High performance pseudo-capacitors based on multicomponent transition metal oxides by local distortion of oxygen octahedra

Abstract

Various metal oxides have been intensively investigated as active materials for pseudo-capacitors, a bridging technology between electric double layer capacitors and rechargeable batteries. A considerable amount of research so far has focused largely on the synthesis of nanostructured metal oxides/chalcogenides and their assembly with conductive nanomaterials even though in-depth understandings regarding crystal chemistry of active materials and suggestions of a new chemical combination are essential steps toward accomplishing higher capacitance. Herein we report that the performance of metal-oxide pseudo-capacitors can be significantly enhanced by engaging MO-type oxides with multiple transition metals: M = Ni, Co, and Mn. According to a series of X-ray absorption near edge spectroscopy (XANES) analyses, the aliovalent states of Co and Mn in contrast to the 2+ state of Ni were identified in the multicomponent TM oxides. In addition, a much wider redox swing of Ni was found as a key feature in the solid-solution oxides. Ab initio density functional theory (DFT) calculations demonstrated that cation vacancies and aliovalent Co and Mn could easily induce permanent local distortions of [$NiO_6$] octahedra. As this type of distortion breaks the degenerate eg level of Ni, the main redox center for pseudo-capacitance, Jahn-Teller lattice instability can be effectively avoided during the $Ni^{2+/3+}$ redox flip, resulting in remarkably enhanced electrochemical capacitance. Our findings highlight the significance of structure？property correlation and further capacitance improvement is also suggested through the optimum control of local distortions.