With the widespread use of lithium-ion batteries in people’s daily lives. The safety performance of lithium-ion batteries is getting more and more attention. For example, the battery explosion of digital camera manufacturer Nikon in 2005, the explosion of Nokia mobile phone battery in 2006 and the battery explosion of Apple’s mobile phone. These accidents have caused huge losses to people’s property and safety.

For larger power battery modules, the safety performance is more prominent. These accidents are mainly caused by the short circuit of the battery during use. Therefore, the safety performance is also a bottleneck restricting the development of lithium-ion batteries to high energy and large scale.

The needle penetration test uses a steel needle with a diameter of 3 mm to 5 mm to insert the battery. Itcausing a short circuit inside the battery, thereby studying the safety performance and heat generation of the battery. The article introduces the influencing factors of the lithium ion battery needle penetration test.

Factors Affecting Battery Needle Penetration Test

Battery Diaphragm

The main function of the separator in the lithium ion battery is to separate the positive and negative electrodes of the battery and perevnt short circuit caused by contact between the two poles. The material is PP film, PE film, two-layer PE/PP composite film or PP/PE/PP three-layer composite film. These materials are not electrically conductive, but have the ability to pass electrolyte ions. The thickness of the diaphragm, the closed cell temperature, the film rupture temperature, the aperture ratio, the direction of the stretch, the wettability, and the absence of coating all affect the safety of the battery to a large extent.

At present, in order to meet the requirements of small internal resistance and strong puncture resistance. The thickness of the lithium ion battery film used in the power tool is generally selected to be less than 25 μm. Besides, the porosity is 40% to 50%, and the pore diameter is 0.03 to 0.12 μm. Although the porosity is too high, the internal resistance of the battery can be reduced. However, the mechanical strength of the diaphragm is poor.

Importance

In the needling test, the diaphragm is easily broken. As a result, a large area contact between the positive and negative electrodes of the battery causes a short circuit. At the same time, when the battery temperature is high, the diaphragm is easy to shrink, posing a danger.

• When the porosity is relatively low, the internal resistance of the battery is relatively large. This will reduce battery power performance to some extent.
• The diaphragm has a large aperture, which easily causes the positive and negative pole contacts to cause a short circuit.
• However, if the aperture is too small, it will affect the internal ion conduction of the battery and affect the battery performance. Therefore, the selection of a separator having a suitable porosity and pore size plays a crucial role in the performance of the battery.

Effect of Electrolyte Flame Retardant on Battery Needling

When the battery is subjected to a violent reaction inside the needle, the reaction of the conductive agent and the binder with the electrolyte in the electrode occupies a large portion. It also determines the safety performance of the battery to a certain extent. Therefore, the selection of a suitable conductive agent and binder is critical to the safety of the battery.

Effect of Electrode Conducting Agent and Adhesive on Battery Needling

In addition to optimizing the various materials inside the battery to improve the safety of the battery. We can also improve the thermal stability and safety of the battery by improving the structural design of the battery.Cylindrical and soft-packed batteries have different needle penetration test results.

• Cylindrical batteries are easily pierced at different speeds of the needle, causing the battery to shoNrt-circuit and explode.

Because the positive and negative electrodes of the cylindrical battery and the diaphragm use a high pressure winding method. This will result in a small positive and negative gap. When the battery encounters a needle stick, the diaphragm cannot extend. The steel pin acts as a conductor to connect the positive and negative electrodes, thereby causing a short circuit.

• Soft pack batteries are greatly affected by the needling speed and are easier to pass through.

Because the soft pack battery is a layered structure, the gap between the positive and negative electrodes is large. When the battery encounters a needling, the diaphragm can extend or even wrap the steel needle. This will make the positive and negative poles not turn on, and will not cause the battery to short circuit or explode.

Therefore, in order to improve the needle safety performance of the battery when manufacturing the battery, it can be operated in the following manner:

• Appropriately increase the gap between the positive and negative electrodes;
• Try to increase the ductility of the diaphragm;

Summary