Meaning
Chemical restructuring at the electrode-electrolyte interface occurs during high-voltage cycling of lithium-ion cells. The surface phase transformation involves the transition of the active cathode material from a layered structure to a high-impedance disordered phase. This reorganization is confined to the outer layers of the active particles but dominates the overall cell resistance.
Degradation Mechanism
Transition metal dissolution and oxygen evolution drive this surface phase transformation in layered oxide cathodes. Under high state of charge, the active material releases oxygen, destabilizing the surface lattice. The transition metal ions migrate into the vacant lithium sites, initiating the transition to a rock-salt or spinel-like structure.
Impedance Impact
Electrical resistance across the active material increases as the surface layer becomes electrochemically inactive. This surface phase transformation blocks the diffusion of lithium ions, resulting in a severe reduction in high-rate performance. Cells affected by this transformation display a noticeable reduction in capacity and a significant rise in operating temperature.
This temperature elevation can trigger secondary reactions in the electrolyte, leading to gas generation and eventual swelling of the pouch cell.
Prevention Technique
Atomic layer deposition of protective oxides effectively blocks direct contact with the liquid electrolyte. This coating suppresses the surface phase transformation by stabilizing the oxygen atoms at the boundary of the cathode. The modification improves the stability of the cell during high-voltage operation.
