seawater batteries. The battery that used CMO displayed a voltage gap of ∼0.53 V, relatively small compared to that of the batteries employing commercial Pt/C (∼0.64 V) and Ir/C (∼0.73 V) nanoparticles and without any catalyst (∼1.05 V) at the initial cycle. This improved performance was due to the large surface area (catalytically active sites) and the high oxidation states of the randomly distributed Co and Mn cations in the CMO. Using a hard carbon anode, the Na-metal-free seawater battery exhibited a good cycle performance with an average discharge voltage of ∼2.7 V and a discharge capacity of ∼190 mAh g–1hard carbon during 100 cycles (energy efficiencies of 74–79%).

77. Hierarchical urchin-shaped α-MnO2 on graphene-coated carbon microfibers: a binder-free electrode for rechargeable aqueous Na–air battery

NPG Asia Materials 8, e294 (2016)

Ziyauddin Khan, Seungyoung Park, Soo Min Hwang, Juchan Yang, Youngsu Lee, Hyun-Kon Song, Youngsik Kim* and Hyunhyub Ko*

With the increasing demand of cost-effective and high-energy devices, sodium–air (Na–air) batteries have attracted immense interest due to the natural abundance of sodium in contrast to lithium. In particular, an aqueous Na–air battery has fundamental advantage over non-aqueous batteries due to the formation of highly water-soluble discharge product, which improve the overall performance of the system in terms of energy density, cyclic stability and round-trip efficiency. Despite these advantages, the rechargeability of aqueous Na–air batteries has not yet been demonstrated when using non-precious metal catalysts. In this work, we rationally synthesized a binder-free and robust electrode by directly growing urchin-shaped MnO2 nanowires on porous reduced graphene oxide-coated carbon microfiber (MGC) mats and fabricated an aqueous Na–air cell using the MGC as an air electrode to demonstrate the rechargeability of an aqueous Na–air battery. The fabricated aqueous Na–air cell exhibited excellent rechargeability and rate capability with a low overpotential gap (0.7 V) and high round-trip efficiency (81%). We believe that our approach opens a new avenue for synthesizing robust and binder-free electrodes that can be utilized to build not only metal–air batteries but also other energy systems such as supercapacitors, metal–ion batteries and fuel cells.

76. Design of an ultra-durable silicon-based battery anode material with exceptional high-temperature cycling stability

Nano Energy, 26, 192-199 (2016)

Hyungmin Park, Sinho Choi, Sung-Jun Lee, Yoon-Gyo Cho, Gaeun Hwang, Hyun-Kon Song, Nam-Soon Choi, Soojin Park*

A new class of Si-based materials is fabricated by synergistic coupling of amorphous ABOx and carbon coating layers onto Si particles. In this system, ABOx stabilizes the SEI layers, while carbon acts as an electrical conducting material. The resulting Si-based anodes exhibit high specific capacities (1667 mA h g−1 (25 °C) and 2021 mA h g−1 (60 °C) at 1 C rate) and highly stable cycling performances (capacity retention of 60% after 500 cycles at 25 °C and 64% after 600 cycles at 60 °C).

75. Organogel electrolyte for high-loading silicon batteries

Journal of Material Chemistry A 4, 8005-8009 (2016)

Yoon-Gyo Cho, Hyungmin Park, Jung-In Lee, Chihyun Hwang, Yuju Jeon, Soojin Park and Hyun-Kon Song*

A cyanoresin organogel electrolyte, characterized by in situ gelation with no initiators or crosslinkers, was used for high-loading silicon batteries of 1.3 mgSi cm−2 (equivalent to 3.3 mA h cm−2). The organogel provided additional cohesion between the silicon particles and maintained electrode integrity even after pulverization, completely suppressing severe crack development and serious electrode thickness changes observed in liquid electrolytes. The capacity retention upon cycling was significantly improved in the organogel when compared with its liquid counterpart.

74. Enhancing Interfacial Bonding between Anisotropically Oriented Grains Using a Glue-Nanofiller for Advanced Li-Ion Battery Cathode

Advanced Materials 28, 4705-4712 (2016)

Hyejung Kim, Sanghan Lee, Hyeon Cho, Junhyeok Kim, Jieun Lee, Suhyeon Park, Se Hun Joo, Su Hwan Kim, Yoon-Gyo Cho, Hyun-Kon Song, Sang Kyu Kwak*, Jaephil Cho*

Formation of a glue-nanofiller layer between grains, consisting of a middle-temperature spinel-like LixCoO2 phase, reinforces the strength of the incoherent interfacial binding between anisotropically oriented grains by enhancing the face-to-face adhesion strength. The cathode treated with the glue-layer exhibits steady cycling performance at both room-temperature and 60 °C. These results represent a step forward in advanced lithium-ion battery via simple cathode coating.

73. Investigation on silicon alloying kinetics during lithiation by galvanostatic impedance spectroscopy

Journal of Power Sources 315, 145–151 (2016)

Younghoon Ko, Chihyun Hwang, Hyun-Kon Song*

Electrochemical parameters characterizing lithiation processes in silicon anodes of lithium ion batteries (LIBs) were measured in situ during a practical charging operation by galvanostatic electrochemical impedance spectroscopy (GS-EIS).

72. Dependency of electrochemical performances of silicon lithium ion batteries on glycosidic linkages of polysaccharide binders

ACS Applied Materials & Interfaces 8, 4042–4047 (2016)

Da-Eun Yoon, Chihyun Hwang*, Na-Ri Kang, Ungju Lee, Dongjoon Ahn, Ju-Young Kim, Hyun-Kon Song*

Molecular structures of polysaccharide binders determining mechanical properties were correlated to electrochemical performances of silicon anodes for lithium ion batteries. Glycosidic linkages (α and β) and side chains (-COOH and -OH) were selected and proven as the major factors of the molecular structures.  Three different single-component polysaccharides were compared: pectin for α-linkages versus carboxylic methyl cellulose (CMC) for β-linkages from the linkage’s standpoint; and pectin as a COOH-containing polymer and amylose as its non-COOH counterpart from the side chain’s standpoint. Pectin was remarkably superior to CMC and amylose in cyclability and rate capability of battery cells based on silicon anodes. The pectin binder allowed volume expansion of silicon electrodes with keeping high porosity during lithiation due to the elastic nature caused by the chair-to-boat conformation in α-linkages of its backbone. Physical integrity of pectin-based electrodes was not challenged during repeated lithiation/delithiation cycles without crack development that was observed in rigid CMC-based electrodes. Covalent bonds formed between carboxylic side chains of pectin and silicon surface oxide prevented active silicon mass from being detached away from electric pathways. However, hydrogen bonds between hydroxyl side chains of amylose and silicon surface oxide were not strong enough to keep silicon mass electrochemically active after cyclability tests.

71. Conductivity-Dependent Completion of Oxygen Reduction on Oxide Catalysts

Angewandte Chemie International Edition 54, 15730-15733 (2015)

Dong-Gyu Lee, Ohhun Gwon, Han-Saem Park, Su Hwan Kim, Juchan Yang, Sang Kyu Kwak, Guntae Kim*, Hyun-Kon Song*

Conductivity makes the difference: Conductive environments surrounding active sites, achieved by more conductive perovskite catalysts (BSCFO, NBSCO) or higher carbon contents, result in a higher number of electrons transferred during complete four-electron (4e) reduction of oxygen, changing the rate-determining step from a two-step 2e process to a single-step 1e process.

70. Breathing silicon anodes for durable high-power operations

Scientific Reports 5, 14433 (2015)

Chihyun Hwang, Sehun Joo, Na-Ri Kang, Ungju Lee, Tae-Hee Kim, Yuju Jeon, Jieun Kim, Young-Jin Kim, Ju-Young Kim,* Sang-Kyu Kwak* & Hyun-Kon Song*

Silicon anode materials have been developed to achieve high capacity lithium ion batteries for operating smart phones and driving electric vehicles for longer time. Serious volume expansion induced by lithiation, which is the main drawback of silicon, has been challenged by multi-faceted approaches. Mechanically rigid and stiff polymers (e.g. alginate and carboxymethyl cellulose) were considered as the good choices of binders for silicon because they grab silicon particles in a tight and rigid way so that pulverization and then break-away of the active mass from electric pathways are suppressed. Contrary to the public wisdom, in this work, we demonstrate that electrochemical performances are secured better by letting silicon electrodes breathe in and out lithium ions with volume change rather than by fixing their dimensions. The breathing electrodes were achieved by using a polysaccharide (pullulan), the conformation of which is modulated from chair to boat during elongation. The conformational transition of pullulan was originated from its α glycosidic linkages while the conventional rigid polysaccharide binders have β linkages.

69. Selectively accelerated Li+ transport to Si anodes via an organogel binder

J. Power Sources 298, 8–13 (2015)

Chihyun Hwang, Yoon-Gyo Cho, Na-Ri Kang, Younghoon Ko, Ungju Lee, Dongjoon Ahn, Ju-Young Kim, Young-Jin Kim and Hyun-Kon Song*

Silicon, a promising high-capacity anode material of lithium ion batteries, suffers from its volume expansion leading to pulverization and low conductivities, showing capacity decay during cycling and low capacities at fast charging and discharging. In addition to popular active-material-modifying strategies, building lithium-ion-rich environments around silicon surface is helpful in enhancing unsatisfactory performances of silicon anodes. In this work, we accelerated lithium ion transport to silicon surface by using an organogel binder to utilize the electroactivity of silicon in a more efficient way. The cyanoethyl polymer (PVA-CN), characterized by high lithium ion transference number as well as appropriate elastic modulus with strong adhesion, enhanced cycle stability of silicon anodes with high coulombic efficiency even at high temperature (60 oC) as well as at fast charging/discharging rates.

