  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.
