Meaning
A disordered, electrochemically inactive phase that forms on the surface of transition metal oxide cathodes represents a primary mechanism of capacity fade in high-voltage lithium cells. This rock-salt layer develops when transition metals migrate to occupy empty lithium sites during cycling or high-temperature storage. The resulting structural reorganization blocks the paths required for lithium diffusion.
Phase Transition
Delithiation at high potentials destabilizes the crystal lattice and triggers oxygen release from the surface. In the presence of this oxygen loss, the transition to a rock-salt layer occurs as the layered structure collapses into a cubic configuration. This irreversible transition is concentrated at the cathode-electrolyte interface.
Performance Deterioration
Internal resistance rises because the collapsed surface structure acts as an insulating barrier to ion transport. The presence of the rock-salt layer reduces the available capacity and slows down charging rates.
Prevention Practice
Surface coatings and element doping are applied to the precursor materials to stabilize the oxygen lattice and suppress metal migration. Minimizing rock-salt layer formation allows cells to operate safely at higher cut-off voltages, which increases the delivered energy density of the battery. Sourcing teams use x-ray photoelectron spectroscopy to verify the thickness of these protective coatings on incoming active materials.
