In the use of electric vehicles, consumers are most worried about the charging time and cruising range. At the current level of technology, it is difficult to have both charging time and cruising range. Therefore, two routes have been developed for power lithium-ion batteries. One is to focus on the specific energy of the cruising range. The important is to increase the cruising range of electric vehicles by continuously increasing the specific energy of the lithium-ion battery. The second is to focus on the fast charging to reduce the charging time. The most important is to improve The fast charging performance of lithium-ion batteries shortens the charging time of electric vehicles. With technological progress and in-depth research on lithium-ion battery materials, the problems encountered in fast charging technology may be solved one by one.
1. How to understand fast charge?
To understand fast charge, a professional term cannot escape-charge and discharge rate C, which can be simply understood as the rate of charge and discharge. The charging and discharging rate of lithium-ion batteries 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 meaning of the industry and CATL, electric vehicle fast charging refers to a charging method with a charging current greater than 1.6C, that is, a technology that takes less than 30 minutes to charge from 0% to 80%. Based on various opinions, the author proposes that the charging rate is less than 1.6C as slow charging, 1.6C-3C is small fast charging, and 3C or higher is fast charging. Most electric passenger cars can achieve "small fast charging", and the charging rate of fast charging 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 athlete running back and forth on 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, which has many micropores for the insertion of lithium ions that reach the negative electrode. The more lithium ions are inserted, the higher the charging capacity.
During fast charging, lithium ions need to be accelerated and inserted into the negative electrode instantaneously. This is a great challenge to the negative electrode's ability to quickly receive lithium ions. The battery of ordinary chemical system will have by-products in the negative electrode during fast charging, which will affect the cycle and stability of the battery. Energy density and power density, in the same battery, can be said to be the two directions of losing the other.
The current technological layout of new energy vehicle companies generally pursues high energy density. When the energy density of the power lithium-ion battery is high enough and the electric capacity of a car is large enough to prevent the so-called "mileage anxiety", the demand for fast charging will be reduced. However, the electricity is large, and it is difficult to be accepted by the market if the cost is not lowered. Therefore, if you can use convenient charging capacity + applicable cruising range under the premise of controlling the battery cost, you can greatly alleviate user anxiety, so that fast charging has value.
2. The application prospects of fast charging of batteries with different technical routes
The speed of charging is closely related to the overall technical and design requirements of power lithium-ion batteries, charging piles, electric vehicles, and power grids. The biggest influencing factor is the battery. We specifically discuss the application trend of different types of power lithium-ion batteries in the direction of fast charging technology. Almost all kinds of cathode materials can be used to make fast-charging batteries, but their applicability and advantages and disadvantages are different.
1. The ternary fast-charging battery is more suitable for electric passenger cars
Ternary batteries are more valued because of their higher energy density. The material itself has excellent electrical conductivity, but the reaction activity is too high, so it poses a greater challenge to the safety of fast charging.
Representative companies of the ternary battery fast charging system include CATL and BAK. CATL has developed "superconducting subnet" and "fast ion ring" technologies, which can charge SOC from 5% to 85% within 15 minutes, with an energy density of 190Wh/kg, and a cycle life of more than 2500 times. The important application area is passenger use. The car will have mass production capacity in 2018.
BAK's latest 3.0 high-energy core introduced in May this year, through the introduction of silicon-based anode materials, high-nickel cathode materials, and specially developed electrolyte, its energy density is as high as 250Wh/kg, and it can achieve an ultra-long cruising range of 500 kilometers. Through the charging strategy design, the charging time is effectively shortened and the charging efficiency is improved. In extreme emergency mode, it can travel 60 kilometers in 10 minutes.
According to the usage habits of fuel vehicles, the charging time must be fully charged within 10-20 minutes, and the charging rate must be at least 3-6C. At present, most of the pure electric passenger cars on the market are fully charged with 80% power in half an hour to one hour, which has improved a lot from the previous two or three hours of charging time, and it is expected to be further compressed to within 20 minutes in the future.
2. Lithium iron phosphate fast charging is 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 materials is relatively low, only one percent of ternary materials. The conductivity of lithium iron phosphate materials should be optimized In order to meet the needs of fast charging. However, the material cost of lithium iron phosphate is relatively low, combined with mature technical background and stable product performance, it has a wider application prospect, representing companies such as CATL and BYD.
