Alteration of oxygen evolution mechanisms in layered LiCoO2 structures by intercalation of alkali metal ions

Abstract

Two oxygen evolution reaction (OER) mechanisms on layered lithium cobalt oxide (LiCoO₂ and simply LCO) were demonstrated by inserting various alkali metal ions. During the OER, we observed the insertion of large cations (Na⁺, K⁺, and Cs⁺) into delithiated LCO from the corresponding electrolyte solutions. The deep intercalation of hydrated Na⁺ and K⁺ resulted in significant structural transformations of delithiated LCO. In addition, the intercalated cations and remaining Li⁺ determined the average Co valence states in the LCO catalysts. Characteristic analyses showed that the increased Co valence state and Co–O covalency enhanced OER activity (LiOH < NaOH < KOH). In addition, density functional theory (DFT) calculations supported the formation of OER active sites as K⁺ was partially intercalated. In contrast, the superior OER activity obtained after Cs⁺ insertion entailed a different process. A sharp rise in the OER activity at a high pH and bent edges of oxide layers imposing surface tensile strain supported the lattice-oxygen-mediated OER. The original LCO bulk structure is mostly preserved with Cs⁺ insertion, offering better catalytic stability.