68. High-performance silicon-based multicomponent battery anodes produced via synergistic coupling of multifunctional coating layers

Energy & Environmental Science 8, 2075-2084 (2015)

Jung-In Lee, Younghoon Ko, Myoungsoo Shin, Hyun-Kon Song, Nam-Soon Choi, Min Gyu Kim* and Soojin Park*

Nanostructured Si-based materials are key building blocks for next-generation energy storage devices. To meet the requirements of practical energy storage devices, Si-based materials should exhibit high-power, low volume change, and high tap density. So far, there have been no reliable materials reported satisfying all of these requirements. Here, we report a novel Si-based multicomponent design, in which the Si core is covered with multifunctional shell layers. The synergistic coupling of Si with the multifunctional shell provides vital clues for satisfying all Si anode requirements for practical batteries. The Si-based multicomponent anode delivers a high capacity of [similar]1000 mA h g−1, a highly stable cycling retention ([similar]65% after 1000 cycles at 1 C), an excellent rate capability ([similar]800 mA h g−1 at 10 C), and a remarkably suppressed volume expansion (12% after 100 cycles). Our synthetic process is simple, low-cost, and safe, facilitating new methods for developing electrode materials for practical energy storage.

67. All-in-one assembly based on 3D-intertangled and cross-jointed architectures of Si/Cu 1D-nanowires for lithium ion batteries

Scientific Reports 5, 8623 (2015)

Chihyun Hwang, Tae-Hee Kim, Yoon-Gyo Cho, Jieun Kim and Hyun-Kon Song*

All-in-one assemblies of separator, electrode and current collector (SECA) for lithium ion batteries are presented by using 1D nanowires of Si and Cu (nwSi and nwCu). Even without binders, integrity of SECA is secured via structural joints based on ductility of Cu as well as entanglement of nwSi and nwCu. By controlling the ratio of the nanowires, the number of contact points and voids accommodating volume expansion of Si active material are tunable. Zero volume expansion and high energy density are simultaneously achievable by the architecture.

66. Multiple roles of superoxide on oxygen reduction reaction in Li+-containing non-aqueous electrolyte: Contribution to formation of oxide as well as peroxide

The Journal of Physical Chemistry C 119, 3472–3480 (2015)

V. S. Dilimon, Dong-Gyu Lee, Sung-Dae Yim and Hyun-Kon Song*

Understanding Li-O2 electrochemistry without ambiguities is the basis required for achieving high energy density with efficient cycling for lithi-um-air batteries. Oxygen reduction reaction (ORR) on carbon is supposed to proceed via a three step mechanism: oxygen (O20) to superoxide (O2- or LiO2) to peroxide (O22- or Li2O2) to oxide (O2- or Li2O). In this work, we provide clear evidences relevant to three controversial issues: (1) whether the superoxide intermediate is really formed; (2) whether the superoxide exists for a significant time period or they are immediately con-verted to the peroxide species; and (3) whether conversion of peroxide to oxide is feasible or the final discharge product of ORR is not oxide but peroxide. ORR on carbon electrode with LiClO4 or LiPF6 in dimethyl sulfoxide (DMSO) as an electrolyte was used as a model system. In addi-tion to conventional voltammetry and Raman spectroscopy, the staircase cyclic voltammetry combined with Fourier transform electrochemical impedance spectroscopy (SCV-FTEIS) was used to investigate the Li-O2 electrochemistry in situ during potential scans. Molecular oxygen was quasi-reversibly reduced to superoxide in the first step. The superoxide was stable enough to be detected in the electrolytes. The superoxide was reduced to peroxide that existed as a surface-adsorbed species. Reduction proceeded further to produce oxide as its final product. Also, Li2CO3 formation resulting from electrolyte decomposition/electrode corrosion was observed only with LiClO4. In addition, we identified a novel chemical route for the oxide formation occurring even at not-enough overpotential: peroxide is further reduced to oxide by the help of superoxide as an in situ formed one-electron reducing agent. This report gives a clear picture of the ORR mechanism on carbon electrode in Li+-containing non-aqueous electrolyte by providing evidences for the formation of superoxide intermediate and oxide for the first time.

65. Highly Porous Piezoelectric PVDF Membrane as Effective Lithium Ion Transfer Channels for Enhanced Self-Charging Power Cell

Nano Energy 14, 77-86 (2015)

Young-Soo Kim, Yannan Xie, Xiaonan Wen, Sihong Wang, Sang Jae Kim, Hyun-Kon Song* and Zhong Lin Wang*  

A self-charging power cell (SCPC) is a structure that hybridizes the mechanisms for energy conversion and storage into one process through which mechanical energy can be directly converted into electrochemical energy. A key structure of an SCPC is the use of a polyvinylidene fluoride (PVDF) piezo-separator. Herein, we have fabricated a piezoelectric β-form PVDF separator with a highly porous architecture by introducing ZnO particles. The electrochemical charge/discharge performance of this SCPC was greatly enhanced at lower discharge rates compared to highly stretched (high-β-content) or less porous PVDF membranes. The lower charge-transfer resistance of this well-developed porous piezo-separator is the main factor that facilitated the transport of Li+ ions without sacrificing piezoelectric performance. This study reveals a novel approach for improving the performance of SCPCs.

64. General Approach for High-Power Li-Ion Batteries: Multiscale Lithographic Patterning of Electrodes

ChemSusChem 7, 3483-3490 (2014)

Sinho Choi†, Tae-Hee Kim†, Jung-In Lee, Jieun Kim, Hyun-Kon Song*, Soojin Park*

We demonstrate multiscale patterned electrodes that provide surface-area enhancement and strong adhesion between electrode materials and current collector. The combination of multiscale structured current collector and active materials (anodes and cathodes) enables us to make high-performance Li-ion batteries (LIBs). When LiFePO4 (LFP) cathode and Li4Ti5O12 (LTO) anode materials are combined with patterned current collectors, their electrochemical performances are significantly improved, including a high rate capability (LiFePO4: 100 mAh g−1, Li4Ti5O12: 60 mAh g−1 at 100C rate) and highly stable cycling (LiFePO4: capacity retention of 99.8 % after 50 cycles at 10C rate). Moreover, we successfully fabricate full cell system consisting of patterned LFP cathode and patterned LTO anode, exhibiting high-power battery performances [capacity of approximately 70 mAh g−1 during 1000 cycles at 10C rate (corresponding to charging/discharging time of 6 min)]. We extend this idea to Si anode that exhibits a large volume change during lithiation/delithiation process. The patterned Si electrodes show significantly enhanced electrochemical performances, including a high specific capacity (825 mAh g−1) at high rate of 5C and a stable cycling retention (88 % after 100 cycle at a 0.1C rate). This simple strategy can be extended to other cathode and anode materials for practical LIB applications.

63. High-yield synthesis of single-crystal silicon nanoparticles as anode materials of lithium ion batteries via photosensitizer-assisted laser pyrolysis

Journal of Materials Chemistry A 2, 18070-18075 (2014)

Seongbeom Kim, Chihyun Hwang, Song Yi Park, Seo-Jin Ko, Hyungmin Park, Won Chul Choi, Jong Bok Kim, Dong Suk Kim, Soojin Park, Jin Young Kim* and Hyun-Kon Song*

Single crystal silicon nanoparticles (Si-NPs) of 20 nm were produced via laser pyrolysis with a virtually complete conversion from SiH4 to Si-NPs. SF6 was used as the photosensitizer to transfer laser beam energy to silicon precursors, dramatically enhancing crystallinity of Si-NPs and their production efficiency. By using their well-developed crystalline structure, the directional volume expansion of Si-NPs was confirmed during lithiation. Lithiation/delithiation kinetics of our Si-NPs was superior to that of their amorphous counterparts due to the footprinted Li+ pathways formed during amorphization.

62. LiFePO4 Nanostructures Fabricated from Iron(III) Phosphate (FePO4 · 2H2O) by Hydrothermal Method

J. Nanoscience Nanotechnology 15, 734-741 (2015)

Viswanathan S. Saji and Hyun-Kon Song

Electrode materials having nanometer scale dimensions are expected to have property enhancements due to enhanced surface area and mass/charge transport kinetics. This is particularly relevant to intrinsically low electronically conductive materials such as lithium iron phosphate (LiFePO4), which is of recent research interest as a high performance intercalation electrode material for Liion batteries. Many of the reported works on LiFePO4 synthesis are unattractive either due to the high cost of raw materials or due to the complex synthesis technique. In this direction, synthesis of LiFePO4 directly from inexpensive FePO4 shows promise. The present study reports LiFePO4 nanostructures prepared from iron (III) phosphate (FePO4 ·2H2O) by precipitation-hydrothermal method. The sintered powder was characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), Inductive coupled plasma-optical emission spectroscopy (ICP-OES), and Electron microscopy (SEM and TEM). Two synthesis methods, viz. bulk synthesis and anodized aluminum oxide (AAO) template-assisted synthesis are reported. By bulk synthesis, micro-sized particles having peculiar surface nanostructuring were formed at precipitation pH of 6.0 to 7.5 whereas typical nanosized LiFePO4 resulted at pH ≥8.0. An in-situ precipitation strategy inside the pores of AAO utilizing the spin coating was utilized for the AAO-template-assisted synthesis. The template with pores filled with the precipitate was subsequently subjected to hydrothermal process and high temperature sintering to fabricate compact rod-like structures.

