JP6449555B2 - Electrochemical capacitor - Google Patents

Description

  The present invention relates to an electrochemical capacitor typified by a lithium ion capacitor.

  In recent years, capacitors have been required to have a high capacity and a long life. Lithium ion capacitors can achieve a longer life than batteries by reducing the discharge depth of the negative electrode or using a polarizable positive electrode. However, like the battery, not only the upper limit voltage but also the lower limit voltage is set. Similar to the battery, the upper limit voltage is determined in consideration of the limit of the potential window of the electrolyte. On the other hand, the lower limit voltage is related to the inherent degradation of the lithium ion capacitor. Lithium ion capacitors use carbon in both the positive and negative electrodes. For this reason, the lithium intercalation reaction at the negative electrode occurring in the lithium ion secondary battery occurs at the positive electrode and the sub member in the lithium ion capacitor. This is related to the deterioration.

  In the invention of Patent Document 1, the positive electrode contains graphite. There is no problem during normal use, but when abnormal use is performed at 2.0 V or less on the basis of lithium, in the presence of lithium ions, graphite undergoes a side reaction on the electrode surface in the same manner as lithium ion batteries. For this reason, depending on the type of graphite, it may lead to deterioration of charge / discharge characteristics due to gas generation.

  In the invention of Patent Document 2, an inorganic material is coated on the negative electrode surface in order to control a side reaction when lithium ions are intercalated (doped) into the negative electrode. However, since the inorganic material has poor electron conductivity, there is a possibility that the electron conduction between the materials is hindered and the characteristics are deteriorated.

JP2012-190929A International Publication No. WO2011 / 0587748

  In view of the above, an object of the present invention is to provide an electrochemical device such as a lithium ion capacitor that can suppress deterioration under a low voltage while improving output characteristics.

As a result of intensive studies by the present inventors, the present invention having the following contents was completed.
The electrochemical device provided by the present invention includes both positive and negative electrodes and an ion conducting medium interposed therebetween. The positive electrode consists of a polarizable electrode. This polarizable electrode contains a conductive aid. The conductive auxiliary agent is composed of a carbon material having a graphite structure and lithium. The lithium is doped in the carbon material. The positive electrode preferably contains 0.1 to 5 wt% of a conductive aid. The proportion of the doped lithium in the total weight of the conductive aid is preferably 0.1 wt% or more. The negative electrode has a carbon material, and the carbon material is a material from which lithium ions can be inserted and removed. The oxygen concentration on the surface of the positive electrode in an analysis in a minute range of 50 μm or less by a photoelectron spectrometer (XPS) is 3 mol% or more.

  According to the present invention, since the positive electrode contains a carbon material having a graphite structure such as graphite, it exhibits excellent electron conduction and excellent output characteristics. In addition, since the carbon material is preliminarily doped with lithium, even when the cell voltage is abnormally lowered and the positive electrode potential is lowered, the electrolytic solution reacts with the carbon material to deposit decomposition products on the surface. Since resistance does not increase and the internal pressure does not increase due to gas generation, it can be used for a long time.

It is a schematic diagram of the microscopic structure of a positive electrode. It is a schematic block diagram of an electrochemical device.

  Hereinafter, the present invention will be described in detail with appropriate reference to the drawings. However, the present invention is not limited to the illustrated embodiment, and in the drawings, the characteristic portions of the invention may be emphasized and expressed, so that the accuracy of the scale is not necessarily guaranteed in each part of the drawings. Not.

  According to the present invention, the electrochemical device is a device that uses an electrochemical reaction of an electrolytic solution, and the type thereof is not particularly limited, and examples thereof include an electric double layer capacitor, a lithium ion capacitor, a redox capacitor, and a hybrid capacitor. . Hereinafter, although the example of a lithium ion capacitor is mainly described, this invention is applicable also to another electrochemical device.

  A lithium ion capacitor is a power storage element that uses a non-aqueous electrolyte containing an electrolyte containing lithium ions, and in the positive electrode, a non-Faraday reaction by adsorption / desorption of anions or cations similar to an electric double layer capacitor, In the negative electrode, it is an electrochemical device that charges and discharges by a Faraday reaction by insertion and extraction of lithium ions, similar to a lithium ion battery. A lithium ion capacitor can achieve both excellent output characteristics and high energy density by performing charge and discharge by a non-Faraday reaction at the positive electrode and a Faraday reaction at the negative electrode.

