Synergetic Anion-Cation Redox Ensures a Highly Stable Layered Cathode for Sodium-Ion Batteries

Sodium-ion batteries are commonly regarded as a promising candidate in large-scale energy storage. Layered iron/manganese oxide cathodes receive extensive attentions due to the element abundance and large theoretical capacity. However, these materials usually undergo obvious degradation of electrochemical performance due to the tendency of Mn dissolution and Fe migration during continuous sodium release and uptake. Herein, a strategy of anion-cation synergetic redox is proposed to suppress the structural deterioration originated from overusing the electrochemical activity of transition-metal ions, and decreased lattice strain as well as superior electrochemical performance are realized simultaneously. Results show that the Na_0.8 Li_0.2 Fe_0.2 Mn_0.6 O₂ (NLFM) electrode is highly resistant to the erosion of moisture that is distinct from the traditional Mn/Fe-based electrodes. Moreover, the NLFM electrode demonstrates solid solution behavior without phase transition during cycles. The ultra-small volume change of 0.85% is ascribed to the negligible manganese dissolution and invisible transition-metal migration. The high-stable layered structure assures superior reversible capacity of ≈165 mA h g^-1 , excellent rate capability, and splendid capacity retention of over 98.3% with 100 cycles. The findings deepen the understanding of the synergy between anion and cation redox and provide new insights to design the high-stable layered cathode for sodium-ion batteries.