Electrodes and energy storage devices

Abstract

To provide electrodes and energy storage devices that can reduce irreversible capacity in high-temperature environments. [Solution] The electrode has a composite layer containing carbon material, and on the surface of the composite layer, the ratio of the atomic concentration of carbon present as OC=O bonds, as determined by the spectrum derived from OC=O bonds, to the total atomic concentration of carbon determined by X-ray photoelectron spectroscopy from spectra derived from CC bonds, CO bonds, OC=O bonds, carbonate ions, and CF bonds is 0.065 or more.

Description

[Technical Field]

[0001] This invention relates to electrodes and energy storage devices containing carbon materials. [Background technology]

[0002] To reduce the irreversible capacitance of electrodes containing carbon materials, the prior art disclosed in Patent Document 1 involves the oxygen peak intensity I of the photoelectron spectrum. O and the peak intensity of carbon I C Ratio I O / I C The value is between 0.5 and 2.5, and the peak intensity is around 288 eV. 288 and peak intensity I around 285 eV 285 Ratio I 288 / I 285 An organic film containing oxygen is formed on the surface of the carbon material such that the ratio is between 0.05 and 1. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 7-245098 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Prior art has the problem of being ineffective in reducing irreversible capacity under high-temperature conditions.

[0005] This invention was made to solve this problem and aims to provide electrodes and energy storage devices that can reduce irreversible capacity in high-temperature environments. [Means for solving the problem]

[0006] A first aspect of the present invention for achieving this objective is an electrode having a composite layer containing a carbon material, wherein on the surface of the composite layer, the ratio of the atomic concentration of carbon present as OC=O bonds, as determined by the spectrum derived from OC=O bonds, to the total atomic concentration of carbon determined by X-ray photoelectron spectroscopy from spectra derived from CC bonds, CO bonds, OC=O bonds, carbonate ions, and CF bonds is 0.065 or more.

[0007] In the second embodiment, the total atomic concentration of carbon is 20% or less, as in the first embodiment.

[0008] In the third embodiment, in the first or second embodiment, the atomic concentration of fluorine determined by X-ray photoelectron spectroscopy on the surface of the composite layer is 20% or less.

[0009] A fourth embodiment is an energy storage device including a positive electrode and a negative electrode, wherein the negative electrode is an electrode according to any of the first to third embodiments. [Effects of the Invention]

[0010] According to the present invention, on the surface of a composite layer containing carbon material, the ratio of the atomic concentration of carbon present as OC=O bonds, as determined by the spectrum derived from OC=O bonds, to the total atomic concentration of carbon determined by X-ray photoelectron spectroscopy from spectra derived from CC bonds, CO bonds, OC=O bonds, carbonate ions, and CF bonds is 0.065 or higher, thus reducing irreversible capacity under high-temperature conditions. [Brief explanation of the drawing]

[0011] [Figure 1] This is a cross-sectional view of an electrode in one embodiment. [Figure 2] This is a cross-sectional view of an energy storage device. [Modes for carrying out the invention]

[0012] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of an electrode 10 in one embodiment. The electrode 10 includes a current collector 11 and a composite material layer 12 attached to the current collector 11. There is no limitation on the material of the current collector 11, but examples include Cu, Al, Cu alloy, Al alloy, and stainless steel. There is no limitation on the shape of the current collector 11. The current collector 11 may be a porous foil provided with a plurality of holes penetrating the current collector 11.

[0013] The composite material layer 12 includes a carbon material 14. The carbon material 14 contains carbon and is not particularly limited as long as it can occlude and release charge carriers. The charge carriers are + Li + Na + K 2+ Ca, etc. are exemplified. The charge carriers are preferably alkali metal ions, and particularly + Li is preferred.

[0014] The carbon material 14 includes graphite, low-crystalline carbon, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon black, carbon nanotubes, and carbon fibers. The carbon material 14 preferably contains particles having a volume-based median diameter of 1 to 30 μm obtained by measuring the particle size distribution by the laser diffraction / scattering method. This is to ensure the capacity of the electrode 10.

