Si–C–O glass-like compound/exfoliated graphite composites for negative electrode of lithium ion battery Original Research Article

Two low molecular weight silicone compounds, a cyclic type having vinyl groups and a chain-type having Si–H bonds, a catalyst for curing, and a catalyst regulator were mixed. The mixture was impregnated into exfoliated graphite (EG) by sorption, and cured in air at 200 °C. By this process cross-linked silicone coatings were formed on graphite flakes. The composites of Si–C–O glass-like compounds and EG were synthesized by heat treatment of this precursor at 1000–1400 °C for 1 h in argon. The composites formed at 1000–1300 °C were amorphous by XRD and had practically the same chemical composition: Si 44–45, C 27–29, O 25–26, H < 0.5, all in mass%. The 29Si MAS-NMR spectra indicated that the compound formed at 1000 °C was mainly composed of siloxane bonds and amorphous silica, whereas in the compound formed at 1300°C, Si–C bonds and amorphous silica were predominant. The insertion/extraction characteristics of lithium ions for the electrode prepared with composite:poly(vinylidene fluoride) = 90:10 mass% were examined in 1 mol L−1 LiClO4 solution of ethylene carbonate:diethyl carbonate = 50:50 vol%. High, 650–700 mA h g−1, capacities and steady cycle performance at 50 mA g−1 were achieved with the composites formed at 1250–1300 °C. Capacities of the composites formed at 1200 °C and lower were initially higher but decreased with increasing number of cycles. The composites formed at 1350 °C showed good cycle performance but the capacity was about 500 mA h g−1 due to the formation of β-SiC. Except for the first cycle, the capacity-potential characteristics were similar to those of hard carbons and the coulomb efficiency was 95–100%. For all the composites the capacity was larger than that of graphite (372 mA h g−1) in the range of 50–200 mA g−1. Due to the large insertion capacity of the first cycle, the efficiency was low (60–70%) at first. By short-circuiting the working electrode to the lithium foil counter electrode for a certain period, the irreversible capacity of the first cycle was almost eliminated. It indicates that direct doping of lithium ions into composites is a promising way to increase the efficiency of the first cycle.