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 reaction and actualizing the appealing high theoretical capacity of ∼270 mAh g⁻¹ for orthosilicates remain challenging. Here, fully reversible two-electron redox reaction in sodium iron orthosilicate cathodes by fluorine doping are reported. Owing 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 insights and opens up a new avenue for the development of high capacity cathode materials for rechargeable batteries.

由于聚阴离子基团对可充电钠离子电池的强烈感应作用，正硅酸盐化合物正成为一类有前景的低成本和本质安全的阴极。然而，实现二电子氧化还原反应并实现原硅酸盐的约270 mAh g ^-1的吸引人的高理论容量仍然具有挑战性。在此，报道了通过氟掺杂在正硅酸钠铁阴极中的完全可逆的两电子氧化还原反应。由于Fe ³⁺ / Fe ⁴⁺氧化还原对的解锁，掺F的Na ₂ FeSiO ₄显示出271 mAh g ^-1的超高容量聚阴离子阴极从未有过报道。基于新建立的三斜晶相晶体结构模型，氟掺杂可以极大地促进电荷重新分布并加速电子交换，在骨架中容纳更多钠离子，并热力学稳定含Fe ⁴⁺的中间相。氟掺杂原硅酸盐的零应变特性确保了其出色的循环稳定性，在200次循环中的容量保持率为93.7％。成功解锁原硅酸盐中捕获的钠离子提供了宝贵的见识，并为开发用于可充电电池的高容量阴极材料开辟了一条新途径。