Safety of Lithium Batteries: Mechanistic Gap Analysis from Materials to Large-Format Cells

Thermal runaway (TR) remains the most critical failure mode in commercial batteries, yet its initiation and propagation in large-format cells are not adequately captured by conventional material- and coin cell-level tests. We conduct a mechanistic gap analysis and identify three primary factors that limit the extrapolation of TR behavior across length scales: intracellular heat and mass transport, gas venting and ejecta dynamics, and degradation under realistic operating conditions. A comparison of lithium-ion and solid-state chemistries demonstrates that architecture-driven heterogeneity, interfacial instability, and scale-dependent transport fundamentally influence TR dynamics at the cell level. While commonly used thermal characterization techniques provide insight into intrinsic reactivity, they fail to account for architecture-specific effects, including electrode crosstalk, pressure buildup, and spatially nonuniform electrochemical aging. We propose a hierarchical, physics-informed safety framework that integrates multiscale experiments with mechanistic modeling, emphasizing the need for cross-scale diagnostics, validated simulations, and standardized safety metrics to guide the design of safer, energy-dense systems.