JPH0997625A - Nonaqueous electrolytic secondary battery and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new nonaqueous electrolytic secondary battery with high voltage, high energy density, excellent charge/discharge characteristics, and long cycle life. SOLUTION: In a nonaqueous electrolytic secondary battery constituted with at least a positive electrode, a negative electrode both capable of absorbing/ releasing a lithium ion, and a lithium ion conductive nonaqueous electrolyte, a conductive layer using carbon as a conductive filler is arranged between the electrode mix of the positive electrode 13 and/or the negative electrode 23 and current collectors 11, 21. The internal resistance of the battery is decreased, drop in the capacity and working voltage in high rate discharge is suppressed, and the secondary battery with long cycle life can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery which uses a lithium ion conductive non-aqueous electrolyte as a positive electrode active material and a negative electrode active material which are capable of inserting and extracting lithium. The present invention relates to an electrode structure having high energy density, excellent high rate charge / discharge characteristics, and good long-term cycle characteristics.

[0002]

Non-aqueous electrolyte batteries using lithium as a negative electrode active material have advantages of high voltage, high energy density, small self-discharge and excellent long-term reliability. It has already been widely used as a power source for industrial use. However, in recent years, with the remarkable development of portable electronic devices, communication devices, and the like, a wide variety of devices that require a large current output for a battery as a power source have appeared, and from the viewpoint of economy and reduction in size and weight of the device,
There is a strong demand for rechargeable / dischargeable secondary batteries with high energy density. The battery using lithium as the negative electrode active material is likely to generate needle-shaped deposits during charging and eventually cause a short circuit inside the battery to cause an accident such as heat generation, rupture, or ignition.

Therefore, as the negative electrode active material, a material represented by a carbon material capable of inserting and extracting lithium ions is used.
Research and development to promote the non-aqueous electrolyte battery having a high energy density into a secondary battery has been actively carried out and partially put into practical use, but energy density, charge / discharge cycle life, reliability, etc. are still insufficient. .

[0004]

Generally, in the above non-aqueous electrolyte secondary battery having a high energy density, the performance at the time of discharging a large current is particularly important. During large current discharge, the voltage drop due to the internal resistance of the battery is greater than during minute current discharge. Generally, in order to minimize this voltage drop, the surface area involved in the electrode reaction is increased,
Measures have been taken to reduce the current density per unit area. Specific examples include a structure in which a flat plate-like elongated electrode is wound around, or strip-shaped electrodes are stacked. In such an electrode, a mixture layer is provided on both sides of a thin current collector such as a metal foil, and the mixture layer is pressure-bonded to the current collector by roll pressing or the like. However, it is still insufficient to be used as a main power source for portable electronic devices.

As a problem of the secondary battery as described above, there is a decrease in capacity during discharge of a large current with respect to a small current discharge capacity, a decrease in discharge voltage during discharge of a large current, and a decrease in discharge voltage / capacity during progress of charging / discharging cycle. And so on. However, a large impedance between the mixture layer and the current collector is an important cause.
It is required to improve the electronic conductivity between the mixture layer and the current collector. Moreover, since the electrodes (active material) expand and contract due to the absorption and desorption of lithium ions during charge and discharge, the adhesion between the mixture layer and the current collector decreases and the impedance increases as the charge and discharge cycle progresses. The discharge voltage drops as shown in (1). The decrease in discharge voltage also causes a decrease in capacity until a certain discharge end voltage is reached. Further, the decrease in capacity is also caused by the fact that the amount of inactive lithium increases with the progress of the cycle, so that the amount of lithium substantially involved in the charge / discharge reaction decreases.

