Brief analysis of the development trend of core technology of power lithium battery

　　A few days ago, big news in the domestic power battery industry broke out frequently. First, SAIC entered the power battery sector, and it married with Ningde Times, the second largest power battery company in China and the third largest power battery company in the world, to establish a power battery system company. Then there was news that BYD would spin off. The power battery department will open its doors or supply to all car companies, and there are reports that this move may change the global market structure. Moreover, the domestic electric vehicle market has ranked first in the world in terms of production and sales for two consecutive years, with a cumulative promotion of more than 1 million vehicles, accounting for more than 50% of the global market. China surpasses the United States to take the top spot in the electric vehicle market. It can be said that the electric vehicle industry has unlimited prospects and rapid development. The key lies in the improvement of power battery technology.

　　The development of electric vehicles requires better batteries. The specific energy, life, safety and price of power batteries are essential to the development of pure electric vehicles. Among them, lithium-ion batteries, which have the advantages of high specific energy and long life, are currently the most practical and valuable electric vehicle batteries, and are widely used in hybrid vehicles, pure electric vehicles and fuel cell vehicles. The current technical level of commercial power batteries and the expected achievable goals in the next 10 years are shown in Figure 1. However, these indicators are often contradictory in actual product production, and battery-related performance needs to be weighed and considered. The improvement of battery performance needs to take into account the performance of electrode materials, electrolytes, and diaphragms. At the same time, the follow-up of assembly technology, battery system grouping, and management technology is also crucial. This article aims to summarize the current development achievements of power batteries with lithium-ion batteries as the core from the aspects of battery material technology, single battery design and manufacturing technology, and battery system technology, while looking forward to the future!

　　Lithium cobalt oxide battery. In recent years, electric vehicle manufacturer Tesla has used this computer battery to drive electric vehicles. This material can also be mixed with lithium manganate to make vehicle power batteries. The domestic NCA precursor has formed a stable production capacity. A few companies have completed the development of NCA cathode materials and are in the process of product promotion. Lithium iron phosphate batteries have high safety and long life. At present, nano-sized power materials and high-density lithium iron manganese phosphate materials are developing rapidly. The performance of high-energy and high-power materials tends to be stable, and the cost is further reduced. Gradually satisfying the domestic market demand and the need for the promotion of new energy vehicles in China at this stage, high-voltage spinel lithium nickel manganese oxide and high-voltage high-specific-capacity lithium-rich manganese-based cathode materials are still under development.

　　Figure 2 Lithium-ion battery electrode material system

　　anode material

　　Anode materials that can be used in power batteries include graphite, hard/soft carbon and alloy materials. Graphite is currently a widely used anode material, and its reversible capacity can reach 360mA·h/g. Amorphous hard carbon or soft carbon can meet the needs of higher rate and lower temperature applications of batteries, and they are beginning to be applied, but they are mainly mixed with graphite. Lithium titanate anode material has the best rate performance and cycle performance, and is suitable for high-current fast charging batteries, but the produced batteries have lower specific energy and higher cost. Nano-silicon was proposed for high-capacity anodes in the 1990s. Improving the capacity of carbon anode materials with a small amount of nano-silicon doping is a hot spot in current research and development. Anode materials with a small amount of nano-silicon or silicon oxide have begun to enter small batches. In the application stage, the reversible capacity reaches 450mA·h/g. However, due to the volume expansion of lithium after being inserted into silicon, the problem that the cycle life will be reduced in actual use needs to be further solved.

　　electrolyte

　　Lithium ion battery electrolyte is generally mixed with high dielectric constant cyclic carbonate and low dielectric constant linear carbonate. Generally speaking, the electrolyte of lithium ion battery should meet the requirements of high ionic conductivity (10-3~10-2S/cm), low electronic conductivity, wide electrochemical window (0~5V), and good thermal stability (-40~60℃) ) And other requirements. Lithium hexafluorophosphate and other new lithium salts, solvent purification, electrolyte preparation, and functional additives technology continue to advance. The current development direction is to further increase its working voltage and improve battery high and low temperature performance. Safe ionic liquid electrolytes and solid electrolytes are under development.

　　diaphragm

　　Polyolefin microporous membrane is the main product in the lithium-ion battery separator market due to its excellent mechanical properties, good electrochemical stability, and relatively low cost [7]. Including polyethylene (PE) single-layer film, polypropylene (PP) single-layer film and PP/PE/PP three-layer composite microporous film. There are many domestic manufacturers that use dry process production, and many companies have mass production of wet process PE diaphragms. With the promotion of ceramic coating technology, high-temperature and high-voltage diaphragms will become the direction of future research and development.

