Durability of the Li_1+<i>x</i>Ti_2–<i>x</i>Al_<i>x</i>(PO₄)₃ Solid Electrolyte in Lithium–Sulfur Batteries

Adoption of cells with a solid-state electrolyte is a promising solution for eliminating the polysulfide shuttle problem in Li–S batteries. Among the various known lithium-ion conducting solid electrolytes, the sodium superionic conductor (NASICON)-type Li_1+<i>x</i>Ti_2–<i>x</i>Al_<i>x</i>(PO₄)₃ offers the advantage of good stability under ambient conditions and in contact with air. Accordingly, we present here a comprehensive assessment of the durability of Li_1+<i>x</i>Ti_2–<i>x</i>Al_<i>x</i>(PO₄)₃ in contact with polysulfide solution and in Li–S cells. Because of its high reduction potential (2.5 V vs Li/Li⁺), Li_1+<i>x</i>Ti_2–<i>x</i>Al_<i>x</i>(PO₄)₃ gets lithiated in contact with lithium polysulfide solution and Li₂CO₃ is formed on the particle surface, blocking the interfacial lithium-ion transport between the liquid and solid-state electrolytes. After the lithium insertion into the NASICON framework, the crystal expands in an anisotropic way, weakening the crystal bonds, causing fissures and resultant cracks in the ceramic, corroding the grain boundaries by polysulfide solution, and leaving unfavorable pores. The assembly of pores creates a gateway for polysulfide diffusion from the cathode side to the anode side, causing an abrupt decline in cell performance. Therefore, the solid-state electrolytes need to have good chemical compatibility with both the electrode and electrolyte, long-term stability under harsh chemical environment, and highly stable grain boundaries