JPH04249860A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To obtain a nonaqueous electrolyte secondary battery able to prevent the occurrence of separation and a crack in a negative electrode mixture layer and having high capacity and an excellent charge and discharge cycle characteristic. CONSTITUTION:In a nonaqueous electrolyte secondary battery, polyvinyliden fluoride (PVDF) is used as a binding agent in a negative electrode mixture layer 1a composed of a carbon material and the binding agent, and also a negative electrode 1 is provided in which a content of the binding agent in a negative electrode mixture is 5-20wt.%.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a nonaqueous electrolyte diode comprising a negative electrode comprising a negative electrode mixture layer containing at least a carbon material and a binder as a negative electrode material carrier, a positive electrode, and a nonaqueous electrolyte. This invention relates to secondary batteries, and particularly to binders.

0002

2. Description of the Related Art The remarkable progress in electronic technology in recent years has led to successive reductions in the size and weight of electronic devices. Along with this, there is an increasing demand for batteries as mobile power sources to be smaller, lighter, and have higher energy density.

Conventionally, aqueous batteries such as lead batteries and nickel-cadmium batteries have been mainstream as secondary batteries for general use. Although these batteries have excellent cycle characteristics, they cannot be said to have fully satisfactory characteristics in terms of battery weight and energy density.

Recently, non-aqueous electrolyte secondary batteries using lithium or lithium alloys as negative electrodes have been introduced as secondary batteries to replace lead batteries and nickel-cadmium batteries, which are insufficient in terms of battery weight and energy density. Research and development is actively underway.

[0005] This battery has excellent features such as high energy density, low self-discharge, and light weight. However, with this battery, as the charge/discharge cycle progresses, lithium crystals grow in the form of dendrites at the negative electrode during charging, and there is a high possibility that these dendrite-like crystals may reach the positive electrode and lead to an internal short circuit. , which was a major obstacle to practical application.

On the other hand, according to a non-aqueous electrolyte secondary battery using a carbon material as a negative electrode active material carrier for the negative electrode,
Lithium previously supported on the carbon material of the negative electrode by chemical or physical methods, lithium contained in the crystal structure of the positive electrode active material, and lithium dissolved in the electrolyte move between the carbon layers in the negative electrode during charging and discharging. Doped and dedoped from between the carbon layers. Therefore, even as the charge/discharge cycle progresses, no dendrite-like crystals are deposited on the negative electrode during charging, making it difficult to cause internal short circuits and exhibiting good charge/discharge cycle characteristics. Furthermore, since it has a high energy density and is lightweight, development is progressing toward practical application.

[0007] Applications of the above-mentioned nonaqueous electrolyte secondary battery include video cameras, laptop computers, and the like. Since many of these electronic devices have relatively large current consumption, the batteries need to be able to withstand heavy loads.

[0008] Therefore, as a battery structure, a spirally wound electrode body structure constituted by winding a strip-shaped positive electrode and a strip-shaped negative electrode in the longitudinal direction with a strip-shaped separator interposed therebetween is effective. According to the battery with this wound electrode body structure,
Since the electrode area is large, it can withstand use under heavy loads.

In the above-described wound electrode body, it is desirable to make the electrode thin in order to increase the electrode area and to fill as much active material or active material support into a limited space as possible. Therefore, the method for manufacturing the strip-shaped electrode is as follows.
A method using paste (slurry) is preferable. In this method, an electrode mixture slurry obtained by dispersing an electrode material containing a binder, an active material (or an active material support), etc. in a solvent is applied to an electrode current collector, and then dried. Accordingly, an electrode mixture layer is formed on the electrode current collector. According to this method, the electrode mixture layer in the strip-shaped electrode can have a thickness of about several μm to several hundred μm.

[0010] Up until now, net-like expanded medals and punched medals with many holes have been often used as electrode current collectors, but these electrode current collectors have heavy load characteristics as described above. It is not suitable for making electrodes thinner. Therefore, as mentioned above, it is preferable to use metal foil as the electrode current collector and to make this metal foil as thin as possible.