[0015] The composite material layer 12 may contain a conductive material 15. This is to reduce the resistance of the electrode 10. The conductive material 15 includes graphite, low-crystalline carbon, graphitizable carbon, non-graphitizable carbon, carbon black, carbon nanotubes, carbon fibers, fibrous or coiled metals, etc.

[0016] The carbon material 14 present on the surface 13 of the composite layer 12 opposite to the surface to which the current collector 11 is attached contains one or more selected from C-C bonds, C-O bonds, O-C=O bonds, carbonate ions, and C-F bonds. The O-C=O bond does not include the O-C=O bond in carbonate ions, and the C-O bond does not include the C-O bond in carbonate ions or O-C=O bonds. This is to exclude the overlap of C-O bonds and O-C=O bonds.

[0017] Information regarding the bonding state of the elements present on the surface 13 of the composite layer 12 is obtained by X-ray photoelectron spectroscopy (XPS). XPS irradiates the surface 13 of the composite layer 12 with X-rays and measures the energy and intensity of the photoelectrons generated from the surface 13 due to the photoelectric effect, thereby identifying the abundance and types of elements present on the surface 13. The X-ray source is exemplified by AlKα rays.

[0018] Since the background overlaps due to the inelastic scattering of the generated photoelectrons in the photoelectron spectrum, the background is subtracted before peak fitting to separate the spectrum into its intrinsic spectrum. The background is usually subtracted using the Shirley method, but the Tougaard method may also be used.

[0019] In peak fitting, the peak top of the C-C bond appears at a binding energy of 285 eV, the peak top of the C-O bond appears at a binding energy of 287 eV, the peak top of the O-C=O bond appears at a binding energy of 289 eV, the peak top of the carbonate ion appears at a binding energy of 290 eV, and the peak top of the C-F bond appears at a binding energy of 291 eV. Assuming that each spectrum can be expressed by a specific function, the parameters of the function that constitutes the synthetic spectrum are adjusted so that the sum of the squares of the differences between the observed spectrum and the synthetic spectrum is minimized, and the optimal values of the parameters are obtained. The functions are exemplified by the Lorentz function, the Gauss function, the Voigt function obtained by convolving the Lorentz function and the Gauss function, and the pseudo Voigt function obtained by adding two functions together.

[0020] The atomic concentration of carbon can be determined by dividing the peak intensity, which is the area of ​​the spectrum after peak fitting (synthetic spectrum), by the relative sensitivity coefficient corresponding to carbon. The atomic concentration of carbon can be determined from the ratio of the peak intensity originating from CC bonds, CO bonds, OC=O bonds, carbonate ions, and CF bonds.

[0021] The bond energy of the OC=O bond is 289 eV, which is greater than the bond energy of the CC bond and CO bond (285-287 eV). Therefore, by increasing the ratio R of the atomic concentration of carbon present as an OC=O bond to the total atomic concentration T of carbon present as CC bonds, CO bonds, OC=O bonds, carbonate ions, and CF bonds, the irreversible capacity (the difference between discharge capacity and charge capacity in one discharge-charge cycle) in high-temperature environments can be reduced. A ratio R of 0.065 or higher is preferable. Examples of high-temperature environments include environments above 85°C.

[0022] A ratio R of 0.2 or less is preferable. This is because the OC=O bond constitutes an inorganic film, and if the ratio R becomes too large, the conductivity of the film will decrease, potentially increasing the resistance of electrode 10.

[0023] To ensure the reduction of irreversible capacity in high-temperature environments, the total atomic concentration T of carbon is preferably 20 atm% or less.

[0024] The atomic concentration of fluorine determined from the XPS spectrum on the surface 13 of the composite layer 12 is preferably 20 atm% or less. This is to reduce the amount of fluorine added.

[0025] The electrode 10 is manufactured, for example, as follows: First, a slurry is prepared in which a carbon material 14 is contained as an essential component, and a conductive material 15, a binder, and a thickener are contained as optional components, and these are dispersed in a dispersion medium.

[0026] Examples of binders include fluororesins, acrylic resins, polyolefins, and rubber. Examples of fluororesins include polytetrafluoroethylene, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymer. Examples of polyolefins include polyethylene and polypropylene. Examples of rubber include styrene-butadiene rubber. Examples of thickeners include carboxymethylcellulose. Examples of dispersion media include organic solvents such as N-methyl-2-pyrrolidone and water.

