Structural Analyses of Amorphous MoS3 Electrode after Charge-Discharge Measurements in All-Solid-State Lithium Secondary Batteries

All-solid-state lithium batteries using inorganic solid electrolytes are expected to be highly safe because of elimination of flammable components such as organic liquid electrolytes. Further improvement of the electrode active materials with high capacity and excellent cyclability is needed. Sulfur is expected to be positive electrode active materials of next-generation batteries because sulfur has a high theoretical capacity. Therefore, high capacity of the batteries can be achieved by using active materials with a large content of sulfur. However, sulfur has low electronic conductivity. We thus focus on a transition metal sulfide 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.

MoS₂ crystal was practically used as a positive electrode in lithium metal batteries [1]. Electrode properties of amorphous MoS₃ prepared by pyrolysis of (NH₄)₂MoS₄ have been investigated in an organic liquid electrolyte [2]. A liquid-type cell with amorphous MoS₃ electrode shows the reversible capacity of 400 mAh g^-1. Thus, amorphous MoS₃ active materials are an attractive positive electrode with high capacities for lithium batteries. In this study, structure and electrochemical properties of amorphous MoS₃in all-solid-state cells were evaluated.

Amorphous 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 Li₂S-P₂S₅electrolytes [3]. In order to analyze the structure of the positive electrode, XRD and HR-TEM were performed for the electrodes before and after the charge-discharge measurements.

The samples prepared by mechanical milling showed no peaks attributable to starting materials in the XRD patterns. Raman bands attributable to amorphous MoS₃ were observed in the milled sample. Those structural analyses suggest that amorphous MoS₃ was prepared by mechanical milling. The initial discharge capacity of the all-solid-state cell with amorphous MoS₃ was approximately 760 mAh g^-1. The cell with amorphous MoS₃ electrode retained the discharge capacity of about 670 mAh g^-1for over 60 cycles. The XRD profile obtained from the electrode after the 1st charge-discharge measurement was almost identical to that from the initial electrode. HR-TEM revealed that amorphous structure was maintained after the 1st cycle.

Reference

[1] M. A. Py et al., Can. J. Phys., 61 (1980) 76.

[2] J. J. Auborn et al., J. Electrochem. Soc.,134 (1987) 580.

[3] A. Hayashi et al., Electrochem. Commun. 5 (2003) 111.