  Disorder-driven degradation phenomena, such as structural phase
transformations and surface reconstructions, can significantly reduce the
lifetime of Li-ion batteries, especially those with nickel-rich layered-oxide
cathodes. We develop a general free energy model for layered-oxide
ion-intercalation materials as a function of the degree of disorder, which
represents the density of defects in the host crystal. The model accounts for
defect core energies, long-range dipolar electrostatic forces, and
configurational entropy of the solid solution. In the case of nickel-rich
oxides, we hypothesize that nickel with a high concentration of defects is
driven into the bulk by electrostatic forces as oxidation reactions at the
solid-electrolyte interface reduce nickel and either evolve oxygen gas or
oxidize the organic electrolyte at high potentials (>4.4V vs. Li/Li+). The
model is used in battery cycling simulations to describe the extent of cathode
degradation when using different voltage cutoffs, in agreement with
experimental observations that lower-voltage cycling can substantially reduce
cathode degradation. The theory provides a framework to guide the development
of cathode compositions, coatings and electrolytes to enhance rate capability
and enhance battery lifetime. The general theory of cation-disorder formation
may also find applications in electrochemical water treatment and ion
separations, such as lithium extraction from brines, based on competitive ion
intercalation in battery materials.
