Temperature-Dependent Dynamic Disproportionation in LiNiO$_2$

The layered oxide LiNiO$_2$ is the model system for high-performance Ni-rich cathodes for lithium-ion batteries. Here we investigate computationally the local electronic and spin structure and dynamics of nickels in LiNiO$_2$. Ab initio molecular dynamics simulations suggest that disproportionation of spin-half ($S = 1/2$) nickels to $S = 0$ and $S = 1$ is favored towards cryogenic temperatures. Upon heating, the spin states of the nickels interconvert between $0$, $1/2$, and $1$ on a sub-picosecond timescale. We propose that the transitions between differing states of the Ni correspond to the heat-activated electronic conduction in LiNiO$_2$. The $S = 1$ nickels are also found to stabilize antisite nickel defects via antiferromagnetic interactions. Finally, we suggest avenues for the experimental verification of our hypothesis.