JPH04363865A - Nonaqueous solvent secondary battery - Google Patents

Abstract

PURPOSE:To provide a nonaqueous secondary battery having an excellent charge/discharge cycle life and a high capacity keeping rate. CONSTITUTION:In a nonaqueous solvent secondary battery having a generating element formed by integrally laminating a positive electrode body consisting of a carbonaceous material supporting lithium or an alkali metal mainly consisting of lithium and a positive electrode body having a composite oxide containing lithium as a positive electrode active material in this order, a lithium silicate is used as the binder of the negative electrode body.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous solvent secondary battery using a carbonaceous material as a negative electrode carrier, and more particularly to an improved negative electrode thereof.

0002

BACKGROUND OF THE INVENTION In recent years, with the development of electronic devices, there has been a demand for the development of secondary batteries that are small, lightweight, have high energy density, and can be repeatedly charged and discharged. This type of secondary battery is known to use lithium or lithium alloy as the negative electrode active material and oxides, sulfides, selenides, etc. of molybdenum, vanadium, titanium, niobium, etc. as the positive electrode active material. There is.

Recently, spinel type LiMn2 O4 and other lithium manganese oxides, which have improved cycle characteristics of manganese oxides having high energy density, have been actively studied.

On the other hand, for positive electrodes, electrode active materials that utilize the intercalation or doping phenomenon of layered compounds, which have a different reaction form from these manganese oxides, are attracting attention. Since these electrode active materials do not cause complex chemical reactions during charging and discharging reactions, they are expected to have extremely excellent charge-discharge cycle characteristics. Among them, a carbonaceous material is used as a negative electrode carrier and Li is used as a positive electrode active material.
CoO2 /LiNiO2, TiS2, MoS2
A battery system using this has been proposed.

[0005] In a battery system in which these oxides, sulfides, and selenides are used as positive electrode active materials and lithium is used as negative electrode active materials, the dissolution and precipitation reactions of lithium, which is the negative electrode active material, are repeated by repeating the cycle. , the problem eventually arises of the formation of acicular lithium dendrite precipitates on the lithium metal. Therefore, in battery systems, as the crystal structure of the positive electrode active material gradually progresses with each repeated charge and discharge, the formation of lithium dendrites on the negative electrode side and the decomposition reaction of the solvent caused by the lithium precipitated on the negative electrode cause the battery to collapse. Lifespan is specified, 500
It has been extremely difficult to manufacture batteries that have a lifespan longer than one cycle and reliability over a long period of time.

Further, in order to reduce lithium dendrites that occur at the negative electrode during charging, studies are underway to use alloys of lithium and metals such as aluminum, lead, and silicon as negative electrode active materials. However, even in this case, repeated charging and discharging causes the alloy to become powdered, and the battery life does not extend much. In order to avoid such problems, attempts have been made to create a secondary battery in which a negative electrode is formed by supporting lithium or an alkali metal mainly composed of lithium on a carbonaceous material obtained by firing various organic compounds. By using such a negative electrode, the precipitation of lithium dendrites is prevented and cycle characteristics are improved, and since metallic lithium is not used, safety is also improved.

However, when a carbonaceous material is used as the negative electrode active material and a metal chalcogen compound such as TiS2 or MoS2 is used as the positive electrode active material, the electromotive force is small (1.0
~1.2V). Therefore, as a positive electrode active material, 3.5V
LiCoO2, LiNiO exhibiting an average operating voltage of about
2, LiCox Ni(1-x)O2, etc. have been studied.

When these positive and negative electrode active materials are used as battery electrodes, they are usually coated with a conductive agent so that the reaction of the active materials can proceed effectively, and with a binder so that the electrode shape can be maintained. A mixture of agents is used. for example,
Cylindrical batteries use electrodes in which a band-shaped positive electrode, a separator, and a negative electrode are formed in a spiral shape. In this case, the strip-shaped electrode is made by depositing a slurry consisting of a mixture of an active material, a conductive agent, a binder, and water or an organic solvent on a substrate made of stainless steel, nickel, titanium, copper, aluminum, etc., or an alloy of these metals. It is created by coating, drying, and rolling. It is desirable that the slurry used at this time has no lumps and is uniformly dispersed.

