Yeryung Jeon

Formation of Ni–Co–MoS2 Nanoboxes with Enhanced Electrocatalytic Activity for Hydrogen Evolution

Nickel and cobalt incorporated MoS2 nanoboxes are synthesized via the reaction between Ni-Co Prussian blue analogue nanocubes and ammonium thiomolybdate. Due to the structural and compositional advantages, these well-defined nanoboxes manifest enhanced electrochemical activity as an electrocatalyst for hydrogen evolution reaction.

A Ge inverse opal with porous walls as an anode for lithium ion batteries

Germanium holds great potential as an anode material for lithium ion batteries due to its large theoretical energy density and excellent intrinsic properties related to its kinetics associated with lithium and electrons. However, the problem related to the tremendous volume change of Ge during cycling is the dominant obstacle for its practical use. The previous research has focused on the improvement in mechanics associated with lithium without consideration of the kinetics. In this study, we demonstrate that the configuration engineering of the Ge electrode enables the improvement in kinetics as well as favorable mechanics. Two types of Ge inverse opal structures with porous walls and dense walls were prepared using a confined convective assembly method and by adjusting Ge deposition parameters in a chemical vapor deposition system. The Ge inverse opal electrode with porous walls shows much improved electrochemical performances, especially cycle performance and rate capability, than the electrode with dense walls. These improvements are attributed to a large free surface, which offers a facile strain relaxation pathway and a large lithium flux from the electrolyte to the active material.

Synergistic Ultrathin Functional Polymer-Coated Carbon Nanotube Interlayer for High Performance Lithium–Sulfur Batteries

Lithium-sulfur (Li-S) batteries have been intensively investigated as a next-generation rechargeable battery due to their high energy density of 2600 W·h kg(-1) and low cost. However, the systemic issues of Li-S batteries, such as the polysulfide shuttling effect and low Coulombic efficiency, hinder the practical use in commercial rechargeable batteries. The introduction of a conductive interlayer between the sulfur cathode and separator is a promising approach that has shown the dramatic improvements in Li-S batteries. The previous interlayer work mainly focused on the physical confinement of polysulfides within the cathode part, without considering the further entrapment of the dissolved polysulfides. Here, we designed an ultrathin poly(acrylic acid) coated single-walled carbon nanotube (PAA-SWNT) film as a synergic functional interlayer to address the issues mentioned above. The designed interlayer not only lowers the charge transfer resistance by the support of the upper current collector but also localizes the dissolved polysulfides within the cathode part by the aid of a physical blocking and chemical bonding. With the synergic combination of PAA and SWNT, the sulfur cathode with a PAA-SWNT interlayer maintained higher capacity retention over 200 cycles and achieved better rate retention than the sulfur cathode with a SWNT interlayer. The proposed approach of combining a functional polymer and conductive support material can provide an optimiztic strategy to overcome the fundamental challenges underlying in Li-S batteries.

Flash-induced reduced graphene oxide as a Sn anode host for high performance sodium ion batteries

Sn is a promising anode material for sodium ion batteries due to its high capacity. However, the fast capacity fading caused by large volume changes limits the employment of Sn anodes. Graphene has been considered as a host for Sn anode materials to improve the cycle performance. However, graphene scaffold preparation with large free spaces is challenging due to the need for a sacrificial template and etching process. Here, we prepared a porous scaffold composed of both reduced graphene oxide and graphene via a camera flash reduction as the host for Sn. The camera flash induces the reduction of the graphene oxide and pores generated by the c-axis popping of the graphene. The mechanical strength of the scaffold is also achieved by adjusting the concentration of graphene which does not react with the flash light. The porosity and mechanical properties of the reduced graphene oxide–graphene scaffold could be controlled by flash irradiation conditions and the mixing ratio between the graphene oxide and graphene. The porous scaffold enables a uniform Sn loading and an improvement in the sodium ion battery performance due to a sufficient free space for accommodating the Sn volume change and mechanical stability.

Si nanotubes array sheathed with SiN/SiOxNy layer as an anode material for lithium ion batteries

Silicon has received high interest as an anode material for lithium ion batteries due to its large theoretical Li storage capacity. However, poor cyclability and low coulombic efficiency of the Si based electrode, caused by the pulverization of the active material and the continuous formation of unstable solid electrolyte interphase (SEI) due to large volume change associated with Li, limits its practical use as an anode material. We have developed a Si nanotube array sheathed with silicon nitride compound to improve the mechanical integrity, resulting in improved electrochemical performance. The SiN/SiOxNy outer shell has excellent mechanical properties, such as a high elastic modulus and hardness. This guides the volume expansion of the Si into the hollow inner space of the tubular structure during charge, which prevents both the pulverization of the Si active material, as well as continuous SEI layer formation by protecting the exposure of fresh Si surface to the electrolyte. Si nanotube array sheathed with silicon nitride electrode compound exhibits improved electrochemical performance, including stable capacity retention and high coulombic efficiencies, over the analogous homogeneous Si nanotube system.