According to data from the State Administration for Market Regulation, at least 40 fire accidents involving new energy vehicles occurred in China in 2018. Since April this year, electric vehicle fire and smoke accidents have occurred one after another. Power battery safety is a sensitive topic, and It is a topic that cannot be avoided.

　　Recently, Wang Zidong, deputy secretary-general of the China Power Battery Innovation Alliance and deputy secretary-general of the China Electric Vehicle Charging Infrastructure Charging Alliance, conducted a multi-dimensional analysis of electric vehicle safety issues at the first China International Electric Vehicle Safety Technology Innovation Conference. He believes that before there is an obvious technological breakthrough in power battery materials, it will be difficult to make further breakthroughs after the specific energy develops to a certain level. At the same time, the negative impact on security is growing. Before mastering the fire rules of lithium batteries, controlling the balance between energy density, safety and long life is an issue that cannot be ignored.

　　The battle over power battery technology routes

　　This problem has been around for a long time, and it is also the game between energy density and safety.

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　　Wang Zidong said, "We must admit that the battery pack is a component containing high-energy substances and is inherently dangerous. Moreover, as the specific energy and specific power of the battery increase, the risk of accidents will increase."

　　Among the many technical routes for lithium batteries, the duel between lithium iron phosphate and ternary is the most stalemate.

　　Lithium iron phosphate has high safety and long life, but its energy density is not as high as that of ternary, but this can be compensated by increasing the battery capacity. Its low-temperature performance is poor, mainly because of its low low-temperature performance and poor material consistency in small-capacity batteries.

　　Ternary batteries have high energy density, good consistency, and good low-temperature performance, but are slightly less safe and have far less cycle life than lithium iron phosphate batteries.

　　"Currently, China has the most mature industrial chain for lithium iron phosphate batteries, and we have mastered many core technologies in related fields, while ternary batteries, represented by Japan and South Korea, are relatively more mature." Wang Zidong believes that this This kind of duel between technical lines is more like China vs. Japan and South Korea.

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　　If evaluated solely from the performance of the power battery itself, he listed 10 dimensions: 1. Safety of the battery pack, 2. Energy density of the battery pack (not a single cell), 3. Cycle life of the battery pack, 4. The cost of the battery pack, 5. Charging rate, 6. Battery cell consistency, 7. Low temperature performance, 8. Group utilization, 9. Convenience of recycling and reuse, 10. The positive and negative electrode materials can be recycled, repaired and reused. .

　　As a technical route that can lead the trend, it cannot have too obvious shortcomings in any of the above aspects. It needs to achieve a balance in all aspects to be a feasible route, rather than a single performance index being high. For example, energy density.

　　Therefore, from the analysis and comparison of the above 10 aspects, in this duel, Sanyuan and lithium iron phosphate fought fiercely and painfully. There were wins and losses, and there were also ties. After these 10 duels, Wang Zidong’s personal referee gave a Simple final conclusion: where safety is a concern and energy density requirements are not very high, lithium iron phosphate batteries are the first choice.

　　Where is the ceiling of power batteries?

　　In Wang Zidong’s view, it is necessary to have a clear understanding of improving the energy density of power batteries: to improve the performance of power batteries for electric vehicles that can be industrialized, it is not only necessary to significantly improve the performance of positive and negative electrode materials, but also needs to be improved in many aspects. Only with relatively large breakthroughs can it be possible to achieve real improvements in power batteries.

　　So from the definition of industrialized power batteries, the goal proposed by the country is: by 2020, electric vehicles can run 400km on a single charge, and the specific energy of a single battery reaches: 300Wh/kg (350), 600Wh/L (700), 0.6 yuan/Wh, the battery system reaches: 220Wh/kg (260), 300Wh/L (380), 1.0 yuan/Wh, cycle life 1500 times (80% DOD).

　　Wang Zidong said that judging from the indicator data, it is still relatively difficult to achieve these indicators.

　　The current product overview of domestic power battery companies is: in terms of lithium iron phosphate, the energy density of large-scale production of energy-type lithium iron phosphate power batteries is roughly between 140-180Wh/kg. In terms of three raw materials, the energy density of large-scale production of ternary cathode lithium-ion power batteries for pure electric drive is roughly between 180-260Wh/kg.

