Lithium iron phosphate

Lithium iron phosphate (LFP) is an inorganic compound with the formula LiFePO
4. It is a gray, red-grey, brown or black solid that is insoluble in water. The material has attracted attention as a component of lithium iron phosphate batteries,[1] a type of Li-ion battery. This battery chemistry is targeted for use in power tools, electric vehicles, and solar energy installations.[2] It is also used in OLPC XO education laptops.

Lithium iron phosphate
Names
IUPAC name
iron(2+) lithium phosphate (1:1:1)
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.124.705
EC Number
  • 604-917-2
Properties
FeLiO
4P
Molar mass 157.757
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Most lithium batteries (Li-ion) used in 3C (computer, communication, consumer electronics) products use cathodes made of other lithium compounds, such as lithium cobalt oxide (LiCoO
2), lithium manganese oxide (LiMn
2O
4), and lithium nickel oxide (LiNiO
2). The anodes are generally made of graphite.

Lithium iron phosphate exists naturally in the form of the mineral triphylite, but this material has insufficient purity for use in batteries.

LiMPO
4

With general chemical formula of LiMPO
4, compounds in the LiFePO
4 family adopt the olivine structure. M includes not only Fe but also Co, Mn and Ti.[3] As the first commercial LiMPO
4 was C/LiFePO
4, the whole group of LiMPO
4 is informally called “lithium iron phosphate” or “LiFePO
4”. However, more than one olivine-type phase may be used as a battery's cathode material. Olivine compounds such as A
yMPO
4, Li
1-xMFePO
4, and LiFePO
4-zM have the same crystal structures as LiMPO
4 and may replace in a cathode. All may be referred to as “LFP”.

Manganese, phosphate, iron, and lithium also form an olivine structure. This structure is a useful contributor to the cathode of lithium rechargeable batteries.[4] This is due to the olivine structure created when lithium is combined with manganese, iron, and phosphate (as described above). The olivine structures of lithium rechargeable batteries are significant, for they are affordable, stable, and can be safely stored as energy.[5]

History and production

Arumugam Manthiram and John B. Goodenough first identified the polyanion class of cathode materials for lithium ion batteries.[6][7][8] LiFePO
4 was then identified as a cathode material belonging to the polyanion class for use in batteries in 1996 by Padhi et al.[9][10] Reversible extraction of lithium from LiFePO
4 and insertion of lithium into FePO
4 was demonstrated. Neutron diffraction confirmed that LFP was able to ensure the security of large input/output current of lithium batteries.[11]

The material can be produced by heating a variety of iron and lithium salts with phosphates or phosphoric acid. Many related routes have been described including those that use hydrothermal synthesis.[12]

Physical and chemical properties

In LiFePO
4, lithium has a +1 charge, iron +2 charge balancing the -3 charge for phosphate. Upon removal of Li, the material converts to the ferric form FePO4.[13]

The iron atom and 6 oxygen atoms form an octahedral coordination sphere, described as FeO
6, with the Fe ion at the center. The phosphate groups, PO
4, are tetrahedral. The three-dimensional framework is formed by the FeO
6 octahedra sharing O corners. Lithium ions reside within the octahedral channels in a zigzag manner. In crystallography, this structure is thought to belong to the Pmnb space group of the orthorhombic crystal system. The lattice constants are: a = 6.008 Å, b = 10.334 Å, and c = 4.693 Å. The volume of the unit cell is 291.4 Å3.

In contrast to two traditional cathode materials - LiMnO
4 and LiCoO
2, lithium ions of LiMPO
4 migrate in the lattice's one-dimensional free volume. During charge/discharge, the lithium ions are extracted concomitant with oxidation of Fe:

Extraction of lithium from LiFePO
4 produces FePO
4 with a similar structure. FePO
4 adopts a Pmnb space group with a unit cell volume of 272.4 3, only slightly smaller than that of its lithiated precursor. Extraction of lithium ions reduces the lattice volume, as is the case with lithium oxides. LiMPO
4's corner-shared FeO
6 octahedra are separated by the oxygen atoms of the PO
43- tetrahedra and cannot form a continuous FeO
6 network, reducing conductivity.

