JP4345920B2 - Electrode for electrochemical device, method for producing the same, and electrochemical device using the electrode - Google Patents

Description

  The present invention relates to an electrode for an electrochemical device, a method for producing the same, and an electrochemical device using the electrode, and more specifically, an 8-titanate nanosheet capable of intercalating metal cations such as Li (lithium) ions. The present invention relates to an electrode for an electrochemical element containing carbon and a carbon material, a manufacturing method thereof, and an electrochemical element such as a lithium ion secondary battery using the electrode.

A lithium ion secondary battery uses a carbon material as an active material for a negative electrode, a composite oxide of Li and a transition metal such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ as an active material for a positive electrode, and a polyolefin microporous film Is used as a separator and Li ion such as LiPF ₆ and LiBF ₄ is used as an electrolyte, Li ions intercalate with the carbon material of the negative electrode during charging, and Li ions transition with Li of the positive electrode during discharging. Charging / discharging is performed by intercalating a complex oxide with metal.
US Pat. No. 4,567,031

In addition, in the lithium ion secondary battery, an active material capable of intercalating Li ions such as MnO ₂ , V ₂ O ₅ , Li ₄ Ti ₆ O ₁₂ , Li ₂ Ti ₃ O ₇ in addition to the above materials. It is also known that a battery can be constructed using

  However, these active materials have a problem that it is difficult to charge and discharge at high speed because Li ion intercalation and deintercalation are slow.

  Further, in an electric double layer capacitor known as the highest capacity among devices capable of high-speed charge / discharge, activated carbon is used as an active material for the positive electrode and the negative electrode. The activated carbon has a very large specific surface area, and charging / discharging is performed by adsorbing ions on the surface of the active material using the large specific surface area. However, only by adsorption on the surface of activated carbon, the capacity obtained is about 1/100 of the intercalation material used in lithium ion secondary batteries, and the capacity obtained for use as a battery is too small. was there.

  Therefore, an object of the present invention is to provide an electrode for an electrochemical element that can be charged and discharged at a high speed and can constitute a high-capacity electrochemical element.

As a result of intensive studies to solve the above problems, the inventors of the present invention have found that 8 titanate nanosheets represented by the chemical formula H ₂ Ti ₈ O ₁₇ .nH ₂ O (n = 0 to 2.0) and carbon When an electrode for an electrochemical element is formed by including a material, the above-mentioned 8 titanate nanosheet is an intercalation material and can be charged / discharged at high speed to form a high-capacity electrochemical element. As a result, the present invention has been completed.

In particular, an 8 titanate nanosheet represented by the chemical formula H ₂ Ti ₈ O ₁₇ .nH ₂ O (n = 0 to 2.0) and a carbon material are combined, and the 8 titanate nanosheet and the carbon material are combined. When an electrode for an electrochemical element is formed using the composite of the above as an active material, the conductivity is improved based on the composite with the carbon material, the load characteristics are further improved, and the capacity capable of high-speed charge / discharge is large. Become.

  Since the electrode for electrochemical devices of the present invention has a short diffusion distance of Li ions in the 8 titanate nanosheet during charge / discharge, it can be charged / discharged at high speed, and the 8 titanate nanosheet or the 8 titanate Since the composite of the nanosheet and the carbon material can intercalate and deintercalate Li ions, a high capacity can be obtained. In particular, when the 8 titanate nanosheet and the carbon material are combined, the combination with the carbon material improves the conductivity and further improves the load characteristics. growing. Therefore, by using these electrodes for electrochemical elements as the electrodes of the electrochemical elements, it is possible to construct a high-capacity electrochemical element that can be charged and discharged at high speed.

  In the present invention, the above-described 8 titanate nanosheet is used as an active material, and it is possible to charge and discharge at high speed and have a high capacity as described above just by mixing a conductive material in order to impart electrical conductivity thereto. Although an element can be provided, it is particularly preferable that the 8 titanate nanosheet and the carbon material are composited and the composite is used as an active material. The case where the composite is used as an active material will be described in detail.