The inactive lithium refers to lithium deposited on the negative electrode current collector during charging or the like, which is a fine powder and is electrochemically inactive. In order to reduce the amount of inactive lithium, there is a method disclosed in, for example, JP-A-6-163030. Carbon as a negative electrode active material is sized,
A carbon fine particle layer of a first layer is coated on a current collector of carbon having a minute particle size, and a carbon particle layer of a second layer having a large particle size is further provided thereon. The deposition of lithium on the current collector is suppressed by coating the current collector with the powder of the minute active material substantially.
However, in the present invention, since the binder for the first and second carbon particle layers and the solvent for forming the slurry are the same, when applying the second carbon particle layer, the solvent causes the first carbon particles to become fine. The major problem is that the binder of the layer is softened and dissolved again, the first carbon fine particle layer is disturbed, the surface of the current collector is exposed, and the second carbon particle layer is directly placed on the current collector. Is.

[0007]

In order to solve the above problems, the present invention provides a non-aqueous electrolyte comprising a positive electrode and a negative electrode capable of absorbing and releasing lithium ions, and a lithium ion conductive non-aqueous electrolyte. The water electrolyte secondary battery is characterized in that a conductive layer containing carbon as a conductive filler is provided between the electrode mixture of the positive electrode and / or the negative electrode and the current collector.

The condition required for the conductive layer is good adhesion to the current collector and the mixture layer. Low impedance. It is insoluble or hardly soluble in the mixture slurry.

It is stable to an electrolyte. Electrochemical reactivity related to charging and discharging with lithium ion is low. And so on. The conductive layer is mainly composed of carbon, which is an electronic conductive medium, and a binder. As the carbon, it is possible to use natural or artificial crystalline graphite, various carbon blacks such as channel black, thermal black and furnace black, and electronically conductive carbon such as glassy carbon. In particular, carbon black and graphite, which are less likely to absorb and release lithium ions, are preferable because they are less likely to expand and contract due to charge and discharge cycles. The shape is fine particles,
Powders such as short fibers are preferable, and an average particle diameter of 5 μm or less is particularly preferable. Further, in order to improve conductivity, it is more preferable to add carbon powder having an average particle diameter of 1 μm or less in the range of 10% or less based on the total weight of carbon powder. In particular, it is preferable to use carbon black such as graphite or acetylene black in combination because good electrical connection to the current collector can be obtained.

The binder functions to obtain the adhesion between the carbon powders in the conductive layer or the current collector and the mixture layer, and a polymer generally used as an adhesive is used. You can Here, as a method of disposing the mixture layer, a method of applying the mixture slurry on the current collector is the mainstream. Therefore, in the present invention in which the conductive layer is provided on the current collector in advance and the mixture slurry is applied thereto, the binder of the conductive layer is dissolved in the solvent of the mixture slurry or is a substance which easily swells. If so, the conductive layer is disturbed and its effect is halved. In particular, N-methyl-2, which is a solvent for polyvinylidene fluoride (PVDF), which is often used as a binder for a mixture.
-Pyrrolidone has high solubility, and selection of a binder for the conductive layer is very important.

The resin used as the binder is thermodynamically thermosetting / thermoplastic / rubber-based, in the form of solvent-necessary / solvent-free / water-soluble / emulsion-based, pressure-sensitive adhesive / film-like, etc. are categorized. Of these, the polymers used as thermosetting resins are often dissolved in an organic solvent in the form of monomers or oligomers when they are applied or coated, and they are highly polymerized by thermosetting (thermal polymerization). Therefore, the adhesion to the current collector and the resistance to organic solvents are greatly improved. Melamine resin, urea resin, phenol resin, epoxy resin, etc., which are typical thermosetting polymers, are very stable against NMP shown above after thermosetting, so that the conductive layer of the present invention can be used. Suitable as a binder. Among them, phenol resin and epoxy resin, which have good adhesion to metal, are very effective. In particular, since it has high adhesion to a copper foil used as a negative electrode current collector, it is excellent as a binder for a negative electrode conductive layer.

