Mixed Polyanionic NaFe$_{1.6}$V$_{0.4}$(PO$_{4}$)(SO$_{4}$)$_{2}$@CNT Cathode for Sodium-ion Batteries: Electrochemical Diffusion Kinetics and Distribution of Relaxation Time Analysis at Different Temperatures

TL;DR

This paper introduces a novel mixed polyanionic NaFe$_{1.6}$V$_{0.4}$(PO$_{4}$)(SO$_{4}$)$_{2}$@CNT cathode for sodium-ion batteries, demonstrating high capacity, excellent rate capability, and long-term stability through detailed electrochemical and thermal analysis.

Contribution

The study provides new insights into the electrochemical kinetics and relaxation processes of this cathode material using distribution of relaxation time analysis at various temperatures.

Findings

Achieved 104 mAhg$^{-1}$ capacity at 0.1 C with 3 V voltage.

Demonstrated stable cycling with 80% capacity retention after 2000 cycles.

Showed high energy density of approximately 155 Wh kg$^{-1}$ at 0.2 C.

Abstract

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 0.1~C with the average working voltage of $\sim$3~V. Intriguingly, a remarkable rate capability and reversibility are demonstrated up to very high current rate of 25~C. The long cycling test up to 10~C shows high capacity retention even after 2000 cycles. The detailed analysis of galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV) data reveal the diffusion coefficient of 10$^{-8}$--10$^{-11}$ 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 5~C. 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 0.2~C for full cell battery having excellent rate capability up to 10~C and long cyclic stability at 1~C.