Lithium iron phosphate
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|Lithium iron phosphate|
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Lithium iron phosphate (LiFePO4), also known as LFP, is a compound used in lithium iron phosphate batteries (related to Li-Ion batteries). It is targeted for use in power tools and electric vehicles. It is also used in OLPC XO education laptops.
Most lithium batteries (Li-ion) used in 3C (computer, communication, consumer electronics) products are mostly lithium cobalt oxide (LiCoO2) batteries. Other lithium batteries include lithium manganese oxide (LiMn2O4), lithium nickel oxide (LiNiO2), and lithium iron phosphate (LFP). The cathodes of lithium batteries are made with the above materials, and the anodes are generally made of carbon.
Avoiding the lithium cobalt oxide cathode leads to a number of advantages. LiCoO2 is one of the more expensive components of traditional li-ion batteries, giving LFP batteries the potential to ultimately become significantly cheaper to produce. Lithium iron phosphate has no known carcinogenicity whereas lithium cobalt oxide does because it contains cobalt, which is listed as a possible human carcinogen by the IARC. LiCoO2 can lead to problems with runaway overheating and outgassing, particularly in the form of lithium polymer battery packs, making batteries that use it more susceptible to fire than LFP batteries. This advantage means that LFP batteries don't need as intense charge monitoring as traditional li-ion. However, LFP batteries tend to have lower (~60%) energy density in comparison to traditional li-ion.
- 1 LiFePO4 introduction
- 2 Principle
- 3 Nomenclature of LiFePO4
- 4 Invention of LFP
- 5 Theory
- 6 Physical and chemical properties
- 7 Rapid development of the LFP industries
- 8 Properties of LFP and development of the industry
- 9 Patent wars
- 10 Lawsuit settlement
- 11 Improvement
- 12 Metal substitution
- 13 Improvement of LFP synthesis processes
- 14 References
- 15 See also
Lithium iron phosphate (molecular formula is LiFePO4, also known as LFP), is used as cathode material for lithium-ion batteries (also called lithium iron phosphate battery). Its characteristic does not include noble elements such as cobalt, the price of raw material is lower and both phosphorus and iron are abundant on Earth which lowers raw material availability issues. The annual production of lithium carbonate available to the automotive industry is estimated at only 30,000 tonnes in 2015. While a natural lithium iron phosphate mineral exists (triphylite) issues with purity and the structure of the material make it unsuitable for use in batteries.
Batteries using this cathode material have a moderate operating voltage (3.3V), high energy storage capacity (170mAh/g), high discharge power, fast charging and long cycle life, and its stability is also high when placed under high temperatures or in a high thermal environment. This seemingly ordinary but, in fact, revolutionary and novel cathode material for lithium-ion batteries belongs to the olivine group. The etymology of its mineral name – triphyllite - is from the Greek tri (three) and phyllon (leaf). This mineral is gray, red-grey, brown, or black. Detailed information about this mineral can be found on the website .
Nomenclature of LiFePO4
The correct chemical formula of LiFePO4 is LiMPO4. LiFePO4 has an olivine crystal structure. The M of the chemical formula refers to any metal, including Fe, Co, Mn, Ti, etc. The first commercial LiMPO4 was C/LiFePO4 and therefore, people refer to the whole group of LiMPO4 as lithium iron phosphate, LiFePO4. However, more than one olivine compounds, in addition to LiMPO4, may be used as the cathode material of lithium iron phosphate. Such olivine compounds as AyMPO4, Li1-xMFePO4, and LiFePO4-zM have the same crystal structures as LiMPO4 and may be used as the cathode material of lithium ion batteries. (All may be referred to as “LFP”.)
Invention of LFP
LiFePO4 was invented and reported by Akshaya Padhi of John Goodenough's group at University of Texas at Austin in 1996 as an excellent candidate for the cathode of rechargeable lithium battery that is inexpensive, nontoxic, and environmentally benign. The reversible extraction of lithium from LiFePO4 and insertion of lithium into FePO4 was demonstrated. The subsequent R&D in the electrochemical energy storage all over the globe has been geared to overcoming the processing and engineering challenges that has led to current use LiFePO4 in rechargeable lithium batteries.