It is also possible to use a polymer which is cured by irradiation with ultraviolet rays or electron beams. Examples include acrylic resins and methacrylic resins. These polymers also form highly grown polymers upon curing,
Its properties are similar to thermosetting resins. It is possible to accelerate the curing reaction by adding a polymerization initiator (sensitizer) such as benzoin, and it can be cured in a shorter time than thermosetting resins. Also, the monomers and prepolymers before curing are liquid. There is an advantage that no solvent is required.

The water-soluble adhesive can obtain the adhesive force of the polymer by a simple operation of only volatilizing the solvent water. Since the water-soluble polymer uses an organic solvent, it has little adverse effect on the environment and the human body. Water-soluble polymers include PVA-based / maleic anhydride-based / polyacrylic acid-based
There are polyacrylamide type. Of these, acrylic acid polymers are preferable because they have high adhesion to the metal current collector. In particular, it has good adhesion to a metal such as aluminum or stainless steel used as a positive electrode current collector, and is excellent as a binder for the positive electrode conductive layer.

In the case of a 4V class lithium ion battery, the positive electrode active
LiCoO as a substance₂, LiNiO₂, LiMn₂O _FourTransition represented by
A composite oxide of transfer metal and lithium is often used. this
Of which LiNiO₂Such as when using an active material that is unstable to water
In the case of a mixture, a water-soluble polymer or PTF is used as a binder for the mixture layer
It is not preferable to use an aqueous emulsion such as E
However, it can be used as a binder for the negative electrode mixture.
You. Binder with a mixture of water-soluble polymer and water-based emulsion
When used as a binder, a water-soluble binder is used as a binder for the conductive layer.
Adhesives are not preferred.

As described above, the binder for the conductive layer of the present invention must be selected in consideration of the adhesion to the current collector and the carbon powder and the combination with the solvent of the mixture slurry. It is also effective to add a stabilizer or a surfactant to the conductive layer slurry which is composed of at least carbon powder and a binder and to which a solvent is added if necessary.

Means for disposing the conductive layer include a method in which the conductive layer slurry is applied on a current collector, dried, and optionally cured, or a carbon powder mixed and dispersed in a binder is formed into a film. Examples thereof include a method of bringing the processed conductive film into close contact with the current collector by a method such as heat fusion. However, the present invention is not limited to this.

The positive electrode active material used in the present invention includes:
Metal chalcogenides such as TiS ₂ , MoS ₂ and NbSe ₃ , as well as MnO ₂ and M
Metal oxides such as oO ₃ , V ₂ O ₅ , Li _x CoO ₂ , Li _x NiO ₂ , and Li _x Mn ₂ O ₄ , conductive polymers such as polyaniline, polypyrrole, polyparaphenylene, polyacene, and graphite Various substances capable of inserting and extracting lithium ions and / or anions such as intercalation compounds can be used.

In particular, the electrode potential for metal lithium such as metal chalcogenide and metal oxide is 2 V or more, more preferably V ₂ O ₅ , MnO ₂ , Li _x CoO ₂ , Li _x NiO ₂ , Li _x Mn ₂ O ₄
Combining a (noble) active material having a high potential of 3 V to 4 V or more and a negative electrode using a (base) active material having a low potential of 1 V or less with respect to metallic lithium as described later. Thereby, a secondary battery having a high energy density can be obtained, which is more preferable.

As the negative electrode active material, metallic lithium, carbonaceous material, Li _x Si, metal oxide, nitride, silicide, carbide, Li _x Si _1-y M _y O _z (0 ≦ x, 0 ≦ y <1,0 <z <2, M is a metal other than alkali metal or a metal other than silicon), and various substances capable of occluding and releasing lithium ions and / or anions such as silicon oxides You can

Particularly, Li _x Si _1-y M _y O _z (0 ≦ x, 0 ≦ y <1,0 <z <2, M is a metal except alkali metal or a metal except silicon) Silicon oxides and the like have a large charge / discharge capacity in the region where the electrode potential with respect to metallic lithium is 1 V or less. Therefore, by combining with a positive electrode using the above positive electrode active material, a secondary battery with high voltage and high energy density can be obtained. Is more preferable because

