JPH09213309A - Method for producing positive electrode for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

(57) Abstract: It is possible to manufacture a positive electrode material having excellent conductivity and less variation by a simple method. A method for producing a positive electrode for a lithium secondary battery, which comprises forming a positive electrode layer on a current collector by using a positive electrode material containing a compound capable of inserting and extracting lithium, a conductive substance, and a binder resin, the method comprising: A mixture containing a substance, a binder resin and a solvent having a solvent concentration of 70 to 90% by weight,
Maximum shear rate is 1.0 × 10 ² [s ^-1 ] or more, 1.0
× 10 ³ [s ⁻¹ ] or less, a kneading step of kneading under a condition of a kneading time of 5 to 100 minutes, and a kneading step of kneading the product obtained through the step and a compound capable of occluding and releasing lithium. A method for producing a positive electrode layer for a lithium secondary battery, comprising:

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention resides in a method for producing a positive electrode for a lithium secondary battery and a lithium secondary battery. More specifically, the present invention resides in a method for producing a positive electrode for a lithium secondary battery, which has high potential, high energy density and excellent cycle characteristics, and a lithium secondary battery using the same.

[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, the development of lithium secondary batteries capable of realizing high voltage, high energy density, and excellent cycle characteristics has been actively pursued.
A lithium secondary battery is generally composed of a positive electrode capable of inserting and extracting lithium, a negative electrode, and a non-aqueous electrolyte solution. For example, a positive electrode containing lithium cobalt oxide, a negative electrode containing a carbon material, and an electrolyte solution are used. In case of the secondary battery,
During charging, lithium ions are occluded from the positive electrode into the negative electrode via the electrolytic solution, and during discharging, lithium ions are released from the negative electrode and occluded into the positive electrode via the electrolytic solution. The characteristics required for this electrode are to have a large capacity for occluding and releasing lithium into the electrode, and to suppress a decrease in each capacity due to repetition (cycle) of storage and release. The characteristics required for the positive electrode of such an excellent lithium secondary battery include the conductivity within the positive electrode layer. The compound capable of occluding and releasing lithium used for the positive electrode is often an oxide having almost no conductivity, and since this alone does not function as the positive electrode, conductivity is usually imparted using a conductive substance such as carbon, It is used as a positive electrode.

[0003]

Such a positive electrode is usually formed by forming a paint using a compound capable of occluding and releasing lithium and a conductive substance using a binder resin and dispersing the both in the paint. Has worked as a. However, it has been conventionally difficult to disperse the particles uniformly, and the conductivity of the positive electrode sometimes decreases. In such a case, not only the charging / discharging characteristics are deteriorated and cycle deterioration is caused, but also the conductivity of the positive electrode is varied among battery products, resulting in variation in product performance. When batteries are connected in series for charging or the like, a battery having poor characteristics may be overcharged as compared with other batteries, which may cause a serious safety problem such as damage or explosion of the battery. The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a lithium secondary battery having excellent cycle characteristics, which makes it possible to produce a positive electrode layer material having excellent conductivity and less variation by a simple method. .

[0004]

Means for Solving the Problems The inventors of the present invention have made extensive studies in view of the above situation, and as a result, in a method for producing a positive electrode layer material, a binder and a conductive substance can be kneaded together with a solvent, and then this and lithium can be absorbed and released. It was possible to provide a positive electrode material for a lithium secondary battery in a good dispersed state by kneading various compounds, and to provide a lithium secondary battery with high potential, high energy density and excellent cycle characteristics. Of.

One of the gist of the present invention is a positive electrode for a lithium secondary battery in which a positive electrode layer is formed on a current collector by a positive electrode layer material containing a compound capable of inserting and extracting lithium, a conductive substance and a binder resin. And a conductive material,
The mixture containing the binder resin and the solvent has a solvent concentration of 70 to 90% by weight and a maximum shear rate of 1.0 × 10.
A kneading step of kneading under a condition of ² [s ⁻¹ ] or more and 1.0 × 10 ³ [s ⁻¹ ] or less and a kneading time of 5 to 100 minutes; A method for producing a positive electrode layer for a lithium secondary battery, which comprises a kneading step of kneading with a compound capable of inserting and extracting. Another aspect of the present invention is a lithium secondary battery comprising a positive electrode capable of inserting and extracting lithium, a negative electrode and a non-aqueous electrolyte solution, wherein the positive electrode is obtained by the manufacturing method as described above. It exists in a characteristic lithium secondary battery. Less than,
The present invention will be specifically described.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Examples of compounds capable of inserting and extracting lithium in the method for producing a positive electrode of the present invention include transition metal oxides, composite oxides of lithium and transition metals, and transition metal sulfides. Here, as the transition metal, Co, N
i, Mn, Fe, etc. are mentioned. Specifically, MnO,
Examples thereof include transition metal oxide powders such as V ₂ O ₅ and TiO ₂ , composite oxide powders of lithium and transition metals such as lithium nickel oxide and lithium cobalt oxide, and transition metal sulfide powders such as TiS ₂ and FeS. -The binder needs to be stable with respect to the electrolytic solution and the like, and weather resistance, chemical resistance, heat resistance, flame retardancy, etc. are desired. For example, polyvinylidene fluoride resin (PVDF), SBR type, etc. may be mentioned.

