Distinct electrochemical behavior of P3 and P2 polytypes of Mn/Ni-based Na-ion battery cathode

A layered sodium-ion battery cathode Na3/4Mn11/16Ni3/16Al2/16O2 (NMNA) prepared in P3 and P2 polytype structures by varying the calcination temperature is systematically investigated for its structural and electrochemical behavior. P3-phase prepared at 650 degrees C delivered a higher initial capacity of 164 mAh/g (similar to 148 mAh/g for P2-phase calcined at 850 degrees C) at 0.1C. In contrast, the P2-phase sample exhibited better rate performance (87 mAh/g vs. 70 mAh/g for P3-phase at 4C) and capacity retention of similar to 47 % after 200 cycles (only similar to 26 % for P2-phase). Electrochemical impedance measurements before and after cycling confirmed that the increase in charge transfer resistance is responsible for the poor rate performance of the P3 phase.