[0027] After applying the prepared slurry to the current collector 11, the slurry is dried to obtain an electrode 10 with an asphalt mixture layer 12 formed on the current collector 11. The dried asphalt mixture layer 12 may be rolled with rollers or the like.

[0028] Next, a charge carrier is supplied to the electrode 10, pre-doping it with an amount of charge carrier equivalent to at least a portion of the irreversible capacitance. Pre-doping the electrode 10 with a charge carrier lowers the potential of the negative electrode and increases the cell voltage, thereby increasing the energy density.

[0029] Pre-doping is exemplified by a method in which a separator is placed between the charge carrier metal and the composite layer 12, the current collector 11 and the metal are short-circuited, and the electrode 10, separator and metal are immersed in an electrolyte. Due to the potential difference between the current collector 11 and the metal, electrons flow from the metal to the current collector 11, and at the same time the metal is ionized, releasing the charge carrier into the electrolyte, and the charge carrier in the electrolyte is supported on the composite layer 12.

[0030] Pre-doping of the charge carrier yields an electrode 10 in which the composite layer 12 has been lithified. A current may be passed between the current collector 11 and the metal during pre-doping. Lithiated composite layer 12 means that some of the atoms constituting the composite layer 12 have been replaced with metal atoms, i.e., the composite layer 12 contains the metal charge carrier.

[0031] By dedoping the lithiated electrode 10, washing it with the solvent used in the electrolyte, and then vacuum drying it at room temperature or under heated conditions, an electrode 10 is obtained in which the proportion R of the atomic concentration of carbon present as OC=O bonds is 0.065 or higher.

[0032] Figure 2 is a cross-sectional view of the energy storage device 20. The energy storage device 20 includes, in order, an electrode 10 (negative electrode), a separator 21, and a positive electrode 22. The energy storage device 20 is an element that involves the interconversion of chemical energy and electrical energy.

[0033] Figure 2 shows an energy storage device 20 having one set of electrodes 10, a separator 21, and a positive electrode 22, but it is not limited to this and may include multiple sets of these components. The energy storage device 20 is not limited to having a stacked structure, but may also have a wound structure in which the electrodes 10 and positive electrode 22 are stacked and wound together via the separator 21.

[0034] Examples of energy storage devices 20 include secondary batteries and electrochemical capacitors. Examples of electrochemical capacitors include redox capacitors that utilize redox reactions of electrodes or redox reactions of ions in a non-aqueous electrolyte, and hybrid capacitors that combine an electric double layer and redox reactions, or combine them with secondary battery materials.

[0035] The positive electrode 22 consists of a current collector 23 and an composite layer 24 superimposed on each other. There are no restrictions on the material of the current collector 23, but examples include metals selected from Ni, Ti, Fe, and Al, alloys containing two or more of these elements, and stainless steel. There are no restrictions on the shape of the current collector 23. The current collector 23 may be a porous foil with multiple holes penetrating it.

[0036] The composite layer 24 contains active materials such as graphite and activated carbon, and conductive materials. Examples of conductive materials include carbon black, acetylene black, Ketjen black, and carbon fiber. The active material can be any material capable of reversibly supporting charge carriers, and is not limited to graphite or activated carbon. The composite layer 24 may also contain a binder that binds the active materials and conductive materials. The binder is the same as the binder contained in the composite layer 12 of the electrode 10.

[0037] The separator 21 is made of a porous material that is durable against the active material and electrolyte contained in the positive electrode 22 and electrode 10, and allows lithium ions to pass through but does not conduct electrons. Examples of separators 21 include nonwoven fabrics and porous membranes made of cellulose, polypropylene, polyethylene, etc.

[0038] The solvent for the electrolyte is not particularly limited as long as it is a liquid within the operating temperature range of the energy storage device 20. Examples of solvents include carbonate esters, aliphatic carboxylic acid esters, phosphate esters, γ-lactones, ethers, nitriles, sulfolanes, dimethyl sulfoxides, fluorescein solvents, and ionic liquids. Mixtures of these may also be used. By using an electrolyte containing an appropriate amount of esters with OC=O bonds, such as propyl acetate, propyl propionate, and ethyl propionate, OC=O bonds are formed in the carbon material 14 of the composite layer 12.