61. Synthesis of a Redox-Active Denpol as a Potential Electrode in Rechargeable Organic Batteries

ChemElectroChem 1, 1618–1622 (2014)

Jonggi Kim, Jieun Kim, Jungho Lee, Hyun-Kon Song* and Changduk Yang*

Together with ROMP strategy, a controlled protocol of a progressive addition of the ultrafast-initiating and more reactive third generation Grubbs catalyst yields a denpol containing multi-anthraquinone (AQ)-terminated dendrons (AQ-ter-Denpol) (Mn = 14.0 kDa) with a unimodal molar mass distribution (PDI = 1.20), fully characterized by 1H NMR. AQ-ter-denpol is investigated as a novel organic cathode material for rechargeable Li-ion batteries with a view to its unique morphology. Our methodologies herein reported establish an opportunity for developing a new generation of organic electrodes based on denpols.

60. Preparation of Co3O4 electrode materials with different microstructures via pseudomorphic conversion of Co-based metal-organic frameworks

Journal of Materials Chemistry A 2, 14393-14400 (2014)

Kyung Joo Lee, Tae-Hee Kim, Tae Kyung Kim, Jae Hwa Lee, Hyun-Kon Song* and Hoi Ri Moon*

To develop high-performance nanostructured metal oxide electrodes, it is important to understand the structural effects on electrochemical performances. Thus, the preparation of metal oxide materials which have well-tailored nanostructures is crucial for the studies. However, while the synthetic strategies to control the size of metal oxide nanoparticles are well-developed, the control of those higher level structures, namely microstructure, is not established very well. Herein, we present the synthesis of two kinds of Co3O4 nanomaterials through pseudomorphic conversion that the macroscopic morphologies of parent MOFs such as plate-like and rod-like shape were well-maintained. Both Co3O4 nanomaterials are composed of almost identical 10 nm-sized primary nanocrystals, but those nanoporous secondary structures and macroscopic morphologies such as plate and rod shapes are different. Those Co3O4 nanomaterials were utilized as an electrode of lithium ion batteries (LIBs), and their electrochemical properties were comparatively studied. It was revealed that the different cyclability and rate capability are attributed to their different microstructures. Pseudo-monolithic integration of the primary and secondary structures at higher level was the governing factor to determine the electrochemical performances of the Co3O4.

59. Conducting polymer-skinned electroactive materials of lithium ion batteries: Ready for mono-component electrodes without additional binders and conductive agents

ACS Applied Materials & Interface 6, 12789–12797 (2014)

Ju-Myung Kim, Han-Saem Park, Jang-Hoon Park, Tae-Hee Kim, Hyun-Kon Song* and  Sang-Young Lee*

Rapid growth of mobile and even wearable electronics is in pursuit of high-energy density lithium-ion batteries. One simple and facile way to achieve this goal is the elimination of non-electroactive components of electrodes such as binders and conductive agents. Here, we present a new concept of mono-component electrodes comprising solely electroactive materials that are wrapped with an insignificant amount (less than 0.4 wt.%) of conducting polymer (PEDOT:PSS or poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate)). The PEDOT:PSS as ultra-skinny surface layer on electroactive materials (LiCoO2 (LCO) powders are chosen as a model system to explore feasibility of this new concept) successfully acts as a kind of binder as well as mixed (both electrically and ionically) conductive film, playing a key role in enabling the mono-component electrode. The electric conductivity of mono-component LCO cathode is controlled by simply varying PSS content and also structural conformation (benzoid-favoring coil strucutre and quinoid-favoring linear or extended coil structure) of PEDOT in the PEDOT:PSS skin. Notably, substantial increase in mass-loading density of LCO cathode is realized with the PEDOT:PSS skin without sacrificing electronic/ionic transport pathways. We envisage that the PEDOT:PSS-skinned electrode strategy opens a scalable and versatile route for making practically meaningful binder-/conductive agent-free (mono-component) electrodes.

58. Surface Complex Formation between Aliphatic Nitrile Molecules and Transition Metal Atoms for Thermally Stable Lithium Ion Batteries

ACS Applied Materials & Interface 6, 8913–8920 (2014)

Young-Soo Kim, Hochun Lee, Hyun-Kon Song*

Non-flammability of electrolyte and tolerance of cells against thermal abuse should be guaranteed for widespread applications of lithium-ion batteries (LIBs). As a strategy to improve thermal stability of LIBs, here, we report on nitrile-based molecular coverage on surface of cathode active materials to block or suppress thermally-accelerated side reactions between electrode and electrolyte. Two different series of aliphatic nitriles were introduced as an additive into a carbonate-based electrolyte: di-nitriles (CN-[CH2]n-CN with n = 2, 5 and 10) and mono-nitriles (CH3-[CH2]m-CN with m = 2, 5 and 10). On the basis of the strong interaction between the electronegativity of nitrile functional groups and the electropositivity of cobalt in LiCoO2 cathode, aliphatic mono- and di-nitrile molecules improved the thermal stability of lithium ion cells by efficiently protecting the surface of LiCoO2. Three factors, the surface coverage , the steric hindrance of aliphatic moiety within nitrile molecule and the chain polarity, mainly affect thermal tolerance as well as cell performances at elevated temperature.

57. Programming galvanostatic rates for fast-charging lithium ion batteries: a graphite case

RSC Advances 4, 16545-16550 (2014)

Younghoon Ko, Yoon-Gyo Cho, Hyun-Kon Song*

Galvanostatically induced lithiation of graphite, as a cathodic process of lithium ion batteries during charging, was investigated in situ by galvanostatic electrochemical impedance spectroscopy (GS-EIS). When lithiation is driven by charge rates slow enough for kinetics of the lithiation process to be considered relatively sluggish, charge transfer resistance (RCT) is slightly reduced as lithium ion intercalation proceeds from the dilute stage to stage 2L. Subsequently, RCT begins to increase during transformation of stage 2L to stage 2, followed by an abrupt increase in RCT observed during transition from stage 2 to stage 1, or after the inter-space of graphites is fully filled with lithium ions. As the ratio of charge rate to lithiated graphite increases, the potential responsible for the transition from stage 2L to stage 2 is shifted to more negative values due to significant polarization. Simultaneously, cells reach cut-off potentials before the transition from stage 2 to stage 1 proceeds. Based on the information regarding RCT profiles obtained by galvanostatic charging processes, a charging strategy is programmed with several different charge rates (C-rates). The capacity of lithiation is significantly enhanced by a C-rate switching (CRS) strategy. As a representative example, 75% of available capacity is charged for 50 minutes by a combination of 2 C, 1 C, and 0.5 C. However, only 12% and 51% of graphite is lithiated within the same time duration by a single charge rate of 0.1 C and 0.5 C, respectively.

56. Enlarging d-spacing of graphite and polarizing its surface charge for driving lithium ions fast

Journal of Materials Chemistry A 2, 7600-7605 (2014)

Tae-Hee Kim, Eun Kyung Jeon, Younghoon Ko, Bo Yun Jang, Byeong-Su Kim,* and Hyun-Kon Song*

Lithium ion transport was accelerated within graphites by controlling its d-spacing as well as its functional groups. By oxidizing bare graphite in a mild condition, expanded graphites (EG* where * = functional groups) were obtained with increasing d-spacing from 0.3359 nm to 0.3395 nm as well as with functional groups formed on the plane or at the edges of graphites. The subsequent thermal reduction of EG* led to insignificant change of d-spacing (0.3390 nm), simultaneously eliminating a portion of the functional groups (EG). The enlargement of d-spacing reduced kinetic hindrance of lithium ion movement within the expanded graphites (EG* and EG) by reserving more space for ionic transport route. In addition, the activation energy of lithium ion intercalation in EG* was reduced by surface charge polarization of graphites induced by hydrogen bonds between oxygen atoms of carbonates in electrolytes and hydrogen atoms of surface functional groups of the expanded graphites, even if degree of graphitization decreased. Re-graphitization induced by the subsequent thermal reduction increased delithiation capacities (QdLi) of EG as an anode for lithium ion batteries especially at high currents:  QdLi at 50C = 243 mAh g-1 for EG versus 66 mAh g-1 for bare graphite.

55. Doubling Capacity of Lithium Manganese Oxide Spinel via Flexible Skinny Graphitic Layer

Angewandte Chemie International Edition 53, 5059–5063 (2014)

Hyun Kuk Noh,* Han-Saem Park, Hu Young Jeong, Sang Uck Lee, and Hyun-Kon Song*

By coating nanoparticular lithium manganese oxide (LMO) spinel with a few layers of graphitic basal planes, the capacity of the material reached up to 220 mA h g−1 at a cutoff voltage of 2.5 V. The graphitic layers 1) provided a facile electron-transfer highway without hindering ion access and, more interestingly, 2) stabilized the structural distortion at the 3 V region reaction. The gain was won by a simple method in which microsized LMO was ball-milled in the presence of graphite with high energy. Vibratory ball milling pulverized the LMO into the nanoscale, exfoliated graphite of less than 10 layers and combined them together with an extremely intimate contact. Ab initio calculations show that the intrinsically very low electrical conductivity of the tetragonal phase of the LMO is responsible for the poor electrochemical performance in the 3 V region and could be overcome by the graphitic skin strategy proposed.

