Lithium-ion battery in the thermal runaway due to high temperature will cause the decomposition of the negative SEI film, the decomposition of the positive active material and the oxidative decomposition of the electrolyte, generating a large amount of gas, resulting in a sharp rise in the gas pressure inside the lithium-ion battery, causing the battery to explode, a large number of high temperatures The release of flammable and toxic gases from the battery can seriously threaten the safety of passengers and property. As the size and capacity of power batteries continue to increase, the gas released by thermal runaway will often multiply, so it is necessary Detailed analysis of the types and quantities of gases released by large-capacity power batteries in thermal runaway to take appropriate protective measures in the design and production of power battery packs.
Recently, SaschaKoch et al. of Daimler AG of Germany conducted detailed analysis on the types, quantities and influencing factors of gas released from thermal runaway of different capacity power batteries. 2, CO, H 2, C2H4, CH 4, C2H6And C 3H6It is the most common seven gases in the thermal runaway of lithium-ion batteries. There is no correlation between the concentration of different gases and the battery capacity. The capacity of the battery is closely related to the total amount of gas released by thermal runaway. The average capacity per Ah is 1.96L. The battery energy density has a significant effect on the thermal runaway trigger temperature. For every 1Wh/L increase in battery volume energy density, the battery thermal runaway trigger temperature drops by 0.42°C.
In general, the amount of gas generated by thermal runaway can be calculated by the following formula, where n is the molar amount of gas, p is the pressure of the gas, V is the volume of the gas, Rm is the ideal gas constant, and T is the absolute temperature, which is also At present, the most widely used method is adopted, but in fact, the gas in the thermal runaway process also has a very large temperature gradient inside the sealed container, which makes it impossible to accurately calculate the volume of the gas.
In order to solve this problem, Sascha Koch chose N 2As a standard gas, N 2The content in the air is 78.084%, usually we think N 2It is an inert gas that does not react in the thermal runaway of a lithium-ion battery, so we can compare the thermal runaway before and after N 2The concentration change is calculated to obtain the amount of gas generated by the thermal runaway of the lithium ion battery, as shown in the following equation. Gas For the number of corresponding gases, the empty volume inside the Vvoid container, C N2Vent And C gasVent For the heat out of control after the container N 2Concentration and concentration of the corresponding gas.
The mass of the gas is relatively simple, and can be calculated by using the volume and molar mass of the gas. As shown in the following formula, mgas is the mass of the gas, Mgas is the molar mass of the corresponding gas, and Vm0 is the molar volume of the ideal gas.
In order to obtain test data for different types of batteries, Sascha Koch tested a total of 51 power batteries, of which 41 were soft pack batteries and 10 were hard shell batteries, all of which were NCM/graphite systems, and the electrolyte lithium salt was LiPF6, and many types of solvents, including EC, DMC, DEC and EMC, the basic information of 51 kinds of batteries is shown in the following table. The 51 types of batteries include 'power type' batteries and 'energy type' batteries, the following figure shows The relationship between the volumetric energy density of the battery and the weight energy density, in which the green line segment is the fitting result, it can be seen from the figure that the volumetric energy density of the 51 power batteries is 2.38 times the weight energy density.
Compared to other kinds of gases, CO 2It has certain peculiarities. In order to simulate the thermal runaway of lithium-ion batteries in practice, the pressure vessel uses an ordinary atmospheric atmosphere, so the gas contains about 21% of O. 2Because the temperature of the gas released by the battery in the thermal runaway is high, most of the combustible gases will be associated with O. 2Reaction occurs, CO is produced twice 2. From the picture below CO and CO 2It can be seen from the concentration curve that at the beginning, the lithium ion battery generates very little gas, at this time CO 2The concentration is very high, but as the gas produced by the battery increases, CO 2The concentration drops rapidly, mainly because of the O inside the pressure vessel. 2The quantity is limited, as the number of combustible gases increases, O 2Exhausted, resulting in CO 2The concentration is also relatively reduced, eventually reaching a stable value, and the concentration of CO along with O 2The consumption is gradually increasing.