[0039] The energy storage device 20 is manufactured, for example, as follows: A slurry is made by dispersing an active material and a conductive material in a binder solution. After applying the slurry onto the current collector 23, it is dried to obtain a green sheet (positive electrode sheet) for the positive electrode 22. After cutting the separator 21, positive electrode sheet and electrode 10 into predetermined shapes, the positive electrode sheet, separator 21, and electrode 10 are stacked in that order, terminals (not shown) are connected to the current collectors 11 and 23 respectively, and the device is sealed in a container (not shown) with an electrolyte to obtain an energy storage device 20 containing the positive electrode 22, separator 21, and electrode 10 (negative electrode) in that order. [Examples]

[0040] The present invention will be described in more detail by reference to examples, but the present invention is not limited to these examples.

[0041] (Preparation of electrolyte solution) Lithium battery-grade ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate were mixed in a ratio of 30:30:40 (by volume), and 1 wt% vinylene carbonate was added to obtain the solvent for Sample No. 1.

[0042] Lithium battery-grade ethylene carbonate, diethyl carbonate, ethyl methyl carbonate, and ethyl propionate were mixed in a ratio of 25:30:40:5 (by volume), and 1 wt% vinylene carbonate was added to obtain the solvent for Sample No. 2.

[0043] Lithium battery-grade ethylene carbonate, diethyl carbonate, ethyl methyl carbonate, and ethyl propionate were mixed in a ratio of 20:30:40:10 (by volume), and 1 wt% vinylene carbonate was added to obtain the solvent for Sample No. 3.

[0044] Lithium battery-grade ethylene carbonate, diethyl carbonate, and ethyl propionate were mixed in a ratio of 30:30:40 (by volume), and 1 wt% vinylene carbonate was added to obtain the solvent for Sample No. 4.

[0045] In samples No. 1-4, lithium bis(fluorosulfonyl)imide (LiFSI) was used as the solvent at a salt concentration of 1 mol / dm³. 3 Each component was added accordingly to obtain the electrolytes for samples No. 1-4.

[0046] (Fabrication of the positive electrode) A slurry was prepared by mixing activated carbon, a binder, and acetylene black (conductive material) in a ratio of 80:10:10 (mass ratio). The slurry was applied to a current collector made of 30 μm thick aluminum foil, dried to form a composite layer, and then punched out into a square shape with sides of 20 mm to obtain the positive electrode. The amount of slurry applied was 5.0 mg / cm³, which is the combined mass of activated carbon, conductive material, and binder. 2 I adjusted it so that it would be as follows.

[0047] (Fabrication of the negative electrode) A slurry was prepared by mixing graphite, binder, thickener, and acetylene black (conductive material) in a mass ratio of 93:5:1:1. The slurry was applied to a current collector made of 8 μm thick copper foil, dried to form a composite layer, and then punched out into a square shape with sides of 25 mm to obtain the negative electrode. The amount of slurry applied was 6.5 mg / cm³, with the combined mass of graphite, binder, thickener, and conductive material being 6.5 mg / cm³. 2 I adjusted it so that it would be as follows.

[0048] (Negative electrode pre-doping) Half-cells were fabricated by sequentially attaching a separator and a metallic lithium foil (counter electrode) pressed onto a copper foil on top of the composite material layer of the negative electrode. After immersing each half-cell in the electrolyte of Samples No. 1-4, they were degassed and sealed. Pre-doping was performed by passing a current between the copper foil of the current collector of the half-cell and the copper foil of the counter electrode, and then charging with a constant current of 0.1C to 0V at 25°C, followed by constant voltage charging to 0.01C. The half-cells immersed in the electrolyte of Samples No. 1-4 were disassembled, and the negative electrodes were removed from each.

[0049] (Cell creation in Sample No. 1) A cell was fabricated by sequentially attaching a separator and a positive electrode to the composite material layer of the negative electrode, which was removed from a half-cell immersed in the electrolyte of Sample No. 1. After injecting the electrolyte of Sample No. 1 into the cell, it was degassed under vacuum. After aging the cell by applying a potential of 3.8V between the positive and negative electrodes at room temperature, the cell was sealed under vacuum to obtain the cell (lithium-ion capacitor) of Sample No. 1.

