Constituent materials Lithium-ion batteries are mainly composed of four basic parts: electrode, electrolyte, separator and container, as shown in Figure 2.2. (1) The electrode is divided into positive electrode material and negative electrode material by its polarity. The most common negative electrode material is graphite; there are many types of positive electrode materials, as shown in Figure 2.3. Cai Shaowei and others have made a comprehensive summary of the types of positive electrode materials and their respective characteristics.u003cpu003eFigure 2.3 Positive electrode materialsu003c/pu003e ( 2) Electrolyte can be divided into two categories: one is an aqueous solution of acid, alkali, and salt; the other is an organic electrolyte solution. Electrolyte is the carrier of lithium ion transfer between positive and negative electrodes, so it determines the battery's capacity, cycle efficiency, rate, high and low temperature discharge, high temperature storage, life and safety performance [23,24]. Organic electrolytes generally consist of the following substances: solvent, electrolyte lithium salt and additives. Commonly used organic solvents are ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate. Different solvents have a great influence on the performance of the battery. For example, a solvent with a large dielectric constant can reduce the attractive force between lithium ions and fully free lithium ions; the flash point of the solvent determines the maximum working temperature of lithium ions [25,26]. Lithium ion salt is a substance that provides lithium ions, and it mainly includes two series: inorganic salt and organic salt. An additive is a substance that can significantly improve the performance of lithium ions. For example, film-forming additives can prevent the solvent from damaging the electrode, and increase the capacity and cycle life of the electrode for lithium insertion and delithiation. In addition, there are anti-overcharge additives, conductivity additives, flame retardant additives and so on. (3) The isolation layer is a layer of organic thin film with abundant tortuous and penetrating micropores. This kind of micropores can prevent the electrolyte with a larger particle size from passing through, but the lithium ions with a smaller particle size can pass freely, forming an electric circuit inside the battery. Therefore, the isolation layer divides the positive and negative electrodes into two parts, which can prevent short circuits inside the battery and play a role in isolation [27-29]. (4) The container is the packaging metal shell of the battery, which mainly plays the role of maintaining the shape of the battery and protecting the inner cell. Generally, the outer shell of the battery is connected with the positive and negative electrodes of the cell. Working principle The charging and discharging process of lithium ion battery is the process of Li+ intercalation and deintercalation in the positive and negative materials accompanied by the absorption and release of energy. Charging a lithium battery pack
: Li+ is deintercalated from the positive electrode material, reaches the negative electrode through the electrolyte, and absorbs electrons and is embedded in the negative electrode material to form a higher energy intercalation compound. discharge: Li+ deintercalates from the negative electrode material and releases electrons, Li+ reaches the positive electrode through the electrolyte, and absorbs electrons into the positive electrode material to form a relatively stable intercalation compound. Figure 2.4 is a schematic diagram of lithium ion intercalation in graphite. u003cpu003eFigure 2.4 Schematic diagram of lithium ion intercalation graphiteu003c/pu003e The cathode and anode materials of lithium-ion batteries are mixed conductor intercalation compounds of ions and electrons, and electrons can only move in the anode and cathode materials. The charge and discharge process requires the interaction of lithium ions and electrons, which requires the lithium ion electrode material to have good conductivity and ion diffusion ability to ensure that electrons are quickly conducted to the external circuit through the electrode tabs during the discharge process, and lithium ions are in the negative electrode. After the material is deintercalated, it rapidly diffuses into the electrolyte; the lithium ions in the electrolyte are delivered to the positive electrode material by the carrier, and combine with electrons to form an intercalation compound. u003c/pu003e
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