Construction of P2/T Biphasic Na_0.44MnO₂ Cathode Material Through Addition of Citric Acid for Sodium‐Ion Batteries

The Na x MnO 2 ( x < 0.44) material with a T‐type tunnel phase shows excellent cycling performance, but low initial specific capacity. The Na x MnO 2 ( x > 0.6) with P2‐type phase exhibits high initial specific capacity, yet poor cycling performance. Layer‐tunnel hybrid materials has served as cathode materials for sodium‐ion batteries (SIBs) due to the integration of high specific capacity and superior cycling performance. Herein, sodium manganite composites is synthesized with tunnel phase containing a small amount of P2‐type layered phase in Na 0.44 MnO 2 materials using citric acid as the dispersant. The citric acid can promote the transition of the T‐P2 phase and contribute to the uniform distribution of elements. The as‐prepared Na 0.44 MnO 2 with layered‐tunnel hybrid phase exhibits the best overall electrochemical performances with a capacity retention of 68% after 300 cycles at 1 C and an initial discharge capacity of 143.7 mA h g −1 , due to combination of the advantages of layered and tunnel structure. This work provides a new strategy for synthesizing transition metal oxides with hybrid phase as the cathode of advanced SIBs.