Organic electrolyte secondary cell

In the present invention, an organic electrolyte secondary cell of the present invention is comprised of a positive electrode, a negative electrode including a carbon material occluding and discharging lithium ion, and an organic electrolyte. In the cell, at least a part of the carbon material is covered with a lithium alkoxide compound having a molecular weight more than 52.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an organic electrolyte secondary cell 
which has a high energy density and reliability as a power source for an 
electronic equipment, for maintaining a memory, for a electric vehicle, 
for storing electric power and the like. 
2. Description of the Related Art 
Accompanying with the drastic miniaturization and lightening of electronic 
equipments, it is highly required to develop a secondary cell as a power 
source thereof which is miniaturized and lightened as well as has a high 
energy density, further is capable of charging and discharging repeatedly. 
In addition, due to environment problems such as air pollution and 
increase of carbon dioxide, it is desired to utilize an electric 
automobile in earliest stages. Accordingly, it is desired to develop an 
excellent secondary cell having features such as high efficiency, high 
power, high energy density and light in weight. Since secondary cell using 
an organic electrolyte which satisfies such requirements has an energy 
density several times as high as that of a conventional cell using an 
aqueous electrolyte, it is desired to put it to practical use. 
As a positive active material of the organic electrolyte secondary cell, 
various types of material have been examined, such as titanium disulfide, 
lithium-cobalt composite oxide, spinel type lithium-manganese oxide, 
vanadium pentoxide and molybdenum trioxide. In these materials, 
lithium-cobalt composite oxide (LiCoO.sub.2) and spinel type 
lithium-manganese oxide (LiMn.sub.2 O.sub.4) conduct charging/discharging 
in extremely high potential more than 4 V (Li/Li.sup.+). Consequently, 
they are used as a positive electrode so as to utilize a cell having a 
high discharge voltage. 
As a negative active material of the organic electrolyte secondary cell, 
lithium, Li-Al alloy and carbon material capable of occluding and 
discharging lithium ion, and the like have been examined. In these 
materials, carbon material has an advantage that a cell having a long 
cycle life can be obtained. 
However, in this kind of cell, since lithium having lower potential is used 
as the negative active material and metal oxide having higher potential is 
used as the positive material, electrolyte is easy to be decomposed. 
Accordingly, it is necessary to consider about this point to select the 
electrolyte, and various kinds of electrolytes have been proposed to use. 
Almost all of the electrolytes are the mixture of a high dielectric 
constant solvent such as propylene carbonate, ethylene carbonate, 
.gamma.-butyrolactone, sulforane, and a low viscosity solvent such as 
1,2-dimethoxyethane, dimethylcarbonate, ethylmethylcarbonate, 
diethylecarbonate. 
On the other hand, as a solute, lithium perchlorate, lithium 
trifluoromethanesulfonate, lithium hexafluorophosphate and the like are 
generally used. Particularly, lithium hexafluorophosphate is popularly 
used in recent, because of high safety and high ion conductive rate of 
electrolyte in which it is dissolved. 
However, when carbon material is used as the negative electrode, a 
reduction decomposition reaction of the electrolyte occurs on the surface 
of the negative electrode with generating gas in the first charging. 
Accordingly, a cell case may be swelled, or a cell capacity may be 
reduced. 
The charge is forwarded to stop generating gas, so that a charge reaction 
to carbon begin to forward. That is, a electrolytic polymerization 
reaction occurs on the surface of the carbon material in the initial stage 
of charge, and a polymer coat is formed on the surface of the carbon 
material. When the coat is formed to some degree, the electrolytic 
polymerization reaction is supressed because of lack of the electron 
conductivity of the coat, thereby forwarding only charging reaction of 
lithium ion. However, since lithium ion is consumed for the polymerization 
reaction in the initial stage, and is not effectively used for charging 
reaction, the capacity of the cell is reduced. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an organic electrolyte 
secondary cell in which reduction of a cell capacity caused by 
decomposition of an electrolyte can be supressed. 
An organic electrolyte secondary cell of the present invention is comprised 
of a positive electrode, a negative electrode including a carbon material 
occluding and discharging lithium ion, and an organic electrolyte, wherein 
at least a part of the carbon material is covered with a lithium alkoxide 
compound having a molecular weight more than 52. 
The organic electrolyte secondary cell according to the present invention 
can suppresses the reduction of a cell capacity caused by decomposition of 
the electrolyte.

DETAILED DESCRIPTION OF THE INVENTION 
The detailed description of the invention will be described as follows 
referring to the accompanying drawing. Although the present invention will 
be described along preferable examples, the present invention is not 
limited by these examples. 
Lithium-cobalt oxide (LiCoO.sub.2), graphite powder as a conductive 
material, and fluorine polymer powder as a binder were sufficiently mixed 
in a weight ration of 90:3:7, thereafter the mixture was pressurized and 
molded to thereby produce a positive electrode. Carbon powder and fluorine 
polymer powder as a binder were mixed in a weight ration of 91:9, 
thereafter the mixture was pressurized and molded to thereby produce a 
negative electrode. The negative electrode was impregnated with 
1,2-ethanediol solution of dilithium-1,2-ethanediol under reduced 
pressure, thereafter it was dried to thereby form 
dilithium-1,2,-ethanediol coat on the surface of the carbon material. 