[0009]

[Problems to be Solved by the Invention] However, batteries using carbonaceous materials as negative electrode carriers have a short charge/discharge cycle life, and have not been able to obtain a sufficient capacity retention rate. In other words, when polytetrafluoroethylene, which has traditionally been used as a binder for battery electrodes, is used, as the charge/discharge cycle progresses, lithium reacts with the binder, polytetrafluoroethylene, to form polytetrafluoroethylene. It decomposes and significantly reduces the binding ability of the negative electrode body. As a result, the conductivity between the current collector and the negative electrode carrier is impaired, and the internal resistance of the battery is significantly increased. Further, there were problems such as falling off of the negative electrode carrier and internal short circuit due to a decrease in binding ability. In addition, the conventionally used binder, which uses a ternary copolymer of ethylene-propylene-cyclic diene, has a binding form that covers the negative electrode carrier, which greatly reduces the internal resistance of the battery. However, it was not possible to obtain sufficient characteristics.

[0010] In addition, the negative electrode support is created by baking an organic compound or by a gas phase method, but the negative electrode support created in this way is active and when stored in an air atmosphere, the surface Functional groups (hydroxyl groups and carbonyl groups) harmful to battery reactions are formed. If these functional groups exist, the lithium metal reduced on the negative electrode carrier during charging will react with these functional groups, and this amount of lithium will not be used for the discharging reaction, resulting in a decrease in charging and discharging efficiency. This results in a decrease in the number of charge/discharge cycles.

[0011] The present invention has been made in order to improve these problems, and aims to provide a non-aqueous solvent secondary battery with high charge/discharge efficiency and excellent charge/discharge cycle life.

0012

[Means for Solving the Problems] That is, the non-aqueous solvent secondary battery of the present invention comprises a negative electrode body made of a carbonaceous material supporting lithium or an alkali metal mainly composed of lithium, a separator, and a lithium-containing composite oxide. A non-aqueous solvent secondary battery comprising a power generation element formed by integrally laminating in this order a positive electrode body having a positive electrode active material, characterized in that lithium silicate is used as a binder for the negative electrode body. do.

The lithium-containing composite oxide used in the present invention is generally synthesized by the following method. That is, lithium and carbonate, nitrate, sulfate, hydroxide, etc. of one or more transition metals selected from Co, Ni, Fe, or Mn are used as starting materials, and these are mixed in a stoichiometric ratio. It is obtained by baking. Note that carbonate is preferred as the starting material. The firing temperature varies somewhat depending on the starting materials, but is usually in the range of 600 to 1,000°C, preferably in the range of 600 to 800°C.

[0014] The carbonaceous material serving as the negative electrode carrier is preferably a carbonaceous material obtained by firing an organic compound in order to improve battery characteristics. The organic compound serving as a raw material for this carbonaceous material is not particularly limited as long as it is commonly used, and phenol resins, particularly novolac resins, polyacrylonitrile, and the like can be used. Moreover, as this carbonaceous material, it is particularly preferable to use a material having characteristics of a fired organic compound body as shown in Japanese Patent Application No. 1-283086.

The properties required of the substance used as a binder for the negative electrode are that it does not react with lithium, it does not react with the electrolyte, it does not deteriorate at the electrode drying temperature,
These include holding active material particles, not losing electrical conductivity, and maintaining flexibility as an electrode.

The lithium silicate according to the present invention satisfies the above-mentioned properties, and further has properties after drying that it is insoluble in water and organic solvents, is nonflammable, and can withstand temperatures of 1,000°C.
It has characteristics such as heat resistance up to 300 degrees and strong adhesive strength. The composition of lithium silicate is SiO2 /Li2
The O ratio is preferably 3.2 to 7.8 in terms of molar ratio. If the above molar ratio is within this range, all of the above properties are excellent and it can be used as the binder of the present invention.

