JP3337077B2 - Manufacturing method of non-aqueous electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a non-aqueous electrolyte secondary battery having a negative electrode mainly composed of a lithium-doped and undoped carbon material.
In particular, it relates to a method for manufacturing a negative electrode.

[0002]

2. Description of the Related Art In recent years, remarkable progress in electronic technology has enabled electronic devices to become smaller and lighter one after another. Along with this, batteries that are smaller and lighter and have a higher energy density are increasingly demanded for batteries as mobile power supplies.

Conventionally, aqueous secondary batteries such as lead batteries and nickel-cadmium batteries have been the mainstream as secondary batteries for general use. These batteries have excellent cycle characteristics, but are not sufficiently satisfactory in terms of battery weight and energy density.

Recently, as a secondary battery, a non-aqueous electrolyte secondary battery using lithium or a lithium alloy for a negative electrode has been replaced by a lead battery or a nickel cadmium battery, which are insufficient in battery weight and energy density. Research and development are actively conducted.

[0005] This battery has excellent features of high energy density, low self-discharge, and light weight. However, this battery has a drawback that, with the progress of the charge / discharge cycle, lithium grows in a dendrite shape at the time of charging at the negative electrode, and this dendrite-like crystal is likely to reach the positive electrode and cause an internal short circuit. , A major obstacle to its practical application.

On the other hand, according to a nonaqueous electrolyte secondary battery using a carbon material as a negative electrode active material carrier for a negative electrode,
Lithium previously supported on the carbon material of the negative electrode by chemical and physical methods, lithium contained in the crystal structure of the positive electrode active material, and lithium dissolved in the electrolytic solution are respectively transferred to the carbon layer at the negative electrode during charge and discharge. Doped and undoped from the carbon layer. For this reason, even if the charge / discharge cycle proceeds, no precipitation of dendrite-like crystals is observed at the time of charging in the negative electrode, and an internal short circuit hardly occurs, and good charge / discharge cycle characteristics are exhibited. In addition, since the energy density is high and the weight is low, development is proceeding toward practical use.

The above-mentioned negative electrode is usually manufactured using a negative electrode mixture obtained by mixing a powdery carbon material and a binder.

[0008] Applications of the non-aqueous electrolyte secondary battery as described above include a video camera and a laptop personal computer. Since many of such electronic devices consume relatively large current, the batteries need to be able to withstand heavy loads.

Therefore, as a battery structure, a spiral wound electrode structure formed by winding a strip-shaped positive electrode and a strip-shaped negative electrode in the length direction thereof through a strip-shaped separator is effective. According to the battery having the wound electrode structure,
Since the electrode area is large, it can withstand heavy load use.

[0010]

In order to provide a secondary battery having excellent performance over a long period of time as a power source for the electronic equipment as described above, it is necessary to minimize the capacity reduction accompanying the progress of the charge / discharge cycle. It is necessary to.

In terms of this capacity, the performance of the conventional non-aqueous electrolyte secondary battery was not always sufficient.

The present inventors have conducted intensive studies on the cause of the capacity reduction in the non-aqueous electrolyte secondary battery. As a result, during the storage period until the carbon material obtained by firing the organic material and the like was used for the production of the negative electrode, Moisture is adsorbed in the fine pores of the carbon material, and this water reacts with lithium ions on the surface of the pores of the carbon material during the charging reaction, and the lithium ions are changed into non-dischargeable lithium compounds. It was found that it decreased as the cycle progressed.

The present invention has been made on the basis of the above-described findings, and an object of the present invention is to improve charge / discharge cycle characteristics in a nonaqueous electrolyte secondary battery having a negative electrode mainly composed of a carbon material. It is an object of the present invention to provide a manufacturing method which can be performed.

