Lithium iron phosphate (LFP) is an inorganic compound with the formula . It is a gray, green, or black solid that is insoluble in water.
Lithium iron phosphate is used as a positive electrode material (cathode) of lithium iron phosphate batteries,Park, O. K.; Cho, Y.; Lee, S.; Yoo, H.-C.; Song, H.-K.; Cho, J., "Who Will Drive Electric Vehicles, Olivine or Spinel?", Energy Environ. Sci. 2011, volume 4, pages 1621-1633. a type of Li-ion battery. The negative electrode (anode) material for these batteries is typically graphite. This battery chemistry is used in power tools, electric vehicles, solar energy installations and large grid-scale energy storage.
Lithium iron phosphate exists naturally in the form of the mineral triphylite, but as found naturally, this material has insufficient purity for use in batteries. Thus, battery-grade LFP powder must be synthesized.
History Arumugam Manthiram and John B. Goodenough first identified the polyanion class of cathode materials for lithium ion batteries. was then identified as a battery cathode material in this class in 1996 by Padhi, Nanjundaswamy, and Goodenough. This work demonstrated reversible extraction of lithium from and insertion of lithium into . Subsequent studies demonstrated the synthesis of nanosized and carbon-coated LFP particles, which are correlated with increased capacity and cycle stability.
See the Commercialization and Intellectual Property section for the history of LFP's commercialization. As of 2021, China produced over 90% of the world's LFP.
Synthesis and production may be synthesized by a variety of methods, including: solid-state synthesis, emulsion drying, sol-gel process, solution coprecipitation, thin-film deposition, mechanochemical activation, microwave-assisted synthesis (in which microwave energy is applied to accelerate chemical reactions, resulting in reduced synthesis time and energy consumption), hydrothermal synthesis, (ultrasonic) spray pyrolysis, and molten state synthesis.
Of these, solid-state synthesis (ceramic method) is the most commonly employed in industrial level production. In a typical synthesis, iron(III) oxalate is reacted with ammonium dihydrogen phosphate ((NH4)(H2PO4)), diammonium hydrogen phosphate ((NH4)2(HPO4)), or phosphoric acid (H3PO4) to synthesize iron(III) phosphate. Iron(III) phosphate is mixed with lithium carbonate (or another lithium source, such as lithium hydroxide), and a carbon source (such as glucose, sucrose or starch, see Carbon coating below), and calcined at 700–800°C. Other lithium, iron, and phosphate precursors can also be used, including iron(II) sulfate and iron(III) oxalate.
The hydrothermal synthesis routes is also industrially relevant, with main advantages being its low cost and ability to perform carbon-coating in one step.
Role of particle size LFP nanoparticles were found to have increased capacity and rate performance compared to larger particles. The particle size was found to have an important influence on the electrode resistance and discharge capacity. Nanosized LFP particles were also found to undergo single-phase Li extraction, which could have important implications for power density.
Carbon coating Coating LFP particles with a thin layer of carbon can be achieved during synthesis via carbothermic reduction. In addition to forming a conductive coating on the surface of otherwise electronically insulating LFP particles, the reductive synthesis environment also suppresses the formation of Fe(III) which can impede Li diffusion within the crystalline lattice. Carbon coating thicknesses of 1-10 nm are typical for increasing the conductivity of particles but allowing for Li intercalation.
Metal substitution and coating Coating LFP with inorganic oxides can also improve the electrochemical performance of LFP. Substituting other metals for the iron or lithium in can also improve performance.
Physical and chemical properties
Structure of In , lithium has a +1 charge, iron has a +2 charge (ferrous), and phosphate ion carries a −3 charge, balancing the charges. The iron atom and 6 oxygen atoms form an octahedral coordination sphere, described as , with the Fe ion at the center. The phosphate groups, , are tetrahedral. The three-dimensional framework is formed by the octahedra sharing O corners. 's corner-sharing octahedra are separated by the oxygen atoms of the tetrahedra and cannot form a continuous network, reducing electronic conductivity.
Lithium ions are octahedrally coordinated by O, and were shown to migrate within one-dimensional channels in the framework in a zigzag manner via neutron diffraction. A nearly close-packed hexagonal array of oxide centers provides relatively little free volume for ions to migrate within. For this reason, the ionic conductivity of is relatively low at ambient temperature.
In crystallography, the crystal structure belongs to the Pmna space group of the orthorhombic crystal system. The lattice constants are approximately a = 10.33 Å, b = 6.01 Å, and c = 4.69 Å, giving a unit cell volume of 291.4 Å3.
Structure of During charge, the lithium ions are extracted concomitant with oxidation of Fe(II) ions to Fe(III):
Extraction of lithium from produces with a similar structure. adopts a Pmna space group with a unit cell volume of 272.4 Å3, an approximately 7% decrease compared to that of its lithiated precursor; extraction of lithium ions reduces the lattice volume. Upon discharge, the lithium ions are reinserted to the host lattice, and Fe(III) is reduced to Fe(II):
This reversible reaction gives rise to a theoretical gravimetric (specific) capacity of 170 mAh/g, and energy density 540 Wh/kg.
