Meaning
Solid-state phase transformations describe the structural shift of lithium transition metal oxides from a highly ordered rhombohedral layered lattice to a highly disordered face-centered cubic rocksalt structure. This R-3m to Fm-3m phase transition occurs primarily at the surface of nickel-rich cathode materials during high-voltage cycling or thermal stress. The structural rearrangement severely degrades the electrochemical performance of the cell.
Crystalline Evolution
The transition proceeds through an intermediate spinel phase as lithium ions are removed and transition metal cations migrate into the vacated sites. During the R-3m to Fm-3m phase transition, the layered structure loses its distinct lithium and transition metal planes, resulting in a random distribution of cations that blocks lithium diffusion. This phase change is irreversible and permanently reduces the active lithium inventory of the battery.
Physical Consequence
The newly formed cubic rocksalt phase on the surface of the cathode particles acts as an insulating barrier to both lithium ions and electrons. Cell impedance increases as the thickness of the transformed surface layer grows.
Analytical Detection
Transmission electron microscopy and synchrotron X-ray diffraction confirm the presence of this transition at the nanometer scale. Sourcing decisions utilize these analytical results to select cathode suppliers whose materials resist this surface transformation.
