Yoshimi Ohzawa

Thermal Stability and Electrochemical Properties of Fluorine Compounds as Nonflammable Solvents for Lithium-Ion Batteries

Differential scanning calorimetry study demonstrated that mixing of fluoro-ethers and fluoro-carbonates improved the thermal stability of 0.67 mol/L LiClO 4 ―ethylene carbonate (EC)/diethyl carbonate (DEC)/propylene carbonate (PC) (1:1:1 by volume). The oxidation currents were smaller in the fluorine compound-mixed electrolyte solutions than in 0.67 mol/L LiClO 4 ―EC/DEC/PC, which also shows a high stability of the fluorine compound-mixed electrolyte solutions against electrochemical oxidation. Electrochemical reduction of fluorine compounds took place at the higher potentials than EC, DEC, and PC, as suggested by the highest occupied molecular orbital and lowest unoccupied p-molecular orbital energies of the fluorine compounds. However, charge/discharge experiments using natural graphite (NG) electrodes showed that the fluorine compounds increased first coulombic efficiencies due to the quick formation of the solid electrolyte interphase on NG in PC-containing solvents.

Electrochemical Behavior of Nonflammable Organo-Fluorine Compounds for Lithium Ion Batteries

The electrochemical behavior of organo-fluorine compounds with antioxidation ability has been investigated. Oxidation currents of fluorine-compound-containing ethylene carbonate (EC)/diethyl carbonate (DEC) solutions were much smaller than those of EC/ DEC and EC/DEC/propylene carbonate (PC) at potentials higher than 6 V vs Li/Li + . Electrochemical reduction of fluorine compounds started at ca. 2 V vs Li/Li + , higher than those for EC, DEC, and PC. However the first coulombic efficiencies for natural graphite electrodes in fluorine-compound-containing EC/DEC mixtures were nearly the same as those in EC/DEC without an increase in irreversible capacities. Furthermore the first coulombic efficiencies in fluorine-compound-containing EC/DEC/PC mixtures were much larger than those in EC/DEC/PC itself. The results show that the fluorine compounds used in the present study can be used as nonflammable solvents for lithium ion batteries.

Electrochemical properties and structures of surface-fluorinated graphite for the lithium ion secondary battery

Surface modification of graphite powder has been performed by elemental fluorine and radiofrequency (rf) plasma fluorination. Both methods give rise to an enlargement of the surface areas of graphite samples and a change of the pore volume distribution. The capacities of surface-fluorinated graphite samples are higher than those of original samples and even more than the theoretical capacity of graphite, 372 mAh g−1, without any reduction of the first colombic efficiencies. The increments of the capacities are ∼5, 10, and 15% for graphite samples with average particle diameters of 7, 25 and 40 μm, respectively.

Effect of fluoride additives on the corrosion of aluminum for lithium ion batteries

Abstract Effect of fluoride additives was investigated in organic solvents containing LiCF 3 SO 3 to prevent the corrosion of aluminum current collector for lithium ion batteries. LiClO 4 was also examined for comparison. Among examined LiBF 4 , LiPF 6 , LiAsF 6 , LiSbF 6 and LiClO 4 , LiBF 4 was the best additive to suppress the corrosion of aluminum because its oxidation potential is close to that of CF 3 SO − 3 anion. Corrosion currents for aluminum in a complex fluoride- or LiClO 4 -added solvents became smaller in the order, LiSbF 6 >LiAsF 6 >LiClO 4 >LiPF 6 >LiBF 4 . Oxidation potential of ClO − 4 is nearly the same as that of CF 3 SO − 3 . However, the corrosion currents were similar to or slightly larger than those observed in LiPF 6 -added solvents. SEM images of electrochemically oxidized aluminum samples indicated that the level of corrosion well coincided with the observed corrosion currents. The corrosion mechanism of aluminum was also proposed.

Electrochemical Behavior of Surface-Fluorinated Natural Graphite in Propylene Carbonate-Containing Solvent

Surface fluorination of natural-graphite samples with average particle sizes of 5, 10, and 15 μm (abbreviated to NG5 μm, NG10 μm, and NG15 μm) was performed by F 2 (3 X 10 4 Pa) at 200 and 300°C for 2 min and electrochemical properties of surface-fluorinated natural-graphite samples were investigated in 1 mol/dm 3 LiClO 4 -ethylene carbonate (EC)/diethyl carbonate (DEC)/propylene carbonate (PC) (1:1:1 in volume). Surface fluorine concentrations were in the range of 11.1-15.3 and 17.7-20.5 atom % for natural-graphite samples fluorinated at 200 and 300°C, respectively, for three natural-graphite samples. The surface areas of NGlO μm and NG15 μm were only slightly increased by fluorination, while the increase in surface area was large in the case of NG5 μm. However, the increase in surface disorder by fluorination was larger for NG10 μm and NG15 μm than for N65 μm. Electrochemical reduction of PC on NG10 μm and NG15 μm was highly reduced by surface fluorination, which led to a large increase in first coulombic efficiencies. The increase in first coulombic efficiencies for surface-fluorinated NG10 μm and NG15 μm is attributed to the increase in their surface disorder and probably actual electrode area by fluorination.

