Influence of Mn Precursor Adjustments on the Structural and Electrochemical Behavior of P2-Type Na_0.65Ni_0.25Mn_0.75O₂ Cathodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are game-changing in large-scale energy storage technology compared to lithium-ion batteries (LIBs) due to abundant reserves, safety, and cost-effectiveness. However, serious issues in Mn-based P2-type cathodes, such as phase transitions, the Jahn–Teller effect, and Mn dissolution, hinder the success of SIBs. Herein, we report that altering the manganese oxide precursors in the solid-state synthesis of P2-type Na0.65Ni0.25Mn0.75O2 (NNMO) leads to structural variations and improvements in electrochemical properties. X-ray diffraction with refined data confirms that all samples are in the P2-type phase, with changes in lattice parameters and cell volume. Raman spectroscopy and electron spin resonance verify the presence of oxygen defects in the P2-type NNMO materials. Furthermore, X-ray photoelectron spectroscopy analysis of the Mn2O3 precursor-used Na0.65Ni0.25Mn0.75O2 (NNMO-2) sample reveals slightly higher Mn4+ and lower Mn3+ mixed valence states compared to other samples. The potential profile and dQ/dV plot of NNMO-2 exhibit solid-solution behavior, delivering an initial discharge capacity of 151 mAh/g and 152 mAh/g at 0.1 C. The sample demonstrates excellent capacity retention of 85.34% and 77.28% after 100 cycles at a 1 C rate, with a Coulombic efficiency exceeding 98% in both tested voltage ranges (1.5–4.0 V and 2.0–4.3 V), attributed to Mn charge compensation. Moreover, the Na-ion diffusion coefficient, estimated to be around 10–10 cm2/s using the galvanostatic intermittent titration technique and the reduced charge transfer resistance, confirmed by impedance measurements, further highlight the electrochemical benefits of the NNMO-2 sample. Overall, the results suggest that the Mn2O3 precursor can be a suitable raw material for solid-state reactions synthesizing P2-type Na0.65Ni0.25Mn0.75O2 cathode materials for sodium-ion battery applications.