54. Nitrile-assistant eutectic electrolytes for cryogenic operation of lithium ion batteries at fast charges and discharges

Energy & Environmental Science 7, 1737-1743 (2014)

Yoon-Gyo Cho, Young-Soo Kim, Dong-Gil Sung, Myung-Su Seo, Hyun-Kon Song*

The charge/discharge characteristics at low temperature (LT = -20 oC) are enhanced by using ethylene carbonate (EC)-based electrolytes with the help of assistant solvents of nitriles. Conventional liquid electrolytes (e.g. a mixture of EC and dimethyl carbonate (DMC), abbreviated by LD) cannot support satisfactory capacity at the low temperature as well as high rates even if electric vehicles require low temperature operation. Introducing propionitrile or butyronitrile (Pn or Bn) into LD (resulting in LDPn or LDBn) as a co-solvent increases significantly the high-rate capacities at -20 oC. For example, LDPn delivers 62 % of available capacity at 1C and 46 % at 3C with 2.7 V cut-off while the control LD provides just 6 % and 4 % at the same rates. The successful operation at -20 oC with the nitrile-assistant electrolytes results from high ionic conductivity, low viscosity and freezing point depression caused by eutectic behavior of carbonates (EC/DMC) and Pn. Based on phase diagram of Pn with EC/DMC, we expect meaningful battery operation up to -110 oC at the eutectic composition with 18 % Pn in 1:1 EC/DMC.

53. Succinonitrile as a Corrosion Inhibitor of Copper Current Collectors for Overdischarge Protection of Lithium Ion Batteries

ACS Appl. Mater. Interfaces 6, 2039-2043 (2014)

Young-Soo Kim, Seon-Ha Lee, Mi-Young Son, Young Mee Jung, Hyun-Kon Song*, and Hochun Lee*


Succinonitrile (SN) is investigated as an electrolyte additive for copper corrosion inhibition to provide overdischarge (OD) protection to lithium ion batteries (LIBs). The anodic Cu corrosion, occurring above 3.5 V (vs Li/Li+) in conventional LIB electrolytes, is suppressed until a voltage of 4.5 V is reached in the presence of SN. The corrosion inhibition by SN is ascribed to the formation of an SN-induced passive layer, which spontaneously develops on the copper surface during the first anodic scan. The passive layer is composed mainly of Cu(SN)2PF6 units, which is evidenced by Raman spectroscopy and electrochemical quartz crystal microbalance measurements. The effects of the SN additive on OD protection are confirmed by using 750 mAh pouch-type full cells of LiCoO2 and graphite with lithium metal as a reference electrode. Addition of SN completely prevents corrosion of the copper current collector in the full cell configuration, thereby tuning the LIB chemistry to be inherently immune to the OD abuses.

52. An inter-tangled network of redox-active and conducting polymers as a cathode for ultrafast rechargeable batteries

Phys. Chem. Chem. Phys. 16, 5295-5300 (2014)

Jieun Kim, Han-Saem Park, Tae-Hee Kim, Sung Yeol Kim, Hyun-Kon Song*


An 1D organic redox-active material is composited with another 1D conductive material for rechargeable batteries. Poly(vinyl carbazole) (or PVK) and Poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (or PEDOT:PSS) are used as the redox-active and conductive 1D materials, respectively. Due to their extreme anisotropic geometry, the two polymers are expected to be inter-tangled with each other, showing an kinetically ideal model system in which each redox-active moiety of PVK is supposed to be directly connected with the conducting pathways of PEDOT:PSS. In addition to the role of conductive agents providing kinetic benefits, PEDOT:PSS works as an efficient binder that guarantees enhanced electrochemical performances with only a tenth of the amount of a conventional binder (polyvinylidene fluoride or PVdF). The benefit of gravimetric energy density gain obtained with the conductive binder comes mainly from efficient spatial coverage of binding volume due to the low density of PEDOT:PSS. Towards realizing flexible all-polymer batteries, a quasi-all-polymer battery half cell is designed with the PVK/ PEDOT:PSS composite with a polymer gel electrolyte.

51. Study on the Electrochemical Kinetics of Manganese Dioxide/Multiwall Carbon Nanotube Composite by Voltammetric Charge Analysis

J. Electrochem. Soc. 161, A137-A141 (2014)

Seung-Beom Yoon, Hyun-Kon Song, Kwang Chul Roh,* Kwang-Bum Kim*


Electrochemical properties of MnO2/multiwall carbon nanotube (MWCNT) composites were investigated by using cavity microelectrodes (CME). Electrochemical kinetics of the MnO2/MWCNT composites were studied by analyzing the scan rate dependence of voltammetric charge, which was measured by cyclic voltammetry (CV) at various scan rates ranging from 20 mV s−1 to 1000 mV s−1. Based on several mathematical models, the relationship between voltammetric charge and scan rate was interpreted systematically. At slow scan rates, ion diffusion in MnO2 dominantly determined the rate of the overall electrochemical process. However, the faradaic reaction of Mn3+/Mn4+ at the MnO2 surface competed with mass transfer in terms of kinetics when the potential scan rate was higher than 400 mV s−1

50. Kinetically enhanced pseudocapacitance of conducting polymer doped with reduced graphene oxide through a miscible electron transfer interface

Nano Energy 3, 1-9 (2014)

Han-Saem Park, Myeong-Hee Lee, Ryeo Yun Hwang, Ok-Kyung Park, Kiyoung Jo, Taemin Lee, Byeong-Su Kim*, Hyun-Kon Song* http://dx.doi.org/10.1016/j.nanoen.2013.10.001

Herein, we report on electrochemical doping of a conducting polymer (CP) with anionically modified graphene nanosheets. The architecture built from reduced graphene oxide (rGO) skeleton skinned by polypyrrole (pPy) enhanced supercapacitor performances especially at high discharge rates superior to those of the same CP with a conventional dopant: e.g., from 141 to 280 F g−1 at 1000C equivalent to ~50 A g−1. At relatively low rates, the graphene-doped pPy reached the theoretical capacitance of pPy, indicating efficient use of whole electroactive mass.

49. Redox-active charge carriers of conducting polymers as a tuner of conductivity and its potential window

Scientific Reports 3, 2454 (2013)

Han-Saem Park, Seo-Jin Ko, Jeong-Seok Park, Jin Young Kim and Hyun-Kon Song*


Electric conductivity of conducting polymers has been steadily enhanced towards a level worthy of being called its alias, “synthetic metal”. PEDOT:PSS (poly(3,4-ethylenedioxy thiophene) doped with poly(styrene sulfonate)), as a representative conducting polymer, recently reached around 3,000 S cm−1, the value to open the possibility to replace transparent conductive oxides. The leading strategy to drive the conductivity increase is solvent annealing in which aqueous solution of PEDOT:PSS is treated with an assistant solvent such as DMSO (dimethyl sulfoxide). In addition to the conductivity enhancement, we found that the potential range in which PEDOT:PSS is conductive is tuned wider into a negative potential direction by the DMSO-annealing. Also, the increase in a redox-active fraction of charge carriers is proposed to be responsible for the enhancement of conductivity in the solvent annealing process.

48. A physical organogel electrolyte: characterized by in situ thermo-irreversible gelation and single-ion-predominent conduction

Scientific Reports 3, 1917 (2013)

Young-Soo Kim, Yoon-Gyo Cho, Dorj Odkhuu, Noejung Park* and Hyun-Kon Song*


Electrolytes are characterized by their ionic conductivity (σi). It is desirable that overall σi results from the dominant contribution of the ions of interest (e.g. Li+ in lithium ion batteries or LIB). However, high values of cationic transference number (t+) achieved by solid or gel electrolytes have resulted in low σi leading to inferior cell performances. Here we present an organogel polymer electrolyte characterized by a high liquid-electrolyte-level σi (~101 mS cm-1) with high t+ of Li+ (> 0.8) for LIB. A conventional liquid electrolyte in presence of a cyano resin was physically and irreversibly gelated at 60 oC without any initiators and crosslinkers, showing the behavior of lower critical solution temperature. During gelation, σi of the electrolyte followed a typical Arrhenius-type temperature dependency, even if its viscosity increased dramatically with temperature. Based on the Li+-driven ion conduction, LIB using the organogel electrolyte delivered significantly enhanced cyclability and thermal stability.

47. Hollow versus nonhollow: the electrochemical preference in a case study of the conversion reaction of Fe3O4

Electrochimica Acta 105, 47-52 (2013)

Tae-Hee Kim and Hyun-Kon Song*


Hollow sphere geometry is compared with the corresponding nonhollow one in terms of its electrochemical benefits. Lithiation of Fe3O4 and its backward reaction is chosen as a case study process. Dimension of the hollow structure is carefully controlled to have the same mass per a single particle as that of its nonhollow counterpart. The comparison shows the possibility that the faradaic performances of hollow geometry could be better than those of the nonhollow in terms of volumetric as well as gravimetric capacities, even if the hollow has void in its centre.

46. Facile Route to an Efficient NiO Supercapacitor with a Three-Dimensional Nanonetwork Morphology

ACS Applied Materials & Interfaces 5, 1596-1603 (2013)

Sun-I Kim, Jung-Soo Lee, Hyo-Jin Ahn, Hyun-Kon Song, Ji-Hyun Jang*


45. Edge-Exfoliated Graphites for Facile Kinetics of Delithiation

ACS nano 6, 10770-10775 (2012)

Jeong-Seok Park, Myeong-Hee Lee, In-Yup Jeon, Han-Saem Park, Jong-Beom Baek,* Hyun-Kon Song*


As high rate charge and discharge characteristics of energy storage devices become more important with the market of electric vehicles intensively growing, the kinetics of lithiation or delithiation of electrode materials for lithium ion batteries are required to be enhanced. Graphites, the most widely used anode materials, have a limited power density at high discharge rates while their alternatives such as silicon and transition metal oxides show even inferior rate capability. This work was motivated from an idea of what if the edge opening of graphite was zipped more open to lithium ions in electrolyte. By edge-selective functionalization, the peripheral d-spacing of graphite (d0) was locally controlled. Larger values of d0 led to higher capacity especially at high discharge rates. Around two-fold enhancement of capacity or energy density was achieved at 50C discharge rate from 110 mAh/g to 190 mAh/g by exfoliating graphite locally in its edge region. Also, the d0 dependency of delithiation kinetics confirmed that the electrochemical step of Li+ influx into or efflux out of interlayer space of graphite is possibly the rate determining step of lithiation or delithiation.

