Revealing Cycling‐Induced Evolution of Intact Sodium Metal Battery Interfaces Using Cryo‐Focused Ion Beam Cross‐Sectioning and Electron Microscopy

Abstract Batteries consist of complex, layered interfaces, and their performance‐limiting mechanisms are best understood through nanoscale structural analysis of both anodes and cathodes in realistic full‐cell architectures. This has been challenging for liquid‐electrolyte‐based batteries due to limitations imposed by handling liquid electrolytes and size constraints in most high‐resolution electron microscopes, while cryogenic focused ion beam (cryo‐FIB) milling has typically been limited to a single electrode. Here, a full‐cell cryo‐FIB milling process is presented that reveals anode, cathode, and seprator interfaces in a liquid electrolyte cell with a sodium metal anode and Na _0.44 MnO ₂ cathode. This full‐cell cryo‐milled battery stack enables visualization of interfaces at both electrodes, allowing characterization of the entire cell while comparing the effects of two solvents, ethlyene carbonat/diethyl carbonate and digylme, in a NaPF ₆ salt‐based electrolyte. It is demonstrated that after moderate cycling (10–50 cycles), degradation pathways differ between carbonate‐ and ether‐based electrolytes. Carbonate‐based cells degrade rapidly, driven largely by electrolyte depletion resulting from excessive solid electrolyte interphase (SEI) formation at the anode. In contrast, diglyme‐based exhibit improved cycling stability but ultimately also experience electrolyte depletion, which instead arises from electrolyte degradation at the cathode. These findings provide insight into solvent‐specific degradation mechanisms relevant to future battery development.