JP2015049984A - Slurry for manufacturing electrode of nonaqueous electrolyte secondary battery, electrode manufactured by using it, and nonaqueous electrolyte secondary battery using that electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slurry for manufacturing the electrode of a nonaqueous electrolyte secondary battery which can prevent formation of a recess in the surface when being applied to a collector, by adding an organic acid to a slurry where lithium titanate and a binder are dispersed into water, and adjusting the pH.SOLUTION: A slurry for manufacturing the electrode of a nonaqueous electrolyte secondary battery is produced by mixing an electrode active material composed of lithium titanate, and a water dispersed polymer binder, and contains an organic acid not containing a heteroatom other than oxygen and having pH of 8 or less. A nonaqueous electrolyte secondary battery having excellent cycle, in which generation of gas is suppressed, can be manufactured by using this electrode.

Description

  The present invention relates to a slurry for preparing an electrode of a nonaqueous electrolyte secondary battery, an electrode manufactured using the slurry, and a nonaqueous electrolyte secondary battery using the electrode.

  Lithium ion storage batteries are currently widely used as power sources for mobile devices. Lithium ion storage batteries are also expected to be used for large power sources such as electric vehicles and power storage because they have a higher energy density than existing nickel-cadmium storage batteries and nickel-hydrogen storage batteries. In particular, non-aqueous electrolyte secondary batteries using transition metal composite oxides such as lithium titanate (LTO) as the electrode active material are expected to have a long life due to good cycle characteristics and are highly safe. (Patent Document 1)

  In current non-aqueous electrolyte secondary batteries, polyvinylidene fluoride is mainly used as a binder used to scour an electrode active material and harden it after drying. This binder is dispersed in N-methyl-2-pyrrolidone which is a non-aqueous organic solvent. However, when an organic solvent is used as the dispersant, it is necessary to recover the solvent in addition to the cost of the solvent itself, so that the cost is high in terms of equipment.

  Therefore, an aqueous dispersion binder such as a styrene-butadiene copolymer may be used instead of polyvinylidene fluoride (see, for example, Patent Document 2).

JP 2012-129095 A JP-A-4-342966 JP 2012-234665 A

  When such a water-dispersed binder and lithium titanate are mixed and applied, there is a problem that the electrode shape becomes non-uniform. That is, when a slurry is made using lithium titanate and a water-dispersed binder and applied to an aluminum foil, a crater-like depression is formed on the surface of the electrode after drying. This makes it difficult to adjust the weight of the electrode per unit area, which affects battery design.

According to Patent Document 3, an attempt is made to solve this problem by mixing an organic phosphonic acid compound and an alkaline earth metal in an electrode slurry containing lithium titanate and water. After the battery was produced, gas generation from the electrode was confirmed.
The present invention provides a slurry for electrode production, a uniform electrode produced using the same, and a non-aqueous electrolyte secondary battery having good cycleability using the electrode and suppressing gas generation. The purpose is to provide.

As a result of research conducted by the present inventors, it has been found that the formation of depressions on the surface of the electrode can be prevented by adjusting the pH by adding acid to a slurry containing lithium titanate and water, and the present invention is completed based on this finding. It came to.
The slurry for electrode preparation of the non-aqueous electrolyte secondary battery of the present invention is a slurry obtained by mixing an electrode active material made of lithium titanate and a water-dispersed polymer binder, and contains hetero atoms other than oxygen. It is characterized by containing no organic acid and having a pH of 8 or less.

The pH of the slurry is preferably 7 or less and 4 or more.
The organic acid is preferably formic acid, acetic acid, citric acid, oxalic acid, maleic acid, fumaric acid, malic acid, tartaric acid, or a mixture of two or more thereof.
The electrode active material is, for example, spinel type Li ₄ Ti ₅ O ₁₂ .
The electrode for a nonaqueous electrolyte secondary battery of the present invention is obtained by coating the slurry on an aluminum current collector and drying it.

The nonaqueous electrolyte secondary battery of the present invention is a nonaqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a nonaqueous electrolyte solution interposed between the positive electrode and the negative electrode, wherein the electrode is the positive electrode or the positive electrode. It is used for either one or both of the negative electrodes.
A plurality of the nonaqueous electrolyte secondary batteries can be connected to form an assembled battery.

