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1、 The development of electric vehicles needs fast charging technology

With the gradual depletion of oil resources, the reduction of resources and environmental pollution caused by automobile exhaust emissions are becoming increasingly serious. New energy vehicles become an important development direction and industry hotspot in the future. All countries in the world are vigorously developing new energy vehicles, and China has included them in the strategic emerging industries of the 7th World War. The development of energy saving and new energy vehicles is one of the important measures to reduce oil consumption and carbon dioxide emissions in China. The central and local governments at all levels have paid high attention to its development, and have successively issued various support and cultivation policies, creating a good policy environment for the development of new energy vehicles.

However, technical problems such as difficulty in charging and long charging time of new energy vehicles have always been obstacles to the promotion of electric vehicles. Many electric vehicle companies have not well considered the charging time while pursuing higher range. In the case that the charging facilities can not fully meet the demand, the core problem is that the charging technology of electric vehicles is not up to standard and can not meet the requirements of rapid charging. The power car that can meet the demand of fast charging has become the dominant demand of the industry, and the new market demand will inevitably lead to new technological innovation and product emergence. Therefore, the power lithium-ion battery manufacturers are also actively adapting to the demand of the industry development and starting to develop power lithium-ion batteries with fast charging function.

Generally, fast charging means that a large amount of electric energy can be charged into the battery in a short time, but there is no uniform regulation on the specific charging time and battery state of charge. According to the early regulations of the California Air Resources Board (CARB), the fast charging time of electric vehicles is 10 minutes (6C). Recently, the promotion of full charge in minutes and seconds is even more common. In terms of the electric vehicle industry, fast charging means that the battery can be fully charged in tens of minutes or even a few minutes, which is different from the 7-8h charging of slow charging.

It is completely feasible for lithium ion battery to realize fast charging, and the internal resistance of the battery will rise slightly in a short time. With the decay of life, the increase of internal resistance of the battery will lead to the decrease of power, which will reduce the battery charging capacity, and the excessive temperature rise will lead to the accelerated decay of life; At present, most lithium-ion power lithium batteries use graphite anode system, which is easy to precipitate active lithium metal during rapid charging at low temperature, which will also lead to a sharp decline in battery life during rapid charging at low temperature, and there is a safety risk of causing internal short circuit in the battery. Therefore, the quick-charge lithium-ion battery for new energy vehicles should be a technology-intensive product with advanced design concept and high manufacturing level.

2、 Fast charging solution

Schematic diagram of lithium ion battery charging process. It can be seen from the figure that the charging process of lithium-ion battery mainly includes: 1. Under the application of external load, lithium ion (Li+) is separated from the cathode material into the electrolyte; Diffusion of 2Li+in electrolyte; 3Li+is embedded in the negative electrode material and accompanied by a series of electron motions. Fast charging should realize the rapid and orderly progress of the above three processes. Therefore, to realize the rapid charging of lithium-ion power lithium batteries requires the joint development of various technologies. First of all, the selection of electrochemical material system for fast charging lithium ion batteries is the key to determine whether fast charging can be achieved and to solve the safety of fast charging; Secondly, the design of fast charging method is the necessary way to realize fast charging. Finally, the design of fast charging battery BMS also directly determines the application of fast charging battery.

1. Selection of fast-charging battery material system

At present, the research focus on the selection of materials system for fast-charging lithium-ion batteries mainly includes three aspects: positive electrode materials, negative electrode materials and electrolyte.

Common cathode materials for lithium ion power lithium batteries include lithium manganate (LMO), ternary materials (NCM and NCA) and lithium iron phosphate (LFP). It is generally believed that higher Li+diffusion coefficient and electronic conductivity can guarantee higher Li+transmission efficiency and lower electronic transmission resistance during charging and discharging.

Figure 2 shows the crystal structure of common cathode materials. The lithium manganate material shown in Figure 2 has a three-dimensional lithium ion diffusion channel, which can always maintain high power performance during charging and discharging.

