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Mobile WebSiteClean energy vehicles have developed rapidly in recent years, and electric vehicle companies led by Tesla have launched a number of technology-rich electric vehicles. Through continuous technological innovation, the performance of electric vehicles has been greatly improved, and electric vehicles have gradually entered people's lives from concept products.
Electric vehicles conform to the trend of scientific and technological progress and the development of the times, and are more and more loved and accepted by more and more people. However, compared with fuel vehicles, electric vehicles still have short-range mileage, slow charging speed, and high costs. The key to solving the problem lies in the "fuel tank" of electric vehicles-power batteries. It can be said that power batteries determine the vitality and competitiveness of electric vehicles. At present, lithium-ion batteries, which are one of the energy storage systems, dominate the development of power batteries because of their advantages such as high voltage, high energy density, long life, and good safety.
Easily "excited" positive and negative electrodes
Lithium-ion batteries can convert electrical energy and chemical energy to each other to achieve energy storage and release. One of the conditions is that the positive and negative electrode materials must be active, easy to oxidize and reduce, and easily participate in chemical reactions to achieve energy conversion. The second is the need for positive and negative materials with potential difference to achieve charge transfer. After a long period of research and exploration, people have found several lithium metal oxides, such as lithium cobaltate, lithium titanate, lithium iron phosphate, lithium manganate, nickel-cobalt-manganese ternary, and other materials, as battery positive active materials.
The negative electrode usually chooses graphite or other carbon materials as the active material, which also follows the above principles. It is not only required to be a good energy carrier, but also to be relatively stable. It also needs to have relatively abundant reserves to facilitate large-scale manufacturing. s Choice.
"Discharge" also needs to be divided into occasions
As described above, lithium ions flow through the electrolyte, and the electrons generated by the reaction do work through an external circuit. Therefore, the battery system must ensure the flow of lithium ions and electrons, that is, it must be a good ionic conductor and an electronic conductor. Many electrochemically active materials are not good electronic conductors, so some conductive materials such as carbon black need to be added. In order to fix the electrode material and the conductive agent together, some binders need to be added. In this case, the electrochemical reaction can only occur where the active material, conductive agent and electrolyte meet.
Although lithium ions flow through the electrolyte, the positive and negative electrodes must be physically separated. To prevent the violent release of energy caused by a short circuit, a material is needed to "isolate" the positive and negative electrodes. This requires the material to have good ion permeability, which can open channels for lithium ions and allow them to pass freely, and at the same time is an insulator of electrons to achieve insulation between positive and negative electrodes. Currently, lithium ion batteries use a porous separator made of polyethylene (PE) and polypropylene (PP).
With these data, we must also consider the battery assembly quality and the weight of the entire vehicle in order to have a rough estimation of the range of the electric vehicle. Taking the Tesla Model S as an example, the battery pack weighs about 1 ton, the battery capacity is about 100 KWh, the entire vehicle mass is about 2.5 tons, and it can reach a range of 600 km. According to recent reports, Tesla ’s third-generation super-charging system has a charging rate of more than 1,000 miles per hour (about 1609 kilometers per hour), recharges up to 75 miles (about 120 kilometers) within 5 minutes, and charges. You can exercise for nearly 270 kilometers in 15 minutes.
"Five minutes of charging and five hundred miles of battery life" is currently unattainable. If this vision is realized, it will undoubtedly shake the dominance of fuel cars. So, "five minutes of charging and five hundred miles of battery life" is really beyond sight?
To achieve this goal, there is a high demand for the charge and discharge speed of the battery. The lithium removal and insertion speed of the positive and negative electrode lithium storage materials and the structural stability during rapid charge and discharge are the main reasons. High-speed charging often heats up the battery, damages its structure, and reduces battery life. This puts forward requirements for the stability and safety of the battery. Although many hydrogen fuel cell concept cars have been reported in recent years, hydrogen fuel cell vehicles need to solve a series of complex problems such as hydrogen production, hydrogen storage, fuel cell engines, car body structure, safety, etc., and it is difficult to commercialize them. Generally speaking, for a long time to come, electric vehicle batteries will still be dominated by lithium-ion batteries. In order to turn "five minutes of charging and five hundred miles of life" into reality, it is necessary to meet the following conditions: (1) The energy density of the material is high, that is, the stored electrical energy is large. (3) When lithium ions are inserted and removed, the reaction between the material and lithium must be very rapid. (3) The material is a good electronic conductor. This will reduce the battery's internal losses and further improve battery performance. (4) Material is stable. During the charging and discharging process, the material does not change the structure or otherwise decompose, and the volume of the material does not expand and deform. (5) Low material cost. This determines the price of batteries and electric vehicles. (6) Materials are environmentally friendly. No pollution to the environment or minimal pollution and controllable.