Incidentally, in the above processes, the solvent was adjusted so that the 
weight ratio of lithium alkoxide became about 1% with respect to the 
carbon material. 
FIGURE is a sectional view of a cell. In the drawing, reference numeral 1 
designates a case also used as a positive electrode terminal which is 
produced by stamping a stainless steel (SUS316); 2, a sealing plate also 
used as a negative electrode terminal which is produced by stamping a 
stainless steel (SUS316); 3, the negative electrode which is attached to 
the inner wall of the sealing plate 2; 5, a separator comprising 
polypropylene to which organic electrolyte is impregnated; and 6, the 
positive electrode. An opening end portion of the case is inwardly 
crimped, and the outer periphery of the sealing plate 2 is clamped via a 
gasket 4, thereby tightly closing and sealing the cell. 
A mixture was used as an organic electrolyte in which an organic solvent 
containing ethylenecarbonate and dimethylecarbonate in a volume ratio of 
1:1 was mixed with lithium hexafluorophosphate in a concentration of 1 
mol/l. The organic electrolyte of about 150 .mu.l was injected into the 
cell. The size of the cell was 20 mm in diameter, and 2 mm in height. Thus 
produced cell was a cell A of the present invention. 
Cell B, C and D of the present invention were produced as similar to the 
above example except using dilithium-1,3-propanediol, 
trilithium-glyceline, and trilithium-1,2,6-hexanetriol instead of 
dilithium-1,2-ethanediol, respectively, and using 1,3-propanediol, 
glyceline, and 1,2,6-hexanetriol instead of 1,2-ethanediol, respectively. 
For comparison, a cell E was produced as similar to the cell of the present 
invention except that the carbon material was not impregnated with alcohol 
solution of lithiumalkoxide. Further, a cell F and G for comparison were 
produced as similar to the cell of the present invention except using 
methoxylithium and ethoxylithium instead of dilithium-1,2-ethandiol, 
respectively, and using methanol and ethanol instead of 1,2-ethandiol, 
respectively. 
Next, in a thermostat at 25.degree. C., these cells were charged by a 
constant current of 2.0 mA until a terminal voltage became 4.2 V. 
Successively, these cells were discharged by the constant current 2.0 mA 
until the terminal voltage became 3 V. The discharge capacities of 
respective cells are indicated in Table 1. 
TABLE 1 
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MOLECULAR WEIGHT 
CELL OF COMPOUND DISCHARGE CAITY (mAh) 
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E -- 20.5 
F 38 20.3 
G 52 20.4 
A 74 21.8 
B 88 21.3 
C 110 23.9 
D 152 23.7 
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As apparent from Table 1, the discharge capacities of the cells A, B, C and 
D of the present invention, which the surface of the carbon material is 
covered with compound having a molecular weight more than 52, is larger 
than that of the comparative cells E, F and G. It may be considered that 
the surface of the carbon material is covered with compound having a 
molecular weight more than 52, so that the decomposition reaction of the 
electrolyte is supressed. 
In the above examples, although the Weight ratio of lithium alkoxyde was 
about 1% with respect to the carbon material, it is not limited thereto. 
The weight ratio lithium alkoxyde is preferably in the range of 0.1 to 10 
weight %, and more preferably, in the range of 0.5 to 5 weight %. If it is 
less than 0.5 weight %, its effect reduces accompanying with reducing 
adding amount thereof. If it is more than 5 weight %, the internal 
resistance of the cell increases accompanying with increasing adding 
amount thereof. 
In the above examples, although lithium-cobalt oxide was used as the 
positive electrode, various types of material such as lithium-nickel 
composite oxide (LiNiO.sub.2), titanium disulfide, manganese dioxide, 
spinel type lithium-manganese oxide, vanadium pentoxide, molybdenum 
trioxide can be used. Also, in the examples, although graphite was used as 
the negative material, the similar effect can be obtained by using a low 
crystalline carbon material. 
The organic solvent and solute are not limited to the above examples. The 
same effect can obtained by using the similar one used in the conventional 
lithium cell. For example, as the organic solvent, the mixture of a high 
dielectric constant solvent such as propylene carbonate, ethylene 
carbonate, .gamma.-butyrolactone, sulforane, and a low viscosity solvent 
such as 1,2-dimethoxyethane, dimethylcarbonate, ethylmethylcarbonate, 
diethylecarbonate can be used. Further, as the electrolyte solute, at 
least one of lithium perchlorate, lithium hexafluoroarsenate, lithium 
tetrafluoroborate, lithium hexafluorophosphate and the like can be used. 
Incidentally, although the cells according to the above examples are 
coin-shaped cells, the present invention can apply to a cylindrical, 
rectangular, paper-shaped cell or the like.