Improved electrochemical performance of P2-type Na_0.67Li_x(Mn_0.5Fe_0.25Co_0.25)_1−xO₂ cathode materials from Li ion substitution of the transition metal ions

In this work, we report the preparation and the enhanced electrochemical performance of P2-type Na_0.67Li_x(Mn_0.5Fe_0.25Co_0.25)_1−xO₂ layered cathode materials with different Li substitutions via a nanomilling-assisted solid-state method. The experimental results show that the introduced lithium ions can greatly improve the performance of the material after entering the transition metal layer. Among these synthesized samples, the Na_0.67Li_0.03(Mn_0.5Fe_0.25Co_0.25)_0.97O₂ electrode exhibits an exceptionally high specific capacity of 190.6 mAh g⁻¹ at 0.1 C rate and a capacity retention of 89.5% after 30 cycles. Obviously, the specific capacity retention is improved by 15.3% compared with the pristine Na_0.67(Mn_0.5Fe_0.25Co_0.25)O₂ without Li incorporation (the specific capacity retention is only 74.2% after 30 cycles). Simultaneously, the sample also shows a large increase in rate performance compared to the pristine material. All of the enhancement can be explained by the fact that the introduced Li ions suppress the phase transition during the charge–discharge process and avoid the volume expansion of the electrode, which significantly reduce the occurrence of micro-cracks within the electrode. The reduced polarization, the decreased internal resistance of the prepared electrodes and the increased sodium ion diffusion coefficient, as confirmed from the CV and EIS measurements, can be regarded as the three major reasons for the enhanced electrochemical performance of the Li ion-substituted materials.