Kimihito Suzuki

Effect of Graphite Surface Structure on Initial Irreversible Reaction in Graphite Anodes

The initial irreversible reaction that occurs in graphite anodes during the first lithium intercalation in lithium rechargeable batteries was studied in view of graphite surface structure. Graphitized mesophase spheres and pitch-based carbon fibers, which show low irreversible capacity, were shown to have turbostatic surface regions and highly graphitized cores using Ar-ion laser Raman spectroscopy. Burning off these surface regions resulted in remarkable increases of initial irreversible capacity. Those results can be explained by a proposed model that a turbostatic structure of the graphite surface region resists drastic swelling of interlayer spaces arising from cointercalation of solvated ions and depresses the side reaction.

A rechargeable lithium battery employing iron Chevrel phase compound (Fe1.25Mo6S7.8 as the cathode

Abstract Powder of iron Chevrel phase compound (Fe 1.25 Mo 6 S 7.8 ) was used as the cathode for lithium secondary battery. 1 M LiClO 4 in PC was used as an electrolyte. The discharge and discharge—charge cycling properties were measured galvanostatically at a constant current density from 7.5 μA cm −2 to 0.7 mA cm −2 . A theoretical energy density of 217 W h kg −1 (only the weight of the cathode and incorporated lithium was considered for the estimation) was obtained at the first discharge when C.D. was fixed at 0.3 mA cm −2 (cut-off 1.0 V). A good rechargeability of more than 1100 times was observed in the case of relatively shallow discharge—charge experiment. Lattice expansion with intercalation of lithium ions in the iron Chevrel phase compound was less than that in copper Chevrel phase compounds which had been investigated previously.

Electrode characteristics of pitch-based carbon fiber as an anode in lithium rechargeable battery

The relations between the electrode characteristics and structures or textures of pitch-based carbon fibers controlled by spinning or stabilization conditions were investigated. The graphitized fiber spun at a lower viscosity, had a higher degree of graphitization and a higher discharge capacity. The graphitized fiber spun at a higher viscosity, exhibited a higher rate capability, suggesting that lithium ions can diffuse more easily in a fiber composing of smaller crystallites than larger ones. The graphitized fiber stabilized at a higher temperature, showed a lower irreversible capacity, which may be due to the relatively less developed graphite structure in the surface region. The graphitized pitch-based carbon fiber prepared with optimum conditions achieved a maximum discharge capacity of 315 mAh/g, an initial irreversible capacity of 10 mAh/g and an initial coulumbic efficiency of about 97%.

Coke powder heat-treated with boron oxide using an Acheson furnace for lithium battery anodes

Abstract Coal tar pitch-based coke power was heat-treated with B 2 O 3 using an Acheson furnace. The heat-treated coke powder contained nitrogen and oxygen probably derived from BN and B 2 O 3 , respectively, and exhibited a large irreversible capacity in the first charge–discharge (lithium dope–undope) cycle. The large irreversible capacity was decreased not by the decomposition of BN or B 2 O 3 but drastically by the increase of dissolved boron concentration. The discharge capacity also correlated well with the concentration of dissolved boron.

Structure of coke powder heat-treated with boron

Abstract Coal tar pitch-based coke powder with a fine mosaic texture was heat-treated with various concentrations of boron powder at 2900°C. Increasing the boron amount led to smaller d 002 and larger d 110 , and made the original fine texture coarser. Some small particles showed specific structures of polyhedrons, of which surfaces are 002 planes of graphite lattice, after heat treatment with boron. The size of the polyhedron increased with boron content. Boron concentration was lower at the surface than at the inner portions of particles for a powder heat-treated with a higher amount of boron, while it depended less on the depth for that heat-treated with a lower amount of boron. The formation mechanism of the polyhedron particle is discussed.