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

Abstract

We report a detailed analysis of the electrochemical investigation of a honeycomb structured Na₂Ni₂TeO₆ material as a cathode for sodium-ion batteries using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge–discharge (GCD) tests and the galvanostatic intermittent titration technique (GITT) to understand diffusion kinetics. We find discharge capacities of 82 and 77 mA h g⁻¹ at 0.05C and 0.1C current rates, respectively, and a mid-working potential of ≈3.75 V and high capacity retention of 80% after >500 cycles at 0.5C 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, in situ EIS measurements show a systematic change in the charge-transfer resistance at different charge/discharge stages as well as after different numbers of cycles. The diffusion coefficient determined using CV, EIS and GITT analysis was found to be in the range of 10⁻¹⁰ to 10⁻¹² cm² s⁻¹, which is considered relatively large for cathode materials. The de-insertion/insertion of Na-ions during electrochemical cycling is consistent with the ratio of Ni³⁺/Ni²⁺ valence states determined by a photoemission study. Moreover, the post-cyclic results of the retrieved active material show a 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.