The precision control of the liquid injection process for lithium battery packs directly affects the capacity, cycle life and safety of the batteries. As the injection volume of electrolyte, wetting effect and environmental control are crucial to battery performance, systematic control is required from the dimensions of equipment, process parameters, environmental management, process monitoring and quality inspection. The following are the specific methods:

Precise control of the electrolyte injection volume

Design of Quantitative Liquid Injection System

High-precision metering pumps or injection needle valves are adopted, and the injection volume is adjusted in real time through a closed-loop feedback system. For example, the pressure changes in the liquid injection pipeline are monitored by pressure sensors, and the pumping speed is dynamically corrected in combination with the data from the flowmeter to ensure that the error of the liquid injection volume of a single battery is controlled within ±0.1%.

Optimization of liquid injection speed

Adjust the injection speed according to the battery capacity and the viscosity of the electrolyte. For large-capacity batteries (such as those above 200Ah), a segmented liquid injection strategy is adopted: in the initial stage, rapid liquid injection (such as 50mL/s) is used to fill large pores, and in the later stage, the flow rate is reduced (such as 10mL/s) to avoid bubble residue.

Multiple liquid injection and standing process

Adopt the "small amount and multiple times" liquid injection method. After each injection, let it stand for 10 to 15 minutes to ensure that the electrolyte fully wets the electrode plates. For instance, injecting the liquid in three installments, with 30%, 30%, and 40% of the total amount injected each time, can reduce the risk of lithium evolution caused by residual electrolyte.

2. Optimization of electrolyte wetting effect

Vacuum-assisted infiltration technology

After liquid injection, place the battery in a vacuum chamber and accelerate the penetration of the electrolyte by vacuuming (vacuum degree ≤-95 kpa). For example, a certain enterprise adopts the "liquid injection - vacuum - pressurization" cycle process (vacuum maintenance for 5 minutes → pressurization to 0.2MPa and maintenance for 3 minutes), which shortens the electrolyte wetting time by 40%.

Coordinated control of temperature and time

Adjust the immersion temperature (usually 25-45℃) and time (4-24 hours) according to the composition of the electrolyte. For instance, when an electrolyte containing LiPF6 is immersed at 35℃ for 12 hours, the interfacial impedance can be significantly reduced.

Porosity matching of electrode sheets

By adjusting the compaction density of the electrode sheet (1.5-1.8g/cm³) and the coating thickness (80-120μm), and optimizing the porosity of the electrode sheet (30%-40%), the rapid penetration of the electrolyte is promoted.

3. Liquid injection environment and sealing control

Low-humidity environmental management

The humidity in the liquid injection workshop should be controlled at ≤1%RH, and a combination of molecular sieve rotor dehumidifiers and nitrogen protection should be adopted. For example, a certain factory minimizes the influence of moisture on the SEI film through a three-stage dehumidification system (dew point ≤-70℃).

Battery sealing performance testing

After liquid injection, the battery leakage rate (≤1×10⁻⁸Pa·m³/s) was detected by a helium mass spectrometer leak detector, and the sealing performance was verified through a negative pressure holding test (vacuum to -80 kpa, hold pressure for 30 minutes, pressure drop ≤5kPa).

Cleanliness control of the liquid injection port

Before liquid injection, treat the battery liquid injection port with a plasma cleaner to remove organic contaminants. Seal immediately after liquid injection to prevent the electrolyte from evaporating or absorbing moisture.

4. Online monitoring and automated adjustment

Visual inspection system

Install a high-speed camera at the liquid injection station to monitor the position of the liquid injection needle (with an accuracy of ±0.05mm) and the height of the electrolyte level in real time, and automatically correct the offset.

Weight feedback closed-loop control

The battery is weighed in real time through a high-precision electronic balance (with a resolution of 0.1mg), compared with the preset liquid injection volume, and the pumping parameters are dynamically adjusted. For example, when the weight deviation exceeds 0.5%, the system automatically compensates for the liquid injection volume.

Data traceability and analysis

Record the parameters such as the liquid injection time, temperature and pressure of each battery, and conduct a correlation analysis in combination with the capacity separation data of the battery cells (capacity and internal resistance) to optimize the process window.

5. Exception Handling and quality verification

Emergency treatment for electrolyte leakage

If electrolyte leakage is detected, immediately isolate the battery and transfer it to a dedicated recovery device to prevent the electrolyte from corroding the equipment or causing a fire.

Non-destructive testing of infiltration effect

The interior of the battery was scanned by industrial CT to observe the uniformity of the electrolyte distribution (porosity ≤2%), or the interface impedance was tested by electrochemical impedance spectroscopy (EIS) (≤50Ω).

Long-term performance verification

The batteries optimized by the liquid injection process were subjected to high-temperature storage (60℃ for 7 days) and cycle tests (500 charge and discharge cycles at 1C) to verify the capacity retention rate (≥90%) and thickness expansion rate (≤10%).

Summary

The precision control of the liquid injection process for lithium battery packs needs to be optimized in a coordinated manner from five aspects: equipment precision, process parameters, environmental management, process monitoring and quality verification. Through measures such as quantitative liquid injection, vacuum immersion, low-humidity environment, online monitoring and long-term performance verification, the consistency and safety of batteries can be significantly improved. For instance, a certain enterprise reduced the capacity dispersion of battery packs from ±3% to ±1.5% through the above-mentioned methods, increased the cycle life by 20%, and decreased the defect rate from 0.8% to 0.2%.