Photo courtesy of interviewees on the graphite channel
"Charging anxiety" and "mileage anxiety" are major obstacles facing the large-scale industrialization of pure electric vehicles. At the 2019 World New Energy Vehicle Conference held recently, a technology called "High Specific Energy Fast Charge Lithium Ion Battery" won the Global New Energy Vehicle Innovation Technology Award.
This technology breaks through the technical bottleneck of the graphite system that cannot be quickly charged. On the basis of maintaining high energy density, high safety, long life and other advantages, it can complete 100% charging in 15 minutes to ensure a 300 km range of electric vehicles.
What's the secret of this technology, how can it achieve the fast charging of electric vehicles, but also let new energy users feel the convenience of "charging for five minutes and talking for two hours" like a mobile phone?
The current fast charging methods in the industry have drawbacks
As we all know, for pure electric vehicles, the charge and discharge performance of the battery system is an important indicator that determines the actual use of the vehicle. High energy density and fast charging capability are not only the technical direction of continuous development of power battery manufacturers, but also the core field of new energy technology.
"Lithium-ion batteries are also known as rocking chair batteries." Cheng Xiaoyan, director of scientific research projects at Ningde Times New Energy Technology Co., Ltd. (hereinafter referred to as Ningde Times), told Science and Technology Daily that the two ends of the rocking chair are the two poles of the battery, and the lithium ions are at the two ends of the rocking chair. Run back and forth. During charging, lithium ions move from the positive electrode of the battery through the electrolyte to the negative electrode. The graphite as the negative electrode has a layered structure, and lithium ions are intercalated into the graphite through the interlayer. The more lithium ions are inserted, the higher the charging capacity.
A key indicator to measure the charging efficiency of electric vehicles is the charge and discharge ratio (C). The charge and discharge rate 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 store a certain amount of energy in the battery, or how fast we can release the energy in the battery. For example, when a battery with a rated capacity of 100 ampere hours is discharged with 20 amperes, the discharge rate is 0.2C. The capacity used after 1 hour of discharge is called 1C discharge; after 5 hours of discharge, it is called 0.2C discharge.
The industry generally believes that fast charging of electric vehicles refers to the charging method with a charging rate greater than 1.6C, that is, the technology from 0% to 80% charging time less than 30 minutes.
As the name implies, to shorten the charging time, it is necessary to continuously increase the charge and discharge rate. The core of fast charging technology is to accelerate the movement of lithium ions between positive and negative electrodes through chemical system and design optimization. However, considering the speed when developing fast charging technology is not enough.
During fast charging, lithium ions need to be accelerated and inserted into the negative electrode instantaneously. This poses a great challenge to the ability of the negative electrode to quickly receive lithium ions. For batteries of ordinary chemical systems, lithium deposits and other by-products will appear in the negative electrode during fast charging, which affects the cycle and stability of the cell. Only negative materials that can withstand fast charging and large current can be used for fast charging.
Cheng Xiaoyan said that in the industry, lithium titanate and amorphous carbon are commonly used as negative electrode active materials in order to achieve fast charging. However, in practical applications, lithium titanate and amorphous carbon inevitably have serious energy density and high cost defects. The conventional design of increasing the amount of conductive material will also affect the energy density of the battery cell. "In recent years, some manufacturers have begun to explore the use of graphite as an active material. However, the problem faced by graphite as a fast charging material is how to quickly release lithium electrons from the positive electrode and then quickly enter the negative electrode."
Cheng Xiaoyan explained that graphite is more like a highway, although the energy density is higher, but lithium electrons can only pass through in sequence.
In other words, graphite is not a material that is naturally suitable for fast charging technology. "But we use technology to break through the bottleneck of the material itself. Its killers are 'fast ion ring' and 'superelectronic network'." Cheng Xiaoyan lamented.
