Analysis of the causes of poor sealing of lithium batteries and on-site troubleshooting solutions

1. Cause Analysis

　　The "nail warping" problem in sealing nail welding is the main defect in sealing nail welding. Its essence lies in stress imbalance or improper mechanical constraint.

　　The defect of lifted nails is manifested as the sealing nails being tilted or half tilted as a whole after welding, with half of the area being higher or lower than the top cover height.

　　1. Insufficient mechanical constraints: uneven distribution or insufficient number of pre-weld points leads to stress concentration during welding;

　　2. The plastic nail is not flat: the plastic nail under the sealing nail is not flattened, and it is lifted up during welding, causing one side to warp;

　　3. Improper fitting method: Mechanical interference fit can easily cause glue overflow and welding stress accumulation.

2. Improvement measures 

       1. Optimize welding mode

　　(1) Multi-point pre-welding: Use three-point positioning pre-welding (one point every 120°) to evenly distribute the pre-welding points to reduce stress concentration during welding.

　　(2) Energy gradient control: Reduce the laser energy in the pre-welding and finishing stages to avoid local overheating that may cause deformation of the sealing pins.

　　2. Adjust the mechanical matching mode

　　(1) Use clearance fit: Use a clearance fit method in which the diameter of the injection hole groove is larger than the rivet to reduce the interference of welding on the rubber nail.

　　(2) Eliminating the glue pressing step: Eliminating the process of press-fitting glue nails to avoid the risk of tilting caused by squeezed glue overflow.

　　3. Process control

　　(1) Check the status of the glue nails: Before welding, use CCD detection or manual inspection to check the flattening status of the glue nails, and rework the glue nails that are not flattened;

　　(2) Fixture accuracy maintenance: Monitor the positioning accuracy of the fixture to ensure that the sealing pins are pressed flat and prevent uneven force caused by welding offset.

　　4. Parameter optimization and testing

　　(1) Intelligent parameter optimization: Combined with the annular spot dual-beam welding technology (center + outer ring power), the temperature gradient distribution is optimized and the stress concentration phenomenon is improved.

　　(2) Lifting nail defect detection: The top cover area height extreme value analysis method is used. If the height extreme difference in the top cover area is ≥0.2mm or the raised area accounts for more than 1/4, it is determined to be a lifting nail defect.

3. On-site investigation plan

　　(1) Check the distribution of pre-welding points: verify whether the number and position of pre-welding points meet the uniform distribution requirements;

　　(2) Measure the pressing height of the nails: Use a 3D line scan camera to detect whether the nails are pressed flat;

　　(3) Shielding gas flow monitoring: Ensure that the nitrogen flow is moderate to avoid air flow turbulence that aggravates weld stress.

　　Through the above comprehensive optimization, the defects of lifting nails can be effectively suppressed and the welding yield can be improved to more than 99.5%.