TECLAB Ding Xia shaw * teclab.cn

Summary:The non-contact non-destructive testing of internal defects in lithium batteries has become increasingly important with the booming development of the new energy industry. This article introduces the basic principle of air coupled ultrasound imaging detection technology and tests Apple phone batteries, soft pack batteries, blade batteries, and lithium battery positive and negative electrode materials separately. The results show that this technology has a very broad application prospect in the lithium battery detection industry.

key word:Air coupled ultrasound, lithium battery testing, non-contact ultrasound testing for lithium deposition

Background:Lithium batteries, as a new type of green energy, are widely used in various fields such as electronic products and new energy vehicles due to their advantages of high energy and long lifespan. It is common to encounter combustion or explosion accidents during the production and service of lithium batteries in daily life, so non-contact non-destructive testing of lithium batteries is crucial. Air coupled ultrasound imaging detection technology is a relatively low-cost, non-contact, and non-destructive imaging detection technology for the interior of batteries, as it does not require coupling agents or immersion of lithium batteries in coupling liquids. It is harmless to humans and the environment.

Principle:At room temperature and pressure, the sound velocity and density of water are 1480 m/s and 1x10 ^ 3 kg/m ^ 3, respectively. The sound velocity and density of air are 340 m/s and 1.3 kg/m ^ 3, respectively. The sound velocity and density of steel are 5900 m/s and 7.85x10 ^ 3 kg/m ^ 3, respectively. Using formula 1-2, it can be concluded that when the steel plate is vertically incident from the air, the sound intensity transmittance is about 3.4x10 ^ -5; When the steel plate is vertically incident from water, the sound intensity transmittance is about 1.3x10 ^ -1. By comparison, it can be seen that the transmittance of the air coupling method is 1/10000 of that of the water immersion method

Acoustic impedance calculation formula (1):

Formula for calculating sound intensity transmittance (2):

From this, it can be seen that there is a very high difference in acoustic impedance between air and the material being tested, and high-frequency sound waves will be absorbed over a very short distance when propagating in air. Therefore, air coupled ultrasound generally uses probes with frequencies ranging from tens of KHz to hundreds of KHz for detection, transmitting and receiving one at a time.

Figure 1. Reflection and transmission models of sound waves vertically incident on lithium batteries

Z1 and Z2 respectively represent the acoustic impedance of air and lithium battery, R12 is the reflectivity from air to lithium battery, and R21 is the emissivity from lithium battery to air. T12 is the sound wave transmittance from air to lithium battery, and T21 is the sound wave transmittance from lithium battery to air. T121 is the transmittance from air to lithium battery, then to air, T12121 is the transmittance from air to lithium battery, then to air, then to lithium battery, and finally to air. The density of lithium batteries is generally about 1.87 x 10 ^ 3 kg/m ^ 3, and the speed of ultrasound in lithium batteries is about 3000 m/s.

According to the formula, T12=1.9998, T21=0.00015, T121=3.151X10 ^ -4, T12121=9.929X10 ^ -8

That is to say, when there are bubbles or delamination defects in the battery, air coupled ultrasound is almost unable to penetrate the normal area.

Figure 2. AIRSCAN+air coupled ultrasound imaging detection system used in the test

Experiment 1: Apple 6 phone battery with a charging capacity of only 30% after 2 years of use

Figure 3. Imaging results of Apple phone battery with only 30% remaining charging capacity after 2 years of use

From the imaging image, it can be seen that the areas with low sound transmittance (blue/green) occupy the majority of the area. Only about 30% of the area with high sound transmittance (red) remains.

Experiment 2: Peeling of Blade Battery Shell

Figure 4. Blade battery air coupled ultrasonic testing

Figure 5. Comparison of air coupled ultrasonic testing results for two blade batteries

The imaging results indicate that the areas with low sound intensity and transmittance are distributed at the edges of the battery, and this defect is likely to occur between the blade battery casing and the battery body. After pressing the battery for a period of time and retesting, it was found that the shape and position of the defect had changed, confirming the previous judgment.

Experiment 3: Lithium Evolution in Soft Pack Batteries

Figure 6. Ultrasonic testing results of air coupling for soft pack batteries

One of the reasons for lithium deposition in lithium batteries caused by rapid charging is that the location and area of lithium deposition can be clearly identified through imaging detection of the soft pack battery after rapid cycling charging.

Experiment 4: Quality inspection of positive and negative electrode welding of lithium batteries

Figure 7. Air coupled ultrasonic testing of welding quality of positive and negative electrodes of lithium batteries

chart8Ultrasonic welding of positive copper foil for lithium batteries (top) and negative aluminum foil (bottom), quality of ultrasonic welding, air coupled ultrasonic testing

Conclusion:

Air coupled ultrasound imaging technology for detecting lithium batteries has good imaging effect, low cost and no radiation. As a non-contact, non-destructive and harmless detection method, it can be widely used for quality control in the lithium battery industry. However, there are various types of internal defects in lithium batteries, such as bubbles, wrinkles, slurry impurities, lithium deposition, etc. It is necessary to accumulate a large amount of testing experience and analyze the causes of defects to provide feedback to the process.