　　2, single cell technology

　　So far, the basic design of lithium-ion batteries is still the same as SONY’s patent application published in 1989. The shapes of the monomers are cylindrical, square metal shell (aluminum/steel) and square soft pack bulk. Cylindrical batteries were originally used in notebook computers. Now Tesla and other companies use 18650 cylindrical batteries for electric vehicles. The prismatic battery generally has a large capacity, and the battery core is made by winding, Z-shaped lamination, winding + lamination, positive-coated film lamination, and lamination + winding. Cylindrical batteries have the most mature technology and lower manufacturing costs, but large cylindrical batteries have poor heat dissipation capabilities, so small cylindrical batteries are generally used. The car battery pack has a large capacity and a large number of batteries, and the management system is more complicated and expensive. The manufacturing process of the wound structure battery in the prismatic battery is relatively simple, but it is mainly suitable for the soft pole piece battery. This method can be used for batteries using lithium iron phosphate and ternary materials in addition to the spinel cathode material. The laminated battery with high reliability and long life is suitable for various material systems. The batteries of the GM Volt plug-in hybrid electric vehicle and the Nissan Leaf pure electric vehicle are manufactured by the laminated technology. By 2015, the specific energy of lithium iron phosphate battery will reach 140W·h/kg, and the specific energy of ternary material mixed lithium manganate battery will reach 180W·h/kg. The specific energy of the small cylindrical battery of NCA is adopted internationally. To reach 240W·h/kg, the specific energy of lithium-ion single cells will further increase in the next few years, and it is expected to reach 300W·h/kg by 2020.

　　3, battery system technology

　　From the perspective of commercial lithium-ion power battery systems, key core technologies include battery pack technology (integrated battery pack, thermal management, collision safety, electrical safety, etc.), battery management system (BMS) electromagnetic compatibility technology, and signal accuracy Measurement (such as cell voltage, current, etc.) technology, accurate estimation of battery state, battery balance control technology, etc.

　　Figure 3 Simple power battery system diagram

　　The other key core components of BMS and battery systems, including sensors, controllers, actuators, and other components, are basically monopolized by powerhouses in automotive electronics technology (Germany, Japan, and the United States). At present, some domestic enterprises have successfully developed smart meters, which can replace foreign current, voltage, and insulation sensors. The primary factor affecting the promotion and application of electric vehicles is the safety and cost of use of lithium-ion power batteries. In addition to the further improvement of the safety, life and consistency of the battery body, battery modular technology and battery pack technology (integrated battery packs, Thermal management, collision safety, electrical safety, etc.) also have a significant gap with foreign countries. At present, the battery pack technology of international automobile companies is relatively mature, and domestic research units have carried out more in-depth research in BMS electromagnetic compatibility technology, accurate signal measurement technology, accurate battery state estimation, and battery balance control technology.

　　The research and development of key technologies for battery power management include comprehensive battery electrochemical models, electrical safety design, battery state estimation, balance management, fault diagnosis and calibration, and charging management. The key technology of battery thermal management and system research and development need to study the heat dissipation effect of different thermal management technologies based on the structural design of the battery pack and the calculation and analysis of battery heat production, so as to obtain a battery thermal management cooling solution with low cost, simple process, and strong safety and reliability. . The light weight of the battery structure needs to take the battery system and the related structure of the vehicle as the research object, consider the mutual coupling characteristics, and carry out the integrated optimization of structural vibration resistance, impact resistance and lightweight from two aspects of structural design optimization and material selection. Design key technology research work. Optimize the design schemes of component materials, structural design, connection, etc. In terms of battery safety, it is necessary to carry out the overall safety scheme design research of the battery system on the basis of electrical safety, mechanical safety and thermal safety, and carry out fault diagnosis and prediction for the battery system , The key technology of thermal safety monitoring, early warning and prevention and control.

　　4. Outlook

　　For a long period of time in the future, lithium-ion batteries will still be the most suitable battery for electric vehicles. Lithium manganate cathode materials, ternary system cathode materials, lithium iron phosphate cathode materials, composite carbon anode materials, ceramic coating separators, electrolyte salts And the development of functional electrolyte technology has supported the progress of battery technology and industrial development. Battery system technology advances in application, and safety and reliability will be further improved in the next few years.

　　Research on the life model of lithium-ion power battery and its influencing parameters, research on the characteristics of battery grouping methods, research on the balancing strategy of high-efficiency and large-capacity lithium-ion battery packs, single battery charging and discharging thermal model and group battery pack temperature field analysis and control methods Research, optimization of fast charging methods for group batteries, etc. are yet to be carried out. The power battery system should be redesigned in conjunction with the entire vehicle product, and the design and manufacturing mode should be upgraded according to the needs of future vehicle power batteries. At the same time, efforts should be made on the entire industry chain of power battery basic materials, battery manufacturing and system technology to improve product quality. Reduce the cost of large-scale production and enhance industrial competitiveness.