Nonconductive Polymers Enable Higher Ionic Conductivities and Suppress Reactivity in Hybrid Sulfide–Polymer Solid State Electrolytes

Hybrid ceramic–polymer solid state electrolytes are promising candidates to enable energy-dense lithium metal batteries by leveraging inorganic high ionic conductivity and flexible polymer mechanical properties. However, studies of hybrid electrolytes using sulfide-type inorganics such as Li3PS4(LPS) have largely focused on combining the inorganic with commercial poly(ethylene oxide) (PEO). PEO has proven to be insufficient for use in hybrid systems because it reacts with LPS and provides a competing pathway for ion transport, therefore producing a hybrid with low conductivity. Our work shows that using nonconductive, nonpolar polyethylene (PE) in hybrid electrolytes with LPS eliminates both polymer and inorganic degradation and remarkably, exhibits higher conductivities than those containing PEO at different polymer and salt concentrations. Using tracer-exchange NMR, we observe that the nonconductive nature of PE allows for ion transport through the inorganic whereas PEO provides a separate, competing pathway for lithium transport. Furthermore, compared to pure LPS, these hybrids enable longer term lithium cycling at 60 °C. Our work shows that the path to enabling conductive and stable sulfide hybrids for solid state lithium-metal batteries may be through the use of nonconductive, nonreactive polymers.