[0017] Lithium silicate also includes carboxymethyl cellulose or its salt (hereinafter referred to as CMC),
It is water-soluble because it can be mixed with water-soluble polymers such as methylcellulose, polyacrylic acid, sodium polyacrylate, and polyvinyl alcohol, and with synthetic rubber latex such as styrene-butadiene rubber latex, acrylonitrile-butadiene rubber latex, and chloroprene latex. It is possible to make a slurry suitable for making the above-mentioned battery electrodes using a synthetic polymer, and furthermore, by using synthetic rubber latex, it is possible to make an electrode with excellent flexibility. When water-soluble polymers are used, the decomposition temperature of these substances is about 120 to 20
Since the temperature is 0° C., even if the electrode drying temperature is raised above the decomposition temperature of the water-soluble polymer used, the lithium silicate maintains its properties and can be used as an electrode.

The amount of binder contained in the electrode is preferably as small as possible as long as the electrode performance can be maintained. If the amount of the binder increases, the internal resistance of the negative electrode increases, which significantly reduces the battery's heavy load discharge capability, which is not preferable. However, when the amount of binder is extremely small, it becomes difficult to maintain the shape of the electrode, and the active material is likely to fall off, causing an internal short circuit. The amount of lithium silicate according to the present invention varies depending on the bulk density, particle size, and particle shape of the negative electrode active material, but is preferably about 1 to 10%.

[0019] As the electrolyte of the non-aqueous electrolyte used in the non-aqueous solvent secondary battery of the present invention, LiPF6, LiClO
4, LiBF4, LiCF3SO3, and other lithium salts. Examples of the solvent for the electrolytic solution include propylene carbonate, ethylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, and 1,2-dimethoxyethane. These solvents can be used singly or in a mixture of two or more, and from the viewpoint of prolonging the charge/discharge cycle life, mixed solvents of propylene carbonate and 1,2-dimethoxyethane, ethylene carbonate and 2-methyltetrahydrofuran are particularly preferred. A mixed solvent of ethylene carbonate and 1,2-dimethoxyethane, a mixed solvent of propylene carbonate and ethylene carbonate are desirable.

[0020]

[Effects of the Invention] By using lithium silicate as a binder in the negative electrode body, the secondary battery of the present invention prevents lithium from reacting with the binder, and as a result, even after repeated charging and discharging cycles. , an increase in battery internal resistance caused by impairing the conductivity between the current collector and the negative electrode carrier, drop-off of the negative electrode carrier due to a decrease in binding ability, and internal short circuits do not occur. Furthermore, since lithium silicate is compatible with water-soluble polymers and synthetic rubber latex, it is possible to use these water-soluble or water-dispersible polymers to create a slurry suitable for creating the battery electrodes mentioned above. Furthermore, using synthetic rubber latex, it becomes possible to create electrodes with excellent flexibility. Furthermore, when water-soluble polymers are used, the decomposition temperature of these substances is approximately 120°C to 2.
Since the temperature is around 00℃, lithium silicate maintains its properties even if the electrode drying temperature is raised above the decomposition temperature of the water-soluble polymer used. Can be disassembled and removed. Furthermore, functional groups on the surface of the negative electrode carrier, which have been a problem in the past, can also be removed. Therefore, a nonaqueous solvent secondary battery with improved charge/discharge efficiency and charge/discharge cycle life and stable battery performance can be obtained.

[0021]

EXAMPLES The present invention will now be explained in detail by way of examples and comparative examples with reference to the drawings. Note that the present invention is not limited to the examples.

Example Commercially available lithium carbonate and cobalt carbonate were weighed so that the molar ratio of Li and Co was Li/Co=1.10, and thoroughly mixed using a mortar. This mixture was placed in an alumina crucible and heat-treated at 800° C. for 6 hours in an electric furnace. After cooling the obtained baked product, it was ground again and similarly heat-treated at 800° C. for 6 hours. Thereafter, it was thoroughly washed with distilled water to wash away unreacted alkaline components. The product was confirmed to be LiCoO2 by powder X-ray method. 90% by weight of this product, 7% by weight of acetylene black as a conductive agent, and 3% by weight of an ethylene-propylene-cyclic diene terpolymer as a binder are kneaded in hexane to form a positive electrode mixture in the form of a slurry. was prepared. After applying this positive electrode mixture onto a 10 μm thick stainless steel substrate and air-drying it,
The mixture was pressurized to a constant thickness, and then a plate-shaped positive electrode having a positive electrode mixture layer with a thickness of 0.26 mm was manufactured.

