How Does the Work Lithium Iron Phosphate Battery Charge and Discharge?

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The lithium iron phosphate (LiFePO4) battery, also known as the LiFePO4 lithium-ion battery, operates by utilizing LiFePO4 as the positive electrode material and carbon as the negative electrode material. It has a single-cell rated voltage of 3.2V, with a charging cut-off voltage ranging from 3.6V to 3.65V. Among all lithium battery packs, it is the most environmentally friendly, has the longest lifespan, the highest safety, and the maximum discharge rate. The working principle is as follows:

  1. Positive Electrode Material: Lithium Iron Phosphate (LiFePO4) is the positive electrode material, where iron ions (Fe3+) are fixed in the lattice, and lithium ions (Li+) are extracted from the positive electrode material during charging and embedded back during discharging.
  2. Negative Electrode Material: Graphite serves as the negative electrode material, with a structure capable of accommodating and releasing lithium ions. During charging, lithium ions move from the positive electrode to the negative electrode through the electrolyte, being embedded into the graphite structure. During discharging, lithium ions exit the graphite, returning through the electrolyte to the positive electrode.
  3. Electrolyte: The electrolyte in the battery is typically an organic solution or polymer film. The electrolyte facilitates the movement of lithium ions between the positive and negative electrodes during the charge and discharge processes.
  4. Separator: The battery is equipped with a separator, which prevents direct contact between the positive and negative electrodes, thereby avoiding short circuits in the battery.
  5. Charging and Discharging Process: During charging, an external voltage is applied to the battery, causing lithium ions to move from the positive electrode, pass through the electrolyte and separator, and embed into the negative electrode material for energy storage. During discharging, when the battery is connected to an external load, lithium ions move from the negative electrode, pass through the electrolyte and separator, and embed into the positive electrode material, releasing energy.

In a lithium iron phosphate battery during charging, Li+ migrates from the (010) surface of the LiFePO4 crystal to the crystal’s surface. Under the influence of an electric field, it enters the electrolyte, traverses the separator, then migrates to the surface of graphene, and finally embeds into the graphene lattice. Simultaneously, electrons flow through the conductive material towards the aluminum foil electrode on the positive side, pass through the pole ear, battery pole column, external circuit, negative pole column, negative pole ear to the negative side’s copper foil collector, and then through the conductive material to the graphite negative electrode. This process balances the charge as lithium ions de-intercalate from LiFePO4, converting it into iron phosphate.

During discharging, Li+ de-intercalates from the graphite crystal, enters the electrolyte, traverses the separator, then migrates to the surface of the LiFePO4 crystal, and finally re-embeds into the (010) surface of the LiFePO4 crystal. Meanwhile, the battery’s charge flows through the conductive material to the copper foil collector on the negative side, passes through the pole ear, battery negative pole column, external circuit, positive pole column, positive pole ear to the positive side’s copper foil collector, and then through the conductive material to the LiFePO4 positive electrode. This process balances the charge on the positive side.

Chemical reaction equations for the lithium iron phosphate battery:

Positive electrode reaction: LiFePO4 → Li1-xFePO4 + xLi+ + xe- ;

Negative electrode reaction: xLi+ + xe- + 6C → LixC6;

Overall reaction: LiFePO4 + 6xC → Li1-xFePO4 + LixC6.

In summary, during charging in a lithium iron phosphate battery, lithium ions (Li+) in the positive electrode migrate to the negative electrode through a polymer separator. During discharging, lithium ions in the negative electrode migrate to the positive electrode through the separator. Lithium-ion batteries are named for the back-and-forth migration of lithium ions during charge and discharge. Lithium iron phosphate batteries have unique advantages, including high working voltage, high energy density, long cycle life, low self-discharge rate, no memory effect, environmental friendliness, and support for seamless scalability. They are well-suited for large-scale energy storage applications in renewable energy power stations, grid peak shaving, distributed power stations, UPS power supplies, emergency power systems, etc., demonstrating promising prospects.

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