Fast ion ring and super electron network accelerate charging
"We use graphite as the main material of the negative electrode, innovatively use hole optimization and 'fast ion ring' technology to create a circle of high-speed channels on the graphite surface, so that lithium ions can be quickly embedded in any position of graphite, greatly improving the lithium ion in the graphite negative electrode. The embedding speed, and the modified graphite takes into account the characteristics of super fast charging and high energy density, and no by-products will appear in the negative electrode during fast charging, affecting the cycle and stability of the cell. "Cheng Xiaoyan said.
In addition, the technical team developed the "superelectronic network" technology to modify the cathode material, combined with the design parameters of the positive and negative pole pieces, such as the crystal orientation and capacity excess coefficient, to optimize the dynamic performance of the electrolyte and the positive and negative electrodes. The battery design parameters are optimally matched.
"Popularly speaking, the 'fast ion ring' and the 'superelectronic network' act on the negative electrode and the positive electrode, respectively, to establish a fast channel for a large number of lithium ions to flow into the negative electrode at the same time, and increase the diffusion rate of lithium ions." Cheng Xiaoyan said.
In addition, in the design of mechanical parts, the team creatively simplified the design of the battery cell top cover, placed the electrode terminal on the side of the top cover plate and reduced the thickness of the terminal, significantly reducing the internal resistance, effectively controlling the fast charging and heat generation, and ensuring fast Increase the energy density by more than 5% while charging reliability.
"Our fast charging technology has a 4C-5C fast charging capability to achieve fast charging in 10-15 minutes. Compared with the fast charging system of lithium titanate anode, it has obvious energy density and cost advantages, and graphite is used in the same industry. Compared with other fast charging technologies for negative electrodes, under the same battery energy density, it can increase the charging speed by 20% -30%, and has better cycle and weather resistance performance. "Cheng Xiaoyan said.
Has been successfully applied to more than 5000 buses
At present, the super iron-lithium fast-charge battery developed by Ningde Times with fast-charging graphite as the main material has been used on more than 5,000 electric buses. The bus is in good operation and has been well received by vehicle companies and bus users. It was rated as the "best word-of-mouth battery for new energy buses" by the Ministry of Communications.
In addition to the energy density, the battery has a greater advantage. In terms of cycle life, taking the bus with more severe application conditions as an example, the average number of full-charge times is about 2 times per day, which is roughly calculated to meet 8 years of operation needs and battery cycle needs. More than 5,600 times. The cycle life of the super iron lithium battery cell can reach 10,000 times, which can not only fully meet the operational needs of electric vehicles, but also can be used for energy storage and other stepped utilization after retirement, creating more economic value.
"In order to ensure the safety and reliability of the fast charging core, we have also developed a special technology to identify the" healthy charging interval "of the chemical system at different temperatures and SOC states, and then perform fast charging within this" healthy charging interval ". It can not only achieve fast charging, but also prevent the battery from being damaged by fast charging, and achieve fast charging, long cycle and safety and reliability. "Cheng Xiaoyan said.
In terms of weather resistance, in order to meet the requirements of low temperature charging in winter in the north and high temperature conditions in summer in the south, the technical team has developed a highly efficient thermal management system to ensure that the battery is in a reasonable temperature range. When the temperature is low, the battery can be quickly heated, and the fast charging mode can be turned on when the temperature reaches the requirement; when the temperature is high, the system will cool the battery, and truly achieve "full climate" fast charging.
"We are developing 2C-2.5C high energy density long-life (cycles greater than 6000 cycles) fast-charged lithium-ion batteries with a single-cell energy density greater than 160 watt-hours per kilogram, and mass production is expected by the end of 2019. This technology is also going to the ternary system The application and development of passenger cars has achieved 350 kilometers of battery life and completed charging within 15 minutes. "Cheng Xiaoyan said that the company also plans to launch ternary power battery products with an energy density of up to 255 watt-hours per kilogram in the next 1-2 years. Quickly charge within 18 minutes. (Reporter Cao Xiuying)
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