<p>Grain boundaries in lithium lanthanum zirconate solid-state electrolytes feature elevated electronic conduction and act as preferential sites for the nucleation of electrically isolated lithium metal during galvanostatic cycling in battery cells. However, the origin of local electronic conductivity variations remains unresolved. Here we show that lithium lanthanum zirconate grain boundaries carry ionic built-in charge, with lithium vacancies accumulating at the interface generating localized electric potentials (−0.15 V at 20 °C). This potential alters carrier distributions near the grain boundary, impeding ionic transport and increasing electronic conduction by a factor of 30 compared with bulk. This imbalance initiates internal lithium metal nucleation during cell operation and accelerates short-circuit failure. To mitigate charge build-up, we propose tailoring the processing oxygen activity and dopant stoichiometry, precisely tuning atomic-scale chemistry and interfacial potential. These modifications homogenize ionic transport and reduce electronic leakage, enabling the intrinsic critical current density to 1 mA cm⁻². Our findings uncover how local defect landscapes shape charge transport and provide a pathway for chemically guided optimization of inorganic solid-state electrolytes at the nanoscale.</p>