The development prospects of fast charging technology

In the use of electric vehicles, consumers are most worried about the charging time and cruising range. Under the current technical level, charging time and cruising range are difficult to achieve, so the power battery has developed two routes, one It is the specific energy class that focuses on the cruising range, mainly by continuously increasing the specific energy of the lithium-ion battery, thereby increasing the cruising range of the electric vehicle. The second is to focus on reducing the charging time, mainly by improving the lithium-ion battery. Fast charging performance, shortening the charging time of electric vehicles. With the advancement of technology and in-depth research on lithium battery materials, the problems that have been encountered in fast charging technology may be solved one by one.

First, how to understand the fast charge?

To understand fast charge, a professional term cannot escape - charge and discharge rate C, can be simply understood as the rate of charge and discharge. The charge-discharge rate of a lithium-ion battery determines how fast we can use to store a certain amount of energy into the battery, or how fast the energy inside the battery is released.

According to the 2018 new energy vehicle subsidy policy, the charging rate is less than 3C, which belongs to the non-fast charge class pure electric bus. The charging rate is higher than (including) 3C, which belongs to the fast charging type pure electric bus. However, the subsidy division of fast charge is only for new energy buses, no passenger cars and logistics vehicles.

According to the definition of the industry and Ningde era, electric vehicle fast charging refers to the charging method with charging current greater than 1.6C, that is, the technology from 0% charging to 80% time less than 30 minutes. The author comprehensively expressed the opinion that the charging rate is less than 1.6C for slow charging, 1.6C-3C for small fast charging, and 3C for fast charging. Most electric passenger cars can achieve "small fast charge", and the charge rate of fast charge buses is mostly concentrated in 3C-5C.

If we compare the lithium-ion battery to a rocking chair, the two ends of the rocking chair are the two poles of the battery, and the lithium ion is like an excellent sportsman, running back and forth at both ends of the rocking chair. When charging, lithium ions are generated on the positive electrode of the battery, and the generated lithium ions move to the negative electrode through the electrolyte. The carbon as the negative electrode has a layered structure, and it has many micropores for intercalating lithium ions reaching the negative electrode. The more lithium ions are embedded, the higher the charging capacity.

At the time of fast charging, lithium ions need to be accelerated to be instantaneously embedded in the negative electrode. This is very challenging for the ability of the negative electrode to quickly receive lithium ions. In the battery of the general chemical system, by-products appear in the negative electrode during fast charging, which affects the cycle and stability of the battery. Energy density and power density can be said to be the two directions in the same battery.

Whether it is national policy orientation or corporate technology layout, the pursuit of high energy density is generally pursued. When the energy density of the power battery is high enough, the load of one car is large enough to avoid the so-called "mileage anxiety", and the demand for fast charging will be reduced. However, the power is large, and it is difficult to be accepted by the market if the cost does not fall. Therefore, if you can control the battery cost, with convenient charging capacity + applicable cruising range, you can greatly alleviate user anxiety, so that there is value in fast charging.

Second, the application of fast charging applications for different technology routes

The speed of charging is closely related to the overall technical and design requirements of power batteries, charging piles, electric vehicles, and power grids. The biggest factor is the battery. We will specifically discuss the application trends of different types of power batteries in the direction of fast charging technology. Almost all kinds of positive electrode materials can be used to make fast-fill batteries, but their suitability and advantages are different.

1, ternary fast charge battery is more suitable for electric passenger cars

The ternary battery is more valued because of its higher energy density. The material itself has excellent electrical conductivity, but the reactivity is too high, which poses a great challenge to the fast charge safety.

The representative enterprises of the ternary battery fast charging system are Ningde era, Bick and so on. Ningde era developed "superconducting subnet" and "fast ion ring" technology, which can achieve SOC charging from 5% to 85% in 15 minutes, energy density 190Wh/kg, cycle life more than 2500 times, the main application field is passenger. The car is expected to have the capacity for mass production in 2018.

The latest 3.0 high-energy core introduced by BAK in May this year, through the introduction of silicon-based anode materials, high-nickel cathode materials, and specially developed electrolytes, has an energy density of nearly 250 Wh/kg, which can achieve a long cruising range of 500 km. Through the charging strategy design, the charging time is effectively shortened and the charging efficiency is improved. In extreme emergency mode, you can drive for 60 kilometers in 10 minutes.

According to the usage habits of the fuel vehicle, it is necessary to fully charge the charging time within 10-20 minutes, and the charging magnification needs to be at least 3-6C. At present, most of the pure electric passenger cars on the market are filled with 80% of electricity in half an hour to one hour. The charging time has increased a lot in the past two or three hours, and it is expected to be further compressed to 20 minutes in the future.

2, lithium iron phosphate fast charge passengers are available

lithium iron phosphate does not have inherent advantages in the field of fast charging. From the material point of view, the intrinsic conductivity of lithium iron phosphate material is relatively low, only one percent of ternary material and it is necessary to optimize the conductivity of lithium iron phosphate material, to match the needs of fast charging. However, the material cost of lithium iron phosphate is relatively low. Combining mature technical background and stable product performance, it has a wide application prospect. Representative enterprises include Ningde era, Waterma and so on.

Limited by the extreme limit of the theoretical energy density, lithium iron phosphate does not have much room for energy density in the future. However, for commercial vehicles such as passenger cars, logistics vehicles, and special vehicles, the lithium iron phosphate system has been adopted, and the improvement of energy density is not necessary, and the fast charging is more and more important.