[0014]

In order to achieve the above object, the present invention comprises a negative electrode mainly composed of a lithium-doped and undoped carbon material and a positive electrode containing lithium. In a method of manufacturing a non-aqueous electrolyte secondary battery, a powdery carbon material is obtained, and then the carbon material is heat-treated. Thereafter, the heat-treated carbon material is used to coat a strip-shaped current collector. together configured to produce the negative electrode by performing the charcoal
The raw material is obtained by carbonizing an organic material.
And the (002) plane spacing (lattice spacing) is 3.70 ° or less.
Top, true density less than 1.70 g / cm ³ and air flow
Exothermic peak above 700 ° C in differential thermal analysis
I tried to use something that was not done . this
In the case, as the positive electrode, a composite metal oxide containing lithium is used.
It is preferable to use an intercalation compound containing a substance or lithium.
Good.

The carbon material used in the present invention is an organic material
Wherein the (002) plane spacing (lattice spacing) is 3.70 ° or more, and the true density is 1.70 g / c.
less than m ³ and 7 by differential thermal analysis in airflow.
Does not have an exothermic peak above 00 ° C
Al, have very good characteristics as a negative electrode material, thus
In addition, a higher capacity battery can be provided.

The carbonaceous material can be produced, for example, by carbonizing an organic material by a method such as firing at a temperature of, for example, about 700 to 1500 ° C. In addition, carbon materials can be generally classified into carbonaceous materials and graphite materials.
As the negative electrode material, a carbonaceous material is preferable as described above.

As a starting material for this carbonaceous material, furan resin composed of a homopolymer or copolymer of furfuryl alcohol or furfural is preferred. Specific furan resins include furfural + phenol,
Polymers composed of furfuryl alcohol + dimethylol urea, furfuryl alcohol, furfuryl alcohol + formaldehyde, furfuryl alcohol + furfural, furfural + ketones and the like can be mentioned. By firing such a furan resin, a carbonaceous material having the above-described properties can be obtained.

Further, a petroleum pitch having a hydrogen / carbon atom ratio of 0.6 to 0.8 is used as a starting material, and an oxygen crosslink for introducing a functional group containing oxygen is applied to the starting material to thereby obtain an oxygen content of 10 to 20. After obtaining the precursor by weight, the carbonaceous material obtained by calcining the precursor also has the above-mentioned properties and is suitable.

Further, when carbonizing the furan resin or the petroleum pitch, it is preferable to add a phosphorus compound or a boron compound since a carbonaceous material having a large doping amount with respect to lithium can be obtained.

The carbon material obtained as described above is preferably ground into a powder so as to have an average particle diameter of several to several tens of μm, and then, using the powdered carbon material, for example, A negative electrode can be manufactured by applying a negative electrode mixture slurry in which a negative electrode mixture obtained by mixing with a binder is dispersed in a solvent to a metal current collector or the like.

Prior to the production of the negative electrode, the powdery carbon material is heat-treated. The temperature of this heat treatment is 300 to
The temperature may be about 1500 ° C, preferably 300 to 1100 ° C, more preferably 500 to 1000 ° C. The atmosphere for the heat treatment is preferably in a vacuum or in an inert gas.

When the negative electrode is manufactured from a negative electrode mixture composed of a powdery carbonaceous material and a binder, for example, as described above, the binder contained in the negative electrode mixture is subjected to heat treatment at the high temperature as described above. Then, the properties of the binder may change,
In the process after obtaining the negative electrode mixture, the heat treatment temperature is restricted. According to the manufacturing method of the present invention, there is no such restriction at all, and the negative electrode can be manufactured by, for example, obtaining the above-described negative electrode mixture after sufficiently removing water from the carbon material.

It is preferable that the carbon material subjected to the heat treatment as described above is used immediately after the treatment (since water may be adsorbed in the fine pores again) in the production of the negative electrode.
Alternatively, it is preferable to store in a dryer and set the relative humidity in the dryer to 2% or less.

The positive electrode active material of the non-aqueous electrolyte secondary battery according to the present invention includes 25 g / g of the carbon material of the negative electrode.
A material containing lithium corresponding to a charge / discharge capacity of 0 mAh or more, for example, a composite metal oxide represented by the general formula LiMO ₂ (where M represents at least one of Co and Ni),
It is preferable to use an intercalation compound containing lithium or the like. In particular, since high voltage and high energy density are obtained and cycle characteristics are excellent, LiCoO ₂ , LiCo
_0.8 Ni _0.2 O ₂ is preferred.

