Off-Stoichiometry, Vacancy Trapping, and Pseudo-irreversible First-Cycle Capacity in LiNiO2

Abstract

We demonstrate that the ubiquitous off-stoichiometry of LiNiO2 in the form of Li1-zNi1+zO2 slows the kinetics of the material by both diminishing the number of ionic charge carriers and increasing the length of diffusion paths. The positive charge of excess Ni in the Li layer, along with the accompanying local chemomechanical strain, creates an attractive potential for Li vacancies, thereby reducing their energy compared to defect-free regions. This attractive field extends over a radius of two lattice sites and also considerably lowers the migration barrier for a Li vacancy to approach the defect, effectively making excess Ni a sink for lithium vacancies. A similar argument can be made for divacancies, which are split by the extra Ni and pinned in the form of single vacancies. In addition to pinning effects, which could vary depending on the Li concentration, excess Ni also constitutes an obstacle to Li migration because it is rather immobile and does not undergo site exchange with an adjacent Li vacancy.