The conductive substance is not particularly limited as long as it can give conductivity by mixing an appropriate amount with a compound powder capable of occluding and releasing lithium, but carbon powder such as acetylene black, carbon black and graphite, Examples thereof include metal powders that are stable at the electrode potential used. When carbon black is used as the conductive material, the oil absorption is 100c
Those having a specific surface area of 100 m ² / g or more and c / g or more are desirable because they show high conductivity in the coating film. This is because carbon black easily forms percolation in the coating film, and a small amount of carbon black provides high conductivity.

The solvent for the coating material forming the positive electrode can be appropriately selected as long as it can dissolve the binder and can be easily dried, and examples thereof include n-methylpyrrolidone and dimethylformamide. In the kneading of the material forming the positive electrode, the order is such that the solution in which the binder resin is dissolved in the solvent and the conductive substance are first kneaded. The conditions are as follows: solvent concentration is 70 to 90% by weight, maximum shear rate is 1.0 × 10 ² [s ⁻¹ ] or more, 1.0 × 10 ³ [s ⁻¹ ].
The following kneading time is 5 to 100 minutes. Next, the product obtained through this step and a compound capable of occluding and releasing lithium are kneaded, but it is preferable to knead at a similar shear rate for 5 to 20 minutes to obtain a positive electrode coating material. As a result, the conductive substance can be dispersed in a good state, and efficient conductivity can be obtained with a smaller amount of the conductive substance.
If the dispersion is inferior to this range, the dispersion of the electroconductive substance will be non-uniform, and the electroconductivity of the positive electrode layer will be inferior and the dispersion will occur among the media. On the other hand, when the dispersion is too strong due to too strong shearing force or too long kneading time, the structure or structure of the conductive substance is broken and the conductivity is lowered. Further, by confirming that the above-mentioned conductive substance has been in a predetermined dispersed state in advance, and then forming the positive electrode coating material, for example, the raw material Lo
It is possible to minimize variations in conductivity that occur due to factors such as the difference in t and the state of the disperser.

Copper foil or aluminum foil is generally used as the current collector. It is preferable to perform roughening treatment on the surface of the current collector in advance because the binding effect will be enhanced. Examples of the surface roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. Examples of the mechanical polishing method include a method of polishing the current collector surface with a polishing cloth paper to which abrasive particles are fixed, a grindstone, an emery buff, a wire brush equipped with a steel wire, or the like. Generally, it is preferable to use a polishing machine such as a buff polishing machine, a portable grinder or a sander. As the abrasive, emery, fused alumina, silicon carbide, boron carbide or the like is used. Examples of the electropolishing method include a method in which electrolysis is performed in an electrolytic solution using a current collector as an anode, and examples of the electrolytic solution include phosphoric acid, sulfuric acid, oxalic acid, citric acid, acetic anhydride, or a mixed solution thereof. The chemical polishing method is a method of immersing a current collector in a polishing solution, and the polishing solution includes an electrolytic solution used in an electrolytic polishing method. Alternatively, a binder resin such as a silane coupling agent may be applied to the surface of the current collector in advance, and a positive electrode may be formed on the bonding resin to suppress separation between the current collector and the positive electrode.

The method of forming the positive electrode on the current collector is not particularly limited, but it is preferable to use a coating method such as comma reverse coating, squeeze coating or lip coating because the viscosity of the coating material is high. The secondary battery of the present invention mainly comprises a positive electrode, a negative electrode and a non-aqueous electrolyte.
Although any conventional negative electrode may be used, the negative electrode is usually formed by coating a current collector containing a carbon powder capable of inserting and extracting lithium and a binder.

Such carbon powder includes natural graphite, artificial graphite, coke, carbon black, vapor grown carbon, carbon fiber, a material obtained by carbonizing an organic polymer compound, or a material obtained by heat-treating or mixing these. Can be mentioned. In particular, as the carbon powder for the negative electrode, one close to lithium potential is preferable, and carbon powder containing graphite as a single component or a main component is preferable. As the method of applying the binder and the negative electrode material used in the production of the negative electrode, the same methods and methods as in the method of producing the positive electrode can be used.