　　From a technical point of view, if the specific energy of the battery pack reaches 260Wh/kg, based on the energy consumption calculation of 10KWh/kg/100km, the weight of the 40KWh battery pack cell of an electric vehicle traveling 400km cannot exceed 99.5kg, and the total weight of the battery pack cannot exceed 153kg, the specific energy of the flexible packaging battery needs to exceed 402Wh/kg, and the difficulty can be imagined.

　　It can be deduced from this that a battery with a specific energy of 350Wh/kg (if it can be produced) needs to be made into a large-capacity (above 80Ah) aluminum alloy hard-shell power battery, which can save the weight of 40KWh after modularization. The total weight of the battery core must be controlled within 114.3kg, which can only account for 74.7% of the weight of the battery pack. The remaining aluminum alloy box (25kg), thermal management system (2kg), connectors and fixings (11.7kg), etc. The weight cannot exceed 38.7kg, the weight ratio of the battery pack must be 25.3%, the total weight of the battery pack must not exceed 153kg, and the specific energy of the battery pack can reach 262Wh/kg.

　　"Why do people think of soft-packaged batteries when they mention high energy density? From the perspective of vehicle engineering, we look at the energy density of the power battery system, not the energy density of individual cells. From individual cells to modules to system integration, the middle There are many links, including connectors between batteries, connecting cables between modules, boxes, fixed frames, support frames, thermal conductive structures, etc., which will add a lot of weight." Wang Zidong pointed out that it is necessary to improve the system energy density and reliability. Optimize performance and security.

　　Safety of power batteries

　　After ten years of accumulation, my country's power battery industry has made great improvements, especially in terms of understanding and understanding of power batteries. It should be said that it is competent for the current use of electric vehicles.

　　In terms of current power battery materials, if there is no obvious technological breakthrough, after the specific energy develops to a certain level, it will be difficult to make further breakthroughs. At the same time, the negative impact on safety is getting bigger and bigger. "Many people have asked me that fuel vehicles often catch fire, and more so than electric vehicles. Why are the requirements for electric vehicles so high?" Wang Zidong said, there is a concept that needs to be clarified: there are patterns in the fires of fuel vehicles. It is related to many known factors. The key point is that the flammable substance in fuel vehicles is fuel, which is sealed in an environment isolated from the outside world and separated from oxygen (combustion accelerant) and fire source. Once this isolation condition is If it breaks (such as aging pipelines, oil leakage and high engine temperatures), accidents will occur.

　　The flammable substance in the power battery system is the electrolyte, which is sealed in the same container environment with the combustion accelerant oxygen (the positive electrode material will decompose to produce oxygen when it encounters high temperatures) and the fire source (internal short circuit and overcharge will generate high temperatures). , so its security uncertainty becomes particularly prominent.

　　Wang Zidong made an analogy about this: which one caused more harm to humans, the cold or the 2003 SARS virus? Of course it was a cold, but humans were afraid of SARS because we had no drugs to treat the SARS virus at the time. Therefore, before mastering the rules of lithium battery fire, controlling the balance between energy density, safety and long life is an issue that cannot be ignored.

　　1. How to understand power batteries?

　　Before the power battery is born, it is necessary to consider in advance: the assembleability design, installability design, maintainability design, adjustability design, recycling and easy disassembly design of the battery module and battery pack (system), etc.

　　The design of these properties is very important. We cannot solve these performance problems by manufacturing all batteries in disaster areas. Lithium batteries are born with explosive tempers. Why do lithium batteries turn into "ticking time bombs"?

　　Lithium-ion batteries are mainly composed of six parts, namely positive electrode, aluminum foil, negative electrode, copper foil, separator, and electrolyte.

　　The electrolyte inside the battery contains a large amount of organic compounds, such as ethylene carbonate, diethyl carbonate, and dimethyl phosphate. Each of these guys has explosive properties, with the words "keep away from fire" written on their faces. In addition, once the positive and negative electrodes of the battery are short-circuited, they will generate a lot of heat and even produce sparks. Therefore, people will naturally think of using something to separate the positive and negative electrodes, so a separator is introduced.

　　After the battery separator is thinned, once this film is damaged, the problem will be serious. Lithium batteries themselves also have hidden dangers of puncturing the separator. This phenomenon is called "dendrites". This problem is a disease written in the genes of lithium batteries. During the use of lithium batteries, some small burrs will form on the electrode surface. These small burrs are called "dendrites", and the dendrites will grow larger and larger, and will eventually penetrate the diaphragm and cause a short circuit.

