Youngsik Kim

Nickel-rich layered lithium transition-metal oxide for high-energy lithium-ion batteries.

High energy-density lithium-ion batteries are in demand for portable electronic devices and electrical vehicles. Since the energy density of the batteries relies heavily on the cathode material used, major research efforts have been made to develop alternative cathode materials with a higher degree of lithium utilization and specific energy density. In particular, layered, Ni-rich, lithium transition-metal oxides can deliver higher capacity at lower cost than the conventional LiCoO2 . However, for these Ni-rich compounds there are still several problems associated with their cycle life, thermal stability, and safety. Herein the performance enhancement of Ni-rich cathode materials through structure tuning or interface engineering is summarized. The underlying mechanisms and remaining challenges will also be discussed.

Graphene–Co3O4 nanocomposite as an efficient bifunctional catalyst for lithium–air batteries

A facile hydrothermal route has been developed to prepare graphene–Co3O4 nanocomposites. The graphene–Co3O4 nanocomposite catalyst demonstrates an excellent catalytic activity toward oxygen-reduction reaction including a considerably more positive half-wave potential (−0.23 V) than that of pristine graphene (−0.39 V), as well as higher cathodic currents. More importantly, this catalyst shows better long-term durability than the commercial Pt/C catalyst in an alkaline solution. The preliminary results indicate that the graphene–Co3O4 nanocomposite is an efficient and stable bifunctional catalyst for Li–air batteries and may be an alternative to the high-cost commercial Pt/C catalyst for the ORR/OER in alkaline solutions.

Corn protein-derived nitrogen-doped carbon materials with oxygen-rich functional groups: a highly efficient electrocatalyst for all-vanadium redox flow batteries

Recent studies on all-vanadium redox flow batteries (VRFBs) have focused on carbon-based materials for cost-effective electrocatalysts to commercialize them in grid-scale energy storage markets. We report an environmentally friendly and safe method to produce carbon-based catalysts by corn protein self-assembly. This new method allows carbon black (CB) nanoparticles to be coated with nitrogen-doped graphitic layers with oxygen-rich functionalities (N-CB). We observed increased catalytic activity of this catalyst toward both V2+/V3+ and VO2+/VO2+ ions, showing a 24% increased mass transfer process and ca. 50 mV higher reduction onset potential compared to CB catalyst. It is believed that the abundant oxygen active sites and nitrogen defects in the N-CB catalyst are beneficial to the vanadium redox reaction by improving the electron transfer rate and giving faster vanadium ion transfer kinetics.

Sodium-metal halide and sodium-air batteries.

Impressive developments have been made in the past a few years toward the establishment of Na-ion batteries as next-generation energy-storage devices and replacements for Li-ion batteries. Na-based cells have attracted increasing attention owing to low production costs due to abundant sodium resources. However, applications of Na-ion batteries are limited to large-scale energy-storage systems because of their lower energy density compared to Li-ion batteries and their potential safety problems. Recently, Na-metal cells such as Na-metal halide and Na-air batteries have been considered to be promising for use in electric vehicles owing to good safety and high energy density, although less attention is focused on Na-metal cells than on Na-ion cells. This Minireview provides an overview of the fundamentals and recent progress in the fields of Na-metal halide and Na-air batteries, with the aim of providing a better understanding of new electrochemical systems.

Highly porous graphitic carbon and Ni2P2O7 for a high performance aqueous hybrid supercapacitor

An aqueous Na-ion based hybrid capacitor has been successfully developed by using highly porous graphitic carbon (HPGC) derived from waste writing paper and a new electrode material as a negative and positive electrode, respectively. HPGC was prepared via hydrothermal carbonization and subsequent KOH activation of waste writing paper which showed a highly porous stacked sheet-like morphology with an exceptionally high BET specific surface area (1254 m2 g−1). HPGC exhibited typical electrical double layer capacitor (EDLC) behavior with a high specific capacitance of 384 F g−1 and good negative working potential (−1.0 V) in an aqueous electrolyte. On the other hand, Ni2P2O7 was synthesized by a simple co-precipitation technique and tested as a cathode material which delivered a maximum specific capacitance of 1893 F g−1 at 2 A g−1 current density. The fabricated HPGC‖Ni2P2O7 hybrid device displayed excellent cyclic stability up to 2000 cycles and delivered a maximum energy density of 65 W h kg−1 at 800 W kg−1 power density in a Na-ion based aqueous electrolyte.

