Investigation of Structural and Electrochemical Modulation in NaFe0.5Co0.5O2 Cathode Material via Zn Substitution for Fe

Sodium-ion batteries are gaining attention as viable alternatives to lithium-ion systems, particularly for large-scale energy storage and cost-effective electric mobility. Advancing high-performance electrode materials, especially cathodes, is the key to driving their commercial viability. Among various strategies, cation doping has shown significant potential to tailor the structural and electronic properties of layered cathode materials, thereby influencing their electrochemical behavior. In this study, the effects of partial Fe³⁺ substitution in NaFe_0.5Co_0.5O₂ were examined by introducing 5% Zn²⁺ (yielding NaFe_0.45Zn_0.05Co_0.5O₂) and by increasing the Co content (NaFe_0.45Co_0.55O₂). The influence of these modifications on the crystal structure, Na⁺ diffusion kinetics, and cycling performance was systematically investigated to clarify the role of doping in tuning electrode properties. Structural analysis revealed that increasing the Co/Fe ratio led to lattice shrinkage along both the a- and c-axes and promoted cation disorder. These changes were associated with reduced capacity fading and a transition from a solid-solution voltage profile to a stepwise voltage profile. For galvanostatic charge–discharge testing, Zn doping in the NaFe_0.45Zn_0.05Co_0.5O₂ cathode enhanced both rate capability and cycling stability, particularly at 1C. This improvement was attributed to a higher Na⁺ diffusion coefficient within the sloping region of the P3 phase. The findings highlight the importance of optimizing redox-active species and structural integrity to improve the layered cathode performance. Zn doping was shown to effectively enhance structural stability and maintain a high capacity while boosting rate performance and long-term cycling durability under high-rate conditions.