High nickel lithium battery safety has become a consensus, but solid-state lithium batteries are now divided
An electric vehicle market that respects energy density has brought huge challenges to the safety of battery packs and complete vehicles. In 2018, there were 52 safety accidents per million electric vehicles in China. In terms of scenes, charging, driving, and parking are all scenes where safety accidents occur.
If the reasons are analyzed, 58% of fire accidents are caused by thermal runaway of lithium batteries. Nearly 90% of thermal runaway is caused by short circuits. At the cell level, the positive and negative materials, electrolyte, and diaphragm are the direct fuse for thermal runaway. After grouping, how to suppress thermal diffusion in structural design, cooling, and electrical control is related to whether the risk of thermal runaway can be reduced or stifled.
From October 16th to 17th, 2019, the 2019 China-Japan-Korea Next Generation New Energy Vehicle Battery Technology Conference was held in Shanghai. The conference is divided into two forums, the topics are battery thermal safety and solutions and solid-state battery key technology and industrialization challenges.
Forum 1, OEMs, power battery companies, well-known universities, laboratories, and testing institutions will discuss the causes and solutions for thermal runaway of high-nickel batteries as the specific energy level of power batteries continues to increase. Forum 2 is about the analysis of different solid-state battery technology routes and status quo.
System to see thermal safety
The full life cycle of a power battery starts from the selection of the material system, to the completion of the battery cell, the molding of modules and PACKs, the battery management after installation and application, to the use in vehicle operation.
The root cause of thermal runaway is the battery cell. The positive and negative electrodes are the "fuse" and the electrolyte is the "fuel storage". It only needs a "spark" to cause thermal runaway or fire.
"Sparks" either come from the inside of the cell or arise from the outside. Internal factors mainly refer to unstable factors generated during battery design and manufacturing; external factors mainly refer to reasons caused by personnel and external conditions during battery transportation, installation, and operation and maintenance.
The thermal safety failure of the battery is mainly caused by local overheating, which causes a short circuit inside the battery, or a micro short circuit causes damage to the battery diaphragm and a larger area short circuit.
Lithium-ion batteries have been upgraded from NCM111 and NCM523 to NCM622 and NCM811. The nickel content of the positive electrode ternary material continues to increase, the oxygen release temperature continues to drop, and the thermal stability of the positive electrode material is getting worse and worse. The decrease in oxygen release temperature means that the lithium battery is more heat-resistant. As the temperature increases, the positive electrode 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 caused by thermal runaway.
Electrochemical abuse is the most headache problem for battery cell factories. Under conditions of abuse such as thermal shock, overcharge, and overdischarge, the active material and electrolyte inside the battery will produce lithium dendrites, which pierce the diaphragm and cause an internal short circuit. Lithium evolution in the negative electrode is a major cause of the growth of lithium dendrites. Therefore, how to prevent lithium dendrites is an important issue.
The short circuit of the positive and negative electrodes caused by the failure of the diaphragm is an important part of 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 diaphragm. Moreover, the enemy facing the diaphragm is lithium dendrites, threatening its integrity and stability.
In addition to battery failure caused by internal short circuit, overcharge, battery aging, etc., mechanical failure under extreme conditions such as external short circuit, extrusion, fire, immersion, and simulated collision will also be converted into internal short circuit and cause electrical failure, which will eventually lead to thermal runaway .
Some failures and performance degradations that may occur during the battery's full life cycle will cause the batteries to be used beyond the safe use range and cause some safety accidents.
Battery factory and OEM 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 materials, separators, electrolyte, battery management, and PACK structure design.
For battery factories, look for high-pressure and high-temperature-resistant flame-retardant electrolytes, high-temperature-resistant single-crystal cathode materials, anode materials that inhibit lithium dendrites, or use NMC811 cathodes coated with safeners to improve dryness. The application of the French diaphragm introduces a ceramic diaphragm to suppress thermal runaway at the cell level.
For OEMs, paying attention to the safety of the battery itself is far from enough. In addition to the problems of the battery itself, battery electrical connection, mechanical safety, charging connection, daily use problems, and rapid handling of problems are the core of electric vehicle safety.
