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 Li 6 PS 5 X (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. Li 6 PS 5 Cl and Li 6 PS 5 Br have identical crystalline structures, but with 6 Li MAS NMR, their resonance peaks have different chemical shifts. Exploiting this with two-dimensional 6 Li- 6 Li exchange NMR on a mixture of Li 6 PS 5 Br and Li 6 PS 5 Cl, 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.