Structural Evaluation of Amorphous MoS3 Positive Electrode Active Materials in All-Solid-State Lithium Secondary Batteries

Abstract

Sulfur is an attractive active material for positive electrode because sulfur has a high theoretical capacity. High capacity of the batteries can be achieved by using active materials with a large content of sulfur. Low electronic conductivity is a drawback of sulfur. We thus focus on a transition metal trisulfides such as TiS3 with high electronic conductivity. Moreover, amorphization of active materials is potentially capable of achieving higher capacity and cyclability because of open and random structure in amorphous materials. Recently, we have found that amorphous TiS3 prepared by ball milling had better cyclability than crystalline TiS3 [1, 2]. Very recently, we have reported that amorphous molybdenum trisulfide (a-MoS3) electrode active materials prepared by mechanical milling showed high reversible capacity (about 670 mAh g-1) for 60 cycles in the all-solid-state lithium batteries with Li2S-P2S5 electrolytes at room temperature [3]. However, the electrochemical reaction mechanism of a-MoS3 has not been identified yet. In order to improve the battery performance, it is important to understand the reaction mechanisms of a-MoS3 in the all-solid-sate lithium batteries. In this study, we investigated microstructures and morphologies of a-MoS3 electrodes before and after charge-discharge processes. a-MoS3 was prepared by mechanical milling of the mixture of molybdenum metal and sulfur. The obtained active materials were applied as a positive electrode to all-solid-state cells with sulfide solid electrolytes. Structures and morphologies of a-MoS3 electrodes before and after charge-discharge tests were analyzed by XRD, XPS, SEM and HR-TEM. The HR-TEM observations of a-MoS3 electrodes during the 1st discharge test revealed that the lattice fringes due to MoS2 layer structure disappeared gradually. After the 1st full discharge, the lattice fringes were not observed and the electron diffraction showed halo pattern. However, the lattice fringes like MoS2 were observed again after the 1st charge. These results of HR-TEM observations suggested that the reversible structural changes of a-MoS3 occurred. On the other hand, Li2S nano crystals were observed in HR-TEM image of a-MoS3 after the 10th discharge. The HR-TEM image after the 10th charge showed no lattice fringes of Li2S and MoS2. It is concluded that MoS3 after the 10th charge was completely amorphous. Reference [1] A. Hayashi, et al., Chem. Lett., 41 (2012) 886. [2] T. Matsuyama, et al., J. Solid State Electrochem., 17 (2013) 2697. [3] T. Matsuyama, et al., J. Mater. Chem. A, submitted for publication Acknowledgment This research was financially supported by ALCA-SPRING project.