Stack Pressure Effect in Li3PS4 and Na3PS4 Based Alkali Metal Solid-State Cells: The Dramatic Implication of Interlayer Growth

Despite the rapid growth in the portfolio of attractive Li+/Na+ conducting solid electrolytes (SEs), the corresponding development of solid-state batteries employing high-energy alkali anodes remains limited. Much of the blame lies with the Li/Na–SE interfacial reactivity and lack of understanding of the impact of fabrication and operating stack pressure—a critical parameter that dictates the alkali metal solid-state battery performance by influencing interfacial contact and mechanical extrusion of the alkali metal through the SE. Here we investigate the stack pressure dependent behavior of the Li/Na cell with prospective SE candidates Li3PS4/Na3PS4. Li and Na metals are soft, and both are prone to creeping under pressure, Na more so than Li. Li3PS4 and Na3PS4 have similar chemistry, comparable ionic conductivity, and are perceived to have comparable chemical stability in contact with the respective alkali metal (Li/Na). Yet, the two systems display unanticipated and contrasting behavior—Li|Li3PS4 is highly prone to mechanically induced shorting due to the extrusion of Li through micropores of the SE pellet, but Na|Na3PS4 is not—owing to the dramatic influence from different alkali metal–SE interfacial mechanistics. This is the first disclosure on the implication of interlayer reactivity on the stack pressure effect in Li/Na solid-state cells, which can potentially guide in further understanding and development of practical alkali metal solid-state batteries.