On the fast charging technology of lithium iron phosphate battery
Some people say that lithium iron phosphate is not suitable for fast charging, because its conductivity is not good enough, and it is easy to generate heat during fast charging. But this is only for pure-phase lithium iron phosphate materials, which have good ion and electron migration rates after grain refinement, surface carbon coating, and unit cell element doping, which is suitable for fast charging. The high safety, high thermal stability and long cycle life of lithium iron phosphate phosphate make it a good choice for fast charging.
Fast charging is the most important technological path for the development of new energy vehicles. It has significant effects on alleviating cruising range anxiety and reducing charging time. Therefore, it has become the market choice of many car companies and battery companies. In the new energy vehicle subsidy adjustment plan released at the end of 2016, the fast-charging pure electric bus was regarded as a separate subdivision product, and detailed subsidy standards were given: according to the electric capacity of the passenger car, a subsidy of 3000 yuan/Kwh was obtained. The three grades of "3C-5C, 5C-15C, 15C+" fast charge rates received 0.8 times, 1 times, and 1.4 times subsidies respectively. In this context, mainstream car companies including Beiqi Foton, Yutong, Xiamen Kinglong and other power battery companies such as Ningde Times, Watermark, Meguli, and Weihong Power have begun to increase their layout in the field of fast charging. From a technical point of view, despite the fact that certain research or new materials can greatly increase the energy density of batteries in the scientific community, in actual production and application, due to cost, technology, process and safety considerations, battery life is still It is a major factor restricting the development of new energy vehicles. Before the battery technology has made a revolutionary breakthrough, choosing fast charging to alleviate the shortcomings of new energy vehicles' insufficient cruising range is a good way to find another way. In fact, the application of fast charging to power batteries has already blossomed everywhere. But in terms of technology, in fact, different routes have the same goal. The core of fast charging technology is to accelerate the speed of lithium ions moving between the positive and negative electrodes through chemical system and design optimization without affecting the life and safety of the battery. The byproducts of the negative electrode will appear in the battery of ordinary chemical system during fast charging, which will affect the cycle and stability of the battery. The general solution to this obstacle is to make the negative electrode capable of withstanding fast charging by using specially designed negative electrode materials (such as hard carbon, soft carbon, carbon-coated graphite, etc.). As for the positive electrode material that is only used as a lithium ion source, there is actually no requirement. In theory, as long as it can provide enough lithium ions, it can be used as a positive electrode. Although the fast charging technology is very different, but the specific application is that the Eight Immortals cross the sea and each show their magical powers. Inventory of mainstream fast-charge batteries on the market, the author divides them into three types: iron phosphate batteries, lithium titanate batteries, and lithium manganate batteries according to battery materials. Some people say that lithium iron phosphate is not suitable for fast charging, because its conductivity is not good enough, and it is easy to generate heat during fast charging. But this is only for pure-phase lithium iron phosphate materials, which have good ion and electron migration rates after grain refinement, surface carbon coating, and unit cell element doping, which is suitable for fast charging. The high safety, high thermal stability and long cycle life of lithium iron phosphate phosphate make it a good choice for fast charging. At present, domestic companies focusing on lithium iron phosphate fast charging technology are represented by CATL and Waterma. As a representative enterprise of lithium iron phosphate batteries, Ningde Times has its own fairly mature and mature system on the way of fast charging. CATL is the only national enterprise in China that can be benchmarked against battery companies in Japan and South Korea. It has the internationalization of resources, customers and talents. The company has strong human resources and an outstanding R&D team. From R&D and design, to production, to testing and verification, and production, with strong resources, it has established the three major advantages of product safety, reliability and durability. Dr. Wang Shengwei, a fast charging expert from the Technology Department of Ningde Times, introduced its fast charging technology at the "2016-2017 China Bus Industry Development Summit Forum". For the positive electrode, CATL has developed the "super electronic network" technology, which makes lithium iron phosphate have excellent electronic conductivity, which can reach 1000 times that of ternary materials. At the same time, the positive electrode of lithium iron phosphate also takes into account the ultra-high performance of the fast charge battery system. Safety and reliability. On the surface of the negative electrode graphite, CATL adopts the "fast ion ring" technology to modify the modified graphite. The modified graphite takes into account the characteristics of super fast charging and high energy density. During fast charging, the negative electrode no longer has by-products, so the lithium ion in the graphite layer is greatly improved. The embedded speed enables it to have 4-5C fast charging capability, realize 10-15 minutes fast charge and charge, and can ensure the energy density of the system level above 70wh/kg, and realize the cycle life of 10,000 times. In addition, the fast charging products currently developed by CATL are equipped with a self-developed thermal management system to fully identify the "healthy charging interval" of a fixed chemical system at different temperatures and SOCs, greatly expanding the operating temperature of lithium batteries. When the battery is in the cold winter in the north, the water heating system heats the battery cell at low temperature. When the battery cell temperature reaches the required level, the fast charge mode is turned on; in the southern summer, the battery system automatically heats up and alarms, and the water cooling system gives The battery core cools down, and it truly achieves "all climate" fast charging. Also as a representative enterprise of domestic lithium iron phosphate batteries, Shenzhen Waterma Battery Co., Ltd. is the first national high-tech enterprise to successfully develop new energy vehicle power batteries in China and take the lead in realizing large-scale production and mass application. Xu Hui, director of Waterma Battery Research Institute, mentioned the unique stunts of Waterma fast charging in the interview. On the positive electrode, Waterma uses lithium iron phosphate with a smaller particle size. At present, the common lithium iron phosphate on the market has a particle size between 300 and 600 nm, while Waterma only uses 100 to 300 nm lithium iron phosphate. Lithium ions will have a faster migration speed and can be charged and discharged with a larger current. At the negative electrode side, artificial graphite with a smaller particle size is also used for carbon coating: small particle size is conducive to the detachment and insertion of lithium ions; carbon coating can optimize and improve the cycle life of the battery, and the microporous carbon structure is conducive to The adsorption and retention of the electrolyte thus play a role in improving the cycle life. In terms of safety, first of all, lithium iron phosphate itself has good thermal stability, that is, the heat emitted in the case of thermal runaway is much smaller than that of ternary materials, only about 1/4. Then set up multiple safety protections in terms of cell structure and grouping. The Waterma battery adopts a cylindrical structure, and is equipped with a vent tube device and a pull-off opening device outside the conventional safety device. The vent pipe device can ensure the balance of internal air pressure, and there is no risk of battery explosion due to excessive local air pressure in the battery cell; and the pull-off opening device will generate gas and disconnect the circuit when the battery is overcharged, overcurrent, or overheated. , Terminate the electrochemical reaction inside the battery, when the internal pressure reaches a certain value, the gas will be released without causing an explosion.
