Unveiling the outstanding full-cell performance of P2-type Na_0.67(Mn_0.44Ni_0.06Fe_0.43Ti_0.0₇)O₂ cathode active material for Na-ion batteries

In this study, we unravel the effect of Ni doping on the half-cell and full-cell performances of the Na_0.67Mn_0.5-xNi_xFe_0.43Ti_0.07O₂ cathode materials where x varies between 0.02 and 0.1. The cyclic voltammetry (CV) analysis of the half-cells is performed at 10 °C, room temperature (RT), and 50 °C to elucidate the redox reaction mechanisms at different temperatures. Among the studied cathodes, the highest specific capacity is obtained fox = 0.06 which delivered a specific capacity of 186 mAh g⁻¹ at C/3-rate. The full cell of Na_0.67Mn_0.44Ni_0.06Fe_0.43Ti_0.07O₂/hard carbon couple is assembled in coin cell format and the specific capacity of the cell at C/2, 1C, and 2C rates are found as 153 mAh g⁻¹, 125 mAh g⁻¹ and 120 mAh g⁻¹, respectively. At the C/2-rate, the excellent capacity retention of the full cell is around 70% after 500 cycles delivering a specific capacity of 103 mAh g⁻¹. Along with the conventional physicochemical characterization methods such as X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Raman and Fourier-transform Infrared Spectroscopies (FTIR), we also utilize X-ray photoelectron spectroscopy (XPS) to bridge the nexus between the performance and the structure properties of the studied materials. Furthermore, we also employ synchrotron-based X-ray Absorption (XAS) to understand the local geometry of the optimized cathode materials in operando.