As the non-aqueous electrolyte of the non-aqueous electrolyte secondary battery according to the present invention, for example, a non-aqueous electrolyte obtained by dissolving a lithium salt in a non-aqueous solvent (organic solvent) can be used.

Here, the organic solvent is not particularly limited. For example, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2
-Diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolan, 4-methyl-1,3
-Dioxolan, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile and the like can be used alone or in combination of two or more.

As the electrolyte to be dissolved in the organic solvent, any of those conventionally known can be used, and LiClO ₄ , L
iAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H
₅ ) ₄ , LiCl, LiBr, CH ₃ SO ₃ Li, CF
₃ SO ₃ Li and the like.

The non-aqueous electrolyte may be a solid, such as a polymer complex solid electrolyte.

[0029]

The water adsorbed in the fine pores of the carbon material is removed by heat-treating the powdery carbon material as described above. As a result, during the charging reaction, the lithium ions do not react with moisture on the surface of the fine pores of the carbon material to change to a lithium compound that cannot be discharged,
It is possible to prevent the capacity from decreasing with the progress of the charge / discharge cycle.

Further, since the above-described heat treatment for the carbon material is performed before the negative electrode is manufactured using the carbon material, other materials and other steps may be adversely affected by the heat treatment during the manufacture of the negative electrode. It won't.

[0031]

FIG. 1 shows an embodiment of the present invention.
This will be described with reference to FIG.

Embodiment 1

FIG. 1 is a schematic vertical sectional view of a non-aqueous electrolyte secondary battery of this embodiment, and FIG. 2 is a perspective view of a strip-shaped negative electrode which can be used in this battery. It was manufactured as follows.

First, the negative electrode 1 was manufactured as follows.
10 petroleum pitch functional groups containing oxygen are used as starting materials.
After oxygen cross-linking to introduce up to 20% by weight, the oxygen-crosslinked precursor is calcined at 1000 ° C. in an inert gas stream to obtain a carbonaceous material having properties close to glassy carbon. Was.

As a result of X-ray diffraction measurement of this carbonaceous material, the (002) plane spacing was 3.76 °, and the true specific gravity was measured by a pycnometer method to be 1.58 g / cm ³ . there were. Further, when a differential thermal analysis was performed in an air stream, no exothermic peak was found at 700 ° C. or higher.

This carbonaceous material is pulverized to an average particle size of 10 μm.
m of the carbonaceous material powder.

The above-mentioned carbonaceous material powder was subjected to a heat treatment at 1000 ° C. for 4 hours in a stream of argon gas. With respect to the carbonaceous material powder after the heat treatment, the sample was reheated at 1000 ° C., the generated water was introduced into the electrolytic cell with a dry argon carrier gas, and the water content was measured by the Karl Fischer method. Met.

The carbonaceous material obtained as described above was used as a negative electrode active material carrier, and 90 parts by weight of the carbonaceous material powder and 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed. A negative electrode mixture was prepared. This adjustment was performed immediately after the above-mentioned heat treatment of the carbonaceous material. This negative electrode mixture was dispersed in N-methyl-2-pyrrolidone as a solvent to form a slurry (paste).

Next, this negative electrode mixture slurry was coated to a thickness of 10 μm.
m, and uniformly coated on both surfaces of the negative electrode current collector 9 which is a strip-shaped copper foil, dried and then compression-molded by a roller press to form a negative electrode on both surfaces of the negative electrode current collector 9 as shown in FIG. A strip-shaped negative electrode 1 having the mixture layer 1a was obtained.

The thickness of the negative electrode mixture layer 1a after molding was the same at 80 μm on both sides, and the width of the strip-shaped negative electrode 1 was 3 μm.
The length was 3.5 mm and the length was 700 mm.

Next, the positive electrode 2 was manufactured as follows.
By calcining for 5 hours at 900 ° C. in air a mixture of lithium carbonate 0.5 molar and cobalt 1 mole carbonate,
LiCoO ₂ was obtained.

Using this LiCoO ₂ as a positive electrode active material, 91 parts by weight of LiCoO ₂ were mixed with 6 parts by weight of graphite as a conductive agent and 3 parts by weight of polyvinylidene fluoride as a binder to form a positive electrode mixture. . This positive electrode mixture was dispersed in a solvent N-methyl-2-pyrrolidone to form a slurry (paste).

