Dendrite formation and growth remains a major obstacle toward high‐performance all solid‐state batteries using Li metal anodes. The ceramic Li_(1+x)Al_(x)Ti_(2−x)(PO₄)₃ (LATP) solid‐state electrolyte shows a higher than expected stability against electrochemical decomposition despite a bulk electronic conductivity that exceeds a recently postulated threshold for dendrite‐free operation. Here, transmission electron microscopy, atom probe tomography, and first‐principles based simulations are combined to establish atomistic structural models of glass‐amorphous LATP grain boundaries. These models reveal a nanometer‐thin complexion layer that encapsulates the crystalline grains. The distinct composition of this complexion constitutes a sizable electronic impedance. Rather than fulfilling macroscopic bulk measures of ionic and electronic conduction, LATP might thus gain the capability to suppress dendrite nucleation by sufficient local separation of charge carriers at the nanoscale.

While great effort has been focused on bulk material design for high-performance All Solid-State Batteries (ASSBs), solid-solid interfaces, which typically extend over a nanometer regime, have been identified to severely impact cell performance. Major challenges are Li dendrite penetration along the grain boundary network of the Solid-State Electrolyte (SSE) and reductive decomposition at the electrolyte/electrode interface. A naturally forming nanoscale complexion encapsulating ceramic Li${}_{1+\mathrm{x}}$Al${}_{\mathrm{x}}$Ti${}_{2-\mathrm{x}}$(PO${}_4$)${}_3$ (LATP) SSE grains has been shown to serve as a thin protective layer against such degradation mechanisms. To further exploit this feature, we study the interfacial doping of divalent Mg${}^{2+}$ into LATP grain boundaries. Molecular Dynamics simulations for a realistic atomistic model of the grain boundary reveal Mg${}^{2+}$ to be an eligible dopant candidate as it rarely passes through the complexion and thus does not degrade the bulk electrolyte performance. Tuning the interphase stoichiometry promotes the suppression of reductive degradation mechanisms by lowering the Ti${}^{4+}$ content while simultaneously increasing the local Li${}^{+}$ conductivity. The Mg${}^{2+}$ doping investigated in this work identifies a promising route towards active interfacial engineering at the nanoscale from a computational perspective.