In the second step, the Li ions on the surface area of the electrode are gradually consumed. The transport speed of Li ions in the electrolyte is slower than the electrochemical reaction rate, and the Li ions on the electrode surface cannot be replenished in a timely manner. Macroscopically, this is manifested as a decrease in external short-circuit current until a plateau current is reached At this point, due to the constant presence of the external wire, polarization persists, manifested as voltage fluctuations around a plateau value.

The third step can be divided into two situations: if the self-protection mechanism of the battery is effective at this time, the current path will be cut off inside the battery. As it no longer forms a complete circuit, the macroscopic manifestation is that the short-circuit current decreases to zero. Since the circuit is cut off inside the battery, the external voltage testing device will no longer sense the voltage of the battery, resulting in a voltage decrease to zero.

If the external wires are manually cut off before the self-protection mechanism of the battery takes effect Due to the disconnection of the circuit, the short-circuit current becomes zero. At this time, the external voltage testing device can still sense the voltage of the battery. After the external current disappears, the polarization of the electrode will gradually disappear, manifested as the voltage rising to near the voltage value before the external short-circuit.

However, high rate discharge can lead to the generation of dead lithium, the formation of lithium dendrites, and damage to the internal materials of the battery, resulting in a decrease in battery capacity and an increase in safety risks.

About the impact of SOC on external short-circuit batteries. The reaction of low SOC batteries during external short circuit is the same as that of high SOC batteries, with the difference being:

• Due to the lower SOC, more Li ions are embedded in the positive electrode material, and there is less Li in the external solution. Therefore, in the first step, the peak current value that the battery can achieve will decrease. In terms of voltage, because polarization has a much greater impact on voltage than Li ion concentration on electrode potential (which can be understood as 10000 negative charges brought to the positive electrode by the external wire, it doesn’t matter whether 10 or 20 Li ions enter the positive electrode now), it is manifested as the voltage of different SOC batteries dropping to similar values

• Assuming that the heat required to trigger the self-protection mechanism of the battery is the same, according to Joule’s law Q=I ² Rt, the current of low SOC batteries is smaller than that of high SOC batteries, resulting in a longer critical time for low SOC batteries

The impact of temperature on external short-circuit batteries

• When the ambient temperature is high, the critical time of the battery will become shorter because the battery’s heat dissipation ability will deteriorate, and a large amount of heat will accumulate inside the battery, allowing it to reach a higher temperature in a short period of time

• For high SOC batteries, when there is an external short circuit, the current is larger, and the influence of external environmental temperature on heat dissipation capacity is limited. Therefore, it is manifested that the critical time of high SOC batteries does not change much at different temperatures

• High temperatures can increase the diffusion rate of substances, resulting in higher plateau current values and shorter critical times for batteries with the same SOC at high ambient temperatures

The impact of external short circuit time on the battery before activating the self-protection device

The longer the time, the longer the process of high rate discharge of the battery, and the greater the damage it causes to the battery When the Joule heat generated causes the diaphragm to fail, the battery will experience an internal short circuit The issues related to internal short circuits will be discussed later

3 Conclusion

So, in the above analysis, we have figured out the phenomena that batteries exhibit when they are short circuited externally and inferred the battery model for external short circuits There are still a few questions that need to be answered:

Firstly, will all batteries have a voltage plateau value of around 1 V after an external short circuit? No, the value of the voltage plateau is determined by the properties of the electrode material itself. In different battery systems, external short circuits can cause the battery to have different voltage plateau values

Secondly, whether an external short circuit will necessarily lead to the failure of battery safety performance (referred to as “leakage” in the article) is not certain. The process of external short circuit is the process of high rate discharge of the battery. Although it may cause damage to the internal materials of the battery and reduce its performance, it will not lead to safety failure in all batteries For power type batteries, the initial design goal is to be able to charge and discharge at high temperatures. High ambient temperatures and high SOC states increase the probability of safety failure during external short circuits