A nearly close-packed hexagonal array of oxides centers provides relatively little free volume for Li+ ions to migrate within. For this reason, the ionic conductivity of Li+ is relatively low at ambient temperate. The details of the lithiation of FePO4 and the delithiation of LiFePO4 have been examined. Two phases of the lithiated material are implicated.[13][14]

Applications
See also: lithium iron phosphate battery

LFP batteries have an operating voltage of 3.3 V, charge density of 170mAh/g, high power density, long cycle life and stability at high temperatures.

LFP's major commercial advantages are that it poses few safety concerns such as overheating and explosion, as well as long cycle lifetimes, high power density and has a wider operating temperature range. Power plants and automobiles use LFP.[15][16]

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 that are capable of subsidiary services of the power network, including spare capacity and frequency adjustment. In China, BAK and Tianjin Lishen are active in the area.

Comparison

Although LFP has 25% less capacity than other lithium batteries due to its operational voltage (3.2 volts vs 3.7 for Co based cathode chemistries), it has 70% more than nickel-hydrogen batteries.

The major differences between LFP batteries and other lithium ion battery types is that LFP batteries contain no cobalt (removing ethical questions about material sourcing) and have a flat discharge curve.

LFP batteries have drawbacks, including higher costs due to lower energy density. The energy density is significantly lower than LiCoO
2 (although higher than the nickel-metal hydride battery).

Lithium cobalt oxide based battery chemistries are more prone to thermal runaway if overcharged and Cobalt is both expensive and not widely available. Other chemistries such as NMC Ni Mn Co have supplanted LiCo chemistry cells in most applications. The original ratio of Ni to Mn to Co was 3:3:3 today, cells are being made with ratios of 8:1:1 whereby the Co content has been drastically reduced.

Intellectual property

The root patents of LFP compounds are held by four organizations. University of Texas-Austin for the discovery of the material. Hydro-Québec, Université de Montréal and the French National Center for Scientific Research (CNRS) for the carbon coating that enhance its conductivity and actually makes LFP suitable for industrial developments.[17] These patents underlie mature mass production technologies. The largest production capacity is up to 250 tons per month. The key feature of Li
1-xMFePO
4 from A123 is the nano-LFP, which modifies its physical properties and adds noble metals in the anode, as well as the use of special graphite as the cathode.

The main feature of LiMPO
4 from Phostech is increased capacitance and conductivity by an appropriate carbon coating. The special feature of LiFePO
4 • zM from Aleees a high capacitance and low impedance obtained by the stable control of the ferrites and crystal growth. This improved control is realized by applying strong mechanical stirring forces to the precursors in high oversaturation states, which induces crystallization of the metal oxides and LFP.

In patent lawsuits in the US in 2005 and 2006, the University of Texas at Austin|University of Texas-Austin and Hydro-Québec claimed that LiFePO
4 as the cathode infringed their patents, US 5910382 and US 6514640. The patent claims involved a unique crystal structure and a chemical formula of the battery cathode material.

On April 7, 2006, A123 filed an action seeking a declaration of non-infringement and invalidity UT's patents. A123 separately filed two ex parte Reexamination Proceedings before the United States Patent and Trademark Office (USPTO), in which they sought to invalidate the patents based upon prior art.

In a parallel court proceeding, UT sued Valence Technology, Inc. ("Valence") - a company that commercializes LFP products that alleged infringement.

The USPTO issued a Reexamination Certificate for the '382 patent on April 15, 2008 and for the '640 patent on May 12, 2009, by which the claims of these patents were amended. This allowed the current patent infringement suits filed by Hydro-Quebec against Valence and A123 to proceed. After a markman hearing, on April 27, 2011 the Western District Court of Texas held that the claims of the reexamined patents had a narrower scope than as originally granted.

