Zr Incorporation Into Lithium Nickel Oxides: Bulk or Surface Substitution?

High-capacity lithium nickel oxides (LNO) are promising cathode active materials for next-generation batteries. However, their practical application is limited by poor cycling stability. Doping the LNO cathode with high-valence elements is an effective strategy to enhance structural stability. In this study, we investigate the mechanism of Zr⁴⁺ incorporation into LNO using experimental and theoretical techniques. Our findings reveal that under oxygen-rich conditions, Zr-doped LNO can be realized as a single phase, but most Zr cations localize at the surface of primary particles. In contrast, under oxygen-deficient or non-equilibrium conditions a secondary layered Li_1-xNi_1+xO₂ phase, characterized by Ni migration into the Li layers (cation ordering), is stabilized coexisting with Zr-doped LNO. This promotes elongation of primary particles at the aggregation surface. The Zr enrichment at the microparticle surface, Zr-induced cation-ordered superstructure, and the formation of elongated grains enhance the chemical stability as well as the structural integrity at atomistic- and micro-scale of the LNO cathode, improving its cycling performance. However, only 1-3% Zr is necessary to maintain the initial capacity and enhance cyclability through surface coating and low bulk doping with Zr. With increasing Zr concentration, an electronically insulating impurity (Li₂ZrO₃) forms, leading to a decrease in the initial capacity.