Lithium secondary battery having a cathode containing galliam

A lithium secondary battery that includes a cathode containing, as an active material, a material that can be doped/undoped with lithium ions; an anode containing, as an active material, a lithium metal, a lithium alloy or a material that can be doped/undoped with lithium ions; and a liquid or solid electrolyte. In this lithium secondary battery, the active material used in the cathode is lithiated nickel dioxide containing gallium. As a result, the lithium secondary battery attains excellent cycle and overcharge resistance characteristics and has a high energy density.

FIELD OF THE INVENTION 
The present invention relates to a lithium secondary battery comprising a 
cathode including, as an active material, a material that can be 
doped/undoped with lithium ions, an anode including, as an active 
material, a lithium metal, a lithium alloy or a material that can be 
doped/undoped with lithium ions, and a liquid or solid electrolyte. 
BACKGROUND OF THE INVENTION 
Lithiated cobalt dioxide and lithiated nickel dioxide are compounds having 
a so-called .alpha.-NaFeO.sub.2 type structure, in which lithium ions are 
regularly arranged in a layered manner alternately with cobalt ions or 
nickel ions, which is arranged vertically to the closest packing layer of 
oxygen ions. Due to this structure, the lithium ions in a layer can 
diffuse comparatively easily. Hence, lithium ions can be electrochemically 
doped or undoped in such a compound. Various studies have been made to use 
these compounds as a cathode in a lithium secondary battery. Such a 
lithium secondary battery is expected to play an important role as a 
high-performance compact secondary battery, which in the future may 
function as a power supply for electric vehicles or as a power storage 
device for load leveling. 
Lithiated cobalt dioxide is already used as a cathode in lithium secondary 
batteries that supply power to some portable telephones and video cameras. 
However, lithiated cobalt dioxide, which is produced from an expensive 
cobalt compound, is inferior to lithiated nickel dioxide, which can be 
produced from an inexpensive and abundant nickel compound. 
However, it is difficult to synthesize lithiated nickel dioxide having a 
large discharge capacity, compared with lithiated cobalt dioxide, because 
the charging/discharging characteristic of lithiated nickel dioxide depend 
largely upon the synthesis method thereof. Specifically, the difficulty is 
that in lithiated nickel dioxide, nickel is easily substituted at lithium 
sites, and therefore, the resultant compound can contain substituted 
nickel unless the synthesis condition is appropriate. Nickel at lithium 
sites inhibits the diffusion of lithium ions, thereby adversely affecting 
the charging/discharging characteristic of the resultant lithiated nickel 
dioxide. 
Recently, various attempts to synthesize lithiated nickel dioxide having a 
large discharge capacity have been made by optimizing the synthesis 
condition. For example, Yamada et al (The 34th Battery Symposium, Lecture 
No. 2A06 (1993)) reported that lithiated nickel dioxide with an Ni 
oxidation number of approximately 3.0 was obtained by firing a mixture of 
LiOH.cndot.H.sub.2 and Ni(OH).sub.2 in oxygen at a temperature of 
700.degree. C.; and then a mixture of the resultant lithiated nickel 
dioxide, acetylene black and polytetrafluoroethylene (hereinafter referred 
to as PTFE) was adhered with pressure to a current collector to 
manufacture a cathode, in which the discharge capacity was found to be 200 
mAh/g through evaluation by a constant capacity charge. They also reported 
that when the charge/discharge was continued to attain the discharge 
capacity of 200 mAh/g, the cycle characteristic was extremely poor and the 
lifetime was approximately ten cycles. It was also reported to be 
necessary to minimize the charge capacity for the constant capacity charge 
in order to attain excellent cycle characteristics. It was further 
reported that a charge capacity of 130 mAh/g or less led to a lifetime of 
100 cycles or more. 
Thus, lithiated nickel dioxide has poor characteristics when it is 
charged/discharged at a high capacity. 
In addition, lithiated nickel dioxide is inferior to lithiated cobalt 
dioxide in energy density when used at the same capacity. This is because 
lithiated nickel dioxide has a lower discharging voltage, which is defined 
as the characteristic of the material to be used. Generally, it is 
effective to increase the discharging voltage, as well as the discharge 
capacity, in order to obtain a secondary battery with a higher energy 
density. However, it has been impossible to increase the discharging 
voltage of pure lithiated nickel dioxide. 
SUMMARY OF THE INVENTION 
The present inventors have conducted studies and found a lithium secondary 
battery that can attain high energy density and excellent cycle 
characteristics, even when charged/discharged at a high capacity, by using 
lithiated nickel dioxide containing gallium as an active material in the 
cathode. 
The lithium secondary battery of this invention comprises a cathode, which 
includes, as an active material, a material that can be doped/undoped with 
lithium ions; an anode, which includes, as an active material, a lithium 
metal, a lithium alloy or a material that can be doped/undoped with 
lithium ions; and a liquid or solid electrolyte. In this lithium secondary 
battery, the active material in the cathode is lithiated nickel dioxide 
containing gallium. 
In a preferable embodiment, the lithiated nickel dioxide containing gallium 
satisfies the following relationship: 
EQU 0&lt;x.ltoreq.0.2 
wherein x is a molar ratio of the gallium to a total amount of the gallium 
and nickel contained in the lithiated nickel dioxide. 
In a more preferable embodiment, the lithiated nickel dioxide containing 
gallium satisfies the following relationship: 
EQU 0&lt;x.ltoreq.0.05 
wherein x is a molar ratio of the gallium to a total amount of the gallium 
and nickel contained in the lithiated nickel dioxide. 
In a most preferable embodiment, the lithiated nickel dioxide containing 
gallium satisfies the following relationship: 
EQU 0.001&lt;x.ltoreq.0.02 
wherein x is a molar ratio of the gallium to a total amount of the gallium 
and nickel contained in the lithiated nickel dioxide. 
In one embodiment, the lithiated nickel dioxide containing gallium is 
obtained by firing a mixture of a lithium compound, a nickel compound, and 
gallium or a gallium compound. 
In another embodiment, the lithiated nickel dioxide containing gallium is 
obtained by dispersing a nickel compound in an aqueous solution including 
a gallium compound and a water-soluble lithium salt, evaporating the water 
content of the resultant solution to obtain a mixture, and firing the 
mixture in an atmosphere containing oxygen. 
In still another embodiment, the gallium compound is gallium nitrate, the 
water-soluble lithium salt is lithium nitrate and the nickel compound is 
basic nickel carbonate. 