44. Hybrid multilayer thin film supercapacitor of graphene nanosheets with polyaniline: importance of establishing intimate electronic contact through nanoscale blending

J. Mater. Chem. 22, 21092-21099 (2012)

Taemin Lee , Taeyeong Yun , Byeongho Park , Bhawana Sharma , Hyun-Kon Song and Byeong-Su Kim*


A hybrid electrode consisting of an electric double-layer capacitor of graphene nanosheets and a pseudocapacitor of the conducting polymer polyaniline exhibits a synergistic effect with excellent electrochemical performance for flexible thin film supercapacitors. This hybrid supercapacitor is constructed by a nanoscale blending method of layer-by-layer (LbL) assembly based on the electrostatic interactions between positively charged polyaniline (PANi) and negatively charged graphene oxide (GO) nanosheets. The hybrid electrode provides not only improved electronic conductivity through the intimate contact with the graphene nanosheet, but also enhanced chemical stability during the charge–discharge process. We also investigated the dependence of the electrochemical performance on the various parameters of LbL assembly such as the number of bilayers and the post-thermal and chemical treatments that could affect the degree of reduction of GO and PANi. We found that after thermal treatment, the LbL-assembled thin film of PANi with GO nanosheets exhibited an excellent gravimetric capacitance of 375.2 F g−1 at a discharge current density of 0.5 A g−1 that outperformed many other hybrid supercapacitors reported to date. The hybrid supercapacitor maintained its capacity up to 90.7% over 500 cycles at a high current density of 3.0 A g−1. This study opens up the possibility for the production of diverse graphene-based hybrid nanocomposites that are promising for future flexible supercapacitors.

43. Catalytic carbonization of an uncarbonizable precursor by transition metals in olivine cathode materials of lithium ion batteries

J. Mater. Chem. 22, 20305-20310 (2012)

Han-Saem Park, Tae-Hee Kim, Myeong-Hee Lee and Hyun-Kon Song*


Herein, we report on catalytic effects of transition metals (Me) in phospho-olivines (LiMePO4) on carbonization of cetyltrimethylammonium bromide (CTAB). Carbon coating is the required process to enhance electronic conductivity of phospho-olivines that are used as cathode materials for lithium ion batteries. Primary particles of phospho-olivines were in situ coated with CTAB and the adsorbed carbon precursor was carbonized to provide electrically conductive pathway. CTAB was successfully carbonized in a significant amount with Fe in phospho-olivines (LiFexMn1-xPO4 with x=1 and 0.5) even if CTAB is thermally decomposed around 300 °C without any residual mass in absence of the phospho-olivines. LiMnPO4 was the most inferior in terms of CTAB adsorption and thermal carbonization. LiNiPO4 and LiCoPO4 showed inefficient conversion of adsorbed CTAB to carbon even if their adsorption ability of CTAB is quite large. Also, the effect of the amount of carbon coating on LiFePO4 was investigated, leading to a conclusion that the carbon thickness balanced between electronic and ionic conductance results in the best electrochemical performances of lithium ion batteries specifically at high discharge rates.

42. Carbon-Coated Single-Crystal LiMn2O4 Nanoparticle Clusters as Cathode Material for High-Energy and High-Power Lithium-Ion Batteries

Angewandte Chemie International Edition 51, 8748-8752 (2012)

Sanghan Lee, Yonghyun Cho, Hyun-Kon Song, Kyu Tae Lee, Jaephil Cho*


41. Graphene Multilayer Supported Gold Nanoparticles for Efficient Electrocatalysts toward Methanol Oxidation

Advanced Energy Materials, 2, 1510-1518 (2012)

Yuri Choi, Minsu Gu, Jongnam Park, Hyun-Kon Song and Byeong-Su Kim*


40. Versatile Double Hydrophilic Block copolymer: Dual Role as Synthetic Nanoreactor and Ionic and Electronic Conduction Layer for Ruthenium Oxide Nanoparticle Supercapacitor

J. Mater. Chem., 22, 11598-11604 (2012)

Eunyong Seo, Taemin Lee, Kyu Tae Lee, Hyun-Kon Song and Byeong-Su Kim*


39. The Current Move of Lithium Ion Batteries towards the Next Phase

Advanced Energy Materials, 2, 860–872 (2012)

Tae-Hee Kim, Jeong-Seok Park, Sung Kyun Chang, Seungdon Choi,* Ji Heon Ryu,* Hyun-Kon Song*


Application targets of lithium ion batteries (LIBs) are moving from small-sized mobile devices of information technology to large-scale electric vehicles (xEVs) and energy storage systems (ESSs). Environmental issues and abruptly increasing power demands are pushing high performance energy storage devices or systems onto markets. LIBs are one of the most potential candidates as the energy storage devices mainly due to their high energy densities with fairly good rate capabilities and a fairly long cycle life. As battery systems become larger in terms of stored energy as well as physical size, the safety concerns should be more seriously cared. Each application target has its own specification so that electrode materials should be chosen to meet requirements of the corresponding application. This report diagnoses the current market trends of LIBs as a primary topic, followed by giving an overview of anode and cathode material candidates of LIBs for xEVs and ESSs based on their electrochemical properties.

38. A polymer electrolyte-skinned active material strategy toward high-voltage lithium ion batteries: polyimide-coated LiNi0.5Mn1.5 spinel cathode material case

Energy & Environmental Science, 5, 7124-7131 (2012)

Ju-Hyun Cho, Jang-Hoon Park, Myeong-Hee Lee, Hyun-Kon Song,* Sang-Young Lee*

Top 10 most read in April, May 2012

A facile approach to the surface modification of spinel LiNi0.5Mn1.5O4 (LNMO) cathode active materials for high-voltage lithium ion batteries is demonstrated. This strategy is based on the nanoarchitectured polyimide (PI) gel polymer electrolyte (GPE) coating. The PI coating layer successfully wrapped a large area of the LNMO surface via thermal imidization of 4-component (pyromellitic dianhydride/biphenyl dianhydride/phenylenediamine/oxydianiline) polyamic acid. In comparison to conventional metal oxide-based coatings, distinctive features of the unusual PI wrapping layer are the highly-continuous surface coverage with nanometer thickness (~ 10 nm) and the provision of facile ion transport. The nanostructure-tuned PI wrapping layer served as an ion-conductive protection skin to suppress the undesired interfacial side reactions, effectively preventing the direct exposure of LNMO surface to liquid electrolyte. As a result, the PI wrapping layer played a crucial role in improving the high-voltage cell performance and alleviating the interfacial exothermic reaction between charged LNMO and liquid electrolyte. Notably, the superior cycle performance (at 55 oC) of PI-wrapped LNMO (PI-LNMO) was elucidated in great detail by quantitatively analyzing manganese (Mn) dissolution, cell impedance, and chemical composition (specifically, lithium fluoride (LiF)) of byproducts formed on the LNMO surface.

37. Optimized evolution of a secondary structure of LiFePO4: Balancing between shape and impurity

Journal of Materials Chemistry, 22, 8228-8234 (2012)

Myeong-Hee Lee, Tae-Hee Kim, Young Soo Kim, Jeong-Seok Park and Hyun-Kon Song*

Top 10 most read in May 2012

Kinetically helpful hollow secondary structure of LiFePO4 olivine (LFP) was optimized by balancing between its impurity and shape. In a thermodynamic process using hydrothermal treatment (+HyT), relatively less amount of hollow structure was developed without any impurity by a sequential precipitation method using the difference of solubility products (Ksp) between two precipitates (Li3PO4 and Fe3(PO4)2). On the other hand, a kinetically controlled process without hydrothermal treatment (-HyT) produced the contrary results of more amount of hollow structure with an impurity (Li3PO4). By removing the impurity of LFP from the latter process (-HyT*), therefore, optimized hollow LFP was obtained without impurities. The resultant cathode material showed enhanced capacities especially at high rate discharges due to its improved accessibility of ions into primary particles of the hollow secondary structure of LFP.

36. Restricted Growth of LiMnPO4 Nanoparticles Evolved from a Precursor Seed

Journal of Power Sources, 210, 1-6 (2012)

Tae-Hee Kim, Han-Saem Park, Myeong-Hee Lee, Sang-Young Lee* and Hyun-Kon Song*


Herein, we report on a novel precipitation method to enable LiMnPO4 olivine (LMP) as a cathode material for lithium ion batteries (LIBs) to reach high capacity at high discharge rates. By confining Mn3(PO4)2 precipitation on surface of a precursor seed of Li3PO4, the size of LMP particles are limited to less than 100 nm for a smaller dimension. The cathode material delivers discharge capacities of 145 mAh g-1 at 0.1C, 112 mAh g-1 at 1C to 62 mAh g-1 at 5C (comparable with top three performances[1-3] in Fig. S1 of Supporting Information). Even if precipitation were one of the versatile strategies to prepare the cathode material, it has not been reported that such a first-tier high performance is obtained from LMP prepared by precipitation methods. When compared with LMP particles synthesized by a conventional co-precipitation method, the performances are recognized to be considerably enhanced. Also, the surface-confined precipitation process described in this work does not involve a ball milling step with a conductive agent such as carbon black[1,2,4-10] so that a low cost synthesis is feasible.