A hybrid solid electrolyte for flexible solid-state sodium batteries

Development of Na-ion battery electrolyte with high-performance electrochemical properties and high safety is still challenging to achieve. In this study, we report on a NASICON (Na3Zr2Si2PO12)-based composite hybrid solid electrolyte (HSE) designed for use in a high safety solid-state sodium battery for the first time. The composite HSE design yields the required solid-state electrolyte properties for this application, including high ionic conductivity, a wide electrochemical window, and high thermal stability. The solid-state batteries of half-cell type exhibit an initial discharge capacity of 330 and 131 mA h g−1 for a hard carbon anode and a NaFePO4 cathode at a 0.2C-rate of room temperature, respectively. Moreover, a pouch-type flexible solid-state full-cell comprising hard carbon/HSE/NaFePO4 exhibits a highly reversible electrochemical reaction, high specific capacity, and a good, stable cycle life with high flexibility.

Carambola-shaped VO2 nanostructures: a binder-free air electrode for an aqueous Na–air battery

Binder-free and bifunctional electrocatalysts have vital roles in the development of high-performance metal–air batteries. Herein, we synthesized a vanadium oxide (VO2) nanostructure as a novel binder-free and bifunctional electrocatalyst for a rechargeable aqueous sodium–air (Na–air) battery. VO2 nanostructures were grown on reduced graphene oxide coated on carbon paper, which had a carambola morphology. We confirmed the bifunctional nature of VO2 nanostructures by analyzing their electrocatalytic activity associated with the oxygen reduction reaction and oxygen evolution reaction. The reaction pathway associated with electrocatalytic activity was also affirmed by computational modeling and simulation studies. Thereafter, an aqueous Na–air cell was built using novel binder-free VO2 nanostructures as the air electrode. The fabricated cell displayed a 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.

Encapsulation of organic active materials in carbon nanotubes for application to high-electrochemical-performance sodium batteries

Sodium (Na) ion batteries are interesting candidates for replacing lithium (Li) ion batteries, primarily due to the abundance of Na in the environment. However, the performance and energy density of a Na-ion battery are inferior to those of a Li-ion battery. Organic active materials can help overcome the drawbacks associated with Na-ion batteries because of their many advantages. However, such organic polymer electrodes are subjected to a high self-discharge and low practical capacity because the polymer electrode easily dissolves in an organic electrolyte and forms an insulating layer. Therefore, in this study, we have designed a unique organic electrode in which an active polymer is encapsulated into a carbon nanotube (CNT) to form an electrode with high polymer content. The CNT is able to retain the active polymer within the electrode structure, providing an effective electronic conduction path. Moreover, the CNT can contain large amounts of active polymer and therefore exhibits superior electrochemical properties without self-discharge, making it well suited for use as a cathode material in a Na-ion battery.

Superior Ion-Conducting Hybrid Solid Electrolyte for All-Solid-State Batteries

Herein, we developed a high-performance lithium ion conducting hybrid solid electrolyte, consisted of LiTFSI salt, Py14 TFSI ionic liquid, and TiO2 nanoparticles. The hybrid solid electrolyte prepared by a facile method had high room temperature ionic conductivity, excellent thermal stability and low interface resistance with good contact. In addition, the lithium transference number was highly increased by the scavenger effect of TiO2 nanoparticles. With the hybrid solid electrolyte, the pouch-type solid-state battery exhibited high initial discharge capacity of 150 mA h g(-1) at room temperature, and even at 1 C, the reversible capacity was as high as 106 mA h g(-1) .

Metal-free hybrid seawater fuel cell with an ether-based electrolyte

In this work, the design of a new metal-free hybrid seawater fuel cell consisting of a flowing seawater cathode and a hard carbon anode was proposed. The electrochemical performance of the cell was investigated with two different electrolytes, i.e., 1 M NaClO4 in ethylene carbonate (EC)/propylene carbonate (PC), and 1 M NaCF3SO3 in tetraethylene glycol dimethyl ether (TEGDME). The TEGDME-based electrolyte showed a good cycle performance for 100 cycles, whereas EC/PC showed poor cycle stability after 30 cycles. Our results showed that a low conducting solid-electrolyte interphase (SEI) was formed with a thick layer, and the PVdF binder was degraded during the redox reaction when the EC/PC-based electrolyte was used. In contrast, the TEGDME-based electrolyte induced the formation of a more efficient SEI layer without degradation of the binder.