Capacity Decay in LiNiO2: An Atomistic Kinetic Picture

High-Ni layered oxides experience significant capacity decay over cycling, but the underlying mechanisms remain controversial. Using atomistic simulations, the electrochemical behavior of the fatigue phase is reproduced: a surface densified phase traps the last 25% of Li at the end of charge, while discharge remains unimpeded. When the Li content falls to 25%, the remaining Li is locked into a superlattice, making the creation of Li vacancies the rate-limiting step for further delithiation. After cycling, the surface densified phase resembles Ni₅O₈, with 25% Ni in the Li layer forming a similar superlattice. Ni pins nearby Li, suppressing Li vacancy formation at the surface and kinetically trapping Li inside. Meanwhile, the Ni₅O₈ phase exhibits high diffusivity for Li interstitials in the superlattice, which explains the minimal resistance increase during discharge at the same Li content. Further densification leads to a surface phase that hinders both the charge and discharge across the entire voltage range.