Low-cost and long-life cathode materials, such as the polyanionic iron-based Alluaudite-type Na2+2δFe2-δ(SO4)3, are crucial for future large-scale energy storage applications. This material is typically synthesized from the hydrated precursor Na2Fe(SO4)2·4H2O. However, the vapor released during the heating of crystal water can lead to reduced crystallinity, increased fraction of Na6Fe(SO4)4 impurities, and potential structural damage to Na2+2δFe2-δ(SO4)3. For the first time, we synthesized a stable Stanfieldite-type Na2Fe(SO4)2 material using a well-designed sol-gel method. This approach effectively mitigates the aforementioned risks by facilitating a unique transition from anhydrous Na2Fe(SO4)2 precursor to Na2+2δFe2-δ(SO4)3 cathode. The enhanced crystallinity, controllable impurity fraction, and reduced migration barriers of the pristine Na2+2δFe2-δ(SO4)3 cathode significantly improve electrochemical performance. Moreover, we constructed Na2+2δFe2-δ(SO4)3/C composite cathodes to optimize their high-rate capacity and cycling retention. At 25 °C, these composites exhibit remarkable high-rate capacity and maintain an impressive 92.2 % capacity retention after 1000 cycles at 10 C. In tests under extreme conditions at −25 °C, 0 °C, and 60 °C, they sustained over 90 % capacity retention after 100 cycles at 1 C or exceeded 95 % after 200 cycles at 2 C. Furthermore, the assembled Na2+2δFe2-δ(SO4)3/C//HC full cells demonstrate superior rate capacity and long-term cycling stability, indicating their promising potential for commercial applications.

中文翻译：

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从无水 Stanfieldite 型 Na2Fe（SO4）2 前驱体过渡到冲积物型 Na2+2δFe2-δ（SO4）3/C 复合阴极：通往经济高效且全气候的钠离子电池的途径


低成本和长寿命的正极材料，如聚阴离子铁基 Alluaudite 型 Na2+2δFe2-δ（SO4）3，对于未来的大规模储能应用至关重要。这种材料通常由水合前驱体 Na2Fe（SO4）2·4H2O 合成。然而，结晶水加热过程中释放的蒸气会导致结晶度降低，Na6Fe（SO4）4 杂质的分数增加，并对 Na2+2δFe2-δ（SO4）3 造成潜在的结构损伤。我们首次使用设计良好的溶胶-凝胶法合成了稳定的 Stanfieldite 型 Na2Fe（SO4）2 材料。这种方法通过促进从无水 Na2Fe（SO4）2 前驱体到 Na2+2δFe2-δ（SO4）3 阴极的独特转变，有效地降低了上述风险。原始 Na2+2δFe2-δ（SO4）3 阴极的增强结晶度、可控杂质分数和降低的迁移势垒显着改善了电化学性能。此外，我们构建了 Na2+2δFe2-δ（SO4）3/C 复合阴极，以优化其高倍率容量和循环保持能力。在 25 °C 下，这些复合材料表现出显著的高倍率容量，并在 10 C 下循环 1000 次后仍保持令人印象深刻的 92.2% 容量保持率。在 −25 °C、0 °C 和 60 °C 的极端条件下进行测试，它们在 1 C 下循环 100 次后保持了超过 90% 的容量保持率，在 2 C 下循环 200 次后保持了超过 95% 的容量保持率。此外，组装的 Na2+2δFe2-δ（SO4）3/C//HC 全电池表现出优异的倍率容量和长期循环稳定性，表明其具有广阔的商业应用潜力。