On August 20th, an electric car caught fire in Liaoyang; on August 23rd, an electric train caught fire in Liaocheng; on August 25th, a Weltmeister car caught fire in Chengdu; and on August 26th, an Ankai electric bus caught fire in Tongling, Anhui… From May to August 2018, there were as many as 16 publicly reported electric vehicle accidents, of which 9 were determined to be caused by “spontaneous combustion while charging.” Other causes included battery malfunction, short circuits in electrical components, and spontaneous combustion while parked. As new energy vehicles enter a period of rapid development, technological breakthroughs and safety challenges follow.
New energy vehicles mainly refer to automobiles that use unconventional vehicle fuels as their power source. Currently, the most common types on the market are pure electric vehicles and hybrid electric vehicles. Similar to ordinary cars that require internal combustion engines, electric vehicles rely on lithium-ion batteries to provide energy for propulsion. Currently, the most important type of lithium-ion battery is the lithium-ion battery.
"Follow scientific principles and avoid blind action," and "Develop safe and reliable technologies first"—these were the calls from academics and industry professionals at the "my country Yichun 2018 Global Lithium Battery Industry Chain Summit Forum" held in Beijing from October 17th to 18th, addressing the issue of safety in lithium battery development.
Lithium-ion batteries evolved from lithium batteries. Lithium batteries have been used in people's lives for a long time; for example, button batteries are a type of lithium battery. The positive electrode material of a lithium battery is manganese dioxide or thionyl chloride, and the negative electrode is metallic lithium. Once assembled, the battery has voltage and does not require further charging. Lithium batteries are generally prohibited from being recharged because lithium dendrites easily form during the charging and discharging process, causing internal short circuits.
In 1992, Sony Corporation of Japan invented a battery that used carbon material as the negative electrode and a lithium compound as the positive electrode. During charging and discharging, only lithium ions are present, without metallic lithium; this is the lithium-ion battery we know today. Subsequently, Sony Energy Development Corporation of Japan and Moli Energy Corporation of Canada successfully developed new types of lithium-ion batteries (hereinafter referred to as "lithium batteries"). Currently, lithium batteries are widely used in various handheld electronic products and electric vehicles.
For lithium batteries, performance is reflected in two indicators: charge/discharge rate, which represents the battery's charging speed, and energy density, which determines a vehicle's driving range. However, blindly pursuing these two indicators largely sacrifices safety.
"Fast charging technology currently has no future."
"At least 60% of these fires occurred during or immediately after charging, indicating a major problem with the charging process," said Wang Zidong, director of the National 863 Electric Vehicle Major Project Power Lithium Battery Testing Center.
During the charging process of lithium batteries, lithium ions repeatedly insert and extract between the two electrodes, without oxidation occurring at the positive and negative electrodes. However, Wang Zidong points out that current charging methods and usage processes utilize redox reactions, which are not the proper charging methods that lithium batteries should follow according to their inherent characteristics. Previous experimental results from Wang Zidong's team showed that using current charging methods can reduce battery life by approximately 30%. Therefore, under these circumstances, Wang Zidong believes that high-current charging should not be considered.
The charge/discharge rate of a lithium battery refers to the charge/discharge current per unit of rated capacity. For example, a battery with a rated capacity of 100Ah charged and discharged at 20A has a charge/discharge rate of 0.2C. Generally, the charging current for lithium batteries is set between 0.2C and 1C. The higher the current, the faster the charging, but the more severe the battery heating. Currently, pure electric vehicles use slow charging, typically between 0.3C and 0.5C. On the other hand, charging with excessively high current will result in insufficient capacity because the electrochemical reactions inside the battery require time. Similar to pouring beer, pouring too quickly will create foam and prevent the battery from filling completely.
Qi Lu, director of the New Energy Materials and Technology Laboratory at Peking University, explained that to achieve an 8-minute charge for today's multi-element cathode metal composite oxide batteries, a rate of approximately 10C would be theoretically required, "which is unimaginable in terms of energy."
These technical bottlenecks are not actually new problems. Qi Lu is one of the pioneers in my country's lithium battery research field. He served as the chief scientist for the clean energy electric vehicle power lithium battery project at the 2008 Beijing Olympics. As early as the Beijing Olympics, they had conducted various experiments to address these issues. At that time, ternary lithium batteries could already be charged in 5 minutes. In the experiments, during the rapid charging process, the heat from ternary lithium batteries could not be released quickly, greatly increasing the possibility of explosion. Considering safety concerns, Qi Lu stated that this technology could not be used in pure electric vehicles, but only in battery hybrid vehicles.
Furthermore, fast charging of lithium-ion batteries faces a very real problem – the city's basic power infrastructure cannot meet the demand. For example, if a bus uses a 150kWh battery and needs 5 minutes to charge, each bus requires 100kW of power. If hundreds or thousands of buses are charging simultaneously, it will put a significant strain on the power grid.
"Today's urban power grids simply cannot do this," Qi Lu said.
Currently, Wang Zidong's team is researching how to adjust the charging method based on battery characteristics during the charging process. After modifying the charging method, the battery life that could only be charged 500 times with the standard charging method can now be increased to 1000 times with the new method, effectively slowing down battery degradation. Therefore, Wang Zidong stated that even with many bottlenecks, lithium batteries will definitely have a charging method that is particularly suitable for them.
