Electronic Structure Modulation and Phase Transformation of Nickel-Cobalt Carbonate Hydroxide Caused by Halogen Doping and Its Effect on Supercapacitor Performance

Nickel-cobalt layered double hydroxides (NiCo-LDHs) have been widely used as supercapacitor materials, and their performance can also be improved by doping engineering. However, the systematic study of halogen anion (X-)-modified NiCo-LDH has rarely been reported. In this study, X- were used to modify nickel-cobalt carbonate hydroxides (NiCo-CHs) via hydrothermal preparation. The halogen doping resulted in changes to the morphology of NiCo-CH. In particular, the electronegativity and electron-withdrawing ability of the halogen were found to be the key to the changes in the metal valence in the NiCo-CHs. Due to the high electronegativity of F, after 12 h of hydrothermal treatment, F-doped NiCo-CH (named F-NCCH12) caused the formation of high-valent M3+ sites, resulting in the production of a NiCo-LDH phase different from other halogen dopings. By varying the hydrothermal reaction time, we identified the mechanism of the structural and phase transition. In addition, by testing the electrochemical properties of NiCo-CHs for their applications as supercapacitors, we found a strong correlation with the doped halogen. Specifically, the supercapacitor performance improved with an increase in the electronegativity of the halogen. Furthermore, density functional theory calculations showed that F doping accelerates the reaction kinetics and improves the conductivity. Finally, we assembled NF@F-NCCH12//AC asymmetric devices, which exhibited an energy density of 35.3 W h kg(-1) at a power density of 375 W kg(-1). Crucially, the stable phase structure resulted in improved cycle performance of the F-NCCH12 electrode material, which showed an initial specific capacitance retention of 91.6% after 10,000 cycles.