In still another embodiment, the lithiated nickel dioxide containing 
gallium is obtained by adding a basic compound to an aqueous solution of 
gallium nitrate to attain a pH of 10 or more, dissolving lithium nitrate 
in the resultant solution, adding basic nickel carbonate thereto, 
evaporating the water content of the solution to obtain a mixture, and 
firing the mixture in an atmosphere containing oxygen. 
In one embodiment, the basic compound is lithium hydroxide. 
In one embodiment, the lithiated nickel dioxide containing gallium has a 
crystallite size of 700 .ANG. or less determined through X-ray powder 
diffraction. 
In one embodiment, the lithium secondary battery is charged at least once 
at the time of the production thereof, at 220 mAh/g or more per weight of 
the lithiated nickel dioxide containing gallium. 
In one embodiment, the anode includes graphite as a sole component or a 
main component of the active material, and the liquid electrolyte 
comprises ethylene carbonate, dimethyl carbonate and ethyl methyl 
carbonate. 
The objective of the present invention is to provide a lithium secondary 
battery having high energy density and excellent cycle and overcharge 
resistance characteristics even when charged/discharged at a high capacity 
.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will now be described in detail. 
A cathode of a lithium secondary battery of this invention includes, as an 
active material, a material that can be doped/undoped with lithium ions. 
The material that can be doped/undoped with lithium ions is lithiated 
nickel dioxide containing gallium. 
As a method for adding gallium to lithiated nickel dioxide, previously 
synthesized lithiated nickel dioxide can be mixed with gallium or a 
gallium compound to be fired. However, in order to simplify the production 
process and homogeneously add a small amount of gallium, it is preferred 
that a lithium compound, a nickel compound, and gallium or a gallium 
compound are mixed with one another to be fired. Alternatively, a nickel 
compound and gallium or a gallium compound can be first mixed with each 
other to be fired, and then the fired mixture can be mixed with a lithium 
compound to be fired again. Similarly, a lithium compound and gallium or a 
gallium compound can be first mixed with each other to be fired, and then 
the fired mixture can be mixed with a nickel compound to be fired again. 
Examples of the lithium compound used in the invention include lithium 
carbonate, lithium nitrate and lithium hydroxide. Examples of the nickel 
compound used in the invention include nickel oxide, nickel hydroxide, 
nickel nitrate, nickel carbonate NiCO.sub.3 .cndot.wH.sub.2 O (wherein 
w.gtoreq.0), basic nickel carbonate xNiCO.sub.3 .cndot.yNi(OH).sub.2 
.cndot.zH.sub.2 O (wherein x&gt;0, y&gt;0 and z&gt;0) and acidic nickel carbonate 
NimH.sub.2 n(CO.sub.3)m+n (wherein m&gt;0 and n&gt;0). As a gallium raw 
material, a gallium compound such as metal gallium, gallium oxide, gallium 
nitrate and gallium hydroxide can be used. In particular, a preferred 
gallium compound is a water-soluble gallium salt such as gallium nitrate. 
The following is a preferred method for mixing the lithium compound, the 
nickel compound and the gallium compound and firing the obtained mixture. 
First, the nickel compound is dispersed in an aqueous solution including 
the gallium compound and the water-soluble lithium salt, and the water 
content of the obtained solution is evaporated. The thus obtained mixture 
is fired in an atmosphere containing oxygen. In this method, the 
water-soluble lithium salt can be homogeneously mixed with the gallium 
compound and the nickel compound. Therefore, the resulting lithiated 
nickel dioxide lacks lithium due to ununiformity in the mixed components. 
As a result of further study, the present inventors found the following 
preferred combination of materials, in which gallium nitrate is preferably 
used as the gallium compound, lithium nitrate as the water-soluble lithium 
salt, and basic nickel carbonate as the nickel compound. When the 
lithiated nickel dioxide containing gallium obtained from these materials 
is used in a cathode, the resultant lithium secondary battery is found to 
have a high energy density. 
In this combination, the aqueous solution of gallium nitrate and lithium 
nitrate is acidic. When basic nickel carbonate is added to this aqueous 
solution, carbon dioxide is generated and gallium hydroxide is 
precipitated therein. Therefore, gallium is actually added to and mixed in 
the solution as gallium hydroxide. 
As a result of further study to improve the above-mentioned method, the 
present inventors found that when the pH of an aqueous solution of gallium 
nitrate is increased to exceed 10, the solution becomes cloudy due to the 
precipitation of gallium hydroxide, and becomes substantially transparent 
at a pH of 11 or more. When the solution has a pH of 10 or more, gallium 
is thought to be dispersed in the solution as gallium hydroxide in the 
shape of an extremely fine colloid, or dissolved in the solution as 
dioxogallate ions GaO.sub.2.sup.-. Accordingly, after a basic compound is 
added to the aqueous solution of gallium nitrate to increase the pH of the 
solution to exceed 10, lithium nitrate is then dissolved in the solution, 
and basic nickel carbonate is also dispersed in the solution. The water 
content of the thus obtained solution is evaporated, and the resultant 
mixture is fired in an atmosphere containing oxygen. When this method is 
adopted, gallium can be homogeneously mixed with the other materials, 
because, as described above, it is dispersed as gallium hydroxide in the 
shape of an extremely fine colloid or dissolved as dioxogallate ions 
GaO.sub.2.sup.- In this method, side production of a composite oxide of 
lithium and gallium, which makes no contribution to charge/discharge, can 
be suppressed, and hence, this method is particularly preferred when only 
a small amount of gallium is to be added. 
Further, when lithium hydroxide, lithium oxide or lithium peroxide is used 
as the basic compound to increase the pH of the gallium nitrate solution, 
lithium nitrate remains as a result of the neutralization reaction between 
such a lithium compound and the solution. Lithium nitrate, however, is 
consumed in and has no harmful effect on the production of lithiated 
nickel dioxide containing gallium. Therefore, the above-mentioned basic 
compounds are preferred, among which lithium hydroxide is most preferable 
because it is inexpensive and easy to handle. 
The above-mentioned mixture is fired preferably in an atmosphere containing 
oxygen, more preferably in oxygen, and most preferably in an oxygen 
stream. 
The firing temperature is preferably in the range between 350.degree. C. 
and 800.degree. C., and more preferably in the range between 600.degree. 
C. and 750.degree. C. When the firing temperature exceeds 800.degree. C., 
the resulting lithiated nickel dioxide includes a larger ratio of a rock 
salt domain, in which lithium ions and nickel ions are irregularly 
arranged, which inhibits reversible charge/discharge. When the firing 
temperature is below 350.degree. C., the generation reaction for lithiated 
nickel dioxide is scarcely effected. 
