  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$_{1-z}$Ni$_{1+z}$O$_2$. 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.
