Durable 4.5 V sodium metal batteries stabilized by negatively charged metal–organic frameworks in gel polymer electrolyte

Abstract

The development of high-voltage sodium metal batteries faces significant challenges, including high desolvation energy and detrimental interfacial side reactions at elevated voltages. These issues critically impede Na⁺ transport kinetics and compromise high voltage stability, but can be addressed by strategically modifying the Na⁺ solvation structure. Herein, we report a metal–organic framework (MOF)-based strategy to reconfigure the Na⁺ solvation structure specifically for enhanced high-voltage stability and Na⁺ transport. A composite gel polymer electrolyte (MPVHF) is engineered by incorporating UiO-66-(COONa)₂ into a polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) matrix, followed by liquid electrolyte infiltration. The densely arrayed carboxylate groups (–COO⁻) on the MOF ligands exert strong electrostatic interactions with Na⁺, effectively weakening the coordination bonds between Na⁺ and solvent molecules. This targeted solvation regulation significantly mitigates interfacial side reactions and promotes the formation of a robust, stable electrode–electrolyte interphase crucial for high-voltage operation. Consequently, the MPVHF electrolyte achieves a wide electrochemical stability window extending to 4.93 V and a high Na⁺ transference number of 0.74. The Na₃V₂(PO₄)₂F₃ (NVPF)||Na full cells employing MPVHF exhibit stable cycling at 4.5 V cut-off for 1300 cycles at 4 C. This work presents an effective approach to tailor Na⁺ coordination and transport for high-voltage and fast-charging sodium metal batteries.