JPH11297327A - Lithium secondary battery - Google Patents

Abstract

(57) [Problem] To provide a lithium secondary battery using a polymer electrolyte, having a high potential, a high energy density, and excellent cycle characteristics. SOLUTION: This is a lithium secondary battery provided with a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode has a current collector containing an active material layer containing a compound capable of inserting and extracting lithium ions, and the negative electrode has a current collector containing an active material layer containing graphite. The positive / negative electrode active material layer and the electrolyte layer are composed of a gel polymer holding an electrolyte, and the negative electrode active material layer is provided with an electronic conductive material in an amount of 1 to 100 parts by weight of the active material. A lithium secondary battery containing 15 parts by weight.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery. More specifically, the present invention relates to a lithium secondary battery using a gel polymer electrolyte instead of an electrolytic solution, and to a lithium secondary battery having high potential, high energy density and excellent cycle characteristics.

[0002]

2. Description of the Related Art In recent years, various devices such as a camera-integrated VTR device, an audio device, a portable computer, and a cellular phone have been reduced in size and weight, and there has been a demand for higher performance of a battery as a power supply for these devices. Is growing. Above all, development of lithium secondary batteries, which have a high voltage and a high energy density as power sources for electric vehicles and are capable of realizing excellent cycle characteristics, has been actively pursued.

A lithium secondary battery comprises a positive electrode capable of inserting and extracting lithium ions, a negative electrode, and a non-aqueous electrolyte solution.
Conventionally, non-aqueous electrolytes have been used as electrolytes for these high-voltage batteries. However, batteries using non-aqueous electrolytes have a risk of filtrate and ignition, and in recent years, in order to improve safety, the development of gel electrolytes containing non-aqueous electrolytes in polymers has been developed. Is being done. In particular, in a secondary battery using lithium metal, heat generation and ignition caused by an internal short circuit due to lithium dendrite deposition that occurs when a liquid electrolyte is used poses a problem, and application of a polymer electrolyte has been desired.

Further, unlike the conventional lithium secondary battery, the above-mentioned polymer electrolyte or the like containing an electrolytic solution in a gel polymer can be used instead of a separator used in this secondary battery system. Therefore, the positive electrode and the negative electrode can be joined and used with the polymer electrolyte interposed therebetween. Since such a polymer is lighter in weight and has shape flexibility as compared with a liquid system, it can be formed into a thin film such as a sheet, for example, and has an advantage that a lightweight and space-saving battery can be produced.

[0005]

Generally, a positive electrode or a negative electrode in a lithium secondary battery is formed by forming a positive electrode active material or a negative electrode active material, a conductive material, and a binder on a current collector such as an aluminum foil or a copper foil. It is manufactured by applying a paint containing resin and the like. Here, in the charging / discharging process of the lithium secondary battery, electrons are exchanged between (the active material in) the lithium ion moving through the electrolyte and the electrode, and thus the electrode of the battery has ionic conductivity and electron conductivity. It is necessary to combine

[0006] Since such a polymer electrolyte is mainly carried out through a non-aqueous electrolyte phase in the electrolyte, the properties such as the decomposition voltage and the like largely depend on the non-aqueous electrolyte. Since the contained polymer matrix maintains the mechanical strength, the fluidity is extremely low and the ionic conductivity is reduced. In particular, in the positive electrode or negative electrode active material layer, the ionic conductivity decreases as the filling amount of the active material increases.

On the other hand, during the charging and discharging process, the active material expands and contracts due to insertion and extraction of lithium ions. In particular, the negative electrode active material repeatedly expands and contracts by about 10% as an interlayer distance when inserting and extracting lithium ions in the charge and discharge process.
When a polymer electrolyte such as the above gel type is used, since the electrolyte is less likely to follow the expansion and contraction of the active material in the electrode than the liquid type, the bonding surface with the active material in the electrode material is small and ionic conduction and electron conduction are low. There is a problem that it cannot be performed sufficiently.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and as a result of intensive studies to obtain a lithium secondary battery having a high potential, a high energy density and excellent cycle characteristics, it was found that By improving the electron conductivity in the active material layer, the battery characteristics were improved, and it was found that a lithium secondary battery with less deterioration of the electrode material, excellent rate characteristics, and excellent cycle characteristics could be obtained. Things.