  The electrode of the nonaqueous electrolyte secondary battery produced by the slurry of the present invention does not generate a depression on the surface, and can be a uniform electrode. A non-aqueous electrolyte secondary battery using the electrode is a battery having a long life because corrosion of aluminum as a current collector can be suppressed. Moreover, since the impurity alkali component mixed in the lithium titanate is removed, the generation of gas is small during cycle use.

Embodiments of the present invention will be described below. The scope of the present invention is defined by the scope of the claims, and is intended to include any modifications within the scope and meaning equivalent to the scope of the claims.
<Electrode>
The slurry for electrode preparation of the non-aqueous electrolyte secondary battery of the present invention is a mixture of an electrode active material mixture dispersed in water, containing an organic acid containing no hetero atom other than oxygen, and adjusted in pH. is there. An electrode of a non-aqueous electrolyte secondary battery can be produced by applying this electrode production slurry to a current collector and drying and solidifying it.

Lithium titanium oxide is used as the electrode active material. In particular, spinel-type Li ₄ Ti ₅ O ₁₂ or a part of titanium substituted with another metal ion is preferable. Although the metal to substitute is not specifically limited, For example, Nb, Al, Ni, Co, Mg, V, Cr, Cu, Mn is mentioned.
A binder dispersed in water is used. For example, at least one selected from the group consisting of polytetrafluoroethylene (PTFE), styrene-butadiene copolymer (SBR), acrylate ester, polyimide, and derivatives thereof may be used. it can. You may add a dispersing agent and a thickener to these.

In the present invention, the amount of the binder contained in the electrode active material is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 1 part by weight or more and 15 parts by weight or less with respect to 100 parts by weight of the electrode active material. If it is the said range, the adhesiveness of an electrode active material and a conductive support material will be maintained, and adhesiveness with a collector can fully be acquired.
The electrode active material may contain a conductive additive as necessary. Although it does not specifically limit as a conductive support material, A carbon material or / and a metal microparticle are preferable. Examples of the carbon material include natural graphite, artificial graphite, vapor-grown carbon fiber, carbon nanotube, acetylene black, ketjen black, and furnace black. Examples of the metal fine particles include copper, aluminum, nickel, and an alloy containing at least one of these. Further, the fine particles of inorganic material may be plated. These carbon materials and metal fine particles may be used alone or in combination of two or more.

The amount of the conductive additive contained in the electrode active material is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 1 part by weight or more and 15 parts by weight or less with respect to 100 parts by weight of the electrode active material. If it is the said range, the electroconductivity of an electrode will be ensured.
The current collector used in the electrode for the nonaqueous electrolyte secondary battery of the present invention is a metal that corrodes in an alkaline solution, such as aluminum and an aluminum alloy. Examples of the shape include a foil shape, a mesh shape, a punching shape, an expanded shape, and a foam structure.

In order to produce the electrode of the present invention, for example, a slurry in which an electrode active material, a conductive additive, and a binder are dispersed in water is supported on a current collector. After filling and applying the slurry to the pores and the outer surface of the current collector, the electrode is produced by removing water.
To the slurry of the present invention, an organic acid containing no heteroatom other than oxygen, such as formic acid, acetic acid, citric acid, oxalic acid, maleic acid, fumaric acid, malic acid, tartaric acid, is added so as to have a predetermined pH. ing.

  The method for preparing the slurry is not particularly limited, but since an electrode active material, a conductive additive, a binder, and water can be uniformly mixed, a stirring granulator, a ball mill, a planetary mixer, a jet mill, and a thin film swirling mixer are used. It is preferable. The method of kneading the electrode active material mixture is not particularly limited, but after mixing the electrode active material and the conductive additive, it may be prepared by adding a binder dispersed in water, or the electrode active material, conductive additive, In addition, water may be added after mixing the binder.

  The method of supporting the slurry on the current collector is not particularly limited. For example, the slurry is dispersed on the current collector and pressurized to form an electrode, and then water is removed. A method of removing water by forming an electrode by pressure bonding to an electric body, a method of removing a solvent after applying the slurry with a doctor blade, a die coater, etc., a method of removing the solvent after adhering the slurry to the current collector by spraying A method of removing the solvent after impregnating the current collector with the slurry is preferable. In particular, a method of forming an electrode by pressure, pressure bonding or the like is preferable. The method for removing water is preferable because it is easy to dry using an oven or a vacuum oven. As the atmosphere, air at room temperature or high temperature, an inert gas, a vacuum state, or the like can be given.

