“Electron/Ion\nSponge”-Like V‑Based Polyoxometalate: Toward High-Performance\nCathode for Rechargeable Sodium Ion Batteries

Abstract

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