Meaning
Phase transformation processes alter layered transition metal oxide crystal structures into electrochemically inactive cubic structures at cathode surfaces. Experiencing a rock salt transition severely reduces lithium ion intercalation capacity and increases surface impedance in layered oxide cathodes. Structural degradation begins at high states of charge when deep delithiation triggers oxygen release and transition metal migration into vacant lithium sites.
The resulting disordered rock salt layer blocks lithium ion diffusion pathways into the active material bulk.
Structural Restructuring
Oxygen loss from the cathode surface drives transition metal cations to occupy octahedral sites within the vacant lithium layer. Irreversible structural changes converting layered structures into rock salt phases destroy the open channels required for fast lithium transport. Structural degradation propagates from particle surfaces inward during repeated high-voltage cycling.
Impedance Impact
Formation of a dense, insulating surface layer increases charge transfer resistance across the electrode-electrolyte interface. The rock salt transition reduces active cathode surface area, causing severe voltage polarizations under high discharge rates. Surface passivating coatings delay structural phase transformations by suppressing oxygen release from the crystal lattice.
Voltage Boundary
High charging voltages above four point three volts accelerate transition metal migration in nickel-rich layered oxide cathodes. Operating outside safe voltage boundaries accelerates phase conversion and capacity loss. Surface doping with elements like aluminum stabilizes lattice oxygen and suppresses phase transitions.