35. Scalable approach to multi-dimensional bulk Si anodes via metal-assisted chemical etching

Energy & Environmental Science, 4, 5013-5019 (2012)

Byoung Man Bang, Hyunjung Kim, Hyun-Kon Song, Jaephil Cho and Soojin Park*


Specific design and optimization of the configuration of micro-scale materials can effectively enhance battery performance, including volumetric density. Herein, we employed commercially available low-cost bulk silicon powder to produce multi-dimensional silicon composed of porous nanowires and micro-sized cores, which can be used as anode materials in lithium-ion batteries, by combining a metal deposition and metal-assisted chemical etching process. Nanoporous silicon nanowires of 5–8 μm in length and with a pore size of 10 nm are formed in the bulk silicon particle. The silicon electrodes having multi-dimensional structures accommodate large volume changes of silicon during lithium insertion and extraction. These materials show a high reversible charge capacity of 2400 mAh g−1 with an initial coulombic efficiency of 91% and stable cycle performance. The synthetic route described herein is simple, low-cost, and mass producible (high yield of 40–50% in tens of gram scale), and thus, provides an effective method for producing high-performance anode materials.

34. Fourier Transform Electrochemical Impedance Spectroscopic Studies on LiFePO4 Nanoparticles of Hollow Sphere Secondary Structures

Journal of The Electrochemical Society, 158, A1267-A1274 (2011)

Geun Gi Min, Younghoon Ko, Tae-Hee Kim, Hyun-Kon Song, Seung Bin Kim, and Su-Moon Park*


Real-time impedance behaviors of LiFePO4 nanoparticles of two different structures have been investigated as cathode materials for lithium ion batteries using real time Fourier transform electrochemical impedance spectroscopy (FTEIS) techniques during potentiodynamic charging and discharging cycles. The effects of their nanostructures were examined employing hollow sphere secondary structured LiFePO4 particles prepared by sequential precipitations and commercially available non-hollow structured LiFePO4 particles. The battery constructed with the hollow sphere secondary structured LiFePO4 cathode material exhibited improved performances during charging and discharging processes as judged from various impedance parameters compared to those observed for the cell using its non-hollow counterpart. These results appear to have resulted from the enhancement of intrinsic capabilities for electron and charge transport characteristics of LiFePO4 by modifying its secondary structures. The real time impedance measurements were shown to be powerful in studying behaviors of battery electrodes during charging and discharging processes.

33. One-dimensional (1D) nanostructured and nanocomposited LiFePO4: its
perspective advantages for cathode materials of lithium ion batteries

Physical Chemistry Chemical Physics, 13, 19226-19237 (2011)

Viswanathan S. Saji, Young-Soo Kim, Tae-Hee Kim, Jaephil Cho and Hyun-Kon Song*


Nanostructured materials have attracted recent research interests as battery materials due to their expected enhancement of properties. Characteristic nanoscale dimension and its structuring guarantees improved charge and mass transfer during charge/discharge processes. Among the potential cathode materials investigated as a substitute to LiCoO2, one of the most promising materials is LiFePO4 with olivine structure (LFP). In this perspective article, the current research and development in the synthesis and electrochemical studies of nanostructured LFP are reviewed with a special emphasis on one-dimensional (1D) nanostructures and nanocompositing with 1D conductive materials. In addition to various examples of 1D LFP with detailed synthetic methods, why 1D nanostructure could be meaningful is discussed in terms of a geometric point of view and the anisotropic lithiation/de-lithiation mechanism of LFP.

32. Organic-skinned Inorganic Nanoparticles: Surface-confined Polymerization of 6-(3-Thienyl)hexanoic Acid Bound to Nanocrystalline TiO2

Nanoscale research letters, 6, 521 (2011)

Viswanathan S. Saji, Yimhyun Jo, Hoi Ri Moon,* Yongseok Jun,* Hyun-Kon Song*


There are many practical difficulties in direct adsorption of polymers onto nanocrystalline inorganic oxide surface such as Al2O3 and TiO2 mainly due to the insolubility of polymers in solvents or polymer agglomeration during adsorption process. As an alternative approach to the direct polymer adsorption, we propose surface-bound polymerization of pre-adsorbed monomers. 6-(3-thienyl)hexanoic acid (THA) was used as a monomer for poly[3-(5-carboxypentyl)thiophene-2,5-diyl]  (PTHA). PTHA-coated nanocrystalline TiO2/FTO glass electrodes were prepared by immersing THA-adsorbed electrodes in FeCl3 oxidant solution. Characterization by ultraviolet/visible/infrared spectroscopy and thermal analysis showed that the monolayer of regiorandom-structured PTHA was successfully formed from intermolecular bonding between neighbored THA surface-bound to TiO2. The anchoring functional groups (-COOH) of the surface-crawling PTHA were completely utilized for strong bonding to the surface of TiO2.

31. Ionic Liquid Modified Graphene Nanosheet Anchoring Manganese Oxide Nanoparticles as Efficient Electrocatalysts for Zn-Air Battery

Energy & Environ. Sci., 4, 4148-4154 (2011)

Jang-Soo Lee, Taemin Lee, Hyun-Kon Song, Jaephil Cho,* and Byeong-Su Kim*


Ionic liquid (IL) modified reduced graphene oxide (rGO-IL) nanosheet anchoring manganese oxide (Mn3O4) are synthesized via a facile solution-based growth mechanism and applied to Zn-air battery as effective electrocatalysts for oxygen reduction reaction (ORR). In this study, IL moiety in these composites increases not only conductivity of the system, but also electrocatalytic activity compared to pristine rGO, together with the synergic effect of facilitating ORR with the intrinsic catalytic activity of Mn3O4. Based on the Koutecky-Levich plot, we suggest that the ORR pathway of these composites is tunable with the relative amount of Mn3O4 nanoparticles supported onto the graphene sheets; for example, ORR mechanism of the system with lower contents of Mn3O4 (19.2%) nanoparticles is similar to a Pt/C electrode, one-step, quasi-4-electron transfer, unlike that with higher contents of Mn3O4 (52.5%) which undergo a classical two-step, 2-electron pathway. We also demonstrate the potential of these hybrid rGO-IL/Mn3O4 nanoparticles as efficient catalysts for ORR in the Zn-air battery with a maximum peak power density of 120 mW/cm2; a higher performance than that from commercial cathode catalysts.

30. Electronegativity-induced enhancement of thermal stability by succinonitrile as an additive for Li ion batteries

Energy & Environ. Sci., 4, 4038-4045 (2011)

Young-Soo Kim, Tae-Hee Kim, Hochun Lee, Hyun-Kon Song*


Succinonitrile (SN, CN-[CH2]2-CN) is evaluated as an additive improving thermal stability in ethylene carbonate (EC)-based electrolyte for lithium ion batteries. Without any sacrifice of performances such as cyclability and capacity, introduction of SN into electrolyte with graphite anode and LixCoO2 cathode leads to (1) reducing the amount of gas emitted at high temperature, (2) increasing onset temperature of exothermic reactions and (3) decreasing the amount of exothermal heat. The improvement of thermal stability is considered to be due to a strong complex formation between surface metal atoms of LixCoO2 and nitrile (-CN) groups of SN, by spectroscopic studies based on photoelectrons induced by X-rays and by considering that the exothermic heat and gas evolution are caused by interfacial reactions between electrolyte and cathode.

29. Precipitation revisited: shape control of LiFePO4 nanoparticles by combinatorial precipitation

J. Phys. Chem. C, 115, 12255–12259 (2011)

Myeong-Hee Lee, Tae-Hee Kim, Young Soo Kim and Hyun-Kon Song*


Tunable precipitation strategy to control the shape of nanoparticles of a three-component system is presented. The strategy is devised from understanding the effects of precursor addition sequences on morphology of resultant precipitates. LiFePO4, one of the most potential candidate as a cathode material of lithium ion batteries for electric vehicles, was used as a representative model of the three (Li, Fe and PO4)-component system. According to the precursor addition sequence, three different precipitation methods were adopted: co-precipitation (Copr) and two different types of sequential precipitations (Seq1 and Seq2). Solubility product (Ksp) of intermediate precipitates (Li3PO4 and Fe3(PO4)2) is the key parameter to help the precipitation processes understood. In Copr, the intermediate precipitates are formed simultaneously under Ksp-governed competition. In Seq1 and Seq2, Li3PO4 precipitates prior to Fe3(PO4)2. When Fe2+ is introduced into the suspension of Li3PO4, the pre-formed precipitate is sacrificed to supply PO43- for Fe3(PO4)2 precipitation due to the stronger tendency (smaller value of Ksp) of precipitation of Fe3(PO4)2. Also, the interaction between a cationic surfactant and PO43- makes the difference between Seq1 and Seq2. As a conclusion of the effects of precursor sequence, the shape of particles spans from spherical nanoparticles through a hollow sphere secondary structure of the same nanoparticles to nano-plates. Each own morphology developed by different precipitation methods leads to different intercalation/de-intercalation behavior of lithium ions in conventional rechargeable battery cells.

28. Suppression of the loss of an electroactive dopant from polypyrrole by using a non-aqueous electrolyte of dopant-phobicity

Journal of Electroanalytical Chemistry 657, 181-186 (2011)

Ryeo Yun Hwang, Sung Yeol Kim, G. Tayhas R. Palmore, Hyun-Kon Song*


Loss of electroactive dopants from conducting polymers (CPs) was investigated by electrochemical quartz crystal microgravimetry (EQCM). pPy[ABTS], polypyrrole doped with ABTS (2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonate), was used as a model system. Two different kinds of electrolytes were used for studying the loss of dopants under electrochemical stimuli: one is a solvent to dissolve the dopant (dopant-philic) while the dopant is insoluble in the other (dopant-phobic). Degradation of polypyrrole backbone as well as loss of the dopant from pPy[ABTS] was observed in the dopant-philic electrolyte. Severe chemical overoxidation of polypyrrole by the most oxidized state of ABTS {(ABTS2+)2-} was emphasized as a factor responsible for de-doping in addition to potential-driven overoxidation or ion exchange. In the dopant-phobic electrolyte, however, the chemical degradation of the polymer film was suppressed.

