In a lithium-ion battery, when lithium metal is used as the negative electrode, the electrolyte reacts with the lithium to form a lithium film on the surface of the lithium metal, which leads to the growth of lithium dendrites, which can easily cause internal short circuits and explosions in the battery.
When the lithium is intercalated in the carbon material, its potential is close to that of lithium, it is not easy to react with the organic electrolyte, and exhibits good cycle performance.
The carbon material is used as the negative electrode, and the lithium in the solid phase undergoes intercalation and deintercalation reactions during charging and discharging.
C6+Li+ +e→LiC6
Lithium-ion battery anode materials include carbon-based and non-carbon-based anode materials. In addition to carbon-based anode materials, the development of non-carbon-based anode materials is also very eye-catching. The classification of carbon-based and non-carbon-based anode materials is listed below.
Carbon-based anode material:
Graphitized carbon anode material
Amorphous carbon material
Modified carbon material
Fullerenes, carbon nanotubes
Kita Carbon Materials
Non-carbon-based anode materials:
Amide
Silicon and silicide
Tin-based oxides and tin compounds
Titanium oxide
New alloy
Nano oxide anode material
Other anode materials
The theoretical capacity of graphitized carbon material is 372mA·h/g, but its preparation temperature is as high as 2800℃. Amorphous carbon is a type of electrode material that is prepared under a low-temperature method and has a high theoretical capacity. There are many preparation methods for amorphous carbon materials, and there are two main ones: ① heat treatment of polymer materials in an inert atmosphere at a lower temperature (<1200°C); ② chemical vapor deposition of small molecular organics. There are many types of polymer materials, such as polyphenylene, polyacrylonitrile, phenolic resin and so on. Small molecule organics include van, hexabenzophene, phenolphthalein and so on. There is no obvious (002) diffraction peak in the X-ray diffraction pattern of these materials, and they are all amorphous structures, composed of graphite crystallites and amorphous regions. There are a large number of microporous structures in the amorphous region. Its reversible capacity is greater than 372mA·h/g under suitable heat treatment conditions, and some even exceed 1000mA·h/g. The main reason is that the micropores can be used as a "warehouse" for reversible lithium storage.
Lithium is intercalated in amorphous carbon materials, first intercalated into the graphite crystallites, and then into the micropores of the graphite crystallites. In the process of intercalation and deintercalation, lithium is intercalated and deintercalated from the graphite crystallites first, and then the lithium in the micropores is intercalated and deintercalated through the graphite crystallites. Therefore, there is a voltage hysteresis phenomenon in the process of lithium intercalation and deintercalation. In addition, because there is no high-temperature treatment, there are defective structures remaining in the carbon material, and lithium first reacts with these structures when intercalated, resulting in low first-time charging and discharging efficiency of the battery; at the same time, because the defective structure is unstable during cycling, the battery capacity decays faster as the number of cycles increases. Although the reversible capacity of amorphous carbon materials is high, these deficiencies have not yet been resolved, so they cannot meet the requirements of practical applications.
New anode materials include thin film anode materials, nano anode materials and new core/shell structure anode materials. Thin-film anode materials are mainly used in micro-batteries, including composite oxides, silicon and their alloys. The main preparation methods include radio frequency magnetron jet, DC magnetron jet and vapor phase chemical deposition, etc., and their application fields are mainly in the microelectronics industry. The development of nano anode materials is to use the nano characteristics of the material to reduce the impact of volume expansion and contraction on the structure during charging and discharging, thereby improving cycle performance. Studies have shown that the effective use of nanometer properties can improve the cycle performance of these anode materials, but there is still a long way to go before practical applications. The key reason is that the nanoparticles gradually combine with the charge and discharge cycle, which loses the unique properties of the nanoparticles, resulting in the destruction of their structure and the attenuation of the reversible capacity. For nano-oxides, the first charge and discharge efficiency is not high, and more electrolyte is required; therefore, nano-materials are mainly concentrated in metals or metal alloys. The thickness of the prepared negative electrode material film is generally not more than 500nm, because too thick a film easily leads to changes in structure and attenuation of capacity.
According to related reports, by improving the surface structure of the deposition substrate, when the film thickness is as high as 615μm, the reversible capacity of the film is still above 1600mA·h/g, and it has better cycle performance. Because it is applied to the chemical sputtering method or the vacuum evaporation method, the preparation process cost is high.
The production of the negative electrode sheet is to mix the negative electrode active material carbon or graphite with about 10% of the adhesive (such as PVDF, or polyimide additives, etc.), and make a paste, evenly coat it on both sides of the copper foil, dry, roll it to 254m, and cut to the specified size as required.
