How LiFePO4 Battery Works?

Structure of lithium iron phosphate batteries

Lithium iron phosphate battery, also known as LiFePO4 battery, uses LiFePO4 with olivine structure as the positive electrode of the battery. It is connected to the positive electrode by aluminum foil, with a polymer diaphragm in the middle that separates the positive and negative electrodes. However, lithium ions can pass through while electrons cannot. On the right is the negative electrode of the battery composed of graphite, which is connected to the negative electrode of the battery by copper foil.

The electrolyte is between the upper and lower ends of the battery, and the battery is enclosed in a metal shell, as shown in Figure 1. During charging, lithium ions in the positive electrode migrate to the negative electrode through a polymer diaphragm; During the discharge process, lithium ions in the negative electrode migrate to the positive electrode through a diaphragm, and lithium-ion batteries are named after the migration of lithium ions during charging and discharging.

Figure 1.

Battery charging

During charging, lithium ions, under the action of electric field force, enter the electrolyte from the surface of lithium iron phosphate crystals, pass through the diaphragm, migrate to the surface of graphite crystals, and then embed into the graphite lattice. After lithium ions are deintercaled from lithium iron phosphate, lithium iron phosphate is converted into iron phosphate, as shown in the following process:

Battery discharge

During discharge, lithium ions detach from graphite crystals, enter the electrolyte, pass through the diaphragm, migrate to the surface of lithium iron phosphate crystals, and then re embed into the lattice of lithium iron phosphate. The process is shown in the following figure:

The nominal voltage of a lithium iron phosphate battery cell is 3.2V, the charging voltage of the cell is 3.7V, and the discharge termination voltage is 2.5V. The battery module consists of a battery cell and a battery management system (BMS), usually consisting of 15 or 16 cells connected in series to form a 48V battery module.

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