The figure below shows the seven gas concentrations that have the highest percentage of lithium-ion batteries released in thermal runaway, CO 2, CO, H 2, C2H4, CH 4, C2H6And C 3H6The ratio of the total concentration of released gas in the thermal runaway of the lithium-ion battery accounted for more than 99%. From the figure below, it can be seen that the most released gas in the thermal runaway is CO. 2, CO and H 2, the volume fraction reached 35.56%, 28.38% and 22.27%, followed by C 2H4And CH 4, the volume fraction reached 5.61% and 5.26%, respectively, the last two gases C2H6 and C 3H6The concentrations are low, 0.99% and 0.52%, respectively.
The gas released from the thermal runaway of lithium-ion batteries is mainly derived from the decomposition of active substances, electrolytes and binders, and the high CO content in the gas. 2The reason for the concentration, Sascha Koch believes that LiPF6 and solvent are mainly decomposed in the electrolyte at high temperature. We know that the positive electrode will decompose and release in the thermal runaway of lithium ion battery. 2, these O 2With O in the air 2Will react with the electrolyte to form CO 2In addition to the sources of CO and CO, there is a small amount of CO. 2Reduction occurs on the surface of the fully charged anode to form CO. H 2Mainly because the binder (such as PVDF, CMC) undergoes a reductive decomposition reaction at the negative electrode, C 2H4The gas is mainly from the decomposition of the SEI film, and the reaction of the EC solvent with the metal Li, and the decomposition of the DMC on the surface of the negative electrode produces CH. 4And C3H6.
From the previous research, it is found that there is no direct relationship between the concentration of different kinds of gases generated by the lithium ion battery in the thermal runaway and the amount of gas generated, but the volume of gas generated in the thermal runaway is closely related to the capacity of the lithium ion battery. The relationship (as shown in the figure below), by fitting the data, found that there is a linear relationship between the amount of gas generated by the lithium-ion battery in the thermal runaway and the battery capacity, and the average capacity of each Ah can produce 1.96L of gas.
It is not only the capacity that affects the thermal runaway process of lithium-ion batteries, but also the energy density has a significant effect on the thermal runaway of lithium-ion batteries. For example, from the following figure a, we can see that as the volumetric energy density of lithium-ion batteries continues to rise, lithium The thermal runaway trigger temperature of the ion battery is also continuously decreasing. From the fitting result, the thermal runaway trigger temperature of the battery will decrease by 0.42 °C for every 1Wh/L of the battery's volumetric energy density. See Figure b below. The higher the thermal runaway trigger temperature of the lithium-ion battery, the smaller the mass loss of the lithium-ion battery in the thermal runaway, and vice versa. From the above analysis, it can be seen that the higher the energy density of the lithium-ion battery, the more the battery thermal stability Poor, the more severe the thermal runaway.
The battery structure also affects the thermal runaway behavior of lithium-ion batteries. For example, it can be seen from the following figure that the mass of gas produced by the soft pack battery accounts for a higher proportion of the mass loss of the battery, while the mass of the gas generated by the hard shell battery accounts for the mass loss. The ratio is relatively low. This is mainly because the hard shell battery can accumulate more pressure inside, and finally release the gas along the pressure relief port. The high pressure gas carries some solid material out of the battery, resulting in an increase in the proportion of solid loss, while soft. The battery structure is low in strength, so the gas is more likely to leak, so it does not carry too much solid material away from the battery.
Sascha Koch's research shows that the gas produced by lithium-ion batteries in thermal runaway is mainly O. 2, CO, H 2, C2H4, CH 4, C2H6 and C 3H6Seven kinds of gases, accounting for more than 99%, the concentration of different gases is independent of the capacity of the battery, but the total amount of gas generated is closely related to the capacity of the battery. The average capacity per Ah is 1.96L, and the thermal stability of the battery is The energy density of the battery is closely related. For every 1Wh/L of the volumetric energy density of the battery, the thermal runaway trigger temperature of the battery will drop by 0.42 °C.