The composite of the 8 titanate nanosheet and the carbon material is obtained by delamination of 4 titanic acid represented by the chemical formula H ₂ Ti ₄ O ₉ .nH ₂ O (n = 0 to 2.0), and delamination thereof. The obtained 4 titanic acid is combined with a carbon material and reconstructed, and then heat treated or electromagnetically treated.

  As a method of delamination of 4 titanic acid, tetraalkylammonium ion such as tetramethylammonium ion, tetraethylammonium ion, tetrapropylammonium ion, tetrabutylammonium ion, decyltrimethylammonium ion, didecyldimethylammonium ion, etc. Can be employed, and conventionally known methods such as washing the product with water and drying can be employed (for example, Journal of Material Chemistry, 2002, 12, 3814-3818, JP-A-2003-201121, JP-A-2001-270022).

  As a method of combining delaminated 4 titanic acid and a carbon material, for example, delaminated 4 titanic acid is dispersed in water to form a sol, and a carbon material such as carbon powder is added to the sol and stirred. A method of obtaining a composite of a 4 titanate nanosheet and a carbon material (hereinafter simply referred to as a “4 titanate nanosheet carbon composite”) by mixing, pouring into an acid or alkaline aqueous solution, and drying. Adopted. As the acid, for example, hydrochloric acid having a concentration of 0.1 mol / l can be used. As the alkaline aqueous solution, for example, a lithium hydroxide aqueous solution having a concentration of 1 mol / l can be used. Further, the 4 titanate nanosheet carbon composite can remove residual alkali by a method such as acid treatment, if necessary. The carbon material used for the composite is not particularly limited as long as it is a substance that can impart electronic conductivity to the composite as an active material by compositing. For example, artificial graphite, natural graphite Acetylene black, ketjen black, carbon black, vapor grown carbon fiber (VGCF), amorphous carbon, carbon fiber, carbon nanotube, fullerene and the like are preferably used.

  In order to construct an electrode for an electrochemical device using the obtained 4 titanate nanosheet carbon composite, first, the 4 titanate nanosheet is applied to or crimped to the current collector, Or it needs to be pelletized. At that time, any method may be used, but in order to utilize the high-speed charge / discharge characteristics more appropriately, it is preferable to employ a method of applying the 4 titanate nanosheet carbon composite to the current collector. The base material used for the current collector may be any material that is generally used as a current collector for electrochemical devices, but Cu, Al, Ni, stainless steel, and the like are preferably used. These base materials can be used as a current collector in a state of being processed into a foil shape, a mesh shape, or the like. Further, for the purpose of improving the adhesion with the current collector, a polymer binder or the like may be added to the tetratitanate nanosheet carbon composite. As the above-mentioned polymer binder, necessary adhesion can be obtained, there is no solubility in the electrolytic solution, and no side reaction such as decomposition by oxidation or reduction is caused in the voltage range to be used, from 4 titanic acid to 8 titanic acid. Any compound can be used as long as it is a compound that is stable with respect to heat treatment and electromagnetic wave treatment during conversion, but a fluorine-based polymer such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, tetrafluoroethylene, It is particularly preferable because of its excellent chemical stability and thermal stability. The electrode body obtained as described above can be converted from 4 titanic acid to 8 titanic acid by heat treatment or electromagnetic wave treatment, whereby the composite of 8 titanate nanosheet and carbon material is activated. An electrode for an electrochemical device as a substance is obtained.