As the electrolyte, γ-butyrolactone, propylene carbonate, ethylene carbonate (E
C), butylene carbonate, dimethyl carbonate,
Diethyl carbonate, methyl formate, 1,2-
LiClO ₄ , LiP as a supporting electrolyte in a single or mixed nonaqueous organic solvent such as dimethoxyethane, tetrahydrofuran, dioxolan, dimethylformamide, etc.
An organic non-aqueous electrolyte in which lithium ion dissociable salts such as F ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ and LiN (SO ₂ CF ₃ ) ₂ are dissolved,
It is preferable to use a polymer solid electrolyte in which the lithium salt is solid-solved in a polymer such as polyethylene oxide or a crosslinked polyphosphazene, or a lithium ion conductive non-aqueous electrolyte such as an inorganic solid electrolyte such as Li ₃ N or LiI. it can.

In particular, the above-mentioned Li _x Si _{1-y is} used as the negative electrode active material.
When using a silicon oxide represented by M _y O _z (0 ≦ x, 0 ≦ y <1,0 <z <2, M is a metal except alkali metal or a metal except silicon), dimethyl is used. Carbonate, diethyl carbonate, ethyl methyl carbonate, etc.
It is preferable to use a mixed solvent of RR'-type alkyl carbonate represented by and EC. Further, it is more preferable that the volume mixing ratio of EC and RR'-type alkyl carbonate is in the range of about 3: 1 to about 1: 3.

[0023]

The present invention is a non-aqueous electrolyte secondary battery comprising at least a positive electrode and a negative electrode capable of occluding and releasing lithium ions, and a lithium ion conductive non-aqueous electrolyte, and an electrode mixture of the positive electrode and / or the negative electrode. By providing a conductive layer containing carbon as a conductive filler between the current collector and the current collector, a decrease in operating voltage and capacity during large current discharge and a decrease in operating voltage and capacity due to repeated charging and discharging are suppressed. By providing the conductive layer, the electronic conductivity between the mixture layer and the current collector can be improved, and the internal resistance of the battery is significantly reduced. As a result, it is possible to suppress a decrease in operating voltage and a decrease in capacity during large current discharge. Furthermore, by coating the current collector with a conductive layer, it is possible to prevent the above-mentioned deposition of inert lithium on the current collector, and to minimize the capacity decrease during the cycle. There is.

Further, when the cycle progresses, the mixture layer is gradually peeled off from the current collector, the internal resistance of the battery is increased, and the operating voltage and the capacity are lowered. This is because the active material expands and contracts as lithium ions are occluded and released in the active material by charging and discharging, and thus the adhesive force between the mixture and the current collector decreases. Therefore, as in the present invention, by providing a conductive layer between the current collector and the mixture layer, the conductive layer functions as an adhesive between the current collector and the mixture layer, so that even when a long-term cycle proceeds. Peeling of the mixture layer can be suppressed.

By providing the conductive layer of the present invention as described above, a highly reliable non-aqueous electrolyte secondary battery with high energy density can be realized. The present invention is described in detail below with reference to examples.

[0026]

【Example】

(Example 1) A prismatic battery was manufactured as shown below and the charge / discharge characteristics were measured. LiB _0.03 Co _0.97 O ₂ composite oxide of lithium, cobalt, and boron 85
By mixing and mixing 8 parts by weight of graphite, which is a conductive agent, with 8 parts by weight of a conductive agent in a mortar, polyvinylidene fluoride (P
VDF) 7 parts by weight N-methyl-2pyrrolidone (NM
P) A positive electrode mixture slurry was prepared by mixing and dispersing it in a solution dissolved in 51.3 parts by weight. An aluminum foil having a thickness of 20 μm was used as a current collector, and a conductive adhesive obtained by dispersing carbon powder in an aqueous solution of an acrylic acid polymer had a thickness after drying of 10 μm.
m and a density of about 0.5 g / cm ³ were applied and dried on both sides of the aluminum foil to form a conductive layer. Next, apply the positive electrode slurry prepared above to both sides of the current collector provided with the conductive layer so that the mixture density after drying and rolling is 3.3 g / cm ³ and the mixture thickness on one side is 60 μm. -It dried and was rolled using the roll press and the positive electrode sheet was produced. The positive electrode sheet thus produced was cut into a size of 27.5 mm × 39 mm to obtain a positive electrode plate. A cross-sectional view of the positive electrode plate is shown in FIG.

