Quantitative Decoupling of Electrode Expansion, Electrolyte Depletion, and Dead Sodium in Low-Temperature Storage Cylindrical Sodium-Ion Cell

Sodium-ion batteries (SIBs) exhibit promising potential for low temperature (LT) energy storage, yet their capacity decay mechanisms under LT conditions remain insufficiently investigated. Herein, we report the coupled mechanisms of Full cell - Electrode - Material/Interface, and quantitative analysis of electrode expansion, electrolyte depletion, and dead sodium of 100% SOC NaCuFeMnO₂ || Hard Carbon cylindrical SIBs during prolonged storage at -20 ℃. Based on multimodal in situ/ex situ characterization methodologies, we found a new capacity fade mechanism of LT storage, which reversible capacity loss (1-3 months) dominated by electrolyte depletion and charge transfer polarization, transitioning to irreversible capacity loss caused by interface deterioration and dead sodium accumulation (>3 months). Interestingly, we quantitatively analyzed the changes in the expansion rate and dead sodium content of the anode electrode at different positions (inner, center, and outer region) and revealed unprecedented heterogeneous degradation patterns: anode electrode degradation dominates system failure, inner region and exhibit higher expansion and greater dead sodium accumulation compared to center and outer regions. Consequently, optimizing cell performance under LT conditions necessitates careful optimization of coating parameters, winding architectures, and electrolyte filling volumes during cell design.