Next, this positive electrode mixture slurry was coated with a thickness of 20 μm.
The positive electrode current collector 10, which is a μm band-shaped aluminum foil, was uniformly coated on both sides and dried. After the drying, compression molding was performed using a roller press to obtain a band-shaped positive electrode 2.

The thickness of the positive electrode mixture after molding was 8 on both sides.
The width of the strip-shaped positive electrode 2 is 31.5 m.
m and length were 650 mm.

Using the strip-shaped negative electrode 1 manufactured as described above, the strip-shaped positive electrode 2, and a pair of strip-shaped separators 3a and 3b made of a microporous polypropylene film having a thickness of 25 μm and a width of 36 mm, Negative electrode 1, separator 3a,
The positive electrode 2 and the separator 3b are laminated in four layers in this order.
The spirally wound electrode body 15 was produced by spirally winding the laminated electrode body having a layered structure many times along the length direction with the negative electrode 1 inside. At this time, the wound final end of the wound electrode body 15 was fixed with an adhesive tape.

The inner diameter of the hollow portion at the center of the spirally wound electrode body 15 was 3.5 mm, and the outer diameter was 19.7 mm. The core 33 is located in this hollow portion.

The spirally wound spirally wound electrode body 15 produced as described above was housed in a nickel-plated iron battery can 5 as shown in FIG.

In order to collect the current of the negative electrode 1 and the positive electrode 2 respectively, a negative electrode lead 11 made of nickel is attached to the negative electrode current collector 9 in advance, and this is led out from the negative electrode 1 and welded to the bottom surface of the battery can 5. Further, the positive electrode lead 12 made of aluminum was attached to the positive electrode current collector 10 in advance, which was led out from the positive electrode 2 and welded to the projection 34a of the safety valve 34 made of metal.

Thereafter, a non-aqueous electrolyte obtained by dissolving LiPF ₆ as a lithium salt in a mixed solvent of propylene carbonate and 1,2-dimethoxyethane at a ratio of 1 mol / l was injected into the battery can 5. The wound electrode body 15 was impregnated.

Before and after this, disk-shaped insulating plates 4a and 4b were disposed in the battery can 5 so as to face the upper end surface and the lower end surface of the wound electrode body 15, respectively.

Thereafter, each of the battery can 5, the safety valve 34 whose outer periphery is in close contact with each other, and the metal battery lid 7 are caulked via the insulating sealing gasket 6 coated with asphalt on the surface thereof. 5 was sealed. Thereby, the battery lid 7 and the safety valve 34 were fixed, and the airtightness in the battery can 5 was maintained. At this time, the lower end of the gasket 6 in FIG. 1 contacts the outer peripheral surface of the insulating plate 4a, so that the insulating plate 4a is in close contact with the upper surface of the spirally wound electrode body 15.

As described above, the diameter 20 mm and the height 4
A 2 mm cylindrical non-aqueous electrolyte secondary battery was manufactured. As shown in Table 1 below, the battery of Example 1 was referred to as Battery A for convenience.
And

The above cylindrical non-aqueous electrolyte secondary battery is
In order to form a double safety device, a safety valve 34, a stripper 36, and an intermediate fitting 35 made of an insulating material for integrating the safety valve 34 and the stripper 36 are provided. Although not shown, this safety valve 3 is
A hole is provided in the battery cover 7 at a cleavage portion that is cleaved when the battery 4 is deformed.

If the internal pressure of the battery rises for some reason, the safety valve 34 is turned around its projection 34a as shown in FIG.
Is deformed upward, so that the connection between the positive electrode lead 12 and the protruding portion 34a is cut off to cut off the battery current, or so that the gas generated in the battery is opened due to the cleavage of the safety valve 34 being opened. Each is configured.

Embodiment 2

In this embodiment, the same carbonaceous material as in the first embodiment is used, and the steps until the powder of the carbonaceous material is obtained are the same as those in the first embodiment. went.

