A hybrid dual-salt polymer electrolyte for sodium metal batteries with stable room temperature cycling performance

Sodium metal batteries (NMBs) are among the most promising alternatives to lithium-ion batteries. However, the thermal instability, potential leakage, and dendrite growth typical of NMBs with organic liquid electrolytes still pose severe limitations to this technology. These challenges can be overcome by replacing the organic liquid electrolytes with solids or gels. In this study, a hybrid dual-salt polymer electrolyte (DSPE), comprising sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and sodium difluoro(oxalate)borate (NaDFOB) salts is reported. The two salts work synergistically: NaTFSI enhances ionic conductivity and thermal stability, while NaDFOB extends the electrochemical window and inhibits the corrosion of the Al current collectors caused by NaTFSI. Furthermore, the DSPE with a NaDFOB/NaTFSI optimized molar ratio of 20% delivers a high ionic conductivity of 0.31 mS cm−1 at 23 °C. At this temperature, the NMB with a Na3V2(PO4)3@carbon positive electrode has excellent cycling performance (104.0 mAh g − 1 at 1 C with 92.4% capacity retention over 300 cycles) and rate capability (93.5 mAh g − 1 at 5 C). X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) characterizations, together with ab initio calculations reveal that stable battery performance is linked to the formation of a NaxBOyFz-containing protective cathode-electrolyte interphase layer. Leveraging the synergistic combination of two salts, this new DSPE formulation opens a new path for next-generation, high-performance NMBs.