As the receiver of Li+in the charging process, the negative electrode is the most important link in the design of fast-charging battery. The negative electrode material must have the ability to quickly accept a large number of embedded Li+, otherwise, during the fast charging process, Li+will deposit and precipitate on the negative electrode surface, forming lithium dendrites, which may cause safety hazards.

Among the current commercialized battery systems, there are two important systems suitable for rapid charging, one is lithium titanate anode system, and the other is composite amorphous carbon system. The crystal structure of lithium titanate, amorphous carbon and graphite anode is shown in Figure 3.

(Li4Ti5O12 spinel lithium titanate anode material Li7Ti5O12 new carbon-coated lithium anode material)

Graphite is the most commonly used anode material, but its layer spacing is small (0.354nm). In the fast charging mode, due to the increase of the interface reaction impedance, the graphite anode is easier to reach the lithium evolution potential than in the slow charging mode. The lithium ion cannot be embedded into the graphite anode normally, but is deposited on the surface of the anode in the form of atoms to form lithium dendrites, which shows that the external state of charge of the fully charged battery may still be 70%. Lithium titanate (LTO) anode is a spinel structure, zero-strain material, lithium ion can be de-inserted without restriction, and it is a nanoparticle, lithium ion migration path is short, high potential does not precipitate lithium, with long life, high magnification, high safety and excellent low-temperature performance. Therefore, amorphous carbon materials and lithium titanate are ideal negative electrodes for quick-charge lithium-ion batteries.

The LTO anode is of spinel structure and has the following characteristics: 1. It has good safety performance. The potential of lithium titanate phase with respect to lithium electrode is 1.55V, and the electrode voltage is high, which can prevent the formation of negative SEI film, and there is no short circuit due to the precipitation of metal lithium. It has good safety performance; 2. Excellent cycle performance, the volume expansion rate of graphite is about 9%, while lithium titanate is a zero-strain material, the crystal structure change is less than 1%, which makes it have excellent cycle performance and stable discharge voltage, and the service life has been reported to be 20000 times; 3. It can be charged and discharged quickly with large current; 4 Good temperature characteristics.

At home and abroad, there are many full battery products using lithium titanate as anode. Japan is the representative of Japan. Toshiba reported that under the condition of rapid charging and discharging, the capacity of lithium titanate battery decreased by less than 10% after about 3000 cycles. It can be used for more than 10 years if it is charged once a day- 30 ℃ can ensure more than 80% of the discharge capacity. According to the report of Argonne Laboratory in the United States, the charge and discharge capacity does not decay at 55 ℃, so it is applicable to a wide range of areas and has excellent energy storage.

In China, the lithium titanate battery technology has been used in batches in high-speed rail and pure electric vehicles, with relatively mature technical methods and application promotion cases.

Amorphous carbon, also known as organic cracked carbon, has no macroscopic crystallographic properties, but there are different degrees of ordered structures in the fine region, called microcrystals. Amorphous carbon is actually graphite with low graphitization degree, which is composed of graphite microcrystals and amorphous regions. From the internal structure, it is a mosaic structure composed of two-dimensional disordered microcrystals with different sizes. The currently mature products of amorphous carbon materials include soft carbon and hard carbon, which are used in the anode system of lithium-ion batteries. Due to the large layer spacing, they can provide a channel for rapid lithium ion removal, and the available potential is high. The safety and low temperature characteristics of the battery are superior to those of the graphite system. However, the existing problem is that the efficiency is very low for the first time, generally only about 60%, resulting in low specific energy, The direction of most applications is hybrid. In foreign countries, Hitachi is the representative. Hitachi's third generation hybrid power product is hard carbon system, with current capacity of 4.4Ah, average voltage of 3.6V, power density of 3000W/kg and energy density of 61Wh/kg; Later, Hitachi launched the fourth generation of products. The overall dimension is 120mm× 90mm× 18mm。 The current capacity is 4.8Ah, the average voltage is 3.6V, and the energy density is 72Wh/kg. The power characteristics of these two generations of products are relatively good, but the energy density is low, and there are problems in the use of pure electric vehicles.

