Reasons for swelling of lithium iron phosphate batteries
I. Overcharging causing swelling of lithium iron phosphate batteries
Overcharging causes all the lithium atoms in the positive electrode material to migrate into the negative electrode material, leading to the deformation and collapse of the originally full grid in the positive electrode. This is a major reason for the decrease in the capacity of lithium iron phosphate batteries. During this process, the number of lithium ions in the negative electrode increases, and excessive accumulation causes lithium atoms to grow into stumps and crystals, resulting in the swelling and bulging of the lithium iron phosphate battery.
II. Bulging caused by over-discharge
During the initial charge and discharge of a liquid lithium-ion battery, the electrode material and the electrolyte react at the solid-liquid interface, forming a passivation layer covering the surface of the electrode material. This passivation layer effectively prevents the passage of electrolyte molecules, but Li+ ions can freely intercalate and deintercalate through it, exhibiting characteristics of a solid electrolyte. Therefore, this passivation film is called the SEI (Sediment Electrode Intake).
The SEI film is not static; it undergoes slight changes during charging and discharging. Importantly, some organic components undergo reversible changes. Over-discharging of a lithium iron phosphate battery pack causes reversible damage to the SEI film. The destruction of the SEI, which protects the negative electrode material, leads to the collapse of the negative electrode material, resulting in the bulging phenomenon in lithium iron phosphate batteries.
III. Manufacturing Level Issues
Swelling in lithium iron phosphate batteries may be due to manufacturing issues with the lithium iron phosphate battery pack, such as uneven electrode coating or a rough production process.
4. Lithium iron phosphate batteries can also bulge if left unused for a long time. This is because air is conductive to some extent. Therefore, leaving the battery unused for too long is equivalent to the positive and negative terminals coming into direct contact, resulting in a slow short circuit.
