Covalency competition induced active octahedral sites in spinel cobaltites for enhanced pseudocapacitive charge storage

Spinel cobaltites are widely presented as promising pseudocapacitive materials, however, a fundamental understanding of their structure-property relationship at an atomic level remains vague. Herein, their geometrical-site-dependent charge storage capability is investigated by substituting Co with inactive Zn and redox-active Mn. Experimental and theoretical analyses reveal that redox-active cations in octahedral sites contribute to enhanced capacitance, intrinsically determined by the covalency competition between tetrahedral and octahedral sites. The Zn2+ incorporation leads to increased occupancy of Co in octahedral sites and 2.9x increased capacitance at 1 A g(-1) current density, whereas the substituted Mn cations mainly sit in octahedral sites which can react with OH- upon cycling and separate on the spinel surface to reconstruct into delta-MnO2 nanosheets, leading to 4x increased capacitance at 1 A g(-1) current density with a detected K+ ion intercalation. Thus, the exposure of redox-active cations in octahedral sites and their intrinsic properties are influential in determining spinel oxides' pseudocapacitive properties. This work provides a general principle to optimize the pseudocapacitive properties of spinel cobaltites by deliberately selecting cations for substitution and controlling their distribution in octahedral/tetrahedral sites. It also offers a fundamental understanding of geometrical-site-dependent activity, and can effectively guide the development of spinel oxides for enhanced pseudocapacitance.