Interphasial Catalytic Anion Reduction for Stable Anode-Free Sodium-Metal Batteries

Anode-free sodium metal batteries (AFSMBs) are promising because of their high energy density, sustainability and affordability. However, their solid electrolyte interphase (SEI) must be improved to mitigate the deterioration of cycling stability due to the zero-sodium inventory at the anode. SEI stabilization for AFSMBs has been widely executed with fluorinated solvents and high-concentration salts, yet their high cost and environmental impact limit practical viability. Herein, we demonstrate interphasial catalytic anion reduction (ICAR) effect that selectively forms a stable NaF-rich SEI for improved cycling performance of AFSMBs. We found that malonic acid, as a molecular electron transfer facilitator, can expedite the anion reduction kinetics by the dipole moment effect that selectively decreases the P-F bond dissociation energy. This process results in enriched inorganic NaF in SEI, thereby enhancing its stability. This ICAR strategy allows an average Coulombic efficiency (CE) of 99.95% over 1000 cycles for sodium plating/stripping. This strategy further enables a high-loading cathode (>14 mg cm^-2) AFSMB with 0.11% capacity decay per cycle over 300 cycles, and a 3-Ah pouch cell delivering 212 Wh kg^-1. Such performance surpasses most previously reported values. This work offers a new avenue for building robust interphases toward next-generation anode-free batteries.