Lithium iron phosphate (LiFePO4) batteries are lithium-ion batteries that use lithium iron phosphate (LiFePO4) as the positive electrode material and carbon as the negative electrode material. A single cell has a rated voltage of 3.2V and a charging cut-off voltage of 3.6V~3.65V. Lithium iron phosphate batteries have advantages such as high operating voltage, high energy density, long cycle life, good safety performance, low self-discharge rate, and no memory effect.
Composition of lithium iron phosphate batteries
The top electrode is LiFePO4 with an olivine structure, serving as the positive electrode of the battery. It is connected to the positive electrode by aluminum foil. To the left is a polymer diaphragm, which separates the positive and negative electrodes, allowing lithium ions (Li⁺) to pass through while electrons (e⁻) cannot. To the right is the negative electrode, composed of carbon (graphite), connected to the negative electrode by copper foil. The electrolyte is located between the top and bottom of the battery, which is then sealed in a metal casing.
Working principle of lithium iron phosphate batteries
During charging, lithium ions (Li) in a lithium iron phosphate battery migrate from the positive electrode to the negative electrode through the polymer separator; during discharging, lithium ions (Li) in the negative electrode migrate from the negative electrode to the positive electrode through the separator. Lithium-ion batteries are named for this migration of lithium ions during charging and discharging.
1. During battery charging, Li₂ migrates from the 010 facet of the lithium iron phosphate crystal to the crystal surface. Under the influence of the electric field, it enters the electrolyte, passes through the separator, and then migrates through the electrolyte to the surface of the graphite crystal, where it is embedded in the graphite lattice. Simultaneously, electrons flow through the conductor to the aluminum foil current collector at the positive electrode, through the tabs, battery terminals, external circuit, negative electrode terminal, and negative tab to the copper foil current collector at the negative electrode, and then through the conductor to the graphite negative electrode, thus achieving charge balance at the negative electrode. After lithium ions are extracted from lithium iron phosphate, lithium iron phosphate is converted to iron phosphate, and its crystal structure changes as shown in Figure 2 above.
2. During battery discharge, Li is extracted from the graphite crystal, enters the electrolyte, passes through the separator, and then migrates through the electrolyte to the surface of the lithium iron phosphate crystal. It then re-intercalates into the lithium iron phosphate lattice via the 010 facet. Simultaneously, the charge flows through the conductor to the copper foil current collector at the negative electrode, through the tab, the negative terminal, the external circuit, the positive terminal, and the positive tab to the aluminum foil current collector at the positive electrode, and then through the conductor to the lithium iron phosphate positive electrode, thus achieving charge balance at the positive electrode.
As we know from the working principle of lithium iron phosphate batteries, the charging and discharging process requires the joint participation of lithium ions and electrons, and the migration rates of lithium ions and electrons must be in balance. This necessitates that the positive and negative electrodes of lithium-ion batteries be mixed conductors of ions and electrons, and that their ionic conductivity and electronic conductivity must be consistent. However, lithium iron phosphate has very poor conductivity. While graphite anodes have better conductivity, to achieve high-rate discharge, the conductivity of the anode still needs to be improved to balance its electronic conductivity with the ability of lithium ions to intercalate and deintercalate from the graphite.
