Peeking across Grain Boundaries in a Solid-State Ionic Conductor

The development of high-performance all-solid-state batteries relies on charge transport in solid electrolytes, where transport across grain boundaries often limits their bulk conductivity. The argyrodite Li6PS5X (X = Cl, Br) solid electrolyte has a high conductivity; however, macroscopic diffusion in this material involves complex jump processes, which leads to an underestimation of the activation energy. Using a comprehensive frequency- and temperature-dependent analysis of the spin-lattice relaxation rates, a complete estimation of Li self-diffusion is demonstrated. Another experimental challenge is quantifying the impact of grain boundaries on the total bulk conductivity. Li6PS5Cl and Li6PS5Br have identical crystalline structures, but with Li-6 MAS NMR, their resonance peaks have different chemical shifts. Exploiting this with two-dimensional Li-6-Li-6 exchange NMR on a mixture of Li6PS5Br and Li6PS5Cl, we observe Li exchange between particles of these two materials across grain boundaries, allowing direct and unambiguous quantification of this often limiting process in solid-state electrolytes.