More specifically, an electron conductive material is added to the negative electrode active material layer in order to prevent the deterioration of the electron conduction accompanying the expansion and contraction of the negative electrode active material during the charging and discharging process of the lithium secondary battery according to the present invention. As a result, it is found that the characteristics aimed at by the present invention are improved, and the cycle characteristics can be improved at a high potential and a high energy density.

The gist of the present invention is a lithium secondary battery provided with a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises an active material layer containing a compound capable of inserting and extracting lithium ions, and the negative electrode comprises an active material layer containing graphite. Are respectively provided on the current collector, the positive / negative electrode active material layer and the electrolyte layer are composed of a gel polymer holding an electrolytic solution, and the negative electrode active material layer contains 1 to 5% by volume of an electron conductive material. Lithium secondary batteries.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium secondary battery of the present invention mainly comprises a positive electrode, a negative electrode and a polymer electrolyte. First, the polymer electrolyte used in the present invention will be described. Generally, a polymer electrolyte containing an electrolyte solution in a gel polymer (hereinafter, this may be simply referred to as a polymer electrolyte) is used as the polymer electrolyte. A non-aqueous electrolytic solution is preferably used as the electrolytic solution to be contained in the gel polymer. In general, a solution obtained by dissolving an electrolyte in a non-aqueous solvent is used.

The electrolyte used for the polymer electrolyte includes:
As the electrolyte, any non-aqueous substance that is stable with respect to the positive electrode active material and the negative electrode active material and that can transfer lithium ions to perform an electrochemical reaction with the positive electrode active material or the negative electrode active material can be used. can do. LiPF ₆ in particular, LiAsF _6, LiSbF _6, L
iBF ₄ , LiClO ₄ , LiI, LiBr, LiC
1, LiAlCl, LiHF ₂ , LiSCN, LiSO
₃ CF _{2 and the} like. Among these, LiP
F ₆ and LiClO ₄ are preferred.

The content of these electrolytes in the electrolyte is as follows:
Generally, it is 0.5 to 2.5 mol / l. The solvent for dissolving the polymer electrolyte is not particularly limited, but a solvent having a relatively high dielectric constant is preferably used. Specifically, cyclic carbonates such as ethylene carbonate and propylene carbonate, acyclic carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, glymes such as tetrahydrofuran, 2-methyltetrahydrofuran and dimethoxyethane, γ-butyl lactone and the like Lactones, sulfur compounds such as sulfolane, and nitriles such as acetonitrile. Of these,
Particularly, a mixed solution of one or more selected from cyclic carbonates such as ethylene carbonate and propylene carbonate, and non-cyclic carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate is preferable.

The above-mentioned electrolyte solution is impregnated with a polymer such as polyethylene oxide, polypropylene oxide, a crosslinked isocyanate of polyethylene oxide, phenylene oxide, or a phenylene sulfide-based polymer to prepare a gel electrolyte.

Next, the electrodes will be described. Generally, a positive electrode or a negative electrode in a lithium secondary battery is obtained by applying a paint containing a positive electrode active material or a negative electrode active material, a binder resin (binder), an electrolyte, a solvent, etc. on a current collector such as an aluminum foil or a copper foil. Manufacturing. In the positive electrode and the negative electrode used in the present invention, the voids in the active material layer are filled with a gel electrolyte, and ion conduction of lithium ions moves to the electrolyte layer through the gel electrolyte. As a result, the nonaqueous electrolytic solution of all of the positive electrode, the negative electrode and the electrolyte layer becomes gel-like, and a safe lithium secondary battery without liquid leakage can be obtained. As the negative electrode active material used for the negative electrode, graphite particles that have a large expansion and contraction of the active material during the charge / discharge process are effective. The particle size of the active material of these negative electrodes may be appropriately selected in consideration of the other components of the battery.
m, especially 15 to 30 μm, the initial efficiency and the laser
This is preferable because battery characteristics such as battery characteristics and cycle characteristics are improved.

