Meaning
Lattice contraction along the vertical axis of layered lithium transition metal oxides occurs during high states of charge when excessive lithium ions are extracted. This phenomenon, known as c-axis collapse, occurs because the electrostatic repulsion between oxygen layers is disrupted at high voltages. The physical contraction reduces the spacing between atomic planes and alters the host structure.
Electrochemical stability of the electrode degrades during cycles.
Structural Mechanics
Extraction of more than eighty percent of lithium ions from the nickel-rich cathode material causes the layer-to-layer spacing to shrink abruptly. In nickel-manganese-cobalt materials, c-axis collapse reduces the lattice parameter by up to three percent. The rapid change in unit cell volume induces severe mechanical stresses within the individual grains.
These microstructural tensions lead to intergranular cracking, exposing fresh grain boundaries to the liquid electrolyte. Electrodes suffer accelerated degradation as a result of these microcracks.
Voltage Degradation
Loss of lattice integrity hinders the diffusion of remaining lithium ions and reduces the operating potential of the cell. Because c-axis collapse alters the electronic band structure of the metal oxide, it increases the internal resistance of the cathode. The cell exhibits higher overpotentials during subsequent discharge cycles.
Over time, the cumulative damage leads to a loss of discharge capacity and a drop in energy density.
Mitigation Strategy
Doping the crystal lattice with non-reactive ions stabilizes the layered structure against severe volume changes. Solid-state modifications prevent the rapid contraction of the c-axis during high-voltage operation.