[0011]

[Problems to be Solved by the Invention] However, since the metal foil as described above has a smooth surface, it is formed by applying a negative electrode mixture slurry to the metal foil as a negative electrode current collector as described above. The negative electrode mixture layer has had problems such as being easily peeled off from the metal foil and cracking during battery manufacture and use. In particular, the above-mentioned peeling is likely to occur when winding the electrode to produce a wound electrode body.

[0012] An object of the present invention is to provide a non-aqueous electrolyte secondary battery in which the negative electrode mixture layer in the negative electrode is free from cracking or peeling.

[0013]

Means for Solving the Problems In order to achieve the above object, the present invention provides a negative electrode 1 comprising a negative electrode mixture layer 1a containing at least a carbon material as a negative electrode active material carrier and an adhesive.
In the nonaqueous electrolyte secondary battery comprising a positive electrode 2 and a nonaqueous electrolyte, the binder is polyvinylidene fluoride, and the content of the binder in the negative electrode mixture is 5% by weight or more. and 20% by weight or less.

Polyvinylidene fluoride is a suitable binder for forming the negative electrode mixture layer. This is because the melting point of polyvinylidene fluoride is relatively high compared to other binders, so it is stable even at high temperatures, and because polyvinylidene fluoride is easily dissolved in solvents, it can be uniformly mixed in the negative electrode mixture. This is because the effect can be obtained by adding a relatively small amount.

[0015] When the negative electrode mixture layer is made from a negative electrode mixture slurry in which a negative electrode mixture consisting of a carbon material and a binder is dissolved in a solvent, the content of the binder in the negative electrode mixture is , is the component amount (% by weight) of the binder in the negative electrode mixture after drying and removing the solvent.

[0016] In the negative electrode, a carbon material is used as a negative electrode active material carrier that can be doped with an alkali metal such as lithium and dedoped. Examples of the carbon material include cokes such as pitch coke and needle coke, and polymers. , carbon fiber, graphite materials, etc. In particular, such carbon materials include (002)
The interplanar spacing (lattice spacing) of the planes is 3.70 Å or more, and the true density is 1.
It is preferable to use a carbonaceous material which has an exothermic peak of less than 70 g/cm 3 and does not have an exothermic peak at 700° C. or higher in differential thermal analysis in an air stream. Since such a carbonaceous material has very good properties as a negative electrode material, a high capacity battery can be obtained.

[0017] The carbonaceous material can be produced, for example, by carbonizing an organic material by a method such as firing at a temperature of about 700 to 1500°C. Note that carbon materials can generally be broadly classified into carbonaceous materials and graphite materials, and either of them can be used, but the above-mentioned carbonaceous materials are preferable.

As a starting material for this carbonaceous material, a furan resin made of furifuryl alcohol or a homopolymer or copolymer of furfural is suitable. Specific furan resins include furfural + phenol, furfuryl alcohol + dimethylol urea, furfuryl alcohol, furfuryl alcohol + formaldehyde,
furfuryl alcohol + furfural, furfural +
Examples include polymers made of ketones and the like. By firing such a furan resin, a carbonaceous material having the above properties can be obtained.

[0019] Also, as a starting material, a hydrogen/carbon atomic ratio of 0
．． After obtaining a precursor with an oxygen content of 10 to 20% by weight by using petroleum pitch of 6 to 0.8 and subjecting it to oxygen crosslinking to introduce an oxygen-containing functional group, this precursor is fired. The carbonaceous material obtained by this method also has the above-mentioned properties and is suitable.

It is also preferable to add a phosphorus compound or a boron compound when carbonizing the furan resin or the petroleum pitch, since it is possible to obtain a carbonaceous material with a large amount of lithium doped.