The volume fraction of graphite in the negative electrode active material layer is 30 to 50 vol%, and more preferably 35 to 45 v
ol% is desirable. If the graphite volume fraction is less than 30 vol%, the battery capacity is low, and 50 vol.
If it exceeds 1%, the fraction of the gelled ionic conductor is too low, and the ionic conductivity in the negative electrode active material layer is reduced, so that charging and discharging at a high rate cannot be performed. Further, the negative electrode of the present invention has an electron conductive material in the negative electrode active material layer of 1 to 15 parts by weight based on 100 parts by weight of the active material.
The feature is that it is contained in parts by weight. In other words, by adding the electron conductive material, it is possible to prevent the deterioration of the electron conduction due to the expansion and contraction of the negative electrode active material during the charge and discharge process of the lithium secondary battery, and to improve the cycle characteristics at a high potential and a high energy density. Can be done.

It is desirable that the added electron conductive material be as small as possible to ensure electron conductivity. The electron conductive material is not particularly limited as long as it is capable of imparting conductivity by being mixed in an appropriate amount with the negative electrode active material. Usually, carbon powder such as carbon black and graphite is used, and carbon black is particularly preferable. As the carbon black, those having a developed structure are preferable, and the DBP oil absorption is 10%.
0 cc / 100 g or more, more preferably 120 cc / 100 g
100 g or more, especially 150 cc / 100 g or more is preferable because it holds the electrolyte. Acetylene black is a particularly preferred carbon powder as the electron conductive material.

The amount of the electron conductive material to be added to the negative electrode active material layer must be such that the electron conductivity is ensured even when the graphite particles expand and contract during charge and discharge. 1 to 15 parts by weight, preferably 3 to 13 parts by weight, more preferably 3 to 13 parts by weight, based on 100 parts by weight of the active material.
8 parts by weight is desirable. If the amount is less than 1 part by weight, sufficient electron conductivity cannot be obtained, and if the amount is more than 15 parts by weight, the amount of the gel-like ionic conductor decreases, and the battery characteristics deteriorate.

The compound capable of inserting and extracting lithium ions as the positive electrode active material used in the positive electrode of the present invention includes, as inorganic compounds, transition metal oxides, composite oxides of lithium and transition metals, transition metal sulfides, and the like. Is mentioned. Here, Fe, Co, Ni, Mn, or the like is used as the transition metal. Specifically, MnO, V ₂ O ₅ , V ₆ O ₁₃ , Ti
Examples include transition metal oxide powders such as O ₂ , composite oxide powders of lithium and a transition metal such as lithium nickel oxide and lithium cobalt oxide, and transition metal sulfide powders such as TiS ₂ and FeS. Examples of the organic compound include a conductive polymer such as polyaniline. These inorganic compounds and organic compounds may be mixed and used as the positive electrode active material.

The particle size of the active material of these positive electrodes may be appropriately selected depending on the other components of the battery, but is usually 1 to 30 μm, particularly 1 to 10 μm, and especially 3 to 10 μm.
The thickness of 8 μm is preferable because battery characteristics such as rate characteristics and cycle characteristics are improved. In addition, an electron conductive material is generally added to the positive electrode, and the electron conductive material is not particularly limited as long as it can be mixed with an appropriate amount of the positive electrode active material to impart conductivity, and is usually carbon black, graphite, or the like. Carbon powder and metal powder stable at the electrode potential to be used. Among them, acetylene black is preferably used.

The weight ratio of the electron conductive material to the active material in the positive electrode is preferably in the range of 98/2 to 90/10.
In the present invention, the content of each component such as an electron conductive material and an active material in the negative electrode active material layer is determined by using a DTA-T
It can be determined by analyzing and identifying using a thermal analysis method such as GA.

As the gel containing the electrolyte forming the active material layers of the positive electrode and the negative electrode, the same material as the above-mentioned electrolyte layer is used. For example, a polyethylene oxide resin or the like can be mentioned. More preferably, it is desirable to introduce an acryl group, a methacryl group or the like into the terminal of the polyethylene oxide resin and to crosslink with heat or ultraviolet rays.

