Lithium iron phosphate battery
|Specific energy||90–160 Wh/kg (320–580 J/g or kJ/kg)|
|Energy density||325 Wh/L (1200 kJ/L)|
|Specific power||around 200 W/kg|
|Time durability||> 10 years|
|Cycle durability||2,000 cycles|
|Nominal cell voltage||3.2 V|
The lithium iron phosphate battery (LiFePO
4 battery) or LFP battery (lithium ferrophosphate), is a type of lithium-ion battery using LiFePO
4 as the cathode material, and a graphitic carbon electrode with a metallic backing as the anode. The specific capacity of LiFePO
4 is higher than that of the related lithium cobalt oxide (LiCoO
2) chemistry, but its energy density is less due to its lower operating voltage. The main drawback of LiFePO
4 is its low electrical conductivity. Therefore, all the LiFePO
4 cathodes under consideration are actually LiFePO
4/C. Because of low cost, low toxicity, well-defined performance, long-term stability, etc. LiFePO
4 is finding a number of roles in vehicle use, utility scale stationary applications, and backup power.
4 is a natural mineral of the olivine family (triphylite). Its use as a battery electrode was first described in published literature by Akshaya Padhi and coworkers of John B. 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 has gained considerable market acceptance.
The chief barrier to commercialization was its intrinsically low electrical conductivity. This problem was overcome by reducing the particle size, coating the LiFePO
4 particles with conductive materials such as carbon nanotubes, or both. This approach was developed by Michel Armand and his coworkers. Another approach by Yet Ming Chiang's group consisted of doping LFP with cations of materials such as aluminium, niobium, and zirconium. 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 AG, Cessna and BAE Systems.
MIT 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) are moved between two electrodes, the anode and the 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.
Negative electrodes (anode, on discharge) made of petroleum coke were used in early lithium-ion batteries; later types used natural or synthetic graphite.
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), LiFePO
4 batteries have a very constant discharge voltage. Voltage stays close to 3.2 V during discharge until the cell is exhausted. This allows the cell 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 3.2 V 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. Along with the good safety characteristics of LFP batteries, this makes LFP a good potential replacement for lead-acid batteries in applications such as automotive and solar applications, provided the charging systems are adapted not to damage the LFP cells through excessive charging voltages (beyond 3.6 volts DC per cell while under charge), 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 2.5 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 LiCoO
2. Since discharge rate is a percentage of battery capacity a higher rate can be achieved by using a larger battery (more ampere 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 LiMn
4 manganese spinel lithium-ion polymer batteries (LiPo battery) or lithium-ion batteries. After one year on the shelf, a LiFePO
4 cell typically has approximately the same energy density as a LiCoO
2 Li-ion cell, because of LFP's slower decline of energy density.
Compared to other lithium chemistries, LFP experiences much slower degradation when stored in a fully charged state. This makes LFP a good choice for standby use.
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 LiCoO
2 and manganese spinel, through omission of the cobalt, with its negative temperature coefficient of resistance that can encourage thermal runaway. The P–O bond in the (PO
ion is stronger than the Co–O bond in the (CoO
ion, so that when abused (short-circuited, overheated, etc.), the oxygen atoms are released more slowly. This stabilization of the redox energies also promotes faster ion migration.
As lithium migrates out of the cathode in a LiCoO
2 cell, the CoO
2 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 LiCoO
2 cells.
No lithium remains in the cathode of a fully charged LiFePO
4 cell. (In a LiCoO
2 cell, approximately 50% remains.) LiFePO
4 is highly resilient during oxygen loss, which typically results in an exothermic reaction in other lithium cells. As a result, LiFePO
4 cells are harder to ignite in the event of mishandling (especially during charge). The LiFePO
4 battery does not decompose at high temperatures.
- Cell voltage
- Minimum discharge voltage = 2.5 V
- Working voltage = 3.0 ~ 3.2 V
- Maximum charge voltage = 3.65 V
- Volumetric energy density = 220 Wh/L (790 kJ/L)
- Gravimetric energy density > 90 Wh/kg (> 320 J/g). Up to 160 Wh/kg (580 J/g).
- 100% DOD cycle life (number of cycles to 80% of original capacity) = 2,000–7,000
- 10% 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
- According to the manufacturer BYD the lithium iron phosphate battery of the electric car e6 is charged at a fast charging station to 80% within 15 minutes, and 100% within 40 minutes.
Higher discharge rates needed for acceleration, lower weight and longer life makes this battery type ideal for bicycles and electric cars. 12V LiFePO4 batteries are also gaining popularity as a second (house) battery for a caravan, motor-home or boat.
Solar-powered lighting systems
LFP's higher (3.2 V) working voltage lets a single cell drive an LED without circuitry to step up the voltage. Its increased tolerance to modest overcharging (compared to other Li cell types) means that LiFePO
4 can be connected to photovoltaic cells without circuitry to halt the recharge cycle. The ability to drive an LED from a single LFP cell also obviates battery holders, and thus the corrosion, condensation and dirt issues associated with products using multiple removable rechargeable batteries.
By 2013, better solar-charged passive infrared security lamps emerged. As AA-sized LFP cells have a capacity of only 600 mAh (while the lamp's bright LED may draw 60 mA), the units shine for at most 10 hours. However, if triggering is only occasional, such units may be satisfactory even charging in low sunlight, as lamp electronics ensure after-dark "idle" currents of under 1 mA.
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 resistance to thermal runaway, there are few barriers for use by amateur home "makers". Motorhomes are often converted to lithium iron phosphate because of the high draw.
Some electronic cigarettes use these types of batteries. Other applications include flashlights, radio-controlled models, portable motor-driven equipment, amateur radio equipment, industrial sensor systems and emergency lighting.
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