The obtained electrode may be used as a negative electrode of a nonaqueous electrolyte secondary battery or may be used as a positive electrode. When used as a positive electrode, lithium ions may be inserted in advance.
<Capacity ratio and area ratio of negative electrode to positive electrode>
As for the ratio of the electric capacity of the positive electrode and the negative electrode in the secondary battery produced using the electrode for nonaqueous electrolyte secondary battery of the present invention, it is desirable to satisfy the following formula (1).

0.7 ≦ B / A ≦ 1.3 (1)
In the above formula (1), A represents the electric capacity per 1 cm ² of the positive electrode, and B represents the electric capacity per 1 cm ² of the negative electrode.
When B / A is less than 0.7, the potential of the negative electrode may become a potential at which the negative electrode current collector reacts with lithium or a lithium deposition potential during overcharge, while B / A is 1.3. If it is larger, a side reaction may occur due to the large amount of the negative electrode active material not involved in the battery reaction.

Although the area ratio of the positive electrode to the negative electrode in the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, it is preferable to satisfy the following formula (2).
1 ≦ D / C ≦ 1.2 (2)
However, C shows the area of a positive electrode, D shows the area of a negative electrode. When D / C is less than 1, for example, when B / A = 1 as described above, the capacity of the negative electrode is smaller than that of the positive electrode, so that the potential of the negative electrode may become a lithium deposition potential during overcharge. On the other hand, when D / C is larger than 1.2, since the negative electrode in the portion not in contact with the positive electrode is large, the negative electrode active material not involved in the battery reaction may cause a side reaction. Although control of the area of a positive electrode and a negative electrode is not specifically limited, For example, in the case of electrode preparation, it can carry out by controlling the coating width.

The area ratio between the separator and the negative electrode used in the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, but preferably satisfies the following formula (3).
1 ≦ F / E ≦ 1.5 (3)
However, E shows the area of a negative electrode and F shows the area of a separator. When F / E is less than 1, the positive electrode and the negative electrode are in contact with each other. When F / E is greater than 1.5, the volume required for the exterior increases, and the capacity density and output density of the battery may decrease.

<Separator>
Examples of the separator used in the nonaqueous electrolyte secondary battery of the present invention include porous materials and nonwoven fabrics. The material of the separator is preferably one that does not dissolve in the organic solvent that constitutes the electrolytic solution. Specifically, a polyolefin polymer such as polyethylene or polypropylene, a polyester polymer such as polyethylene terephthalate, cellulose, or glass. Inorganic materials.

The thickness of the separator is preferably 1 to 500 μm. If it is less than 1 μm, it tends to break due to insufficient mechanical strength of the separator and cause an internal short circuit. On the other hand, when it is thicker than 500 μm, the load characteristics of the battery tend to be reduced due to the increase in the internal resistance of the battery and the distance between the positive and negative electrodes. A more preferable thickness is 10 to 300 μm.
<Nonaqueous electrolyte>
The non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery of the present invention is not particularly limited, but a polymer is impregnated with an electrolytic solution in which a solute is dissolved in a non-aqueous solvent, or an electrolytic solution in which a solute is dissolved in a non-aqueous solvent. A gel electrolyte or the like can be used.

  The non-aqueous solvent preferably includes a cyclic aprotic solvent and / or a chain aprotic solvent. Examples of the cyclic aprotic solvent include cyclic carbonates, cyclic esters, cyclic sulfones and cyclic ethers. Examples of the chain aprotic solvent include chain carbonates, chain carboxylic acid esters and chain ethers. In addition to the above, a solvent generally used as a solvent for nonaqueous electrolytes such as acetonitrile may be used. More specifically, dimethyl carbonate, methyl ethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane, sulfolane, dioxolane, propionic acid Methyl and the like can be used. These solvents may be used alone or as a mixture of two or more. However, in view of the ease of dissolving the solute described below and the high conductivity of lithium ions, a mixture of two or more of these solvents. Is preferably used. A gel electrolyte in which an electrolyte is impregnated in a polymer can also be used.