The design of special electrolyte for quick-charge battery is generally based on two aspects: one is to improve the conductivity of the electrolyte and reduce the Li+diffusion resistance; the other is to prepare high concentration electrolyte to reduce the concentration polarization at high magnification. In addition, adding additives to the electrolyte is also a common method to improve its fast charging performance. After determining the positive and negative electrode materials and the electrolyte system, the electrode design of quick-charge batteries often adopts the thinning method. By reducing the coating thickness, the diffusion path of lithium ion is reduced, which is conducive to improving the rapid charging capacity of the battery. At the same time, in order to reduce the resistance of the electrode plate, the content of the conductive agent in the electrode material will also be increased, so as to reduce the internal resistance of the battery and improve the charging capacity of high current.

2. Design of fast charging method

The traditional charging concept based on lead-acid batteries does not allow rapid charging. Before the early 1960s, the charging rule of lead-acid batteries was that it was not allowed to use a current of more than 1C for charging, otherwise a large amount of gas would be produced. With the progress of science, researchers began to explore methods that can be used for fast charging, and made many achievements. It has been proved that all kinds of batteries can achieve 3~10min fast charging and reach 30%~70% state of charge. At present, the fast charging methods developed at home and abroad mainly include large current constant current method, multi-stage constant current method, large current limit constant voltage method, current limit open constant voltage method, pulse charging method (including ordinary pulse charging, pulse charging with negative pulse and slow pulse charging) and special fast charging method developed according to the basic charging theory of specific battery system.

Mengguli has some innovation in the research of fast charging method. On the premise of ensuring the safety of the battery, the ICA analysis method is used to find the voltage range with the maximum capacity. In this range, the high current charging is used. When the cut-off potential is reached, the current reduction charging is used to ensure the fast charging while taking into account the safety and life.

3. Fast charging battery management system

Lithium ion power lithium battery does not appear in the form of single battery in practical application, but is arranged in the battery box in a certain structure and sequence after a series of series and parallel connection, supplemented by battery management system and other electrical components, so as to achieve stable and safe power storage and release. Advanced fast charging technology also requires the efficient cooperation of battery management system. Therefore, the research work of quick-charging lithium ion power battery must include the development of battery management system and the optimization of battery box structure.

The battery management system (BMS) is the link between the battery and the user. It can realize the collection function of the dynamic data of the electric vehicle and monitor the various dynamic data of the vehicle operation in real time. It can be said that the BMS management system is the brain of the lithium-ion power lithium battery module, and even the brain of the electric vehicle. It must be able to effectively monitor and accurately predict the state of the battery pack and the whole vehicle, prevent the abuse of the power lithium battery, and cut off the power supply in time before the accident danger occurs, reduce the accident level and reduce the accident loss. Especially in the fast charging mode, in order to realize the conversion of a large amount of electric energy in a short time, it is necessary to accurately monitor and quickly predict the real-time state of each battery during the fast charging process.

In addition, with the increase of charging current, the heat production of the battery increases and the temperature rise increases, which requires optimizing the structure of the battery and strengthening the heat dissipation of the battery box. At present, the commonly used cooling methods of battery box are air-cooled and liquid-cooled. The thermal management system of lithium-ion batteries should not only strengthen heat dissipation but also keep heat at low temperature, especially for fast-charging batteries. At low temperature, the ionic conductivity of the electrode material and electrolyte decreased to varying degrees, which increased the difficulty of fast charging of the battery. The guarantee of fast charging at low temperature can be considered from two aspects. First, the battery can be quickly heated by the heating system, and the battery can be operated at a lower temperature with thermal insulation measures; The second is to develop fast-charging lithium-ion power lithium battery suitable for wide temperature range, that is, fully consider the increase of battery polarization and the decrease of conductivity at low temperature in the battery design stage.