This lithium battery’s cathode material of olivine composition is already being mass produced by several up source professional material manufacturers. It is expected to widely expand the applications in the field of lithium batteries, and take it to the new fields such as electric bicycles, gas-electric hybrid vehicles and automation vehicles; In Tokyo Japan, a research group led by Professor Atsuo Yamada of Tokyo University of Technology, published a report on August 11, 2008 issue of Natural Materials which included the following statement: the lithium-ion iron phosphate battery will be used as the power source for environmental-friendly electric cars, which have great future prospects. The Tokyo University of Technology and North East University research group is led by Professor Atsuo Yamada. The group uses neutron irradiation phosphate iron, and then analyzes the interaction between neutron and materials to study the motion status of lithium-ion in iron phosphate. The researchers concluded that in the lithium iron phosphate, lithium-ion extended in accordance with a certain straight direction, has a different motion pattern with the existing lithium-ion electrode materials such as cobalt. This is a coincidence with the original assume theory, the analysis results with the use of neutron diffraction, confirms that lithium iron phosphate (molecular formula is LiFePO4, also known as LFP) is able to ensure the security of large input/output current of lithium battery.
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.
Rapid development of the LFP industries
At present, the root patents of the LFP compounds are held by three professional material companies: Li1-xMFePO4 by A123, LiMPO4 by Phostech and LiFePO4 • zM by Aleees. These patents have been translated to very mature mass production technologies. The largest production capacity is up to 250 tons per month. The key feature of Li1-xMFePO4 from A123 is the nano-LFP, which converts the originally less conductive LFP into commercial products by modification of its physical properties and addition of noble metals in the anode material, as well as the use of special graphite as the cathodes. The main feature of LiMPO4 from Phostech is the increased capacitance and conductivity by appropriate carbon coating; the crucial feature of LiFePO4 • zM from Aleees is the LFP with 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.
These breakthroughs and fast development in upstream materials have drawn the attention of lithium battery factories and the automobile industry. It has prompted the developments of batteries and hybrid vehicles. LFP batteries and ordinary lithium batteries are both environmentally benign. The major differences between these two are that the LFP batteries do not have such safety concerns as overheating and explosion, have 4 to 5 times longer cycle lifetimes than the lithium batteries, have 8 to 10 times higher discharge power than the lithium batteries (which can produce an instant high current), and have, under the same energy density, 30 to 50% less weight than the lithium batteries. The development of the LFP battery is highly valued by corporations such as the Department of Defense of the United States (for their hybrid tanks and Hummers), General Motors, Ford Motor, Toyota Motor, etc.
Properties of LFP and development of the industry
That being said, the market of hybrid vehicles is the determinant. It is the stable and safe olivine structure of LFP material that makes LFP favorable in lithium batteries. Different from other cathode material like Li-Co of layered structures and Li-Mn of spinel structures, LFP of olivine structures has strong oxygen covalent bonds and does not explode upon the short-circuit of lithium batteries. This feature might not be the most important for other mobile IT products but it is for lithium batteries installed on vehicles.
According to US AABC’s statistics, one out of 70,000 hybrid vehicles (PHEV, HEV, BEV) using batteries containing cobalt or manganese will explode if they have the same incidence rate as the lithium batteries of notebooks and cell phones. This number is beyond the wildest estimation of automakers. What they give top priority is safety rather than capacity. The reason is simple: It is too expensive to recall automobiles, tens of thousands of times more expensive than recalling notebooks. Therefore, safety has to be weighed against battery life.
Although LFP has 25% less capacity than other lithium batteries due to its material structure, it has 70% more performance than nickel-hydrogen battery. LFP’s improved capacity and stability draw automakers’ interests. For them, LFP can meet both the requirements of safety and battery life. Hence, hybrid vehicles are the critical market.
