Physical and Chemical Properties
The chemical formula of lithium iron phosphate is LiFePO4, in which lithium has +1 valence, iron has +2 valence and phosphate has -3 valence. The central iron atom together with its surrounding 6 oxygen atoms forms a corner-shared octahedron - FeO6 - with iron in the center. The phosphorus atom of the phosphate forms with the four oxygen atoms an edge-shared tetrahedron - PO4 - with phosphorus in the center. A zigzag three-dimensional framework is formed by FeO6 octahedra sharing common-O corners with PO4 tetrahedra. Lithium ions reside within the octahedral channels in a zigzag structure. In the lattice, FeO6 octahedra are connected by sharing the corners of the bc face. LiO6 groups form a linear chain of edge-shared octahedra parallel to the b axis. A FeO6 octahedron shares edges with two LiO6 octahedra and one PO4 tetrahedron. In crystallography, this structure is thought to be the Pmnb space group of the orthorhombic crystal system. The lattice constants are: a=6.008A, b=10.334A, and c=4.693A. The volume of the unit lattice is 291.4 A3. The phosphates of the crystal stabilize the whole framework and give LFP good thermal stability and excellent cycling performances.
Different from the two traditional cathode materials - LiMnO4 and LiCoO2, lithium ions of LiMPO4 move in the one-dimensional free volume of the lattice. During charge/discharge, the lithium ions are extracted from/inserted into LiMPO4 while the central iron ions are oxidized/reduced. This extraction/insertion process is reversible. LiMPO4 has, in theory, a charge capacity of 170mAh/g and a stable open-circuit voltage of 3.45V. The insertion/extraction reaction of the lithium ions is shown below: LiFe(II)PO4 <-> Fe(III)PO4 + Li + e- (1)
The extraction of lithium from LiFePO4 produces FePO4 with similar structures. FePO4 also has a Pmnb space group. The lattice constants of FePO4 are a=5.792A, b=9.821A and c=4.788A. The volume of the unit lattice is 272.4 A3. Extraction of lithium ions reduces the lattice volume, as is the case of lithium oxides. The corner-shared FeO6 octahedra of LiMPO4 are separated by the oxygen atoms of the PO43- tetrahedra and cannot form a continuous FeO6 network. Electron conductivity is reduced as a result. On the other hand, a nearly close-packed hexagonal oxygen atom array provides a relatively small free volume for lithium ion motion and therefore, lithium ions in the lattice have small migration speeds at ambient temperate. During charge, lithium ions and corresponding electrons are extracted from the structure, and a new phase of FePO4 and a new phase interface are formed. During discharge, lithium ions and the corresponding electrons are inserted back into the structure and a new phase of LiMPO4 is formed outside the FePO4 phase. Hence, the lithium ions of spherical cathode particles have to go through an inward or an outward structural phase transition, be it extraction or insertion . A critical step of charge and discharge is the formation of the phase interface between LixFePO4 and Li1-xFePO4. As the insertion/extraction of lithium ions proceeds, the surface area of the interface shrinks. When a critical surface area is reached, the electrons and ions of the resulting FePO4 have low conductivity and two-phase structures are formed. Thus, LiMPO4 at the center of the particle will not be fully consumed, especially under the condition of large discharge current.
The lithium ions move in the one-dimensional channels in the olivine structures and have high diffusion constants. Besides, the olivine structures experiencing multiple cycles of charge and discharge remain stable and the iron atom still resides in the center of the octahedron. Therefore, putting the limit of electron conductivity aside, LiMPO4 is a good cathode material with excellent cycling performances. During a charge, the iron atom in the center of the octahedron has a high spin state.
Read more about this topic: Lithium Iron Phosphate
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