Lithium Penetration in Solid-State Batteries: A Literature-Based Reassessment of a Commonly Mechanical Failure Narrative

Lithium penetration through solid electrolytes is commonly discussed as a mechanical problem: if the electrolyte is sufficiently stiff or hard, lithium filaments should be suppressed. This framing has been historically influential, especially after the mechanical-stability treatment introduced by Monroe and Newman. However, a growing body of evidence suggests that a purely mechanical interpretation is incomplete. Recent studies indicate that lithium initiation can occur through deposition into subsurface pores or defects, that grain boundaries can localize electronic potential changes and preferential lithium growth, and that percolating pores can provide low-overpotential pathways for filament propagation. At the same time, the role of stack pressure appears more conditional than a simple mechanical-blocking model would predict: pressure can improve contact, but it does not uniformly suppress failure and can, in some regimes, coincide with accelerated penetration or short-circuiting. This manuscript presents a conservative literature-based reassessment of the problem. It argues that many observations are more consistently interpreted if lithium penetration is treated as an electrochemically amplified microstructural failure process, in which mechanics remains important but is not the sole or always primary causal variable. Limited analogies from other field-driven solid-state systems are also discussed, not as direct proof, but as structural support for the plausibility of path-selective filament growth in confined media. This upload contains the manuscript PDF together with the source package used for preparation, including LaTeX and BibTeX files. Version 3 adds the final conceptual schematic as Figure 1 and integrates it into the manuscript as a literature-based conceptual illustration.