  The electrochemical device of the present invention includes both positive and negative electrodes and an ion conducting medium interposed therebetween. The positive electrode consists of a polarizable electrode. The polarizable electrode is an electrode that can take a potential range in which no current flows even when a potential is applied. Examples thereof include an electrode formed by bonding a platinum electrode, a gold electrode, a carbon electrode, activated carbon, and the like with a binder. In the present invention, a conventionally known polarizable electrode can be used without particular limitation.

The polarizable electrode as the positive electrode includes a carbon material having a graphite structure. When the graphite structure is viewed microscopically, carbon atoms form a layered structure formed by forming strong covalent bonds, and the layers are connected by weak van der Waals bonds. More specifically, in the graphite structure, three of the four valence electrons of carbon form SP ² hybrid orbitals and form a layered hexagonal network surface. The remaining one valence electron is a π-electron and is oriented perpendicular to the hexagonal network surface to form a van der Waals bond between the surfaces. Examples of the carbon material having a graphite structure include natural graphite, artificial graphite, and graphene.

  The carbon material having a graphite structure is doped with lithium. FIG. 1 is a schematic diagram of a microscopic structure of a positive electrode. In the embodiment shown in FIG. 1, a polarizable electrode is formed by applying activated carbon 1 bound by a binder 4 to an aluminum foil 5 as a substrate. The positive electrode includes a carbon material 3 having a graphite structure and lithium 2 doped in the carbon material 3. The carbon material 3 and the lithium 2 doped therein constitute a conductive auxiliary agent.

  The proportion of doped lithium in the total weight of the conductive aid is preferably 0.1 wt% or more, more preferably 0.1 to 0.5 wt%. The weight percentage of the doped lithium can be measured by ICP emission spectroscopy (Inductively Coupled Plasma Atomic Emission Spectroscopy) as follows. The graphite whose weight has been measured in advance is burned to dissolve the residue in pure water. The content of the solution is measured by ICP issue analysis. The amount of lithium contained in the solution is measured from the content, and is defined as the amount of lithium doped in the graphite. Since graphite can remove lithium adhering to the surface by washing with pure water in advance, doped lithium can be measured correctly.

  The ratio of the conductive auxiliary agent to the total weight of the positive electrode is preferably 0.1 to 5 wt%. Within the above range, an excellent improvement effect of electronic conduction is likely to be exhibited, and an electrochemical device exhibiting good output characteristics is easily obtained. The weight ratio of the conductive auxiliary agent can be measured by X-ray diffraction as follows. As the activated carbon used for the polarizable electrode, a carbon material having low crystallinity is generally used. On the other hand, examples of the carbon material used for the conductive additive include amorphous carbon having low crystallinity called carbon black and graphite having high crystallinity. Since the graphite used in the present invention has high crystallinity, the content can be quantified with an 002 diffraction line unique to graphite by an X-ray diffractometer. Since the peak and intensity ratio of the related diffraction line may differ depending on the type of graphite, the analysis is performed after identifying the type of graphite by confirming the ratio with the 004 diffraction line and 110 diffraction line peculiar to graphite. This makes it possible to appropriately measure the content of graphite contained in the electrode.

  There is no particular limitation on the form of doping of the carbon material 3 with the lithium 2, and it is only necessary that the lithium 2 is not separated from the carbon material 3 during use. The method of doping the lithium 2 into the carbon material 3 is not particularly limited, and examples include mixing and stirring the carbon material 3 and the lithium 2 in a medium that does not contain moisture. In addition, conventional techniques related to lithium doping can be referred to as appropriate.

  The present inventors have newly found that the degree of lithium doping is reflected in the oxygen concentration on the graphite surface. If the carbon material 3 is doped with a desired amount of lithium 2 as in the present invention, the oxygen concentration on the surface of the graphite becomes 10 mol% or more, preferably 12 to 20 mol%. On the other hand, in the conventional electrode not doped with lithium, even if lithium is undesirably mixed into the electrode, in this case, the oxygen concentration on the surface of the activated carbon occupying most of the positive electrode is at most about 2 mol%. Only 10 mol% as in the present invention. In other words, unless the carbon material 3 is intentionally doped with a desired amount of lithium 2, the oxygen concentration on the graphite surface never happens to be 10 mol% or more. Even when activated carbon and graphite are mixed to form an electrode, the oxygen concentration on the cathode surface increases due to the influence of graphite doped with lithium, but the ratio of the activated carbon and graphite that occupies most of the cathode makes the entire cathode surface The effect on oxygen concentration is small. It is possible to evaluate the influence by repeatedly performing microanalysis of a very narrow range of electrodes. The oxygen concentration at the time of forming an electrode needs to be 3% or more by performing XPS microanalysis described later, and preferably 3 to 5%. If it is less than 3%, the effect of the present invention is not seen because the graphite content is low or the lithium doping rate is low. When it greatly exceeds 5%, since the graphite content is high, the activated carbon content is low and the energy density is low. In addition, since the amount of doping treatment is large, impurities formed in the treatment process may be deposited to hinder the reaction.

