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 (Δc=[Formula presented]×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.