Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-03-04
2004-07-06
Hendrickson, Stuart (Department: 1754)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C252S510000
Reexamination Certificate
active
06759169
ABSTRACT:
TECHNICAL FIELD
The present invention relates to graphite powder used for negative electrodes of lithium ion secondary cells (batteries). More particularly, this invention relates to the graphite powder, which is capable of improving the discharge/charge efficiency and the discharge capacity of the cell and a method for producing the same.
BACKGROUND ART
As the material for a negative electrode of lithium ion secondary cells, carbon materials such as graphite and carbon are used in practice owing to the advantage in that the formation of needle crystals of lithium dendrite can be avoided during the charging and discharging. In order to adjust the particle size of the carbon mate rial to a level as desired, they are subjected to dry grinding in most cases. That is, they are ground by using a jet mill or the like to obtain their particles of a certain particle size by means of recovering with a cyclone. The obtained particles are then sieved to produce the particles in a certain range in particle size to be used. The particles of carbon material obtained by dry grinding have many active sites on the minute surfaces that are newly formed in the grinding. The carbon material having the above-mentioned active sites often cause the decomposition of electrolytes or the formation of irreversible lithium compound. For this reason, the high capacity and high reversibility required of the carbon material for lithium ion secondary cell are influenced adversely. In other words, the high Coulomb efficiency and the long-term storage life are adversely influenced; as a result, the characteristic properties of obtainable lithium ion secondary cells are impaired.
In order to solve these problems, the use of graphite material substantially containing none of fine graphite particles of 10 &mgr;m or less in particle size is disclosed in Japanese Laid-Open Publication No. H06-52860. Furthermore, disclosed in Japanese Laid-Open Patent Publication No. H06-295725 is graphite material for negative electrodes, which graphite material has an average particle size in the range of 10-30 &mgr;m and a specific surface area (BET value) in the range of 1-10 m
2
/g, and the quantities of graphite particles of less than 10 &mgr;m and more than 30 &mgr;m in particle sizes are limited to a value not more than 10%.
Furthermore, it is disclosed in Japanese Laid-Open Patent Publication No. H09-213335 to incorporate at least one element selected from the group of Mg, Al, Si, Ca, Sn and Pb into the carbon material for the negative electrode of lithium ion secondary cells by means of mixing the carbon material with at least one member of Mg
2
S, Al
4
C
3
, tin oxalate and CaC
3
which is followed by the treatment of sintering.
Still further, disclosed in Japanese Laid-Open Patent Publication No. H09-249407 is a material for negative electrode of lithium cells, which material is prepared by forming mechano-chemically a graphite composite using graphite particles and solid element particles of Li, Al, Sn, Pb and Cd.
Furthermore, disclosed in Japanese Laid-Open Patent Publication No. H08-45548 is that the material for negative electrode of lithium secondary cell is added or plated with 3-10 wt. % of at least one kind of metallic powder element selected from the group consisting of gold, silver, copper, nickel and chromium.
However, in the carbon materials (including graphite) which are obtained by dry grinding, the formation of active site on the surfaces of particles cannot be avoided, as described above. Furthermore, in the case of the carbon material prepared by dry grinding, the carbon material substantially contains fine particles even when the particle size is controlled by sieving or cyclone recovering, because the particle size is not controlled in the stage of the primary particles. That is, in the fine particles of less than 1 &mgr;m in particle size obtained by dry grinding have high surface energy so that the particles aggregate during the dry grinding process to form the secondary particles of several tens of &mgr;m in particle diameter.
DISCLOSURE OF INVENTION
The present invention has been accomplished in order to solve the above-described problems. Several graphite materials such as natural graphite, artificial graphite, kish graphite, mesophase carbon micro-beads (MCMB), mesophase carbon micro-fiber (MCF) and resin carbonized graphite which can occlude and release lithium ions, are used as the graphite materials for negative electrodes of lithium ion secondary cells. The carbon material proposed by the present invention is prepared by causing the surfaces of graphite material to adsorb or to be coated by 0.01 to 10 wt. % (on the basis of graphite material) of a material having surface active effect (hereinafter referred to as “surface active effect material”) which is at least one member elected from the group consisting of starch derivatives having a basic structure of C
6
H
10
O
5
, viscous polysaccharides having a basic structure of C
6
H
10
O
5
, water-soluble cellulose derivatives having a basic structure of C
6
H
10
O
5
and water-soluble synthetic resins.
Furthermore, the present invention provides the graphite material for the negative electrodes of lithium ion secondary cells, in which the graphite powder is coated with or adsorbs the above-mentioned surface active effect material can further contain 50 to 30,000 ppm of at least one alkali metal element or alkaline earth metal element selected from the group consisting of lithium, calcium, magnesium, sodium and potassium.
Another aspect of the present invention is to provide a method for producing the graphite powder for negative electrodes of lithium ion secondary cells, which method comprises the steps of adding graphite powder into an aqueous solution of a surface active effect material; dispersing the mixture with stirring; then filtering and drying the mixture, thereby obtaining the graphite material, in which 0.01 to 10 wt. % on the basis of the graphite material, of a surface active effect materials are adsorbed or coated. The above surface active effect material is, as described above, at least one member selected from the group consisting of starch derivatives having a basic structure of C
6
H
10
O
5
, viscous polysaccharides having a basic structure of C
6
H
10
O
5
, water-soluble cellulose derivatives having a basic structure of C
6
H
10
O
5
and water-soluble synthetic resins and the above graphite powder is exemplified by natural graphite, artificial graphite, kish graphite, mesophase carbon micro-beads (MCMB), mesophase carbon micro-fiber (MCF) and resin carbonized graphite which are able to occlude and release lithium ions.
In addition, by using the water containing lithium calcium, magnesium, sodium and potassium in the above method, the present invention further provides another method for producing graphite powder for negative electrodes of lithium ion secondary cells, in which the graphite powder contains 50 to 30,000 ppm of at least one alkali metal element or alkaline earth metal element selected from the group consisting of lithium, calcium, magnesium, sodium and potassium.
In this description, the above graphite powder used for negative electrodes of lithium ion secondary cells is sometimes simply referred to as “graphite powder” and the ordinary graphite which is used as the raw material for the graphite powder of this invention will be referred to “graphite material”.
The graphite powder adsorbing or coated with various compounds in the present invention can be prepared by mixing graphite powder into an aqueous solution of the above compounds and by dispersing with stirring, which are followed by filtration and drying.
The graphite materials used in the present invention are exemplified by natural graphite, artificial graphite, kish graphite, mesophase carbon micro-beads (MCMB), mesophase carbon micro-fiber (MCF) and resin carbonized graphite. That is, they are optionally selected according to the structure of lithium ion secondary cell of the combination of positive electrode material, electrolyte and separators. Any of
Ohzeki Katsutomo
Oyama Shigemi
Shirahige Minoru
Birch & Stewart Kolasch & Birch, LLP
Hendrickson Stuart
Hitachi Powdered Metals Co. Ltd.
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