Unraveling diffusion kinetics of honeycomb structured Na$_2$Ni$_2$TeO$_6$ 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 −1 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 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.