The firing time is preferably 2 hours or more, and more preferably 5 hours 
or more. 
The amount of gallium to be added is preferably as small as possible, since 
gallium or gallium compounds are expensive. Specifically, when the molar 
ratio of gallium to be mixed with the total amount of gallium and nickel 
compound is taken as x, the relationship of 0&lt;x.ltoreq.0.2 is preferably 
satisfied. When x is less than 0.05, the diffraction peak in X-ray powder 
diffraction of a composite oxide of lithium and gallium, which makes no 
contribution to reversible charge/discharge, has a smaller intensity, or 
there appears to be no diffraction peak for the composite oxide of lithium 
and gallium. This is advantageous in regard to the discharge capacity per 
volume or weight. Therefore, the relationship of 0&lt;x.ltoreq.0.05 is more 
preferred. Further, taking both the discharge capacity and the cycle 
characteristic into consideration, the relationship of 0.001&lt;x.ltoreq.0.02 
is most preferred. 
Furthermore, the present inventors examined the relationship between the 
cycle characteristic and a crystallite size obtained through the X-ray 
powder diffraction of lithiated nickel dioxide containing gallium. As a 
result, the inventors found that lithiated nickel dioxide with a 
crystallite size of 700 .ANG. or less exhibits excellent cycle 
characteristics and overcharge resistance. Thus, the present invention was 
accomplished. 
The cathode of the lithium secondary battery of the present invention 
includes, as an active material, the above-mentioned lithiated nickel 
dioxide containing gallium, and can further include, as additional 
components, a carbonaceous material as a conductive substance and a 
thermoplastic resin as a binder. 
Examples of the carbonaceous material include natural graphite, artificial 
graphite and cokes. Examples of the thermoplastic resin include 
poly(vinylidene fluoride) (hereinafter referred to as PVDF), PTFE, 
polyethylene and polypropylene. 
The anode of the present lithium secondary battery includes a lithium 
metal, a lithium alloy or a material that can be doped/undoped with 
lithium ions. Examples of the material that can be doped/undoped with 
lithium ions include carbonaceous materials such as natural graphite, 
artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers 
and fired products of organic polymer compounds. Among the carbonaceous 
materials, the graphitic materials, such as a natural graphite or an 
artificial graphite, are more preferable than others for the anode, 
because of the flatness of their charging/discharging potential and 
because of their low average working potential, which, when combined with 
a cathode, provides high energy density. The carbonaceous material can be 
in any shape, including a flake, a sphere, a fiber or an agglomerate of 
fine powder. The anode can further include a thermoplastic resin as a 
binder, if necessary. Examples of the thermoplastic resin include PVDF, 
PTFE, polyethylene and polypropylene. 
The electrolyte of the present lithium secondary battery can be liquid or 
solid. An example of the liquid electrolyte includes a nonaqueous liquid 
electrolyte in which a lithium salt is dissolved in an organic solvent. An 
example of the solid electrolyte includes a so-called solid electrolyte. 
The lithium salt to be dissolved in the nonaqueous liquid electrolyte is 
one of, or a combination of two or more of, LiClO.sub.4, LiPF.sub.6, 
LiAsF.sub.6, LiSbF.sub.6, LiBF.sub.4, LiCF.sub.3 SO.sub.3, LiN(CF.sub.3 
SO.sub.2).sub.2, Li.sub.2 B.sub.10 Cl.sub.10, lower aliphatic lithium 
carboxylate and LiAlCl.sub.4. 
Examples of the organic solvent include carbonates, such as propylene 
carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate and 
ethyl methyl carbonate; ethers, such as 1,2-dimethoxyethane, 
1,3-dimethoxypropane, tetrahydrofuran and 2-methyltetrahydrofuran; esters, 
such as methyl formate, methyl acetate and .gamma.-butyrolactone; 
nitriles, such as acetonitrile and butyronitrile; amides, such as 
N,N-dimethylformamide, and N,N-dimethylacetamide; carbamates, such as 
3-methyl-2-oxazolidone; and sulfur-containing compounds, such as 
sulfolane, dimethylsulfoxide and 1,3-propane sulfone. Generally, a 
combination of two or more of these compounds is used. In particular, a 
mixed solvent including a carbonate is preferably used, and a mixed 
solvent of a combination of a cyclic carbonate and a noncyclic carbonate 
or a combination of a cyclic carbonate and an ether is more preferably 
used. Among the mixed solvents of cyclic carbonates and non-cyclic 
carbonates, the solvents which comprise ethylene carbonate, dimethyl 
carbonate, and ethyl methyl carbonate are more preferable than others, 
because they provide a wide operating temperature range, excellent drain 
capability and they hardly decompose when they are used with an anode 
including said graphitic materials. 
Examples of the solid electrolyte include a polymer electrolyte, such as 
polyethylene oxide polymer compounds and polymer; compounds including at 
least one of a polyorganosiloxane branch and a polyoxyalkylene branch; a 
sulfide type electrolyte, such as Li.sub.2 S--SiS.sub.2, Li.sub.2 
S--GeS.sub.2, Li.sub.2 S--P.sub.2 S.sub.5 and Li.sub.2 S--B.sub.2 S.sub.3, 
and an inorganic compound type electrolyte including sulfide such as 
Li.sub.2 S--SiS.sub.2 --Li.sub.3 PO.sub.4 and Li.sub.2 S--SiS.sub.2 
--Li.sub.2 SO.sub.4. Further, a so-called gel type electrolyte, in which a 
nonaqueous liquid electrolyte is held by a polymer, can be used. 
The shape of the present lithium secondary battery is not particularly 
specified, and can be in any shape including paper, a coin, a cylinder and 
a rectangular parallelepiped. 
According to the present invention, the lithium secondary battery can 
attain excellent cycle characteristics even when charged/discharged at a 
high capacity, and also can attain a high energy density, because it has a 
higher discharging voltage than a conventional lithium secondary battery 
using pure lithiated nickel dioxide. Although it is not clear why the 
present lithium secondary battery attains these excellent characteristics, 
the reason is considered to be that the structure of lithiated nickel 
dioxide is stabilized when charged/discharged, especially when deeply 
charged, by incorporation of gallium into the crystal structure of 
lithiated nickel dioxide in some way, or by suppressing the growth of the 
crystallite of lithiated nickel dioxide by gallium. 
