Introduction
In recent years, tremendous interest has been focused on the usage of silicon (Si) as anode material for lithium-ion batteries (LIB). Silicon has received increasing attention because it is abundantly available, cost-effective, eco-friendly, has the highest theoretical specific capacity (4200 mA*H*g-1) and satisfactory working potential (~0.4 V vs Li+/Li).1-3 Simultaneously, electrode materials based on Si-Li alloys suffer from the main drawback – appearance of the strong anisotropic mechanical strain at lithium/delithium process, that leads to degradation of the electrochemical system and its huge volume change resulting the shunt contact loss, irreversible capacity fast loss, low initial coulombic efficiency, i.e. decreasing in quantitative and qualitative cycling characteristics. 4-7
Covering Si particles with carbon is one of the most effective solutions to overcome the above disadvantages. Silicon-carbon (Si/C) electrodes have a higher reversible capacity and number of charge/discharge cycles compared to the uncovered Si. 8, 9It was found that the best performance characteristics have electrode materials with carbon content approximately 30% per mass. Such experimental data were confirmed by our previous quantum-chemical study concerning lithium/delithium process modeling using СmSi13 (m = 0, 6, 10, 13, 18, 26, 39, 45) model clusters. 10
Today various methods exist for the Si/C composites synthesis and any of them provide deposition or adsorption carbon atoms or their associates on the surface of Si producing more complex fragments of carbon nanostructure. 11-13 A wide variety of carbon nanostructures (graphene, CNT, fullerenes, etc.) have been used for the synthesis of Si/C composites. 2, 14, 15 Fullerene is a good alternative for Si coating because of its high electron affinity, high chemical, and mechanical stability, which is useful for electrochemical devices. 16 Moreover, fullerene could form a thin polymeric film on Si that acts as a buffer layer to reduce the volume expansion and to improve the LIB kinetic properties.17-19 In addition, fullerene could be easily modified via attaching different functional groups (carboxyl, ester, piperazine) for decreasing the Si/C composite solubility or via doping of the other elements. 20-22 Simultaneously, it is well known, that after the metal intercalation to the fullerene cage the obtained material becomes soluble in the polar organic electrolyte solution that limited the use of pure C60 anodes. Synthesis of the fullerene-type composite material with Si-C covalent bonds could be a great alternative to solve this problem.
The quantum-chemical calculation is a powerful tool for the study of various characteristics of Si/C anode materials. A lot of different clusters via various theoretical methods have been investigated earlier.23-26 Continuing on our previous studies10, 27, 28 and to expand our knowledge on the role of the electronic structure of Si/C nanocomposites to the lithium/delithium process, here we explore a series of SinCm clusters with various atomic ratios. In total, we report diffusion of Li in the Si/C systems and volume change under lithiation via first-principle calculations.