27. Who will drive electric vehicles, olivine or spinel?

Energy & Environmental Science, 4, 1621-1633 (2011)

Ok Kyung Park, Yonghyun Cho, Sanghan Lee, Ho-Chun Yoo, Hyun-Kon Song,* Jaephil Cho*


Lithium iron phosphate olivine (LFP) and lithium manganese oxide spinel (LMO) is competitive and complementary to each other as cathode materials for lithium ion batteries especially for hybrid electric vehicles and electric vehicles. The interests in the materials due to their low cost and high safety have pushed research and development forward and toward high performance in terms of rate capability and capcity retention or cyclability at high temperature of around 60 oC. From the view point of basic properties, LFP shows a higher gravimetric capacity while LMO has better conductivities electrically and ionically. Accoding to our comparison experiments, depending on the material properties and operational potential window, LFP was favored for fast charging while LMO led to better discharge performances. Capacity fading at high termparue due to metal dissolution was revealed to be the most problematic issue of LFP and LMO-based cells for EVs (with thicker electrodes) in the case of no additive in electrolyte and no coating to prevent metal dissolution on cathode materials. Various strategies to enhance properties of LFP and LMO are ready for reallizing EVs in the near future.

26. The effect of introducing a buffer layer to polymer solar cells on cell efficiency

Solar Energy Materials & Solar Cells, 95, 1119-1122  (2011)

Gi-Hwan Kim, Hyun-Kon Song*, Jin Young Kim*


The effect of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as a buffer layer was investigated in polymer solar cells (PSCs). Four different types of PEDOT:PSS were used: PH, PH500 and their DMSO (dimethylsulfoxide)-doped counterparts. The efficiency of PSCs was independent of the electric conductivity of the buffer layer as a bulk property while it was significantly related to interfacial properties between the buffer layer and a bulk-heterojunction (BHJ) layer. The interfacial properties included charge transfer resistance, hole mobility and contact angle of the solution of BHJ on the buffer layer. Lower charge transfer resistance, higher hole mobility and smaller contact angle led to the higher fill factor (up to 72%), enabling highly efficient PSCs with efficiency = 4.25%.

25. Quantitative control of neuron adhesion at a neural interface using a conducting polymer composite with low electrical impedance

ACS Applied Materials & Interfaces, 3, 16-21 (2011)

Sung Yeol Kim, Kwang-Min Kim, Diane Hoffman-Kim, Hyun-Kon Song, G. Tayhas R. Palmore*


Tailoring cell response on an electrode surface is essential in the application of neural interfaces. In this paper, a method of controlling neuron adhesion on the surface of an electrode was demonstrated using a conducting polymer composite as an electrode coating. The electrodeposited coating was functionalized further with biomolecules-of-interest (BOI), their surface concentration controlled via repetition of carbodiimide chemistry. The result was an electrode surface that promoted localized adhesion of primary neurons, the density of which could be controlled quantitatively via changes in the number of layers of BOI added. Important to neural interfaces, it was found that additional layers of BOI caused an insignificant increase in electrical impedance, especially when compared to the large drop in impedance upon coating the electrode with the conducting polymer composite.

24. Ladder-type heteroacene polymers bearing carbazole and thiophene ring units and their use in field-effect transistors and photovoltaic cells

J. Mater. Chem., 21, 843-850 (2011)

Ravi Kumar Cheedarala, Gi-Hwan Kim, Shinuk Cho, Junghoon Lee, Jonggi Kim, Hyun-Kon Song, Jin Young Kim and Changduk Yang*


A family of ladder-type ?ð-excessive conjugated monomer (dicyclopentathienocarbazole (DCPTCz)) integrating the structural components of carbazole and thiophene into a single molecular entity is synthesized and polymerized by oxidative coupling to yield poly(dicyclopentathienocarbazole) (PDCPTCz). Moreover, through the careful selection of 2,1,3-benzothiadiazole unit as a ?ð-deficient building block, the dicyclopentathienocarbazole-based donor–acceptor copolymer (poly(dicyclopentathienocarbazole-alt-2,1,3-benzothiadiazole) (PDCPTCz-BT)) is prepared by Suzuki polycondensation. The optical, electrochemical, and field-effect charge transport properties of the resulting polymers (PDCPTCz and PDCPTCz-BT) are not only characterized in detail but also their bulk-heterojunction (BHJ) solar cell in combination with PC71BM are evaluated. The values of field-effect mobility (µ) for PDCPTCz and PDCPTCz-BT are 8.7 ¡¿ 10−6 cm2 V−1 s−1 and 2.7 ¡¿ 10−4 cm2 V−1 s−1, respectively. A power conversion efficiency (PCE) of 1.57% is achieved on the PDCPTCz-BT/PC71BM device, implying that the push–pull copolymers based on ladder-type dicyclopentathienocarbazole as an electron-donating moiety are promising for organic electronic devices.

23. A Hollow Sphere Secondary Structure of LiFePO4 Nanoparticles

Chemical Communications, 46, 6795-6797 (2010)

Myeong-Hee Lee, Jin-Young Kim, Hyun-Kon Song*


We report on the evolution of a hollow sphere secondary structure of spherical nanoparticles by a solubilization-reprecipitation mechanism based on the difference of solubility products (Ksp) of two different precipitates. Carbon-coated nanoparticles of olivine structure LiFePO4 served as the primary nano-blocks to build the secondary nano-architecture.

22. Recent Progress in Nanostructured Cathode Materials for Lithium Secondary Batteries

Advanced Functional Materials, 20, 3818-3834 (2010) Feature Article

Hyun-Kon Song, Kyu Tae Lee, Min Gyu Kim, Linda F. Nazar,* Jaephil Cho*


Diversified and extended applications of lithium ion batteries demand the development of more enhanced materials that can be achieved by sophisticated synthetic methods. Combination of novel materials with strategic design of their shape in a nanometer scale enables a breakthrough to overcome problems that present technologies have. In this feature article, Mn-based and polyanion-based cathodic cathode materials with nano-scale features for lithium ion batteries are overviewed as the materials coming after conventional bulk cathodic cathode materials. Various synthetic methods coupled with nanostructuring as well as the benefits obtained from the nanostructure are described.

21. Enhancing the stability and performance of a battery cathode using a non-aqueous electrolyte

Electrochemistry Communications 12, 761–764 (2010)

Sung Yeol Kim, Sujat Sen, Hyun-Kon Song, G. Tayhas R. Palmore*


For conductive polymers to be considered materials for energy storage, both their electroactivity and stability must be optimized. In this study, a non-aqueous electrolyte (0.2 M LiClO4 in acetonitrile) was studied for its effect on the charge storage capacity and stability of two materials used in batteries developed in our laboratory, polypyrrole (pPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) doped with 2,2¡?-azinobis(3-ethylbenzothiaxoline-6-sulfonic acid (ABTS). The results are compared to the performance of these materials in an aqueous electrolyte (0.2 M HCl/aq). Loss of ABTS dopant was eliminated principally due to the low solubility of ABTS in acetonitrile, resulting in cathode materials with improved stability in terms of load cycling and performance.

20. Direct Electron Transfer of Glucose Oxidase and Carbon Nanotubes Entrapped with Biocompatible Organic Materials

Mol. Cryst. Liq. Cryst. 519, 82–89 (2010)



Efficient electron transfer between redox enzymes and electrodes is essential for enzyme-based biosensors, biofuel cells, and bioelectronic devices. Generally glucose oxidase (GOx) requires mediators for electrical communication with electrodes because the redox center of GOx is deeply buried in the insulating protein shell. In the present work, direct electron transfer (DET) between GOx and electrodes was attempted. GOx and carbon nanotubes (CNTs) were immobilized on a glassy carbon (GC) electrode by using biocompatible polymer, chitosan (CHI). Cyclic voltammograms revealed that the CHI=GOx=CNT-GC electrode showed a pair of well-defined redox peaks in 0.1Mphosphate buffer solution (pH 7.0) saturated with argon. Under the same conditions, no redox peak was observed in the absence of CNTs. The formal redox potential was 450mV (vs. Ag=AgCl), which agreed well with that of FAD=FADH2, the redox center of GOx. This result clearly shows that the DET between the GOx and the electrode was achieved. The use of thin CNTs significantly improved the DET efficiency of the GOx. It was found that the GOx immobilized on the electrode retained catalytic activity for the oxidation of glucose.

19. Colloidal Nanoparticles as a Wireless Booster for Electroenzymatic Reactions

Small 5, 2162-2166 (2009)

Sahng Ha Lee, Keehoon Won, Hyun-Kon Song*, Chan Beum Park*


Nanoparticles in a wireless form are employed to overcome
the extremely low efficiency of electroenzymatic synthesis reactions. The nanoparticle-mediated electrochemical regeneration of cofactor (NADH) is used in the enzymatic conversion of -ketoglutarate to L-glutamate (see picture). The use of colloidal nanoparticles in electrolyte provides a new strategy for electroenzymatic catalysis.

18. Electrochemical Regeneration of NADH Enhanced by Platinum Nanoparticles

Angew. Chem. Int. Edn. 47, 1749-1752 (2008)

Hyun-Kon Song, S. H. Lee, K. Won, J. H. Park, J. K. Kim, H. Lee, S. –J. Moon, D. K. Kim, C. B. Park*


Nanotechnology-inside wireless chemical communication: Platinum nanoparticles (nPt) in electrolyte enhance electron transfer from electrode to NAD+ in electrolyte during an indirect electrochemical regeneration of NADH. The intermediate nPt-Hads, formed at negative potential, helps the primary mediator M¡?s turnover by donating proton and electron in a kinetically favorable way.