In a battery using pure lithiated nickel dioxide, when it is charged by 
using lithium as a counter electrode, an open circuit voltage (hereinafter 
referred to as OCV) increases as the charge capacity increases. It is 
observed that when the charge capacity exceeds approximately 200 mAh/g, 
the OCV no longer increases but remains constant at 4.18 V, a potential 
against lithium. Whereas in the battery using lithiated nickel dioxide 
containing gallium, the OCV does not remain constant at 4.18 V, but 
further increases as the charge capacity increases. Further, the 
charging/discharging curve of the battery using pure lithiated nickel 
dioxide has several inflection points that can be regarded to correspond 
to the change of its crystal structure, while the charging/discharging 
curve of the battery using lithiated nickel dioxide containing gallium is 
smooth and it is difficult to identify such an inflection point in this 
curve. From these facts, it is thought that the gallium added to the 
lithiated nickel dioxide affects the crystal structure or the crystallite 
size of the lithiated nickel dioxide in some way. 
The present lithium secondary battery using lithiated nickel dioxide 
containing gallium as an active material for the cathode exhibits a high 
energy density and has an excellent cycle characteristics even when 
charged/discharged at a high capacity. The lithium secondary battery, 
however, exhibited a comparatively large overvoltage at an initial stage, 
the reason for which is not apparent. Therefore, at first, the battery had 
a problem in that several charging/discharging cycles were required to 
settle the overvoltage, namely, the rise of the discharge capacity of the 
battery was delayed. 
The present inventors made further study to overcome this problem, and 
found that a quick rise in discharge capacity can be attained by 
decreasing the overvoltage at the initial stage through conducting at 
least one charge at 220 mAh/g per weight of lithiated nickel dioxide 
containing gallium at the time of the production thereof. 
The present invention will now be described in more detail by way of 
examples, which do not limit the invention. 
EXAMPLES 
An electrode and a plate battery for a charging/discharging test were 
manufactured as follows, unless otherwise mentioned: 
To a mixture of lithiated nickel dioxide or lithiated nickel dioxide 
containing gallium, as an active material, and acetylene black, as a 
conductive substance, was added a 1-methyl-2-pyrrolidone solution 
(hereinafter referred to as NMP), including PVDF as a binder, so as to 
attain a composition ratio among the active material, the conductive 
substance and the binder of 91:6:3 (weight ratio). The resultant solution 
was kneaded to obtain a paste. The paste was coated over a #200 stainless 
mesh, which was to work as a current collector, and the mesh bearing the 
paste was dried under vacuum at a temperature of 150.degree. C. for 8 
hours. Thus, an electrode was produced. 
By using the thus obtained electrode, a plate battery was manufactured 
together with a liquid electrolyte described below, a polypropylene 
microporous membrane as a separator and metal lithium as a counter 
electrode (i.e., an anode). The liquid electrolyte used was a solution in 
which LiClO.sub.4 was dissolved at a proportion of 1 molar/litter in a 1:1 
mixture of propylene carbonate (hereinafter referred to as PC) and 
1,2-dimethoxyethane (hereinafter referred to as DME) (hereinafter this 
liquid electrolyte is referred to as LiClO.sub.4 /PC+DME.); or a solution 
in which LiPF.sub.6 was dissolved at a proportion of 1 molar/litter in a 
30:35:35 mixture of ethylene carbonate (hereinafter referred to as EC), 
dimethyl carbonate (hereinafter referred to as DMC) and ethyl methyl 
carbonate (hereinafter referred to as EMC) (hereinafter this liquid 
electrolyte is referred to as LiPF.sub.6 /EC+DMC+EMC) was used. 
X-ray powder diffraction was conducted by using a RAD-IIC system 
(manufactured by Rigaku Corporation) under the following condition: 
X-ray: CuK.alpha.(monochromatized by a graphite curved monochromator) 
Voltage-current: 40 kV-30 mA 
Range of measured angle: 2.theta.=15.degree. to 140.degree. 
Slit: DS-1.degree., RS-0.15 mm, SS-1.degree. 
Step: 0.02.degree. 
Counting time: 1 second 
The data obtained were processed by using an MXP system (manufactured by 
Mac Science Co., Ltd.) as follows: first, a true profile was obtained by a 
Stokes method, and then a crystallite size was calculated by a Warren & 
Averbach method. 
EXAMPLE 1 
First, 2.09 g of gallium nitrate (Ga(NO.sub.3).sub.3 .cndot.9H.sub.2 O; 
manufactured by Kojundo Chemical Laboratory Co., Ltd.; 3N graded reagent), 
7.23 g of lithium nitrate (manufactured by Wako Pure Chemical Industries, 
Ltd.; guaranteed graded reagent) were dissolved in 15.1 g of water. Then, 
11.91 g of basic nickel carbonate (NiCO.sub.3 .cndot.2Ni(OH).sub.2 
.cndot.4H.sub.2 O (manufactured by Wako Pure Chemical Industries, Ltd.; 
graded reagent) was added to and homogeneously dispersed in the obtained 
solution. The water content of the resultant solution was evaporated, and 
the mixture obtained was charged in a tubular furnace having an alumina 
core tube and fired in an oxygen stream of 50 cm.sup.3 /min. at a 
temperature of 660.degree. C. for 15 hours. At this point, the molar ratio 
x of gallium to the total amount of gallium and nickel was set to be 0.05. 
By using the thus obtained powder, a plate battery (in which a liquid 
electrolyte was LiClO.sub.4 /PC+DME) was manufactured and subjected to a 
charging/discharging test under a maximum charging voltage of 4.3 V, a 
minimum discharging voltage of 2.5 V and a constant current of 0.17 
mA/cm.sup.2. During the charging/discharging test, a resting time of 2 
hours was provided between the termination of the charge and the start of 
the discharge so as to settle an overvoltage, and the voltage immediately 
before the start of the discharge was regarded as the OCV after the 
termination of the charge. The OCVs after the termination of the charge in 
the 5th cycle and the 20th cycle were both 4.22 V. 
FIG. 1 shows the variation of the discharge capacity up to the 30th cycle. 
The discharge capacity in the 30th cycle was 171mAh/g, and thus, the 
battery exhibited excellent cycle characteristics. 
FIG. 2 shows the variation of the average discharging voltage up to the 
30th cycle. The average discharging voltage in the 30th cycle was kept at 
3.80 V. 
FIG. 3 shows the discharging curve in the first cycle. In this graph, the 
discharge capacity is normalized for comparison. This graph reveals that 
the discharging curve obtained by using lithiated nickel dioxide 
containing gallium is smooth and that it is difficult to identify an 
inflection point that is conventionally observed in using pure lithiated 
nickel dioxide. 
