Electron-deficient borate anions tailoring the electronic structure of defect-rich amorphous nickel–cobalt cathode for superior capacitive energy storage

Designing transition metal nickel–cobalt-based battery-type electrode materials driven by anions is crucial for achieving rapid OH⁻ ion transport under electrochemical activation conditions, thereby improving capacitance performance. Herein, borate anions are selected through theoretical calculations, and two-dimensional (2D) defect-rich amorphous nickel–cobalt-based borate is synthesized via a facile chemical reduction method. Under potentiostatic modification, activated products (NCB-G-E) are obtained. In situ Raman spectra reveal that electron-deficient borate extracts electrons from metal centers, facilitating the oxidation state transition of Ni and Co. Theoretical calculations show that in situ adsorbed borate regulates the d-band centers of metal sites, enhancing OH⁻ intermediate adsorption. Meanwhile, borate anion adsorption accelerates the deprotonation and activation processes. Electrochemical tests demonstrate that NCB-G-E displays superior capacitance performance, with a high quality specific capacity of 383.3 mA h g⁻¹ and 65% retention rate at 30 A g⁻¹, surpassing most nickel–cobalt-based electrodes. The assembled asymmetric supercapacitor presents an impressive energy density of 68.2 Wh kg⁻¹ and good cycling stability. This work highlights the role of electron-deficient borate in tuning metal band structure and promoting oxidation state transition through synergistic defect advantages, offering new prospects for advanced battery-type energy storage materials.