Lithium Iron Phosphate (LiFePO4) batteries are safer than ordinary Li-ion batteries, have a range of large cell sizes that can provide 5~100AH capacity, and have a much longer cycle life than conventional batteries. Among them, cylindrical lithium iron phosphate batteries are one of the hottest products in all series, and they have many excellent features:
●High energy density, 270~340Wh/L; this means long working time.
●Stable discharge voltage.
●Good consistency between different cells in the same batch.
●Long cycle life, 80% of the capacity remains after 2,000 times of charge and discharge.
●Quick charge, can be charged within 1 hour.
● safety and high temperature performance.
Lithium iron phosphate is a lithium-ion battery because energy is stored in the same way, it moves and stores lithium ions instead of lithium metal. These cells and batteries not only have high capacity, but also provide high power. At present, high-power lithium iron phosphate batteries are now a reality, and they can be used as energy storage units or power sources.
In addition, the lithium iron phosphate battery is one of the most vigorous batteries ever. Laboratory test data shows that it has up to 2,000 charge/discharge cycles due to the extremely strong crystal structure of iron phosphate, which does not decompose during charge and discharge due to repeated intercalation and deintercalation of lithium ions.
Table 1 Main parameters of charge and discharge of lithium iron phosphate battery.
While small-capacity lithium-ion (polymer) batteries containing lithium cobalt oxide (LiCoO2) offer the best mass and volumetric energy density, lithium-cobalt oxide is very expensive and unsafe for high-capacity lithium-ion batteries. More recently, lithium iron phosphate has become the "best choice" for commercial lithium-ion (and polymer) battery materials for high-capacity and high-power applications such as laptops, power tools, electric wheelchairs, electric bicycles, electric vehicles, and electric buses .
LiFePO4 batteries have mixed properties: they are as safe as lead-acid batteries (LA Battery) and as powerful as lithium-ion batteries. Listed below are the advantages of large format lithium-ion (and polymer) batteries containing lithium iron phosphate.
regular charging
During the traditional lithium-ion charging process, conventional lithium-ion batteries containing lithium iron phosphate require two steps to fully charge.
Step 1: Use constant current (CC) charging to reach a state of charge (SOC) of about 60~70%; when the charging voltage of each cell reaches 3.65V (this is the upper limit of the effective charging voltage), go to step 2, Constant voltage (CV) charging. Going from CC to CV means that the charging current is limited by the current the battery can accept at that voltage, so the charging current tapers off, just like charging a capacitor through a resistor, and the voltage taps off to its final value.
In terms of time: Step 1 (60~70% SOC) takes about one to two hours; Step 2 (30~40% SOC) also takes about two hours.
Since an overvoltage can be applied to the LiFePO4 battery without decomposing the electrolyte, it can be charged through step 1 of CC only to achieve 95% SOC, or CC+CV charging to achieve 100% SOC. This is similar to the safe forced charging of lead-acid batteries, with a minimum total charging time of about two hours.
Large overcharge tolerance and safer performance
Lithium cobalt oxide batteries have a narrow overcharge tolerance of about 0.1V above the 4.2V per cell charge voltage value, which is also the upper limit of charge voltage. Continuous charging beyond 4.3V will either degrade battery performance (eg shorten cycle life) or cause a fire or explosion.
Lithium iron phosphate batteries have a much wider overcharge tolerance of about 0.7V than their charge voltage of 3.5V per cell. A lithium iron phosphate battery can be safely overcharged to 4.2V per cell, but higher voltages will start to break down the organic electrolyte. However, 12V 4-cell series battery packs are typically charged using lead-acid battery chargers, either AC powered or using the car's alternator, these chargers have a maximum voltage of 14.4V. It works fine, but the lead acid charger reduces the float voltage to 13.8V, so it usually stops charging before the lithium iron phosphate battery pack reaches 100% full capacity, so special lithium iron phosphate chargers are required to be reliable to 100% capacity.
Due to the increased safety factor, these battery packs are the first choice for high capacity and high power applications. From the point of view of large overcharge tolerance and safety performance, lithium iron phosphate batteries are similar to lead-acid batteries.
Three times higher energy density than lead-acid batteries
Lead-acid batteries are aqueous systems. When discharging, the rated single-cell voltage is 2V. Lead is a heavy metal with a specific capacity of only 44Ah/kg. In contrast, lithium iron phosphate batteries are non-aqueous systems and, when discharged, have a rated voltage of 3.2V. Its specific capacity exceeds 145Ah/kg. Therefore, the weight energy density of lithium iron phosphate battery is 130Wh/kg, which is four times that of lead-acid battery (35Wh/kg), and the three times higher energy density makes lithium iron phosphate battery more favored than lead-acid battery for technical purposes.
lower cost
The large overcharge tolerance and self-balancing characteristics of lithium iron phosphate batteries can simplify the design of battery protection and balance circuit boards, and reduce costs. The one-step charging process allows LiFePO4 batteries to be charged using simpler conventional chargers instead of expensive professional Li-Ion battery chargers.
longer cycle life
Compared with the lithium cobalt oxide battery with a cycle life of 400 cycles, the cycle life of the lithium iron phosphate battery is extended to 2,000 cycles.
High temperature performance
Lithium cobalt oxide batteries are dangerous to operate at high temperatures (eg, 60°C). However, lithium iron phosphate batteries work better at high temperatures, providing an additional 10% capacity due to higher lithium-ion conductivity.
The main advantages of lithium iron phosphate batteries include:
●Like nickel-based rechargeable batteries (unlike other lithium-ion batteries), the discharge voltage of lithium iron phosphate batteries is very stable. The voltage remains around 3.2V during discharge until the cell is depleted.
●This allows the cells to provide almost full power before being fully discharged.
●It can greatly simplify or even eliminate the need for voltage regulator circuits.
● Lithium iron phosphate cells are extremely difficult to ignite when mishandled (especially during charging), although any fully charged battery can only dissipate the overcharged energy as heat. Therefore, it is still possible for the battery to fail due to misuse. Usually lithium iron phosphate batteries do not decompose at high temperatures.
• LiFePO4 cells have a slower rate of capacity loss (or longer shelf life) than Li-ion battery chemistries such as cobalt for lithium cobalt oxide or manganese for LiMn2O4.
● Because the energy density of lithium iron phosphate declines more slowly, after one year on the shelf, lithium iron phosphate batteries typically have roughly the same energy density as lithium cobalt oxide lithium ion batteries.
