Revealing Cycling-Induced Evolution of Intact Sodium Metal Battery Interfaces Using Cryo-Focused Ion Beam...

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_0.44MnO₂ 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 NaPF₆ 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.