Mechanical ball milling
The particle size of the carbon material has a greater relationship with its charge and discharge performance. Generally, the carbon powder can be treated by ball milling. Different ball milling methods and process conditions result in different particle size, bulk density, specific surface area, and microcrystalline defect density, which will affect its electrochemical performance.
Grinding can increase the reversible capacity of graphite. For example, graphite has a reversible capacity of 700mA·h/g after 150h ball milling, but the potential curve has a certain hysteresis, the irreversible capacity is also larger (580mA.h/g), and the capacity decays faster. The increase in capacity is due to the increase in the number of micropores, microcavities, etc.; the increase in irreversible capacity is due to the increase in surface area; the voltage lag is due to the existence of interstitial carbon atoms; the deterioration of the cycle performance is because the movable and certain bonded interstitial carbon atoms make the micropores disappear and the electrolyte penetrates into the micropores, and agglomerate particles are formed during the lithium intercalation and deintercalation process. After grinding, graphite transforms into a metastable carbon interstitial phase, which has a structure similar to that of pyrolytic carbon.
After the flake graphite is vibrated and sheared and ground, the grinding method has an effect on the irregularity, morphology and structure of the carbon material. The energy transferred depends on the grinding method used. Generally, the energy transferred by vibration grinding is relatively large. Shear grinding has little effect on the graphitization of carbon materials, but after 80h vibration grinding, the graphite structure is similar to that of hard carbon. The surface dangling bonds generated during grinding form a structure similar to hard carbon between adjacent free bonds, thereby increasing the capacity of the graphite electrode. In this material, two lithium (Li2C6) (about 700mA·h/g) can be inserted into every 6 carbons, and the irreversible capacity is 320mA·h/g. Grinding reduces the number of graphite layers and increases defects.
Mechanical grinding can also introduce the rhombohedral phase into the hexagonal graphite, thereby reducing the layer exfoliation of the graphite in the electrolyte. For example, after 15 minutes of barrel grinding, sufficient rhombohedral phase can be introduced. After grinding, graphite can maintain high crystallinity and high reversible capacity. At the same time, the reversibility of the electrode can be improved due to the reduction of peeling.
Mechanical grinding can improve the electrical performance of graphite electrodes, and different grinding methods and time can be selected according to the performance requirements of the target. However, graphite that has been milled by ball milling and vibration milling has a fast capacity decay and a voltage lag.
Carbon nanotubes are mixed with graphite and form nano-scale micropores with graphite to increase the space for intercalation of Li and increase its reversible capacity to 341.8mA·h/g, and the doped carbon nanotubes act as a bridge to avoid the formation of "islands", facilitate the insertion and extraction of lithium ions, and enhance the conductivity of the material. Silanization changes the surface chemistry, surface area and graphite morphology. After oxidizing with O2 at 420°C for 16 hours, silanization is carried out. Since the silane film can be penetrated by the organic electrolyte, it is more conducive to the formation of SEI film under silane and reduces the irreversible capacity loss of the material.
A passivation film is formed on the surface of the carbon material, which can increase the reversible capacity of the material. A layer of crystalline Li2CO3 or LiOH film is pre-deposited and crystallized on the graphitized MCMB material. The test shows: the surface of the modified crystal film is relatively uniform, dense and smooth, and adheres closely to the material, basically without cracks, and significantly changes the compatibility with the PC-based electrolyte. The capacity in the PC-based electrolyte reaches 210mA·h/g and 270mA·h/g, respectively, and its capacity and cycle performance are shown in Figure 1.
Figure 1- Schematic diagram of capacity cycle performance of MCMB modified in 1molL LiClO4+PCDEC electrolyte
The modification of the passivation film can also be obtained by treating the graphite with n-butyl lithium, hexane, lithium tea and molten metal lithium and then reacting with the electrolyte. The reversible capacity of the treated graphite can be increased to 430mA·h/g.