4. Introduction to technical progress of quick-charge lithium-ion battery

Research institutions and new energy companies have invested great efforts in the research and development of practical fast charging technology, power lithium batteries for fast charging and charging facilities for fast charging, and have achieved many results.

Qualcomm and Texas Instruments have played a leading role in fast charging technology. Qualcomm's QuickCharge 2.0 is an important fast charging technology developed from its 1.0 technology and applied to 3C products. It is reported that mobile devices using this technology can reduce the charging time by 75%. Its core idea is to increase the charging current limit and increase the charging power by simultaneously increasing the voltage and current. MaxLife rapid charging technology produced by Texas Instruments in the United States uses innovative battery aging systematization model, which can significantly shorten the charging time. Moreover, the laboratory test data also shows that the technology can extend the service life of the battery by 30%. Using the Impedance Track battery capacity measurement technology, MaxLife algorithm can accurately predict and prevent the charging conditions that lead to battery aging. There are three important technical advantages: 1. The battery capacity measurement technology based on Impedance Track can prevent the battery aging caused by high rate rapid charging, and accurately control the charging voltage and current and the charging end time; 2. It can reduce software overhead, reduce the total bill of materials cost, reduce space, and improve battery safety and heat dissipation management; 3 The charging algorithm can be automatically adjusted for different platforms and batteries with higher capacity.

Invented by EeshaKhare, the fast charging technology that can charge smart phones in 20~30s won the highest award at the 2013 International Science Expo. The invention is a high-performance supercapacitor composed of hydrogenated titanium dioxide and polyaniline nanorods. This capacitor can load a lot of energy in a small space. It can not only charge fast, but also save electricity for a long time. In addition, the charging cycle of the device reaches 10000 times, while the traditional rechargeable battery only has 1000 times.

The above fast charging technologies are derived from 3C products such as mobile phones, and cannot be applied to the fast charging of power lithium batteries for electric vehicles. However, many of its design concepts and control algorithms can be used for reference. Taking the impedance tracking battery capacity measurement technology of MaxLife rapid charging technology as an example, it has changed the current detection method of voltage or current used by common battery management systems. Therefore, the analysis of other types of battery rapid charging technology can enrich the design ideas and benefit the development of lithium ion power battery rapid charging technology.

In terms of research and development of power lithium batteries that adapt to the fast charging mode, Mengguli has successfully developed high-performance power lithium batteries that can be charged at a high rate of 100C. When the newly developed power lithium battery is charged at 100C, it can charge about 16% of the capacity in only 6s, and then charge more than 70% of the capacity in 1.7min. Of course, 100C fast charging is a level that can be achieved in the laboratory, which does not mean that it will be used in practical applications, but it shows that there is much room for improvement in the rate characteristics of lithium-ion batteries. The development of this product means that the super charging technology can be realized from the perspective of power lithium battery. As long as the external infrastructure conditions meet the demand of supplying large current, the super charging of electric vehicles can be realized. The technology disclosed by Mengguli also proves that lithium ion power battery may have greater potential to be explored.

As an electric vehicle company, Tesla has also done a lot of work in the development and application of rapid charging technology. At present, the Tesla super charging pile can charge 50% of the battery in 20 minutes (200 km), and the battery can be fully charged in 80 minutes. This is due to Tesla's unique charging gun design: use Tesla's super charging pile to charge. The sensor in the charging gun will detect the temperature change of the battery in the vehicle at any time. If the battery temperature is too high, the charging gun will immediately send a signal to reduce the charging intensity and reduce the battery temperature; At the same time, the cooling system in the battery panel also makes corresponding response synchronously, so as to strengthen the cooling force synchronously. In other words, Tesla's charging gun seems to have nerves in it. It can sense the temperature change of the battery in the car and automatically adjust the charging intensity according to the temperature of the battery. Tesla's simple charging process is a highly collaborative work of the charging gun, battery cooling system and charging pile. Based on such technical advantages, JBStrubal, the co-founder of Tesla, said confidently that the charging time of Tesla would be reduced to 5~10min in the future (the endurance of Tesla ModelS would exceed 400km).

