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Electric vehicles frequently catch fire: balancing energy density and safety is the direction of battery development

"In 2027, global power tram shipments will reach 1 trillion watt-hours, last year's shipments were 1,060 watt-hours, and there will be a 5.7-fold increase in China in 2018-2025." July 12, Chinese Academy of Sciences Academician Ouyang Minggao pointed out at the summit forum “Creating a New Life for Travel” that the cost of power batteries will continue to decline as the scale increases.

The data shows that the lithium-ion battery cell energy density reached 265 watt-hours, 2.2 times higher than in 2012; the price dropped from more than 80% from 5 yuan to 0.8 yuan.

The decline in battery costs is accompanied by an increase in sales of electric vehicles. On July 10, the sales and sales data of the automobile industry released by the China Automobile Association in the first half of 2019 showed that in the first half of this year, the total production and sales of China's new energy (5.530, 0.03, 0.55%) vehicles totaled 614,000 and 617,000, respectively. They increased by 48.5% and 49.6% respectively. Among them, the production and sales of pure electric vehicles were 493,000 and 490,000 respectively, up 57.3% and 56.6% respectively, accounting for 80% of the sales of new energy vehicles. The growth of production and sales of pure electric vehicles has also become the main driving force for China's new energy vehicle market.

At the same time, with the popularization of electric vehicles, as of June this year, the country has built a total of 410,000 public charging piles, 590,000 private charging piles, and a total of 1 million charging facilities. The national unified market is basically formed. Promoted throughout the country, the power battery recycling system has gradually matured.

However, with the rapid growth of the electric vehicle market and the rapid development of the power battery industry, frequent fire accidents have also caused widespread concern and discussion in the society, and have also adversely affected the development of the industry.

According to incomplete statistics, there were 52 electric vehicle safety accidents in China in 2018. There were 19 electric vehicle fire accidents from January to May this year. 29% of these accidents occurred during charging, and 19% were in When it happened, 19% occurred during parking. In addition, as of May this year, China’s new energy vehicles recalled 123,000 vehicles, of which 50% were due to three power failures. In power battery failures, design reasons accounted for 40%, manufacturing reasons accounted for 60%.

After more than ten years of rapid development, the current power battery has reached a "crossroads." On the one hand, the demand for power batteries for new energy vehicles is huge; on the other hand, power battery companies are also facing a series of challenges such as subsidies, "white list" cancellations, and frequent safety incidents.

Battery safety first place

As the core component of electric vehicles, the power battery is not only related to the competitiveness of the OEM's electric vehicle products, but also closely related to the safety of consumers' driving. "To develop safe, energy-saving and emission-reducing electric vehicles with mature and safe batteries." On July 10, at the first China International Electric Vehicle Safety Technology Innovation Conference, Yang Yusheng, an academician of the Chinese Academy of Engineering, emphasized.

In Yang Yusheng's view, there are four main reasons for the frequent occurrence of burning accidents in electric vehicles.

The first is battery technology. Because lithium-ion batteries contain flammable solvents, there is no absolutely safe battery.

Second is the reason for the development of new energy vehicles. The development of the national key development of long-distance pure electric vehicles has triggered mileage anxiety, safety anxiety, charging anxiety, price anxiety, and subsequent battery anxiety. Yang Yusheng believes that the self-ignition probability of the battery pack is not only related to the battery nature, but also proportional to the total battery. The longer the mileage, the more batteries need to be installed, the higher the burning probability of the battery pack, and the pure electric vehicle pursues the development route of long mileage. It is one of the incentives for frequent car burning.

In addition, the state's subsidy policy is linked to pure electric mileage, car companies pursue "multiple battery backup subsidies", and too much pursuit of energy density induces companies to chase high-energy batteries. The “double-hook” subsidy policy has led to over-loading of batteries and rapid energy-enhancing, which has led to a decline in security.

Finally, Yang Yusheng pointed out that the current development of electric vehicles cannot grasp the main aspects of major contradictions and contradictions. "Now the mileage is regarded as the main aspect of the contradiction. Regardless of safety, we are desperately trying to continuously improve the mileage requirements. Safety is a secondary aspect. This is the reason for the frequent method of burning a car. This is also the root cause of problems in development routes, policies and technologies. ”

China Power (24.610, 0.73, 3.06%) Deputy Secretary-General of the Battery Innovation Alliance and Deputy Secretary-General of China's Electric Vehicle Charging Infrastructure Charging Alliance, Wang Dongdong also believe that the new energy vehicle safety accident is mainly caused by the thermal runaway of the power battery, but the thermal runaway It is only the result that the cause of thermal runaway is complicated and the source of the accident is difficult to be clear.

In theory, battery energy density is inversely proportional to safety. Enterprises blindly pursue high energy density, and safety issues will be exposed. Although it is not clear how much the fire event has occurred and the pursuit of energy density, as high-nickel ternary batteries enter the market, new energy vehicles face higher safety technical requirements.

From the perspective of safety technology, any of the directors of the lithium ion laboratory of Tsinghua University believes that the main reason for the fire of the power battery is that the reliability of the product is not high.