According to statistics, HEV, PHEV, and BEV would have, in 2008, a market of at least 7 hundred million US dollars worldwide, and at least 5 billion US dollars by 2012. From 2008 to 2015, the sales of hybrid vehicles worldwide will increase by at least 12%. In 2012, the sales of hybrid vehicles in the US will exceed 1 million. Production of hybrid vehicles in Japan will increase 6.6% from 2008 to 2011. Over all, the market for hybrid vehicle batteries for will expand 10.4% from 2010 to 2015 and the markets of hybrid vehicle parts will increase 17.4%.
In addition to compact vehicles, bus makers will also try to incorporate LFP batteries into their products. BAE has announced that their HybriDrive Orion 7 hybrid bus will use about 180KW LFP battery cells. Power plants are also using LFP now. AES in the US has developed multi-trillion watt battery systems that are capable of subsidiary services of the power network, including spare capacity and frequency adjustment.
A major competitor to LiFePO4 is lithium manganese spinel, which GM has chosen to use for the Chevrolet Volt, a gas-electric hybrid vehicle.
Before this new generation of materials can be used as the power source for electric bicycles, gas-electric hybrid vehicles and automation vehicles there lies one large obstacle: patents. Many of the companies that entered the field in the early stages have already received patents, which may result in other companies entering the market at a later time running into legal trouble.
At present, the root patents of the LFP compounds are held by the three professional material companies: Li1-xMFePO4 by A123, LiMPO4 by Phostech and LiFePO4 • zM by Aleees. And these patents have been developed into very mature mass production technologies. The largest production capacity is up to 250 tons per month. The key feature of Li1-xMFePO4 of A123 is the nano-LFP, which converts the originally less conductive LFP into commercial products by modification of its physical properties and addition of noble metal in the anode material, as well as the use of special graphite as the cathodes. The main feature of LiMPO4 of Phostech is the increased capacitance and conductivity by appropriate carbon coating; the crucial feature of LiFePO4 • zM of Aleees is the LFP with the high capacitance and low impedance obtained by the stable control of the ferrites and the 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.
These breakthroughs and fast development in upper source materials, has drawn the attention of lithium battery factories and the automobile industry. It has led some to surmise that this technology when applied to lithium batteries and gas-electric hybrid vehicles will give lead to a bright future for hybrid vehicles. LFP batteries and ordinary lithium batteries are both environmentally friendly. The major differences between these two are that the LFP batteries do not have such safety concerns as overheating and explosion, that the LFP batteries have 4 to 5 times longer cycle lifetimes than the lithium batteries, that the LFP batteries have 8 to 10 times higher discharge power than the lithium batteries (which can produce an instant high current), and that the LFP batteries have, under the same energy density, 30 to 50% less weight than the lithium batteries. The development of LFP battery is highly valued in the industry, and has been developed for the United States Department of Defense's gas-electric hybrid tanks and Hummers, General Motors, Ford Motor, Toyota Motor and so on.
From a development point of view, the U.S. auto industry estimates that by 2010, there will be over four million gas-electric hybrid vehicles on American roads. General Motors of the United States has decided to work towards the "large-scale production of electric cars" to break the domination of Japanese manufacturers. As U.S. consumers are under the extremely high pressure of skyrocketing oil prices, General Motors believe that the future auto market must be able to use all kinds of energy, and the electric car will be the key to success. Therefore, at the 2007 North American International Auto Show, GM unveiled the Plug-in Hybrid Electric Vehicle(PHEV) concept car "Chevrolet Volt Concept" and with the development of new GM hybrid system ( E-FLEX), one ordinary household power supply can be connected to the car for charging the lithium iron phosphate battery. When the Volt Concept reaches mass production, each car will able to reduce 500 gallons (1,900 liters) of gasoline consumption each year, and will reduce carbon dioxide output by 4400 kg.
