Why does the capacity of the LiFePO4 battery decrease?

Why does the capacity of the LiFePO4 battery decrease?

1. Structural changes of cathode material

The cathode material is an important source of lithium ion batteries. When lithium ions are removed from the cathode, in order to maintain the electrically neutral state of the material, the metal elements will be inevitably oxidized to a highly oxidized state, accompanied by changes in components. The transformation of components easily leads to phase transfer and changes in bulk phase structure. The phase transformation of electrode materials will cause changes in lattice parameters and lattice mismatch, and induced stress will lead to grain fracture and crack expansion of the material, resulting in mechanical damage to the material structure, which leads to the degradation of the electrochemical performance of lithium-ion batteries.

2. Structure of anode materials

Carbon materials and lithium titanate are commonly used materials for commercial lithium-ion batteries. In this paper, typical anode graphite is used for analysis. The decay of lithium-ion battery capacity is first seen in the formation stage, where SEI consumes some lithium ions on the surface of the cathode.
With the use of Li-ion batteries, the change of graphite structure also leads to the decrease of battery capacity. It is found that although the recycled carbon material maintains the morphology and structure of graphite, the half-height and width of the crystalline surface of carbon material increases, leading to the decrease of grain size in the c-axis direction, and the change of crystal structure leads to the cracking of carbon material, which destroys the SEI film on the cathode surface and promotes the repair of SEI film, and the overgrowth of SEI film consumes the active lithium, leading to the irreversible decay of lithium-ion battery capacity.

3. Oxidative decomposition of electrolyte

The performance of electrolyte has a significant impact on the specific capacity, lifetime, charge and discharge performance, operating temperature range and safety performance of lithium-ion batteries. Electrolyte importantly includes solvent, electrolyte and additives. The decomposition of solvent and electrolyte will lead to the loss of battery capacity, and the decomposition and side reaction of electrolyte is an important factor in the capacity decay of lithium-ion battery. With the recirculation of lithium-ion battery, the decomposition of electrolyte and the interfacial reaction with positive and negative electrode materials will lead to capacity decay, no matter what positive and negative electrode materials or processes are used.

4. Rapid charge and discharge

When fast charging, the current density is too high, the negative electrode is severely polarized, and the lithium-ion battery deposition is more obvious, making the copper foil brittle at the junction of copper foil and carbon active material and prone to cracking, and the spontaneous winding of the core is restricted by the fixed space, and the copper foil cannot extend freely to appear under pressure. Under pressure use, due to insufficient expansion space, the copper foil fractures and the original crack spreads and grows.

5. Long-term deep charging and discharging

Discharge should be transferred to the internal structure, one is the electrolyte is too easy to volatilize, the second is the lithium-ion battery negative overreaction will cause changes in its dielectric film, resulting in a decline in delamination capacity, resulting in permanent loss of capacity.

Charging important from the voltage stability and nighttime grid voltage rise, has stopped the charger, voltage rise, will lead to overcharging the battery, resulting in structural changes in the cathode material, capacity loss, decomposition and oxygen release, electrolyte severe oxidation reaction, and then combustion explosion; electrolyte organic solvent / electrolyte lithium salt decomposition; negative lithium over-discharge may lead to negative copper trap dissolution, the formation of copper dendrites on the cathode.

6. Temperature

Temperature is undoubtedly one of the key factors affecting the life of lithium-ion batteries. Too high or too low temperature will lead to the reduction of active lithium ion content, thus reducing the life of lithium-ion batteries.


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