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What are the steps in the development of power battery technology?

2017-7-28 14:42:00 次流覽

    The day before, the domestic battery industry news frequently burst, first starting SAIC battery plate, and the marriage of the second major domestic and the world's third largest power battery enterprises in Ningde times, set up power battery systems; then there is news that BYD will spin off battery department, open the door to all car prices or supply, the move reported or will change the global market pattern. Moreover, the domestic electric car market for two consecutive years, production and sales ranked first in the world, the cumulative promotion of more than 1 million vehicles, accounting for more than 50% of the global market share. China has overtaken the United States to take the lead in the electric car market. It can be said that the future of the electric vehicle industry is unlimited and developing rapidly. The key lies in the improvement of the technology level of power batteries.

    The development of electric vehicles requires better batteries. The specific energy, life, safety and price of the power storage battery are essential to the development of pure electric drive vehicles. The lithium ion battery which has the advantages of high specific energy, long life is the electric vehicle battery the most practical value, are widely used in hybrid vehicles, pure electric vehicles and fuel cell vehicles. At present, the technical level of commercial power batteries and the expected target for the next 10 years are shown in Figure 1. In actual production, these indicators are often contradictory, and the performance of the battery needs to be weighed and considered. The improvement of battery performance needs to take into account the properties of electrode materials, electrolytes and separators. Meanwhile, assembly technology, battery systems, and management techniques are also critical. The purpose of this paper is to summarize the achievements in the development of power batteries with lithium-ion batteries as the core from the aspects of battery materials technology, single cell design and manufacturing technology, battery system technology, etc., and look forward to the future!

Figure 1 technical indicators of existing power batteries and development targets for the next 10 years [1]

   1. lithium battery materials technology
       Positive and negative materials

    The lithium anode material system is very rich (Figure 2), at present, the research of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium nickel cobalt manganese cathode materials has become mature. LiCoO2 material than the capacity of 200-210 mA h/g, the material density and sheet compaction density were highest in the existing commercial cathode materials, lithium cobalt oxide / graphite system can improve the charging voltage of 4.40 V, already can meet the demand of intelligent mobile phone and tablet computer with high volume energy density soft package battery [3]. The cost of lithium manganate material is low, the production process is simple, the thermal stability is high, the overcharge resistance is good, the discharge voltage platform is high, and the safety is high. The utility model is suitable as a low cost battery for light electric vehicles, but the theoretical capacity is relatively low, and the dissolution of manganese elements in the circulation process may affect the service life of the batteries in a high-temperature environment. Domestic lithium manganate material mainly meets the demand of mobile power supply, power tools and electric bicycle market, and has a trend towards low-end development. NCM three layered cathode material is mainly used in battery, in addition to the application of nickel, cobalt and manganese each accounted for 1/3 LiNi1/3Co1/3Mn1/3O2 in the power battery in mature [4], high capacity LiNi0.5Co0.2Mn0.3O2 has entered batch application, general and mixed lithium manganate used in electric vehicle battery. Lithium nickel cobalt oxide aluminum doped (NCA) energy density close to high voltage cobalt acid lithium battery, in recent years, Tesla electric car manufacturers will drive electric vehicles for the computer battery, the material can be mixed with the lithium manganate used for battery car manufacturing, domestic NCA precursor has formed a stable production capacity, a few companies have complete the development of NCA cathode materials, in the process of product promotion. The lithium iron phosphate battery with high safety and long service life of [5], the fast power type nanometer material and lithium manganese phosphate material development speed and high density, high performance and high energy power type material tends to be stable, to further reduce the cost, gradually to meet the needs of the domestic market demand and the current China promotion of new energy vehicles, high voltage spinel lithium nickel manganese oxide and high voltage high capacity rich cathode materials of lithium manganese base is still under development.

 

Figure 2 electrode material system of lithium ion battery [2]

  Negative electrode material
   The negative electrode materials used for power batteries include graphite, hard / soft carbon and alloy material. Graphite is widely used as negative electrode material, and reversible capacity has reached 360 mA. H/g. Amorphous hard carbon or soft carbon can meet the needs of higher rate and lower temperature applications of batteries, and begin to be applied, but mainly with graphite. Lithium titanate anode material has the best rate performance and cycle performance. It is suitable for large current fast charge battery, but it has lower specific energy and higher cost. Nano silicon in 1990s was introduced for high capacity anode [8], to enhance the capacity of carbon anode materials by a small amount of nano silicon doping is a hotspot of current research and development of anode materials, adding a small amount of nano silicon or silicon oxide has been used in small batch, reversible capacity reached 450 mA - h/g. However, because of the volume expansion of lithium embedded in silicon, the cycle life will be reduced in practical use, which needs further solution.

    electrolyte
    The electrolyte of lithium ion battery is usually mixed with a linear carbonate with a high dielectric constant and a low dielectric constant linear carbonate [6]. Generally speaking, the electrolyte of lithium-ion batteries should meet the requirements of high ionic conductivity (10-3~10-2, S/cm), low electron conductivity, wide electrochemical window (0~5, V), good thermal stability (-40~60 DEG C) and so on. [2]. Six lithium fluoride and lithium salt, other new solvent purification, electrolyte preparation and functional additive technology continues to progress, the current direction of development is to further improve the high temperature performance of the voltage and improve the battery safety, ionic liquid electrolyte and solid electrolyte is developed.

