  In search of the potential cathode materials for sodium-ion batteries and to
understand the diffusion kinetics, we report the detailed analysis of
electrochemical investigation of honeycomb structured Na$_{2}$Ni$_{2}$TeO$_{6}$
material using cyclic voltammetry (CV), electrochemical impedance spectroscopy
(EIS), galvanostatic charge-discharge (GCD) and galvanostatic intermittent
titration technique (GITT). We found the discharge capacities of 82 and 77
mAhg$^{-1}$ at 0.05~C and 0.1~C current rates, respectively, and the
mid-working potential of $\approx$3.9~V at 1~C and high capacity retention of
80\% after 500 cycles at 0.5~C as well as excellent rate capability. The
analysis of CV data at different scan rates reveals the pseudo-capacitive
mechanism of sodium-ion storage. Interestingly, the {\it in-situ} EIS
measurements show a systematic change in the charge-transfer resistance at
different charge/discharge stages as well as after different number of cycles.
The diffusion coefficient extracted using CV, EIS and GITT lies mainly in the
range of 10$^{-10}$ to 10$^{-12}$ cm$^{2}$s$^{-1}$ and the
de-insertion/insertion of Na$^+$-ion concentration during electrochemical
cycling is consistent with the ratio of Ni$^{3+}$/Ni$^{2+}$ valence state
determined by photoemission study. Moreover, the post-cyclic results of
retrieved active material show very stable structure and morphology even after
various charge-discharge cycles. Our detailed electrochemical investigation and
diffusion kinetics studies establish the material as a high working potential
and long life electrode for sodium-ion batteries.
