For lithium-ion battery safety tests, acupuncture tests are often the most distressing. The energy of the entire battery will be released in a short time through this short-circuit point (up to 70% of the energy will be released within 60 seconds). The temperature of the short circuit point rises sharply in a short period of time, which in turn triggers a chain reaction that leads to thermal runaway. Due to the sealed structure of the lithium-ion battery, our previous studies on the process of acupuncture of Li-ion batteries can only stay on the battery when they take the risk. Smoke, when fires and explosions and other external observations, the internal reaction process of lithium ion batteries during acupuncture can only be inferred. Therefore, most of the improvement strategies proposed here are based on 'reasonable assumptions'.
Recently, Tokihiko Yokoshima and others from Japan's Waseda University designed a method to directly observe the internal reaction of the lithium ion battery in the process of acupuncture, realizing the real-time observation of the internal structure of the pole piece in the lithium ion battery during the acupuncture experiment, and the process of internal gas production. And then better guide us for lithium ion battery safety design.
The observation method used by Tokihiko Yokoshima is shown in the following figure. The X-rays emitted from the point X-ray source pass through a specially-structured soft-pack battery and then imaged in the right X-ray camera and CT camera. The X-ray camera can achieve High-speed imaging of lithium-ion batteries inside, while CT cameras can achieve high-resolution imaging.
In order to facilitate the analysis of the effect of the short circuit process on lithium-ion batteries, Tokihiko Yokoshima equates the lithium-ion battery with the structure shown in the figure below. Each pair of positive and negative electrodes forms a battery cell, and multiple battery cells are connected in parallel to form a single battery cell. , Acupuncture experiments will cause these battery cells to have a short circuit, and the number of short-circuited battery cells is related to the number of battery poles piercing through. When only two electrodes are short-circuited, not only will the battery cell short circuit, but also Seriously, other battery cells connected in parallel with it will also be short-circuited through this short-circuit point, which means that the entire battery will pass through this short-circuit point and generate a lot of heat. From the structural features of lithium-ion batteries, we can easily see that The larger the battery capacity is, the smaller the short circuit point is, the more serious the consequences are. That is to say, the smaller the needle diameter used in the acupuncture experiment is, the slower the needle speed is, the higher the risk of thermal runaway will be.
The figure below shows the structure of a 60mAh small module (kn) above the 60mAh (ad), 420mAh (ej) and 860mAh cells obtained by computer tomography before and after the test. The lower capacity can be seen in the figure. The 60mAh battery left only pinholes after the acupuncture experiment, the cell structure did not change significantly, and the battery did not suffer thermal runaway. After the short circuit test of the 420mAh battery, the distance between the electrodes inside the cell increased significantly. In the acupuncture experiment, obvious gas production appeared in the battery, but it was not serious. In the 860mAh battery, a large amount of gas blew out from the inside of the battery during the acupuncture experiment. The battery also suffered from flatulence, and the cell could be seen from the CT map. The interlayer spacing between the internal electrode layers increased significantly, and the positive electrode of the first layer was completely damaged, indicating that the 860mAh capacity battery had a thermal runaway phenomenon inside the lithium ion battery during the acupuncture experiment.
The following figure shows the needle punching process of a 420mAh battery shot with X-rays. We see that as the needle is inserted into the battery within a distance of 0.2mm, a short circuit point is formed inside the battery, followed by the first and second layer electrodes in the battery core. The distance between them began to increase, indicating that at the moment the cell began to produce gas due to a short circuit, but after 200ms, the distance between the two electrodes began to fall again, and the gap between the electrode layers returned to its original size. From the shape of the steel needle At this point, the radius of curvature of the tip of the steel needle increased from 20um to 100um, indicating that the needle had become blunt at this time, mainly because the large current of the short circuit melted the sharp tip of the steel needle and disconnected the internal short circuit of the battery. The change in the external battery voltage can also confirm this result. During the whole process, the voltage of the battery was first reduced from 4.2V to 3.6V, then back to 3.8V, and stabilized at 3.8V. This shows that the battery first occurred during the acupuncture process. Short circuit, but then the short-circuit point was disconnected, indicating that the steel needle has been partially melted.
The following figure shows an image of a two-layer electrode of an 860mAh battery shot with X-rays. We have seen that after the short circuit in the lithium ion battery caused by the steel needle, the first five layers of the electrode are all affected and the short-circuited battery generates a lot of heat. The electrolyte between the first 5 layers of electrodes boils and vaporizes, the distance between the pole pieces increases significantly, and white smoke can be observed to leak from the short circuit point. The radius of curvature of the tip of the steel after the short-circuit test Increasing from 20um to 200um indicates that the current generated by the 860mAh battery during the short circuit is greater, but as the tip of the steel needle melts, the short-circuit point is also rapidly disconnected, and eventually the battery voltage tends to be stable.
The figure below shows the short circuit video of the 860mAh battery being punctured on the 7th layer. It can be seen that due to the blunt steel needle, the steel needle did not actually penetrate the pole piece at the beginning, but only caused the deformation of the electrode, and then the electrode was punctured. The shape becomes released, and at the same time, the high temperature of the short-circuit point also causes vaporization of the electrolyte between the pole pieces, resulting in an increase in the spacing between all the pole pieces. From the outside, it can be observed that the lithium ion battery emits white smoke. The voltage of the battery sees a rapid drop in the voltage of the battery after the short circuit occurs, but then the voltage rebounds and stabilizes, indicating that the short circuit point is rapidly disconnected after the short circuit occurs.
Although the battery tested in the above process was stable, the temperature of the battery and the steel needle continued to rise slowly. The battery still emitted white smoke. After 32 seconds, the pole piece began to move toward the steel needle. The depth of the tablet is getting bigger and bigger, and the smoke released by the battery is also getting bigger and bigger. The battery has flatulence and the temperature rapidly rises to 100°C. After 38s, the battery runs out of control, the temperature rises rapidly, and the battery voltage drops instantly. This shows that the initial battery The short-circuit point is not completely cut off. There is still a current passing through the short-circuit point to heat the electrolyte. After 32s, the vaporized electrolyte pushes the pole piece toward the needle, causing the resistance between the pole piece and the steel needle to rapidly decrease. The occurrence of a secondary short circuit eventually led to thermal runaway.
For a long time, our understanding of the lithium-ion battery acupuncture experiment is based on the external observation of the image and the collected voltage and other information to infer its internal response. For the first time, Tokihiko Yokoshima's method allows us to 'see directly'. The structural changes in the internal lithium-ion battery, let us have a more in-depth understanding of the entire process of acupuncture experiments, to help us design a more secure lithium-ion battery is of great significance.
