A comprehensive review on double-redox reaction towards high-performance polyanionic sodium-ion batteries

• • Double-redox reactions in vanadium-based polyanionic cathodes enable multi-electron transfer, achieving specific capacities exceeding 150 mAh/g, a critical threshold for commercial viability in grid-scale storage. • • Manganese-based polyanionic cathodes, such as Na4MnV(PO4)3, exhibit reversible double-redox reactions at high voltages (>4.0 V vs Na+/Na), but suffer from Jahn-Teller distortion, which can be mitigated by doping with elements like Cr, Zr, or W to maintain structural integrity over 1000 cycles. • • Doping strategies, including Cr substitution in Na4MnV(PO4)3, improve high-voltage performance by suppressing irreversible phase transitions, leading to capacity retention above 80% after 500 cycles at 1C rate. • • The review highlights that achieving double-redox reactions requires careful regulation of the local coordination environment and electronic structure, as demonstrated by the three-electron reaction in Na2TiV(PO4)3, which delivers a high energy density of ~400 Wh/kg, surpassing conventional single-electron cathodes.