3. Lithium manganate battery is suitable for plug-in hybrid bus

The lithium manganate battery has the characteristics of power performance, discharge rate performance, low temperature performance, high voltage frequency, and the cost advantage of lithium manganate is gradually highlighted under the madness of raw materials in the ternary upstream. However, there is still a need to improve in terms of energy density and high temperature performance. In recent years, lithium manganate fast-charge batteries accounted for a significant increase in the field of plug-in hybrid buses, representing companies such as CITIC Guoan Mengli, Yipeng New Energy, and Weihong Power.

However, the lithium manganate battery has poor cycle performance under high temperature conditions, and the high temperature performance of the lithium manganate battery can be improved by the positive electrode doping, but the modified lithium manganate material is not the "original lithium manganate". In the industry, “multi-component composite materials” are commonly used. The positive electrode adopts a ternary material and a lithium manganate mixed system, and the negative electrode uses a porous composite carbon to further improve the performance of fast charging, but the safety still needs to be focused and continuously improved.

4, lithium-titanate fast charge battery for pure electric bus

The lithium titanate power battery is named after the anode material, and the positive electrode is made of ternary material. Zhuhai Yinlong, Weihong Power and Tianjin Jiewei are typical enterprises. From the performance point of view, the lithium titanate battery has superior low-temperature performance, safety and recycling performance, and has been recognized by the industry as the rate performance of the fast-charge battery. However, the current outstanding problems of lithium titanate have two points: First, the energy density is relatively low. Under the pressure of policy and market demand for increasing energy density, the current market share of lithium titanate accounts for the entire power battery market. Second, due to the impact of high-cost small metal materials such as titanium, nickel, and cobalt, the cost of lithium titanate batteries is significantly higher than other systems.

Lithium titanate batteries are significantly better in cycle life than other systems of fast-fill batteries, which are determined by the nature of the material itself, ie, "zero strain". However, its disadvantage is obvious, the energy density is low, and the energy density is only about half of the ternary system. In addition, the price is too high, and most of them are currently applied in fast charging buses. It is urgent to seek higher voltage cathode materials and matching electrolytes to solve this defect.

5, fast filling new direction - titanium niobium oxide anode material

Titanium strontium oxide is developed based on lithium titanate. The main advantage is that the theoretical capacity of titanium strontium oxide is about 280 mAh/g with respect to the theoretical capacity of lithium titanate is 175 mAh/g.

In October 2017, Toshiba officially announced that it has successfully developed a new generation of lithium-ion batteries for vehicles, which is expected to be commercially available in 2019. The battery uses titanium bismuth oxide material, which is a subversive advancement compared to current technologies such as ternary and lithium iron phosphate. The new battery has the advantages of high energy density and fast charging efficiency. It can reach 90% of electricity in just 6 minutes of charging, and can travel 320 kilometers. At present, lithium batteries take an average of 30 minutes to charge up to 80%.

In addition, the concept of "graphene battery" has been hot, but there are controversies in the industry. In the application of lithium batteries, graphene is mainly used as a negative active material and a conductive additive. In terms of fast charge capacity, the use of graphene as a conductive agent or the coating of lithium iron phosphate/ternary lithium material with graphene can achieve a better fast charging effect. However, the indicators such as comprehensive cost and process difficulty are still very challenging.

Third, fast charge product market prospects

High energy density, fast charging and low price are the ideal power battery products that users are most expecting. However, "fish and bear's paw can't have both". Under the existing lithium-ion battery system, the five most important indexes of power battery, such as rate performance, energy density, life, safety and price, are fixed in a relatively stablespecial chart. Inside, to improve any one indicator, other indicators will be relatively lost.

At present, fast-charged power batteries are mainly used in new energy buses. Because they are highly selective to cities and audiences, cities or units with relative financial support tend to be fast-filled battery buses. However, from the perspective of market development potential, the growth rate and market size of passenger cars and special logistics vehicles will be higher than that of passenger cars in the future. Therefore, the consumption structure of fast-filled power batteries will shift to these two types of vehicles in the future.

According to the battery China data, in 2017, the output of fast-fill passenger cars in China was 6,486, and the installed capacity of batteries reached 597.52 MWh, accounting for 6% of the total number of new energy buses. Among them, the charging rate of fast-fill passenger cars is up to 6.42C. The output of the 3C-5C model is 4,771 units, and the installed capacity of the battery is 480.68MWh; the output of the 5C-10C model is 1,715 units, and the installed capacity of the battery is 116.84MWh. At present, the fast charge rate of fast-fill passenger cars is mainly concentrated between 3C-5C. From the perspective of battery type, the battery material of the fast-filling passenger car in 2017 is mainly lithium titanate, and the installed capacity is 571.54Mwh, accounting for 95.65%.

According to the four types of power battery shipments in 2017, 1.54GWh of lithium manganate is used for plug-in hybrid vehicles, partially meeting the requirements of small fast charge, and the 16GWh ternary battery part meets the requirements of small fast charge. Overall, the ternary fast-charge battery is suitable for passenger cars, lithium iron phosphate, lithium titanate and other fast-charge batteries are suitable for passenger cars. Lithium manganate fast-charge batteries are suitable for plug-in hybrid cars, titanium bismuth oxide or fast charge new direction.

The page contains the contents of the machine translation.