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Role of Anode Composition and Electrolyte Interactions on the Thermo-Electrochemical Stability of Sodium-Ion Batteries Sodium-ion batteries (SIBs) are promising for large-scale energy storage, owing to their resource abundance and low cost. However, long-term stability is constrained by complex interfacial interactions and microstructural degradation. This study investigates the mechanistic coupling between anode composition, electrolyte chemistry, and solid electrolyte interphase (SEI) evolution in full-cell SIBs employing sodium vanadium phosphate (NVP) cathodes. Pure tin (Sn), hard carbon (HC), and Sn–HC composite anodes were systematically evaluated with carbonate ester- and ether-based electrolytes. Microscopic and spectroscopic analyses reveal that Sn-rich electrodes undergo significant pulverization and unstable SEI formation, whereas HC maintains structural integrity and forms kinetically stable SEI. On the other hand, the Sn–HC composite mitigates Sn’s mechanical degradation while enhancing capacity retention. Electrochemical analysis highlights the critical role of electrolyte choice in modulating redox reversibility and interfacial integrity. Accelerating rate calorimetry (ARC) links interphase behavior to distinct thermal decomposition pathways and self-heating rate. These findings provide mechanistic insights into the electro-chemo-mechanical degradation processes dictating the long-term stability and thermal safety of SIBs. https://pubs.acs.org/doi/10.1021/acsami.5c19946