Unveiling the anionic redox chemistry in phosphate cathodes for sodium-ion batteries

Anionic redox chemistry has aroused increasing attention in sodium-ion batteries (SIBs) by virtue of the appealing additional capacity. However, up to now, anionic redox reaction has not been reported in the mainstream phosphate cathodes for SIBs. Herein, the ultrathin VOPO${}_4$ nanosheets are fabricated as promising cathodes for SIBs, where the oxygen redox reaction is first activated accompanied by reversible ClO${}_4$${}^{-}$ (from the electrolyte) insertion/extraction. As a result, the VOPO${}_4$ cathode harvests a record-high capacity (168 mAh g${}^{-1}$ at 0.1 C) among its counterparts ever reported. Moreover, the ClO${}_4$${}^{-}$ insertion efficiently expands the interlayer spacing of VOPO${}_4$ and accelerates the ion diffusion, enabling an unprecedentedly high rate performance (69 mAh g${}^{-1}$ at 30 C). Via systematic ex situ characterizations and theoretical computations, the anionic redox chemistry and charge storage mechanism upon cycling are thoroughly elucidated. This study opens up a new avenue toward high-energy phosphate cathodes for SIBs by triggering anionic redox reactions. (© 2023 Wiley-VCH GmbH)