Graphite is an excellent anode material for lithium batteries
Lithium-ion batteries refer to two different types of secondary battery systems that use lithium intercalation compounds that can reversibly intercalate and deintercalation lithium ions as the positive and negative electrodes of the battery. During charging, lithium ions are de-intercalated from the positive electrode and are embedded into the negative electrode through the electrolyte and the separator; when discharged, lithium ions are de-intercalated from the negative electrode and are inserted into the positive electrode through the electrolyte and the separator. The negative electrode of a lithium-ion battery is made by mixing a negative electrode active material, a binder, and an additive, and pasting the paste on both sides of a copper foil, drying and rolling.
The advantages of graphite as a negative electrode material
Graphite is an allotrope of carbon, and the two are closely related. Graphite is the most stable form of carbon. (Diamond is a metastable allotrope of carbon. Although its hardness is much higher than graphite, and it is the hardest substance in nature, its stability is lower than graphite.)
The term "graphite" comes from the Greek "graphein." The material is resistant to high temperatures and corrosion, has good electrical conductivity, thermal conductivity, and stable chemical properties, and is lighter than aluminum. In addition to being used as a negative electrode material for lithium-ion batteries, high-quality graphite can also be used in different fields such as fuel cells, solar cells, semiconductors, light-emitting diodes, and nuclear reactors.
In general, graphite has the advantages of high electronic conductivity, the small volume change of the layered structure before and after lithium intercalation, high lithium intercalation capacity, and low lithium intercalation potential. It has become the mainstream commercial lithium-ion battery anode material.
How to obtain graphite
There are two ways to get graphite: one is natural ore, and the other is the synthesis of coal tar. The graphite materials used in lithium-ion batteries are generally prepared by blending 55% synthetic graphite with 45% low-purity natural graphite.
Manufacturers once favored synthetic graphite because the uniformity and purity of synthetic graphite were better than natural graphite. Now it is different. The application of modern chemical purification methods makes it possible to obtain natural graphite with a purity of 99.9% after heat treatment. In comparison, the purity of synthetic graphite is 99%, so the former is more popular.
Graphite lithium insertion mechanism
Graphite has good electrical conductivity, high crystallinity, and well-layered structure, which is very suitable for repeated insertion-deintercalation of lithium ions. It is currently the most widely used and most mature anode material. After lithium ions are intercalated between graphite layers, LixC6 (0 ≤ x ≤ 1) is formed, and the theoretical capacity can reach 372mAh / g (x = 1). The reaction formula is: xLi ++ 6C + xe- → LixC6
The intercalation of lithium ions changes the deposition mode between the graphite layer and the layer from ABAB to AAAA.
Modification of graphite
Because the graphite layer spacing (d≤0.34nm) is smaller than the crystal surface layer spacing (0.37nm) of the graphite lithium intercalation compound LixC6, the graphite layer spacing changes during the charge and discharge process, which quickly causes the graphite layer to peel, powder, and also occur. Lithium ions and organic solvent molecules are embedded in the graphite layer, and the organic solvent is decomposed, which affects the cycle performance of the battery.
Through graphite modification, such as oxidizing and coating polymer pyrolytic carbon on the surface of graphite to form composite graphite with core-shell structure, it can improve the charge and discharge performance of graphite and increase the specific capacity.