Inclusion of Anion Additives in the Inner Solvation Shell to Regulate the Composition of Solid Electrolyte Interphase

An innovative approach to electrolyte engineering in carbonate electrolytes is introduced by incorporating high donor number dual anion additives into the conventional electrolyte system (1 M NaPF₆ EC:PC). The active engagement of anions in the primary solvation shell effectively hinders the reduction of solvent molecules by reducing the Lowest Unoccupied Molecular Orbital (LUMO) of Na⁺-solvent-anion complex as compared to the LUMO of pure solvents or Na⁺-solvent complex. The participation of anions leads to the formation of a thinner and an inorganic-rich Solid Electrolyte Interphase on the hard carbon anode enhancing Initial Coulombic Efficiency and significantly improving its kinetics. Moreover, the system with dual anion additives exhibits oxidative stability up to 4.5 V, effectively mitigating the undesired side reactions at high voltage operation of the layered sodium nickel manganese oxide cathode. The addition of dual anion additives proves instrumental in suppressing structural degradation and transition metal dissolution during the long cycling performance of the layered oxide cathode in a sodium-ion full cell. The synergistic effects of this dual anion additive added electrolyte on both the anode and the cathode ultimately ensured prolonged cycling of the sodium-ion full cell. The electrolyte engineering approach outlined in this study opens the door to advancing next-generation high-voltage sodium batteries.