   The diaphragm
   Because of its excellent mechanical properties, good electrochemical stability and relatively low price, polyolefin microporous membrane is the main market for the lithium-ion battery separator Market at present. [7]. It consists of polyethylene (PE) monolayer, polypropylene (PP) monolayer and PP/PE/PP three layer composite microporous membrane. Domestic manufacturers using dry process more, wet process PE diaphragm has also been a number of enterprises can mass production. With the development of ceramic coating technology, the diaphragm with high temperature resistance and high voltage will be the future research and development direction.

   2. single cell technology
   Since the basic design of lithium ion battery with SONY company in 1989, [8] released the same patent application, single shape, square cylindrical metal shell (aluminum / steel) and square soft bulk, originally used in notebook computer battery, 18650 cylindrical battery now Tesla and other companies to use for electric vehicles. The square battery has a large capacity, and the electric core is made by winding, Z, lamination, winding + lamination, positive plate, lamination, lamination and winding. The cylindrical core technology is the most mature and the manufacturing cost is low, but the heat dissipation of the large cylindrical battery is poor, so the small cylindrical battery is usually used. The battery capacity is large, the battery is large, and the management system is complex and expensive. The structure of the winding structure of the square battery is simple, but it is mainly suitable for the soft pole battery, and the lithium iron phosphate battery and the three element material can be used by the method except the spinel positive electrode material. The high reliability and long life stacked battery is suitable for all kinds of material systems. The batteries of the universal Volt plug-in hybrid electric vehicle and the Leaf pure electric vehicle are fabricated by lamination process. To 2015, the lithium iron phosphate battery than the energy of 140 W h/kg, three yuan material mixed lithium manganate battery than the energy of 180 W - h/kg, international NCA using small cylindrical battery than the energy of 240 W - h/kg, the next few years, lithium ion battery energy ratio to further improve, is expected to 2020 can reach a maximum of 300 W - h/kg.

    3. battery system technology
     From the commercialization of lithium ion power battery system perspective, the key technology including battery technology (integrated battery, thermal management, safety, electrical safety), battery management system (BMS) electromagnetic compatibility measurement technology, signal (such as single voltage and current) technique, the exact state of battery estimation, battery equalization control technology [9].


Fig. 3 simple power battery system diagram

     BMS and other key components of the battery system include sensors, controllers, actuators and other components, basically by the automotive electronics technology powers (Germany, Japan, the United States) monopoly. At present, some enterprises in China have successfully developed smart meters, which can replace foreign current, voltage and insulation sensors. The primary factors affecting the popularization and application of electric vehicles is the safety and cost of lithium ion battery, in addition to the cell body to further enhance the safety of life and the consistency of the battery, modular technology, battery technology (integrated battery, thermal management, safety, electrical safety) are also obvious the gap with foreign countries. At present, the international automotive industry battery technology is relatively mature, the domestic research units in the BMS electromagnetic compatibility technology, signal measurement technology, accurate estimation of battery state and battery equalization control technology has carried out in-depth research [2].

   The research and development of key technologies for battery power management include integrated battery electrochemical model, electrical safety design, battery state estimation, equalization management, fault diagnosis and calibration, charging management and so on. According to the structure design of battery and battery heat calculation analysis of battery thermal management system and key technology research, research on different thermal management technology of heat temperature effect, has low cost, simple process, battery thermal management solutions strong safety and reliability. Root cell structure lightweight with related structure and vehicle battery system as the research object, considering the coupling effect between the integrated optimization of the key technologies in the design work from the two aspects of the optimization of structural design and material selection of the structure of anti vibration, impact resistance and light weight. For parts materials, structure design, connection design optimization, on the overall design of the battery safety safety need to carry out the battery system based on electrical safety, mechanical safety and thermal safety on the battery system to carry out fault diagnosis, prediction, prevention and control of thermal safety monitoring and early warning and key technology.

     4. outlook
     The future for a long period of time, the electric car battery lithium ion battery is the most suitable, the development of lithium manganate cathode materials, three element system of cathode materials, lithium iron phosphate cathode materials, composite carbon anode materials, ceramic coating membrane, electrolyte salt and functional electrolyte technology support the battery technology progress and industrial development. Battery technology advances in applications, and security and reliability will be further improved over the next few years.

     Study on life model and model parameters of lithium ion battery, battery research groups, high efficiency and high capacity lithium ion battery research strategy equilibrium, electrothermal model and group battery pack temperature field analysis and control method of single battery charging, the battery group optimization fast charging method to carry out research. Battery power system should be combined with the vehicle product redesign and according to the needs of the future vehicle power battery design and manufacturing mode upgrade, the power battery based materials, battery manufacturing and system technology of the whole industry chain at the same time, efforts to improve product quality, reduce the scale of production costs, improve the competitiveness of the industry.





  



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