Characteristic requirements of cathode materials
1) The positive electrode material should be able to reversibly insert and extract a large number of bond ions, and have a potential platform, so that the energy efficiency in the charge and discharge process can be enhanced.
2) The positive electrode material should be light and dense, so that the capacity per unit weight or unit volume is high; it should also have high electronic and ionic conductivity to ensure high power.
3) Maintain high cycle efficiency. Side reactions on the positive and negative electrodes that have nothing to do with lithium leaving the cycle will reduce the cycle efficiency.
4) Since the irreversible phase transition of the crystal structure will shorten the cycle life, these phase transitions should be avoided during the charge and discharge cycle of the positive electrode material. If the material lattice has a large volume change, the active material will fall off from the current collector, which will reduce the battery capacity.
5) In order to prevent it from reacting with the electrolyte, the cathode material should have electrochemical and thermal stability.
6) The particles of the cathode material must be spherical particles with a narrow particle size distribution, so that the electrode can have a good contact with the aluminum box when preparing the electrode, and at the same time improve the contact between the material particles and increase the electrical conductivity.
How the cathode material works
3d transition metals are often used as cations for cathode materials. Compared with the 4d and 5d transition metals, the 3d transition metal element has a higher electrode potential, and its capacity per unit weight or volume is also higher due to the lighter weight and smaller size. Chalcogen elements, especially oxygen, have a more stable structure relative to halogen elements, and are more suitable as anions for positive electrode materials.
During the charging or discharging process, in order to maintain the electrical neutrality of the transition metal oxide, the process of inserting or removing cations from the electrode material and transporting through the electrolyte must occur quickly. In order to ensure that the rapid charge and discharge based on the movement of cations can be performed in a wide range of redox potential or to minimize the change in the crystal structure of the positive electrode material. It is necessary to select low-valent cations that are small in size and form only weak bonds with the active material. When the coordination number (Cordinalion Number, CN) is 6, the ionic radius of beryllium ion (Be2+) is 0.45 Å, which is smaller than that of lithium ion, which is 0.7 Å, but because of its high valence, it has stronger bonding force with oxygen. Compared with other cations, the repulsive force is larger, which makes the transmission of beryllium ions slow in the lattice, so its compound is not suitable as a cathode material. On the other hand, lithium is a suitable cathode material element because its size is smaller than other candidate elements except beryllium, and its charge is low. The standard reduction potential of lithium is -3.040V, which is much lower than the 1.847V of beryllium.
Common cathode materials are lithium transition metal oxides, such as layered LiMO2 and spinel type LiMn2O4) and lithium transition metal phosphates, such as olivine type LEMPO4. Research on the use of these different structural materials by mixing, surface coating and forming aliases is extensive. For example, in a cheap, stable, low-conductivity but high-capacity cathode material coated with a layer of expensive, high-conductivity but low-capacity material, a high-capacity and high-capacity material can be obtained. Power characteristics of the material. Table 1 shows various cathode materials and their characteristics including discharge capacitance and potential.
|positive electrode||Theoretical capacity/(mAh/g)||Actual capacity/(mAh/g)||Average voltage/(V vsLi/Li+ )||True density/(g/ce)|