Lithium iron phosphate (LiFePO4) batteries are safer than lithium-ion batteries, available in a variety of large battery sizes ranging from 5 to 100 AH, and have a much longer cycle life than traditional batteries.
Cylindrical LiFePO4 batteries are among the most popular products in the entire series, boasting many powerful features:
High energy density, 270 to 340 Wh/L; this means long operating time.
• Stable discharge voltage
• Good consistency between different cells in the same order
• Long cycle life, 80% capacity remaining after 2000 cycles
Fast charging, fully charged in one hour.
• Safe and high-temperature resistant performance
Lithium iron phosphate (LFP) is a type of lithium-ion battery because energy is stored, moved, and stored in the same way as lithium ions, rather than lithium metal. These batteries and assemblies not only have high capacity but can also provide high power. High-power LFP batteries are now a reality. They can be used as rechargeable batteries or power sources.
Furthermore, lithium iron phosphate batteries are among the longest-lasting batteries ever made. Laboratory test data shows up to 2000 charge/discharge cycles. This is due to the extremely robust crystal structure of lithium iron phosphate, which prevents decomposition during the repeated accumulation and unpacking of lithium ions during charging and discharging.
While small-capacity lithium-ion (polymer) batteries containing lithium cobalt oxide (LiCoO₂) offer the best mass and volumetric energy density, LiCoO₂ is very expensive and unsafe for large-scale lithium-ion batteries. Recently, lithium iron phosphate (LiFePO₄) has emerged as the “best choice” material for commercial lithium-ion (and polymer) batteries in high-capacity and high-power applications such as laptops, power tools, wheelchairs, e-bikes, electric vehicles, and electric buses.
LiFePO4 batteries possess hybrid characteristics: they are as safe as lead-acid batteries and as powerful as lithium-ion batteries. The advantages of large-size lithium-ion (and polymer) batteries containing lithium iron phosphate (LiFePO4) are as follows:
A. Regular charging
In a typical lithium-ion battery charging process, lithium iron phosphate (LiFePO4) batteries require two steps to be fully charged: Step 1 uses constant current (CC) to reach approximately 60%-70% state of charge (SoC); Step 2 occurs when the charging voltage reaches 3.65V per cell, which is the upper limit of the effective charging voltage. Switching from constant current (CC) to constant voltage (CV) means that the charging current is limited by the current the battery can accept at that voltage, therefore the charging current gradually decreases, much like a capacitor being charged through a resistor will eventually reach its final voltage asymptotically.
Based on process timing, step 1 (60%-70% SoC) takes approximately one to two hours, and step 2 (30%-40% SoC) takes another two hours.
Because overvoltage can be applied to LiFePO4 batteries without decomposing the electrolyte, a single CC charge can reach 95% SoC, or a CC+CV charge can reach 100% SoC. This is similar to the safe forced charging method for lead-acid batteries. The shortest total charging time is approximately two hours.
LiCoO2 batteries have a very narrow overcharge tolerance, exceeding the 4.2V charging voltage plateau (which is also the upper limit of the charging voltage) of each cell by about 0.1V. Continuous charging above 4.3V will damage battery performance, such as cycle life, or cause fire or explosion.
The overcharge tolerance of LiFePO4 batteries is much wider than the 3.5V charging voltage plateau per cell, approximately 0.7V. LiFePO4 batteries can be safely overcharged to 4.2V per cell, but higher voltages will begin to break down the organic electrolyte. Nevertheless, lead-acid battery chargers are typically used to charge 12V four-cell series-connected battery packs. The maximum voltage of these chargers, whether powered by AC or using a car's alternator, is 14.4V. This works fine, but lead-acid chargers will reduce their voltage to 13.8V for float charging, thus typically terminating the LiFePO4 battery pack before it reaches 100%. Therefore, a special LiFePO4 charger is needed to reliably reach 100% capacity.
Due to the increased safety margin, these battery packs are more suitable for high-capacity and high-power applications. In terms of large overcharge tolerance and safety performance, LiFePO4 batteries are similar to lead-acid batteries.
Lead-acid batteries are aqueous systems. The nominal voltage of a single cell during discharge is 2V. Lead is a heavy metal; its specific capacity is only 44Ah/kg. In contrast, lithium iron phosphate (LiFePO4) batteries are non-aqueous systems with a nominal voltage of 3.2V during discharge. Their specific capacity is greater than 145Ah/kg. Therefore, the gravimetric energy density of LiFePO4 batteries is 130Wh/kg, four times that of lead-acid batteries (35Wh/kg). This four-fold increase in density makes LiFePO4 batteries more attractive than lead-acid batteries in technological applications.
The large overcharge tolerance and self-balancing characteristics of LiFePO4 batteries simplify battery protection and balancing circuit boards, reducing their cost. The one-step charging process allows for charging LiFePO4 batteries with simpler conventional power sources, instead of expensive specialized lithium-ion battery chargers.
E. Longer cycle life
Compared to LiCoO2 batteries with a cycle life of 400 cycles, LiFePO4 batteries have a cycle life extended to 2000 cycles.
F. High-temperature performance
LiCoO2 batteries are detrimental to operation at high temperatures, such as 60°C. However, LiFePO4 batteries perform better at high temperatures, with a 10% increase in capacity due to higher lithium-ion conductivity.
Main advantages of lithium iron phosphate batteries:
Like nickel-based rechargeable batteries (unlike other lithium-ion batteries), LiFePO4 batteries have a very constant discharge voltage. The voltage remains close to 3.2V during discharge until the battery is depleted.
This allows the battery to provide almost full power before it discharges.
It can greatly simplify or even eliminate the need for voltage regulation circuits.
• Lithium iron phosphate batteries are more difficult to ignite under improper handling (especially during charging), although any fully charged battery can only dissipate overcharged energy as heat.
Therefore, the possibility of battery malfunction due to misuse still exists. It is generally believed that LiFePO4 batteries do not decompose at high temperatures.
• Compared to lithium-ion battery chemistry such as LiCoO2 cobalt or LiMn2O4 manganese, LiFePO4 batteries have a slower rate of capacity loss (or a longer calendar life).
• After a year on the market, the energy density of LiFePO4 batteries is usually roughly the same as that of LiCoO2 lithium-ion batteries because the energy density of LiFePO4 decreases more slowly.