　　The thinner the diaphragm, the flammable electrolyte, the undercurrent of dendrites that will grow on their own, and the decomposition of materials at high temperatures that will automatically separate out oxygen, the entire lithium battery is like a powder barrel, combustion accelerant, and lighter in a small room inside, and then separated by a layer of plastic wrap, which would make anyone shudder to think about it. Now, the most important thing is to control the "lighter".

　　2. How to ensure the safety of the power battery system?

　　The safety of the battery system must be solved by the battery core. To ensure the safety of the battery core, it must use more stable materials and safer designs.

　　Wang Zidong said that now we are deliberately lowering the safety requirements of battery cells to reduce costs and increase energy density. It is difficult to pass the thermal runaway propagation experiment. The safety of the entire vehicle should still be evaluated based on the basic safety requirements at the source. Make sure to use a thicker separator to ensure the safety of the battery core. The design of the battery core to increase energy density should not be achieved by thinning the separator thickness.

　　Safe management when charging the battery pack is key! Since the most critical and core issues in the use of power lithium batteries in groups are: first, safety, and second, lifespan. Especially during fast charging, the difference between batteries in the battery pack increases. How to solve the service life of the battery pack faces huge challenges. .

　　Factors that affect the safe use and cycle life of batteries, in addition to the battery's own craftsmanship and product quality, are also a crucial issue: safety control and thermal management technology when charging batteries in groups. Without perfect battery group safety control and thermal management technology, the safety and long life cycle of the battery cannot be guaranteed. Therefore, the power battery charging management system is as important as the safety of the battery itself.

　　3. Fast charging technology has high requirements on power batteries

　　Regarding the issue of charging speed, everyone hopes to achieve fast charging. The current charging speed of high-energy power batteries can replenish 80% of the energy in about 40-60 minutes. For urban commuting transportation, it does not constitute the real use of electric vehicles. There are barriers to use, but there may be some problems for those who hope to use electric vehicles to solve transportation problems between cities.

　　From fast charging to ultra-fast charging (200-400kW), 90% of energy can be replenished within 10 minutes, which will effectively shorten the gap between electric vehicles and internal combustion engine vehicles.

　　Wang Zidong said that the current design plan is to reduce the thickness of the electrodes, change the battery structure, and select materials more suitable for fast charging. These will increase the production cost of power batteries, reduce their energy density, and also reduce the power battery system. The service life needs to be optimized from an overall consideration.

　　On the other hand, how to reduce the differences between single cells during the fast charging process of the battery pack requires reasonable thermal management system design to increase the service life of the power battery.

　　4. How to solve the safety problem of power battery?

　　Wang Zidong believes that new energy vehicle safety accidents are mainly caused by thermal runaway of power batteries. Thermal runaway is not only the result, but also the causes are complicated. The source of the accident is difficult to identify, and safety issues should be highly valued.

　　The industry continues to reflect on safety issues, and the blind pursuit of high energy density has become the focus. Professionals point out that theoretically, battery energy density is inversely proportional to safety. When companies pursue high energy density, safety issues are exposed. Although it is not clear that fire incidents have occurred How relevant it is to the pursuit of energy density, but as high-nickel ternary batteries enter the market, new energy vehicles are facing higher safety technical requirements.

　　How to strike a balance between high energy density and improved safety has become a major problem that needs to be solved in the current industry. Various companies are improving overall safety from multiple levels of single cell, module design and battery pack structural design.

　　Improving the safety of power batteries is mainly done at three levels, including single cell, module design and battery pack structural design to improve overall safety. In terms of single cells, additives can be added to the electrolyte. It can be improved by reducing its flammability, improving the temperature resistance of the separator, or improving the stability of the cathode material. In terms of module design, safety can be improved by strengthening temperature control design, BMS charging management, or changing the single connection method. At the vehicle level, The battery can be positioned to better dissipate heat, improve charging methods, and reduce safety hazards caused by improper charging.

　　Since the battery pack is a component containing high-energy substances, it is inherently dangerous. Moreover, as the specific energy and specific power of the battery increase, the risk of accidents will increase. Therefore, it is necessary to study the contradictory balance between energy density and safety, including the balance of material performance, the balance of battery module structure, the balance of the battery pack system level, the balance of cost acceptability, and the balance in the multi-level utilization process. , balance in the recycling process of power battery materials.

　　Research on solutions includes: optimization of material performance matching, optimization of battery module structure design, integrated design of battery pack system and body, control of production and manufacturing costs, promotion of multi-level utilization to dilute application costs, and encouragement of repair and reuse of power battery materials.

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