Meaning
Structural phase transitions at cathode surfaces can create a highly resistive disordered phase that impedes lithium-ion transport during cell cycling. This inactive region, referred to as a rocksalt surface layer, forms when transition metal ions migrate into vacant lithium sites at high states of charge and elevated temperatures. It represents a major degradation mode that limits the power capability of nickel-rich batteries.
Diffusion Impedance
The rearrangement of metal ions at the boundary of the cathode particle blocks the channels needed for lithium insertion. As a rocksalt surface layer grows, the lithium diffusion coefficient in that region drops by several orders of magnitude. This localized rise in resistance forces the cell to reach its cutoff voltage prematurely during discharge.
This early cutoff decreases the usable capacity and reduces the overall energy output of the battery pack.
Phase Transformation
Exposure to high voltages drives the extraction of lithium ions, leaving the layered structure unstable. Under these conditions, the release of active oxygen from the lattice triggers the phase transition that forms the rocksalt surface layer. This reorganization converts the outer nanometers of the cathode from a layered structure to a disordered rocksalt phase.
This irreversible reaction is accelerated by high temperatures and contact with acidic species in the electrolyte.
Coating Mitigation
Material designers apply thin metal oxide coatings to the cathode powder to prevent the structural degradation of the active particles. Sourcing departments seek cells that use these surface-modified materials, as they resist the formation of the rocksalt surface layer during rapid charge-discharge profiles. These protective barriers maintain the structural transition pathways and delay the onset of capacity loss.
Utilizing such stable cathodes ensures long-term power retention in high-performance electric vehicles.