As the current collector of the positive electrode, an aluminum foil is generally used. A copper foil is used as a current collector of the negative electrode. It is preferable that the surface of the current collector be subjected to a roughening treatment in advance, since the binding effect is enhanced. Examples of the surface roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. As the mechanical polishing method, abrasive cloth paper to which abrasive particles are fixed,
A method of polishing the surface of the current collector with a wire brush provided with a grindstone, emery buff, steel wire, or the like can be given.

The method for forming the positive electrode or negative electrode active material layer on the current collector is not particularly limited. However, since the paint has a high viscosity, a converse coat, a squeeze coat,
A coating method such as lip coating is used.

The shape of the lithium secondary battery of the present invention can be any of various shapes such as a cylindrical shape, a box shape, a paper shape, and a card shape without leakage of the electrolytic solution since a gel electrolyte is used. .

[0026]

As described above, the feature of the present invention is to improve the electron conductivity of the negative electrode active material layer in a lithium secondary battery using a polymer electrolyte. Hereinafter, the present invention will be described specifically.

[0027]

EXAMPLES The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples unless it exceeds the gist. According to the composition shown below, the positive electrode coating material was coated on an aluminum base material and the negative electrode coating material was coated on a copper base material, and a positive electrode and a negative electrode for a lithium secondary battery were evaluated. The following were used as raw materials for the positive electrode paint and the negative electrode paint. Positive electrode active material LiCoO ₂ powder (manufactured by FMC) Electronic conductive material Acetylene black DBP oil absorption 115 cc / 100 g (manufactured by Denki Kagaku Kogyo) Negative electrode active material SFG15: particle diameter 15 μm graphite (manufactured by TIMCAL) Binder Photomer 4050: Acrylic group at terminal Polyethylene oxide resin (Henkel) Electrolyte PC: Propylene carbonate (Mitsubishi Chemical) EC: Ethylene carbonate (Mitsubishi Chemical) Crosslinking initiator Trignox42 (Akuzo Nobel)

Example 1 (Positive electrode coating composition) LiCoO ₂ 100.0 parts Acetylene black 5.0 parts Photomer 4050 5.0 parts Trignox 42 0.1 parts PC 15.0 parts EC 15.0 parts The above positive electrode material was kneaded. A dispersion treatment was performed to form a paint, which was applied on an aluminum foil having a thickness of 20 μm using a doctor blade so that the film thickness became 150 μm. After that, the coating film is
The sheet was crosslinked at a temperature of ° C, and a sheet coated with a positive electrode material was obtained. Thereafter, the positive electrode shown in Table 1 was prepared by punching into a predetermined shape.

(Negative electrode paint composition) SFG15 100.0 parts Acetylene black 6.7 parts Photomer 4050 10.0 parts Trignox 42 0.1 parts PC 16.0 parts EC 16.0 parts (Volume fraction in negative electrode active material layer of SFG15) 41vo
The above-mentioned negative electrode material is kneaded and dispersed to form a paint, and is coated on a copper foil having a thickness of 20 μm with a doctor blade to a thickness of 15%.
It was applied so as to have a thickness of 0 μm. Thereafter, the coating film was crosslinked at 120 ° C. to obtain a sheet on which the electrode material was applied.

(Electrolyte composition) LiPF ₆ 12.0 parts Photomer 4050 9.0 parts Trignox 0.5 part PC 40.0 parts EC 39.0 parts The above electrolyte material is kneaded, and a doctor is placed on the negative electrode active material layer.
Coating was performed using a blade so that the film thickness became 100 μm. Thereafter, the coating film was cross-linked by ultraviolet rays to obtain a sheet in which an electrolyte layer was applied on the negative electrode layer.

The sheet coated with the electrolyte paint on the negative electrode active material layer obtained as described above and the positive electrode active material layer were combined, laminated and pressed to obtain a sheet-like battery. .

