The lithium iron phosphate battery (LiFePO4 battery) or LFP battery (lithium ferrophosphate), is a type of rechargeable battery, specifically a lithium-ion battery, using LiFePO4 as the cathode material, and a graphitic carbon electrode with a metallic backing as the anode. The specific capacity of LiFePO4 is higher than that of the related lithium cobalt oxide (LiCoO2) chemistry, but its energy density is less due to its lower operating voltage. The main drawback of LiFePO4 is its low electrical conductivity. Therefore, all the LiFePO4 cathodes under consideration are actually LiFePO4/C. Because of low cost, low toxicity, well-defined performance, long-term stability, etc. LiFePO4 is finding a number of roles in vehicle use, utility scale stationary applications, and backup power.
LiFePO4 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 LiFePO4 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.
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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
The LiFePO4 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 3.2 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, as well as the potential for the thermal runaway characteristic of cobalt-content rechargeable lithium cells manifesting itself.
LiFePO4 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 LiCoO2 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 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 LiCoO2 battery of the same capacity) can be used.
LiFePO4 cells experience a slower rate of capacity loss (aka greater calendar-life) than lithium-ion battery chemistries such as LiCoO2 cobalt or LiMn2O4 manganese spinel lithium-ion polymer batteries (LiPo battery) or lithium-ion batteries. After one year on the shelf, a LiFePO4 cell typically has approximately the same energy density as a LiCoO2 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.
LiFePO4 Safety
One important advantage over other lithium-ion chemistries is thermal and chemical stability, which improves battery safety. LiFePO4 is an intrinsically safer cathode material than LiCoO2 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 (PO4)3−ion is stronger than the Co–O bond in the (CoO2))−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 LiCoO2 cell, the CoO2 undergoes non-linear expansion that affects the structural integrity of the cell. The fully lithiated and unlithiated states of LiFePO4 are structurally similar which means that LiFePO4 cells are more structurally stable than LiCoO2 cells.
No lithium remains in the cathode of a fully charged LiFePO4 cell. (In a LiCoO2 cell, approximately 50% remains.) LiFePO4 is highly resilient during oxygen loss, which typically results in an exothermic reaction in other lithium cells. As a result, LiFePO4 cells are harder to ignite in the event of mishandling (especially during charge). The LiFePO4 battery does not decompose at high temperatures.
LiFePO4 vs Lead Acid
In applications where weight is a consideration, Lithium batteries are among the lightest options available. In recent years Lithium has become available in several chemistries; Lithium-Ion, Lithium Iron Phosphate, Lithium Polymer and a few more exotic variations.
LiFePO4 (also known as Lithium Iron Phosphate) batteries are a huge improvement over lead acid in weight, capacity and shelf life. The LiFePO4 batteries are the safest type of Lithium batteries as they will not overheat, and even if punctured they will not catch on fire. The cathode material in LiFePO4 batteries is not hazardous, and so poses no negative health hazards or environmental hazards. Due to the oxygen being bonded tightly to the molecule, there is no danger of the battery erupting into flames like there is with Lithium-Ion. The chemistry is so stable that LiFePO4 batteries will accept a charge from a lead-acid configured battery charger. Though less energy-dense than the Lithium-Ion and Lithium Polymer, Iron and Phosphate are abundant and cheaper to extract so costs are much more reasonable. LiFePO4 life expectancy is approximately 5-20 years depending on usage.
Lithium-Ion batteries and Lithium Polymer batteries are the most energy dense of the Lithium batteries, but they are lacking in safety. The most common type of Lithium-Ion is LiCoO2, or Lithium Cobalt Oxide. In this chemistry, the oxygen is not strongly bonded to the cobalt, so when the battery heats up, such as in rapid charging or discharging, or just heavy use, the battery can catch fire. This could be especially disastrous in high pressure environments such as airplanes, or in large applications such as electric vehicles. To help counteract this problem, devices that use Lithium-Ion and Lithium Polymer batteries are required to have extremely sensitive and often expensive electronics to monitor them. While Lithium Ion batteries have an intrinsically high energy density, after one year of use the capacity of the Lithium Ion will have fallen so much that the LiFePO4 will have the same energy density, and after two years LiFePO4 will have significantly greater energy density. Another disadvantage of these types is that Cobalt can be hazardous, raising both health concerns and environmental disposal costs. The projected life of a Lithium-Ion battery is approximately 3 years from production.
Lead Acid is a proven technology and can be relatively cheap. Because of this they are still used in the majority of electric vehicle applications and starting applications. Compared to Lead-Acid batteries the only disadvantage of the LiFePO4 batteries is that they really do not perform well below about 0 degrees Celsius. However, since capacity, weight, operating temperatures and CO2 reduction are large factors in many applications, LiFePO4 batteries are quickly becoming an industry standard. Although the initial purchase price of LiFePO4 is higher than lead acid, the longer cycle life can make it a financially sound choice.