Electrolyte Reconfiguration by Cation/Anion Cross-coordination for Highly Reversible and Facile Sodium Storage

Hard carbon (HC) materials are promising anodes for sodium-ion batteries (SIBs) owing to low cost, high specific capacity and low working potential. However, the poor compatibility of the electrolyte with HC leads to low initial coulombic efficiency (ICE) and sluggish Na⁺ transport kinetics. Here, we propose an electrolyte reconfiguration strategy based on the hard and soft acid and base (HSAB) theory by introducing methyltriphenylphosphonium bromide (MTPPB). MTPPB can realize a spontaneous cross-coordination solvation structure with NaPF₆ by selective affinity, synchronously optimizing the interfacial chemistry and sodium storage process. The advantages of the chemical π-π bridging of MTPP⁺-HC and interaction of MTPP⁺-PF₆ ^- contribute to preferential and oriented reduction of PF₆ ^-, forming a low-resistance supramolecular SEI. Additionally, Na⁺-Br^- coordination weakens the Na⁺-solvent interactions, facilitating Na⁺ de-solvation kinetics. Consequently, the HC||Na cell achieves a superior ICE of 96.6 %, desirable rate capability under 25 °C and invisible capacity decay after 500 cycles at 1 C under -20 °C. The Na₄Fe₃(PO₄)₂P₂O₇||HC pouch battery displays a high ICE of 90.3 % and a 15 % increment of energy density under 25 °C. This work provides a guidance through electrolyte reconfiguration engineering for designing practical HC-based SIBs with high energy/power density and long-life span in the extended operating-temperature range.