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 degrees 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 (29)Si nuclear magnetic resonance ((29)Si 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 SiO(2)C(2), SiO(3)C, and Slat units, respectively. The corresponding (29)Si 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 SiO(4) units decrease during repeated cycles. The remaining ones can reversibly transform to Li-silicate (Li(2)SiO(3)) when lithium is inserted, while the lost ones irreversibly transform to Li-silicate (Li(4)SiO(4)). However, the SiOC(3) units of the material are totally irreversible with lithium because they nearly disappear in the first discharge process, and lead to the formation SiC(4) units. (C) 2011 Elsevier B.V. All rights reserved.