Unraveling diffusion kinetics of honeycomb structured Na₂Ni₂TeO₆ as a high-potential and stable electrode for sodium-ion batteries

Abstract

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 Na₂Ni₂TeO₆ 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 at 0.05~C and 0.1~C current rates, respectively, and the mid-working potential of 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 cm²s^-1 and the de-insertion/insertion of Na-ion concentration during electrochemical cycling is consistent with the ratio of Ni/Ni 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.