Takashi Achiha

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 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.

Effect of Conductive Additives and Surface Fluorination on the Electrochemical Properties of Lithium Titanate ( Li4 / 3Ti5 / 3O4 )

The effect of conductive additives and surface fluorination on the charge/discharge behavior of Li 4/3 Ti 5/3 O 4 was investigated. Mixtures of acetylene black (AB) and vapor-grown carbon fiber (VGCF) gave larger first charge capacities at a high current density of 600 mA/g than those obtained when AB or VGCF was used as a conductive additive by improving the electric contact between Li 4/3 Ti 5/3 O 4 particles and a nickel current collector. The surface fluorination of Li 4/3 Ti 5/3 O 4 by NF 3 improved the thermal stability of the electrolyte solution without decreasing first charge capacities and first coulombic efficiencies.

Electrochemical Properties of Natural Graphite Fluorinated by ClF3 and NF3 in Propylene Carbonate-Containing Solvent

Surface fluorination of natural graphite samples with average particle sizes of 5, 10, and 15 μm (NG5, NG10, and NG15 μm) was performed by ClF 3 and NF 3 (1 X 10 5 Pa) at 200 and 300°C for 5 min, and charge/discharge behavior of fluorinated samples was investigated in 1 mol/dm 3 LiClO 4 -ethylene carbonate (EC)/diethyl carbonate (DEC)/propylene carbonate (PC) (1:1:1 in volume). Mesopores with a diameter of 2 nm were increased by the surface fluorination, while those with diameters of 2.5 and 3 nm were reduced. Surface disorder of natural graphite samples also increased with increasing fluorination temperature and particle size. These surface structure changes by surface fluorination would have reduced the decomposition of PC, leading to an increase in first coulombic efficiencies. The increase in first coulombic efficiencies was larger at a current density of 150 mA/g than 60 mA/g, reaching ca. 10 and 20% for NG10 and NG15 μm fluorinated by ClF 3 .