The solute is not particularly limited. For example, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , LiBOB (Lithium Bis (Oxalato) Borate), LiN (SO ₂ CF ₃ ) _2, etc. are dissolved in the solvent. It is preferable because it is easy to do. The concentration of the solute contained in the electrolytic solution is preferably 0.5 mol / L or more and 2.0 mol / L or less. If it is less than 0.5 mol / L, the desired lithium ion conductivity may not be exhibited. On the other hand, if it is higher than 2.0 mol / L, the solute may not be dissolved any more. The non-aqueous electrolyte may contain a trace amount of additives such as a flame retardant and a stabilizer.

<Nonaqueous electrolyte secondary battery>
The positive electrode and the negative electrode of the nonaqueous electrolyte secondary battery of the present invention may be in the form in which the same electrode is formed on both sides of the current collector, and the positive electrode is formed on one side of the current collector and the negative electrode is formed on one side. In other words, in the case of a bipolar type, in order to prevent a liquid junction between the positive electrode and the negative electrode through the current collector, the conductive material and / or the insulating material is interposed between the positive electrode and the negative electrode. Has been placed. In the case of a bipolar electrode, a separator is disposed between the positive electrode side and the negative electrode side of the adjacent bipolar electrode, and the positive electrode and An insulating material is disposed around the negative electrode.

  The nonaqueous electrolyte secondary battery of the present invention may be one obtained by winding or laminating a separator disposed between the positive electrode side and the negative electrode side. The positive electrode, the negative electrode, and the separator are impregnated with a nonaqueous electrolyte that is responsible for lithium ion conduction. When a non-aqueous electrolysis gel is used, the electrolyte may be impregnated in the positive electrode and the negative electrode, or may be in a state only between the positive electrode and the negative electrode. If the positive electrode and the negative electrode are not in direct contact with the gel electrolyte, it is not necessary to use a separator.

The amount of the nonaqueous electrolyte used in the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, but is preferably 0.1 mL or more per 1 Ah of battery capacity. If it is less than 0.1 mL, the conduction of lithium ions accompanying the electrode reaction may not catch up, and the desired battery performance may not be exhibited.
The nonaqueous electrolyte may be added to the positive electrode, the negative electrode, and the separator in advance, or may be added after winding or laminating a separator disposed between the positive electrode side and the negative electrode side. When using a gel-like non-aqueous electrolyte, it may be gelled after impregnation with a monomer, or may be placed between the positive electrode and the negative electrode after gelling in advance.

  The non-aqueous electrolyte secondary battery of the present invention may be wound or laminated with a laminate film after the laminate is wound, or may be rectangular, elliptical, cylindrical, coin-shaped, button-shaped, or sheet-shaped. It may be packaged with a metal can. The exterior may be provided with a mechanism for releasing the generated gas or the like. Further, a mechanism for injecting an additive for recovering the function of the deteriorated nonaqueous electrolyte secondary battery from the outside of the battery may be provided. The number of stacked layers can be stacked until a desired battery capacity is exhibited. In the case of stacking, pressure may be applied in the stacking direction of the electrodes. Pressure may be applied inside the cell or may be applied from the outside of the exterior.

  The nonaqueous electrolyte secondary battery of the present invention can be made into a secondary battery module by connecting a plurality of nonaqueous electrolyte secondary batteries. The module of the present invention can be manufactured by connecting in series or in parallel as appropriate depending on the desired size, capacity, and voltage. Further, a control circuit may be attached to the secondary battery module in order to confirm the state of charge of each battery and improve safety.

(1) Fabrication of negative electrode Li ₄ Ti ₅ O ₁₂ as a negative electrode active material was obtained from literature (“Zero-Strain Insertion Material of Li [Li1 / 3Ti5 / 3] O4 for Rechargeable Lithium Cells” J. Electrochem. Soc., Volume 142 , Issue 5, pp. 1431-1435 (1995)).
That is, first, titanium dioxide and lithium hydroxide are mixed so that the molar ratio of titanium and lithium is 5: 4, and then this mixture is heated at 800 ° C. for 12 hours in a nitrogen atmosphere to obtain a negative electrode active material. Produced.

