Multifunctional sulfonate additive induced CEI layer enables ultra‐stable PEO based solid‐state sodium batteries

Abstract Polyethylene oxide (PEO)‐based solid polymer electrolytes are considered as promising material for solid‐state sodium metallic batteries (SSMBs). However, their poor interfacial stability with high‐voltage cathode limits their application in high‐energy–density solid‐state batteries. Herein, a uniform, sulfur‐containing inorganic–organic composite cathode–electrolyte interphase layer was in situ formed by the addition of sodium polyvinyl sulfonate (NaPVS). The 5 wt% NaPVS‐Na 3 V 2 (PO 4 ) 3 (NVP)|PEO‐sodium hexauorophosphate (NaPF 6 )|Na battery shows a higher initial capacity of 111.2 mAh·g −1 and an ultra‐high capacity retention of 90.5% after 300 cycles. The 5 wt% NaPVS‐Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) |PEO‐NaPF 6 |Na battery with the high cutoff voltage of 4.2 V showed a specific discharge capacity of 88.9 mAh·g −1 at 0.5C for 100 cycles with a capacity retention of 79%, which is much better than that of the pristine‐NVPF (PR‐NVPF)|PEO‐NaPF 6 |Na battery (33.2%). The addition of NaPVS not only enhances the diffusion kinetics at the interface but also improves the rate performance and stability of the battery, thus bolstering its viability for high‐energy applications. In situ phase tracking further elucidates that NaPVS effectively mitigates self‐discharge induced by the oxidative decomposition of PEO at high temperature. This work proposes a general strategy to maintain the structural stability of the cathode–electrolyte interface in PEO‐based high‐performance SSMBs.