Surface-Structure Change and Charge/Discharge Behavior of Petroleum Cokes Surface-Modified by Thermally Activated ClF3 and NF3

Heat-treatment of oxygen-containing petroleum coke (PC) at a high temperature (∼2800°C) gave rise to closure of edge planes by carbon-carbon bond formation. To change the surface structure, surface modification of PC and those heat-treated at 1860, 2300, and 2800°C (PC1860, PC2300, and PC2800) has been performed by ClF 3 and NF 2 at 200-500°C. No surface fluorine was detected except in one sample treated by NF 3 , while small amounts of surface chlorine were found in all samples treated by ClF 3 . Small amounts of nitrogen were detected in two samples treated by NF 3 . Brunauer-Emmett-Teller surface areas and total mesopore volumes were reduced by surface modification. Transmission electron microscopic observation revealed that closed-edge planes of graphitized PC were destroyed and opened by surface modification with ClF 2 and NF 3 . Removal of closed-edge planes increased first-charge capacities of many PC samples heat-treated at 1860-2800°C by ∼63 mAh/g (∼29.6%) at 150 mA/g.

Preparation of high-temperature filter by pressure-pulsed chemical vapour infiltration of SiC into carbonized paper-fibre preforms

SiC was partially infiltrated into three types of carbonized paper-fibre preforms using pressure-pulsed chemical vapour infiltration from SiCl4(4%)–CH4(4%)–H2 at 1100 °C (A-type preforms, source fibres of filter paper; BH- and BL-type preforms, source fibres of recycled paper). The porosity of the preforms decreased linearly with the number of pulses. After 10 000 pulses, the porosity of A-, BH- and BL-type samples was 77,78 and 85%, respectively. Average pore sizes of A-, BH- and BL-type samples after 10 000 pulses were about 5.0, 2.7 and 7.0 μm, respectively. On an A-type sample of 10 mm Φ and 5 mm long after 10 000 pulses, pressure drop along the direction of axial air flow was 10 kPa at a face velocity of 0.8 m s−1. The order of pressure drop was BH,>, A >, BL. Flexural strength of A-type sample reached 10 MPa after 15,000 pulses.

Preparation of SiC-based cellular substrate by pressure-pulsed chemical vapor infiltration into honeycomb-shaped paper preforms

Using a pressure-pulsed chemical vapor infiltration technique, SiC was infiltrated from a SiCl4 (4%)–CH4 (4%)–H2 gas phase into carbonized paper preforms at 1100°C. SiC-based cellular substrates with cell wall thicknesses of 25, 50 and 100 μm were obtained by using honeycomb-shaped paper preforms as the templates. The reduction of both wall thickness t and cell pitch d of SiC-based honeycomb substrate successfully led to an increase in geometric surface area per unit volume S, keeping pressure drop ΔP at constant, besides; ΔP decreased without lowering S by the reduction of t and the increase in d. The pressure drop in the prepared honeycomb substrates depended on S2α−3 value, where α was open frontal area fraction. The compressive strength of the honeycomb substrates with t of 25 μm was about 7 MPa. The strength increased in proportion with 1 − α, which corresponded to the volume fraction of the cell wall in the honeycomb substrate.

Pressure-pulsed chemical vapour infiltration of pyrolytic carbon to porous carbon preforms or two-dimensional-carbon/SiC particulate preforms from a gas system CH4-H2-N2

Pyrolytic carbon (pyrocarbon) was infiltrated to porous carbon preforms or two-dimensionally woven carbon fiber (2D-C)/SiC particulate preforms using pressure-pulsed chemical vapour infiltration from 20–100% CH4 diluted by H2 or N2. Pyrocarbon deposited from the gas system CH4-N2 had a laminar microstructure oriented parallel to the macrosurface of the preform or the surface of the carbon fiber, whereas, laminar structure was not observed clearly in the pyrocarbon from the gas system CH4-H2. After 4 × 104 pulses with a reaction time of 0.4 s per pulse at 1423 K from 50% CH4-H2 or at 1373 K from 50% CH4-N2, residual porosity of 2D-C/SiC particulate preforms decreased from 30% to 10%. Flexural strength of 2D-C/SiC particulate preforms increased from 32 to 125 MPa.

Surface Structure And Electrochemical Characteristics Of Graphite Fluorinated By Elemental Fluorine And Plasma Treatment Using Cf 4

Surface modification of graphite powder has been performed by elemental fluorine and radiofrequency plasma fluorination. Both of the methods enlarge the surface areas of graphite samples and change their pore volume distribution. Surface-fluorinated graphite samples demonstrate the capacities higher than those of original samples and even the theoretical capacity of graphite, 372 mAhg −1 without reducing the first coulombic efficiencies. The increments of the capacities are ∼5, ∼10 and ∼15% for graphite samples with average particle diameters of 7, 25 and 40 μm, respectively.