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

   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.