Discussion on the Effect of Battery on Fast Charging of New Energy Vehicle

Electric cars in use, the consumer the most worried about or charging time and range of the problem, under the current technical level, the charging time and range seldom meet together, so the power battery has developed two routes, one is the concentration range of specific energy, mainly through continuously improve the specific energy of lithium ion battery, thereby increasing range of electric vehicles; The second is the fast charging pie, which focuses on reducing the charging time, mainly by improving the quick charging performance of lithium ion batteries and shortening the charging time of electric vehicles. With the progress of technology and in-depth research on lithium battery materials, the problems encountered by the rapid charging technology may be solved one by one.

I. how to understand fast charging?

To understand fast charge, a technical term is inescapable -- charge and discharge ratio C, can be simply understood as the rate of charge and discharge. The charge and discharge rate of lithium ion battery determines how fast we can store a certain amount of energy in the battery, or how fast we can release the energy in the battery.

According to the subsidy policy for new energy vehicles in 2018, those with the charging ratio less than 3C belong to the category of non-fast-charging pure electric buses, while those with the charging ratio higher than 3C belong to the category of fast-charging pure electric buses. However, the subsidy division of quick charge only applies to new energy passenger cars, and there is no standard for passenger cars and logistics vehicles.

According to the definition of the industry and Ningde era, quick charging of electric vehicles refers to a charging mode with a charging current of more than 1.6c, that is, from 0% charging to 80% charging time of less than 30 minutes. Based on the opinions of many parties, the author puts forward that charging ratio less than 1.6c is slow charging, 1.6c-3c is small quick charging, and above 3C is fast charging. Most electric passenger cars can achieve "small and quick charging", and the charging ratio of fast charging passenger cars is mostly concentrated in 3c-5c.

If we think of the lithium ion battery as a rocking chair, the two ends of the rocking chair are the two poles of the battery, and the lithium ion is like a good athlete, running back and forth between the two ends of the rocking chair. During charging, lithium ions are formed on the positive electrode of the battery, and the lithium ions generated move to the negative electrode through the electrolyte. As a negative carbon, it has a layered structure with many tiny pores for lithium ions to be embedded in. The more lithium ions that are embedded, the higher the charging capacity.

For fast charging, lithium ions need to be embedded into the negative electrode at an accelerating moment. This challenges the anode's ability to absorb lithium ions quickly. By-products will appear in the negative electrode of the battery of common chemical system during the quick charging, which will affect the circulation and stability of the cell. Energy density and power density can be regarded as two directions in the same battery.

Both national policy orientation and enterprise technology layout generally pursue high energy density. When the power density of the battery is high enough and a car is loaded with enough power to avoid "range anxiety," the need for quick charging is reduced. However, it is difficult to be accepted by the market if the cost has not been reduced due to the large quantity of electricity. Therefore, if the battery cost can be controlled under the premise of convenient charging capacity + applicable range, user anxiety can be greatly alleviated, so that quick charging has the value of existence.

Ii. Application prospect of quick charging of batteries with different technical routes

The speed of charging is closely related to the overall technical and design requirements of power battery, charging pile, electric vehicle, power grid, etc., among which the biggest factor is the battery. Let's discuss the application trend of different types of power batteries in quick charging technology. Almost all kinds of anode materials can be used to make fast rechargeable batteries, but their applicability and advantages are different.

1. Ternary quick charge battery is more suitable for electric passenger cars

Ternary batteries are paid more attention to because of their high energy density. The materials themselves have excellent electrical conductivity, but their re-activity is too high.

The representative enterprises of ternary battery quick charging system are Ningde times and bick, etc. Ningde times has developed "superconducting electronic network" and "fast ion ring" technology, which can realize charging SOC from 5% to 85% within 15 minutes, energy density 190Wh/kg, cycle life of more than 2500 times, the main application field is passenger cars, it is expected to have the capacity of mass production in 2018.

With the introduction of silicon anode material, high nickel anode material and specially developed electrolyte, the 3.0 high-energy core launched by bick in May this year has an energy density up to nearly 250Wh/kg, and can achieve an ultra-long range of 500km. The charging strategy design can effectively shorten the charging time and improve the charging efficiency. In extreme emergency mode, the car can travel 60 kilometers on a 10-minute charge.

According to the usage habits of fuel oil vehicles, the charging time should be fully charged within 10-20 minutes, and the charging ratio should be at least 3-6c. At present, most of the pure electric passenger cars on the market are filled with 80% of the power in half an hour to one hour, which has improved a lot compared with the previous two or three hours' charging time, and it is expected to be further compressed into 20 minutes in the future.

2. Lithium iron phosphate quick charge product can be used

Lithium iron phosphate does not have innate advantages in the field of rapid charging. From the perspective of materials, the intrinsic conductivity of lithium iron phosphate is relatively low, which is only 1% of ternary materials. Therefore, it is necessary to optimize the conductivity of lithium iron phosphate to meet the demand of rapid charging. However, the material cost of lithium iron phosphate is relatively low. Combined with the mature technical background and stable product performance, lithium iron phosphate has a relatively broad application prospect, representing enterprises such as Ningde times and watma.

Limited by the extreme value of theoretical energy density, lithium iron phosphate has little room to play in energy density in the future. However, for commercial vehicles such as passenger cars, logistics vehicles and special vehicles, lithium iron phosphate system has been selected, the improvement of energy density is not necessary, and quick charge is more and more important.

