lithium ion battery cells 18650 are now being studied as thermoelectrics, but there are serious differences within the industry

　　Forum 1, OEMs, power battery companies, well-known universities, laboratories, and testing institutions discussed the causes and solutions of thermal runaway of high-nickel batteries as the specific energy level of power batteries continues to improve. The second forum is an analysis of different solid-state battery technology routes and current status.

　　System looks at thermal safety

　　The entire life cycle of a power battery starts from the selection of the material system, to the completion of the battery cell, the molding of the module and PACK, the battery management after the vehicle is installed, and the use in the vehicle operation.

　　The root cause of thermal runaway is the battery core. The positive and negative electrodes are "fuses" and the electrolyte is the "fuel reservoir". It only needs a "spark" to cause thermal runaway or fire.

　　"Sparks" may come from inside the battery core, or may be caused by external factors. Internal factors mainly refer to unstable factors generated during the battery design and manufacturing process; external factors mainly refer to causes caused by personnel, external conditions, etc. during battery transportation, installation, operation and maintenance.

　　The thermal safety failure of the battery is mainly caused by local overheating, which leads to an internal short circuit in the battery, or a micro short circuit that causes damage to the battery separator and a larger area of short circuit.

　　Lithium-ion batteries have been upgraded from NCM111 and NCM523 to NCM622 and NCM811. The nickel content of the cathode ternary material continues to increase, the oxygen release temperature continues to decrease, and the thermal stability of the cathode material becomes worse and worse. The decrease in oxygen release temperature means that lithium batteries are less heat-resistant. As the temperature increases, the cathode material changes from a layered structure to a spinel structure, and then forms rock salt and releases active oxygen. The growth of rock salt and the release of oxygen are the fundamental problems in thermal runaway.

　　Electrochemical abuse is the biggest headache for battery cell manufacturers. Under abuse conditions such as thermal shock, overcharge, and overdischarge, the active material and electrolyte inside the battery will produce lithium dendrites, pierce the separator, and cause an internal short circuit. Lithium evolution from the negative electrode is a major cause of lithium dendrite growth. Therefore, how to prevent lithium dendrites is an important issue.

　　Diaphragm failure leading to short circuit of the positive and negative electrodes is an important link in thermal runaway. When the safety film of the SEI film is destroyed, the electrolyte reacts with the electrode to generate heat, which will melt the separator. Moreover, the enemy facing the diaphragm is lithium dendrites, which threatens its integrity and stability.

　　In addition to battery failure caused by internal short circuit, overcharge, battery aging, etc., mechanical failure under very extreme conditions such as external short circuit, extrusion, fire, immersion, and simulated collision will also be converted into electrical failure caused by internal short circuit, eventually leading to thermal runaway. .

　　Some failures and performance degradation that may occur during the entire life cycle of the battery will cause the battery cells to be used beyond the safe use range, causing some safety accidents.

　　Battery factories and OEMs work together

　　The internal and external causes of thermal runaway require the cooperation of battery manufacturers and OEMs to provide an overall solution, including positive and negative electrode materials, separators, electrolytes, battery management, and PACK structural design.

　　For battery manufacturers, they are looking for high-pressure and high-temperature resistant flame-retardant electrolytes, high-temperature resistant single crystal cathode materials, negative electrode materials that inhibit lithium dendrites, or use coated NMC811 cathodes with added safety agents to improve dryness. The application of French diaphragm and the introduction of ceramic diaphragm can suppress thermal runaway at the cell level.

　　For OEMs, it is not enough to pay attention to the safety of the battery itself. In addition to the problems of the battery itself, battery electrical connections, mechanical safety, charging connections, daily use problems, and quick handling of problems when they occur are all the core of electric vehicle safety.

　　OEM's power battery safety protection system is designed and verified from four aspects: cell, module, BMS and system. On the one hand, the battery manufacturers themselves ensure safety from the design and manufacturing stages. On the other hand, OEMs consider mechanical, electrical, and thermal safety from the perspective of module safety, such as safety clearances, force design, and protection.

　　In terms of assembly structure, OEMs must consider the various working conditions of the entire vehicle, as well as cooling pipelines, new cooling technologies, early warning of thermal runaway, and anti-proliferation. At the same time, they must think about active fire extinguishing and how to extinguish the fire through the external structure.

