In recent years, clean energy vehicles have been developing rapidly, and electric car companies led by Tesla have launched a variety of electric cars with a strong sense of technology. Through continuous technical innovation, the performance of electric cars has been greatly improved, and electric cars have gradually entered people's lives from conceptual products.
Electric cars are in line with the trend of technological progress and development of the times, and are loved and accepted by more and more people. However, there are still problems such as short range, slow charging speed and high cost compared with fuel cars. The key to solve the problem lies in the "fuel tank" of electric vehicles - the power battery, which can be said to determine the vitality and competitiveness of electric vehicles. At present, as one of the energy storage system, lithium-ion batteries dominate the development of power batteries, because of its high voltage, high energy density, long life and better safety advantages.
What is a lithium-ion battery?
A lithium-ion battery is a secondary battery that can be repeatedly charged and discharged. Its main components are: positive electrode, negative electrode, diaphragm and electrolyte. As shown in the figure below, when charging, lithium ions are removed from the positive electrode and enter the negative electrode through the electrolyte, while the released electrons are transferred from the external circuit to the negative electrode to maintain the charge balance; when discharging, lithium ions are removed from the negative electrode and enter the positive electrode through the electrolyte, while the electrons arrive at the positive electrode from the negative electrode through the external circuit. During each charge and discharge cycle, lithium ion (Li+) acts as a carrier of electric energy, moving back and forth from positive pole → negative pole → positive pole, reacting with positive and negative materials, converting chemical energy and electric energy to each other and realizing charge transfer, which is the basic principle of lithium ion battery.
Easily "excited" positive and negative electrodes
One of the conditions for a lithium-ion battery to be able to convert electrical energy and chemical energy to each other and then realize the storage and release of energy is that the positive and negative electrode materials should be active, easy to oxidize and reduce, and "easy" to participate in chemical reactions to realize energy conversion. The second is the existence of a potential difference between the positive and negative electrode materials to achieve charge movement. 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 the active material of the battery cathode.
The negative electrode usually choose graphite or other carbon materials to do the active material, is also to follow the above principles, both require a good energy carrier, but also to be relatively stable, but also a relatively abundant reserves, to facilitate large-scale manufacturing, carbon is a relatively optimal choice.
"Discharge" also needs to be divided into occasions
As mentioned above, lithium ions flow through the electrolyte, while the electrons generated by the reaction do work through the external circuit. Therefore, the battery system must ensure the flow of lithium ions and electrons, i.e., it must be a good conductor of ions and a good conductor of electrons. Many electrochemically active materials are not good electron conductors, so some conductive materials such as carbon black need to be added. To hold the electrode material and the conductive agent together, some binder needs to be added as well. In this case, the electrochemical reaction can only occur where the active material, the conductive agent and the electrolyte meet.
While the lithium ions flow through the electrolyte, the positive and negative electrodes must be physically separated. In order to prevent a short circuit from causing a violent release of energy, a material is needed to "isolate" the positive and negative electrodes. This requires a material with good ion flux, which opens a channel for lithium ions to pass freely, and an insulator for electrons to insulate the positive and negative electrodes. The current lithium-ion batteries use polyethylene (polyethylene, PE), polypropylene (polypropylene, PP) prepared into the porous diaphragm.