[0050] (Cell creation in Sample No. 2) Sample No. 2 was obtained in the same manner as Sample No. 1, except that a separator and a positive electrode were sequentially attached to the composite material layer of the negative electrode taken from the half-cell immersed in the electrolyte of Sample No. 2, and the electrolyte of Sample No. 2 was injected into the cell.

[0051] (Cell creation in Sample No. 3) Sample No. 3 was obtained in the same manner as Sample No. 1, except that a separator and a positive electrode were sequentially attached to the composite material layer of the negative electrode taken from the half-cell immersed in the electrolyte of Sample No. 3, and the electrolyte of Sample No. 3 was injected into the cell.

[0052] (Cell creation in Sample No. 4) Sample No. 4 was obtained in the same manner as Sample No. 1, except that a separator and a positive electrode were sequentially attached to the composite material layer of the negative electrode taken from the half-cell immersed in the electrolyte of Sample No. 4, and the electrolyte of Sample No. 4 was injected into the cell.

[0053] (Measurement of initial capacity) Cells in samples No. 1-4 were charged at a constant current of 1C to 3.8V at room temperature. Next, each cell was charged at a constant voltage of 3.8V for 30 minutes, then switched to constant voltage discharge at a 1C rate, and the slope of the discharge curve was calculated. The initial capacity of each cell was calculated by dividing the discharge current value by the slope of the discharge curve.

[0054] (Measurement and determination of cell capacity after high-temperature testing) A high-temperature test was conducted by placing each cell in a constant temperature bath at 85°C and applying a voltage of 3.8V between the positive and negative electrodes for 1000 hours. After the high-temperature test, the cells were cooled to room temperature and charged and discharged at a 1C rate to calculate the cell capacity. Cells whose capacity after the high-temperature test was 90% or more of the initial capacity were classified as A, and those whose capacity after the high-temperature test was less than 90% of the initial capacity were classified as B.

[0055] (Quantitative analysis using XPS) After the high-temperature test, the cell was disassembled, and the removed negative electrode was washed by immersing it in dimethyl carbonate for 5 minutes to remove the electrolyte contained in the negative electrode. The washed negative electrode was stored in a transfer vessel under an argon atmosphere. After introducing the negative electrode into the XPS analyzer, the surface of the composite layer was irradiated with X-rays. The XPS conditions were: X-ray: monochromatic AlKα rays, pass energy: 280 eV, analysis area: 100 μmΦ.

[0056] After removing the background from the observed C1s narrow spectrum using the Shirley method, we assumed that the peak top for the CC bond appeared at bond energy 285eV, the peak top for the CO bond appeared at bond energy 287eV, the peak top for the OC=O bond appeared at bond energy 289eV, the peak top for the carbonate ion appeared at bond energy 290eV, and the peak top for the CF bond appeared at bond energy 291eV, and then performed peak fitting of the C1s spectrum using the Voigt function.

[0057] The atomic concentrations of carbon present as CC bonds, CO bonds, OC=O bonds, carbonate ions, and CF bonds were determined by dividing the peak intensity (the area of ​​the spectrum after peak fitting) by a relative sensitivity coefficient corresponding to carbon. Furthermore, the atomic concentration of fluorine was determined.

[0058] Table 1 shows the results of quantitative analysis of the negative electrode by XPS for samples No. 1-4, including the total atomic concentration (atm%) of carbon present as CC bonds, CO bonds, OC=O bonds, carbonate ions, and CF bonds (T), the ratio (ratio) of the atomic concentration of carbon present as OC=O bonds to the total T, the atomic concentration of fluorine (atm%), and the results of the high-temperature test of the cells for samples No. 1-4.

[0059] [Table 1]

[0060] As shown in Table 1, samples No. 2 and 3 received a rating of A, while samples No. 1 and 4 received a rating of B. Samples No. 2 and 3 exhibited lower irreversible capacity under high-temperature conditions compared to samples No. 1 and 4. In samples No. 2 and 3, the ratio of carbon atoms present as OC=O bonds to the total carbon atom concentration T was 0.065 or higher, whereas in samples No. 1 and 4, the ratio of carbon atoms present as OC=O bonds to the total carbon atom concentration T was less than 0.065. Since OC=O bonds have higher bond energy than CC bonds and CO bonds, it is presumed that samples No. 2 and 3, with a larger proportion of carbon atoms present as OC=O bonds, exhibited reduced irreversible capacity under high-temperature conditions compared to samples No. 1 and 4.

