Impact of Salts and Linear Carbonates on the Performance of Layered Oxide/Hard Carbon Sodium-Ion Pouch Cells with Alkyl Carbonate Electrolytes

This study examines the influence of electrolyte salts and solvents on the performance of O3 layered oxide NaMn0.39Fe0.31Ni0.22Zn0.08O2/hard carbon sodium-ion pouch cells with polyethylene terephthalate (PET) jellyroll tape. A significant enhancement in cell performance between 2.0 and 3.8 V was observed across various temperatures (20, 40, and 55 °C) by substituting NaPF6 with NaFSI, including reduced impedance growth, minimized gas generation, and supressed jellyroll tape decomposition. Ultra-high precision coulometry revealed that the use of NaPF6 resulted in increased unwanted parasitic reactions associated with tape decomposition, e.g., capacity fade and charge endpoint capacity slippage. Teardown of sodium-ion pouch cells after cycling in DMC-based electrolytes revealed a severe decomposition of the PET tape with NaPF6 but not with NaFSI. Gas chromatography shows significantly more electrolyte decomposition products with NaPF6 as opposed to NaFSI. DEC-based electrolyte showed less capacity fade, less electrolyte decomposition products, and less tape decomposition after cycling than DMC-based electrolyte. The electrolyte additive DTD can prevent parasitic reactions in DMC- and NaPF6-based electrolyte. Overall, the choice of salts and linear carbonates in alkyl carbonate electrolytes plays a crucial role in determining the overall cycling performance of the layered oxide/hard carbon sodium-ion cells with PET jellyroll tape.