　　OEMs generally think about how to improve the safety design of battery packs from the system level. Whether it is the positive and negative electrode materials, electrolyte, separator, the structural design, cooling, thermal management, and prevention and warning of the PACK after grouping, they are all the objects of OEM analysis.

　　The safety of lithium batteries is a big topic, which involves all aspects from materials, production to applications. Ensuring the thermal safety of electric vehicles requires full cooperation from OEMs, battery factories, and testing agencies. Starting from analyzing the mechanism of thermal runaway, we will explore new technologies to delay the occurrence of thermal runaway.

　　Different sounds of lithium ion battery cells 18650

　　The advancement of electric vehicles indicates that power batteries will not retreat in terms of energy standards. The application of high-potential positive and negative materials has become a trend, and NCM811 and silicon carbon anodes are increasingly appearing in the technical routes of battery factories. But the risk of fire still threatens the application of high-nickel batteries. Therefore, battery manufacturers and OEMs have turned their attention to flame-retardant, high-voltage-resistant solid electrolytes, hoping to solve the balance problem between specific energy and safety.

　　However, at this China-Japan-Korea Conference, Chinese and Japanese guests had very different views on the research and application of lithium ion battery cells 18650, challenging the industry's inherent views on lithium ion battery cells 18650. While the high-nickel safety solutions scene is working together, the solid-state battery scene is moving forward with divergence.

　　The views of Japanese experts can be summarized as follows: Toyota sulfide is still in the research and development stage, and mass production is impossible at the current technical level. Its original intention of developing lithium ion battery cells 18650 was to reduce the number of batteries used in hybrid vehicles. External misconceptions are that lithium ion battery cells 18650 are used in electric vehicles. This is the difference between Toyota's internal thinking and external public opinion.

　　In terms of safety, lithium ion battery cells 18650 can also produce lithium dendrites, and their safety is very worrying. And its safety cannot be judged by whether the electrolyte is flammable. The most important issue is the direct contact between the positive and negative electrodes with high energy density.

　　One reason all-lithium ion battery cells 18650 may increase energy density is by reducing external materials. But this isn’t just a property unique to all-lithium ion battery cells 18650.

　　In terms of fast charging, Toyota papers and most researchers have not confirmed any evidence that all-lithium ion battery cells 18650 can be charged quickly. They all said that lithium dendrites will form during charging. The more people know about all-lithium ion battery cells 18650, the more they deny that they can charge quickly.

　　Most of Toyota's patents in the past ten years are about impedance. It has been studying this issue since ten years ago, and it is still a big problem until now.

　　Domestic battery factory’s perspective: The spread of real fires is directly related to organic liquid electrolytes. Solid electrolytes, ranging from polymers to ceramic electrolytes, can improve battery safety to varying degrees. lithium ion battery cells 18650 have been improved in terms of safety and energy density compared to traditional lithium-ion batteries in the past. The premise is that we must have good technology to solve the interface problem and ensure that the solid electrolyte can adapt to the battery design and meet the high ratio. Energy battery requirements.

　　We think lithium ion battery cells 18650 do have advantages in some aspects. When the separator and electrolyte are replaced with solid substances, it will be safer. When the safety threshold of the entire system is raised, the system can use high-potential positive and negative electrode materials, such as metallic lithium negative electrodes, and will have higher energy density in the future.

　　The current idea is to be as compatible as possible with existing lithium battery equipment and lithium battery technology, and to reduce costs as much as possible. Because lithium ion battery cells 18650 have high energy density and high safety, they may first find some applications in some special situations.

　　The energy density advantage of lithium ion battery cells 18650 is relatively not obvious at the cell level, but is more prominent at the PACK level. By 2021, by using active materials with higher utilization rates, the energy density of lithium ion battery cells 18650 at the cell level will be equal to that of liquid batteries, and then gradually surpass it.

　　Although experts at home and abroad have disputes about the energy density and safety of lithium ion battery cells 18650, they basically believe that in order to solve some of the shortcomings of liquid batteries, the commercial application of lithium ion battery cells 18650 is a long process. Therefore, lithium ion battery cells 18650 can be introduced from the motorcycle and consumer electronics fields first, and then enter the electric vehicle field when the three dimensions of safety, performance, and cost are mature.

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