Lithium iron phosphate (LiFePO₄), the core cathode material for lithium iron phosphate batteries, has become a key material in the lithium-ion battery field due to its unique crystal structure. Its crystal structure belongs to the orthorhombic olivine type, where lithium ions diffuse in one-dimensional channels. This structure gives LiFePO₄ high stability, long cycle life, and excellent safety, but it also results in poor ionic and electronic conductivity.
The LiFePO₄ crystal framework is composed of PO₄³⁻ tetrahedra and FeO₆ octahedra, with lithium ions occupying the interstices between the octahedra, forming a chain-like structure. Within this spatial framework, the PO₄³⁻ tetrahedra are covalently linked to the FeO₆ octahedra, forming a stable network. Because the FeO₆ octahedra are connected by corner-sharing rather than edge-sharing on the bc planes, electrons cannot conduct continuously between the octahedra, resulting in extremely low electronic conductivity. Furthermore, the PO₄³⁻ tetrahedra restrict the volume change of the crystal lattice, forcing lithium ions to diffuse only along the one-dimensional channels along the b-axis. This restricted diffusion path further reduces ion migration efficiency.
Lithium iron phosphate's strong P-O covalent bonds are key to its thermal stability. Under high temperatures, the PO₄³⁻ tetrahedrons inhibit oxygen release, allowing the material to maintain structural integrity even at temperatures exceeding 300°C. Its decomposition temperature is significantly higher than that of ternary materials. This property makes the lithium iron phosphate battery less susceptible to fire and explosion under extreme conditions such as puncture, overcharging, and short circuits. Its safety is significantly superior to other lithium-ion battery systems, making it particularly suitable for energy storage power stations and public transportation, where safety requirements are stringent.
The charge and discharge process of lithium iron phosphate is based on a two-phase transition between LiFePO₄ and FePO₄. During charging, lithium ions are extracted from the LiFePO₄ lattice to form FePO₄. During discharge, lithium ions are reinserted into the FePO₄, returning to LiFePO₄. This process is accompanied by a redox reaction between Fe²⁺ and Fe³⁺, resulting in a stable voltage plateau around 3.2V and a flat curve. Because the two-phase structure coexists as a solid solution below 200°C, there is no obvious phase transition during charge and discharge, minimizing volume change (to only approximately 6.81%) and reducing mechanical stress, resulting in a cycle life of thousands of cycles.
To overcome the inherently poor conductivity of lithium iron phosphate, material modification technology is crucial. Reducing particle size through nanocrystallization shortens the lithium ion diffusion path; carbon coating or metal ion doping (such as Mg²⁺ and Al³⁺) can build a conductive network and improve electronic conductivity. For example, the use of composite conductive agents reduces internal resistance and improves charge and discharge efficiency. Furthermore, spherical morphology design increases tap density, optimizes electrode processability, and improves coating uniformity.
The crystal structure characteristics of lithium iron phosphate batteries directly influence their application scenarios. Its high safety and long cycle life make it a top choice for energy storage systems, electric buses, and low-speed electric vehicles. However, the low rate capability resulting from the one-dimensional lithium ion diffusion pathways limits its application in high-end electric vehicles and fast charging applications. Through CTP (cell-free pack) and blade battery technologies, the volumetric energy density of lithium iron phosphate batteries has been improved, gradually addressing their energy density shortcomings.
The crystal structure characteristics of lithium iron phosphate determine its market positioning as a "safety-first, cost-sensitive" battery. With material modification and system design optimization, the shortcomings of lithium iron phosphate batteries in energy density, fast charging performance, and low-temperature adaptability are gradually being addressed. In the future, as global demand for safe and sustainable energy storage grows, lithium iron phosphate batteries are expected to play a more important role in the new energy sector.
