In the previous four days of continuous use, the original translation of the document "Thermal runaway mechanism of lithium ion battery for electric vehicles: A review", the first author Xuning Feng. This article mainly collates several of the literature Points.
1 Power lithium battery, demand growth and energy density increase in parallel
For a long time in the future, as the battery energy density increases, the risk of thermal runaway will show an upward trend.
Figure 1. EV production and lithium-ion battery demand for electric vehicles.
Figure 2. Blueprint for the development of lithium-ion batteries for purely electric vehicles: The need for longer battery life and subtext is a material with lower thermal stability.
Figure 2 shows a roadmap for a lithium-ion battery for EVs. The goal is to achieve no less than 300 Wh·kg-1 at the battery level by 2020 and 200 Wh·kg-1 at the battery pack level, which indicates that electric The total range of cars can be extended to 400 km or more. To achieve this goal, the cathode material may have to change from LiFePO4 (LFP*) and Li'Ni1/3Co1/3Mn1/3'O2 (NCM111) to Ni-rich NCM cathodes. , such as LiNi0.6Co0.2Mn0.2O2 (NCM622), LiNi0.8Co0.1Mn0.1O2 (NCM811) or Li-manganese-rich oxide, etc., and the anode material may have to be changed from carbon (including graphite, including C) For a mixture of Si and C.
2 Observe the safety of lithium battery electric vehicles from the perspective of probability
From a probabilistic point of view, the self-induced failure of lithium-ion batteries is present, but at a very low level. Self-induced internal short circuits, also known as spontaneous internal short circuits, are considered as possible causes of failure of the Boeing 787 battery (Table 2 Accidents 4 & 5). For EV, the vehicle-level self-induced failure rate can be calculated by P = 1 - (1-p) ^ (mn), where P is the failure rate considering m EVs, where each EV battery The group contains n cells. Take Tesla Model S as an example, n = 7104. Suppose that the self-induced failure rate p of 18,650 cells is 0.1 ppm, then when the number of EVs is equal to m = 10,000, the failure rate P = 0.9992, indicating that the failure rate is about 1 out of 10,000 products. Compared with conventional cars (in the United States, there are 7.6 fire accidents per 10,000 fuel vehicles'13), the probability of an EV accident seems to be high. Low more.
3 Abuse of power lithium-ion battery
Machinery abuse
Under the action of the external force, the lithium battery cells and the battery pack are deformed, and the relative displacement occurs in different parts of the battery. This is the main external feature of mechanical abuse. The main forms for the battery core include collision, extrusion and puncture. Consider the battery pack level. , also need to consider vibration problems.
Deflection of the battery pack is likely to occur during a car collision. The placement of the battery pack on the EV affects how the battery pack responds during a collision '15'. The deformation of the battery pack can lead to dangerous consequences: 1) The battery diaphragm is torn and occurs Internal short circuit (ISC); 2) Leakage of flammable electrolytes and possible ignition. The study of battery pack extrusion behavior requires multiscale studies from material level, cell level to battery pack level.
The article summarizes the effects of the mechanical properties of materials on the consequences of mechanical abuse, and the various methods for predicting mechanical abuse using computer modeling simulations. Mechanical abuse often leads to internal short circuits, external short circuits, electrolyte leakage, and thus thermal effects. The process, therefore, the establishment of the mechanical-electrical-thermal coupling model in computer modeling is the most realistic form of the lithium battery mechanical abuse model, and it is also an urgent need for thermal runaway prediction. The computer simulation partner may wish to explore in this direction .
In machinery misuse, the most dangerous one is the puncture. When the conductor is inserted into the battery body, the positive and negative electrodes are directly short-circuited. Compared to the collision and extrusion, only a probabilistic internal short circuit occurs. The heat generation during the puncture process is more severe, causing thermal runaway. The probability is higher. Previously, puncture was considered an alternative test method for ISC. However, the repeatability of the needle test is being challenged by the battery manufacturer. Some people think that the lithium ion battery with higher energy density will never pass the standard. The nail puncture test. Improving the repeatability of the puncture test or finding an alternative test method is still an open and challenging issue for lithium ion battery safety research.
It is worth mentioning that in the January of this year after the publication of the article, the National Standard relating to the abuse of machinery, issued a draft of the “Safety Requirements for Lithium-ion Batteries for Electric Vehicles” for comments, recommending that the 'single acupuncture' test be suspended. This should be part of the 'change' that the author foresees.