On Dec 9th, 2008, the European Patent Office revoked Dr. Goodenough’s patent numbered 0904607. This decision basically reduced the patent risk of using LFP in European automobile applications. The decision is believed to be based on the lack of novelty.[18]

The first major large settlement was the lawsuit between NTT and the University of Texas-Austin (UT). In October 2008,[19] NTT announced that they would settle the case in the Japan Supreme Civil Court for $30 million. As part of the agreement UT agreed that NTT did not steal the information and that NTT would share its LFP patents with UT. NTT’s patent is also for an olivine LFP, with the general chemical formula of A
yMPO
4 (A is for alkali metal and M for the combination of Co and Fe), now used by BYD Company. Although chemically the materials are nearly the same, from the viewpoint of patents, A
yMPO
4 of NTT is different from the materials covered by UT. A
yMPO
4 has higher capacity than LiMPO
4. At the heart of the case was that NTT engineer Okada Shigeto, who had worked in the UT labs developing the material, was accused of stealing UT’s intellectual property.

As of 2020, an organisation named LifePO+C claims to own the key IP and offers licenses. It is a consortium between Johnson Matthey, the CNRS, University of Montreal, and Hydro Quebec.

Research

Power density

LFP has two shortcomings: low conductivity and low lithium diffusion constant, both of which limit the charge/discharge rate. Adding conducting particles in delithiated FePO
4 raises its electron conductivity. For example, adding conducting particles with good diffusion capability like graphite and carbon[20] to LiMPO
4 powders significantly improves conductivity between particles, increases the efficiency of LiMPO
4 and raises its reversible capacity up to 95% of the theoretical values. LiMPO
4 shows good cycling performance even under charge/discharge current as large as 5C.[21]

Stability

Coating LFP with inorganic oxides can make LFP’s structure more stable and increase conductivity. Traditional LiCoO
2 with oxide coating shows improved cycling performance. This coating also inhibits dissolution of Co and slows the decay of LiCoO
2 capacity. Similarly, LiMPO
4 with an inorganic coating such as ZnO[22] and ZrO
2,[23] has a better cycling lifetime, larger capacity and better characteristics under rapid discharge. The addition of a conductive carbon increases efficiency. Mitsui Zosen and Aleees reported that addition of conducting metal particles such as copper and silver increased efficiency.[24] LiMPO
4 with 1 wt% of metal additives has a reversible capacity up to 140mAh/g and better efficiency under high discharge current.

Metal substitution

Substituting other metals for the iron or lithium in LiMPO
4 can also raise efficiency. Substituting zinc for iron increases crystallinity of LiMPO
4 because zinc and iron have similar ion radii.[25] Cyclic voltammetry confirms that LiFe
1-xM
xPO
4, after metal substitution, has higher reversibility of lithium ion insertion and extraction. During lithium extraction, Fe (II) is oxidized to Fe (III) and the lattice volume shrinks. The shrinking volume changes lithium’s returning paths.

Synthesis processes

Mass production with stable and high quality still faces many challenges.

Similar to lithium oxides, LiMPO
4 may be synthesized by a variety of methods, including: solid-phase synthesis, emulsion drying, sol-gel process, solution coprecipitation, vapor-phase deposition, electrochemical synthesis, electron beam irradiation, microwave process, hydrothermal synthesis, ultrasonic pyrolysis and spray pyrolysis.

In the emulsion drying process, the emulsifier is first mixed with kerosene. Next, the solutions of lithium salts and iron salts are added to this mixture. This process produces nanocarbon particles.[26] Hydrothermal synthesis produces LiMPO
4 with good crystallinity. Conductive carbon is obtained by adding polyethylene glycol to the solution followed by thermal processing.[27] Vapor phase deposition produces a thin film LiMPO
4.[28] In flame spray pyrolysis FePO4 is mixed with Lithium carbonate and glucose and charged with electrolytes. The mixture is then injected inside a flame and filtered to collect the synthesized LiFePO
4.[29]

See also

References
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