Another commonly used method to improve the performance of carbon materials is to introduce metal or non-metal into the material for doping. This doping changes the microstructure and electronic state of the carbon, thereby affecting the lithium insertion behavior of the carbon material.
(1) Introduce non-metallic elements. Boron is a group IIIA element, and its introduction can be divided into two types: atomic form and compound form. The introduction of atom form is mainly used to prepare carbon materials by vapor deposition (CVD), introduce boron-containing alkanes or other borides, and obtain carbon materials in which boron atoms and carbon atoms are deposited together through cracking. The introduction of the compound form is to directly add borides, such as B2O3, H3BO3, etc., to the precursor of the carbon material, and then conduct pyrolysis. The introduction of boron can increase the reversible capacity of carbon materials. This is due to the lack of electrons of boron, which can increase the binding energy of lithium and carbon materials, that is, from E0 to E0+Δ (E is the binding energy when lithium is inserted into graphite to form LixC6). Its influence on the charging voltage is mainly between 1.1~1.6V. In addition, the above two methods have slightly different effects on the capacity of the obtained carbon material. The boron content of the former basically increases linearly with the increase of boron content before 9%, while the latter is the maximum at 1.0%~2.0%, and will reduce its irreversible capacity.
The main reason why the composite of silicon and carbon can increase the reversible capacity is that the introduction of silicon can promote the diffusion of lithium in the carbon material and effectively prevent the generation of dendrites; and the silicon is distributed in the nanometer level in carbon materials, and nano-silicon itself has electrochemical properties and can be reversibly intercalated and deintercalated with lithium.
It was first believed that nitrogen exists in two forms in carbon materials, namely chemical nitrogen and lattice nitrogen. The former is prone to irreversible reaction with lithium, which increases irreversible capacity. Therefore, it is considered that carbon materials doped with nitrogen atoms are not suitable as negative electrode materials for lithium ion batteries. However, with the same chemical vapor deposition method and the same raw material (pyridine), the results obtained are different. The charging and discharging results show that with the increase of nitrogen content, the reversible capacity increases and exceeds the theoretical capacity of graphite. In polymer pyrolysis carbon, the reversible capacity of carbon materials also increases with the increase of nitrogen content, and nitrogen atoms exist in the form of ink sheet nitrogen (located in the ink film molecule, its N1s electron binding energy is 398.5eV) and conjugated nitrogen (-C=N- which is not incorporated into the ink film molecule, its N1s electron binding energy is 400.2eV).
The influence of the introduction of phosphorus on the electrochemical behavior of carbon materials varies with different precursors. The introduction of phosphorus into petroleum coke mainly affects the surface structure of carbon materials. The surface is where the phosphorus atoms are combined with the edge faces of the carbon material, but because the radius of the phosphorus atoms is larger than that of the carbon atoms, this combination increases the interlayer spacing of the carbon material, which is beneficial to the insertion and extraction of lithium. If after adding H3PO4, without direct heat treatment, H3PO4 reacts with the precursor first, and then heat treatment, so that phosphorus can be completely incorporated into the structure of the carbon material. XRS results show that phosphorus exists in a single form. On the one hand, it bonds with carbon materials, and on the other hand, it bonds with oxygen atoms due to the low heat treatment temperature (<1200°C). The introduction of phosphorus not only affects the electronic state of the carbon material, but also affects the structure of the carbon material. This effect varies with different precursors, but the introduction at higher temperatures (>800°C) can increase the reversible capacity of carbon materials.
XPS measurement results show that sulfur atoms exist in carbon materials in the form of C-S, S-S and sulfuric acid esters. The electron binding energies corresponding to the sulfur atom S2p are 164.leV, 165.3eV, and 168.4eV, respectively, indicating that the charging capacity of the carbon materials obtained after the introduction of sulfur has been greatly improved. The charging curve also shows that the platform performance before 0.5V after the introduction of sulfur is better.
(2) Doping with metal elements. Potassium is introduced into the carbon material by forming the intercalation compound KC8, which is then assembled into a battery. Since the interlayer spacing (0.341nm) of the carbon material after lithium is removed from the carbon is larger than that of pure graphite (0.336nm), it is conducive to the rapid insertion of lithium and can form LiC6 intercalation compounds with a reversible capacity of 372mA·h/g. In addition, with KC8 as the negative electrode, the choice of positive electrode material is relatively wide, and some low-cost, lithium-free materials can be used.
The introduction of aluminum and gallium can increase the reversible capacity of carbon materials. The reason is that they form a solid solution with carbon atoms and have a planar structure. Since aluminum and the grafted pz orbitals are empty orbitals, more lithium can be stored and the reversible capacity can be improved.
Transition metals such as vanadium, nickel, and cobalt are mainly added to the precursor in the form of oxides and then subjected to heat treatment. Because they act as catalysts in the heat treatment process, they are beneficial to the formation of graphite structures and the increase of interlayer spacing, thereby increasing the reversible capacity of carbon materials and improving the cycle performance of carbon materials.
The doping process of copper and iron is more complicated, usually their chloride reacts with graphite to form an intercalation compound, which is then reduced with LiAlH4. After such treatment, on the one hand, the interlayer spacing is increased, on the other hand, the tip position of the graphite is improved, so that the electrochemical performance of the carbon material is improved. In the obtained dopant compound CxM (M=Cu, Fe), if x<24, there will be too much M, and there will be few insertion positions of lithium in the graphite, which will reduce the capacity; on the contrary, when x>36, the first irreversible capacity is large and the overdischarge resistance is poor.