Revealing the electrolyte suitability optimization and failure mechanism of sodium-ion pouch cells with Na3.5V1.5Mn0.5(PO4)3 polyanionic cathode

Polyanionic cathodes are promising for sodium-ion batteries (SIBs) in large-scale energy storage applications, but there is almost no research on the failure mechanisms of SIBs with polyanionic cathodes. Herein, we investigate the failure mechanisms of the pouch cells with the Na_3.5V_1.5Mn_0.5(PO₄)₃ (NVMP) polyanionic cathode and hard carbon (HC) anode. Firstly, the sodium salts are studied in same electrolyte solvents by combining theoretical calculation and electrochemical tests, and NaClO₄ is verified as suitable for NVMP//Na half-cells, while NaPF₆ is optimal for the cells containing HC electrode. Secondly, the charging cut-off voltages significantly affect the cycling performance. For the NVMP pouch cells cycled within 1.5 ∼ 4.2 V, slight electrolyte oxidation occurs during the final stage of each charge cycle, leading to macroscopic “swelling” after 500 cycles. Finally, for the NVMP pouch cells cycled more than 3000 cycles within 1.5 ∼ 4.0 V, the main failure mechanism is the “sodium deficiency” and the irreversible deactivation of manganese ions in the waste NVMP cathode, rather than the oxidation of the spent separator or the “sodium precipitation” on the spent HC anode. This work addresses critical gaps in failure mechanism research and lays a foundation for the large-scale application of SIBs with polyanionic cathodes.