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In this study, we unravel the effect of Ni doping on the half-cell and full-cell performances of the Na 0.67 Mn 0.5-x Ni x Fe 0.43 Ti 0.07 O 2 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 0.67 Mn 0.44 Ni 0.06 Fe 0.43 Ti 0.07 O 2 /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. • Ni doping on P2 type Na 0.67 Mn 0.5-x Ni x Fe 0.43 Ti 0.07 O 2 cathode active material was studied. • Na 0.67 Mn 0.44 Ni 0.06 Fe 0.43 Ti 0.07 O 2 cathode material delivered a specific capacity of 186 mAh g-1 at C/3-rate. • Full-cell containing Na 0.67 Mn 0.44 Ni 0.06 Fe 0.43 Ti 0.07 O 2 /Hard Carbon couple revealed 70% capacity retention after 500 cycles. [ABSTRACT FROM AUTHOR]

LiCoO 2 is the most widely used and extensively studied cathode material for Li-ion batteries. The studies based on the improvement of the performance have focused on the structural and electrochemical properties of LiCoO 2 . However, significantly less attention has been paid to its magnetic properties and their effects on battery performance. For the first time to our knowledge, we report a thermally induced magnetic spin state transition from low spin (LS) to intermediate spin (IS) at ∼800 K in bulk LiCoO 2 via magnetic susceptibility measurements. We quench the LiCoO 2 from above the spin state transition temperature (∼810 K) into liquid nitrogen to preserve the IS state at room temperature. We use this quenched sample (q-LiCoO 2 ) as an active cathode material. We observe a significant improvement (∼15% better capacity retention after 200 cycles at 1C) in cell performance of q-LiCoO 2 compared to the ref-LiCoO 2 which is used as a reference material. Our results show that it is possible to tailor the magnetic properties and electronic structure of cathode materials to achieve better battery performance. [ABSTRACT FROM AUTHOR]