Uncovering the Nonequilibrium Evolution Mechanism between Na_2+2δFe_2−δ(SO₄)₃ Cathode and Impurities in the Na₂SO₄-FeSO₄·7H₂O Binary System for High-Voltage Sodium-Ion Batteries

Sodium-ion batteries are increasingly recognized as ideal for large-scale energy storage applications. Alluaudite Na_2+2δFe_2−δ(SO₄)₃ has become one of the focused cathode materials in this field. However, previous studies employing aqueous-solution synthesis often overlooked the formation mechanism of the impurity phase. In this study, the nonequilibrium evolution mechanism between Na_2+2δFe_2−δ(SO₄)₃ and impurities by adjusting ratios of the Na₂SO₄/FeSO₄·7H₂O in the binary system is investigated. Then an optimal ratio of 0.765 with reduced impurity content is confirmed. Compared to the poor electrochemical performance of the Na_2.6Fe_1.7(SO₄)₃ (0.765) cathode, the optimized Na_2.6Fe_1.7(SO₄)₃@CNTs (0.765@CNTs) cathode, with improved electronic and ionic conductivity, demonstrates an impressive discharge specific capacity of 93.8 mAh g⁻¹ at 0.1 C and a high-rate capacity of 67.84 mAh g⁻¹ at 20 C, maintaining capacity retention of 71.1% after 3000 cycles at 10 C. The Na_2.6Fe_1.7(SO₄)₃@CNTs//HC full cell reaches an unprecedented working potential of 3.71 V at 0.1 C, and a remarkable mass-energy density exceeding 320 Wh kg⁻¹. This work not only provides comprehensive guidance for synthesizing high-voltage Na_2+2δFe_2−δ(SO₄)₃ cathode materials with controllable impurity content but also lays the groundwork of sodium-ion batteries for large-scale energy storage applications.