Understanding the Effect of Zn Doping on Stability of Cobalt-Free P2-Na_0.60Fe_0.5Mn_0.5O₂ Cathode for Sodium Ion Batteries

In this work, we report a sol-gel synthesis-based Zn-doped Na_0.6Fe_0.5Mn_0.5O₂ (NFM) cathode and understand the effect of Zn doping on the crystal structure and electrochemical performances such as discharge capacity and rate capability. Detailed X-Ray diffraction (XRD) pattern analysis indicated a decrease in the Na-layer thickness with Zn doping. Small amount of Zn²⁺ dopant (i.e., 2 at.%) slightly improved cycling stability, reversibility, and rate performances at higher discharge current rates. For example, at 1 C-rate (1 C = 260 mAh/g), the Zn²⁺-doped cathode retained a stable reversible capacity of 72 mAh/g, which was ~16% greater than that of NFM (62 mAh/g) and showed a minor improvement in the capacity retention of 60% compared to 55% for the pristine NFM after 65 cycles. Slight improvement in the electrochemical performance for the Zn-doped cathode can be attributed to a better structural stability, which prevented the initial phase transition and showed the presence of electrochemical active Fe^3+/4+ even after 10 cycles compared to NFM.