Lithium iron phosphate battery
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|This article is outdated. (June 2014)|
|Specific energy||90–110 Wh/kg (320–400 J/g)|
|Energy density||220 Wh/L (790 kJ/L)|
|Specific power||around 2400 W/kg|
|Energy/consumer-price||0.7–3 Wh/US$ (US$0.11–0.56/kJ)|
|Time durability||>10 years|
|Cycle durability||2,000 cycles|
|Nominal cell voltage||3.2 V|
The lithium iron phosphate (LiFePO
4) battery, also called LFP battery (with "LFP" standing for "lithium ferrophosphate"), is a type of rechargeable battery, specifically a lithium-ion battery, which uses LiFePO
4 as a cathode material. LiFePO
4 batteries have somewhat lower energy density than the more common LiCoO
2 design found in consumer electronics, but offers longer lifetimes, better power density (the rate that energy can be drawn from them) and are inherently safer. LiFePO
4 is finding a number of roles in vehicle use and backup power.
4 is a natural mineral of the olivine family. Its use as a battery electrode was first described in published literature by John Goodenough's research group at the University of Texas in 1996, as a cathode material for rechargeable lithium batteries. Because of its low cost, non-toxicity, the natural abundance of iron, its excellent thermal stability, safety characteristics, electrochemical performance, and specific capacity (170 mA·h/g, or 610 C/g) it gained some market acceptance.
Its key barrier to commercialization was intrinsically low electrical conductivity. This problem was overcome by reducing the particle size, coating the LiFePO
4 particles with conductive materials such as carbon, and doping the result with cations of materials such as aluminium, niobium, and zirconium. This approach was developed by Yet-Ming Chiang and his coworkers at MIT. Products are now in mass production and are used in industrial products by major corporations including Black and Decker's DeWalt brand, the Fisker Karma, Daimler, Cessna and BAE Systems.
MIT has introduced a new coating that allows the ions to move more easily within the battery. The "Beltway Battery" utilizes a bypass system that allows the lithium-ions to enter and leave the electrodes at a speed great enough to fully charge a battery in under a minute. The scientists discovered that by coating lithium iron phosphate particles in a glassy material called lithium pyrophosphate, ions bypass the channels and move faster than in other batteries. Rechargeable batteries store and discharge energy as charged atoms (ions) form between two electrodes, the anode and cathode. Their charge and discharge rate are restricted by the speed with which these ions move. Such technology could reduce the weight and size of the batteries. A small prototype battery cell has been developed that can fully charge in 10 to 20 seconds, compared with six minutes for standard battery cells.
Advantages and disadvantages
4 battery uses a lithium-ion-derived chemistry and shares many advantages and disadvantages with other Lithium-ion battery chemistries. However, there are significant differences.
LFP chemistry offers a longer cycle life than other lithium-ion approaches.
Like nickel-based rechargeable batteries (and unlike other lithium ion batteries), LiFePO4 batteries have a very constant discharge voltage. Voltage stays close to 3.2V during discharge until the battery is exhausted. This allows the battery to deliver virtually full power until it is discharged. And it can greatly simplify or even eliminate the need for voltage regulation circuitry.
Because of the nominal 3.2V output, four cells can be placed in series for a nominal voltage of 12.8 V. This comes close to the nominal voltage of six-cell lead-acid batteries. And, along with the good safety characteristics of LFP batteries, this makes LFP a good potential replacement for lead-acid batteries in many applications such as automotive and solar applications, provided the charging systems are adapted not to damage the LFP cells through excessive charging voltages, temperature-based voltage compensation, equalisation attempts or continuous trickle charging. The LFP cells must be at least balanced initially before the pack is assembled and a protection system also needs to be implemented to ensure no cell can be discharged below a voltage of 2.0 V or severe damage will occur in most instances.
The use of phosphates avoids cobalt's cost and environmental concerns, particularly concerns about cobalt entering the environment through improper disposal.
4 has higher current or peak-power ratings than LiCoO
The energy density (energy/volume) of a new LFP battery is some 14% lower than that of a new LiCoO
2 battery. Also, many brands of LFPs, as well as cells within a given brand of LFP batteries, have a lower discharge rate than lead-acid or LiCoO2. Since discharge rate is a percentage of battery capacity a higher rate can be achieved by using a larger battery (more ampère-hours) if low current batteries must be used. Better yet, a high current LFP cell (which will have a higher discharge rate than a lead acid or LiCoO
2 battery of the same capacity) can be used.
4 cells experience a slower rate of capacity loss (aka greater calendar-life) than lithium-ion battery chemistries such as LiCoO
2 cobalt or LiMn2O4 manganese spinel lithium-ion polymer batteries or lithium-ion batteries. After one year on the shelf, a LiFePO
4 cell typically has approximately the same energy density as a LiCoO2 Li-ion cell, because of LFP's slower decline of energy density. Thereafter, LiFePO
4 likely has a higher energy density.
