Liquid electrolytes have reached their research limits, and many scientists are now focusing on solid electrolytes. But Professor Meng Ying, director of the Center for Sustainable Power and Energy and the Energy Storage and Conversion Laboratory at the University of California, San Diego, led her team to go in the opposite direction, studying gaseous electrolytes and making breakthroughs. These gaseous electrolytes can liquefy under a certain pressure and are more resistant to freezing. In the new research, they selected two liquefied gases - fluoromethane and difluoromethane - from a large number of gas candidates to make electrolytes for lithium batteries and supercapacitors respectively, extending the minimum operating temperature of lithium batteries from minus 20 to The operating temperature of supercapacitors extends from minus 40 degrees below zero to minus 80 degrees below zero. Moreover, these electrolytes can still work efficiently after returning to normal room temperature. In addition to setting low-temperature operating records, these gaseous electrolytes also overcome the common thermal runaway problem in lithium batteries and have greater safety advantages. Thermal runaway is a vicious cycle of heat in the battery. When the battery is working, the temperature will rise, starting a series of chemical reactions. The heat generated by these reactions will in turn further heat the battery, causing the battery to expand and be destroyed.

　　1. In actual product production, these indicators are often contradictory, and battery-related performance needs to be weighed and considered. Improving battery performance requires taking into account the performance of electrode materials, electrolytes, and separators. At the same time, the follow-up of assembly technology, battery system grouping, and management technology are also crucial. This article aims to summarize the current development achievements of power batteries with lithium-ion batteries as the core from aspects such as battery material technology, single battery design and manufacturing technology, and battery system technology, while looking forward to the future!

　　1. Lithium battery material technology positive and negative electrode materials Lithium battery positive and negative material systems are very rich. At present, research on positive electrode materials such as lithium cobalt oxide, lithium manganate, lithium iron phosphate, and lithium nickel cobalt manganese has become mature. The specific capacity of lithium cobalt oxide material is 200-210mA·h/g. Its material true density and electrode plate compacted density are the highest among existing cathode materials. The charging voltage of commercial lithium cobalt oxide/graphite system can be increased by 4.40V. It can already meet the demand for high-volume energy-density soft-pack batteries for smartphones and tablets. Lithium manganate has low raw material cost, simple production process, high thermal stability, good overcharge resistance, high discharge voltage platform and high safety. It is suitable as a low-cost battery for light electric vehicles, but it has problems such as relatively low theoretical capacity, and the possible dissolution of manganese during the cycle, which affects the life of the battery in high-temperature environments. Domestic lithium manganate materials mainly meet the needs of the mobile power supply, power tools and electric bicycle markets, and have a tendency to develop towards the low end.

　　NCM ternary layered cathode materials are mainly used in power batteries. In addition to LiNi1/3Co1/3Mn1/3O2, which each accounts for 1/3 of nickel, cobalt, and manganese, its application in power batteries is relatively mature. LiNi0.5Co0 with higher capacity .2Mn0.3O2 has also entered batch applications and is generally used in electric vehicle batteries mixed with lithium manganate. The energy density of aluminum-doped lithium nickel cobalt oxide (NCA) can be close to high-voltage lithium cobalt oxide batteries. In recent years, electric vehicle manufacturer Tesla has used this computer battery to drive electric vehicles. The material can also be used with lithium manganate Hybrids are used to manufacture vehicle power batteries. Domestic NCA precursors have formed 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, nanoscale 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 meeting the needs of the domestic market and 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. Negative electrode materials The negative electrode materials that can be used in power batteries include graphite, hard/soft carbon and alloy materials. Graphite is currently a widely used negative electrode material, and its reversible capacity can reach 360mA·h/g.

　　Amorphous hard carbon or soft carbon can meet the needs of batteries for higher rate and lower temperature applications and is beginning to be used, but it is mainly mixed with graphite. Lithium titanate anode material has optimal rate performance and cycle performance and is suitable for high-current fast-charging batteries, but the battery produced has low specific energy and high cost. Nano-silicon was proposed to be used in high-capacity anodes in the 1990s. Doping a small amount of nano-silicon to increase the capacity of carbon anode materials is a current research and development hotspot. Anode materials that add 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 caused by lithium being embedded in silicon, the problem of reduced cycle life during actual use needs to be further solved. Electrolyte Lithium-ion battery electrolyte is generally a mixture of 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 additive technology continue to advance. The current development direction is to further increase its operating voltage and improve the high and low temperature performance of the battery. Safe ionic liquid electrolytes and solid electrolytes are under development. Separator Polyolefin microporous membrane is currently the main variety 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 manufacturers in China that use the dry process for production, and there are many companies that can mass-produce wet process PE separators. As ceramic coating technology is promoted, high temperature and high voltage resistant diaphragms will become the future research and development direction.

