Compositional Tuning of Fe/Mn and Fe/Ni Ratios in P3-Type Cathodes Enables High Energy Density Sodium-Ion Batteries

Layered oxide cathodes are promising candidates for sodium-ion batteries due to their high theoretical capacity and structural tunability. However, irreversible high-voltage redox reactions and structural degradation hinder their practical deployment. In this study, a series of Na_0.8(Mn-Fe-Ni)O₂ cathodes with systematically varied Fe/Mn and Fe/Ni ratios is investigated to uncover the role of transition metal composition in governing redox behavior, phase transitions, and long-term performance across 2.0-4.0 V and 2.0-4.4 V. Structural analyses reveal that increasing Fe/Mn ratio expands Na-O₂ layer spacing and strengthens TM-O bonds, indicating reduced anionic activity and improved structural stability. Na_0.8Mn_0.53Fe_0.25Ni_0.22O₂ delivers the highest specific capacity (153 mAh g^-1), specific energy (500.3 Wh kg^-1), and reversible high-voltage redox activity, retaining 92.6% of its capacity after 100 cycles at 0.2C (2.0-4.4 V). Operando Synchrotron X-ray diffraction confirms P3/O3↔P3″/O3 transformations with minimal lattice strain ( $\text{Δ}c=\frac{c_{4.4V}-c_{2.0V}}{c_{4.4V}}\times 100\%=-1.81\%$ for O3, +1.00% for P3), contributing to enhanced high-voltage cyclability in Na_0.8Mn_0.53Fe_0.25Ni_0.22O₂. Meanwhile, Na_0.8Mn_0.64Fe_0.14Ni_0.22O₂ exhibits exceptional cycling performance (99% retention) in the 2.0-4.0 V range, benefiting from a P3↔P3' transition. These findings highlight the critical role of Fe/Mn and Fe/Ni tuning in balancing redox reversibility and structural integrity, offering a rational design strategy for high-energy, long-life sodium-ion cathodes.