Understanding how off-stoichiometry promotes cation mixing in LiNiO_2

Although LiNiO_2 is chemically similar to LiCoO_2 and offers a nearly identical theoretical capacity, LiNiO_2 and related Co-free Ni-rich cathode materials suffer from degradation during electrochemical cycling that has prevented practical use in Li-ion batteries. The observed capacity decay of LiNiO_2 has been attributed to the formation of structural defects via Li/Ni cation mixing that reduces cyclability and leads to poor capacity retention. Herein, we investigate the kinetics and thermodynamics of Li/Ni mixing in ideal LiNiO_2 and off-stoichiometric Li_1-zNi_1+zO_2. We find that ideal LiNiO_2 is stable against cation mixing with similar characteristics as LiCoO_2. Li/Ni mixing is promoted by extra Ni in the Li layers that cannot be avoided in synthesis. Our study elucidates the crucial role of extra Ni atoms on Li sites in the cation mixing mechanism, an insight that can inform the development of Co-free cathode materials.