Meaning
Solid-state transport processes involve the undesirable movement of transition metal ions from their original octahedral sites in the transition metal layer into empty sites in the lithium layer during battery cycling. This nickel cation migration occurs predominantly at high states of charge where the vacant lithium sites provide a low-energy pathway for the displaced ions. The phenomenon leads to structural degradation and capacity fade in high-energy density lithium-ion cells.
Structural Degeneration
The displacement of divalent transition metal ions into the lithium pathways causes local lattice contraction and blocks the diffusion channels of lithium ions. When nickel cation migration proceeds unchecked, it triggers a phase transition from the layered structure to a disordered rocksalt phase. This structural transformation diminishes the rate capability of the cathode material because the lithium diffusion coefficient in the rocksalt phase is several orders of magnitude lower than in the layered framework.
Driving Force
High operating voltages extract a large fraction of lithium ions, creating electrostatic instability that encourages transition metals to shift. The migration rate increases at elevated temperatures because thermal energy helps the cations overcome the activation energy barrier associated with passing through intermediate tetrahedral sites.
Industrial Consequence
Material designers evaluate this atomic rearrangement to determine the long-term stability of nickel-rich cathode chemistries. Minimizing the migration through atomic doping or surface modification extends the cycle life of cells destined for electric vehicles.