A negative electrode was prepared in the same manner. 45 parts by weight of commercially available silicon monoxide (SiO) as a negative electrode active material and 40 parts by weight of graphite as a conductive agent were crushed and mixed in a mortar, and 15 parts by weight of a crosslinkable acrylic resin as a binder was added to 300 parts by weight of water. Was mixed and dispersed in a solution dissolved in a part to prepare a negative electrode mixture slurry. A copper foil having a thickness of 10 μm was used as a current collector, and a conductive adhesive obtained by dispersing carbon powder in a solution of phenol resin ethoxyethyl acetate was dried to a thickness of 10 μm.
m, density of about 0.45 g / cm ³ , coating and drying on both sides of copper foil.
It was cured to form a conductive layer. The negative electrode slurry prepared above is applied to both sides of the current collector provided with the conductive layer so that the mixture density after drying and rolling is 1.6 g / cm ³ and the mixture thickness on one side is 27 μm. After drying, rolling was performed using a roll press. The negative electrode sheet thus manufactured was cut into a size of 27.5 × 39 mm to obtain a negative electrode plate.

Seventeen positive electrode plates and eighteen negative electrode plates were alternately laminated with a separator, which is a porous film permeable to lithium ions, interposed therebetween (the outermost side is a negative electrode coated with a mixture on only one side) and made of stainless steel. The battery was inserted into the battery case, the lead was taken, the electrolytic solution was injected, and the battery was sealed to manufacture a prismatic battery.

The battery thus produced was subjected to three charge / discharge cycles under the conditions of a constant current of 20 mA, an end-of-charge voltage of 4.2 V and an end-of-discharge voltage of 2.7 V. The discharge characteristic of the second cycle is shown in FIG. Next, charging voltage 4.2V,
Charge at constant current and constant voltage for 5 hours at maximum charging current of 200mA,
A charging / discharging cycle was performed under the conditions of a discharge current of 400 mA and a discharge end voltage of 2.7 V. The discharge characteristics of the second cycle are also shown in FIG.

Example 2 A prismatic battery was prepared in the same manner as in Example 1, except that the mixture slurry was directly applied to the current collector without providing a conductive layer on the current collector for both the positive electrode and the negative electrode. Then, the same charge / discharge cycle was performed. The discharge characteristics obtained as a result are shown in FIG.

As is apparent from FIGS. 3 and 4, the provision of the conductive layer of the present invention markedly increases the capacity and further suppresses the decrease in the discharge capacity during high rate discharge relative to the discharge capacity during low rate discharge. Good results have been obtained. This reflected that the contact resistance between the electrode mixture and the current collector was reduced by providing the conductive layer between the current collector and the current mixture layer, and the internal resistance of the battery was reduced. The result. Furthermore, when the charging / discharging cycle was performed 300 times under the above-mentioned charging / discharging conditions, it was confirmed that deterioration of the battery could be suppressed also by providing the conductive layer in terms of cycle characteristics.

The above embodiments merely show one example of the present invention, and the present invention is not limited to this. Especially,
The shape of the battery may be cylindrical or coin-shaped, and the conductive layer,
There are various methods such as laminating the mixture layers processed into sheets.

