Cationic vacancies and interface engineering on crystalline–amorphous gamma-phase Ni–Co oxyhydroxides achieve ultrahigh mass/areal/volumetric energy density flexible all-solid-state asymmetric supercapacitor

Abstract

Construction of gamma-phase transition metal oxyhydroxides for electrode materials is an effective strategy for improving electrochemical properties. However, the preparation of gamma-phase transition metal oxyhydroxides with various defects remains an arduous challenge. Herein, gamma-phase Ni–Co oxyhydroxides with cationic vacancy defects and crystalline–amorphous interfaces are synthesized successfully via electrochemical reconstruction to achieve an ultrahigh mass/areal/volumetric energy density, flexible all-solid-state asymmetric supercapacitor (ASC). The Ni–Co oxyhydroxides consist of γ-NiOOH, γ-CoOOH and NiOOH phases; the γ-NiOOH and γ-CoOOH phases can effectively improve electronic conductivity and theoretical capacitance due to the abundant Ni⁴⁺/Co⁴⁺. The cationic vacancy defects and crystalline–amorphous interfaces not only endow the gamma-phase Ni–Co oxyhydroxides with abundant electrochemical active sites, but also enable fast electron transfer between the electrode and electrolyte. Therefore, the as-obtained gamma-phase Ni–Co oxyhydroxides display a high capacitance of 20.9 F cm⁻² at 4 mA cm⁻² and 3483 F g⁻¹ at 0.67 A g⁻¹, as well as excellent rate characteristics (90.5% capacitance retention at a high current density of 240 mA cm⁻² and 40 A g⁻¹). Density functional theory calculations reveal that Ni–Co oxyhydroxides with cationic vacancies can increase the adsorption energy of H₂O, which is beneficial for the capture of H₂O to occur for subsequent charge storage reactions. Furthermore, the assembled Ni–Co oxyhydroxides//active carbon all-solid-state ASC shows ultrahigh mass/areal/volumetric energy density of 92.6 W h kg⁻¹/3.3 mW h cm⁻²/19.5 mW h cm⁻³ (at 1156 W kg⁻¹/34.6 mW cm⁻²/204.1 mW cm⁻³) and possesses excellent electrochemical stability with 91% retention after 7000 cycles.