Irreversible and Discharge Capacities Depending on Dissolved Boron Concentration of Coke Powder

Abstract

Well-graphitized coke powder was further heat-treated at 2200°C with various amounts of boron in Ar to prepare coke powders containing different amounts of dissolved boron. The particle size distribution, specific surface area, crystallite size, and the texture observed with transmission electron microscopy were less dependent on the concentration of residue boron, ͓B͔ res , after the heat-treatment. The dissolved boron concentration, ͓B͔ sol , was quantitatively estimated from the potential curve of the first lithium doping. ͓B͔ sol was equal to ͓B͔ res when ͓B͔ res Ϲ1.0 wt % and slightly decreases with the increase of ͓B͔ res when ͓B͔ res м1.0 wt %. The irreversible capacity of the first lithium dope-undope cycle monotonously decreased, became minimum at about 0.4-0.5 wt % ͓B͔ sol , and again slightly increased with the increase of ͓B͔ sol . The discharge capacity of the first cycle was monotonously raised with the increase of ͓B͔ sol . The mechanisms of irreversible capacity and discharge capacity depending on ͓B͔ sol are discussed.Boron-doped graphite powders were studied as anode materials of lithium-ion secondary batteries. Mesophase pitch-derived carbon fibers heat-treated with B 4 C exhibited higher reversible capacities in lithium dope-undope cycles. 1,2 The higher reversible capacity of boron-doped graphite powder would mainly come from the improved crystallinity by boron during heat-treatment. 1-5 Boron is believed to be dissolved in carbon materials with replacing carbon, 6-10 elevating diffusion of carbon, and increasing the crystallite size in heat-treatment, 4,5,11 while other mechanisms of improved crystallinity by boron doping have been proposed. 5,11 The effect of dissolved boron in graphite was studied electrochemically and theoretically by Dahn et al. 12 They simulated the potential curve of lithium dope of a boron-doped graphite uner the condition of small amount of lithium intercalated, assuming that boron substituted for carbon acts as an electron acceptor, lowers the Fermi level, and somewhat shifts the potential curve of lithium dope curve higher. 12 The theory has been supported by some experiments. 3,12,13 Suzuki et al. reported that pitch coke powders heat-treated with boron in Ar using a laboratory furnace possess significantly reduced irreversible capacities in the first lithium dope-undope cycle, as well as larger reversible capacities, while pitch coke graphitized without boron exhibits a larger irreversible capacity. 14,15 Hamada et al. prepared boron-doped pitch coke powder using an Acheson furnace, which is available in mass production of anode materials for lithium-ion secondary batteries but cannot be free from N 2 or CO 2 gas. 16,17 The obtained coke powder contained BN film on the particle surface and B 2 O 3 , and exhibited a large irreversible capacity in the first lithium dope-undope cycle. Oxidation of the BN to B 2 O 3 at 1100°C in CO 2 and the following conversion of B 2 O 3 to B 4 C at 1400 or 1600°C in Ar had less affect on the larger irreversible capacity, but heat...