Predictive Modeling of Gradient Interfaces in Solid‐State Lithium Metal Batteries: A Multiscale Framework with Experimental Relevance

Solid‐state lithium metal batteries (SSLMBs) hold promise for next‐generation energy storage but suffer from interfacial instabilities that limit performance and lifespan. A critical challenge remains: understanding how engineered interphases affect ion transport and battery behavior across scales. Here, a multiscale modeling framework that quantitatively links microscale lithium‐ion concentration gradients at functional interphases with full‐cell electrochemical performance is presented. Using coupled FiPy‐based diffusion simulations and PyBaMM‐based cell modeling, a Li–LiAl–LiF interfacial layer with a gradual change in composition—known as a gradient interphase is analyzed, and it is shown that its parabolic concentration profile—arising from steady‐state diffusion—directly governs voltage stability, impedance spectra, and power delivery. The framework reproduces key experimental signatures such as Warburg impedance and voltage flattening, while outperforming conventional models in predicting performance stability. This is the first integrated simulation tool to connect interfacial morphology to system‐level metrics in SSLMBs, offering a predictive path toward the design of stable, high‐efficiency interfaces for solid‐state energy storage.