"Electron/Ion Sponge"-Like V-Based Polyoxometalate: Toward High-Performance Cathode for Rechargeable Sodium Ion Batteries

One key challenge facing room temperature Na-ion batteries lies in identifying earth-abundant, environmentally friendly and safe materials that can provide efficient Na⁺ storage sites in Na-ion batteries. Herein, we report such a material, polyoxometalate Na₂H₈[MnV₁₃O₃₈] (NMV), with entirely different composition and structure from those cathode compounds reported before. Ex-situ XPS and FTIR analyses reveal that NMV cathode behaves like an "electron/Na-ion sponge", with 11 electrons/Na⁺ acceptability per mole, which has a decisive contribution to the high capacity. The extraordinary structural features, evidenced by X-ray crystallographic analysis, of Na₂H₈[MnV₁₃O₃₈] with a flexible 2D lamellar network and 1D open channels provide diverse Na ion migration pathways, yielding good rate capability. First-principle calculations demonstrate that a super-reduced state, [MnV₁₃O₃₈]^20-, is formed with slightly expanded size (ca. 7.5%) upon Na⁺ insertion compared to the original [MnV₁₃O₃₈]^9-. This "ion sponge" feature ensures the good cycling stability. Consequently, benefiting from the combinations of "electron/ion sponge" with diverse Na⁺ diffusion channels, when revealed as the cathode materials for Na-ion batteries, Na₂H₈[MnV₁₃O₃₈]/G exhibits a high specific capacity (ca. 190 mA h/g at 0.1 C), associates with a good rate capability (130 mA h/g at 1 C), and a good capacity retention (81% at 0.2 C). Our results promote better understanding of the storage mechanism in polyoxometalate host, enrich the existing rechargeable SIBs cathode chemistry, and enlighten an exciting direction for exploring promising cathode materials for Na-ion batteries.