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

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-Na₃V₂(PO₄)₃ (NVP)|PEO-sodium hexauorophosphate (NaPF₆)|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-Na₃V₂(PO₄)₂F₃ (NVPF) |PEO-NaPF₆|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-NaPF₆|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.