The positive electrode active material in the positive electrode may be a transition metal oxide such as manganese dioxide or vanadium pentoxide, a transition metal chalcogenide such as iron sulfide or titanium sulfide, or a composite compound of these and lithium. For example, a composite metal oxide represented by the general formula LiMO2 (where M represents at least one of Co and Ni) can be used. In particular, LiCoO2 can obtain high voltage, high energy density, and has excellent cycle characteristics.
, LiCo0.8 Ni0.2 O2 and other lithium-cobalt composite oxides and lithium-cobalt-nickel composite oxides are preferred.

[0022] As the non-aqueous electrolyte, for example, a non-aqueous electrolyte in which an electrolyte (lithium salt) is dissolved in a non-aqueous solvent (organic solvent) can be used.

[0023] The organic solvent here is not particularly limited, but includes, for example, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-dimethoxyethane, etc.
-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3
-Dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, etc. can be used alone or in combination of two or more.

[0024] Furthermore, any conventionally known electrolyte can be used as the electrolyte to be dissolved in the organic solvent, such as LiClO4,
LiAsF6, LiPF6, LiBF4, LiB
(C6 H5)4, LiCl, LiBr, CH3
There are SO3 Li, CF3 SO3 Li, etc.

[0025] Furthermore, the non-aqueous electrolyte may be solid, such as a polymer complex solid electrolyte.

[0026]

[Function] Since polyvinylidene fluoride has properties as a binder and is contained in an appropriate amount in the negative electrode mixture, peeling and cracking of the negative electrode mixture layer in the negative electrode is less likely to occur.

[0027]

EXAMPLES Experimental examples for determining the appropriate content of the binder in the present invention and examples according to the present invention will be described below with reference to FIGS. 1 and 2. Note that FIG. 1 is a schematic longitudinal cross-sectional view of the non-aqueous electrolyte secondary battery of this example,
FIG. 2 is a perspective view of a strip-shaped negative electrode that can be used in this battery.

Experimental Example 1 A negative electrode 1 as shown in FIG. 2 was prepared for an experiment in the following manner. After carrying out oxygen crosslinking to introduce 10 to 20% by weight of oxygen-containing functional groups into petroleum pitch as a starting material, this oxygen-crosslinked precursor was heated in an inert gas stream for 10% by weight.
By firing at 00°C, a carbonaceous material with properties similar to glassy carbon was obtained.

[0029] As a result of X-ray diffraction measurement of this carbonaceous material, the spacing of the (002) plane was 3.76 Å.
Moreover, when the true specific gravity was measured by the pycnometer method, it was 1.58 g/cm3. Further, when differential thermal analysis was performed in an air stream, it was found that there was no exothermic peak above 700°C. This carbonaceous material was pulverized to obtain carbonaceous material powder with an average particle size of 10 μm.

Carbonaceous material powder 9 obtained as above
7 parts by weight and polyvinylidene fluoride (PV) as a binder.
DF) was mixed with 3 parts by weight to prepare a negative electrode mixture. This negative electrode mixture was mixed with a solvent (N-
(using methyl-2-pyrrolidone) to form a slurry (paste).

Next, this negative electrode mixture slurry was applied to a thickness of 10 μm.
After applying it uniformly to one side of the negative electrode current collector 9, which is a strip-shaped copper foil of m, the solvent in the negative electrode mixture slurry is dried, and after this drying, compression molding is performed using a roller press machine, as shown in FIG. A strip-shaped negative electrode 1 having a negative electrode mixture layer 1a (solid line portion in FIG. 2) on one side of a negative electrode current collector 9 was obtained.

Note that the film thickness of the negative electrode mixture layer 1a after molding is 8.
The strip-shaped negative electrode 1 had a width of 41.5 mm and a length of 280 mm.

Experimental Examples 2, 3, 4, 5, and 6 In the negative electrode mixture in Experimental Example 1 described above, as shown in Table 1 below, carbonaceous materials were used at 95%, 90%, 85%, 80%, and 75%, respectively.
Five types of strip-shaped negative electrodes 1 as shown in FIG. Created.