Limited by the extreme limit of the theoretical energy density, lithium iron phosphate has little room for energy density in the future. However, as far as commercial vehicles such as passenger cars, logistics vehicles, and special vehicles are now using lithium iron phosphate systems, the increase in energy density is not necessary, and fast charging is increasingly showing its importance.
3. Lithium manganate battery is suitable for plug-in hybrid buses
Lithium manganate battery has the characteristics of power performance, discharge rate performance, good low temperature performance, and high voltage frequency. In addition, under the trend of skyrocketing ternary upstream raw materials, the cost advantage of lithium manganate is gradually becoming prominent. However, energy density, high-temperature performance, etc. still need to be improved. In recent years, the proportion of lithium manganese oxide fast charge batteries in the field of plug-in hybrid buses has increased significantly.
However, lithium manganate batteries have poor cycle performance under high temperature conditions. The high temperature performance of lithium manganate batteries can be improved by positive electrode doping, but the modified lithium manganate material is no longer the "original lithium manganate". Commonly used "multiple composite materials" in the industry, the positive electrode uses a mixed system of ternary materials and lithium manganate, and the negative electrode uses 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 is suitable for pure electric bus
Lithium titanate power lithium-ion battery is named after the negative electrode material, and the positive electrode adopts ternary material. The typical companies are Zhuhai Yinlong, Weihong Power, and Tianjin Gateway. From the performance point of view, lithium titanate batteries have superior low-temperature performance, safety and cycle performance, and their rate performance as a fast-charge battery has also been affirmed by the industry. However, lithium titanate currently has two outstanding problems: First, the energy density is relatively low. Under the pressure of policy and market requirements to continuously increase energy density, the current market share of lithium titanate is in the entire power lithium-ion battery market. Accounted for relatively low. Second, affected by 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 than other fast-charging batteries in terms of cycle life, which is determined by the characteristics of the material itself, that is, the "zero strain" characteristics. But its disadvantages are obvious, the energy density is low, and the energy density is only about half of the ternary system. In addition, the price is relatively high, and most of them are currently used in fast-charging buses. In the future, it is urgent to seek higher voltage cathode materials and matching electrolytes to solve this defect.
5. New direction of fast charging-titanium niobium oxide anode material
Titanium niobium oxide is developed on the basis of lithium titanate. The important advantage is that the theoretical capacity of lithium titanate is 175mAh/g, and the theoretical capacity of titanium niobium oxide is about 280mAh/g.
In October 2017, Toshiba officially announced that it has successfully developed a new generation of lithium-ion batteries for vehicles, which will be commercially available in 2019. The battery uses titanium niobium oxide material, and has achieved disruptive progress compared with the current ternary and lithium iron phosphate technologies. The new battery has the advantages of high energy density and fast charging efficiency. It can reach 90% of the power in just 6 minutes and can travel 320 kilometers. At present, it takes an average of 30 minutes for lithium-ion batteries to charge to 80% capacity.
In addition, the concept of "graphene battery" has been relatively hot, but there are also controversies in the industry. In the application of lithium-ion batteries, graphene is important as a negative electrode active material and conductive additive. In terms of fast charging capability alone, using graphene as a conductive agent or coating lithium iron phosphate/ternary lithium materials with graphene can achieve a better fast charging effect. However, from the perspective of comprehensive cost, process difficulty and other indicators, it is still very challenging.
3. Market prospects of fast charging products
With high energy density, fast charging, and low price, this is the ideal power lithium-ion battery product that users are most looking forward to. However, "fish and bear's paw cannot have both." Under the existing lithium-ion battery system, the five most important indicators of power lithium-ion batteries such as rate performance, energy density, life, safety, and price, are all fixed in a relatively stable In the chart, if any indicator is improved, other indicators will suffer relatively.
At present, fast-charging power lithium-ion batteries are mainly used in new energy buses. Because of their strong selectivity for cities and audience units, that is, cities or units with relative financial support, they are more inclined to fast-charging battery buses. However, from the perspective of market development potential, the future growth rate and market size of passenger cars and special logistics vehicles will be higher than those of passenger cars. Therefore, the consumption structure of fast-charging power lithium-ion batteries in the future will shift to these two types of models.
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