Comparative Example 1 
First, 94.1 g of lithium nitrate (manufactured by Wako Pure Chemical 
Industries, Ltd.; guaranteed graded reagent) was dissolved in 150 g of 
water, and then 163.0 g of basic nickel carbonate (NiCO.sub.3 
.cndot.2Ni(OH).sub.2 .cndot.4H.sub.2 O (manufactured by Wako Pure Chemical 
Industries. Ltd.; graded reagent) was added thereto and homogeneously 
dispersed. The water content of the resultant solution was evaporated, and 
the mixture obtained was charged in a tubular furnace having an alumina 
core tube and fired in an oxygen stream of 50 cm.sup.3 /min. at a 
temperature of 720.degree. C. for 5 hours. 
By using the thus obtained powder, a plate battery (in which a liquid 
electrolyte was LiCLO.sub.4 /PC+DME) was manufactured and subjected to a 
charging/discharging test under a maximum charging voltage of 4.3 V, a 
minimum discharging voltage of 2.5 V and a constant current of 0.17 
mA/cm.sup.2. During the charging/discharging test, a resting time of 2 
hours was provided between the termination of the charge and the start of 
the discharge so as to settle an overvoltage, and the voltage immediately 
before the start of the discharge was regarded as the OCV after the 
termination of the charge. The OCVs after the termination of the charge in 
the 5th cycle and the 20th cycle were both 4.18 V. 
The variation of the discharge capacity up to the 30th cycle is shown in 
FIG. 1. The discharge capacity in the 30th cycle was 161 mAh/g, which was 
largely degraded as compared with the discharge capacity in the initial 
stage of the test. 
The variation of the average discharging voltage up to the 30th cycle is 
shown in FIG. 2. The average discharging voltage in the 30th cycle was 
3.70 V, which was lower by 0.1 V than that of the battery of Example 1. 
The discharging curve in the first cycle is shown in FIG. 3. In this 
graph, the discharge capacity is normalized for comparison. The discharge 
capacity of this battery has a complicated shape having several infection 
points that are thought to correspond to changes in the crystal structure. 
This discharging curve is apparently different from that of the battery 
using lithiated nickel dioxide containing gallium. 
Comparative Example 2 
By using the powder of lithiated nickel dioxide obtained in Comparative 
Example 1, a plate battery (in which a liquid electrolyte was LiClO.sub.4 
/PC+DME) was similarly manufactured and subjected to a 
charging/discharging test under a maximum charging voltage of 4.2 V, a 
minimum discharging voltage of 2.5 V and a constant current of 0.17 
mA/cm.sup.2. 
The variation of the discharge capacity and that of the average discharging 
voltage up to the 10th cycle are shown in FIGS. 1 and 2, respectively. The 
discharge capacity in the 10th cycle was 169 mAh/g. Thus, when the maximum 
charging voltage is decreased, cycle characteristics are improved, whereas 
the discharge capacity is decreased. The average discharging voltage in 
the 10th cycle was 3.73 V, which was lower by approximately 0.1 V than 
that of the battery of Example 1. 
EXAMPLE 2 
First, 0.235 g of gallium oxide (manufactured by Kojundo Chemical 
Laboratory Co., Ltd.; 3N graded reagent), 3.62 g of lithium nitrate 
(manufactured by Wako Pure Chemical Industries, Ltd.; guaranteed graded 
reagent), and 5.96 g of basic nickel carbonate (NiCO.sub.3 
.cndot.2Ni(OH).sub.2 .cndot.4H.sub.2 O (manufactured by Wako Pure Chemical 
Industries, Ltd.; graded reagent) were homogeneously mixed in an agate 
mortar. The mixture obtained was charged in a tubular furnace having an 
alumina core tube and fired in an oxygen stream of 50 cm.sup.3 /min. at a 
temperature of 660.degree. C. for 15 hours. At this point, the molar ratio 
x of gallium to the total amount of gallium and nickel was set to be 0.05. 
By using the thus obtained powder, a plate battery (in which an liquid 
electrolyte was LiClO.sub.4 /PC+DME) was manufactured and subjected to a 
charging/discharging test under a maximum charging voltage of 4.3 V, a 
minimum discharging voltage of 2.5 V, and a constant current of 0.17 
mA/cm.sup.2. During the charging/discharging test, a resting time of 2 
hours was provided between the termination of the charge and the start of 
the discharge so as to settle an overvoltage, and the voltage immediately 
before the start of the discharge was regarded as the OCV after the 
termination of the charge. The OCVs after the termination of the charge in 
the 5th cycle and the 20th cycle were both 4.22 V. 
The discharge capacity in the 30th cycle was 165 mAh/g and the average 
discharging voltage in the 30th cycle was 3.78 V. 
Comparative Example 3 
By using the powder obtained in Comparative Example 1, a plate battery (in 
which a liquid electrolyte was LiClO.sub.4 /PC+DME) was manufactured and 
subjected to a charging/discharging test using charge by a constant 
current and voltage and discharge by a constant current under the 
following condition: 
Maximum charging voltage: 4.3 V 
Charging time: 6 hours 
Charging current: 1 mA/cm.sup.2 
Minimum discharging voltage: 2.5 V 
Discharging current: 0.17 mA/cm.sup.2 
The above-mentioned charge by a constant current and voltage will be 
described in more detail. At first a battery is charged up to a maximum 
charging voltage (4.3 V in this Comparative Example) with a constant 
charging current (1 mA/cm.sup.2 in this Comparative Example), then the 
charging current is decreased to keep the voltage of the battery as high 
as the maximum charging voltage. The charge is terminated when total 
charging time reaches a predetermined charging time (6 hours in this 
Comparative Example). 
The discharge capacity up to the 20th cycle is shown in FIG. 4. As compared 
with the battery of Comparative Example 1, this battery was degraded 
faster with cycles, because it was charged with a constant current and 
voltage under a more severe condition, namely, it was supplied with a 
voltage of 4.3 V for a longer time, although it was discharged in the same 
manner as in Comparative Example 1. 
EXAMPLE 3 
By using the powder obtained in Example 1, a plate battery (in which a 
liquid electrolyte was LiClO.sub.4 /PC+DME) was manufactured and subjected 
to a charging/discharging test using charge by a constant current and 
voltage and discharge by a constant current under the same condition as in 
Comparative Example 3. 
The variation of the discharge capacity up to the 20th cycle is shown in 
FIG. 4. Although the capacity is slightly decreased, with the 10th cycle 
being a peak, this battery exhibited superior cycle characteristics as 
compared with the battery containing no gallium (i.e., Comparative Example 
3), even when the battery was more severely charged with a constant 
current and voltage. 
