Internal short circuit is one of the most important topics in power battery safety research. At present, battery companies often use acupuncture to trigger internal short circuits to check battery safety. However, the needling experiments are not repeatable, so it is urgent to develop a controllable and repeatable internal short circuit research method. Previously, NASA, UL and other research institutions have proposed different methods of internal short circuit research, and the domestic voice in this field is very low, which is very inconsistent with China's position in the international lithium ion battery. Academician Ouyang Minggao's team at Tsinghua University has been working in the field of battery safety for many years. For the study of battery short circuit, the team creatively proposed to use the shape memory alloy's temperature-sensitive characteristics to embed the shape memory alloy inside the battery to achieve a controllable battery short circuit study. The results were published in the Journal of The Electrochemical Society under the title of Internal Short Circuit Trigger Method for Lithium-Ion Battery Based on Shape Memory Alloy.
One. Four internal short-circuit modes and experimental shape memory alloys
According to the current understanding, the short-circuit mode in the battery can be simply divided into four types: positive-negative short-circuit, positive-Cu current short-circuit, Al current-negative, and Al current-Cu current. Among the above four internal short-circuit modes, the positive-negative short-circuit is the most common, but it is generally considered that the most dangerous is the Al current-negative short-circuit. Specific explanation can be found in Analysis of internal short-circuit in a lithium ioncell, Journal of Power Sources 194 (2009) 550–557.
Shape memory alloy used in the experiment. The triangular area of the alloy remains horizontal under low temperature conditions, and the triangular arrow rises up at high temperatures. The alloy is placed at a specific position inside the battery during the battery manufacturing process, and the alloy can be activated by heating, and the triangle arrow is raised to trigger a short circuit in the battery.
two. The effect of experimental small soft-pack battery and shape memory alloy on battery electrical performance
Diagram of the process of placing the shape memory alloy inside the battery.
Effect of shape memory alloy on battery internal resistance and capacity.
In order to test the effect of the above-mentioned shape memory alloy triggering the internal short circuit, the author used NMC small soft-pack battery with a capacity of 1 Ah for verification. The specific information of the small soft-pack battery is shown in Table 1. It is worth pointing out that the shape memory alloy can be placed in a suitable position according to the needs of the experiment, and all four internal short-circuit modes can be realized. In addition, the introduction of shape memory alloys has no effect on battery internal resistance and capacity (Figure 4).
three. Shape memory alloy triggers battery Al current collector-negative short circuit
The shape memory alloy triggers the change in voltage and temperature of the battery's external center during the short-circuit process of the Al current collector-negative electrode of the battery.
The short circuit between the Al current collector and the negative electrode is the most likely to cause thermal runaway of the battery among the four classic short circuit modes. To this end, the authors used shape memory alloys to study the characteristics of the battery in short-circuit mode. The experiment was performed in a thermal insulation box. When the temperature inside the box reached 70 ℃, the shape memory alloy functioned and caused the battery to have an Al current collector-negative electrode short circuit. From the voltage and temperature curves of the two experiments in Figure 5, the maximum thermal runaway temperatures of the battery are 383 ℃ and 393 ℃, respectively. In Figure 6, it can be seen that the Al current collector-negative electrode short circuit causes the soft-pack battery to swell first, then spray a large amount of activity, and finally emit smoke.
four. Shape memory alloy triggers battery positive-negative short circuit
The shape memory alloy triggers the battery's positive-negative short-circuit voltage and changes in the battery's external center temperature.
As mentioned above, the positive-negative short-circuit is relatively minor among the four classic short-circuit modes. To this end, the author also studied the short-circuit behavior using shape memory alloys. It can be seen from the voltage and temperature curves in Figure 7 that the battery voltage slowly decreases after the positive-negative short circuit, and the maximum surface temperature of the battery does not exceed 80 ℃. Since the temperature around ℃ is different, it confirms the correctness of the danger ranking of the previous classic short circuit models. At the same time, the good repeatability of the method can be seen from the change trend of the voltage and temperature curves.
Fives. Acupuncture triggers battery positive-negative short circuit
Acupuncture experiments were performed on the same small soft-packed battery.
In order to further illustrate the advantages of using the shape memory alloy method to trigger a short circuit in the battery, the author performed three sets of tests on the battery using the traditional needling method. As shown in FIG. 8, the results of the three groups of tests are quite different. Only three of the three groups of batteries have experienced thermal runaway. The repeatability of the experiment is much lower than the shape alloy method proposed in this study.