Example 2 A sheet battery was obtained in the same manner as in Example 1 except that the amount of graphite in the negative electrode active material layer was changed to 140.0 parts (volume: 50 vol%). Example 3 The procedure of Example 1 was repeated, except that the amount of graphite in the negative electrode active material layer was changed to 53.0 parts (volume: 27 vol%).
A battery was obtained. Example 4 A sheet battery was obtained in the same manner as in Example 1, except that the amount of graphite in the negative electrode active material layer was changed to 200.0 parts (volume: 58 vol%). Example 5 The particle size of graphite used in the negative electrode active material layer was 40
A sheet battery was obtained in the same manner as in Example 1 except that the thickness was changed to μm. Example 6 The particle diameter of graphite used in the negative electrode active material layer was 40.
A sheet-like battery was obtained in the same manner as in Example 2 except that the thickness was changed to μm. Example 7 The DBP oil absorption of the carbon black used in the negative electrode active material layer was 78 cc / 100 g (manufactured by Mitsubishi Chemical Corporation).
Except that the product name is # 44), the procedure is the same as in Example 1.
A battery was obtained. Example 8 The procedure of Example 1 was repeated, except that the carbon black used in the negative electrode active material layer had an DBP oil absorption of 165 cc / 100 g (trade name # 3250, manufactured by Mitsubishi Chemical Corporation). A battery was obtained. Comparative Example 1 A sheet battery was obtained in the same manner as in Example 1, except that acetylene black was not added to the negative electrode active material layer. Comparative Example 2 A sheet battery was obtained in the same manner as in Example 1, except that 16.0 parts of acetylene black was used in the negative electrode active material layer. Comparative Example 3 A sheet battery was obtained in the same manner as in Example 1, except that 100.0 parts of coke was used as the negative electrode active material.

[0033]

[Table 1] Table 1 活 Active material (negative electrode) Electronic conductive material (Negative electrode) Rate characteristics Cycle characteristics Content Particle size Content Oil absorption (vol%) (μm) (parts by weight) (cc / 100g) (%) (times) ─────────── ───────────────────────── Example 1 41 15 6.7 120 95 500 or more Example 2 50 15 4.8 120 98 500 or more Example 3 27 15 12.7 120 90 200 Example 4 58 15 3.4 120 70 500 or more Example 5 41 40 6.7 120 85 300 Example 6 50 40 4.8 120 83 500 or more Example 7 41 15 6.7 78 68 200 Example 8 41 15 6.7 165 95 500 or more Comparative Example 1 42 15 0-25 50 or less Comparative Example 2 39 15 16.0 12 40 100 Comparative Example 3 41 15 6.7 120 70 300 ────────────────────────────────────

The evaluation was performed under the following conditions. (1) Rate characteristics When the charge / discharge is C / 24 (current value for full charge / discharge in 24 hours) at 4.1 V constant voltage charge, and when the discharge capacity at 2.7 V discharge is 100, C / C 24, the capacity was compared when the battery was discharged at C / 2 (current value for fully charging and discharging in 2 hours) after charging at 4.1 V constant voltage. (2) Cycle Characteristics When the initial discharge capacity was set to 100, the cycle characteristics were evaluated by the number of cycles at which the capacity retention became 80% or less.

[0035]

According to the lithium battery of the present invention, high rate charge / discharge characteristics can be obtained despite the use of a gel electrolyte, and the cycle characteristics are excellent without impairing the battery capacity.

Claims (5)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode collects an active material layer containing a compound capable of inserting and extracting lithium ions, and the negative electrode collects an active material layer containing graphite. The positive / negative electrode active material layer and the electrolyte layer are composed of a gel polymer holding an electrolytic solution, and the negative electrode active material layer is provided with an electron conductive material in an amount of 1 part by weight per 100 parts by weight of the active material. A lithium secondary battery containing up to 15 parts by weight. 2. The lithium secondary battery according to claim 1, wherein graphite has a particle size of 10 to 50 μm and is contained in the negative electrode active material layer in an amount of 30 to 50 vol%. 3. The lithium secondary battery according to claim 1, wherein the electron conductive material is carbon black. 4. The carbon black has a DBP oil absorption of 10
4. The weight is 0 cc / 100 g or more.
4. The lithium secondary battery according to 1. 5. The lithium secondary battery according to claim 1, wherein at least the gel polymer of the negative electrode active material layer is a polymer having an alkylene oxide skeleton.