EXAMPLE 4 
First, 108.6 g of lithium nitrate (manufactured by Wako Pure Chemical 
Industries, Ltd.; guaranteed graded reagent) was dissolved in 150 g of 
water. Then, to the resultant solution were added 9.06 g of gallium 
hydroxide (Ga(OH).sub.3 ; manufactured by Kojundo Chemical Laboratory Co., 
Ltd.; 3N graded reagent) and 190.5 g of basic nickel carbonate (NiCO.sub.3 
.cndot.2Ni(OH).sub.2 .cndot.4H.sub.2 O (manufactured by Wako Pure Chemical 
Industries, Ltd.; graded reagent) and homogeneously dispersed. The water 
content of the resultant solution was evaporated. The mixture obtained was 
charged in a tubular furnace having an alumina core tube and fired in an 
oxygen stream of 50 cm.sup.3 /min. at a temperature of 660.degree. C. for 
15 hours. At this point, the molar ratio x of gallium to the total amount 
of gallium and nickel was set to be 0.05. 
By using the thus obtained powder, a plate battery (in which a liquid 
electrolyte was LiClO.sub.4 /PC+DME) was manufactured and subjected to a 
charging/discharging test using charge by a constant current and voltage 
and discharge by a constant current under the same conditions as in 
Example 3. 
The variation of the discharge capacity up to the 20th cycle is shown in 
FIG. 4. Although the discharge capacity in the initial stage was small and 
the rise of the discharge capacity was delayed, the battery exhibited 
excellent cycle characteristics from the 8th cycle on, even when charged 
with the constant current and voltage. 
The X-ray diffraction of the obtained powder is shown in FIG. 5. In this 
powder, a small amount of a composite oxide of lithium and gallium, 
LiGaO.sub.2, was found. EXAMPLE 5 
First, 13.64 g of gallium nitrate (Ga(NO.sub.3).sub.3 .cndot.9H.sub.2 O; 
manufactured by Kojundo Chemical Laboratory Co., Ltd.; 3N graded reagent) 
and 12.06 g of lithium nitrate (manufactured by Wako Pure Chemical 
Industries, Ltd.; guaranteed graded reagent) were dissolved in 19.3 g of 
water. Then, 16.72 g of basic nickel carbonate (NiCO.sub.3. 2Ni(OH).sub.2 
.cndot.4H.sub.2 O (manufactured by Wako Pure Chemical Industries, Ltd.; 
graded reagent) was added to and homogeneously dispersed in the solution 
obtained, and the water content of the resultant solution was evaporated. 
The mixture obtained was charged in a tubular furnace having an alumina 
core tube and fired in an oxygen stream of 50 cm.sup.3 /min. at a 
temperature of 660.degree. C. for 15 hours. At this point, the molar ratio 
x of gallium to the total amount of gallium and nickel was set to be 0.2. 
By using the thus obtained powder, a plate battery (in which a liquid 
electrolyte was LiClO.sub.4 /PC+DME) was manufactured and subjected to a 
charging/discharging test using charge by a constant current and voltage 
and discharge by a constant current under the same conditions as in 
Example 3. 
The variation of the discharge capacity up to 20th cycle is shown in FIG. 
4. Although the discharge capacity was as small as 120 mAh/g, the battery 
exhibited excellent cycle characteristics even when charged with a 
constant current and voltage. 
The result of the X-ray diffraction of the powder obtained is shown in FIG. 
5. In this powder, a large amount of the composite oxide of lithium and 
gallium, LiGaO.sub.2, was found. The cycle characteristic was improved by 
the addition of gallium in this battery, but it had a small discharge 
capacity per weight due to the presence of a large amount of the compound 
making no contribution to the charge/discharge. It is thought that the 
compound blocks a conductive path in the battery, thereby making the 
discharge capacity small. 
EXAMPLE 6 
First, 6.97 g of gallium nitrate (Ga(NO.sub.3).sub.3 .cndot.9H.sub.2 O; 
manufactured by Kojundo Chemical Laboratory Co., Ltd.; 3N graded reagent) 
was dissolved in 33.5 g of water. The pH of the solution obtained was 
1.47. Then, 2.61 g of lithium hydroxide monohydrate (LiOH.cndot.H.sub.2 O; 
manufactured by Wako Pure Chemical Industries, Ltd.; guaranteed graded 
reagent) was added to and dissolved in the solution. The resultant 
solution became cloudy and then became transparent. The pH of the solution 
at this point was 11.16. Then, 20.67 g of lithium nitrate (manufactured by 
Wako Pure Chemical Industries, Ltd.; guaranteed graded reagent) was 
dissolved in the solution, and 42.34 g of basic nickel carbonate 
(NiCO.sub.3 .cndot.2Ni(OH).sub.2 .cndot.4H.sub.2 O (manufactured by Wako 
Pure Chemical Industries, Ltd.; graded reagent) was successively added 
thereto and homogeneously dispersed. The water content of the resulting 
solution was evaporated, and the mixture obtained was charged in a tubular 
furnace having an alumina core tube and fired in an oxygen stream of 50 
cm.sup.3 /min. at a temperature of 660.degree. C. for 15 hours. The molar 
ratio x of gallium to the total amount of gallium and nickel was set to be 
0.05. 
By using the thus obtained powder, a plate battery (in which a liquid 
electrolyte was LiClO.sub.4 /PC+DME) was manufactured and subjected to a 
charging/discharging test using charge by a constant current and voltage 
and discharge by a constant current under the same conditions as in 
Example 3. 
The variation of the discharge capacity up to the 20th cycle is shown in 
FIG. 4. This battery had a quicker rise in discharge capacity than that of 
Example 4, and exhibited superior cycle characteristics to that of Example 
3 from the 7th cycle on. 
The result of the x-ray diffraction of the powder obtained is shown in FIG. 
5. In this powder, no diffraction line for the composite oxide of lithium 
and gallium, LiGaO.sub.2, was found. 
Comparative Example 4 
By using the powder obtained in Comparative Example 1, a plate battery (in 
which a liquid electrolyte was LiPF.sub.6 /EC+DMC+EMC) was manufactured 
and subjected to a charging/discharging test using charge by a constant 
current and voltage and discharge by a constant current under the 
following condition: 
Maximum charging voltage: 4.3 V 
Charging time: 8 hours 
Charging current: 0.3 mA/cm 
Minimum discharging voltage: 3.0 V 
Discharging current: 0.3 mA/cm.sup.2 
The variation of the discharge capacity up to 20th cycle is shown in FIG. 