On the other hand, the carbonaceous material that is the negative electrode carrier is prepared by baking a novolak resin at 950°C in a nitrogen atmosphere, then heating it to 2,000°C to carbonize it, and crushing it to obtain an average particle size of 10 μm. It was obtained by powdering.

Lithium silicate with a SiO2/Li2O molar ratio of 4.5 was used as a binder. Furthermore, CMC was used as a thickener to obtain an appropriate viscosity when applying the mixture containing the negative electrode carrier as a main component to the substrate. This CMC has a degree of etherification: 1, an average degree of polymerization: 1,00
0, average molecular weight: 50,000, and was neutralized to form an ammonium salt. These, the negative electrode carrier, the binder and the thickener were in a ratio of 95:3:2, and the negative electrode carrier 1
Distilled water of 80% to 00% was added and kneaded to create a negative electrode mixture slurry. This negative electrode mixture was applied onto a 20 μm thick nickel substrate and dried to produce a strip-shaped negative electrode having a 0.2 mm thick negative electrode mixture layer. After that, 200
Vacuum drying was performed at ℃ for 10 hours. The following battery preparation was performed in a glove box with a relative humidity of 1% or less.

Using the positive and negative electrodes thus obtained, a non-aqueous solvent secondary battery of AA size as shown in FIG. 1 was assembled. That is, the non-aqueous solvent secondary battery 1 is
It has a bottomed cylindrical stainless steel container 3 with an insulator 2 disposed at the bottom and serving also as a negative electrode terminal. This container 3 houses an electrode group 4. This electrode group 4 has a structure in which a band-like material in which a negative electrode 5, a separator 6, and a positive electrode 7 are laminated in this order is spirally wound so that the negative electrode 5 is located on the outside. The separator 6 is formed from a polypropylene porous film impregnated with an electrolytic solution. Each electrolytic solution was made by adding lithium hexafluorophosphate (LiPF6) as an electrolyte to a mixed solvent of propylene carbonate and 1,2-dimethoxyethane (volume ratio 50:50).
．． Contains 5 molar concentrations.

An insulating plate 8 with an opening at the center is arranged above the electrode group 4 in the container 3. Said container 3
An insulating sealing body 9 is hermetically caulked and fixed to the container 3 at the upper opening of the container 3 . In the central opening of this insulating plate 8,
The positive electrode terminal 10 is fitted. This positive electrode terminal 10 is
It is connected to the above-mentioned positive electrode 7 via a positive electrode lead 11. Note that the negative electrode 5 is connected to the container 3, which is a negative electrode terminal, via a negative electrode lead (not shown).

Comparative Example 1 A non-aqueous solvent secondary battery was assembled in the same manner as in Example except that polytetrafluoroethylene was used as the binder for the negative electrode.

Comparative Example 2 A terpolymer of ethylene-propylene-cyclic diene was used as the binder for the negative electrode, no thickener was used, and the drying conditions were 100%
A nonaqueous solvent secondary battery was assembled in the same manner as in the example except that it was vacuum dried at ℃ for 10 hours.

Comparative Example 3 A non-aqueous solvent secondary battery was assembled in the same manner as in Example except that the drying conditions for producing the negative electrode were vacuum drying at 100° C. for 10 hours.

Regarding the four types of non-aqueous solvent secondary batteries of Examples, Comparative Examples 1, 2, and 3 assembled in this manner, 2
Charging and discharging evaluation was performed in a voltage range from 4.3 V to 3.0 V at a constant temperature of 0° C. and a constant current of 100 mA. The results are shown in FIG. In FIG. 2, A is the discharge capacity retention curve of the battery of Example according to the present invention, B is the battery of Comparative Example 1, C is the battery of Comparative Example 2, and D is the discharge capacity retention curve of the battery of Comparative Example 3.