[0061] As shown in Table 1, samples No. 2 and 3 had a total carbon atomic concentration T of 20% or less. It is presumed that having a total carbon atomic concentration T of 20% or less contributed to the reduction of irreversible capacity under high-temperature conditions.

[0062] Generally, as the atomic concentration of fluorine increases, the irreversible capacity under high-temperature conditions tends to decrease. However, as shown in Table 1, samples No. 2 and 3 showed reduced irreversible capacity under high-temperature conditions even at low fluorine atomic concentrations of 20% or less. Since samples No. 2 and 3 can reduce irreversible capacity under high-temperature conditions without adding large amounts of fluorine, the consumption of fluorine-related materials can be reduced.

[0063] Although the present invention has been described above based on embodiments, it can be easily inferred that the present invention is not limited in any way to the above embodiments, and that various improvements and modifications are possible without departing from the spirit of the present invention.

[0064] In the embodiment, the energy storage device 20 was described as comprising an electrode 10 (negative electrode) with a composite material layer 12 provided on one side of a current collector 11, and a positive electrode 22 with a composite material layer 24 provided on one side of a current collector 23, but it is not necessarily limited to this. For example, it is certainly possible to apply each element of the embodiment to an energy storage device that comprises electrodes (so-called bipolar electrodes) with a composite material layer 12 and a composite material layer 24 provided on both sides of the current collector 11, respectively. By alternately stacking bipolar electrodes and separators 21 and housing them in a container (not shown), a so-called bipolar structure energy storage device can be obtained.

[0065] In the embodiment described, an electrode 10 was provided with a composite material layer 12 on one side of the current collector 11, but it is not necessarily limited to this. It is certainly possible to provide the composite material layer 12 on both sides of the current collector 11.

[0066] In the embodiment, a carbon material 14 is formed into an electrode 10, which is then immersed in an electrolyte solution. An electric current is passed through the electrode using a metal charge carrier as the counter electrode. The electrode 10 is then pre-doped with the charge carrier into the composite layer 12 of the electrode 10, followed by dedoping in a high-temperature environment of 85°C, and finally washing and drying with a solvent contained in the electrolyte solution to obtain the electrode 10. However, the invention is not limited to this method. It is certainly possible to fabricate the electrode 10 by other methods.

[0067] Another example involves applying a surface treatment to the carbon material 14 to form an organic film having CO bonds or OC=O bonds on its surface, and then using the carbon material 14 with the organic film to form the composite layer 12. After forming the composite layer 12 using the carbon material 14 with the organic film into the form of an electrode 10, it is naturally possible to further treat the carbon material 14 by pre-doping or de-doping the composite layer 12 of the electrode 10 with a charge carrier. [Explanation of symbols]

[0068] 10 electrodes (negative electrode) 12 Composite layer 13 Surface 14 Carbon materials 20 Energy Storage Devices 22 Positive electrode

Claims

1. An electrode having a composite layer containing a carbon material, An electrode in which, on the surface of the composite layer, the ratio of the atomic concentration of carbon present as O-C=O bonds, as determined by the spectrum derived from O-C=O bonds, to the total atomic concentration of carbon determined by X-ray photoelectron spectroscopy from spectra derived from C-C bonds, spectra derived from C-O bonds, spectra derived from O-C=O bonds, spectra derived from carbonate ions, and spectra derived from C-F bonds, is 0.065 or more.

2. The electrode according to claim 1, wherein the total atomic concentration of carbon is 20% or less.

3. The electrode according to claim 1, wherein the atomic concentration of fluorine determined from the spectrum obtained by X-ray photoelectron spectroscopy on the surface of the composite layer is 20% or less.

4. An energy storage device including a positive electrode and a negative electrode, The negative electrode is an electrode according to any one of claims 1 to 3, in this energy storage device.

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