Approaching the Theoretical Capacity Limit of Na2FeSiO4-Based Cathodes with Fully Reversible Two-Electron Redox for Sodium-Ion Battery

Abstract Orthosilicate compounds are emerging as a promising class of low-cost and intrinsically safe cathodes due to the strong inductive effects of polyanion groups for rechargeable sodium ion batteries. However, enabling two-electron redox and actualizing the appealing high theoretical capacity of ∼270 mAh g-1 for orthosilicates remains challenging. Here, fully reversible two-electron redox in sodium iron orthosilicate cathodes by fluorine doping are reported. Due to the unlocking of the Fe3+/Fe4+ redox couple, F-doped Na2FeSiO4 displays exceptionally high capacity of 271 mAh g-1 that has never been reported for polyanionic cathodes. Based on the newly built crystal structure model of triclinic phase, fluorine doping is demonstrated to greatly promote charge redistribution and accelerate the electron exchange, hosting more sodium ions in the framework and stabilizing Fe4+ containing intermediate phases thermodynamically. The zero-strain characteristics of fluorine-doped orthosilicate ensure its excellent cycling stability with 93.7% capacity retention over 200 cycles. The successful unlocking of the trapped sodium in orthosilicates provides valuable insight and opens up a new avenue for the development of high capacity cathode materials for rechargeable batteries.