Anion competitive coordination for solvation structure regulation in high-voltage sodium-ion batteries

Electrolytes with high stability against high-voltage cathodes are critical for improving the energy density of sodium-ion batteries (SIBs), while the conventional electrolytes encounter severe electrolyte decomposition and electrode-electrolyte interface degradation at high voltages. Although solvent-mediated weakly solvated electrolytes have demonstrated enhanced high-voltage stability through the preservation of Na+-anion coordination, they often involve intricate solvent molecular design and complex synthesis with increased cost, posing challenges for low-cost SIBs. Here, we introduce a straightforward strategy involving multiple sodium salt anion competition coordination to engineer a weakly solvated electrolyte (denoted as MUL). Such an anion-dominated electrolyte is capable of forming an ultra-thin and stable electrode-electrolyte interface that facilitates rapid ion transport and significantly mitigates electrolyte decomposition, interface degradation, and harmful particle cracking issues. Consequently, the MUL electrolyte enables O3-NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode to maintain 78.6% capacity retention and excellent rate performance after 200 cycles. Notably, when integrated into a 1.5 Ah NFM||hard carbon (HC) pouch cell, the MUL electrolyte exhibits 91.1% capacity retention after 100 cycles at 1C, underscoring its superior practical applicability. By prioritizing the optimization of the primary salt rather than the solvent, our research presents an innovative pathway to enhance the high-voltage stability of SIB electrolytes.