The realization of Tesla's concept of 5min ultra-fast charging of electric vehicles may not be far away. Recently, StoreDot Israel announced that it had successfully developed a variety of automobile super charging technologies, including battery packs and special charging piles. The introduction shows that StoreDot's super-charging technology can replenish the power of electric vehicles with a range of 480 km in 5 minutes, which is similar to the time when traditional vehicles fill up a tank of fuel. StoreDot CEO Gordon Myersdorf pointed out that the battery used in this super charging technology has adopted a lot of innovative designs, and the company has developed a new technology multifunctional electrode (MFE). MFE uses conductive polymers and metal oxides as battery materials. The former allows the battery to receive rapid charging, while the latter is used to slowly flow rapidly polymerized lithium ions into the electrode. This fast and slow process not only ensures the rapidity of charging, but also prevents electrode collapse or short life. In addition, StoreDot's quick-charge batteries have modified all components of traditional lithium-ion batteries (including positive pole, negative pole, battery separator, etc.), so that they can meet the requirements of rapid charging. The internal resistance of this fast-charging battery is very small, and the heat generated during charging is very small. Its life is about three times that of ordinary lithium-ion battery, and its cost is only 20%~30% higher than that of lithium-ion battery. StoreDot plans to launch the prototype of this super charging system in 2016 and strive to achieve commercialization in 2017.

It can be predicted that with the continuous progress of science and technology, the application of various new fast charging technologies mentioned above will be gradually popularized, and there will inevitably be many more novel fast charging technologies. The emergence of new technologies depends not only on the leap-forward creation of leaders or companies, but also on the continuous absorption of innovative energy from existing technologies, mutual learning and mutual promotion.

3、 Application of quick-charge lithium-ion batteries in China

At present, the application of quick-charging lithium-ion power lithium battery is mainly concentrated in the field of electric bus, and there is no successful application example of electric taxi. In the process of promotion and application of quick-charge electric buses, some companies led by CATL, Weihong, Zhuhai Yinlong and Mengguli have worked in their respective technical routes for many years and become leaders in the field of quick-charge batteries by virtue of their core competitiveness.

The energy density of CATL's lithium iron phosphate/quick-charging graphite battery system is about 70~75kWh/kg, which can realize 10~15min charging, and the cycle life can reach more than 10000 times. Moreover, the battery system is relatively safe due to its low cost. However, even though the lithium iron phosphate system has solved the problems of poor conductivity and slow ion diffusion rate, it still has some inherent defects such as low tap density, poor low-temperature performance and poor monomer consistency.

The energy density of the ternary material/porous hard carbon battery system of Weihong Company is about 60~65Wh/kg, which can realize 10~15min charging, and the cycle life is 5000~6000 times. The cost is high. The disadvantage is that the side reaction may be large under high voltage.

Supported by the Beijing Municipal Science Committee and undertaken by Mengguli, the research project on the key technologies for the industrialization of quick-charging power lithium batteries has successfully completed all the work, and the quick-charging battery system with a 10-minute quick-charging capacity can be realized. This research achievement won the first prize of Beijing Science and Technology Award in 2015. This article introduces two examples of the quick-charging project, so as to have a deeper understanding of the application and promotion of the quick-charging lithium-ion battery technology.

1. Beijing Xiaoying Fast Charging Pure Electric Project

The Beijing Xiaoying quick-charge pure electric bus project has been put into operation since January 2015. It adopts the Futian Ouhui 12m model and carries the Mengguli quick-charge lithium battery. The battery system has 8 boxes of batteries, which are assembled by 7 and 144 strings. The battery system capacity and voltage are 533V/245Ah, and the total power is 130KWh. It is important to use 16 chargers with a power of 380kW and a maximum output current of 400A.

