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 gamma-NiOOH, gamma-CoOOH and NiOOH phases; the gamma-NiOOH and gamma-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.

Automated summary

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 as-obtained gamma-phase Ni-Co oxyhydroxides display a high capacitance and excellent rate characteristics. Density functional theory calculations reveal that Ni-Co oxyhydroxides with cationic vacancies can increase the adsorption energy of H2O, which is beneficial for subsequent charge storage reactions.