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 NaFe$_{1.6}$V$_{0.4}$(PO$_{4}$)(SO$_{4}$)$_{2}$@CNT composite as a cathode. The specific capacity of 104 mAhg$^{-1}$ is observed at 0.1C with the average working voltage of $\sim$3V. Intriguingly, a remarkable rate capability and reversibility are demonstrated up to very high current rate of 25C. The long cycling test up to 10C shows high capacity retention even after 2000 cycles. The detailed analysis of galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV) data reveal the diffusion coefficient of 10$^{-8}$--10$^{-11}$ cm$^{2}$s$^{-1}$. We find excellent stability in the thermal testing between 25--55$^\circ$C temperatures and 80% capacity retention up to 100 cycles at 5C. Further, we analyse the individual electrochemical processes in the time domain using the novel DRT technique at different temperatures. The {\it 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 $\approx$155 Wh kg$^{-1}$ (considering the total active material loading of both the electrodes) at 0.2C for full cell battery having excellent rate capability up to 10C and long cyclic stability at 1C.