Class: phospho-0livine, phosphate mineral, insertion solid, natural
Related Structures: 0livine, triphylite
Notable Properties: reductive insertion, battery electrode
The parent mineral of lithium iron phosphate (LiFePO4) is olivine, a magnesium/iron silicate with the similar formula (Mg,Fe)2SiO4. Olivine exists as a solid solution of the pure magnesium phase, forsterite, and the purely ferrous phase fayalite.
Lithium iron phosphate itself is also naturally occurring (triphylite) and is found as a solid solution with magnesium analogue lithiophilite. Here lithium occupies positions in the olivine structure that are equivalent to the iron sites. Both iron and lithium occupy octahedral holes. The lithium sites are arranged to form infinite 1-D tunnels that promote lithium mobility through the crystal lattice. This oxygen atoms form an hcp lattice where Li and Fe occupy half of the octehedral sites and one eighth of the tetrahedra sites are occupied by phosphorous. This olive lattice is preferable to spinels of similar composition when the oxyanion is relatively small (P, Si and smaller). Other transition metals such as manganese and cobalt can form the same structure and also insert lithium with varying degrees of reversibility. The insertion reaction is a phase change type transformation whereLixFePO4 and FePO4coexist during the reduction insertion reaction.
Lithium iron phosphate is best known as a rechargeable battery cathode material that have been commercially realized with applications primarily in electric vehicles and power tools. The seminal report on the electrochemical performance of lithium iron phosphate emerged from J. B. Goodenough’s lab in 1997. The materials as also been explored for commercial implementation in grid level storage devices. Given the push to commercialization, the actual synthesis, phase, and surface modification found in rechargeable batteries has been heavily optimized.
Formula: LixFePO4 0 < x < 1
Space Group: Pbnm
Lattice parameters: 6.008 Å, 10.334Å, 4.693 Å
Structure Notes: 1-D tunnels amenable to lithium ion conduction
Reaction Notes: Phase change type reductive insertion of lithium, reversible