6. Further, a capacity holding ratio R1 from the 1st cycle to the 10th 
cycle (i.e., the discharge capacity in the 10th cycle/the discharge 
capacity in the 1st cycle) and a capacity holding ratio R2 from the 11th 
cycle to the 20th cycle (i.e., the discharge capacity in the 20th 
cycle/the discharge capacity in the 11th cycle) are listed in Table 1 
below. Although the battery was degraded less and less through the cycles, 
the discharge capacity was largely decreased as compared with that at the 
initial stage. 
TABLE 1 
______________________________________ 
X R1 R2 Crystallite size (.ANG.) 
______________________________________ 
Comparative 4 
0 0.783 0.862 810 
Example 7 0.05 1.239 1.023 320 
0.02 1.124 1.005 330 
0.01 1.048 0.979 370 
0.005 1.047 0.969 360 
0.001 1.058 0.949 430 
______________________________________ 
EXAMPLE 7 
Gallium nitrate (Ga(NO.sub.3).sub.3 .cndot.6.2H.sub.2 O (manufactured by 
Kojundo Chemical Laboratory Co., Ltd.; 3N graded reagent), lithium nitrate 
(manufactured by Wako Pure Chemical Industries, Ltd.; guaranteed graded 
reagent) and basic nickel carbonate (NiCO.sub.3 .cndot.2Ni(OH).sub.2 
.cndot.4H.sub.2 O (manufactured by Wako Pure Chemical Industries, Ltd.; 
graded reagent) were weighed so as to achieve molar ratios x of gallium to 
the total amount of gallium and nickel of 0.05, 0.02, 0.01, 0.005 and 
0.001, and the thus weighed compounds were mixed to obtain the following 
five kinds of mixtures: Each desired amount of gallium nitrate was 
dissolved in water, and lithium hydroxide monohydrate (LiOH.cndot.H.sub.2 
O; manufactured by Wako Pure chemical Industries, Ltd.; guaranteed graded 
reagent) was added thereto to adjust the pH of the resultant solution. The 
pH was adjusted to be 11.1, 11.2, 11.4, 11.2 and 10.6 in the solutions 
having molar ratios x of 0.05, 0.02, 0.01, 0.005 and 0.001, respectively. 
Then, basic nickel carbonate was added to the respective solutions and 
homogeneously dispersed, and the water content of the solutions were 
evaporated. Each of the resultant mixtures was charged in a tubular 
furnace having an alumina core tube and fired in an oxygen stream of 50 
cm.sup.3 /min. at a temperature of 660.degree. C. for 15 hours. At this 
point, the molar ratio of lithium to the total amount of gallium and 
nickel was set to be 1.05 in each solution. 
By using the thus obtained powder, plate batteries (in which a liquid 
electrolyte was LiPF.sub.6 /EC+DMC+EMC) were manufactured and subjected to 
a charging/discharging test using charge by a constant current and voltage 
and discharge by a constant current under the same conditions as in 
Comparative Example 4. 
The variation of the discharge capacity of each battery up to the 20th 
cycle is shown in FIG. 6. Further, a capacity holding ratio R1 from the 
1st cycle to the 10th cycle (i.e., the discharge capacity in the 10th 
cycle/the discharge capacity in the 1st cycle) and a capacity holding 
ratio R2 from the 11th cycle to the 20th cycle (i.e., the discharge 
capacity in the 20th cycle/the discharge capacity in the 11th cycle) are 
listed in Table 1 above. 
The results shown in FIG. 6 and Table 1 reveal that the batteries exhibited 
excellent cycle characteristics as compared with a battery containing no 
gallium, regardless of the molar ratio x. Further, in all the batteries, 
the discharge capacity was rather small at the initial stage of the test 
and increased as the test proceeded, also regardless of the molar ratio x. 
The reason for this phenomenon is not clear, but the phenomenon is found 
to correspond to gradually settling a comparatively large overvoltage at 
the initial stage. The speed at which the overvoltage is settled, the 
discharge capacity attained when increase capacity is stabilized, and the 
cycle characteristics attained thereafter depend upon the molar ratio x. 
Specifically, as the molar ratio x decreases from 0.05, settlement of the 
overvoltage occurs faster (i.e., it is settled by approximately the 5th 
cycle), and the discharge capacity attained when the increase capacity is 
stabilized becomes larger. With regard to cycle characteristics, the 
capacity is not decreased at all when 0.02.ltoreq.x.ltoreq.0.05, while it 
is slightly decreased when x&lt;0.02. Accordingly, a preferable range of the 
molar ratio x is 0.001&lt;x.ltoreq.0.02, taking both the discharge capacity 
and the cycle characteristics into consideration. 
Comparative Example 5 
The powder obtained in Comparative Example 1 was subjected to X-ray powder 
diffraction, thereby obtaining a crystallite size in accordance with the 
above-mentioned method. The obtained result is listed in Table 1. By using 
this powder, a plate battery (in which the liquid electrolyte was 
LiPF.sub.6 /EC+DMC+EMC) was manufactured and subjected to an overcharging 
test in the following manner: 
The battery was charged by a constant current and voltage under a condition 
of a maximum charging voltage of 4.2 V, a charging current of 0.3 
mA/cm.sup.2 and a charging time of 40 hours. Then, the battery was 
discharged by a constant current under a condition of a minimum 
discharging voltage of 3.0 V and a discharging current of 0.3 mA/cm.sup.2. 
At this point, the charge capacity was 263 mAh/g and the discharge 
capacity was 221 mAh/g. When the charge/discharge was repeated under the 
same condition, the charge capacity and the discharge capacity became 219 
mAh/g and 214 mAh/g, respectively. In this manner, the battery using this 
powder was excessively charged, which degraded the efficiency of the 
charge/discharge and decreased the discharge capacity. 
EXAMPLE 8 
The five kinds of powder obtained in Example 7 were subjected to X-ray 
powder diffraction, thereby obtaining crystallite sizes in accordance with 
the above-mentioned method. The results obtained are listed in Table 1. By 
using powders wherein the molar ratios of x are 0.02 and 0.005, plate 
batteries (in which a liquid electrolyte was LiPF.sub.6 /EC+DMC+EMC) were 
manufactured and subjected to an overcharging test as follows: 
The batteries were charged by a constant current and voltage under a 
maximum charging voltage of 4.3 V, a charging current of 0.3 mA/cm .sup.2 
and a charging time of 40 hours, and then were discharged by a constant 
current of a minimum discharging voltage of 3.0 V and a discharging 
current of 0.03 mA/cm.sup.2. The charging voltage in this example was 4.3 
V, which is a more severe condition than in Comparative Example 5, where 
the charging voltage was 4.2 V. 
