China's Ni-MH batteries breaks through the "7 series aluminum" technical restricted zone

Key core technologies are related to the initiative of innovation and development, as well as national economic security, special security and other security. We must strive to achieve independent control of key core technologies and firmly grasp the initiative of innovation and development in our own hands.

Many potential consumers are deterred by electric vehicles due to problems such as short driving range and high cost.

The energy density of lithium-ion power batteries has become a bottleneck for its industrialization. For this reason, the United States, Japan, South Korea and other countries have formulated relevant industrial policies, and their goals are all aimed at "energy density of 300Wh/kg in 2020". Recently, with the support of national key projects, the research and development team of CATL Co., Ltd. has overcome key core technologies such as high-nickel ternary materials and silicon-carbon negative electrode materials, and took the lead in developing battery samples with a specific energy (mass energy density) of 304Wh/kg, winning the championship in this international competition.

Opening up the "Ren and Du Meridians" to make up for the shortcomings of positive electrode materials

Lithium-ion power batteries are currently the most widely used power batteries for new energy vehicles and are the core part of new energy vehicles. Its advantages lie in high energy density and long cycle life, while its technical difficulties lie in high requirements for stability and safety and complex preparation process. This core production technology has always been in the hands of a few countries in the world.

The energy density of a battery refers to the amount of electrical energy released per unit volume or mass of the battery. "At present, the improvement of energy density has become the biggest bottleneck restricting the development of lithium-ion batteries, and it faces many world-class problems." Wu Kai, chief scientist of CATL, said that battery manufacturers can achieve the effect of expanding power capacity by increasing the size of the battery, but the "fatness" or "lengthening" of the battery cell is only a temporary solution, not a fundamental solution.

What exactly limits the energy density of lithium batteries?

Wu Kai introduced that the chemical system behind the battery is the main reason. Generally speaking, the four parts of a lithium battery are very critical: positive electrode, negative electrode, electrolyte, and diaphragm.

Since the energy density of the negative electrode material is currently much greater than that of the positive electrode, the positive electrode material has become the "short board of the barrel" - the lower limit of the energy density of lithium-ion batteries depends on the positive electrode material, so to improve the energy density, the positive electrode material must be continuously upgraded. However, my country started late in the development of high-nickel materials, with relatively weak technical accumulation, and relatively backward preparation processes and equipment conditions.

"The stable supply of high-performance high-nickel cathode materials in batches is one of the key technical difficulties in the development of high-energy-density power batteries." Wu Kai said that for this reason, CATL relied on major scientific research platforms such as the National Engineering Research Center and the Key Laboratory of Fujian Province, and through collaborative development with upstream and downstream cooperative units in the industrial chain, optimized the conditions for the synthesis of raw materials, improved structural stability, adjusted the microstructure, and controlled the morphology and size distribution of materials, and gradually realized the large-scale production and application of domestic high-nickel materials.

Compared with similar materials from Japanese and Korean competitors, domestic high-nickel materials currently have the characteristics of high reversible capacity, high compaction density, and relatively stable surface and bulk structures. They will break the technology monopoly of Japan and South Korea, improve the technical level of the domestic industrial chain and the core competitiveness of domestic power batteries, and eliminate the "first roadblock" on the road to innovation.

Subverting tradition to solve the shortcomings of negative electrode materials

Negative electrode materials are also one of the core materials of lithium-ion batteries. Currently, most of them use graphite as negative electrode materials. With the continuous upgrading of the demand for driving range, traditional graphite anodes can no longer meet the market's expectations for battery energy density.

According to calculations, the specific capacity of silicon-based anode materials can reach 10 times that of graphite anodes, and they are regarded as "substitutes" for the latter. The application of traditional silicon-based materials mainly adopts carbon coating technology, that is, a layer of carbon material is compounded on the surface of silicon materials. Wu Kai introduced that, due to the volume change of silicon materials during charging and discharging as high as 300%, the carbon material coated on the surface will break and fall off after multiple cycles, and the protective effect on silicon materials will be greatly weakened, resulting in poor battery cycle performance.

This world-class problem has been plaguing the industry like a "ghost" for more than 10 years.

CATL abandoned the traditional carbon coating technology and turned to the research of artificial electrolyte interface membrane coating technology. It took more than two years to apply this technology to the preparation of silicon materials and develop a new type of artificial electrolyte interface membrane-coated silicon-carbon composite anode material with independent intellectual property rights. Its cycle performance is significantly better than foreign products, knocking down the "second roadblock" on the road of innovation.

"Compared with carbon materials, the artificial electrolyte interface membrane has a stronger binding force with silicon materials, better elasticity, and is not easy to break or pulverize. It plays a good protective role on silicon materials. Therefore, it can greatly improve the interface stability of silicon materials in the cycle, thereby improving the cycle life of the battery." Wu Kai said that this move will promote my country to fully master the core technologies in many aspects such as material modification and precursor synthesis, realize the localization of key material technologies, and provide important guarantees for the gradual commercialization and promotion of silicon-carbon composite negative electrodes.

Perfect "slimming" is the first to use special-grade "7 series aluminum"

Under the condition that energy consumption remains unchanged and volume and weight are limited, the cruising range of new energy vehicles mainly depends on the energy density of the battery pack.

"This tests the ability of researchers to 'slim down' the battery pack." Wu Kai said that CATL used the special-grade "7 series aluminum" for the first time in the lower box of the battery pack. "7 series aluminum", the "combat aluminum" among aluminum, is often used to manufacture aircraft landing gear, with the characteristics of lightness, firmness and safety.

Wu Kai told reporters that the application of "7 series aluminum" also has many risks, especially stress corrosion (metal materials break in certain specific media due to the combined effect of corrosive media and stress).

"The industry generally believes that this is the technical difficulty of '7 series aluminum', or even a technical forbidden zone." Wu Kai said that for this reason, they have controlled the stress corrosion index at the highest level in the industry through hundreds of experiments and related process improvements. At present, CATL has successfully developed the "7 series aluminum" lower box and has mass-produced it.

So far, the company's battery pack lower box lightweight design has reached the world's leading level. This new energy density Ni-MH batteries can increase the energy load (total power of the battery loaded) of the B-class pure electric car battery compartment by about 50% without adding additional space on the existing basis; the energy of the on-board Ni-MH batteries system is increased by 50%; the weight of the whole vehicle can be reduced by 250 kilograms on the existing basis, so that the standard operating range of this model can be increased to more than 600 kilometers.

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