  By using, as a positive electrode or a negative electrode, an electrode using the composite of the thus obtained 8 titanate nanosheet and carbon material (hereinafter simply referred to as “8 titanate nanosheet carbon composite”) as an active material. An electrochemical element can be constructed. The counter electrode in the case where an electrode using the above 8 titanate nanosheet carbon composite as an active material is used as a positive electrode includes metallic lithium, LiAl alloy, metal that forms an alloy with Li such as Sn, Si, amorphous carbon, artificial Carbon materials such as graphite, natural graphite, fullerene, and nanotubes can be used. However, in order to utilize the high-speed charge / discharge characteristics of the 8 titanate nanosheet carbon composite of the present invention more appropriately, the counter electrode is made of metal such as metal lithium, LiAl alloy, Sn, Si or other metal that forms an alloy with Li. It is preferable to use a system.

When an electrode using the 8 titanate nanosheet carbon composite as an active material is used as the negative electrode, an active material having a potential of 2 V or more on the basis of Li / Li ⁺ reference is used for the counter electrode. Any electrode can be used. For example, the counter electrode includes LiCoO ₂ , LiNiO ₂ , LiMnO ₂ having a layered structure, LiMn ₂ O _{4 having} a spinel structure, LiFePO _{4 having} a olivine structure, a complex oxide of Li and a transition metal, V ₃ O ₆ , MnO ₂ , Metal oxides such as WO ₃ , β-FeOOH having a tunnel-like structure, or conductive polymers such as polyaniline, polypyrrole, polythiophene, polyacetylene, polyacene, and polyparaphenylene can be used.

  In addition, an electrode including a mixture of 8 titanate nanosheets and a carbon material is used for either a positive electrode or a negative electrode in the same manner as an electrode using the 8 titanate nanosheet carbon material composite as an active material. The counter electrode can be the same as that of the electrode using the 8 titanate nanosheet carbon material composite as an active material.

  The 8 titanate nanosheet in the electrode for electrochemical devices of the present invention is preferably as thin as possible. Specifically, the thickness is preferably 100 nm or less.

  As described above, the 8 titanate nanosheet and the carbon material may be combined at the stage of the 4 titanate nanosheet before being converted into the 8 titanate nanosheet, but the amount of the composite is particularly limited. Although not carried out, from the viewpoint of improving the conductivity by combining with the carbon material and suppressing the capacity reduction due to the decrease of the 8 titanate nanosheet, the mass ratio of the 8 titanate nanosheet and the carbon material is 40:60. ~ 90: 10 is preferred.

In constructing an electrochemical element using the electrode for an electrochemical element of the present invention, an electrolytic solution in which a Li salt is dissolved in an organic solvent is used. The Li salt may be any one as long as it dissociates in a solvent to form Li ⁺ ions and does not cause a side reaction such as decomposition in the voltage range used as the device. For example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ and other inorganic compounds, LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiN (SO ₂ CF _a ) (SO ₂ C ₄ F ₉ ), LiC (SO _{2 CF 2) 3, LiC (} SO 2 C 2 F 5) 2, LiPF 8-n (C 2 F 5) n ( n is an integer from 1 to _{6), LiSO 3 CF 3,} LiSO 3 C 2 F 5 Organic compounds such as LiSO ₃ C ₄ F ₈ can be used.

  The organic solvent may be any one that dissolves the Li salt and does not cause side reactions such as decomposition in the voltage range used as the device. For example, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene Cyclic carbonates such as carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate, cyclic esters such as γ-butyrolactone, chain ethers such as dimethoxyethane, diglyme, triglyme and tetraglyme, dioxane, tetrahydrofuran, 2- Cyclic ethers such as methyltetrahydrofuran, nitriles such as acetonitrile, propionitrile, and methoxypropionitrile can be used alone or in combination of two or more. It is also possible to use. However, in order to obtain good characteristics, it is preferable to use a combination capable of obtaining high conductivity such as a mixed solvent of ethylene carbonate and chain carbonate.

  These electrolytes contain additives such as vinylene carbonate or derivatives thereof, benzene or derivatives thereof, 1,3-propane sultone, diphenyl for the purpose of improving safety, cycleability, high temperature storage properties and the like. -Additives such as disulfide or a derivative thereof, biphenyl or a derivative thereof can be appropriately added.