Regarding the six types of strip-shaped negative electrodes 1 obtained in Experimental Examples 1 to 6 above, the negative electrode current collector 9 of the negative electrode mixture layer 1a
In order to investigate the adhesion to the film, the following bending test was conducted. That is, the strip-shaped negative electrode 1 was bent 180 degrees in the length direction so that the metal foil of the negative electrode current collector 9 was on the inside and the negative electrode mixture layer 1a was on the outside, so that it was completely folded in half. The negative electrode mixture layer 1a at this bent portion was observed to check for cracks and peeling. The results and the practicality of whether the negative electrodes obtained in each experimental example can actually be used are shown in Table 1 below.

[0035]

[Table 1]

As shown in Table 1 above, the content of the binder (PVDF) in the negative electrode mixture after drying the solvent is 5 to 2.
It was found that within the range of 0% by weight, the negative electrode mixture layer 1a would not peel off from the negative electrode current collector 9 and no cracks would occur.

[0037] Also, when the content of the binder was 3% by weight, peeling was observed in the negative electrode mixture layer 1a, but this was probably due to the lack of binder and the insufficient effect of the binder. In addition, cracks were observed in the case of 25% by weight, but this is thought to be because too much binder was present and the ductility of the negative electrode mixture layer 1a decreased.

Example 1 The non-aqueous electrolyte secondary battery shown in FIG. 1 was manufactured as follows.

First, the negative electrode 1 was produced as follows. Using the negative electrode mixture (containing 95% by weight of carbonaceous material and 5% by weight of PVDF) in Experimental Example 2 described above, a negative electrode mixture slurry obtained in the same manner as in Experimental Example 1 described above was made into a copper negative electrode assembly. After coating both sides of the current collector 9, the solvent in the negative electrode mixture slurry is dried, and after drying, compression molding is performed using a roller press machine to apply the negative electrode mixture to both sides of the negative electrode current collector 9, as shown in FIG. A strip-shaped negative electrode 1 having a layer 1a (portions indicated by solid lines and broken lines in FIG. 2) was obtained. The thickness of the negative electrode mixture layer 1a was 80 mm on both sides, and the width of the negative electrode 1 was 41.5 mm, and the length was 280 mm.

[0040] In addition to copper, metal foils such as stainless steel copper, nickel, and titanium can be used for the negative electrode current collector.

Next, the positive electrode 2 was produced as follows. By mixing 0.5 mole of lithium carbon and 1 mole of cobalt carbon and baking the mixture in air at 900°C for 5 hours,
LiCoO2 was obtained.

This LiCoO2 is used as a positive electrode active material, and 91 parts by weight of this LiCoO2 is mixed with 6 parts by weight of graphite as a conductive agent and 3 parts by weight of polyvinylidene fluoride as a binder.
A positive electrode mixture was prepared by mixing parts by weight. This positive electrode mixture is dispersed in a solvent (using N-methyl-2-pyrrolidone) of the same weight as the positive electrode mixture to form a slurry (paste).
I made it.

Next, this positive electrode mixture slurry was coated to a thickness of 20 mm.
After uniformly coating both sides of the positive electrode current collector 10, which is a μm band-shaped aluminum foil, the solvent in the positive electrode mixture slurry is dried, and after this drying, compression molding is performed using a roller press machine to form the positive electrode current collector 10. A strip-shaped positive electrode 2 having positive electrode mixture layers 2a on both sides was obtained.

The film thickness of the positive electrode mixture layer 2a after molding is 80 μm on both sides, which is the same, and the width of the strip-shaped positive electrode 2 is 39 μm.
．． 5 mm, and the length was 230 mm.

Using the strip-shaped negative electrode 1 produced as described above, the strip-shaped positive electrode 2, and a pair of strip-shaped separators 3a and 3b made of microporous polypropylene film with a thickness of 25 μm and a width of 44 mm, Negative electrode 1, separator 3a,
The positive electrode 2 and the separator 3b are stacked in four layers in this order, and these four layers are stacked in this order.
A wound electrode body 15 was produced by spirally winding a laminated electrode body having a layered structure many times along its length with the negative electrode 1 facing inside. At this time, the final winding end of the wound electrode body 15 was fixed with an adhesive tape. In the process of manufacturing the wound electrode body as described above, the wound electrode body 15 was successfully manufactured without any peeling of the negative electrode mixture layer 1a from the negative electrode 1.

