Compositional tuning of Fe/Mn and Fe/Ni ratios in P3-type cathodes enables high energy density sodium-ion batteries

Abstract

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<inf>0.8</inf>(Mn–Fe–Ni)O<inf>2</inf> 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-O<inf>2</inf> layer spacing and strengthens TM–O bonds, indicating reduced anionic activity and improved structural stability. Na<inf>0.8</inf>Mn<inf>0.53</inf>Fe<inf>0.25</inf>Ni<inf>0.22</inf>O<inf>2</inf> 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<inf>0.8</inf>Mn<inf>0.53</inf>Fe<inf>0.25</inf>Ni<inf>0.22</inf>O<inf>2</inf>. Meanwhile, Na<inf>0.8</inf>Mn<inf>0.64</inf>Fe<inf>0.14</inf>Ni<inf>0.22</inf>O<inf>2</inf> 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. © 2025 Elsevier B.V., All rights reserved.