  Also, instead of organic solvents, room temperature molten salts such as ethyl-methylimidazolium trifluoromethylsulfonium imide, hebutyl-trimethylammonium trifluoromethylsulfonium imide, pyridinium trifluoromethylsulfonium imide, guanidinium trifluoromethylsulfonium imide, etc. It can also be used.

  Further, the above electrolyte solution is crosslinked with polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyethylene oxide, polypropylene oxide, ethylene oxide-propylene oxide copolymer, ethylene oxide chain in the main chain or side chain. An electrolytic solution obtained by adding a host polymer capable of forming a gel electrolyte such as a polymer to be gelled can also be used.

  Also, instead of electrolyte, polymers such as polyethylene oxide, siloxane polymers containing ethylene oxide groups in the side chains, (meth) acrylates containing ethylene oxide groups in the side chains, polyethylene carbonates, polypropylene carbonates, polycarbonates containing anhydroglucitol groups It is also possible to use a polymer electrolyte comprising lithium salt.

Further, instead of the electrolytic solution, Li ₂ S—SiS ₂ —Li ₄ SiO ₄ , Li ₂ S—SiS ₂ —P ₂ S ₅ —LiI, LiO ₂ —Al ₂ O ₃ —TiO ₂ —P ₂ O _5, etc. Inorganic glass electrolyte, NASICON type such as LiTi ₂ (PO ₄ ) ₃ , perovskite type such as La _0.57 Li _0.28 TiO ₃ , LISICON type such as Li _3.25 Ge _0.25 P _0.75 S ₄ , Li _3.4 Si _0.4 P _0.6 S ₄ An inorganic solid electrolyte can also be used.

  An electrochemical element is constructed using these constituent materials, and the form of the element at that time may be any shape and form such as a conventionally known cylindrical shape, square shape, coin shape or laminate type. There is no particular limitation. For example, when producing a cylindrical electrochemical element, a sheet-like positive electrode and negative electrode produced through application or pressure bonding to a current collector are wound through a separator made of a polyolefin microporous film. An electrode having a wound structure is inserted into a bottomed cylindrical can made of a material such as stainless steel or aluminum, a positive and negative electrode terminal is attached, an electrolyte is injected, and the electrode is sealed. As a sealing method, a method such as caulking through a gasket or laser welding can be employed. In addition, when producing a laminate type electrochemical element using an aluminum laminate film having aluminum as a core material as an exterior material, the wound body is wound not in a cylindrical shape but in an oval shape to form a bag. It is produced by inserting it into a bag of aluminum laminate film, taking out the terminal, injecting an electrolyte, and sealing the laminate film by heat fusion or the like. Moreover, when producing a coin-shaped electrochemical element, a pellet-shaped electrode formed by pressing and a counter electrode are placed in a can via a separator, an electrolyte is injected, a counter electrode can is covered, and a sealing gasket is interposed. It is produced by caulking.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples. In the following examples and the like,% indicating the concentration of a solution or the like is mass% unless otherwise specified.

Example 1
The electrode for an electrochemical element of Example 1 will be described in the order of [4 delamination of titanic acid], [composite with carbon material], and [production of electrode].
[4 Titanic acid delamination]
4 titanic acid dihydrate (H ₂ Ti ₄ O ₉ .2H ₂ O) was dissolved in an ethylamine aqueous solution so that the mass ratio was 4 titanic acid: ethylamine = 1: 8, and the mixture was stirred at room temperature for 24 hours. Centrifugation and vacuum drying were carried out to obtain ethylamine intercalate 4 titanate (hereinafter referred to as “C ₂ N-HT ₄ ”).

This C ₂ N-HT ₄ was dissolved in a 10% aqueous tetrabutylammonium hydroxide solution at a molar ratio of C ₂ N-HT ₄ and tetrabutylammonium in a ratio of 1: 2, and stirred at room temperature for 24 hours. Centrifugation and vacuum drying were performed to obtain tetratitanic acid intercalated with tetrabutylammonium (hereinafter referred to as “TBA-HT ₄ ”). The TBA-HT ₄ was stirred in pure water for 10 minutes to perform delamination to obtain a sol of 4 titanate nanosheets.