The charge capacity and the discharge capacity of the battery using the 
powder where the molar ratio of x was 0.02, were 246 mAh/g and 191 mAh/g, 
respectively. The charge capacity and the discharge capacity of the 
battery using the powder where the molar ratio of x was 0.005, were 247 
mAh/g and 207 mAh/g, respectively. 
When the charge/discharge was repeated under the same condition, the charge 
capacity and the discharge capacity of the battery using the powder where 
the molar ratio of x was 0.02, became 191 mAh/g and 191 mAh/g, 
respectively. The charge capacity and the discharge capacity of the 
battery using the powder where the molar ratio of x was 0.005, became 206 
mAh/g and 206 mAh/g, respectively. 
Thus, it was found that the battery using the powder with a crystallite 
size of 700 .ANG. or less, was not excessively charged, even when supplied 
with a high voltage of 4.3 V for a long time, which is not the case with 
the battery of Comparative Example 5. Further, it was found that the 
efficiency of the charge/discharge of such a battery was not degraded, and 
that the discharge capacity was substantially maintained. In this manner, 
an excellent overcharge resistance can be attained by using powder with a 
crystallite size of 700 .ANG. or less. 
EXAMPLE 9 
In Example 7, the initial charge capacities of the batteries using the 
powder where the molar ratio of x was 0.05, 0.02, 0.01, 0.005 and 0.001, 
were 217 mAh/g, 218 mAh/g, 237 mAh/g, 235 mAh/g and 238 mAh/g, 
respectively. The rise of the discharge capacity was quicker in the 
battery with the initial charge capacity of 220 mAh/g or more. Therefore, 
the relationship between the charge capacity and the speed at which 
discharge capacity rose was studied by using the battery using the powder 
where the molar ratio of x was 0.02. 
By using the powder where the molar ratio of x was 0.02, obtained in 
Example 7, two plate batteries (in which a liquid electrolyte was 
LiPF.sub.6 /EC+DMC+EMC) were manufactured. The cycle characteristics of 
the batteries were determined under the same condition as in Example 7, 
except that the initial charging time was respectively set to be 15 hours 
and 24 hours (which was 8 hours in Example 7). 
The variation of the discharge capacity up to the 15th cycle is shown in 
FIG. 7. In this graph, the capacity is normalized for comparison by using 
the capacity in the 15th cycle as 1. When the discharge capacity was 
220mAh/g or more, discharge capacity rose at an improved speed. 
Specifically, it was found that the discharge capacity rose by 
approximately the 5th cycle. 
Thus, when the charge capacity at the time of the production of a battery 
is set to be 220 mAh/g or more per weight of lithiated nickel dioxide 
containing gallium, the initial overvoltage can be decreased and the 
discharge capacity rises at an increased speed. 
Although the battery was charged at 220 mAh/g or more only once in the 1st 
cycle in this example, the battery can be charged twice or more in the 2nd 
or a later cycle. 
As described above, the present lithium secondary battery has excellent 
cycle characteristics and overcharge resistance even when 
charged/discharged at a high capacity, and can attain a high energy 
density, because it exhibits a high discharging voltage. Accordingly, the 
present lithium secondary battery is extremely valuable in industry. 
EXAMPLE 10 
An electrode and a plate battery for a charging/discharging test were 
manufactured as follow: 
To a mixture of the powder obtained in Example 1, as an active material; 
and an artificial flaky graphite (KS15 manufactured by Ronza Co., Ltd.), 
as a conductive substance; was added an NMP solution including PVDF, as a 
binder; so as to attain a composition ratio among the active material, the 
conductive substance and the binder of 87:10:3 (weight ratio). The 
resulting solution was kneaded to obtain a paste. 
The paste was coated on one surface of an aluminum foil, 20 .mu.m thick, 
and vacuum-dried at 150 .degree. C. for 8 hours and pressed to obtain a 
sheet. Then, the sheet was cut to obtain a cathode measuring 1.3 
cm.times.1.8 in size. 
Natural graphite powder (occurrence: Madagascar), which was heat-treated at 
3000 .degree. C., and has a number-average particle size of 10 .mu.m, a 
specific surface area of 9 m.sub.2 /g according to a nitrogen adsorption 
method, a true specific gravity of 2.26, an interlayer spacing d.sub.002 
of 3.36 .ANG. in X-ray diffraction, and an ash content of 0.05% by weight; 
was mixed with graphitic carbon black (TB3800 manufactured by Tokai Carbon 
Co., Ltd.), and subjected to graphitization at 2800.degree. C. so as to 
attain a composition ratio among the natural graphite powder and the 
carbon black powder of 95:5 (weight ratio). 
One part by weight of a silane coupling agent (A186 manufactured by Nippon 
Unicar Co., Ltd.) was added to the mixture. After sufficiently mixing 
them, the mixture was vacuum-dried at 150.degree. C. to obtain mixed 
graphite powder treated with the silane coupling agent. 
Then, to the mixed graphite powder treated with the silane coupling agent 
was added an NMP solution, including PVDF as a binder, so as to attain a 
composition ratio of the mixed graphite powder treated with the silane 
coupling agent and the binder of 97:3 (weight ratio). The resultant 
solution was kneaded to obtain a paste. The paste was coated on one 
surface of copper foil, 10 .mu.m thick, and vacuum-dried at 150.degree. C. 
for 8 hours and pressed to obtain a sheet. Then, the sheet was cut to 
obtain an anode 1.5 cm.times.2 cm size in size. 
By using the thus obtained cathode and anode, a plate battery was 
manufactured together with a liquid electrolyte (1M LiPF.sub.6 
/EC+DMC+EMC) and a polypropylene microporous membrane as a separator. 
The plate battery obtained above was subjected to a charging/discharging 
test using a constant current and voltage, and discharge by a constant 
current under the following condition: 
Maximum charging voltage: 4.2 V 
Charging time: 1 hour 
Charging current: 7.7 mA 
Minimum discharging voltage: 2.75 V 
Discharging current: 7.7 mA 
The potential of the cathode of the present battery went above 4.2 V (vs. 
Li/Li.sup.+) at the last stage of charging under the experimental 
conditions owing to the mixed graphite powder treated with the silane 
coupling agent as an anode active material. 
The discharge capacity of the 1st, 10th, 11th and 20th cycles were 3.67, 
3.79, 3.86 and 3.76 mAh, respectively. The capacity holding ratios R1 and 
R2 were 1,032 and 0.967, respectively, and thus, the battery exhibited 
excellent cycle characteristics.