[0033]

INDUSTRIAL APPLICABILITY As described above, the present invention provides a positive electrode and / or a positive electrode and / or a negative electrode capable of inserting and extracting lithium ions, and a non-aqueous electrolyte secondary battery comprising at least a lithium ion conductive non-aqueous electrolyte. Since the conductive layer containing carbon as a filler is provided between the electrode mixture of the negative electrode and the current collector, the internal resistance of the battery can be reduced, and the decrease in capacity and operating voltage during large current discharge can be suppressed.

Deterioration due to the deposition of inactive lithium on the current collector can be suppressed, which is extremely stable and the cycle life is improved. And so on. As a result, a high-quality secondary battery having high energy density and good cycle characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of the structure of a positive electrode plate implemented in the present invention.

FIG. 2 is a cross-sectional view showing an example of the structure of a negative electrode plate implemented in the present invention.

FIG. 3 is an explanatory diagram comparing the discharge characteristics of the prismatic batteries manufactured in Example 1 carried out in the present invention.

FIG. 4 is an explanatory diagram comparing the discharge characteristics of the prismatic batteries manufactured in Example 2 carried out in the present invention.

[Explanation of symbols]

 11 Positive Electrode Current Collector 12 Positive Electrode Conductive Layer 13 Positive Electrode Mixture Layer 21 Negative Electrode Current Collector 22 Negative Electroconductive Layer 23 Negative Electrode Mixture Layer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location H01M 4/66 H01M 4/66 A (72) Inventor Seiji Yahagi 1-chome Nakase, Mihama-ku, Chiba-shi, Chiba 8 Seiko Electronic Industry Co., Ltd. (72) Inventor Tsugio Sakai 1-8 Nakase Nakase, Mihama-ku, Chiba Chiba Electronics Seiko Electronics Co., Ltd.

Claims (11)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising at least a positive electrode and a negative electrode capable of inserting and extracting lithium ions, and a lithium ion conductive non-aqueous electrolyte, and an electrode mixture of the positive electrode and / or the negative electrode. A non-aqueous electrolyte secondary battery, characterized in that a conductive layer containing carbon as a conductive filler is disposed between the battery and the electric body. 2. The conductive layer is composed of at least carbon and a resin as a binder, and the resin is insoluble or sparingly soluble in a mixture slurry prepared by adding a solvent to the mixture and kneading. The non-aqueous electrolyte secondary battery according to claim 1. 3. The binder for the conductive layer is at least one resin selected from acrylic acid polymers, phenol resins and epoxy resins, and carbon is graphite and / or carbon black powder, particles and short fibers. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the non-aqueous electrolyte secondary battery is one or more selected from the above. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the binder of the positive electrode conductive layer is an acrylic acid polymer. 5. The positive electrode mixture is composed of at least a positive electrode active material, a conductive agent, and a binder, and the positive electrode active material is a lithium-containing composite transition metal oxide. The non-aqueous electrolyte secondary battery described in 2, 3, and 4. 6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the current collector of the positive electrode is aluminum, aluminum alloy, titanium or stainless steel. 7. The non-aqueous electrolyte secondary battery according to claim 1, wherein the binder of the negative electrode conductive layer is at least one selected from phenol resins and epoxy resins. . 8. The negative electrode mixture is composed of at least a negative electrode active material and a binder, and the negative electrode active material is a carbonaceous material and / or an oxide of silicon. , 3 and 7 non-aqueous electrolyte secondary battery. 9. The non-aqueous electrolyte secondary battery according to claim 1, wherein the current collector of the negative electrode is copper or a copper alloy. 10. The non-aqueous electrolyte contains at least one selected from propylene carbonate and ethylene carbonate, and an R · R′-type alkyl carbonate represented by Chemical formula 1. ~ 9 non-aqueous electrolyte secondary battery. Embedded image 11. A step of disposing a conductive layer on the current collector,
A step of mixing and kneading at least an electrode active material, a binder, a solvent and, if necessary, a conductive agent to prepare a mixture slurry, and a step of disposing the mixture on the conductive layer provided on the current collector. A method for producing a non-aqueous electrolyte secondary battery, characterized by comprising at least: