Lithium iron phosphate (LFP) batteries are lithium-ion batteries that use lithium iron phosphate as the positive electrode material. The negative electrode is also graphite. The electrolyte is primarily lithium hexafluorophosphate. These batteries can be charged and used immediately regardless of their state, without needing to be fully discharged before recharging, making them the safest lithium batteries currently available.
Lithium iron phosphate battery charging and discharging principle
The charging and discharging reactions of lithium iron phosphate batteries occur between the LiFePO4 and FePO4 phases. During charging, lithium ions gradually detach from LiFePO4 to form FePO4, and during discharging, lithium ions insert into FePO4 to form LiFePO4.
When the battery is charging, lithium ions migrate from the lithium iron phosphate crystal to the crystal surface. Under the action of the electric field, they enter the electrolyte, then pass through the separator, and then migrate through the electrolyte to the surface of the graphite crystal, where they are then embedded in the graphite lattice.
Meanwhile, electrons flow through the conductor to the aluminum foil current collector at the positive electrode, then through the tab, the positive terminal of the battery, the external circuit, the negative terminal, and the negative tab to the copper foil current collector at the negative electrode, and finally through the conductor to the graphite negative electrode, thus balancing the charge at the negative electrode. After lithium ions are extracted from lithium iron phosphate, lithium iron phosphate is converted into iron phosphate.
When the battery discharges, lithium ions are extracted from the graphite crystals, enter the electrolyte, pass through the separator, migrate through the electrolyte to the surface of the lithium iron phosphate crystals, and then re-inserte into the lithium iron phosphate lattice.
Meanwhile, electrons flow through the conductor to the copper foil current collector at the negative electrode, then through the tab, the negative terminal of the battery, the external circuit, the positive terminal, and the positive tab to the aluminum foil current collector at the positive electrode, and finally through the conductor to the lithium iron phosphate positive electrode, thus balancing the charge at the positive electrode. After lithium ions are inserted into the iron phosphate crystal, iron phosphate is converted into lithium iron phosphate.
Features of lithium iron phosphate batteries
High energy density: According to reports, the energy density of square aluminum-cased lithium iron phosphate batteries produced in 2018 was around 160Wh/kg. In 2019, some excellent battery manufacturers were able to achieve a level of 175-180Wh/kg. Some outstanding manufacturers adopted stacking technology and made the capacity larger, which may have achieved 185Wh/kg.
Good safety performance: The positive electrode material of lithium iron phosphate battery has relatively stable electrochemical performance, which determines that it has a stable charge and discharge platform. Therefore, the battery structure will not change during the charge and discharge process, and it will not burn or explode. Even under special conditions such as short circuit, overcharge, compression, and puncture, it is still very safe.
Long cycle life: Lithium iron phosphate batteries generally have a 1C cycle life of 2,000 times, and even more than 3,500 times. The energy storage market requires more than 4,000-5,000 times to ensure a service life of 8-10 years, which is higher than the more than 1,000 cycles of ternary batteries. The cycle life of long-life lead-acid batteries is around 300 times.
