Evaluation of Mechanical Properties of Li2S-P2S5-based Glass Electrolytes with Li2O, P2O5 or LiI

Abstract

Sulfide glasses are expected as solid electrolytes for all-solid-state batteries because of high lithium ion conductivities. The sulfide electrolyte powders pressed at ambient temperature have much lower grain boundary resistance compared to oxide electrolytes. The all-solid-state batteries using Li2S-P2S5 solid electrolytes have thus good interfaces between solid electrodes and solid electrolytes because of a favorable formability of sulfide electrolytes [1]. On the other hand, a high capacity of the batteries was retained during charging and discharging for hundreds of cycles [2]. This would be due to the fact that sulfide solid electrolytes maintain electrode/electrolyte contacts against volume change of electrode materials during Li insertion/extraction. Consequently, these facts indicate the superiority of sulfide solid electrolytes in terms of mechanical properties. However, their mechanical properties have rarely been reported because such compounds are generally unstable in air. Recently, our group has succeeded in evaluating the elastic modulus of Li2S-P2S5 sulfide electrolytes [3]. The Young’s moduli of Li2S-P2S5 glasses are about 18-25 GPa and these moduli are lower than those of oxide solid electrolytes such as cubic Li7La3Zr2O12, which is reported as 149.8 ±0.4 GPa at a relative density of ~97% [4].Various properties such as stability or conductivity of Li2S-P2S5 glasses are changed by adding a third component to Li2S-P2S5 glasses. We reported that the addition of oxides as a third component to Li2S-P2S5 sulfide glasses improved the electrochemical stability and chemical stability in air [5,6]. On the other hand, lithium-ion conductivities of Li2S-P2S5 sulfide glasses are increased by adding LiI [7]. These sulfide glasses are thus attractive electrolytes to apply for all-solid-state batteries because of the superior stability or conductivity. However, their mechanical properties have not been investigated.In this study, (70-x)Li2S·xLi2O·30P2S5 (x=0, 10, 20 mol%), 70Li2S·(30-y)P2S5·yP2O5 (y=0, 3, 10 mol%) and (100-z)(0.75Li2S·0.25P2S5)·zLiI (z=0, 5, 10, 20, 30 mol%) sulfide glass electrolytes were prepared by mechanical milling. The formability of the glass powders was investigated by powder compression tests. A partial substitution of oxides for sulfides lowered the formability of glasses, while the addition of LiI to sulfides improved the formability. Furthermore, Young’s moduli of these glasses were evaluated by ultrasonic wave velocity measurements. The Young’s moduli increased with an increase in the P2O5 content, while those decreased with an increase in the LiI content.