Smart MOF separator towards long-cycling high-energy-density sodium-metal batteries

Abstract The practical advancement of sodium-metal batteries (SMBs) is hindered by unstable anode/cathode interfaces and suboptimal ion transport kinetics. Traditional separators, functioning as passive elements, fail to effectively regulate ionic flux and interfacial chemistry, leading to low Na+ transference numbers, pronounced concentration polarization, and unstable electrode interphases. This study introduces a novel smart separator based on medium-entropy metal-organic framework (EMOF). By incorporating ion-confinement mechanisms within sub-nanochannels, it synergistically regulates Na+ transport and interfacial reaction kinetics. The EMOF separator establishes a hierarchical capillary system: macroscale channels ensure rapid electrolyte wettability, while nanoconfinement channels guide uniform Na+ flux distribution. Its multicomponent medium-entropy architecture endows channel surfaces with specific electrostatic environments that effectively anchor PF6− anions, elevating the Na+ transference number to 0.81. Significantly, this configuration further promotes PF6− decomposition, promoting the in-situ formation of inorganic-rich and stable electrode-electrolyte interphase layers. Leveraging these synergistic mechanisms, SMBs employing the EMOF separator in Na3V2(PO4)2O2F systems achieve an ultra-high energy density of 296.5 Wh kg−1 based on the mass of electrode materials, and outstanding fast-charging capability, maintaining 87.1% capacity retention even at 10 C rate, showcasing superior comprehensive performance. This work transcends the conventional paradigm of separators as inert barriers, establishing a new design framework for high-performance SMBs.