Mixed Polyanionic NaFe1.6V0.4(PO₄)(SO₄)₂@CNT Cathode for Sodium-ion Batteries: Electrochemical Diffusion Kinetics and Distribution of Relaxation Time Analysis at Different Temperatures

We report the electrochemical sodium-ion kiinetics and distribution of relaxation time (DRT) analysis of a newly designed mixed polyanionic NaFe1.6V0.4(PO₄)(SO₄)₂@CNT composite as a cathode. The specific capacity of 104 mAhg⁻¹ is observed at 0.1~C with the average working voltage of ~3~V. Intriguingly, a remarkable rate capability and reversibility are demonstrated up to very high current rate of 25~C. The long cycling test up to 10~C shows high capacity retention even after 2000 cycles. The detailed analysis of galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV) data reveal the diffusion coefficient of 10⁻⁸--10⁻¹¹ cm²s⁻¹. We find excellent stability in the thermal testing between 25--55^∘C temperatures and 80% capacity retention up to 100 cycles at 5~C. Further, we analyse the individual electrochemical processes in the time domain using the novel DRT technique at different temperatures. The { ex-situ} investigation shows the stable and reversible structure, morphology and electronic states of the long cycled cathode material. More importantly, we demonstrate relatively high specific energy of ≈155 Wh kg⁻¹ (considering the total active material loading of both the electrodes) at 0.2~C for full cell battery having excellent rate capability up to 10~C and long cyclic stability at 1~C.