Meaning
Surface degradation involves the formation of a dense and electrochemically inactive layer on cathode particles. The rock salt phase conversion results in a crystal structure that lacks the open channels needed for lithium ion movement. Drivers for this transformation include the loss of oxygen from the lattice at high states of charge or elevated temperatures.
The resulting layer acts as a physical barrier that increases internal resistance and reduces capacity while hindering the flow of current.
Atomic Migration
Cation mixing occurs when transition metal ions occupy sites originally reserved for lithium. During rock salt phase conversion, the nickel or cobalt ions move into the lithium layers, creating a disordered arrangement. This blockage prevents ions from entering or leaving the particle effectively.
Ion Transport
Diffusion rates through the rock salt structure are several orders of magnitude lower than in the layered phase. As the rock salt phase conversion spreads from the surface into the bulk, the power capability of the battery drops. High resistance at the surface forces the cell to reach its cutoff voltage earlier during discharge.
Capacity Loss
Fewer lithium ions can be stored when the host structure is no longer accessible. The irreversible nature of the rock salt phase conversion means that the lost capacity cannot be recovered by slow charging. Protective coatings are often applied to the particles to prevent the initial oxygen loss that triggers this phase change.
