Meaning
Phosphate minerals with the chemical formula LiFePO4 are the stable starting material for many lithium ion battery cathodes. The triphylite phase is the lithiated form of iron phosphate which crystallizes in an olivine structure. It is valued for its thermal stability and long cycle life.
Crystal Structure
Atomic arrangements in this material consist of corner sharing iron oxide octahedra and edge sharing phosphate tetrahedra. In the triphylite phase, lithium ions reside in one dimensional channels that run through the lattice. This geometry dictates the speed at which ions can move in and out of the electrode.
Lithium Site
Occupancy of the octahedral sites by lithium atoms provides the charge storage capacity of the cell. During discharge, lithium ions return to the triphylite phase and restore the original lattice volume. The stability of these sites ensures that the material does not easily collapse even after many cycles.
Ionic Conductivity
Electronic and ionic transport are relatively low in the bulk mineral compared to other cathode types. To compensate for this, particles of the triphylite phase are usually coated with a thin layer of conductive carbon. This coating allows for efficient electron transfer while the small particle size reduces the distance ions must travel.
Carbon addition is a standard requirement for commercial viability.
