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Lithium iron phosphate

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Update time : 2023-02-02 11:57:58

Lithium iron phosphate or lithium ferro-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, solar energy installations[2] and more recently large grid-scale energy storage.[3]

Most lithium batteries (Li-ion) used in consumer electronics products use cathodes made of other lithium compounds, such as lithium cobalt oxide (LiCoO
2
), lithium manganese oxide (LiMn
2
O
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.[4] 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
y
MPO
4
Li
1−x
MFePO
4
, and LiFePO
4−z
M
 have the same crystal structures as LiMPO
4
, and may replace it 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.[5] 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 used to store energy.[6]

History and production

Arumugam Manthiram and John B. Goodenough first identified the polyanion class of cathode materials for lithium ion batteries.[7][8][9] LiFePO
4
 was then identified as a cathode material belonging to the polyanion class for use in batteries in 1996 by Padhi et al.[10][11] 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.[12]

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.[13]

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 FePO
4
.[14]

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 Å3, 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 PO3−
4
 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 temperature. The details of the lithiation of FePO
4
 and the delithiation of LiFePO
4
 have been examined. Two phases of the lithiated material are implicated.[14][15]

Applications

LFP cells have an operating voltage of 3.3 V, charge density of 170 mAh/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.[16][17]

BAE has announced that their Hybri Drive Orion 7 hy brid 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, originating from the electrically insulating properties of LFP, as well as the lower maximum charge/discharge voltage. 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 geographically available. Other chemistries such as nickel-manganese-cobalt (NMC) have supplanted LiCo chemistry cells in most applications. The original ratio of Ni to Mn to Co was 3:3:3, whereas today, cells are being made with ratios of 8:1:1 or 6:2:2, whereby the Co content has been drastically reduced.

LiFePO4 batteries are comparable to sealed lead acid batteries and are often being touted as a drop-in replacement for lead acid applications. The most notable difference between lithium iron phosphate and lead acid is the fact that the lithium battery capacity is independent of the discharge rate. With very high discharge rates, for instance 0.8C, the capacity of the lead acid battery is only 60% of the rated capacity. Therefore, in cyclic applications where the discharge rate is often greater than 0.1C, a lower rated lithium battery will often have a higher actual capacity than the comparable lead acid battery. This means that at the same capacity rating, the 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.[18]


Research

Power density

LFP has two shortcomings: low conductivity (high overpotential) 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[22] 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. However, addition of conductive additives also increases the "dead mass" present in the cell that does not contribute to energy storage. LiMPO
4
 shows good cycling performance even under charge/discharge current as large as 5C.[23]

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[24] and ZrO
2
,[25] 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.[26] LiMPO
4
 with 1 wt% of metal additives has a reversible capacity up to 140 mAh/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 ionic radii.[27] Cyclic voltammetry confirms that LiFe
1−x
M
x
PO
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 stability 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[vague], 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.[28] Hydrothermal synthesis produces LiMPO
4
 with good crystallinity. Conductive carbon is obtained by adding polyethylene glycol to the solution followed by thermal processing.[29] Vapor phase deposition produces a thin film LiMPO
4
.[30] 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
.[31]

Effects of temperature

The effects of temperature on lithium iron phosphate batteries can be divided into the effects of high temperature and low temperature.

Generally, LFP chemistry batteries are less susceptible to thermal runaway reactions like those that occur in lithium cobalt batteries; LFP batteries exhibit better performance at an elevated temperature. 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.[32]

However, this is only true for a short cycling timeframe. Later yearlong study has shown that despite LFP batteries having double the equivalent full cycle, the capacity fate rate increased with increasing temperature for LFP cells but the increasing temperature does not impact NCA cells or have a negligible impact on the aging of NMC cells.[33] This capacity fade is primarily due to the solid electrolyte interface (SEI) formation reaction being accelerated by increasing temperature.

LFP batteries are especially affected by decreasing temperature which possibly hamper their application in high-latitude areas. The initial discharge capacities for LFP/C samples at temperatures of 23, 0, -10, and -20 °C are 141.8, 92.7, 57.9 and 46.7 mAh/g with coulombic efficiency 91.2%, 74.5%, 63.6% and 61.3%. These losses are accounted for by the slow diffusion of lithium ions within electrodes and the formation of SEI that come with lower temperatures which subsequently increase the charge-transfer resistance on the electrolyte-electrode interfaces.[34] Another possible cause of the lowered capacity formation is lithium plating. As mentioned above, low temperature lowers the diffusion rate of lithium ions within the electrodes, allowing for the lithium plating rate to compete with that of intercalation rate. The colder condition leads to higher growth rates and shifts the initial point to lower state of charge which means that the plating process starts earlier.[35] Lithium plating uses up lithium which then compete with the intercalation of lithium into graphite, decreasing the capacity of the batteries. The aggregated lithium ions are deposited on the surface of electrodes in the form of “plates” or even dendrites which may penetrate the separators, short-circuiting the battery completely.[36]