Lithium battery cells need to be aging process after manufacturing, which is a key step in the battery production process. The purpose is to stabilize the chemical and physical state of the cell through static and specific environmental conditions (such as temperature and time) to ensure its performance, safety and consistency. The following are the main causes and effects of battery cell aging:
The core purpose of battery aging:
1.Stabilize SEI film (solid electrolyte interface film) After the first charge and discharge (formation) of the battery cell, an SEI film will form on the negative electrode surface, but its structure is unstable at the beginning. During the aging process, the SEI film gradually improves and passivates, reducing side reactions in subsequent cycles (such as electrolyte decomposition) and improving cycle life.
2.Release internal stress After winding/stacking and injecting, the internal materials (such as electrodes and diaphragms) will generate stress due to mechanical deformation or uneven electrolyte infiltration. The aging process allows the stress to be slowly released by standing still to avoid diaphragm wrinkles or pole piece micro-short circuits due to stress concentration.
3.Exhaust gas A small amount of gas (such as CO₂, H₂) may be generated during the formation and injection process. During the aging period, the gas escape is accelerated by standing still or in a high temperature environment to prevent the battery cell from bulging or poor interface contact.
4.Screening defective cells During the aging process, the voltage, internal resistance and self-discharge rate of the cells are monitored to screen out abnormal cells (such as micro short circuits and electrolyte leakage) to prevent unqualified products from entering the subsequent processes.
5.Balanced electrolyte distribution After injection, the electrolyte needs to fully infiltrate the electrodes and diaphragm pores. Aging provides time for the electrolyte to be evenly distributed, reduce local dry areas, and improve ion transmission efficiency.
Specific forms of aging process
- Room temperature aging (room temperature standing) conditions: Standing at 25~30℃ for 24~72 hours. Function: Preliminary stabilization of SEI film, release of mechanical stress, and exhaust of gas.
- High temperature aging (accelerated aging) conditions: Standing at 45~60℃ for 12~48 hours. Function: Accelerate SEI film stabilization and gas escape, shorten production cycle, commonly used for high energy density or thick electrode cells.
- Charge and discharge cycle aging conditions: Perform 1~3 small current charge and discharge cycles on the cell. Function: Further activate electrode materials, optimize interface contact, and detect capacity consistency.
Impact of aging on battery performance
Improve cycle life: A stable SEI film can reduce the loss of active lithium and the continuous decomposition of electrolyte, delaying capacity decay.
Reduce self-discharge rate: Through aging, cells with excessive self-discharge (such as voltage drop > 5mV/day) are screened out to avoid thermal runaway of the battery pack due to differences in cells.
Improve safety: Discharging gas and releasing stress can reduce the risk of cell expansion and internal short circuit, and reduce the probability of thermal runaway.
Improve consistency: After aging, the voltage and internal resistance of the cell are more concentrated, which is conducive to battery pack matching and balanced management.
Risks of insufficient battery aging
If the aging time is omitted or shortened, it may lead to:
- SEI film instability: electrolyte is continuously consumed during the cycle, and the capacity drops.
- Gas residue: the battery cell swells, causing safety hazards (such as leakage of soft-pack batteries).
- Micro short circuit is not detected: local heating after grouping, accelerated aging and even thermal runaway.
- Uneven electrolyte infiltration: polarization increases and rate performance decreases.
Optimization direction of aging
- Dynamic aging monitoring: Use online monitoring systems (such as voltage, temperature, and air pressure sensors) to track the aging process in real time.
- Gradient temperature control: Adjust the temperature in stages (such as high temperature to accelerate gas discharge first, and then stabilize the SEI film at room temperature).
- Material and process improvement: Shorten the aging time by optimizing the electrolyte formula (such as adding film-forming additives) or pre-lithiation process.
Battery cell aging is a key step in balancing production efficiency and product quality. By adjusting time, temperature and environmental conditions, the reliability, safety and life of lithium batteries can be significantly improved. Batteries of different systems (such as lithium iron phosphate vs. ternary) and forms (cylindrical/square/soft pack) need to have targeted aging processes to achieve optimal performance.
