Polymorphism and Doping of NaxFeyMn1-yO2 Cathode Materials for Sodium-Ion Batteries

Sodium-ion batteries present a sustainable and cost-effective alternative to lithium-ion batteries. In line with sustainability principles, this study explores NaxFeyMn1-yO2 synthesised via biotemplating approach, systematically examining factors influencing polymorph formation, including calcination conditions, precursor oxidation states, and oxygen content. Thermal-dependent phase behaviour revealed tuneable and reversible O3-P3 phase ratios., with P3 phase formation favoured by increased precursor oxidation states in oxygen-rich environments.
Electrochemical performance showed strong dependence on phase composition. An O3-rich phase Na0.9Fe0.5Mn0.5O2, with 80% O3, delivered an initial capacity of 134 mAh/g and 82% capacity retention, while a P3-rich phase achieved 165 mAh/g with satisfactory retention. Al doping in materials enhanced Na diffusion pathways, with Na0.9Al0.1Fe0.45Mn0.45O2 demonstrating superior performance—154 mAh/g initial capacity and 76% retention after 50 cycles.
These findings highlight the critical interplay between structural polymorphism and battery performance, offering a pathway for optimising sodium-ion cathodes through controlled phase formation and strategic doping. Further studies could explore polymorph behaviour and structural evolution to advance sodium-ion battery technologies.