OEM's power battery safety protection system is designed and verified from four aspects: monomer, module, BMS and system. On the one hand, battery manufacturers themselves ensure safety from the design and manufacturing links. On the other hand, OEMs consider mechanical, electrical, and thermal safety from the perspective of module safety, such as safety clearance, force design, and protection.
In terms of assembly structure, OEMs must consider various operating conditions of the vehicle, as well as cooling pipelines, new cooling technologies, early warning of thermal runaway, and non-proliferation. At the same time, they must consider active fire extinguishing and how to extinguish fires through external structures.
OEMs generally think about how to improve the design of battery pack safety from the system level. Whether it is positive and negative electrode materials, electrolytes, diaphragms, the structural design, cooling, thermal management, and precautionary warnings of the PACK after the group 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 the cooperation of OEMs, battery factories, and testing institutions to analyze the mechanism of thermal runaway and explore new technologies to delay the occurrence of thermal runaway.
Different sounds of solid-state batteries
The forward movement of electric vehicles indicates that the specific energy standard of power batteries will not go backwards. 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-pressure-resistant solid-state electrolytes, hoping to solve the problem of the balance between specific energy and safety.
However, at this China-Japan-Korea conference, the views of the Chinese and Japanese guests on the research and application of solid-state batteries are very different, challenging the industry's inherent views on solid-state batteries. Relative to the concerted efforts of the high-nickel safety solution site, the solid-state battery site is moving forward in differences.
Japan’s 30-year solid-state battery expert Dr. Tadahiko Kubota, Japan’s former Toyota and Honda battery core expert Ogi Eiki, comments on the current state of solid-state battery research can be described as "pessimistic". It is quite difficult for solid-state batteries to be applied to electric vehicles. On the other hand, domestic battery factories such as Qingtao, Weilan, Huineng, Guoxuan Hi-Tech, the Chinese Academy of Sciences, Tongji University, and Shanghai Jiaotong University are all working tirelessly on solid-state batteries.
The opinions of Japanese experts can be summarized as follows: Toyota Sulfide is still in the research and development stage, and mass production is impossible with the current level of technology. Its original intention of developing solid-state batteries was to reduce batteries for hybrid vehicles. The outside world mistakenly believes that solid-state batteries are used in electric vehicles. This is the difference between Toyota's internal thinking and external public opinion.
In terms of safety, solid-state batteries can also produce lithium dendrites, and the safety is very worrying. And judging its safety cannot be judged by whether the electrolyte is flammable. The most important problem is the direct contact between the positive electrode and the negative electrode with high energy density.
All-solid-state batteries may increase energy density, one of the reasons is that external materials can be reduced. But this is not just a characteristic characteristic of all-solid-state batteries.
In terms of fast charging, Toyota's paper and most researchers have not confirmed any evidence that all solid-state batteries can be fast charged. They all said that lithium dendrites are formed during charging. The more people who understand all-solid-state batteries, the more they deny that it can be charged quickly.
Most of Toyota's patents in the past decade are related to impedance. It has been studying this problem since ten years ago, and it is still a big problem.
Views of domestic battery factories: 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. In terms of safety and energy density, solid-state batteries have been improved compared to conventional traditional lithium-ion batteries in the past. The premise is that we must have good technology to solve the problem of the interface, and ensure that the solid electrolyte can adapt to the battery design and meet the high ratio Energy battery requirements.
We believe that solid-state batteries do have advantages in some aspects. When the diaphragm and electrolyte are replaced with solid substances, it will have higher safety. When the safety threshold of the entire system is increased, this system can use high-potential positive and negative materials, such as lithium metal negative electrodes, and will have a higher energy density in the future.
The current thinking is to be compatible with existing lithium battery equipment and lithium battery technology as much as possible, and to reduce the cost as much as possible. Because solid-state batteries have high energy density and high safety, they may be used first in some special situations.
The energy density advantage of solid-state batteries is relatively not obvious at the cell level, and is more prominent at the PACK level. By 2021, solid-state batteries will use active materials with higher utilization rates, and the energy density at the cell level will be the same as that of liquid batteries, and then gradually surpass it.
Although domestic and overseas experts have disputes about the energy density and safety of solid-state batteries, they basically believe that the commercial application of solid-state batteries is a long process in order to solve some of the shortcomings of liquid batteries. Therefore, solid-state batteries can be imported 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.
