Cross-linking Si_xO_y Cages with Carbon by Thermally Annealing Polyhedral Oligomeric Silsesquioxane: Structures, Morphology, and Electrochemical Properties as Lithium-Ion Battery Anodes

Chemically cross-linked Si_xO_y cage clusters embedded in N-doped carbon were prepared by thermal treatment of octaammonium polyhedral oligomeric silsesquioxane (POSS; (SiO_1.5C₃H₆NH₃Cl)₈) in an inert environment. Transmission electron microscopic studies show that by annealing at 900 °C, Si-rich clusters with a size of approximately 5 nm were formed and uniformly distributed in a lighter matrix. XPS analysis indicates that the sample obtained from annealing at 900 °C (P900) has a chemical composition of Si_6.5O_12.0C_9.3N_0.5 and it also contains substantial amounts of Si−C and C−O bonds, sp² C, and a small number of C=N bonds, in addition to Si−O bonds. Together with thermogravimetric analysis, this suggests that the clusters are likely to be formed by chemical cross-linking several POSS cages through Si−C or C−O bonds, whereas the matrix may be N-doped carbon that results from carbonization of some detached side-chains. When used as a lithium-ion battery anode, P900 exhibits stable and high cycling and rate capacity. The capacity can be maintained for at least 1000 cycles. This is much better than results for anodes made from silicon oxides, which could be ascribed to 1) the less compact but fairly stable Si−O active sites brought by the cage structure and the cross-linking; 2) the small size of the clusters that ensures smooth and complete lithium insertion/extraction; and 3) the “island–sea” morphology for the clusters-in-carbon hybrid that enhances interfacial interactions and facilitates charge conduction.