External short circuit generally refers to a short circuit caused by direct contact between the positive and negative electrodes of a battery. External short circuit (ESC) can cause temperature rise, and if it lasts long enough, it may damage the battery.

1 Test

18650 NCM battery was used in the test

In the first test, the ambient temperature of the battery was fixed at 25 ℃, and the variables were different SOC.

The testal data is shown in the following figure; The entire process of external short circuit of the battery is divided into two stages according to the change of current:

(1) The first stage: rapid rise stage. The current rapidly rises to its peak, while the voltage drops to a low level;

(2) Second stage: Continuous current stage. The current will decrease at a faster rate, and then a current plateau will appear. The current plateau is maintained for a period of time, the current will decrease to zero rapidly. The trend of voltage changes is the same, with a voltage plateau appearing when it drops to around 1V, and after a certain period of time, the voltage rapidly drops to zero.

The time required to achieve self-protection of the battery is set as the critical time.

Summarize the testal phenomena as follows:

During the rapid rise phase, except for 10% SOC, the current of the remaining batteries in other SOC states will rise to 60-80A in about 0.1 seconds, equivalent to discharge within the range of 30-40 C. During the rapid rise phase, the voltage of batteries in all SOC states will drop from 4.2 V to around 1.0 V in approximately 0.1 seconds.

During the sustained current phase, the higher the SOC of the battery platform, the higher the current, but the shorter the duration.

During the sustained high current phase, the higher the SOC, the shorter the duration of the battery voltage plateau. After the voltage drops to zero and is left to stand for 80-100 seconds, there will be a voltage rebound.

In the third test, the battery environment temperature was fixed, SOC was set to 20%, 50%, and 80%, and the variables were different external short-circuit times. The testal data is shown in the following figure, and the time required to achieve self-protection of the battery is set as the critical time. Summarize the testal phenomena as follows:

• Batteries with low SOC have a larger critical time point during external short circuits.
• After an external short circuit, the capacity of the battery has decreased.

The conclusions we can draw from three tests are:

Assuming no leakage occurs after a short circuit outside the lithium-ion battery

(1)At the same temperature, the higher the SOC of a battery, the higher the peak value of the short-circuit current during the rising phase at the moment of external short circuit (0.1s level); The higher the current plateau during the sustained current phase, the shorter the critical time.

(2)At the same temperature, at the moment of external short circuit (0.1s level), the short-circuit voltage drops to a certain value, and the voltage difference between batteries with different SOC after the drop is not significant.

(3)At the same temperature, batteries with higher SOC release less capacity during external short circuits.

(4)For the same SOC, the higher the temperature, the shorter the critical time for external short circuit of the battery; When the SOC is high, the critical time of the battery changes less at different temperatures.

(5)For the same SOC, the higher the temperature, the greater the value of the sustained current plateau.

(6)For the same SOC, the higher the temperature, the smaller the discharge capacity during the critical time after the external short circuit of the battery. After an external short circuit, the battery capacity decreases slightly.

During the external short circuit process, if the battery leaks (due to the failure of internal materials and the onset of thermal reactions)

(1)Batteries with higher ambient temperature and SOC are more prone to leakage during external short circuits.

(2)After the leakage, the value of the battery’s current plateau decreases, the peak temperature of the temperature rise increases, and the released capacity decreases.

2 Analysis

Analyzing the testal phenomena and conclusions, construct an external short circuit model for lithium-ion batteries (the following content is the author’s speculation, not testally proven, watch with caution)

The external short circuit of lithium-ion batteries is shown in the following figure: use a wire with a resistance of m Ω to connect the positive and negative terminals of the battery.

The premise is that there is no leakage during the external short circuit of lithium-ion batteries. As shown in the figure below, the internal reactions of high SOC batteries during the external short circuit process are divided into three steps.

The first step is that at the moment when the external wire contacts the positive and negative electrodes, the Li ions on the electrode surface area quickly complete the process of extraction and insertion Macroscopically, it manifests as the appearance of a large current.

Due to the accumulation of a large amount of negative charges on the positive electrode after connecting the external wire, the positive electrode potential decreases,; Similarly, the potential of the negative electrode increases due to the accumulation of a large amount of positive charges, which is a phenomenon of polarization (readers who are not clear about polarization can refer to the author’s previous article – Lithium ion Battery – Lithium ion Battery – The test voltage at the end of discharge is V1, and after standing for a period of time, the voltage is measured again as V2. Why is V2 greater than V1?)

Due to polarization, it manifests macroscopically as a rapid increase in voltage The speed control steps for this step are the electrochemical reaction rate and diffusion rate. The external short circuit of lithium-ion batteries is shown in the following figure: use wires with a resistance of m Ω to connect the positive and negative poles of the battery.