The metal/alloy-carbon composite material solves the problem of low electronic conductivity caused by the use of inactive metals, and also obtains outstanding electrochemical characteristics by minimizing volume expansion. The properties of metal/alloy-carbon composite materials as negative electrode materials are related to their microstructure and manufacturing methods. Although Sn and Si are common elements that make up metals or alloys in these composite materials, due to the low melting point of Sn, the use of Sn in the design and manufacture of carbon composite materials is restricted.
Figure 1 shows the microstructure and schematic diagram of the Sn-Co-C composite. In this composite material, Sn forms an alloy with Co and C. Although the alloying between the three elements is difficult to achieve, the alloying of Sn-Co-C has been experimentally confirmed. The alloying of Sn with Co and C and the presence of carbon suppress the volume expansion. Batteries based on Sn-Co-C composite materials have been commercialized on cameras.
However, the complexity of synthesis and the inadequacy of increased capacity limit this composite material to special-purpose applications.
In general carbon composite materials, Si and Si alloys are coated with carbon, and Si is dispersed in the carbon material. The carbon layer on the surface of the active material improves the electronic conductivity between the particles and the electrochemical characteristics of the electrolyte. The material can be prepared by thermally decomposing the carbon precursor on the surface of Si and Si alloy or simultaneously thermally decomposing and depositing Si and carbon precursor when the temperature is higher than 1000°C. Figure 2 shows a schematic diagram of a composite material of Si and carbon.
In the composite material obtained by the above method, Si particles are dispersed in the continuous phase of carbon. The composite material reduces the deterioration of the electrode caused by the volume expansion of the Si particles, but the use of carbon leads to a shortened cycle life and an irreversible reaction in the early stage. In Figure 2b, the carbon layer is applied to the Si-C composite material by the CVD method, which reduces the irreversible reaction and improves the cycle life. This is more effective than the method in Figure 2a, but it also requires higher manufacturing costs, and reduces the energy density of the electrode, which is not conducive to a significant increase in capacity.
Another method is to use a composite material of silicon and graphite, which is formed by ball milling and coated with a layer of graphite on the surface of the silicon particles. This method not only prevents the increase of irreversible capacity, but the presence of the graphite layer also inhibits the volume expansion of silicon particles, and finally obtains the high capacity and superior performance of the electrode. Figure 3 shows a schematic diagram of the composite material of si and graphite.
The silicon particles are coated with a graphite layer by ball milling, and an additional carbon layer is covered to form a silicon-graphite composite material. The carbon coating allows the silicon particles to be evenly distributed and reduces the volume expansion caused by the reaction of the Si particles with lithium during the charge and discharge process. Through the combination of graphite and Si particles, both electronic conductivity and ionic conductivity have been enhanced. The carbon layer on the surface of the material is formed by first mixing the carbon precursor and the silicon-graphite composite material, and then performing thermal decomposition and carbonization at 1000°C. The use of Si alloy (Si/M, M: transition metal) instead of pure Si enhances the electrical conductivity of electrons and reduces the volume change.