Mechanism of (de)lithiation The phase diagram of was determined by Dodd, Yazami and Fultz, as well as by Delacourt et al. The phase separation between the lithiated phase and delithiated phase results in a wide compositional miscibility gap, and a flat voltage curve when is used in a battery. The details of the phase separation mechanism have been investigated extensively.Malik, R.; Abdellahi, A.; Ceder, G., "A Critical Review of the Li Insertion Mechanisms in Electrodes", J. Electrochem. Soc. 2013, volume 160, pages A3179-A3197. At high current rates on discharge, phase separation is suppressed, which could provide insight into the material's high power density.
Applications
Lithium iron phosphate batteries are important for many applications, including power tools, battery energy storage systems, uninterruptible power systems (UPS), power plants, and electric vehicles.Rechargeable Li-ion batteries based on Olivine-structured (LiFePO4) cathode materials - Kumar et al., Nov 15, 2015, retrieved April 1, 2020 According to the IEA, LFP batteries made up half of the world's electric vehicle sales, driven largely by China.
LFP batteries have an operating voltage of ~3.3 V, which is low compared the ~3.7 V average operating voltage of Li-ion batteries utilizing lithium transition metal oxide cathode materials (such as lithium cobalt oxide (), lithium nickel oxide (), NMC, or NCA). This leads to lower energy density for LFP batteries. However, LFP batteries have better safety characteristics (including high temperature stability), higher power density and longer cycle life than other Li-ion battery cathode chemistries. Additionally, LFP batteries do not contain nickel or cobalt, which lowers the cost of the battery, and sidesteps ethical issues associated with cobalt extraction.
BAE has announced that their HybriDrive Orion 7 hybrid bus uses about 180 kW LFP battery cells. AES has developed multi-trillion-watt battery systems capable of providing ancillary services to the power network, including spare capacity and frequency regulation. In China, BAK and Tianjin Lishen are active in the area.
Safety is a crucial property for certain applications. For example, in 2016, an LFP-based energy storage system was installed in Paiyun Lodge on Mt.Jade (Yushan) (the highest alpine lodge in Taiwan). As of 2024, the system is still operating safely.
Comparison to other battery chemistries In addition to the aforementioned differences between LFP and other Li-ion battery cathodes, LFP exhibits a flat discharge voltage curve. The voltage curves of lithium transition metal oxides are sloped. The flat voltage curve of LFP gives a nearly constant voltage throughout the entire use of the battery (discharge), but presents issues for state of charge estimation by a device's battery management system.
LiFePO4 batteries are comparable to sealed lead acid batteries and are often touted as a drop-in replacement for lead-acid applications. The most notable difference between lithium iron phosphate and lead acid is that lithium battery capacity shows only a small dependence on discharge rate. With high discharge rates, for instance, 0.8C, the capacity of the lead-acid battery is only 60% of its rated capacity. Therefore, in cyclic applications where the discharge rate is often greater than 0.1C, a lower-rated lithium battery will usually have a higher actual capacity than the comparable lead-acid battery. This means that, at the same capacity rating, lithium will cost more, but a lower-capacity lithium battery can be used for the same application at a lower price. The cost of ownership when considering the lifecycle further increases the value of the lithium battery when compared to a lead acid battery, but they have much poorer performance at lower temperatures, as covered in the section on effects of temperature.
Power density In the initial report of its electrochemical properties, Padhi et al. claimed that LFP had low power density. Various modifications to the material, including nanosizing particles, coating particles with carbon, and doping with other elements contributed to improved kinetics of (de)lithiation and improved cycling stability, such that LFP is now considered a battery cathode material with high power performance. LFP / graphite batteries have high power performance, capable of being discharged at >5C (1C = discharge in 1 hour, and 5C = discharge in 12 minutes).
Effects of temperature The effects of temperature on lithium iron phosphate batteries can be divided into high- and low-temperature impacts.
Generally, LFP batteries are less susceptible to thermal runaway reactions like those in lithium cobalt batteries; they exhibit better performance at elevated temperatures. Research has shown that at room temperature (23 °C), the initial capacity loss approximates 40-50 mAh/g. However, at 40 °C and 60 °C, the capacity losses approximate 25 and 15 mAh/g respectively, but these capacity losses were spread over 20 cycles instead of a bulk loss like that in the case of room temperature capacity loss.
However, this is only true for a short cycling timeframe. A later year-long study has shown that, despite LFP batteries having double the equivalent complete cycle, the capacity fade rate increased with increasing temperature for LFP cells, but higher temperature did not affect NCA cells or had a negligible impact on the ageing of NMC cells. This capacity fade is primarily due to the solid electrolyte interface (SEI) formation reaction being accelerated by increasing temperature.