Effect of Li2SO4 on the properties of Li2S-P2S5 glass-ceramic solid electrolytes

Glass-ceramic solid electrolytes of (100−x)(78Li2S·22P2S5)·xLi2SO4 (x=0, 4, and 5) were prepared by high-energy mechanical milling and subsequent heat-treatment. The low thermal stability of the metastable thio-LISICON II analog phase of the 78Li2S·22P2S5 glass-ceramics was identified by gradual transformation to a thio-LISICON III analog phase over a narrow temperature range of heat treatment. However, the addition of 5 mol% Li2SO4 into the 78Li2S·22P2S5 glass-ceramics suppressed the formation of the thio-LISICON III analog phase and enhanced the structural stability of the thio-LISICON II analog phase at higher heat treatment temperatures. From X-ray diffraction (XRD) and differential thermal analysis (DTA) results, the enhanced thermal stability of the thio-LISICON II analog phase in the Li2SO4 added 78Li2S·22P2S5 glass-ceramics is closely related to the lithium content of the glass matrix and the incorporated tetrahedral SO42− anions in the crystal lattice of the thio-LISICON II analog phase.