One important advantage over other lithium-ion chemistries is thermal and chemical stability, which improves battery safety. LiFePO
4 is an intrinsically safer cathode material than LiCoO2 and manganese spinel. The Fe-P-O bond is stronger than the Co-O bond, so that when abused, (short-circuited, overheated, etc.) the oxygen atoms are much harder to remove. This stabilization of the redox energies also helps fast ion migration.
As lithium migrates out of the cathode in a LiCoO2 cell, the CoO2 undergoes non-linear expansion that affects the structural integrity of the cell. The fully lithiated and unlithiated states of LiFePO
4 are structurally similar which means that LiFePO
4 cells are more structurally stable than LiCoO2 cells.
No lithium remains in the cathode of a fully charged LiFePO
4 cell—in a LiCoO2 cell, approximately 50% remains in the cathode. LiFePO
4 is highly resilient during oxygen loss, which typically results in an exothermic reaction in other lithium cells.
As a result, lithium iron phosphate cells are much harder to ignite in the event of mishandling (especially during charge) although any fully charged battery can only dissipate overcharge energy as heat. Therefore, failure of the battery through misuse is still possible. It is commonly accepted that LiFePO4 battery does not decompose at high temperatures. The difference between LFP and the LiPo battery cells commonly used in the aeromodelling hobby is particularly notable.
- Cell voltage
- Min. discharge voltage = 2.5V 
- Working voltage = 3.0 ~ 3.3 V
- Max. charge voltage = 3.6V
- Volumetric energy density = 220 Wh/dm3 (790 kJ/dm3)
- Gravimetric energy density > 90 Wh/kg (> 320 J/g)
- Sony Fortelion: 71% capacity after 10000 cycles with 100% DoD
- 100% DOD cycle life (number of cycles to 80% of original capacity) = 2,000–7,000
- 90% DOD cycle life (number of cycles to 80% of original capacity) > 10,000 
- Cathode composition (weight)
- Cell configuration
- Experimental conditions:
- Room temperature
- Voltage limits: 2.0–3.65 V
- Charge: Up to C/1 rate up to 3.6 V, then constant voltage at 3.6 V until I < C/24
Higher discharge rates needed for acceleration, lower weight and longer life makes this battery type ideal for bicycles and electric cars.
BYD, also a car manufacturer, plans to use its lithium iron phosphate batteries to power its PHEV, the F3DM and F6DM (Dual Mode), which will be the first commercial dual-mode electric car in the world. It plans to mass-produce the cars in 2009. In October 2014, BYD announced a 60-foot, 120-passenger battery-electric bus with a range of more than 170 miles that uses lithium iron phosphate batteries.
Rimac Automobili have developed an advanced LFP battery system with integrated battery management and liquid cooling systems, primarily for their Concept One electric supercar which will enter production but also for commercial availability of the battery system.
ZBoard electric skateboards use LFP batteries, offering ranges up to 20 miles.
Golfskatecaddy Golf Skate Caddy electric single person golf vehicle use LFP batteries, allowing a full 18 holes of golf.
EV-Fleet electric pickup trucks use a 50kWh LFP battery for a 100+ mile range.
Solar garden and security light systems
Single "14500" (AA sized) LFP cells are now used in some solar powered path lights instead of 1.2V NiCd/NiMH.
LFP's higher (3.2V) working voltage can allow a single cell to drive an LED without needing a step-up circuit. The increased tolerance to modest overcharging (compared to other Li cell types) means that LiFePO4 could be connected to photovoltaic cells without complex circuitry. A single LFP cell also alleviates corrosion,condensation & dirt issues notoriously associated with battery holder and cell to cell contacts - such poor connections often especially plague outdoor systems using multiple removable NiMH cells.
More sophisticated LFP solar charged passive IR security lamps are also emerging (2013). As AA sized LFP cells have a capacity of only ~600 mAh (while the lamp's bright LED may draw 60mA) only 10 hours run time may be expected. However- if triggering is only occasional -such systems may cope even under low sunlight charging conditions,as lamp electronics ensure after dark "idle" currents of under 1mA.
LiFePO4 powered solar lamps are visibly brighter than ubiquitous outdoor solar lights,and performance overall is considered more reliable.
One Laptop per Child
Many home EV conversions use the large format versions as the car's traction pack. With the efficient power to weight ratios, high safety features and the chemistry's refusal to go into thermal runaway, there are few barriers for use by amateur home "makers".
Some electronic cigarettes use these types of batteries.
Two torch/flashlight manufacturers Mag Instruments and LED Lenser have products which utilise these batteries.
RC model cars may use these batteries, especially as RX and TX packs as a direct replacement of NiMh packs or LiPo packs without need for voltage regulator, as they provide 6.6v nominal voltage over 7.4v of LiPo packs, which is little higher and may require to be regulated down to 6.0v.
- Lithium battery
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