17. A biopolymer composite that catalyzes the reduction of oxygen to water

Chem. Mater. 19, 1565-1570 (2007)

Jiangfeng Fei, Hyun-Kon Song, and G. Tayhas R. Palmore*


A biopolymer composite consisting of polypyrrole, ABTS, and laccase (PAL) was electrodeposited onto the surface of an electrode and shown to catalyze the reduction of dioxygen to water under acidic conditions. The catalytic activity of this biopolymer composite is highest at pH 4, decreasing with increasing pH. The activity of laccase immobilized within this polymer composite was found to be higher than laccase dissolved in solution when methanol was present or at elevated temperatures.

16. Redox-active Polypyrrole: Toward polymer-based batteries

Adv. Mater. 18, 1764-1768 (2006)
Hyun-Kon Song and G. Tayhas R. Palmore

Highlighted in Research News at Materials Today 9, p10 (2006)


The redox-active conducting polymer battery (pPy[IC] pPy[ABTS]), consisting of two conducting polymer electrodes incorporated with different electroactive dopants, was developed. Polypyrrole (pPy) was used as the conducting polymer with indigo carmine (IC) and 2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonate) (ABTS) as dopants. The pPy[IC] pPy[ABTS] showed dramatically enhanced performance at high power density (energy density = 8 Wh kg-1 at power density = 10^2 to 10^4 W kg-1).

15. Combined topographical and chemical micropatterns for templating neuronal networks

Biomaterials 27, 5734-5739 (2006)
J. Zhang, S. Venkataramani, H. Xu, Y-K Song, Hyun-Kon Song, G.T.R. Palmore, J. Fallon and A.V. Nurmikko


In-vitro neuronal networks with geometrically defined features are desirable for studying long-term electrical activity within the neuron assembly and for interfacing with external microelectronic circuits. In standard cultures, the random spatial distribution and overlap of neurites makes this aim difficult; hence many recent efforts have been made on creating patterned cellular circuits. Here, we present a novel method for creating a planar neural network that is compatible with optical devices. This method combines both topographical and chemical micropatterns onto which neurons can be cultured. Compared to other reported patterning techniques, our approach and choice of template appears to show both geometrical control over the formation of specific neurite connections at low plating density and compatibility with microelectronic circuits that stimulate and record neural activity.

14. Micropatterns of positive guidance cues anchored to polypyrrole doped with polyglutamic acid: A new platform for characterizing neurite extension in complex environments

Biomaterials 27, 473-484 (2006)
Hyun-Kon Song, Beth Toste, Katherine Ahmann, Diane Hoffman-Kim, and G. Tayhas R. Palmore


This paper describes a method for preparing substrates with micropatterns of positive guidance cues for the purpose of stimulating the growth of neurons. This method uses an oxidizing potential, applied to a micropattern of indium tin oxide in the presence of pyrrole and polyglutamic acid, to electrodeposit a matrix consisting of polypyrrole doped with polyglutamic acid. The resulting matrix subsequently can be modified with positive guidance cues via standard amide coupling reactions. Cells adhered to the micropatterned substrates can be stimulated electrically by the underlying electrodeposited matrix while they are in contact with positive guidance cues. This method can be extended to include both positive and negative guidance cues in a variety of combinations. To demonstrate the suitability of this method in the context of nerve guidance, dorsal root ganglia were grown in the presence of a micropatterned substrate whose surface was modified with molecules such as polylysine, laminin, or both. Cell adhesion and neurite extension were found to occur almost exclusively in areas where positive guidance cues were attached. This method is easy to execute and is of general utility for fundamental studies on the behavior of neurons in the presence of complex combinations of guidance cues as well as advanced bio-electronic devices such as neuronal networks.

13. Electrochromism of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) Incorporated into Conducting Polymer as a Dopant

Chem. Mater., 17, 2232-2233 (2005)
Hyun-Kon Song, Eun Ju Lee and Seung M. Oh


Polypyrrole films doped with ABTS {2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate)} show multi-color electrochromism: the anodic coloration from brown at 0.0 V to greenish blue at 0.8 V due to oxidation of ABTS to its radical; and the cathodic coloration to yellow (-1.0 V) based on the reduction of polypyrrole itself.

12. Conductive Polypyrrole via Enzyme Catalysis

J. Phys. Chem. B., 109, 19278-19287 (2005)
Hyun-Kon Song and G. Tayhas R. Palmore


Laccase catalyzes the polymerization of pyrrole into a conducting polymer using dioxygen as the terminal oxidant. This finding is significant because it identifies an enzymatic route, and thus an environmentally benign method, for preparing a technologically important polymer. In addition, the rate of oxidation of pyrrole increases when the redox molecule, ABTS {2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonate)}, is included in a reaction medium that contains laccase. This increase in rate occurs because laccase catalyzes the oxidation of ABTS to ABTS•. In addition to laccase, the biocatalytically-generated ABTS• oxidizes pyrrole to its corresponding radical cation to yield polypyrrole. Moreover, oxidation of pyrrole by ABTS• regenerates ABTS for subsequent biocatalytic turnover. Including ABTS in the reaction medium has two important consequences on the final product: (a) the reaction proceeds fast enough to form polymeric films instead of oligomeric precipitates; and (b) ABTS remains within the polymeric film as a redox-active dopant. The charge transport properties of the resulting polymers, both with and without ABTS as the counter anion, are compared to those of other conducting materials including polypyrrole prepared electrochemically or chemically.

11. Electrochemical Porosimetry: Deconvolution of Distribution Functions

Electrochemistry Communication, 8, 1191-1196 (2006)
Hyun-Kon Song, Jong H. Jang, Jae Jeong Kim, Seung M. Oh

Discrete Fourier transform (DFT) was used to deconvolute distribution functions involved in Fredholm integral equation of the first kind. Fourier-transformed distribution function Nk was obtained by convolution theorem. The noise was removed by low-pass filtering the power spectral density Nk in Fourier space. Then, the distribution function nn was obtained from the noise-free Nk by inverse DFT. Several examples were tested: adsorption isotherm, and impedances for faradaic/resistive and non-faradaic/capacitive systems. The ¡°continuous periodicity¡± was the required property of the functions in the original space (the observed function p(x) and the kernel q(x)) for successful deconvolution. For continuous distributions, noise reduction was possible by low-pass filtering without a loss of information. On the other hand, for discontinuous distributions, the noise reduction process led to damping of the distributions.

10. Electrochemical Porosimetry

Journal of the Electrochemical Society, 151, E102-E109 (2004)
Selected in Tech Highlights at Interface 13, p16 (2004)
Hyun-Kon Song, Joo-Hwan Sung, Yong-Ho Jung, Kun-Hong Lee, Le H. Dao, Myung-Hwan Kim and Hyuk-Nyun Kim

For more detailed information: Official Homepage of Electrochemical Porocimetry

We have developed the electrochemical porosimetry (ECP) analyzing microstructures of porous electrodes in situ, which can give geometric information most meaningful in electrochemical systems. The methodology is based on a model that relates electrochemical impedance data with microstructural information. Pore length (lp), as well as pore size distribution (PSD), could be obtained by fitting the model to the experimental impedance data of a porous electrode. This geometric information was validated for the microporous, mesoporous and macroporous samples. Also, the ECP could be used as a nondestructive probe to investigate the construction of electrochemical devices.

9. Electrochemical Porosimetry

Electrically Based Microstructural Characterization III (Editors: R.A. Gerhardt, A. Washabaugh, M.A. Alim, G.M. Choi)- MRS Proceedings Volume 699, R7.7. (2002).
Hyun-Kon Song, Kun-Hong Lee

8. The Effect of Pore Size Distribution on the Frequency Dispersion of Porous Electrodes

Electrochim. Acta, 45, 2241-2257 (2000)
Hyun-Kon Song, Hee-Young Hwang, Kun-Hong Lee and Le. H. Dao


7. The Effect of Pore Size Distribution on the Electrochemical Impedance of Porous Electrodes

Electrochim. Acta, 44, 3513-3519 (1999)
Hyun-Kon Song, Yong-Ho Jung, Kun-Hong Lee and Le. H. Dao


6. Enhancement of Heat and Mass Transfer in Silica-Expanded Graphite Composite Blocks for Adsorption Heat Pumps-Part I. Characterization of the composite blocks

Int. J. Refrigeration, 23, 64-73 (2000)
T.-H. Eun, H.-K. Song, J. H. Han, K.-H. Lee and J.-N. Kim

5. Enhancement of Heat and Mass Transfer in Silica-Expanded Graphite Composite Blocks for Adsorption Heat Pumps-Part II. Cooling system using the composite blocks

Int. J. Refrigeration, 23, 74-81 (2000)
T.-H. Eun, H.-K. Song, J. H. Han, K.-H. Lee and J.-N. Kim

4. Adsorption of carbon dioixde on the chemically modified adsorbents

Int. J. Environ. Conscious Des. Manuf., 7, 53-56 (1998)
Hyun-Kon Song and Kun-Hong Lee

3. Adsorption of Carbon Dioxide on the Chemically Modified Silica Adsorbents

J. Noncrystalline Solids, 242, 69-80 (1998)
Hyun-Kon Song, Kil Won Cho and Kun-Hong Lee

2. Adsorption of Carbon Dioxide on Chemically modified Carbon Adsorbents

Sep. Sci. and Tech., 33, 2039-2057 (1998)
Hyun-Kon Song and Kun-Hong Lee

1. Manufacture of Biodegradable packaging Foams from Agar by Freeze-Drying

J.Mat. Sci., 32, 5825-5832 (1997)
Jong-Pil Lee, Kun-Hong Lee and Hyun-Kon Song

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