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

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 Ni 4+ /Co 4+ . 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 H 2 O, which is beneficial for the capture of H 2 O 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. Novel gamma-phase Ni-Co oxyhydroxides nanoplate networks with cationic vacancy defects and crystalline-amorphous interfaces achieve ultrahigh mass/areal/volumetric energy-density flexible all-solid-state asymmetric supercapacitor.