[Composite with carbon material]
Vapor growth carbon fiber (hereinafter referred to as “VGCF”) was added to the sol of 4 titanate nanosheets so that the mass ratio was TBA-HT ₄ : VGCF = 93.3: 50, and stirred for 24 hours. After mixing, the resulting mixture was poured into 1 mol / l hydrochloric acid, stirred for 48 hours, washed and dried to obtain a 4-titanate nanosheet carbon composite.

[Production of electrodes]
The tetratitanate nanosheet carbon composite was mixed with polytetrafluoroethylene as a binder at a ratio of 10 parts by mass with respect to 90 parts by mass of the 4 titanate nanosheet carbon composite. An electrode using an 8 titanate nanosheet carbon composite as an active material was obtained by heat-bonding to 200 ° C. for 2 hours. The chemical formula of the obtained 8 titanate nanosheet is H ₂ Ti ₈ O ₁₇ · H ₂ O, the mass ratio of 8 titanate nanosheet to VGCF in the composite is 50:50, and the amount of active material on the electrode is It was 20 mg / cm ³ . In addition, about the obtained electrode, as will be described in detail later, a tripolar cell is assembled using the electrode, and load characteristics and cycle characteristics in high-speed charge / discharge are examined using the cell.

Example 2
An electrode was produced in the same manner as in Example 1, except that the composite of TBA-HT ₄ and VGCF obtained in Example 1 was applied as it was in a paste form to a Ni net and dried without drying. did.

Example 3
A sol obtained by stirring TBA-HT ₄ in pure water for 10 minutes without being combined with carbon in Example 1 was poured into 1 mol / l hydrochloric acid as it was and stirred for 48 hours, followed by washing and drying. And the 4 titanic acid nanosheet was obtained.

The obtained 4 titanate nanosheet and acetylene black (hereinafter referred to as “AB”) were mixed at a mass ratio of 50:50, and 10% by mass of polytetrafluoroethylene was added and mixed as a binder. The mixture was pressure-bonded to a Ni net and heat treated at 200 ° C. for 2 hours to convert 4 titanic acid to 8 titanic acid to obtain an electrode. The chemical formula of the obtained 8 titanate nanosheet is H ₂ Ti ₈ O ₁₇ · H ₂ O, the mass ratio of 8 titanate nanosheet to AB is 50:50, and the amount of active material on the electrode is 20 mg / cm ^3. Met.

Comparative Example 1
Instead of delaminated 4 titanic acid, fibrous 4 titanic acid having a fiber length of 20 μm and a diameter of 800 nm was used, and the fibrous 4 titanic acid and VGCF were mixed at a mass ratio of 55:50. Similarly, an electrode was prepared by mixing with polytetrafluoroethylene and press-bonding to a Ni net and heat-treating at 200 ° C. for 2 hours. The chemical formula of the obtained 8 titanic acid was H ₂ Ti ₈ O ₁₇ · H ₂ O, and the amount of active material on the electrode was 20 mg / cm ₂ .

Comparative Example 2
An electrode was produced in the same manner as in Comparative Example 1 except that AB was used instead of VGCF in Comparative Example 1.

  Using the obtained electrodes for electrochemical elements of Examples 1 to 3 and Comparative Examples 1 and 2, respectively, a tripolar cell was assembled as shown below, and the cell was used for high-speed charge / discharge. The load characteristics and cycle characteristics were investigated. The assembly of the triode cell, the evaluation method for load characteristics, and the evaluation method for cycle characteristics are as follows.

Assembling the tripolar cell:
Using the electrodes of Examples 1 to 3 and Comparative Examples 1 and 2 obtained as described above, metallic lithium was used for the counter electrode and the reference electrode, a polyethylene microporous membrane was used for the separator, and the electrolyte was 1M. A tripolar cell was assembled using a LiClO ₄ PC solution (1 mol / l of LiClO ₄ dissolved in propylene carbonate (PC)).