Facing such strong and unstoppable development, some industrial banks, venture capital funds and investment companies, have focused on the overall arrangement on the upper source material companies. In addition to the above-mentioned three companies, besides A123 in the United States, ActaCell Inc. just received 5,800,000 U.S. dollars funding from Google.org, Applied Materials (AMAT) Venture Capital and other venture capital firms. ActaCell’s main focus is to carry out the study outcome of University of Texas to the market. One of the early innovators in LFP was Inanovation, Inc. Inanovation helped develop processes with Phostech and is one of the few remaining LFP battery development companies in the United States after the purchases of A123, Altairnano, and the downsizing of Enerdel. Professor Arumugam Manthiram has done a long-term study of development of spinel-based structure and superconducting materials. He served as a research assistant at UT, and then was promoted to professor. In recent years he discovered that when adding the expensive conductive polymers in the lithium iron phosphate (LFP), the grams capacity 166Ah/g of lithium iron phosphate (LFP) can be made in the laboratory, and then applied the microwave method to speed up the ceramic powder process of lithium iron phosphate (LFP). As to whether or not to circumvent the lithium iron phosphate (LFP) patents of A123, Aleees and Phostech by adding the conducting polymer, it is unclear at this current stage.
However, the pace of the lower source industry is not slowing down at all, in Europe, BOSCH committed to the public by continuously expanding the automation and electric powered vehicle development in 2008. Some people in Europe believe the applications of the technologies are very limited. The traditional reciprocating engine may still have an advantage of 20 years, but eventually the vehicle electric vehicles will be able to catch up.
BOSCH has a proud history of automotive technology research and development, and their own R&D department, which as a result of not looking to purchase technology from other corporations has been busy developing its own anti-lock brake and TCS tracking control system. They will be redesigned with a gas-electric hybrid computer program and will be featured in the VW Touareg and the PORSCHE Cayenne hybrid from BOSCH which was launched on the market in 2010.
BOSCH was one of the first companies that decided to focus and maintain a leading edge in fuel technology. Finally, others in the industry are beginning to wake up as the automotive safety becomes concerned about safety and now that alternative forms of energy are beginning to try to catch up. BOSCH believes they need to deeply explore the field of electric power, as it is going to be widespread technology worldwide.
BOSCH and South Korea SAMSUNG are cooperating to develop lithium batteries and carry out mass production at a cost of about 4,000,000 U.S. dollars. Although it is predicted that it will take about four to five years to move into the matured stage, BOSCH in any case will continue to invest in this effort in order to maintain its position as the top leader in the automobile technology.
Another European automotive components assembler Continental, announced that their lithium iron phosphate (LFP) partners are A123 Systems and Johnson Controls-Saft. Continental will supply the batteries for Mercedes Benz. For dealings with Bosch, they may consider doing it themselves or purchasing from A123. For the security of the supply chain, they bought stocks from a small battery factory Enax in Japan, but the company is only capable of producing small voltage products.
GS YUASA in Japan is a rising company that has announced the result of their work on the application of the anode of large-scale battery unit with its independently developed carbon-load of lithium iron phosphate (LFP). The tests results for external size of 115mm × 47mm × 170mm square shaped "LIM40" industrial battery unit indicated that even with the 400A large current discharge, the capacity is nearly not reduced. The original products without using the carbon load, had a 400A discharge unit that actually only had half the capacity of a 40A discharge. In addition, the trial product was usable in temperatures as low temperature as -20℃.
In China, the two heavy-weight lithium battery manufacturers: BAK and Tianjin Lishen, also announced their building plans of the special LFP factories, which will have annual outputs of 20,000,000 lithium iron phosphate (LFP) batteries, will be completed at the end of 2008 and early 2009 respectively. The total amount of investment in their construction is 600million dollars. As for the upper source cooperative companies, they have yet to be found in the newspaper; the speculation is that they will be cooperating with one of the three lithium iron phosphate (LFP) vendors which has a production factory in Asia.