"Battery product test verification is seriously insufficient, the test cycle is insufficient, and the test methods are not perfect. Many factories may soon leave the factory after production." He Xiangming pointed out that many battery factories do not consider the reliability of such products at all. The battery is in many people's Inside the concept is the battery of the laboratory, not the battery of an industrial product. There is very little research on actual reliability in the entire research industry.

However, the current cause of the fire in the power battery car is unknown. Is it the quality control in the production process? Or is it a problem in later use? Or what kind of scenes have problems? Wang Zidong believes that these problems are still unclear.

Power battery route

In the face of electric vehicle safety issues, Yang Yusheng proposed to use extension technology to resolve the contradiction between safety and mileage. The specific technical route includes the use of micro-small pure electric vehicles as a breakthrough, pure electric vehicles, micro-small, large and medium-sized vehicles to develop pure electric drive extended programs.

"Micro-miniature electric vehicles can use low-speed vehicles with lead-acid batteries, or high-speed vehicles with lithium iron phosphate batteries. The extended-range electric vehicles can solve the five major anxieties of pure electric vehicles, especially safety anxiety, and marketization is the most feasible." Yu Sheng said.

Of course, frequent electric vehicle fire accidents have also led to a dispute over the development of lithium iron phosphate batteries and ternary batteries.

"Lithium iron phosphate batteries and ternary batteries have been vying for cost, safety and energy density. We must admit that the battery pack is a very high-energy component and has a dangerous nature. It is said that this battery is not dangerous at all. This is impossible.” Wang Zidong pointed out, “As the battery's specific energy and specific power increase, the risk of a safety accident increases, which is related to certain degrees, so we have to choose a balanced optimization.”

It is understood that the lithium iron phosphate battery is characterized by high safety, high rate charge and discharge characteristics and long cycle life, but the energy density is not as good as the ternary battery; and the ternary battery has high energy density, good consistency and good low temperature. However, the safety is poor and the cycle life is not as good as that of lithium iron phosphate batteries.

"The car power battery is not only the weight ratio energy, but more importantly, the volume ratio energy. The volume ratio energy determines how far the car can really run. In this sense, the lithium ion battery has an advantage." Ouyang Minggao It is believed that the application of solid-state lithium-ion batteries will need to wait a few more years.

Ouyang Minggao pointed out that with the withdrawal of subsidies, the scale of phosphoric acid batteries in the sedan market will increase significantly, especially for vehicle manufacturers represented by BYD (55.350, 1.09, 2.01%), which will focus on phosphoric acid batteries. Although the energy is lower, the system has a larger difference than the energy; in addition, the life expectancy is doubled, and now the cost can be controlled within 5 cents, which is much lower than the ternary battery.

"Comprehensively, the phosphoric acid battery will rebound sharply, but the ternary battery is still our focus. In the future, we need to raise the safety balance point from 1:1:1, or 5:2:3 of the ternary battery to 8:1: 1, that is, eight yuan heat battery."

In the future development direction of electric vehicles, Ouyang Minggao recommends that cities develop small electric vehicles, and large and medium-sized electric vehicles use pure electric plug-in type--developing hybrid power that can be externally charged, and urban vehicles choose pure electric, high-speed long-distance use The motor is combined in parallel with the hybrid.

In the battery technology, the new and old forces of the lithium battery industry such as CATL, Panasonic, Samsung SDI, Moeneng Technology, Wanxiang A123, and Micro-Power are planning to introduce the lamination process after 2022. Honeycomb Energy also officially launched a square laminated battery on July 9, and on the power battery technology route, based on NCM three yuan independent development of quaternary batteries, cobalt-free batteries, solid-state batteries and other routes.

For the next generation of car-grade power batteries, Lu Languang, director of the State Key Laboratory of Safety and Energy Conservation at Tsinghua University, bluntly stated: "The winding process has the advantage of high production efficiency and controllable interlaminar tightening force, but due to the existence of radius of curvature, especially the core The inner center has a small radius of curvature, resulting in small N/P, uneven charging and discharging stress, inconsistent deformation, and easy lithium deposition. The lamination process is reversed, and the parts are evenly stressed and not easily deformed. The laminated piece is more suitable for deformation. Large electrode materials help to ensure consistent deformation during charge and discharge cycles and avoid problems such as lithium deposition."

He Xiangming believes that the focus of the next study is to find ways to reduce the damage caused by electric vehicle combustion accidents. "There are many kinetics of chemical reactions in the battery. This is a blank space. I think everyone should pay more attention to control. Secondly, we talk about the inherent safety technical specifications of the battery. We must clearly define the goal as battery safety. What are the sexual goals? Many companies have no goals."

Mo Ke, founder of True Lithium Research, believes that electric vehicles should be able to break through 16 million in 2025 and 35 million in 2030. At present, it seems that only lithium-ion batteries can do this. In the future, there is a great possibility that more highly-new batteries will emerge. Technology, but not currently seen. Therefore, how to balance energy density and safety at the current level of technology is the direction of car companies and power battery companies.

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