Analysis of power battery fast charging technology: Lithium iron phosphate is not suitable?

If new energy vehicles want to truly achieve the same experience as traditional gasoline vehicles, shortening the charging time, improving the cruising range and the convenience of charging are unavoidable topics. Dr. Wang Shengwei, the head of Ningde Times New Energy Technology Co., Ltd. (CATL) fast charge project, said that the "superconducting subnet" and "fast ion ring" technologies developed by CATL have been combined to develop lithium iron phosphate fast charging cores. After the 4C high-rate fast charge, the pure electric bus can be 100% charged in 15 minutes.

On September 27th, in the eighth open class with the theme of “Charging for five minutes, the era of two hours driving is coming?”, Wang Shengwei delivered a speech explaining the principle and characteristics of the fast-filled lithium-ion battery, and introduced the CATL Maintenance programs and fast-filled lithium-ion batteries are progressing.

Dr. Wang Shengwei, Head of Ningde Times New Energy Technology Co., Ltd. (CATL) Fast Charge Project

In principle, the bottleneck of the battery fast charge is at the negative electrode, and the common chemical system will produce by-products at the negative electrode during fast charge, which affects the cycle and stability of the battery. Once lithium ions are blocked in the negative electrode, it is very dangerous, but as long as lithium ions enter the inside of the graphite, there is basically no problem. As a result, CATL has developed a “fast ion ring” technology that creates a highway network on the graphite surface and inside, allowing lithium ions to be quickly embedded anywhere in the graphite.

"It can achieve both lithium ion density and fast charge. This is like repairing the city high speed to alleviate traffic congestion. 'Fast ion ring' graphite is this mechanism. We have hundreds of millions of shipments on mobile phones, reliable. Sex has been verified." Wang Shengwei is introduced.

Lithium iron phosphate is not suitable for fast charging?

Some people in the industry think that lithium iron phosphate is not suitable for fast charging. In Wang Shengwei's view, this problem has two sides. From the material point of view, the intrinsic conductivity of the lithium iron phosphate material is relatively low, only one percent of the ternary material, and the conductivity optimization of the lithium iron phosphate material is required to meet the demand for fast charging.

Wang Shengwei said that after the transformation of CATL, lithium iron phosphate is not the "original lithium iron phosphate", even better than the ternary material. On the positive electrode, CATL developed the "superconducting sub-network" technology, which makes lithium iron phosphate have excellent electronic conductivity, up to 1000 times that of ternary materials.

He pointed out that the current cycle performance of lithium iron phosphate fast-charged batteries is better than that of uncharged batteries. In principle, there is no contradiction between cycle performance and charging. Then why is it not good to use a fast charge cycle for a conventional battery? That's because the traditional battery is not designed for fast charge. When it is fast charged, there will be by-products or side reactions. After more side reactions, the cycle will naturally deteriorate. Once the dynamics of the fast charge is resolved, its cycle will not at least deteriorate. At the same time, the fast charge battery is not only high in dynamics, but also because the energy density is lower than that of the non-fast charge, the energy density is low, the electrolyte is much, and the cycle becomes better. In fact, they currently develop lithium iron phosphate fast rechargeable batteries can reach 10,000 cycles.

Guarantee fast charging in "healthy charging range"

“CATL After years of research, we believe that the healthy charging interval is related to the irreversible reaction speed of charging. For fixed chemical systems, it is affected by temperature and voltage. The speed of irreversible reaction determines the battery during the cycle. The capacity attenuation and the speed of impedance increase, so the focus of our research is how to identify the speed of irreversible reaction, fast charging in this "healthy charging range" range, can achieve fast charging, and can not let the battery fast charge The damage is done with both fast charge and long cycle."

At low temperatures, CATL proposes two types of battery protection. One is charging in a temperature range, slow charging at low temperature, and charging in the “healthy charging interval” of the battery to better protect the battery. The second is to use the hydrothermal system to heat the battery at low temperature. When the temperature of the battery reaches the requirement, the fast charge mode can be turned on to achieve a "healthy" fast charge at low temperature.

Similarly, the battery also needs to avoid the effects of high temperatures because it has a redox reaction itself, and the higher the temperature, the more intense the reaction. In this regard, CATL's solution is to use a low voltage system of 3.65V, which has weak positive electrode oxidation, less side reactions and slower speed. There is also water cooling control temperature to avoid high temperature conditions.

High energy density battery fast charge leading international

In terms of “longer cruising range”, CATL's technical plan is to achieve 300 watt-hours per kilogram of battery energy density before 2020. After 2025, there is a plan to further increase energy density. In terms of "shorter charging time", CATL is already developing a fast-charged battery with an energy density of 190Wh/kg for 15 minutes. The battery is expected to have mass production capacity in 2018, and its main application field is passenger cars.

"For the high energy density fast charging battery of the car, we have the technical reserve advantage, and the domestic battery companies have the opportunity to lead the Japanese and South Korea." Wang Shengwei said, "In terms of charging speed, the charging of ordinary cars is basically Below 1C, it will be charged to 80% for nearly 50 minutes. Our products can be charged 80% in 12 minutes, and the charging speed has a sharp contrast. Even for heavy-duty heavy user taxis, it is filled twice a day. It is necessary to charge 700 times a year. According to the taxi's use period of 4 years or so, 2800 times of charging should be enough. The cycle of our high-energy fast-battery battery is expected to be more than 3,000 times, which should meet the needs of passenger car applications. ."

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