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 6Li MAS NMR, their resonance peaks have different chemical shifts. Exploiting this with two-dimensional 6Li–6Li 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.