Mechanical Degradation by Anion Redox in LiNiO2 Countered via Pillaring

Abstract

A promising next-generation high-energy cathode material, LiNiO₂ (LNO) has failed to realize commercialization due to severe capacity degradation during cycling. The dual mechanisms of surface oxygen evolution due to anion redox and anisotropic volume change upon delithiation synergistically pulverize and degrade the material. Detailed Density Functional Theory (DFT) modeling and analysis of the anisotropic structural changes associated with crack formation in LiNiO₂ (LNO) reveals the link of mechanical behavior to charge transfer and oxygen redox activity upon deep charge cycling (>4.2 V vs Li/Li⁺). In the two-phase region and H2–H3 transition from 66% to 100% delithiation, oxygen of [NiO₆] octahedra is discovered to undergo redox in growing the Li-deficient regions, causing c-lattice mechanical weakening and collapse as the Li-slab becomes depleted. Li-site dopants are investigated to locally compensate against anion redox, resulting in enhanced coulombic repulsion and supporting the interslab layer thickness even at 100% depth of charge. Ionic size and oxidation state of M in Li_x-yM_yNiO₂ are found to fundamentally impact stabilization capability, moderating the anisotropic strain and volume expansion asynchronously. Optimization of mixed doping composition may then enable “zero strain” high-Ni Li(Ni,Co,Mn)O₂ (NCM) or LNO.