Composite graphite particles and lithium rechargeable battery using the same
The invention provides composite graphite particles, comprising a core material consisting of graphite having a interlayer distance d(002) of 0.337 nm or less and a surface layer consisting of graphite in which the intensity ratio I D /I G (R value) between the peak intensities (I D ) in a range of 1300 to 1400 cm −1 and ( I G ) in a range of 1580 to 1620 cm −1 as measured by Raman scattering spectroscopy is 0.3 or higher, wherein the peak intensity ratio I 110 /I 004 between the peak intensities (I 110 )of face (110) and (I 004 )of face (004) obtained by XRD measurement on the graphite crystal is 0.15 or higher when the graphite has been mixed with a binder and pressure-molded to a density of 1.55 to 1.65 g/cm 3 , which composite graphite particles are useful in a secondary battery negative electrode having a high capacity, high charge/discharge characteristics and an excellent charging property, and also provides a paste for negative electrode, a negative electrode and a lithium secondary battery using the composite graphite.
1. Composite graphite particles, comprising a core material consisting of graphite having an interlayer distance d(002) of 0.337 nm or less, a surface layer consisting of graphite in which the intensity ratio I D /I G (R value) between the peak intensity (I D ) in a range of 1300 to 1400 cm −1 and the peak intensity (I G ) in a range of 1580 to 1620 cm −1 as measured by Raman scattering spectroscopy is 0.4 or higher, wherein the peak intensity ratio I 110 /I 004 between the peak intensity (I 110 )of face (110) and the peak intensity (I 004 ) of face (004) obtained by XRD measurement on the graphite crystal is 0.15 or higher when the graphite has been mixed with a binder and pressure-molded to a density of 1.55 to 1.65 g/cm 3 , and vapor-grown carbon fiber attached on the surface layer via carbon, wherein the vapor grown carbon fiber is attached on the surface layer via carbon by a process of mixing the vapor grown carbon fiber with an organic compound, polymerizing the organic compound in the mixture and then carbonizing the organic compound to form the surface layer, wherein the surface layer graphite is obtained by thermally treating the organic compound at a temperature of 1000 to 2700° C., wherein the organic compound is at least one selected from a group consisting of phenol resin, polyvinylalcohol resin, furan resin, cellulose resin, polystyrene resin, polyimide resin and epoxy resin, wherein the coating amount of the organic compound serving as raw material for the surface layer is in a range of 15 to 30% by mass based on the core material, and wherein the R value of the core material is smaller than that of the surface layer graphite.
2. The composite graphite particles according to claim 1 , wherein the crystallite diameter in the c-axis direction Lc of the core material graphite is 100 nm or more.
3. The composite graphite particles according to claim 1 , wherein the core material graphite is a natural graphite.
4. The composite graphite particles according to claim 1 , wherein in particle size distribution measurement by laser diffraction method, the particle size of 90% or more of the total core material is within a range of 5 to 50 μm.
5. The composite graphite particles according to claim 1 , wherein the BET specific surface area is in a range of 0.5 to 30 m 2 /g.
6. The composite graphite particles according to claim 1 , wherein the interlayer distance d(002) is 0.337 nm or less and the crystallite diameter in the c-axis direction Lc is 100 nm or more.
7. The composite graphite particles according to claim 1 , wherein in particle size distribution measurement by laser diffraction method, the particle size of 90% or more of the particles is within a range of 5 to 50 μm.
8. A method for producing the composite graphite particles as claimed in claim 1 , comprising a step of allowing the organic compound to attach to the surface of the core material consisting of a graphite having an interlayer distance d(002) of 0.337 nm or less, a step of polymerizing the organic compound at 200 to 400° C. and a step of conducting a thermal treatment at a temperature of 1000 to 2700° C.
9. A paste for negative electrode, comprising the composite graphite particles as claimed in claim 1 , a binder and a solvent.
10. A negative electrode, wich is obtained by spreading the paste for negative electrode as claimed in claim 9 on a collecter, drying the spread paste and pressure-molding the dried paste.
11. A lithium secondary battery comprising the negative electrode as claimed in claim 10 as a constituent.
12. The lithium secondary battery according to claim 11 , comprising a nonaqueous electrolytic solution and/or nonaqueous polymer electrolyte, wherein the nonaqueous electrolytic solution and/or nonaqueous polymer contains at least one nonqueous solvent selected from a group consisting of ethylene carbonate, diethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone and vinylene carbonate.