Meaning
Disordered cubic lattices represent a common degradation product on the surface of transition metal oxide cathodes. The rocksalt phase arises when the layered transition metal oxide transforms into a highly stable structure where lithium and transition metal ions are randomly distributed. This phase transition is irreversible and hinders lithium transport.
Structural Transition
Repeated cycling forces transition metals to migrate into lithium sites, creating the rocksalt phase. This structural reorganization occurs primarily at the particle surface, where direct contact with the electrolyte destabilizes the lattice. The transformation propagates deeper as the cell experiences more cycles at high potentials.
Impedance Growth
Cathode impedance increases rapidly once this inactive barrier forms at the electrode boundary. The rocksalt phase possesses very low ionic conductivity because the random arrangement of ions blocks the paths needed for lithium diffusion. Consequently, the battery suffers from increased polarization and reduced rate capability.
The increased resistance translates into higher internal heating during high-current discharges, which accelerates secondary degradation pathways across the entire cell. Cells displaying this surface behavior exhibit shortened runtimes even at low discharge rates.
Thermal Stability
Thermal runaway risk is modified because this phase is highly stable and lacks active oxygen. When the surface of the cathode is covered by the rocksalt phase, it acts as a protective shield against further interaction with the electrolyte. This stability reduces the rate of heat generation during thermal abuse tests.