Further, as the non-aqueous electrolyte, the above electrolyte is used.
Stable to the positive electrode active material and the negative electrode active material, and
Lithium ions serve as the positive electrode active material or the negative electrode active material
It is a non-aqueous substance that can move to perform an electrochemical reaction.
You can use any of them, specifically
LiPF₆, LiAsF₆, LiSbF₆, LiB
F _Four, LiClO_Four, LiI, LiBr, LiCl, L
iAlCl, LiHF_Two, LiSCN, LiSO_ThreeCF
_TwoAnd the like. Among these, especially LiPF ₆,
LiClO_FourIs preferred. Solvent that dissolves this electrolyte
Is arbitrary, but a solvent having a relatively high dielectric constant is preferably used.
It is. Specifically, ethylene carbonate, propylene carbonate
Carbonates such as carbonates, dimethyl carbonate
, Diethyl carbonate, ethyl methyl carbonate
Acyclic carbonates such as gypsum, tetrahydrofuran,
2-methyltetrahydrofuran, dimethoxyethane, etc.
Glymes, lactones such as γ-butyl lactone, sulphates
Sulfur compounds such as foran, nitriles such as acetonitrile
Examples thereof include one kind or a mixture of two or more kinds.
You. Among these, especially ethylene carbonate,
Cyclic carbonates such as ropylene carbonate, dimethy
Leucarbonate, diethyl carbonate, ethyl methyl
Selected from acyclic carbonates such as carbonates
One or a mixed solution of two or more is suitable.

In the battery of the present invention, as a solid electrolyte, the electrolyte solution is impregnated with a polymer such as polyethylene oxide, polypropylene oxide, an isocyanate cross-linked product of polyethylene oxide, phenylene oxide, or a polymer of phenylene sulfide. Solid electrolyte, Li ₃ N, LiBCl ₄ , Li ₄ SiO
It is also possible to use an inorganic solid electrolyte with a lithium glass such as ₄ , Li ₃ BO _3, or the like. In the lithium secondary battery of the present invention, in addition to the above constituent requirements, a separator having a function of allowing passage of lithium ions may be used to prevent contact between both electrodes, hold a nonaqueous electrolytic solution, and the like. Examples of the separator include porous films such as polyethylene and polypropylene, non-woven fabrics or woven fabrics. The thickness of the separator is preferably about 10 to 200 μm. The lithium secondary battery of the present invention can have various shapes such as a cylindrical shape, a box shape, a paper shape, and a card shape.

[0014]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to the examples shown below as long as the gist thereof is not exceeded. A positive electrode coating material was prepared according to the following composition and applied on an aluminum substrate to obtain a positive electrode for a Li secondary battery, which was evaluated. The following were used as the raw materials for the positive electrode paint. Positive electrode material LiCoO ₂ conductive material Acetylene black (particle size 40 nm, oil absorption 125 cc / 100 g, BET 60 m ² / g) Binder PVDF (manufactured by Mitsubishi Chemical) Solvent NMP: n-methylpyrrolidone

Example 1 (Best Mode) In the first dispersion step, the following compositions were kneaded. PVDF 42.0 wt% Acetylene black 58.0 wt% NMP 350.0 wt% After the binder was dissolved in the solvent in the order of kneading, acetylene black was added and mixed. The mixed composition was kneaded with a kneader (“Agitator Mill” manufactured by NETZSCH) at a solvent concentration of 78% and a maximum shear rate of 3.0 × 10 ² [s.
⁻¹ ], and the kneading time was 60 minutes. In the second step, the positive electrode material and the solvent were added to the composition obtained in the first step, and the following composition was used, and the kneading time was 20 minutes. did.

Final positive electrode coating composition LiCoO ₂ 88.0 wt% Acetylene black 7.0 wt% PVDF 5.0 wt% NMP 100.0 wt% The above positive electrode coating material is applied on one side of a 20 μm thick aluminum foil using a doctor blade and then vacuumed. After drying, the back surface was similarly coated on the back surface to obtain a sheet having both surfaces coated with the electrode material. Next, the press pressure is 500 kgf / cm ²
Was pressed to prepare a sheet-shaped positive electrode having a film thickness of 100 μm and punched into a predetermined shape to prepare a positive electrode. As the negative electrode material, coke powder having a BET specific surface area of 10 m ² and an average particle size of 12 μm was used.

