Air stability is a crucial factor in the practical application of a battery material, as it profoundly affects the material's preparation, storage, and electrode fabrication processes. Sodium iron sulfate cathodes, despite their attractive attributes in cost and electrochemical performance, are widely believed to be unstable upon air exposure because of the sulfate group. Here we report remarkable air-stability of the Na2Fe(SO4)2-based (NFS) cathodes (minimal decay in 20 % RH air for 60 days, 91.9 % capacity retention after 3500 cycles in half cells) and their outstanding cycle performance in practically relevant pouch-type full cells (∼100 Wh kg-1 specific energy, >1000 cycle-life). Although the NFS cathodes do react with moisture H2O to produce Na2Fe(SO4)2⋅4H2O but the hydration is spatially confined at the NFS particles’ surface and not propagating into their bulk. Further, the structural changes are reversible when the surface-hydrated NFS particles are heated in the typical electrode vacuum-drying process, avoiding extra treatment and additional cost. This work reveals the promising properties of the NFS cathode materials towards high-performance and sustainable Na-ion batteries.

中文翻译：

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用于长循环寿命钠离子电池的 Na2Fe（SO4）2 阴极的自限性和可逆表面水合


空气稳定性是电池材料实际应用中的关键因素，因为它会深刻影响材料的制备、储存和电极制造过程。硫酸铁钠阴极尽管在成本和电化学性能方面具有吸引力的属性，但由于硫酸盐基团，人们普遍认为在空气暴露时不稳定。在这里，我们报告了基于 Na2Fe（SO4）2 （NFS） 的阴极显着的空气稳定性（在 20 % RH 空气中 60 天的最小衰减，半电池循环 3500 次后保持 91.9% 的容量）及其在实际相关的软包型全电池中的出色循环性能（∼100 Wh kg-1 比能量，>1000 循环寿命）。尽管 NFS 阴极确实与水分 H2O 反应产生 Na2Fe（SO4）2⋅4H2O，但水合在空间上被限制在 NFS 颗粒的表面，而不是传播到它们的本体中。此外，在典型的电极真空干燥过程中加热表面水合的 NFS 颗粒时，结构变化是可逆的，从而避免了额外的处理和额外的成本。这项工作揭示了 NFS 正极材料对高性能和可持续钠离子电池的良好特性。