Analysis of Charge-Discharge Mechanism of Titanium Polysulfide Electrode Materials for Lithium/Metal-Polysulfide Secondary Batteries
Authors: Sakuda Atsushi , Takeuchi Tomonari , Ohara Koji , Kawaguchi Tomoya , Nakanishi Koji , Fukuda Katsutoshi , Arai Hajime , Uchimoto Yoshiharu , Ogumi Zempachi , Ohta Toshiaki , Kageyama Hiroyuki , Shikano Masahiro , Sakaebe Hikari 
Published: February 2020

Development of high capacity electrode active materials has been desired to enhance the energy density of secondary batteries. The challenge to achieve the revolutionary energy density of batteries is the development of new electrode active materials with breakthrough charge-discharge mechanism such as charge and discharge with more than two electron processes, and the use of redox of anions.
Metal sulfides are attractive candidates for positive-electrode materials because of the high capacity [1, 2]. The metal sulfide electrides have been studied for more than 30 years. Their high capacities originate from charge and discharge processes that involve more than one electron; for example, crystalline TiS3 charges and discharges with high reversible capacities of greater than 300 mAh g−1. Recentry we have proposed metal polysulfide electrodes, which are new concept electrode materials prepared mechanochemically, such as amorphous metal polysulfides [3-7], cubic Li2TiS3 and Li3NbS4 [8, 9]. Among them, the amorphous TiS4 showed a high initial discharge capacity of 688 mAh g−1 [5]. This capacity corresponds to more than 4.0-electron process. The determination of the structure and charge-discharge mechanism of the amorphous titanium polysulfide electrodes will benefit to the proposal of the new concept for electrode material design.
In this study, the charge-discharge mechanism of the amorphous titanium polysulfides was investigated by pair distribution function (PDF) analysis, X-ray absorption fine structure (XAFS) measurement, and first-principles molecular dynamics (MD) simulation.
Acknowledgements
This work was supported by the “Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING Project)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan.
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