Grain-Boundary Stability in Solid-State Batteries Beyond Ionic Conductivity: Electronic Blocking as a Threshold-Dependent Design Criterion

This preprint is the second article in a literature-based series on lithium penetration in solid-state batteries. The first article argued that lithium penetration is more consistently interpreted as an electrochemically amplified microstructural failure process than as a purely mechanical failure of electrolyte hardness. The present article advances a narrower claim: once threshold-dependent localized failure conditions become relevant, grain-boundary stability should not be evaluated by ionic conductivity alone. Grain-boundary electronic blocking—or, more precisely, ionic-to-electronic selectivity—must be treated as an explicit design criterion. The manuscript does not propose a proprietary material solution or map the full intervention space. Its contribution is interpretive and methodological: to identify a missing criterion in the current framing of grain-boundary optimization, while preserving a conservative, regime-dependent extension of the classical mechanical view. The paper argues that grain boundaries can become electrochemically distinct threshold sites, so a conductivity-only metric can misclassify improvement by recording transport gains while overlooking increased local permissivity for internal deposition. This work is based entirely on publicly available experimental literature and does not report new experimental data. It is intended as a preprint for scholarly discussion and citation.