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 the end of charge, while discharge remains unimpeded. When the Li content falls to 25%, the remaining Li are locked into a superlattice, making the creation of 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. These Ni pin nearby Li, suppressing 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 charge and discharge across the entire voltage range.