A binder-driven cathode-electrolyte interphase via a displacement reaction for high voltage Na₃V₂(PO₄)₂F₃ cathodes in sodium-ion batteries

Sodium super ionic conductor (NASICON)-structured Na₃V₂(PO₄)₂F₃ (NVPF) is a promising cathode for application in sodium-ion batteries (SIBs) because of its high working potential (3.7 V and 4.2 V vs. Na/Na⁺) and structural stability. Nonetheless, interfacial instability deteriorates its electrochemical performance. Therefore, to overcome this limitation, we introduced a sodium polyacrylate (NaPAA) binder for NVPF cathodes. The NaPAA binder effectively suppresses electrolyte decomposition by uniformly covering NVPF particles. Furthermore, the sodium carboxylate group of R-COONa in the NaPAA binder can react with the HPO₂F₂ intermediate generated by the hydrolysis of NaPF₆ and be converted into R-COOH and NaPO₂F₂via the displacement of Na⁺ by H⁺. This results in the formation of a stable and Na-ion conductive NaPO₂F₂-rich cathode-electrolyte interphase (CEI) layer. In addition, the NaPAA-based electrode exhibits desirable cycling and rate performances compared to those of conventional poly(vinylidene difluoride)-based electrodes. This study provides new insights into the design of CEI layers by introducing chemical functional groups in the binder for high-performance SIB cathodes.