As a result, by 2010, the competition landscape of lithium iron phosphate (LFP) industry in Europe, Asia and the United States is developing. With the high safety and stability of lithium iron phosphate(LFP) materials, the level of technology from each factory seems to be less important. The only decisive factor is the market price. According to general estimates, the union of lithium iron phosphate (LFP) will be able to lower battery price to 0.35 U.S. dollars per watt hours by 2010, will be able to take the lead in the rapid development of gas-electric hybrid vehicles and lithium battery bicycles, coming out as the ultimate winner.
Professor Goodenough at UT Austin, who discovered LFP of olivine structures more than ten years ago, probably would not expect that a micro material made of lithium iron phosphate (commonly used in fertilizers) could have such huge development and rapidly revolutionize many important industries. This prosperous development also elicits patent problems.
In the patent lawsuits in the US in 2005 and 2006, UT and Hydro-Québec claimed that every battery using LiFePO4 as the cathode and the cathode material used in some lithium ion batteries infringed their patents, US patent No 5910382 and 6514640. The ‘382 and ‘640 patents claimed a special crystal structure and a chemical formula of the battery cathode material.
On April 7, 2006, A123 Systems, Inc. (“A123") - a company that commercializes LFP products - filed an action seeking a declaration of non-infringement and invalidity with respect to two patents, U.S. Patent No. 5,910,382 ('382) and U.S. Patent No. 6, 514,640 ('640) owned by UT. Meanwhile A123 also separately filed two ex parte Reexamination Proceedings before the United States Patent and Trademark Office (USPTO), in which they sought to invalidate the patents-in-suit based upon prior art.
In a parallel court proceeding, UT also sued Valence Technology, Inc. ("Valence") - a company that commercializes LFP products - alleging infringement of its '382 and '640 patents.
The USPTO issued a Reexamination Certificate for the '382 patent on April 15, 2008 and a Reexamination Certificate for the '640 patent on May 12, 2009, by which the claims of these patents were amended. This allows the current patent infringement suits filed by Hydro-Quebec against Valence and A123 to proceed. After a markman hearing, the Western District Court of Texas held on April 27, 2011, that the claims of the reexamined '382 and '640 patents have a narrower scope than as originally granted. This will most likely affect the outcome of any future LFP patent war involving these patents.
On Dec 9th, 2008, European Patent Office revokes Dr. Goodenough’s LiMPO4 patent, patent number 0904607. This decision basically reduces the patent risk of using lithium iron phosphate in automobile application in Europe. The reason of this decision is believed to be based on the lack of novelty. While UT can still appeal the EPO decision, this result encourages the electric vehicle makers to pursue on lithium iron phosphate battery technologies in Europe.
While the patent war of LFP formulae and crystal structures is still going, it has involved many famous manufacturers of lithium batteries, such as Panasonic, ASEC (an energy supply subsidiary of Renault Samsung Motors), Johnson Controls-SAFT, Toshiba, Hitachi, Aleees, Enerdel, Altairnano, Mitsui Zosen, LG, Johnson controls, AESC, Valence, SAFT, ABB, E-one Moli. They are all trying to win this LFP patent war. The US government, too, has invested 55 million US dollars in LFP development.