Electrical abuse
The electrical abuse of lithium batteries generally includes external short circuit, overcharge, and over-discharge. Several of them are most likely to develop into thermal runaway.
External short circuit, when two conductors with differential pressure are connected outside the cell, an external short circuit occurs. The external short circuit of the battery pack may be due to deformation caused by a car collision, flooding, conductor contamination, or electric shock during maintenance. Compared with puncturing, usually, the heat released by an external short circuit does not heat the battery. From the external short circuit to thermal runaway, the important link in the middle is excessive temperature. When the heat generated by the external short circuit does not dissipate well, the battery temperature As it rises, high temperature triggers thermal runaway. Therefore, cutting off the short-circuit current or dissipating the excess heat is a method to suppress the external short circuit from further damage.
Overcharging, due to its full energy, is the most dangerous type of electrical abuse. Heat and gas generation are two common features in the overcharging process. Heat comes from Ohmics and side reactions. First, due to excessive lithium insertion, Lithium dendrite grows on the surface of the anode. The point in time when the lithium dendrite begins to grow is determined by the stoichiometric ratio of the cathode and the anode. Second, excessive deintercalation of lithium causes the cathode structure to collapse due to heat and oxygen release (oxygen release from the cathode of the NCA). '38'). The release of oxygen accelerates the decomposition of the electrolyte and generates a large amount of gas. As the internal pressure increases, the exhaust valve opens and the battery starts to vent. After the active material in the cell comes into contact with the air, a violent reaction occurs. A lot of heat. Overcharge protection can be done from both voltage management and material adjustment.
Figure 5. The results of overcharged TR in commercial lithium-ion batteries.
Over discharge, voltage inconsistencies between the batteries within the battery pack are unavoidable. Therefore, once the BMS fails to specifically monitor the voltage of any single battery, the battery with the lowest voltage will be over-discharged. Over-discharge abuse mechanisms and other Different forms of abuse, their potential danger may be underestimated. During overdischarge, the battery with the lowest voltage in the battery pack may be forcibly discharged by other batteries connected in series. During forced discharge, the pole reverses and the battery voltage becomes negative. Causes overheating of the overdischarged battery. Overdischarge induced dissolution of dissolved copper ions through the membrane and the formation of copper dendrites with a lower potential at the cathode side. As growth continues to increase, the copper dendrite may penetrate the membrane, causing severe ISC.
Figure 6. Overdischarge, internal short circuit due to dissolution and deposition of copper current collectors
Hot abuse
Local overheating may be a typical case of thermal abuse that occurs in the battery pack. Thermal abuse rarely occurs independently and is often developed from mechanical abuse and electrical abuse, and is ultimately a direct trigger for thermal runaway. In addition to mechanical/electrical In addition to overheating caused by abuse, overheating may be caused by loose contact connections. The loosening of the battery connection has been confirmed. Thermal abuse is also the situation that is currently being simulated the most. Use the device to control the heating of the battery to observe its response during heating. .
Internal short circuit
The internal short circuit, the direct contact of the positive and negative electrodes of the battery, of course, the degree of contact is different, and the resulting follow-up reaction is also very different. Usually the large-scale ISC caused by mechanical and heat abuse will directly trigger TR. In contrast, the internal self-developed internal short circuit , To a lesser extent, it generates little heat and does not immediately trigger TR. Energy release rate varies with the extent of diaphragm rupture and the length of time from ISC to TR. Spontaneous ISC is believed to be derived from the manufacturing process. Pollution or defects. It takes days or even months for the pollution/defects to develop into spontaneous ISCs. The mechanism during prolonged incubation is quite complicated.
Figure 8. Three-stage internal short circuit.
4 Overview of chain reactions during thermal runaway and energy release diagrams
The mechanism of TR can be explained by the chain reaction shown in Figure 9. Once the temperature rises abnormally under abuse conditions, the chemical reactions will occur one after another, forming a chain reaction. The heat-temperature-reaction (HTR) cycle is The root cause of the chain reaction. It needs to be clear that the abnormal heating causes the temperature of the cell to rise, and initiates side reactions, for example, SEI decomposition. The side reactions release more heat to form the HTR cycle. The HTR cycle circulates at extremely high temperatures until electricity. The core experiences TR.
