I. Analysis of the causes of low voltage failure in the formation process
1. Insufficient pre-formulation stage:
Impurity residue: When the pre-formation cutoff voltage is insufficient, impurities such as moisture and metal particles in the electrode cannot be effectively removed. The residual impurities will consume active lithium, resulting in a decrease in the initial coulombic efficiency and a drop in the voltage plateau.
SEI film defects: If the pre-formation voltage does not reach the reaction potential of the electrolyte additive, the formation path of the SEI film is disturbed, resulting in a loose or insufficiently dense film structure, which increases the lithium-ion transport impedance.
2. Incorrect charging cutoff voltage setting:
Excessive closed-circuit charging voltage: This leads to excessive lithium removal from the positive electrode active material during overcharging, lithium deposition and dendrite formation on the negative electrode, and even puncture of the separator, causing an internal short circuit and a sharp drop in voltage.
Insufficient charging voltage: If the lithium iron phosphate battery does not reach 3.65V, the lithium intercalation reaction of the graphite in the negative electrode is not fully activated, resulting in low utilization of the active material in the positive electrode and insufficient discharge capacity.
3. Abnormal contact with metallic foreign objects or interfaces:
Metal impurities are introduced: foreign objects such as metal dust (e.g., copper particles) are mixed into the positive electrode or between the separator, causing internal micro-short circuits and directly pulling down the voltage.
Poor electrode contact: Gas generated during formation is not discharged in time, resulting in uneven contact between the diaphragm and the electrode, increased local polarization, and the formation of lithium compound deposits (white spots).
4. Process parameters out of control:
Abnormal temperature: High temperature accelerates electrolyte decomposition, generating excessive CO and other gases, which affects electrode wetting; low temperature makes electrolyte desolvation difficult, resulting in insufficient lithium ion insertion.
Pressure loss: When no clamp pressure is applied, gas buildup increases the lithium-ion transport distance, leading to increased impedance and reduced capacity.
II. The impact of low voltage defects on battery cells
1. Deterioration in cycle performance:
Unstable SEI film or lithium dendrite growth can lead to increased battery internal resistance, intensified polarization at the charge and discharge ends, and a significant decrease in capacity retention after cycling (e.g., a 4.0V cutoff voltage battery has a capacity retention of only 88.6% after 800 cycles).
2. Security Risks:
Lithium dendrites in the negative electrode may puncture the separator, causing an internal short circuit and triggering thermal runaway; oxygen produced by the decomposition of the positive electrode accelerates the decomposition of the electrolyte, catalyzing gas production reactions, leading to battery swelling or even explosion.
3. Increased self-discharge:
When SEI film defects or metallic impurities are present, the negative electrode and electrolyte side reaction continuously consume lithium, and the battery static voltage decays rapidly (e.g., when the discharge cutoff voltage is <1.5V, the temperature rise rate reaches 20°C/s, and the voltage returns to zero).
4. Increased costs:
Low-voltage defective products need to be reworked or scrapped, which prolongs the formation time (e.g., the high-temperature formation step ① requires 40 minutes at 80°C to charge to 3.7V), increasing energy consumption and production costs.
