Uncovering the Nonequilibrium Evolution Mechanism between Na2+2δFe2-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 Na2+2 delta Fe2-delta(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 Na2+2 delta Fe2-delta(SO4)(3) and impurities by adjusting ratios of the Na2SO4/FeSO4 center dot 7H(2)O 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 Na2.6Fe1.7(SO4)(3) (0.765) cathode, the optimized Na2.6Fe1.7(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 Na2.6Fe1.7(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 Na2+2 delta Fe2-delta(SO4)(3) cathode materials with controllable impurity content but also lays the groundwork of sodium-ion batteries for large-scale energy storage applications.