Meaning
Chemical reduction processes in lithium-ion battery cathodes involve the decrease in the formal oxidation state of nickel ions at the surface, typically from tetravalent or trivalent states to the more stable divalent state. This nickel valence reduction occurs when the highly reactive surface nickel species react with the liquid electrolyte or undergo thermal degradation. The transition results in the formation of resistive surface phases that hinder lithium transport.
Reaction Mechanism
The unstable tetravalent nickel generated at high states of charge tends to extract electrons from organic solvent molecules. During this redox reaction, nickel valence reduction occurs concurrently with the release of active oxygen from the cathode lattice. The resulting divalent nickel ions then easily migrate into the empty lithium sites due to the similarity in ionic radii between divalent nickel and lithium ions.
Electrochemical Impact
The accumulation of reduced nickel species on the cathode surface increases the charge-transfer resistance of the electrode. This rising resistance reduces the discharge voltage of the cell and decreases the overall energy efficiency during prolonged cycling.
Mitigation Strategy
Sourcing engineers select cathode materials treated with surface coatings or concentration gradients to suppress this chemical degradation. Protecting the reactive nickel from direct contact with the electrolyte preserves the high-voltage performance of the battery.