The inner diameter of the hollow portion at the center of this wound electrode body 15 was 3.5 mm, and the outer diameter was 13.9 mm. Note that the winding core 33 is located in this hollow portion.

The 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 current from the negative electrode 1 and the positive electrode 2, a nickel negative electrode lead 11 is attached to the negative electrode current collector 9 in advance, led out from the negative electrode 1, and welded to the bottom of the battery can 5. In addition, an aluminum positive electrode lead 12 was attached to the positive electrode current collector 10 in advance, led out from the positive electrode 2, and welded to the protrusion 34a of the metal safety valve 34.

Thereafter, a non-aqueous electrolyte in which lithium salt LiPF6 was dissolved at a ratio of 1 mol/l in an equal volume mixed solvent of propylene carbonate and 1,2-dimethoxyethane was injected into the battery can 5. The wound electrode body 15 was impregnated.

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

After that, the battery can 5, the safety valve 34 whose outer peripheries are in close contact with each other, and the metal battery lid 7 are each caulked through an insulating sealing gasket 6 whose surface is coated with asphalt. 5 was sealed. As a result, the battery lid 7 and the safety valve 34 were fixed, and the airtightness inside the battery can 5 was maintained. Further, at this time, the lower end of the gasket 6 in FIG. 1 comes into contact with the outer peripheral surface of the insulating plate 4a, so that the insulating plate 4a comes into close contact with the upper surface side of the wound electrode body 15.

[0052] As described above, a diameter of 14 mm and a height of 5
A 0 mm cylindrical non-aqueous electrolyte secondary battery was manufactured. As shown in Table 2 below, the battery of Example 1 was used as battery A for convenience.
shall be.

[0053] The cylindrical non-aqueous electrolyte secondary battery has the following characteristics:
In order to constitute a double safety device, a safety valve 34, a stripper 36, and an intermediate fitting body 35 made of an insulating material for integrating the safety valve 34 and the stripper 36 are provided. Although not shown, the safety valve 34 has this safety valve 3
A hole is provided in the battery lid 7 to form a cleavage portion that ruptures when the battery 4 is deformed.

In the unlikely event that the internal pressure of the battery rises for some reason, the safety valve 34 will move around its protrusion 34a as shown in FIG.
By deforming upward, the connection between the positive electrode lead 12 and the protrusion 34a is severed and the battery current is cut off, or the cleavage part of the safety valve 34 is ruptured and the gas generated in the battery is exhausted. They are each composed of .

[0055] Furthermore, as a solvent for preparing the negative electrode mixture slurry or positive electrode mixture slurry as described above, various solvents can be used as long as they can dissolve polyvinylidene fluoride used as a binder. be. in particular,
Ketones such as methyl ethyl ketone and cyclohexanone,
Esters such as methyl acetate and methyl acrylate, amides such as dimethylformamide, dimethylacenamide and N-methylpyrrolidone, amines such as diethyltriamine and N-N dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc. cyclic ethers etc. can be used.

Example 2 This example was the same as Example 1 except that the negative electrode mixture (containing 90% by weight of carbonaceous material and 10% by weight of PVDF) obtained in Experimental Example 3 above was used. A cylindrical nonaqueous electrolyte secondary battery was manufactured using the following methods. This battery is designated as battery B as shown in Table 2 below.

Example 3 This example was the same as Example 1 except that the negative electrode mixture (containing 85% by weight of carbonaceous material and 15% by weight of PVDF) obtained in Experimental Example 4 above was used. A cylindrical nonaqueous electrolyte secondary battery was manufactured using the following methods. This battery is designated as battery C as shown in Table 2 below.

