Impact of P3/P2 mixed phase on the structural and electrochemical performance of Na0.75Mn0.75Al0.25O2 cathode

The biphasic nature is known to enhance the structural stability and electrochemical properties of layered oxide cathodes. Herein, we have synthesised Na0.75Mn0.75Al0.25O2 (NMA) with different P2/P3 phase fractions, tuned by modifying the calcination temperature. Rietveld refinement of XRD data showed a continuous reduction in the P3 phase fraction with increased calcination temperature. NMA sample calcined at 650 °C (NMA-650) with ∼52 % P3 and 48 % P2 phases showed good rate performance and a high specific capacity of 150 mAh g−1 at 0.1 C with 80 % capacity retention after 200 cycles between 1.5 and 4.2 V. The enhanced electrochemical performance is attributed to the material's morphology and improved structural durability. Interestingly, operando Synchrotron XRD data of NMA-650 and NMA-750 (sample calcined at 750 °C) showed a continued existence of the P3 phase in both compounds during charging-discharging in the 1.5–4.2 V range. Further, the P3 unit cell showed only a 0.5 % increase in volume compared to 1.5 % in NMA-750 during cycling. The enhanced electrochemical properties of NMA-650 are attributed to the synergistic effect of P3/P2 phase coexistence in the material.