Amorphous Molybdenum Oxysulfide As a Highly-Reversible Anode Material for Na-Ion Batteries
Authors: Kim Tae-Hee , Shin Jaewook , Lee Kug-Seung , Cho EunAe 
Published: February 2020

Rechargeable Na-ion batteries (NIBs) have attracted great attention as a promising alternative for lithium-ion batteries (LIBs) due to the earth-abundant and cheap Na resource. In particular, NIBs can be applied for large-scale energy storage and conversion for renewable energy sources such as solar and wind energies. However, the larger radius of Na+ than that of Li+ results in more sluggish diffusion kinetics and more serious damage to the structure of the electrode materials, which cause low capacity, poor rate capability, and cyclic stability. These disadvantages prevent NIBs from practical application, and therefore, it is crucial to search for suitable Na+ storage materials with high performance.

Recently, conversion reaction based electrode materials such as transition metal (TM) oxide, sulfide, and phosphide, which have mostly been studied as high-capacity electrodes for LIBs, have been investigated for possible high-capacity NIBs anodes. Similar to conversion reaction with Li, all possible valance states of transition metal (TM) in the materials can be utilized, enabling more than one electron redox reaction per TM, which leads to a generally high specific capacity. However, intrinsic poor conductivity which causes large hysteresis and low reversibility and huge volume change during sodiation/desodiation which deteriorate cycling stability plague many conversion reactions.

Over the past decade, nanostructured materials have been widely used to solve these problems of conversion reaction based electrodes by shortening both electronic and ionic pathway and relieving stress during sodiation/desodiation. However, fabricating nanostructured materials usually involves multiple complicated steps, and large surface area of the nanostructure accelerates electrolyte decomposition which induces large irreversible capacity and contact loss between the electrode material and current collector upon repeated cycling.

Nowadays, there have been several reports showing that the intrinsically isotropic nature in amorphous materials, which can mitigate the large volume changes associated with charge/discharge process, greatly improves the cycling stability of rechargeable batteries. In addition, amorphous materials commonly show faster reaction kinetics, better reversibility and narrower voltage hysteresis than crystalline materials. However, there have been neither sufficient explanations nor experimental evidences explaining the origin of the high reversibility and cycling stability of the amorphous electrode.

Herein, we propose to design a binder- and conductive agent-free amorphous MoOxSy anode for Na-ion batteries using an ultra-simple and facile electrodeposition process. As well as electrochemical performance measurements, we investigated the structure and phase evolution change of the a-MoOxSy during first sodiation and desodiation to better understand the mechanisms underlying the conversion process and the electrochemical behavior. X-ray absorption fine structure (XAFS) measurements, sensitive to short-range order, were used to track the structure and phase evolution of the electrodes during the conversion process. In addition, using TEM analysis, it was clearly proposed that the microscopic mechanisms involved in the reaction of conversion-reaction based electrode, such as the nucleation and evolution of the metal phases and their phase distribution on the nanoscale, have a great impact on the electrochemical performances of the electrodes.