Example 4 This example was the same as Example 1 except that the negative electrode mixture (containing 80% by weight of carbonaceous material and 20% by weight of PVDF) obtained in Experimental Example 5 above was used. A cylindrical nonaqueous electrolyte secondary battery was manufactured using the following methods. This battery is designated as Battery D as shown in Table 1 below.

Comparative Example As a comparative example for confirming the effects of the present invention, the following battery was manufactured. That is, a cylindrical nonaqueous electrolyte secondary was prepared in the same manner as in Example 1, except that the negative electrode mixture (containing 80% by weight of carbonaceous material and 25% by weight of PVDF) obtained in Experimental Example 6 above was used. A battery was created. This battery is designated as Battery E as shown in Table 1 below.

A wound electrode body was prepared in the same manner as in Example 1, except that the negative electrode mixture (containing 97% by weight of carbonaceous material and 3% by weight of PVDF) obtained in Experimental Example 1 above was used. However, the negative electrode mixture layer 1a peeled off at the negative electrode portion with a small radius of curvature near the start of winding, and the wound electrode body could not be completed. Moreover, the above-mentioned Examples 2 to 4
And in the comparative example, in the manufacturing process of the wound electrode body, the negative electrode 1
No peeling of the negative electrode mixture layer 1a was observed at all, and the wound electrode bodies 15 were successfully manufactured.

For the above five types of batteries A, B, C, D, and E, after setting the charging upper limit voltage to 4.1 V and constant current charging at 500 mA for 2 hours, the final voltage was set to 2 with a constant load of 18 Ω. Charge and discharge cycles were repeated to discharge to .75V. The capacity after 10 charge/discharge cycles was measured as the initial capacity, and the discharge capacity after 100 cycles was further measured. The ratio of the discharge capacity at 100 cycles to the initial capacity (capacity at 100 cycles/initial capacity) was defined as the capacity retention rate. The results are shown in Table 2 below.

[0062]

[Table 2]

As shown in Table 2 above, batteries A to D in which the content of polyvinylidene fluoride as a binder in the present example is 5 to 20% by weight are different from those in the comparative example in which the content is 25%. It can be seen that the discharge capacity is increased compared to Battery F, and the capacity retention rate is significantly improved. It is considered that no peeling or the like occurred in the negative electrode mixture layer 1a during charging and discharging of these batteries A to D.

From Tables 1 and 2, it is found that the content of polyvinylidene fluoride as a binder in the negative electrode mixture is suitably within the range of 5 to 20% by weight. Furthermore, it is possible to obtain a battery that does not cause peeling or cracking in the negative electrode mixture layer during use, has a high capacity, and has excellent charge/discharge cycle characteristics.

Although the battery of this example was a cylindrical nonaqueous electrolyte secondary battery using a spirally wound electrode body, the present invention is not limited to this. It may be cylindrical or the like, and can also be applied to button- or coin-shaped non-aqueous electrolyte secondary batteries.

[0066]

[Effects of the Invention] Since the present invention is configured as described above, it is possible to prevent peeling and cracking in the negative electrode mixture layer during battery manufacturing and use, thereby improving battery productivity and battery life. This also makes it possible to increase the capacity and prevent the capacity from decreasing as the charge/discharge cycle progresses. Therefore,
In addition to the conventionally known features of light weight and high energy density, it is possible to provide a non-aqueous electrolyte secondary battery that has high capacity and excellent charge/discharge cycle characteristics.

[Brief explanation of the drawing]

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 before manufacturing a wound electrode body in the battery shown in FIG. 1;

[Explanation of symbols]

1 Negative electrode 1a Negative electrode mixture layer 2 Positive electrode

Claims (1)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising: a negative electrode comprising a negative electrode mixture layer containing at least a carbon material as a negative electrode active material support and a binder; a positive electrode; and a non-aqueous electrolyte. The binder is polyvinylidene fluoride, and the content of the binder in the negative electrode mixture is 5% by weight.
A non-aqueous electrolyte secondary battery characterized in that the content is above and 20% by weight or less.