The above-mentioned powdery carbonaceous material is placed in a vacuum for 30 minutes.
Heat treatment was performed at 0 ° C. for 10 hours. A cylindrical nonaqueous electrolyte secondary battery was manufactured in the same manner as in Example 1, except that the heat-treated carbonaceous material was used as the negative electrode active material carrier. This battery was used as shown in Table 1 below.
And

The water content of the carbonaceous material after the heat treatment was measured by the same method as in Example 1 and found to be 393 ppm.

Comparative Example

As a comparative example for confirming the effect of the present invention, the following battery was manufactured. In other words, except that the same carbonaceous material as in Example 1 was used, and the steps until the powder of the carbonaceous material was obtained were the same as those in Example 1, and thereafter, the carbonaceous material without heat treatment was used. A cylindrical non-aqueous electrolyte secondary battery was manufactured in the same manner as in Example 1. This battery is referred to as Battery C as shown in Table 1 below.

The water content of the above-mentioned carbonaceous material powder was measured by the same method as in Example 1 and found to be 1405 ppm.

For the three types of batteries A, B, and C, the charging upper limit voltage was set to 4.1 V, the battery was charged at a constant current of 1 A for 2 hours, and the final voltage was 2.75 V at a constant load of 7.5 Ω.
The charge / discharge cycle for discharging the battery was repeated. The capacity at 10 charge / discharge cycles was measured as the initial capacity, and the discharge capacity at 100 cycles was further measured. 1
The ratio between the discharge capacity at the time of 00 cycles and the initial capacity (capacity at the time of 100 cycles / initial capacity) was defined as the capacity retention rate. The results are shown in Table 1 below.

[0063]

[Table 1]

From the results shown in Table 1 above, the non-aqueous electrolyte secondary batteries A and B according to the production method in which the present invention is applied and the carbon material powder is subjected to heat treatment and then the negative electrode 1 is produced,
It can be seen that the capacity retention ratio is greatly improved as compared with the nonaqueous electrolyte secondary battery C according to the conventional manufacturing method.

This is because the carbonaceous material powder as the negative electrode material is reheated at a high temperature (300 to 1500 ° C.) separately from the firing of the starting material for obtaining the carbonaceous material, so that the carbonaceous material can be finely divided. It is considered that the effect of removing the water adsorbed in the pores appeared. It is considered that the moisture adsorbed in the fine pores of the carbonaceous material causes lithium ions to react with lithium ions on the surface of the fine pores during the charging reaction to generate a lithium compound that cannot be discharged, thereby causing a reduction in capacity. This is clear from the residual moisture values of the batteries A, B, and C. The carbonaceous material used for the negative electrode is a so-called glassy or nearly glassy material, and has pores with a very small diameter as compared with activated carbon and other carbon materials. For this reason, it is difficult to remove moisture once adsorbed in the micropores. Conventionally, the negative electrode mixture slurry was applied to the negative electrode current collector 9 and then dried at a temperature of, for example, about 80 to 200 ° C. If the drying temperature is higher than this temperature, the properties of PVDF as a binder may change, which is not preferable. Therefore, the moisture trapped in the fine pores of the carbonaceous material could hardly be removed at the conventional drying temperature. On the other hand, according to the production method of the present invention, moisture can be sufficiently removed from the fine pores of the carbonaceous material without adversely affecting the binder and other production steps.

In Examples 1 and 2, the heat treatment after obtaining the powdery carbonaceous material was performed in an atmosphere heating furnace and a vacuum heating furnace. Various apparatuses such as an induction heating furnace and a forced hot air circulation furnace can be used. The temperature at which the carbonaceous material powder is heated may be in the range of 300 to 1500 ° C., preferably 300 ° C.
To 1100 ° C, more preferably 500 to 1000 ° C.

In the method of heat-treating the carbonaceous material as described above, the sample is heated at 1000 ° C., the generated moisture is introduced into the electrolytic cell with a dry argon carrier gas, and the water content is measured by the Karl Fischer method. In the method of measuring the value, the carbonaceous material is subjected to a heat treatment so that the moisture value becomes 2000 ppm or less, preferably 1200 ppm or less. As a result, a non-aqueous electrolyte secondary battery having good charge / discharge cycle characteristics can be efficiently manufactured with small variations.

