Unveiling Phase Synergy and Heterogeneity in O3-P2 Biphasic Sodium Layered Oxides

Sodium-ion batteries (SIBs) present a cost-effective and sustainable alternative to lithium-ion batteries, with layered oxide cathodes offering high specific capacity, energy density, and cycle life. Phase engineering along with compositional tuning, plays a critical role in optimizing the electrochemical properties of layered transition metal oxides. In this study, we report a systematic phase-engineering strategy to enhance the electrochemical performance of NaₓMn₀.₅Ni₀.₄Fe₀.₁O₂ (MNF541) through controlled modulation of sodium content (x = 1, 0.85, and 0.65), yielding O3, P2, and O3-P2 biphasic materials. The O3-P2 composite, containing only 13.5% of the P2 phase, exhibited a high reversible capacity of 134 mAh g⁻¹ at C/2, an exceptional coulombic efficiency of 99.91%, and outstanding cycling stability, maintaining 99.7% of its initial capacity after 400 cycles within the 4.0 - 2.0 V window. The biphasic system also exhibited superior rate performance and reduced charge-transfer resistance at all states of charge. When cycled up to 4.3 V, the O3-P2 material outperformed pure O3 in both capacity (>190 mAh g⁻¹) and cycle life, underscoring the pivotal role of minor phase synergy in stabilizing high-voltage operation. Multiscale structural and spectroscopic analyses, including AFM and micro-Raman mapping, revealed the intimate coexistence of O3 and P2 domains within individual O3-P2 particles, along with discrete O3-rich regions that are more prone to structural degradation during cycling. X-ray absorption near-edge structure (XANES) analysis indicated that Ni, Fe, and Mn predominantly exist in +2, +3, and +4 oxidation states, respectively. Ni-centered redox processes dominate below 4.0 V, while Fe oxidation becomes more prominent at higher voltages, as confirmed by low-temperature Mössbauer spectroscopy. Spatial heterogeneity in Fe oxidation states across the electrode, revealed through micro-XANES mapping, coupled with inconsistencies observed in bulk XANES spectra, highlights the critical role of spatially resolved characterization and low-temperature spectroscopies in capturing the formation and dynamic evolution of transient Fe⁴⁺ species, and elucidating their influence on electrochemical performance.