Negative electrode coating composition Coke 90.0 wt% PVDF 10.0 wt% NMP 100.0 wt% The above negative electrode coating material was applied to one side using a doctor blade on a copper foil having a thickness of 20 μm and whose surface was roughened with waterproof paper, and then vacuumed. After drying, the back surface was similarly coated on the back surface to obtain a sheet having both surfaces coated with the electrode material. Next, pressing is performed at a pressing pressure of 500 kgf / cm ² to obtain a film thickness of 100.
A sheet-shaped negative electrode having a thickness of μm was prepared and punched into a predetermined shape to prepare a negative electrode. Next, a polyethylene separator having a thickness of 20 μm is sandwiched between the positive electrode and the negative electrode obtained here,
A battery was prepared by impregnating a solution (concentration: 1 mol / l) in which LiPF ₆ was dissolved in an equal volume mixture of ethylene carbonate and diethyl carbonate as an electrolytic solution.

Example 2 (changing kneading time, shearing force and solvent concentration) In the first step of preparing the positive electrode coating material, the conditions for kneading the mixture of acetylene black and the binder solution were:
Maximum shear rate is 3.0 × 10 ² [s ^-1 ], kneading time 3
A battery was prepared and evaluated in the same manner as in Example 1 except that kneading was performed for 0 minutes.

Example 3 (changed oil absorption amount and surface area of positive electrode carbon black) The conductive material constituting the positive electrode coating material was replaced by acetylene black, and conductive carbon black "# 3050" (manufactured by Mitsubishi Chemical Co.) was used. 40nm, DBP oil absorption 180cc / 10
A battery was prepared and evaluated in the same manner as in Example 1 except that the specific surface area was 0 g and the specific surface area was 50 m ² / g.

Comparative Example 1 (mixed in one piece) The method for preparing the positive electrode coating material was not divided into two steps, and the positive electrode material,
A battery was prepared and evaluated by kneading under the same kneading conditions as in Example 1 except that the conductive material, the binder and the solvent were simultaneously mixed to prepare the positive electrode coating material.

Comparative Example 2 In the first step of preparing the positive electrode coating material, the conditions for kneading the mixture of acetylene black and the binder solution were:
A battery was prepared and evaluated in the same manner as in Example 1 except that kneading was carried out at a shear rate of 5.0 × 10 ¹ [s ⁻¹ ] at the maximum and a kneading time of 10 minutes.

Evaluation method (Evaluation of electroconductivity) A positive electrode sheet was prepared in the same manner as in the example except that the positive electrode coating material prepared as described above was replaced with an aluminum foil and a PET film having a thickness of 100 μm was used. It was measured. Surface resistivity is in accordance with JIS standard
An AC voltage was applied to both ends of the medium, and it was determined by a high resistance measuring machine.

(Initial discharge capacity [mAH / g]) The initial discharge capacity was calculated by the discharge capacity per positive electrode weight. For the evaluation, five identical batteries were prepared from the same sheet, and variations in the initial discharge capacity were also evaluated. (Cycle characteristics) The cycle characteristics were evaluated by the capacity retention rate (%) after 30 cycles when the initial discharge capacity was 1. For evaluation, five same batteries were prepared from the same sheet, and variations in cycle characteristics were also evaluated.

Evaluation results Surface specific resistance [Ω / □] Initial discharge capacity [mAH / g] Cycle characteristics (%) Example 1 6k 130 ± 5 80 ± 2 2 7k 125 ± 8 78 ± 3 3 9k 120 ± 8 79 ± 2 Comparative Example 1 50k 110 ± 15 75 ± 5 2 30k 115 ± 15 75 ± 5

[0025]

The lithium battery using the positive electrode obtained by the production method of the present invention has a high conductivity and a small variation even though the conductive material of the positive electrode layer is small. It is a product with excellent characteristics.

Claims (3)

[Claims] 1. A compound capable of inserting and extracting lithium and conductivity.
Collector with a positive electrode material containing a conductive substance and a binder resin
Manufacture of positive electrode for lithium secondary battery having positive electrode layer formed thereon
Manufacturing method, which is a conductive substance, a binder resin and a solvent
A mixture containing a solvent concentration of 70-90% by weight of the mixture,
Maximum shear rate is 1.0 × 10^Two[S ^-1] Or more, 1.0
× 10^Three[S^-1] Below, kneading time is 5 to 100 minutes
Kneading step of kneading below, and those obtained through the step
Kneader for kneading and a compound capable of occluding and releasing lithium
And a positive electrode layer for a lithium secondary battery, characterized in that
Manufacturing method. 2. The conductive substance has a particle diameter of 50 nm or less, DB
P oil absorption of 100cc / 100g or more and specific surface area of 10
The production method according to claim 1, wherein the carbon black is 0 m ² / g or less. 3. A lithium secondary battery comprising a positive electrode capable of inserting and extracting lithium, a negative electrode and a non-aqueous electrolyte solution, wherein the positive electrode is obtained by the manufacturing method according to claim 1. A lithium secondary battery characterized by the above.