Although the battery diaphragm material is inside the battery and does not affect the battery's energy storage and output, its mechanical properties play a vital role in the battery's performance and safety performance. This is especially true for lithium-ion batteries, so battery manufacturers have begun to pay more and more attention to the performance of the separator when designing batteries. When the battery is designed, the separator will not affect the performance of the battery, unless the performance and safety of the battery are affected due to the uneven nature of the separator or other reasons.
Improper use of the battery (such as short circuit, overcharge, etc.) causes its temperature to rise, which may increase the resistance of the diaphragm by 2-3 orders of magnitude. The diaphragm is not only required to be able to interrupt the current at around 130°C, but also to maintain its softening integrity at higher temperatures. If the diaphragm can completely interrupt the current, the battery may continue to heat up during the overcharge test, resulting in thermal runaway. The softening integrity of high temperature is also very important for the safety performance of the battery during long-term overcharge or long-term exposure to high temperature environment.
Figure 1 is a typical short circuit curve of a 18650 lithium-ion battery with a current-blocking separator. The cathode material is LiCoO2 and the anode is MCMB carbon anode material. The battery does not have other safety devices that can act before the diaphragm interrupts the current, such as active current blocking devices (CID), positive temperature coefficient resistors (PTC), etc. At the moment when a small shunt resistor is used to short-circuit the battery externally, the battery starts to heat up due to a large current flowing through the battery. The current interruption function of the diaphragm acts at about 130°C to prevent the battery from progressing and heating up. The battery current begins to decrease. This is due to the increase in the internal resistance of the battery due to the current interruption function of the diaphragm. The current interruption function of the battery diaphragm prevents the thermal runaway of the battery.
Figure 1 - 18650 Li-ion battery short circuit curve
When the battery charging control system fails to feed back the battery voltage correctly or the battery charger is damaged, the battery will be overcharged at this time. When overcharge occurs, the lithium ions remaining in the positive electrode material continue to be extracted and inserted into the negative electrode material. If the maximum lithium insertion capacity of the carbon negative electrode is reached, the excess lithium will be deposited on the carbon negative electrode material in the form of metallic lithium, which greatly reduces the thermal stability of the battery. Because Joule heat is proportional to PR, the amount of heat generated will increase substantially under higher charging and discharging conditions. As the temperature increases, several exothermic reactions inside the battery (such as the reaction between lithium and battery electrodes, the thermal decomposition reaction of the positive and negative materials, and the thermal decomposition reaction of the electrolyte, etc.) may occur. The current interruption function of the separator works when the battery temperature reaches near the melting point of polyethylene, as shown in Figure 2. The CID and PTC of the 18650 lithium-ion battery are removed, and the diaphragm is left for an overcharge test. As in Figure 1, the current reduction is due to the increase in the internal resistance of the battery. Once the micropores of the separator collapse or close due to their softening, the battery can no longer be charged and discharged. If you continue to overcharge, although the diaphragm can maintain its current interruption characteristics, the battery is not allowed to heat up at this time.
Figure 2 - 18650 type lithium ion battery diaphragm current interruption function curve diagram during overcharging
To prevent internal short circuits, the diaphragm cannot allow any dendrites to penetrate. When an internal short circuit occurs in a battery, if this failure does not happen instantaneously, then the diaphragm is the only device that can prevent the battery from thermally running out of control. However, if the heating rate is too fast and the fault occurs instantaneously, the diaphragm will not be able to interrupt the current; if the heating rate is not very high, the current interruption function of the diaphragm can play a role in controlling the heating rate to further prevent thermal runaway of the battery.
During the acupuncture test, an instantaneous internal short circuit occurs when the nail is driven into the battery. This is because the current between the loop formed between the nail and the electrode generates a lot of heat. The contact area between the nail and the electrode varies according to the depth of the needle penetration. The shallower the needle penetration, the smaller the contact area, and the greater the local current density and heat generation. When the locally generated heat causes the electrolyte and electrode materials to decompose, thermal runaway will occur. On the other hand, if the battery is completely penetrated, the increase in the contact area will reduce the current density. Since the contact area between the electrode and the nail is smaller than the contact area between the electrode and the metal current collector, the internal short-circuit current is much larger than the external short-circuit.
Figure 3 is a needle prick test diagram of an 18650 battery with a separator current interruption function, in which the positive electrode material is LiCoO2 and the negative electrode material is carbon. It can be clearly seen that when the nail passes through, the voltage suddenly drops from 4.2V to 0V, and the temperature of the battery rises. When the heating rate is low, it will stop heating when the battery temperature is close to the current interruption temperature of the diaphragm [shown in Figure 3(a)]; if the heating rate is too fast, the battery will continue to heat up when the diaphragm current interruption temperature is reached, and the diaphragm current interruption will lose its function [Figure 3(b)]. In this case, the current interruption of the diaphragm is too late to take effect to prevent thermal runaway of the battery. Therefore, the role of the diaphragm in the simulated acupuncture and impact tests is only to delay the thermal runaway caused by the internal short circuit. The diaphragm with high temperature softening integrity and current interruption function needs to pass the internal short circuit test. Thin separators (<20μm) used in high-capacity batteries must also exhibit various properties similar to thicker separators. The mechanical strength loss of the separator needs to be balanced by the design of the battery, and the properties of the separator in the horizontal and vertical directions must also be consistent to ensure the safety of the battery during abnormal use.
At present, efforts are being made to research and develop the application of microporous membranes in battery separators, and gel electrolytes and polymer electrolytes are also being further explored. Especially in the development of polymer electrolyte, there is no volatilization of liquid organic electrolyte in the battery, so the safety of the battery is higher.
Figure 3-The relationship between the internal short-circuit voltage temperature and time during acupuncture simulation of the 18650 lithium-ion battery; (a) The battery passed the acupuncture test; (b) The battery failed the acupuncture test