In Examples 1 and 2 according to the present invention,
Immediately after the heat treatment of the carbonaceous material powder, the mixture was mixed with a binder to prepare a negative electrode mixture, but the heat-treated carbonaceous material powder was dried in an atmosphere having a relative humidity of 2% or less, such as a desiccator. After storage in a container, the mixture may be mixed with a binder at an appropriate time to prepare a negative electrode mixture.

In this embodiment, the manufacturing method of the present invention is applied to a cylindrical non-aqueous electrolyte secondary battery using a wound electrode body. However, the present invention is not limited to this. The present invention may be applied to a non-aqueous electrolyte secondary battery such as a cylindrical type, or may be applied to the manufacture of a button-type or coin-type non-aqueous electrolyte secondary battery. Also, a graphite material can be used as the carbon material.

[0070]

According to the present invention, a method of manufacturing a non-aqueous electrolyte secondary battery including a negative electrode mainly composed of a lithium-doped and undoped carbon material and a lithium-containing positive electrode, respectively. In, after obtaining a powdery carbon material, the carbon material is subjected to a heat treatment, and thereafter,
The negative electrode was manufactured by applying the heat-treated carbon material to the belt-shaped current collector, and the anode was manufactured by performing the application to the belt-shaped current collector using the powdered carbon material. The non-aqueous electrolyte secondary battery in which the decrease in capacity due to the progress of the charge / discharge cycle is reduced without adversely affecting other materials of the negative electrode and other manufacturing processes of the negative electrode. Therefore, in addition to the conventionally known high energy density and high capacity, the above-mentioned nonaqueous electrolyte secondary battery having excellent charge / discharge cycle characteristics can be manufactured. Ma
In addition, since the negative electrode was manufactured by applying to the belt-shaped current collector using the powdered carbon material after the heat treatment , it was possible to manufacture a high-strength negative electrode by a simple process, In particular, the strength of a spirally wound electrode body that can be manufactured by spirally winding a large number of times is large, and its manufacture is easy. Further, since a band-shaped current collector having an extremely low water content can be used, the water content of the negative electrode as a whole can be extremely reduced in combination with the low residual water content of the powdery carbon material after the heat treatment. For this reason, the charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery can be further improved. Further
In addition, as a carbon material, a material obtained by carbonizing an organic material
And the plane spacing (lattice spacing) of the (002) plane is 3.70.
Å, true density less than 1.70 g / cm ³ and air
Exothermic peak above 700 ° C by differential thermal analysis in airflow
Is not used, so even higher capacity
A battery can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view of a cylindrical nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

FIG. 2 is a perspective view showing a strip-shaped negative electrode of the battery shown in FIG. 1 before a wound electrode body is manufactured.

[Explanation of symbols]

 1 negative electrode 1a negative electrode mixture layer

──────────────────────────────────────────────────続 き Continued from the front page (72) Inventor Tomoaki Sato 1-1, Shimosugishita, Takakura, Hiwada-cho, Koriyama-shi, Fukushima Prefecture 1 Sony Energy Tech Koriyama Plant (56) References JP-A-59-18579 JP, A) JP-A-4-79170 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 10/40 H01M 4/58

Claims (2)

(57) [Claims] 1. A method for producing a non-aqueous electrolyte secondary battery comprising a negative electrode mainly composed of a lithium-doped and undoped carbon material and a lithium-containing positive electrode, the method comprising: After obtaining the material, the carbon material is subjected to heat treatment, and thereafter, the negative electrode is manufactured by applying the heat-treated carbon material to a belt-shaped current collector .
And the carbon material is obtained by carbonizing an organic material.
And the plane spacing (lattice spacing) of the (002) plane is 3.70.
Å, true density less than 1.70 g / cm ³ and air
Exothermic peak above 700 ° C by differential thermal analysis in airflow
A method for producing a non-aqueous electrolyte secondary battery, characterized by using a non-aqueous electrolyte secondary battery. 2. The method for producing a non-aqueous electrolyte secondary battery according to claim 1 , wherein a lithium-containing composite metal oxide or an intercalation compound containing lithium is used as the positive electrode. .