100 parts by weight of the negative electrode active material (Li ₄ Ti ₅ O ₁₂ ), 6.8 parts by weight of the conductive additive (acetylene black), and 6.8 parts by weight in terms of solid content of the SBR binder were mixed. A water-dispersed slurry was prepared. Further, to this slurry, organic acids such as acetic acid, maleic acid, formic acid, citric acid, oxalic acid, fumaric acid, malic acid and tartaric acid were added to adjust the pH. This pH-adjusted slurry was applied to an aluminum foil and then vacuum-dried at 80 ° C. to produce an electrode for a working electrode. The electrode was cut into 30 mm × 30 mm.

(2) Production of half-cell Lithium metal was punched into 30 mm × 30 mm as a counter electrode. Using these electrodes, a working electrode / separator (Celgard # 2500, polypore) / lithium metal was laminated in this order and surrounded by a laminate sheet. In the laminate sheet, 1 mL of 1 mol / L of LiPF ₆ dissolved in a non-aqueous solvent of ethylene carbonate / dimethyl carbonate = 30/70 vol% was put into a half cell.

(3) Measurement After this half-cell was left at 25 ° C. for one day, it was connected to a charge / discharge test apparatus (HJ1005SD8, manufactured by Hokuto Denko).
In the cycle characteristic test, 25 ° C., 0.5 mA constant current charging, and 0.5 mA constant current discharging were repeated 200 times. The charge end voltage and discharge end voltage at this time were 3.0 V and 1.0 V, respectively. The first discharge capacity was 165 mAh / g, and the capacity per unit area was 3.1 mAh / cm ² .

Table 1 shows the 200th discharge capacity (capacity maintenance ratio) when the first discharge capacity is 100.
The amount of gas generated is estimated by immersing the first and 200th cells in a solution and measuring the volume change. The presence or absence of a dent was confirmed visually.

As shown in Comparative Examples 1 and 2 and Examples 1 to 4, when acetic acid is used as the organic acid, a depression is observed in the electrode when the pH is 10 or more. No depression was observed below pH = 8. Further, when the pH was high, the gas generation amount tended to increase.
As shown in Examples 5 to 8, even when maleic acid was used as the organic acid, no depression of the electrode was observed at pH = 8 or less, the cycleability was good, and the gas generation was small.

As shown in Examples 9 to 14, when the neutral pH was 7, no depression of the electrode was observed regardless of the type of various organic acids. These electrodes also have good cycle characteristics and small gas generation.
As shown in Comparative Examples 3 and 4, when an organic acid containing a heteroatom other than oxygen is used, the depression of the electrode can be suppressed by setting pH = 7, but good results in gas generation are obtained. It was not obtained. This is considered that the residual hetero atom induces a side reaction.

  For these reasons, if the slurry is mixed with an organic acid that does not contain heteroatoms other than oxygen and the pH is adjusted to 8 or less, the electrode will not dent even if an aqueous binder is used, and even better cycling and gas generation It was found that a deterrent effect can be imparted.

Claims (8)

A slurry for preparing an electrode of a non-aqueous electrolyte secondary battery comprising a mixture of an electrode active material comprising lithium titanate and a water-dispersed polymer binder,
A slurry for preparing an electrode for a non-aqueous electrolyte secondary battery, comprising an organic acid containing no heteroatom other than oxygen and having a pH of 8 or less.   The slurry according to claim 1, having a pH of 7 or less and 4 or more.   The slurry according to claim 1 or 2, wherein the organic acid is formic acid, acetic acid, citric acid, oxalic acid, maleic acid, fumaric acid, malic acid, tartaric acid, or a mixture of two or more thereof. The slurry according to any one of claims 1 to 3, wherein the electrode active material is spinel type Li ₄ Ti ₅ O ₁₂ .   The electrode for nonaqueous electrolyte secondary batteries obtained by apply | coating the slurry in any one of Claim 1 to 4 to an aluminum electrical power collector, and drying. A non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte interposed between the positive electrode and the negative electrode,
A non-aqueous electrolyte secondary battery using the electrode according to claim 5 for either one or both of the positive electrode and the negative electrode.   A nonaqueous electrolyte secondary battery using the electrode according to claim 6 as the negative electrode.   An assembled battery formed by connecting a plurality of the nonaqueous electrolyte secondary batteries according to claim 7.