[0031] In the battery of Comparative Example 1, the discharge capacity decreased significantly as the charge/discharge cycle progressed. When we disassemble the battery that has been evaluated and observe the surface condition of the negative electrode, we find that
In the battery of Comparative Example 1, the negative electrode active material was peeled off from the substrate, and this is thought to be because lithium and polytetrafluoroethylene, which is a binder, reacted and the binder decomposed. In the electrode in such a state, electrical contact between the substrate and the negative electrode carrier particles is no longer established, and the amount of the negative electrode carrier that undergoes a battery reaction is reduced, resulting in a decrease in discharge capacity.

The battery of Comparative Example 2 has a low initial discharge capacity. This is due to the low drying temperature of 100°C. Comparative example 1
Although the decrease in discharge capacity is small compared to that of , the discharge capacity gradually decreases as the charge/discharge cycle progresses. When this battery was disassembled after the evaluation test, a considerable amount of lithium was deposited on the electrode surface. This is because a ternary copolymer of ethylene-propylene-cyclic diene was used as the negative electrode binder, which covered the surface of the negative electrode carrier, reducing the contact area between the negative electrode carrier and the electrolyte. It is thought that the negative electrode capacity decreased, and the negative electrode became below the lithium deposition potential, causing lithium to precipitate, resulting in a decrease in discharge capacity.

Furthermore, the battery of Comparative Example 3 had an extremely low first discharge capacity. This is because the lithium metal reduced on the negative electrode carrier during the first charge reacts with the functional groups remaining on the surface of the negative electrode carrier, so that lithium cannot be used as an active material during the next discharge. From the second cycle, the battery recovers to a capacity retention rate comparable to that of the battery of the example, but as the cycle progresses further, the difference in capacity retention rate between the two becomes larger. This is because the electrode drying temperature is below the decomposition temperature of CMC used as a thickener, so the water-soluble polymer remains in the electrode, increasing the internal resistance of the negative electrode and reducing the battery's heavy load discharge capacity. This is thought to be because it is decreasing.

[0034] In contrast to the batteries of these comparative examples, the batteries of the examples have good charging performance because the lithium silicate maintains its properties even when the electrode drying temperature is raised above the decomposition temperature of the water-soluble polymer used. Water-soluble polymers that are not related to discharge performance can be decomposed and removed, and the functional groups on the surface of the negative electrode carrier, which were a problem in the past, can also be removed, so there is no problem of low capacity at the first discharge, and even after that. It can be seen that a high discharge capacity is maintained. As described above, the non-aqueous solvent secondary battery of this example exhibits a high discharge capacity even after repeated charging and discharging cycles.

[Brief explanation of drawings]

FIG. 1 is a partial cross-sectional view showing a non-aqueous solvent secondary battery of an example.

FIG. 2 is a characteristic diagram showing changes in discharge capacity retention rate with respect to the number of charge/discharge cycles in non-aqueous solvent secondary batteries of Examples and Comparative Examples.

[Explanation of symbols]

1 Non-aqueous solvent secondary battery 2 Insulator 3 Stainless steel container 4 Electrode group 5 Negative electrode 6 Separator 7 Positive electrode 8 Insulating board 9 Insulating sealing board 10 Positive terminal 11 Positive electrode lead

Claims (2)

[Claims] [Claim 1] A negative electrode body made of a carbonaceous material supporting lithium or an alkali metal mainly composed of lithium, a separator, and a positive electrode body having a lithium-containing composite oxide as a positive electrode active material are integrated in this order. What is claimed is: 1. A non-aqueous solvent secondary battery comprising a power generation element laminated with a lithium silicate battery, characterized in that lithium silicate is used as a binder for the negative electrode body. 2. SiO2 of the lithium silicate
The nonaqueous solvent secondary battery according to claim 1, wherein the /Li2O ratio is 3.2 to 7.8 in terms of molar ratio.