After more than one year of operation, the maximum operating mileage of the vehicle has reached 64000 km, and the average operating mileage is about 45000 km. Each vehicle will return to the station after running for about 60 km (as shown in Figure 4) for charging, with an average charge of about 4 kWh per minute and an average charge time of about 15 minutes (see Table 3 for details).

During the rapid charging process of the battery system, the average temperature rise is 6 ℃, and the battery system works stably. The thermal management design provides a good working temperature environment for the battery system, ensures that the battery system is always in a reasonable working temperature environment, and ensures the stable operation of the vehicle.

Through the return inspection of Xiaoying fast-charging battery, from the perspective of capacity attenuation, the capacity attenuation rate of Xiaoying fast-charging battery is 5.2% during the 14 months of operation (see Table 4 for details), which is equivalent to about 4.5% in the first year of operation. In theory, the capacity decay of the battery is the largest at the initial stage, and gradually flattens at the later stage. Therefore, it can be basically judged that the battery of the Xiaoying fast-charging project can guarantee a five-year capacity retention rate of more than 80%.

2. Zhangjiakou Chongli and Chicheng fast charging pure electric bus project

As a new energy demonstration project for the 2022 Winter Olympics, Zhangjiakou, Hebei Province, has carried a total of 41 quick-charging pure electric buses with Mengguli 25Ah batteries (3 parallel 156 strings, 43kWh) since the end of 2015, distributed in Chongli County and Chicheng County, Hebei Province. Chongli County is the ski competition site for the Winter Olympic Games, while Chicheng County is the hot spring tourist destination.

Because Chongli County and Chicheng County are located in the northern mountains, and the minimum temperature in winter is lower than - 20 ℃, some vehicles are equipped with the battery with wide temperature range that is newly designed and developed by Mengguli, which can meet the service requirements of - 30 ℃, have more excellent power performance at low temperature, lower heat generation of the battery, and improve the cycle life.

The vehicle with wide temperature range battery needs about 20 minutes to fully charge from 30% SOC to 160A current at low temperature (average temperature is about - 10 ℃, battery temperature is 0 ℃~10 ℃). If the charger power is increased, the charging time will be further shortened. The wide temperature range battery allows 3C charging strategy between 0~10 ℃ (see Figure 5 for details). It can meet the charging time requirements without heating and only taking thermal insulation measures, It is very suitable for popularization and application in the north and northeast cold areas.

4、 Conclusion

Although the application of quick-charging lithium-ion power lithium battery is still in a small range of use, the quick-charging mode has brought great convenience to the operation of pure electric buses. First, the configuration capacity of battery pack has been reduced, reducing the cost; Secondly, with the reduction of the volume and weight of the battery, the space utilization rate and safety of the vehicle have been improved, and the fast charging has also greatly shortened the charging time and increased the operating income. However, the comprehensive promotion and application of fast charging technology still faces various problems, such as high current charging has high requirements on local power grid, low range of fast charging battery, small range of state subsidies, and the inability to meet the purchase tax exemption conditions, which increases the cost of car purchase, and the technical standards for fast charging high-power charging piles have not been unified, and the compatibility policy has not yet been introduced. These limits the application of quick-charge lithium batteries.

However, in the foreseeable future, the market scale of quick-charging lithium-ion power lithium battery is huge. First of all, the advantages of domestic urban buses are very obvious. Stimulated by the government subsidy policy, the scale of urban buses may reach hundreds of millions of kilowatt-hours by 2020; Secondly, under the pressure of environmental protection, European cities will have a stronger desire for the electrification of vehicles, and the difference in the price of oil in Europe is very large, and the power supply is relatively sufficient, so it is expected that the fast charging market will rise rapidly.

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