Uncovering the Nonequilibrium Evolution Mechanism between Na 2+2 δ Fe 2- δ (SO 4 ) 3 Cathode and Impurities in the Na 2 SO 4 -FeSO 4 ·7H 2 O 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+2 δ Fe 2- δ (SO 4 ) 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+2 δ Fe 2- δ (SO 4 ) 3 and impurities by adjusting ratios of the Na 2 SO 4 /FeSO 4 ·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 Na 2.6 Fe 1.7 (SO 4 ) 3 (0.765) cathode, the optimized Na 2.6 Fe 1.7 (SO 4 ) 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 2.6 Fe 1.7 (SO 4 ) 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+2 δ Fe 2- δ (SO 4 ) 3 cathode materials with controllable impurity content but also lays the groundwork of sodium-ion batteries for large-scale energy storage applications.