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

Lithium manganate battery has the characteristics of power performance, discharge rate performance, good low temperature performance, high voltage frequency, and in the situation of three upstream raw materials soaring, the cost advantage of lithium manganate is gradually highlighted. However, it still needs to be improved in energy density, high temperature performance and other aspects. In recent years, the proportion of lithium manganate fast-rechargeable batteries in the field of plug-in hybrid buses has increased significantly, and the representative enterprises are citic guomeng guli, yepeng new energy and micro macro power.

However, the cycling performance of lithium manganate battery is poor under high temperature conditions, and the high temperature performance of lithium manganate battery can be improved by positive pole doping. However, the modified lithium manganate material is no longer the "original lithium manganate". The industry commonly used "multiple composite materials", the positive pole using ternary materials and lithium manganate mixed system, the negative pole using porous composite carbon, further improve the performance of fast charge, but the safety still need to focus on and constantly improve.

4. Lithium titanate quick rechargeable battery is suitable for pure electric bus

Lithium titanate power battery is named after the negative electrode material, the positive pole USES the ternary material, takes Zhuhai Yinlong, micro macro power, Tianjin jie wei as the typical enterprise. In terms of performance, lithium titanate battery has excellent low-temperature performance, safety and good recycling performance. As a fast-rechargeable battery, its multiplier performance has also been recognized by the industry. However, lithium titanate has two prominent problems at present: first, the energy density is relatively low, and under the pressure of constantly increasing energy density required by policies and the market, the current market share of lithium titanate is relatively low in the whole power battery market. Secondly, the cost of lithium titanate battery is significantly higher than other systems due to the influence of high-cost small metal materials such as titanium, nickel and cobalt.

Lithium titanate battery is obviously superior to other systems in terms of cycle life, which is determined by the material characteristics, namely "zero strain" characteristics. But its disadvantage is obvious, the energy density is low, the energy density is only half of the ternary system. In addition, due to the high price, most of them are used in quick confiscation and delivery. In the future, it is urgent to seek for more high-voltage anode materials and matching electrolyte to solve this defect.

5. New direction of quick charge -- titanium and niobium oxide cathode material

Titanium niobium oxide is developed on the basis of lithium titanate. Its main advantage is that the theoretical capacity of titanium niobium oxide is around 280mAh/g compared with the theoretical capacity of 175mAh/g of lithium titanate.

In October 2017, Toshiba officially announced that it had successfully developed a new generation of automotive lithium-ion batteries, which are expected to be commercially available in 2019. The battery USES titanium niobium oxide material, compared with the current ternary, lithium iron phosphate and other technologies, to achieve disruptive progress. The new battery has the advantages of high energy density and fast charging efficiency. It only takes 6 minutes to charge up to 90% of the battery and can travel 320 kilometers. Lithium batteries currently take an average of 30 minutes to charge up to 80 percent.

In addition, the concept of "graphene batteries" has long been a hot topic, but it is also controversial within the industry. In lithium batteries, graphene is mainly used as a cathode active material and conductive additive. In terms of quick charging ability, using graphene as a conductive agent or covering lithium iron phosphate/lithium ternary material with graphene can achieve better quick charging effect. However, the comprehensive cost, process difficulty and other indicators, there is still a great challenge.

Iii. Market prospect of quick charge products

With high energy density, fast charging and low price, this is the ideal power battery product that users most expect. However, "you can't have your cake and eat it too." under the existing lithium ion battery system, the most important five indicators of power battery, such as multiplier performance, energy density, service life, safety and price, are all fixed in the relatively stablespecial chart. If any indicator is improved, the other indicators will suffer relatively.

At present, quick charge power battery is mainly used in new energy bus, because of its strong selectivity to cities and audience units, that is, cities or units with relative financial support, tend to fast charge battery bus. However, from the perspective of market development potential, the growth rate and market scale of passenger cars and special logistics vehicles will be higher than that of passenger cars in the future, so the consumption structure of fast charging power batteries will shift to these two types of models in the future.

According to China's battery data, China produced 6,486 fast charging buses in 2017, and the installed capacity of batteries reached 597.52MWh, accounting for 6% of the total number of new energy buses. Among them, the charging rate of fast charging bus products is the highest at 6.42c. The production volume of 47771 vehicles and the battery capacity of 480.68mwh in the 3c-5c model were multiplied. The production of the model 5c-10c is 1715, and the battery capacity is 116.84MWh. At present, the quick charging ratio of fast charging bus is mainly concentrated between 3C and 5C. From the perspective of battery type, lithium titanate was the main battery material of fast charging bus in 2017, with the loading capacity of 571.54Mwh, accounting for 95.65%.

According to the shipments of 4 types of power batteries in 2017, 1.54GWh lithium manganate is partly used in plug-in hybrid vehicles, partly meeting the requirements of small quick charge, and 16GWh ternary battery vehicles partly meeting the requirements of small quick charge. On the whole, ternary quick rechargeable batteries are suitable for passenger cars, lithium iron phosphate, lithium titanate and other fast rechargeable batteries are suitable for passenger cars, lithium manganese acid fast rechargeable batteries are suitable for plug-in hybrid cars, titanium niobium oxide or fast rechargeable new direction.

The page contains the contents of the machine translation.