Evaluation method of load characteristics:
Cells assembled using the electrodes of Examples 1 to 3 and Comparative Examples 1 and 2 were charged at a current density of 100 mA / g and a discharge current density of 50 mA / g to 2.00 A / g at 25 ° C. The density is fixed at 100 mA / g, and the discharge current density is appropriately varied from 50 mA / g (0.05 A / g) to 2.00 A / g]. The discharge capacity at the 10th discharge was measured. The result is shown in FIG. The load characteristics can be evaluated from the discharge capacity at a discharge current density of 1.00 A / g or more, which is the high discharge current density region of FIG.

Evaluation method of load characteristics:
The cells assembled using the electrodes of Examples 1 to 3 and Comparative Examples 1 to 2 were charged at a charge current density and discharge current density of 100 mA / g at 25 ° C. and a voltage of 1.2 to 3.6 V. The discharge capacity was measured for each cycle. The result is shown in FIG. The cycle characteristics can be evaluated from the decrease in the discharge capacity accompanying the increase in the number of cycles in FIG.

  As shown in FIG. 1, Examples 1 to 3 have less load capacity reduction and excellent load characteristics when discharged at a high current density of 1.00 A / g or more compared to Comparative Examples 1 and 2. It was. Further, as shown in FIG. 2, the decrease in the discharge capacity accompanying the increase in the number of cycles in Examples 1 to 3 was not different from that in Comparative Examples 1 and 2, and no particular deterioration in cycle characteristics was observed. In addition, Examples 1-2 which used what compounded 8 titanate nanosheet and carbon material as an active material used the thing which mixed Comparative Examples 1-2 and 8 titanate nanosheet and carbon material. In comparison with Example 3, the discharge capacity was always high, and it was shown that the composite of the 8 titanate nanosheet and the carbon material can provide a higher capacity during high-speed charge / discharge.

It is a figure which shows the load characteristic at the time of the high-speed charge / discharge of the cell comprised using the electrode for electrochemical elements of Examples 1-3 and Comparative Examples 1-2, and using the metal Li for the counter electrode and the reference electrode. It is a figure which shows the cycling characteristics at the time of the high-speed charge / discharge of the cell comprised using the electrode for electrochemical elements of Examples 1-3 and Comparative Examples 1-2, and using the metal Li for the counter electrode and the reference electrode.

Claims (6)

Electrochemistry characterized in that it contains at least an 8 titanate nanosheet represented by the chemical formula H ₂ Ti ₈ O ₁₇ .nH ₂ O (n = 0 to 2.0) and a carbon material for imparting conductivity. Element electrode. For an electrochemical device characterized in that a composite of 8 titanate nanosheets represented by the chemical formula H ₂ Ti ₈ O ₁₇ .nH ₂ O (n = 0 to 2.0) and a carbon material is used as an active material. electrode. The electrode for an electrochemical element according to claim 1, comprising an 8 titanate nanosheet having a thickness of 100 nm or less. The carbon material includes at least one selected from the group consisting of artificial graphite, acetylene black, ketjen black, carbon black, vapor grown carbon fiber, amorphous carbon, carbon fiber, carbon nanotube, and fullerenes. The electrode for an electrochemical element according to any one of? 4 titanic acid represented by the chemical formula H ₂ Ti ₄ O ₉ .nH ₂ O (n = 0 to 2.0) was delaminated, and the delaminated 4 titanic acid was combined with a carbon material and reconstructed. A composite of an 8 titanate nanosheet and a carbon material, wherein the 4 titanate is converted into 8 titanate by being bonded to the current collector by pressure bonding or coating and then heat-treated. For producing an electrode for an electrochemical element using a material as an active material. An electrochemical element using the electrode for an electrochemical element according to claim 1.

Images