Figure 9 shows the chain reaction mechanism of a lithium ion battery using a NCM/graphite electrode and a PE-based ceramic-coated separator in the TR process '70'. During the entire temperature rise, the SEI decomposes, the reaction between the anode and the electrolyte, The melting of the PE matrix, the decomposition of the NCM cathode, and the decomposition of the electrolyte occur sequentially. Once the ceramic coating of the diaphragm collapses, a large number of internal short-circuits instantaneously release the battery's electrical energy, causing TR to ignite the electrolyte. Figure 9 is just a chain reaction mechanism during TR Qualitative interpretation. In order to quantify the HTR loop of the chain reaction, the respective thermodynamics of various component materials are necessary.
Based on the previously reviewed TR mechanism '33,63,71', we present a graphical illustration of the chain reaction mechanism during TR, called the energy release diagram. This energy release diagram, first proposed in the literature, is used for quantitative consideration of thermal runaway development. The process of defining thermal runaway conditions.
Figure 9. Qualitative explanation of the chain reaction during thermal runaway.
The energy release map is described in detail as follows:
Take the LFP decomposition of the electrolyte as an example. The key features of the chemical reaction include the characteristic temperature, heating power (Q), which expresses the heat release rate and enthalpy (Δh), and 焓 represents the total energy released during the reaction. Characteristic temperatures include reactions from Temperature (Tonset), Peak Temperature (Tpeak), and Temperature Termination (Tend). The x-axis of Figure 10 shows the characteristic temperature, so the reaction zone is in a certain area in the horizontal direction. The color has a hilly area (green indicates LFP The chemical kinetics of the reaction decomposition of LFP and electrolyte is determined. The shape of the mountainous region is uniquely determined by Tonset, Tpeak, Tend, and Q. Q determines the height of the hill region, and Δh determines the vertical position of the mountain. According to the legend, all Chemical kinetics can be depicted in the energy release diagram in Figure 10, where the kinetics of all the different reaction processes can be compared.
It is necessary to emphasize a premise: This energy release diagram is for a 100% SOC cell, and the decomposition of the anode and cathode materials is considered in combination with the electrolyte.
Figure 10. Energy release diagram of a lithium-ion battery.
5 Improve battery resistance to thermal runaway
In the process of thermal runaway, what happens to the anode, what happens to the cathode, how does the diaphragm from shrinking to melting, and causes a large-scale internal short circuit. See (For continued, continued four) for details.
A discussion of how to prevent thermal runaway from causing catastrophic consequences. Starting from the aspects of improving the safety of the three major components of electrode materials, electrolytes and separators, various electrode modification methods, electrolyte additives and new electrolyte systems, and safer separators are described. Type (continued).
6 Reduce the risk of thermal runaway
Here is mainly from the perspective of controlling the spread of thermal runaway. The previous article "Robustness of Power Battery Pack Structure Design, Absolutely There Are Ways You Didn't Pay Attention to (completely)", the security involved in structural design, a large part It is also from the perspective of preventing the spread of thermal runaway. The article "Thermal runaway mechanism of lithium ion battery for electric vehicles: A review" on Energy Storage Materials' specifically raises questions about the escape time. The evacuation time of the car is less than 30 seconds, and the evacuation time of a bus with a length of 12 meters is 5 minutes. It is ensured that such an escape time is reserved to a certain degree to ensure that no one is trapped during the accident. Therefore, the serious TR is 5 It is not allowed to spread within minutes. 'This number can serve as a quantitative reference for our system design security.
7 Summary
The literature provides a comprehensive review of the mechanisms of thermal runaway of commercial lithium-ion batteries used in electric vehicles. It describes current research findings on thermal runaway phenomena, causes, and coping strategies. Abuses include mechanical abuse, electrical abuse, and thermal abuse. Internal short circuits are The most common features of all abuse conditions. Thermal runaway follows the chain reaction mechanism in which the decomposition reactions of the battery component materials occur one after the other. A reaction energy kinetics of all battery component materials is proposed. A new type of energy release diagram to explain the mechanism of chain reaction during thermal runaway. Two cases were used to further clarify the relationship between internal short circuit and thermal runaway. Finally, a three-level protection concept was proposed to help reduce the risk of thermal runaway.
