During the “May 1st” period, an electric bus crash occurred in the parking lot of the Crab Island Resort in Chaoyang District, Beijing, and 89 new energy vehicles were burned. According to the price of one million electric buses, the fire loss was estimated to be nearly 100 million yuan. Although after the incident, the traffic police said that the fire caused by the accumulation of catkins, but people still have deep doubts about the battery safety of new energy vehicles.

In recent years, under the background of China’s determination to vigorously develop new energy vehicles, especially after 2014, under the strong series of policies including high subsidies at the national and local levels, the new energy automobile industry Development is unstoppable. With the increase in the number of commercial applications of electric vehicles, the spontaneous combustion and fire explosion of lithium-ion battery for electric vehicles have occurred. It is now a critical time for the development of the electric vehicle industry. Any electric vehicle safety accident is not only correct. Manufacturing companies have a major impact, but also a heavy blow to the entire industry. The power battery is the core component of the electric car. When the electric car catches fire or explodes, the first thing people think of is the battery problem. Is the battery of the electric car safe now?

Battery mechanism analysis lithium titanate material has better safety performance

From the perspective of battery mechanism, the ignition explosion of lithium-ion batteries is caused by the thermal runaway of lithium-ion batteries. The root cause is the intense chemical exothermic reaction between the electrodes and the electrolyte under suitable conditions. At present, the cathode materials used in commercial applications of lithium ion power batteries are lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate and lithium manganate; the negative electrode materials mainly include graphite or its composite materials and lithium titanate; The electrolyte is mainly composed of an organic solvent and a lithium salt, and the main component of the separator is polypropylene, polyethylene or a composite material of the two.

For the positive electrode material, the lithium iron phosphate requires the highest temperature for thermal decomposition, and the heat released by the decomposition is the least, which is the safest positive electrode material. In the anode material, the reaction temperature of the graphite anode and the electrolyte is the lowest, and it is most likely to react with the electrolyte, and the heat released is the highest, and the temperature at which the lithium titanate anode is decomposed is not only higher than the reaction between graphite and electrolyte, but also released. The heat is much lower than the heat released by the graphite reaction, so the lithium titanate material is much safer than the graphite negative electrode.

In addition, in the charge and discharge cycle of the power battery, some physical and chemical changes occur in the battery constituent materials, which may cause the battery performance to decline. These changes will affect the safety of the battery to some extent. In fact, in the actual use after the battery is produced, an accident such as a fire of the battery occurs. The existing research on the safety of fresh batteries without circulation does not fully explain the mechanism of the accident. The electrochemical or abuse behavior of different materials, different capacities, and different designs of battery cycles may vary. According to the different anode materials used, the power batteries commonly used on the market are divided into two categories, carbon anode system and lithium titanate anode battery.

Since the potential of the carbon negative electrode is very close to the potential of the metal lithium after lithium insertion, the deposition of metallic lithium generally occurs on the surface of the negative electrode. When lithium ions migrate to the surface of the negative electrode, part of the lithium ions do not enter the negative electrode active material to form a stable compound, but electrons are deposited on the surface of the negative electrode to become metallic lithium, and lithium dendrites are gradually formed. As the number of cycles increases, the internal polarization increases. Lithium dendrites grow through the circulation of lithium-ion batteries, which may penetrate the diaphragm and cause short-circuit between the positive and negative electrodes. After nearly a thousand cycles of the battery, due to changes in the material structure of the battery (continuous expansion and contraction) and accumulation of side reaction products (lithiation), many bumps appear on the surface of the carbon negative electrode. This change not only affects the structure and activity of the electrode, but also deforms the diaphragm and increases its likelihood of damage. The battery is prone to thermal runaway in short circuit, extrusion, and overcharge experiments.

Compared with the carbon anode material, the lithium titanate material is called “zero strain material”, has high stability, and has a higher lithium insertion potential (1.55V vs. Li+/Li), which fundamentally eliminates metal lithium dendrites. Produced, reducing the risk of internal short circuits in the battery. The reaction between lithium titanate and electrolyte is low, and almost no SEI film is formed, so it has good cycle stability and safety, and its normal temperature cycle life can reach more than 25,000 times. At very high temperatures, lithium titanate absorbs the oxygen generated by the decomposition of the cathode, reducing the risk of thermal runaway and improving battery safety. The replacement of carbon material with lithium titanate as the negative electrode of lithium ion battery provides a solid foundation for ensuring the safety of lithium ion battery and improving the cycle performance and service life of the battery.

Lithium titanate firmly adheres to battery safety mission

The safety of the power battery depends more on the material itself and the manufacturing process, and the high safety of the lithium titanate battery effectively solves the problem that the safety hazard brings about the promotion and application of the new energy battery. Lithium titanate batteries have unparalleled advantages in terms of the most important safety. Following the new national standard, the lithium titanate battery has undergone "cruel" experiments such as acupuncture, electric drill, and cutting, and has not experienced any fire or explosion, and has withstood the test of safety testing. The market acceptance of lithium titanate battery technology has been continuously improved. At present, it has formed a three-pronged situation in the power lithium battery market together with the ternary lithium battery and lithium iron phosphate battery.

Lithium titanate battery is the longest and safest battery in lithium battery. The lithium titanate battery has a long cycle life and can achieve a charge/discharge cycle of more than 10,000 times, which is higher than that of a normal lithium battery. Moreover, the rapid charging performance of lithium titanate is very good, and the charging rate is not only 6C equivalent to lithium iron phosphate and ternary, but also the rate charging of the electrochemical supercapacitor 10C. This proves on the other hand that the use of lithium titanate is very extensive.

At present, the domestic market technology of lithium titanate battery is relatively mature. It is mainly used in buses and shuttle buses in the application of electric vehicles. The domestic production of lithium titanate materials is relatively early, such as Yinlong, Weihong, etc. Toshiba in foreign countries in 2007 There is a lithium-ion battery based on a lithium titanate negative electrode. Among them, the lithium titanate battery represented by Yinlong has already overcome the “five major problems” recognized in the industry, achieving 6-minute fast charging, wide temperature resistance (-50°C-+60°C), 30-year cycle life, no The fire does not explode, and has excellent characteristics such as high safety and high efficiency. However, the low energy density of lithium titanate battery is its biggest disadvantage. At present, the energy density difference between lithium titanate battery and lithium iron phosphate battery is also shrinking. It is reported that the fourth generation of high-energy density lithium titanate battery of Yinlong has a 40% lower cost and a 60% increase in energy density compared with the third generation. Titanium hydrogen technology developed by Yinlong will also effectively solve the problem of battery energy density, and through the combination of lithium titanate battery and fuel cell , successfully solve the problem of new energy vehicle cruising range.

In the future, electric vehicles will be the development trend of the country's "blue sky defense war", and it is also the focus of attention inside and outside the industry. More support for operating enterprises can increase customers' confidence in electric vehicles, and it can also promote the development of new energy vehicles in China. . From the recent electric vehicle safety accidents, we should be alert to solve the problem of battery safety. The step of greatly increasing the energy density can be appropriately slowed down. The reliability and durability of the battery should be placed in a more important position. High energy means high risk, and power battery safety is above everything else.

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