Meaning
Crystalline structures with the formula FePO4 appear when lithium ions are removed from an olivine lattice. The heterosite phase is the fully delithiated state of lithium iron phosphate cathode materials. It maintains the same symmetry as the starting material but with a reduced unit cell volume.
Delithiation State
Removal of lithium during the charging process forces the iron ions to change their oxidation state. The heterosite phase forms once the lithium concentration drops below a certain critical level. This transition occurs through a two-phase reaction mechanism where the lithiated and delithiated regions coexist.
Crystal Lattice
Volumetric changes during the transition from triphylite to heterosite create mechanical strain at the phase boundary. High concentrations of the heterosite phase can lead to the formation of defects or dislocations within the cathode particles. These defects can eventually impede the movement of ions during subsequent cycles.
Phase Boundary
Interface movement between the two distinct crystal structures governs the rate at which a battery can be charged. Because the heterosite phase has a different lattice parameter than the lithiated phase, a coherent interface must be maintained to prevent particle fracture. This mechanical constraint limits the maximum current density for stable operation.
Structural integrity is maintained by limiting the speed of the phase front.
