Analysis of the reasons for low voltage defects in lithium battery formation process and their impact on battery cells

Impurity residue: When the pre formation cut-off voltage is insufficient, impurities such as water and metal particles in the electrode cannot be effectively removed. The residual impurities will consume the active lithium, resulting in a decrease in the first coulombic efficiency and a decrease in the voltage plateau.

SEI film defect: If the pre formation voltage does not reach the reaction potential of the electrolyte additive, the formation path of the SEI film is disrupted, resulting in a loose or insufficient density of the film structure, increasing the lithium ion transport impedance.

High closed charging voltage: This can cause excessive lithium stripping of the positive electrode active material during overcharging, resulting in dendrite formation of lithium on the negative electrode, and even causing internal short circuits by piercing the separator, leading to a sharp drop in voltage.

Insufficient charging voltage: When the lithium iron phosphate battery does not reach 3.65V, the negative electrode graphite lithium insertion reaction is not fully activated, resulting in low utilization of the positive electrode active material and insufficient discharge capacity.

Introduction of metal impurities: Metal dust and other foreign objects (such as copper particles) are mixed between the positive electrode or separator, causing internal micro short circuits and directly lowering the voltage.

Poor electrode contact: The gas generated during the formation process is not discharged in a timely manner, the contact between the diaphragm and the electrode is uneven, local polarization increases, and lithium compound deposition (white spots) is formed.

Abnormal temperature: The high temperature accelerates the decomposition of the electrolyte, generating excessive gases such as CO, which affects the infiltration of the electrode; Low temperature causes difficulty in desolvation of electrolyte and insufficient insertion of lithium ions.

Pressure deficiency: When no fixture pressure is applied, gas accumulation increases the distance of lithium ion transmission, impedance increases, and capacity decreases.

The instability of SEI film or the growth of lithium dendrites can lead to an increase in internal resistance of the battery, intensified polarization at the end of charge and discharge, and a significant decrease in capacity retention after cycling (such as a 4.0V cut-off voltage battery with a capacity retention rate of only 88.6% after 800 cycles).

Negative lithium dendrites may puncture the separator, causing internal short circuits and leading to uncontrolled heat generation; The oxygen generated by the decomposition of the positive electrode accelerates the decomposition of the electrolyte, catalyzes the gas production reaction, and causes the battery to swell or even explode.

When SEI film defects or metal impurities are present, the negative electrode and electrolyte side reaction continue to consume lithium, and the static voltage of the battery accelerates the decay (such as when the discharge cut-off voltage is less than 1.5V, the temperature rise rate reaches 20 ° C/s, and the voltage returns to zero).

Low voltage defective products need to be reworked or scrapped, extending the formation time (such as high-temperature formation step ①, which takes 40 minutes to charge to 3.7V at 80 ° C), increasing energy consumption and production costs.