Qi Lu believes that the most suitable approach at this stage is to use charging cables per parking space, allowing for charging for two or three hours, five or six hours, or even overnight – something that charging technology can easily achieve. By first developing safe and reliable charging methods, we can promote the steady, safe, and healthy development of electric vehicles.
Energy density and safety are a contradiction.
In February 2018, the Ministry of Finance, the Ministry of Industry and Information Technology, and two other ministries jointly issued the "Notice on Adjusting and Improving the Fiscal Subsidy Policy for the Promotion and Application of New Energy Vehicles," which canceled subsidies for pure electric vehicles with a range of less than 150 kilometers, while increasing subsidies for pure electric vehicles with a range of more than 300 kilometers to 34,000 yuan and subsidies for models with a range of more than 400 kilometers to 50,000 yuan.
Wang Yunshi, director of the my country Transportation Energy Center at the University of California, Davis, analyzed that this means that once pure electric vehicles achieve a driving range similar to gasoline vehicles, the longer the driving range, the better. The new policy may hope to promote the development of lithium-ion batteries by imposing requirements on the energy density of power lithium-ion battery systems.
The energy density (Wh/kg) of a lithium battery refers to the amount of energy that a unit weight of battery can store, and it is mainly determined by the battery's material properties. Assuming the energy density of a typical lead-acid battery is approximately 40Wh/kg, if a passenger car were to travel more than 200 kilometers using a lead-acid battery, nearly one ton of battery would be required. Therefore, provided the battery weight is kept within a certain range, the higher the battery's energy density, the longer the car's driving range.
While energy density should ideally be as high as possible, batteries are small devices with highly concentrated energy. When more energy is packed into a smaller volume, improper use, such as overheating or sudden violent impact, can have consequences comparable to a bomb.
According to the latest data released by the New Energy Research Institute's True Lithium Research, after June 2018, the installed capacity of battery packs with a capacity of 120Wh/kg accounted for approximately 95%, while in June of the previous year, this proportion was only 7.3%. In other words, the progress in energy density of domestic battery systems has far exceeded that of overseas countries at an "astonishing speed".
Percentage of installed capacity of battery packs with different energy densities in my country's electric passenger vehicle production, 2017-2018.8. Source: Zhenli Research.
Despite the significant increase in the number of high-energy-density battery packs installed, the problem of balancing energy density and safety remains unresolved.
"Today, energy density is undoubtedly inversely proportional to battery safety, and we haven't solved this problem yet," Qi Lu said.
Currently, the theoretical energy density of domestically produced lithium batteries in my country is between 300 and 400 Wh. Since there is no way to break through this upper limit, the energy density can be increased by reducing the wear on the separator, thereby expanding the material space. The purpose of the separator is to separate the positive and negative electrodes of the battery, preventing short circuits caused by contact between the electrodes, and allowing electrolyte ions to pass through.
"This is the simplest, but also the most dangerous method," Wang Zidong said.
Wang Zidong explained that his research team had previously learned about the Samsung Note7 fire incidents and found that the separator used in the Samsung Note7 battery was about 45 to 46 micrometers thick. Under the condition of using the same material, some power lithium battery manufacturers are even considering using separators that are 10 micrometers or 8 micrometers thick. In his opinion, such ideas are "very bold".
Since it is unavoidable for particles to fall off during the battery manufacturing process, the separator will inevitably have some minor "injuries". Without breakthroughs in materials, the ultra-thin separator, flammable electrolyte, and the surging dendrites can create a potential explosion hazard if risks are taken in this stage.
"Before we understand the ignition patterns of lithium batteries, balancing energy density, safety, and lifespan is an issue we cannot ignore," said Wang Zidong.
In reality, the seemingly contradictory issue of energy density and safety not only plagues the development of lithium battery technology in my country but also troubling industry leaders in South Korea and Japan. Seunghoon Han, an analyst at Deutsche Securities Korea, stated that no company can claim its technological path is entirely definitive, yet each believes its batteries are the safest. Given that many other industries have addressed safety issues through standardization and regulation, the safety challenges facing the lithium battery industry today may also be resolved through standardization. These developmental bottlenecks do not preclude the continued focus on fast charging technology and increasing energy density as future technological directions.
"Only by standardizing and regulating safety indicators can technological development more easily determine what is safer and what is unsafe," Han said.
On the other hand, high energy density means high-density materials, and high-density materials will determine the amount of electrical energy that can be stored. When the thickness of a material reaches the safety limit but is still lower than people's expectations, many people turn their attention to finding new materials. Wang Zidong believes that without a breakthrough in materials, the problem of high energy density will remain stagnant for a long time, possibly 10 or 50 years.
However, Qi Lu is not optimistic about the recently popular graphene and nanomaterials. He stated that these materials, including the previously used lithium iron sulfate, are actually low-density materials, while ternary and ternary materials have much higher densities, and in the future, the density can even be made higher.
"From a materials science perspective, graphene has good electrical conductivity, but can the concepts be applied to batteries and then to electric vehicles?" Qi Lu said. "Nanomaterials won't have any specific applications in this field."
Regardless of whether it's fast charging or high energy density, Qi Lu believes we must remain vigilant and avoid complacency. Especially for technologies like solid-state lithium batteries that can guarantee energy density and are safe to use, we shouldn't have too many expectations for the industrialization of these batteries before electrolyte materials with good conductivity become available.
"We still need to go all out to develop our viable technologies, and I think the most important thing is our 'technology of tomorrow'," Qi Lu said.
