The introduction of oxygen into silicon is mainly to alleviate the volume effect of silicon and improve the cycle performance of the material. For the negative electrode of lithium ion battery, due to the good chemical affinity between Li+ and O during the lithium insertion process, the electrochemical irreversible phase Li2O is easily generated, thereby increasing the first irreversible capacity of the material. Therefore, in the preparation and modification of negative electrode materials, it is generally necessary to avoid introducing too much oxygen-containing material.
Someone has studied several kinds of silicon oxide SiOx (x=0.8, 1.0, 1.1) and found that as the oxygen content in silicon oxide increases, the specific capacity of the battery decreases, but the cycle performance improves; as the oxide particles are reduced to below 30nm, adhesion between the particles will occur during the charging and discharging of the battery, which reduces the cycle performance.
There are two views on the lithium insertion mechanism of SiOx (0<x<2): one view is that during the process of lithium insertion, SiO and Li+ form LixSiO, and the other view is that at a higher potential, Li first reacts with O in the SiOx molecule to form the irreversible compound Li2O; as the lithium insertion process proceeds, a silicide of lithium is formed with silicon at a lower potential.
In addition to O, the stable non-metallic elements that can form lithium-intercalation properties with silicon also include B elements. B and silicon form a compound SiBx (x=3.2~6.6). Studies have shown that the first lithium insertion capacity of SiBx can reach 922mA·h/g, but its delithiation capacity is only 440mA·h/g, and its reversibility is lower than that of silicon oxide. However, the first discharge capacity of SiBx is as high as 1500mA·h/g, the first charge and discharge efficiency is also up to 82%, and the capacity retention rate after 20 cycles is 95%.
In view of the serious volume effect of silicon materials, in addition to alloying or other forms of silicide (SiOx, SiBx, etc.), another effective method is to prepare silicon-containing composite materials. The synergistic effect between the components of the composite material is used to achieve the purpose of complementing each other's advantages. Carbon-based anodes have small volume changes during charging and discharging, have good cycle performance, and are a mixed conductor of ions and electrons, so they are often selected as the matrix material (ie, dispersion carrier) of high-capacity anode materials. The lithium-intercalation potential of silicon is similar to that of carbon materials, such as graphite MCMB. Therefore, Si and C are usually combined to improve the volume effect of Si, thereby improving its electrochemical stability. Since both silicon and carbon have high stability at room temperature, it is difficult to form a complete interface bond. Therefore, the preparation of Si/C composite materials is generally synthesized by high-temperature solid-phase reaction, CVD and other high-temperature methods. Si and C will form an inert phase of SiC when the temperature exceeds 1400°C. Therefore, the order of the C matrix in the Si/C composite prepared in the high temperature process is low.
Si/C composite materials are mainly divided into the following three categories according to the distribution of silicon in carbon (as shown in Figure 1).
Figure 1 - Schematic diagram of different types of Si/C composite materials
(1) Covered type. The coated type is the so-called core-shell structure, and the more common structure is a carbon layer wrapped with silicon. The presence of the carbon layer coated with silicon particles can minimize the direct contact between the electrolyte and the silicon, thereby improving the decomposition of the electrolyte caused by the dangling bonds on the silicon surface. On the other hand, Li+ has to overcome the resistance of the carbon layer and the Si/C interface layer in the solid phase to react with silicon. Therefore, the lithium insertion depth of silicon can be controlled to a certain extent through an appropriate charging and discharging system, thereby reducing the structural damage of silicon and improving the cycle stability of the material.
(2) Embedded type. Among the Si/C composite materials, the most common is the embedded structure. The silicon powder is uniformly dispersed in a dispersed carrier such as carbon and graphite to form a stable and uniform two-phase or multi-phase composite system. During the charge and discharge process, silicon is the active center of electrochemical reaction. Although the carbon carrier has the ability to deintercalate lithium, it mainly acts as a transmission channel for ions and electrons and a structural support. The preparation of this system mostly adopts high-temperature solid-phase reaction, which is obtained by uniformly dispersing silicon in a polymer that can be cracked and carbonized at high temperature, and then obtained by high-temperature solid-phase reaction. The electrochemical performance of this type of system is mainly determined by factors such as the performance of the carrier and the Si/C molar ratio. Generally speaking, the higher the order of the carbon matrix, the more thorough the dehydrogenation, the lower the Si/C molar ratio, the more obvious the coordination between the two components, and the better the cycle performance. However, since inert SiC is easily generated in the high temperature process of Si/C, which makes the silicon lose its electrochemical activity, the disorder of the carbon matrix has become a bottleneck problem for the embedded Si/C composite to further improve its electrochemical performance. The use of silicon powder and graphite with a high degree of order can be directly used as reaction precursors. The nano silicon powder prepared by high-energy ball milling is dispersed in the Si/C composite system C1-xSix in the carbon matrix, which can improve the cycle performance of silicon within a certain range. The value in C1-xSix determines the initial capacity of the material. For example, the initial lithium insertion capacity of C0.8Si0.2 is as high as 1089mA·h/g. After 20 cycles, the capacity is 794mA·h/g, showing good cycle performance. Since the stability of silicon and graphite itself determines that it is difficult to form a complete interface bond between the two, increasing the milling time can increase the degree of synergy between the two, but the increase of the milling time will cause the precursors to react with each other, forming an inert SiC phase. When silicon powder is subjected to high-energy ball milling, an amorphous powder with a high specific surface area can be obtained, and then graphite powder is added to it for ball milling. On the one hand, it can increase the specific surface area of silicon powder and reduce the absolute volume expansion during the process of lithium insertion. On the other hand, it can reduce the possibility of SiC formation due to long-term high-energy ball milling, thereby greatly improving the cycle performance of the material.
(3) Molecular contact type. Both the coated and molecular contact Si/C composite materials use pure silicon powder as the reaction precursor to enter the composite system. Molecular contact type composite material, silicon and carbon are both a highly dispersed system of molecular contact formed by organic precursors containing silicon and carbon after treatment, which is a relatively ideal dispersion system(as shown in Figure 2). Nano-scale active particles are highly dispersed in the carbon layer, which can overcome the volume expansion of silicon to the greatest extent.
Figure 2 - The model of nanoparticles dispersed in the carbon matrix before graphitization
Using vapor deposition method, using benzene, SiCl4 and (CH3)2Cl2Si as precursors can prepare molecular contact Si/C composite materials. The initial capacity of this material varies with the atomic percentage of silicon, generally ranging from 300 to 500 mA·h/g. When the content of silicon is less than 6%, its capacity changes linearly with the atomic percentage of silicon, and its lithium insertion capacity is much smaller than the actual lithium insertion capacity of silicon. Approximately 1.5 molecules of lithium ions per molecule of silicon atom can be inserted. This may be due to the inevitable formation of part of the inert SiC in the gas phase reaction.
Studies have shown that there are a large number of H, O and other heteroatoms in the precursor molecular silicon source and carbon source, which reduces the structural stability of the material and increases the first irreversible capacity during the lithium insertion process. In addition, the high reactivity of the gas-phase precursor also limits the relative content of silicon in the system.
The Si/C material prepared by high-temperature pyrolysis using silicon-containing polymer and pitch as the precursor has a much simpler preparation process than vapor deposition, and the reversible capacity of the product is higher, reaching more than 500mA·h/g. However, a large amount of active elements such as S and O remain in the prepared system. These elements form a Si-O-S-C glass body with a network structure in the system, which irreversibly consumes the lithium source in the process of lithium insertion, resulting in the first irreversible capacity of the material being high.