Mechanism of lithium storage in Si-O-C composite anodes

A Si-O-C composite material is prepared by pyrolyzing a copolymer of phenyl-substituted polysiloxane and divinylbenzene at 800 °C under a hydrogen atmosphere. The material has a high delithiation capacity about 965.3 mA h g⁻¹ in the first cycle and retains 660 mA h g⁻¹ after 40 cycles at 50 mA g⁻¹. The differential capacity curves of the anode show that there are several reduction peaks between 0.2 and 0.6 V existing all the time during repeated cycles. By comparing ²⁹Si nuclear magnetic resonance (²⁹Si MAS NMR), Si (2p) X-ray photoelectron spectroscopy (XPS) of the anode in the original, fully lithiated, and fully delithiated state, the reduction peaks are related to lithium reversible insertion into SiO₂C₂, SiO₃C, and SiO₄ units, respectively. The corresponding ²⁹Si MAS NMR resonances shift to high field and their binding energies of the Si (2p) XPS peak increase in the fully lithiated state, and then both turn to the opposite direction in the fully delithiated state. The SiO₄ units decrease during repeated cycles. The remaining ones can reversibly transform to Li-silicate (Li₂SiO₃) when lithium is inserted, while the lost ones irreversibly transform to Li-silicate (Li₄SiO₄). However, the SiOC₃ units of the material are totally irreversible with lithium because they nearly disappear in the first discharge process, and lead to the formation SiC₄ units.