Electrochemical Performance and Structural Analyses of Amorphous TiS3 Positive Electrode in All-Solid-State Batteries

Abstract

Development of all-solid-state rechargeable lithium batteries attracts much attention because of their high safety, long cycle life, versatile geometry and high energy density. To increase energy density of the batteries, the use of active materials with high capacity and the increase of active material content are important. Sulfur-rich amorphous transition metal sulfides such as TiS3 are promising positive electrodes with high capacity and high conductivity [1]. Amorphous TiS3 (a-TiS3) was prepared by mechanical milling of crystalline TiS3 (c-TiS3), which was prepared via solid-phase reaction in advance. The 80Li2S·20P2S5 (mol%) glass-ceramics and Li-In alloy were respectively used as a solid electrolyte and a negative electrode. A composite positive electrode was prepared by mixing of a-TiS3, the electrolyte, and acetylene black with the weight ratio of 40:60:6. All-solid-state cells with the composite positive electrode exhibited a high capacity of about 560 mAh per gram of a-TiS3 at 25oC and retained better cyclability than the cell with c-TiS3 [2]. XRD patterns and the Raman spectra of the a-TiS3 electrode after the 1st and 10th charge–discharge measurements were similar to those before the measurement [2,3]. In addition, the high resolution-TEM images after the 10th charge-discharge tests showed no periodic lattice fringes, indicating that the a-TiS3 electrode after 10 cycles did not have fine crystals with nanometer size. The most part of a-TiS3 electrode after charge-discharge measurements maintained amorphous structure. Furthermore, the cell with a-TiS3 positive electrode including no carbon conductive additives and solid electrolytes operated as a rechargeable battery and exhibited a reversible capacity for 10 cycles of about 510 mAh per gram of a-TiS3 [4]; the maximum content of active material in a positive electrode layer was achieved because of high conductivity of a-TiS3. The electronic structures of sulfur in a-TiS3 before and after charge-discharge tests were analyzed by X-ray photoelectron spectroscopy (XPS) with a monochromatic AlKa source (1486.6 eV). The samples were mounted on a sample stage in a dry Ar glove box and they were transferred to an analysis chamber using an Ar filled transfer vessel without exposure to air atmosphere. S2p XPS revealed that a reversible sulfur redox in a-TiS3 mainly appeared during charge-discharge process and contributed to its good capacity retention. It is noteworthy that amophization of TiS3 active material is effective in developing all-solid-state lithium batteries with higher reversible capacity and better cyclability. AcknowledgementThe research was financially supported by the Japan Science and Technology Agency (JST), Advanced Low Carbon Technology Research and Development Program (ALCA), Specially Promoted Research for Innovation Next Generation Batteries (SPRING) Project, and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.