Construction of P2/T Biphasic Na0.44MnO2 Cathode Material Through Addition of Citric Acid for Sodium-Ion Batteries

The Na_xMnO₂ (x < 0.44) material with a T-type tunnel phase shows excellent cycling performance, but low initial specific capacity. The Na_xMnO₂ (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_0.44MnO₂ 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_0.44MnO₂ 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.