Because this novel material could make an important energy storage contribution to PHEV, HEV, and BEVs, significant interest has developed in its patent history. The first challenge of commercial products is patent infringement. Many of the pioneering companies in this field have exhaustive and thorough patent maps of various olivine formulations and preparations. Follow on patents often fall within these patent maps. The first major case of an expensive settlement is the lawsuit between NTT Japan and the University of Texas-Austin (UT). In October 2008, NTT announced that they would settle the case in the Japan Supreme Civil Court with UT, by paying to UT 30 million US dollars. As part of the agreement UT agreed that NTT did not steal the information and NTT will share its NTT’s patents of LFP materials with UT. NTT’s patent is also for an olivine LiFePO4 (LFP), with the general chemical formula of AyMPO4 (A is for alkali metal and M for the combination of Co and Fe.). This compound is what BYD Company is using now. (BYD gained substantial media exposure after Warren Buffett’s announcement of investing in BYD’s LFP hybrid vehicle project.) Although chemically the materials are nearly the same, from the viewpoint of patents, AyMPO4 of NTT is different from the initial LiMPO4 materials covered by the UT. A main difference is that the AyMPO4 has higher capacity than LiMPO4, although since the patents were matter of composition based, the differences in performance were not totally germane. At the heart of the case was that NTT engineer - Okada Shigeto - who worked in the labs at UT developing the material - was suspected of stealing UT’s business secrets and used them when he returned to Japan.
LFP has two shortcomings inhibiting market penetration: low conductivity and low lithium diffusion constant, both of which limit the rate at which batteries can be charged and discharged. Researchers all over the world are working on improving the conductivity of LiMPO4. A123 is working around the problem of LFP’s extremely low conductivity (10-10 ~ 10-9 S/cm) by coating and replacing the material and converting the material into nano particles. Adding conducting particles in delithiated FePO4 raises its electron conductivity. For example, adding conducting particles with good diffusion capability like graphite and carbon  to LiMPO4 powders significantly improves conductivity between particles, increases the efficiency of LiMPO4 and raises its reversible capacity up to 95% of the theoretical values. LiMPO4 shows good cycling performance even under the condition of as large charge/discharge current as 5C.
Besides, coating LFP with inorganic oxides can make LFP’s structure more stable and increase conductivity. Traditional LiCoO2 with oxide coating shows improved cycling performance. This coating also inhibits dissolution of Co and slows the decay of LiCoO2 capacity. Similarly, LiMPO4 with inorganic coating, such as ZnO and ZrO2, has a better cycling lifetime, larger capacity and better characteristics under the condition of a large discharge current. The addition of a conductive carbon in LiMPO4 increases the efficiency of LiMPO4, too. Mitsui Zosen Japan and Aleees reported that addition of other conducting metal particles, such as copper and silver, also increased LiMPO4’s efficiency. LiMPO4 with 1 wt. % of metal additives has a reversible capacity up to 140mAh/g and better characteristics under the condition of large discharge current.
Substituting other metals for the iron or lithium in LiMPO4 can also raise its efficiency. A123 and Valence reported the substitution of magnesium, titanium, manganese, zirconium and zinc. Take zinc substitution for example. Substituting zinc for iron increases crystallinity of LiMPO4 because zinc and iron have similar ion radii. Cyclic voltammetry also confirms that LiFe1-xMxPO4, 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.
Improvement of LFP synthesis processes
Similar to lithium oxides, LiMPO4 may be synthesized by the following methods: 1. solid-phase synthesis, 2. emulsion drying, 3. sol-gel process 4. solution coprecipitation, 5. vapor phase deposition, 6. electrochemical synthesis, 7. electron beam irradiation, 8. microwave process 9. hydrothermal synthesis, 10. ultrasonic pyrolysis, 11. spray pyrolysis, etc. Different processes have different results. For example, 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 carbon particles of nano sizes. Hydrothermal synthesis produces LiMPO4 with good crystallinity. Conductive carbon is obtained by adding polyethylene glycol to the solution followed by thermal processing. Vapor phase deposition produces a thin film LiMPO4.
LFP batteries also have their drawbacks. There are ongoing international patent suits regarding this technology, and mass production with stable and high quality still faces many challenges. The current low production levels mean that LFP batteries tend to cost more than their LiCoO2 equivalents. The energy density of LFP batteries is significantly lower than LiCoO2 (although well higher than its main competitor for safety and lifespan, the nickel-metal hydride battery), and the market acceptance for large batteries is rather low in certain applications, making LFP batteries harder to commercialize.
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