　　2. Single cell technology So far, the basic design of lithium-ion batteries is still the same as that announced by SONY in 1989. The shapes of single cells include cylindrical, square metal shells (aluminum/steel) and square soft packs. The original cylindrical batteries Mainly used in laptops, 18650 cylindrical batteries are now used in electric vehicles by companies such as Tesla.

　　Square batteries generally have a larger capacity, and the cells are produced by rolling, Z-shaped lamination, winding + lamination, positive electrode coating lamination, lamination + winding, etc. Cylindrical battery cell technology is the most mature and has lower manufacturing costs. However, large cylindrical batteries have poor heat dissipation capabilities, so small cylindrical batteries are generally used. Vehicle battery packs have a large capacity and a large number of batteries, and the management system is complex and expensive. Among square batteries, the production process of the winding structure battery is relatively simple, but it is mainly suitable for soft electrode sheet batteries. This method can be used for batteries using lithium iron phosphate and ternary materials in addition to spinel cathode materials.

　　Laminated batteries with high reliability and long life are suitable for various material systems. The batteries of GM Volt plug-in hybrid electric vehicle and Nissan Leaf pure electric vehicle are manufactured using the lamination process. By 2015, the specific energy of lithium iron phosphate single cells will reach 140W·h/kg, and the specific energy of ternary material mixed lithium manganate single cells will reach 180W·h/kg. The specific energy of small cylindrical batteries using NCA internationally is Reaching 240W·h/kg, the specific energy of lithium-ion single cells will further increase in the next few years, and is expected to reach a maximum of 300W·h/kg by 2020.

　　3. Battery system technology From the perspective of commercialized 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, accurate measurement of signals (such as cell voltage, current, etc.) technology, accurate estimation of battery status, battery balance control technology, etc. The BMS and other key core components of the battery system, including sensors, controllers, actuators and other components, are basically monopolized by the automotive electronics technology powers (Germany, Japan, and the United States).

　　At present, some domestic companies 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 further improvements in the safety, lifespan and consistency of the battery itself, battery modular technology, battery group technology (integrated battery pack, Thermal management, collision safety, electrical safety, etc.) also have obvious gaps with foreign countries. At present, the battery pack technology of international automobile companies is relatively mature, and domestic research units have conducted relatively in-depth research on BMS electromagnetic compatibility technology, precise signal measurement technology, accurate battery status estimation, and battery balance control technology.

　　The research and development of key battery power management technologies include comprehensive battery electrochemical models, electrical safety design, battery state estimation, equalization management, fault diagnosis and calibration, and charging management. The research and development of key battery thermal management technologies and systems need to study the heat dissipation and temperature equalization effects of different thermal management technologies based on the structural design of the battery pack and battery heat generation calculation and analysis, and obtain a battery thermal management and cooling solution with low cost, simple process, strong safety and reliability .

　　To reduce the weight of the battery structure, we need to take the battery system and the related structure of the vehicle as the research object, consider the coupling characteristics between each other, and carry out integrated optimization of structural vibration resistance, impact resistance and lightweight from two aspects: structural design optimization and material selection. Design key technology research work. Optimize component materials, structural design, connection and other design plans. In terms of battery safety, it is necessary to carry out overall safety plan design research on the battery system based on electrical safety, mechanical safety and thermal safety, and carry out fault diagnosis and prediction for the battery system. , thermal safety monitoring, early warning and key technologies for prevention and control.

　　4. Looking forward to a long period of time in the future, lithium-ion batteries will still be the most suitable electric vehicle batteries, including lithium manganate cathode materials, ternary system cathode materials, lithium iron phosphate cathode materials, composite carbon anode materials, and ceramic coating separators. The development of , electrolyte salt and functional electrolyte technology supports the progress of battery technology and industrial development.

　　As battery system technology advances in application, safety and reliability will be further improved in the coming years. Research on the life model and model influencing parameters of lithium-ion power batteries, research on battery grouping characteristics, research on balancing strategy of high-efficiency and large-capacity lithium-ion batteries, single battery charge and discharge thermal model and group battery pack temperature field analysis and control methods Research, research on optimized fast charging methods for group batteries needs to be carried out.

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