Meaning
Surface degradation phases consist of a nanometer-thin, disordered cubic structure that forms on the surface of layered transition metal oxide cathode particles during electrochemical operation. This rocksalt reconstruction layer arises from the migration of transition metal cations into the lithium channels, driven by the loss of lattice oxygen at high voltages. The resulting phase behaves as a transport barrier that increases cell impedance.
Formation Mechanism
Electrolyte oxidation at the active interface creates an acidic environment that promotes transition metal dissolution and oxygen loss from the cathode lattice. As oxygen departs, the surface region stabilizes itself by rearranging into the thermodynamically favored rocksalt reconstruction layer. This layer prevents efficient lithium insertion because the cubic structure lacks the open two-dimensional pathways found in the original layered material.
Electrochemical Impact
The continuous growth of this surface layer leads to capacity decline and reduced rate capability in lithium-ion cells. Cell designers monitor the growth rate of this phase to estimate the calendar life of batteries under high-voltage storage conditions.
Suppression Method
Depositing ultra-thin protective coatings or doping the surface with electrochemically inactive elements suppresses the initial oxygen release. This intervention prevents the cascade of atomic rearrangements that build the unwanted surface phase.
