Anion-cation synergistic interactions for low-temperature and fast-charging performance in sodium batteries

Battery polarization increases dramatically at low temperatures (≤20 °C) and high charging rates (>1C), making polarization reduction critical for improving both low-temperature and fast-charging performance. In this study, we explore the synergistic effect of anion-cation regulation on the solvation structure to mitigate battery polarization and enhance the low-temperature kinetic performance of electrolytes. As confirmed by a series of temperature-dependent probes (Raman, NMR, and FTIR) and molecular dynamics (MD) simulations, the stabilization of an anion-rich solvation structure via cation-anion synergistic interactions suppresses solvent penetration into the inner solvation shell, effectively lowering the desolvation energy barrier and suppressing dendrite formation. This enables stable cycling at −20 °C and 3C while also supporting operation at −60 °C. The Na‖Na symmetrical cell demonstrates outstanding cycling stability, with over 7500 hours of stripping/plating durability at −40 °C and a current density of 0.5 mA cm⁻². Additionally, the Na₄Fe₃(PO₄)₂P₂O₇‖Na half cells retain an ultra-high capacity of 88.7% after 1500 cycles at −20 °C and 3C. Under harsher conditions (−40 °C and 0.5C), the NFPP‖Na battery with 1 M-BG2-LP electrolyte endures over 3000 cycles, maintaining 94.4% capacity retention and an average coulombic efficiency of 99.6%. Furthermore, the Na₄Fe₃(PO₄)₂P₂O₇‖hard carbon pouch batteries exhibit excellent low-temperature performance, with a capacity retention of 93.4% after 500 cycles at −40 °C and 0.3C. This work demonstrates a promising pathway for developing robust energy storage solutions suitable for extreme environmental conditions.