  Although the mechanism of the relationship between doping the carbon material 3 of the positive electrode 2 with lithium 2 and the oxygen concentration on the surface of the positive electrode is not necessarily clear, it is generally estimated as follows. Doping proceeds by allowing graphite and lithium to coexist in a solvent that does not contain moisture. Since lithium has a property of easily bonding to oxygen, it is considered that lithium is inserted into the carbon in the process of doping lithium into the carbon and oxygen is bonded to the carbon surface in the air. On the other hand, when the device is formed, the element does not come into contact with the outside air, so that the amount of oxygen on the carbon surface does not increase during the doping process.

  In order to increase the oxygen concentration on the positive electrode surface, for example, the amount of lithium doped can be increased, or the weight ratio of the lithium-doped carbon material to the positive electrode can be increased to lower the oxygen concentration on the positive electrode surface. For this purpose, the reverse method to the above is mentioned. The oxygen concentration on the positive electrode surface is measured by a photoelectron spectrometer (XPS) in an atmosphere containing no oxygen. The specific measurement method is as follows. The device is disassembled in a glove box filled with argon gas with an oxygen concentration of 5 ppm or less, and the positive electrode is taken out. The extracted positive electrode is washed with the same solvent as the electrolyte used in the device. At this time, a solvent having a water content of 20 ppm or less is used as a cleaning solvent. The washed positive electrode is dried under reduced pressure at room temperature. The dried positive electrode is fixed to the sample stage of the measuring apparatus, sealed in a special container that can be introduced into the apparatus without being exposed to the atmosphere, and introduced into the apparatus. The measurement is carried out after the apparatus is preliminarily filled with argon gas without water and oxygen. By setting the measurement range to 0.1 mm or less, an oxygen concentration of 3 mol% or less, which is the oxygen concentration of activated carbon, is shown on average, but a high value of 3 mol% or more is shown in a region containing a large amount of graphite. It can be confirmed that the graphite is doped with lithium ions by measuring the maximum value measured at 20 points with a measurement range of φ50 μm. Although it is a qualitative technique, it is possible to analyze that the oxygen concentration of graphite in the positive electrode is high by using an energy dispersive X-ray spectrometer (EDS).

  The negative electrode includes a carbon material from which lithium ions can be inserted and removed. Examples of such a carbon material include graphite, hard carbon, and soft carbon.

  By doping the carbon material 3 with lithium 2 described above, the potential of the positive electrode in the organic solvent is preferably 1.8 to 2.7 V. The reference electrode at this time is an electrode formed by attaching lithium to a nickel net.

  The negative electrode only needs to contain a carbon material from which lithium ions can be inserted and removed. For other components, the overall configuration of the negative electrode, the manufacturing method, etc., the conventional electrode structure may be referred to as appropriate. Examples described later can also be referred to as examples of production. The ratio of the carbon material from which lithium ions can be desorbed / inserted in the entire negative electrode is preferably 80 wt% or more, and more preferably 90 to 99 wt%.

  The electrochemical device includes the above-described positive and negative electrodes and an ion conductive medium interposed between these electrodes. FIG. 2 is a schematic configuration diagram of an electrochemical device. In the form of FIG. 2, a separator 30 is sandwiched between the positive electrode 10 and the negative electrode 20. Examples of the ion conductive medium include an electrolytic solution interposed between the positive electrode 10 and the negative electrode 20, a separator impregnated with the electrolytic solution, and the like.

The structure of the electrolytic solution can refer to the prior art in electrochemical devices as appropriate, and examples of the electrolyte in the electrolytic solution include LiN (SO ₂ C ₂ F ₅ ) ₂ , LiBF ₄ , and LiPF ₆ . Examples of the non-aqueous electrolyte solution for dissolving these electrolytes include cyclic carbonates represented by ethylene carbonate (EC) and propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate ( Examples thereof include chain carbonates represented by MEC), lactones such as γ-butyrolactone (γBL), and mixed solvents thereof.

  In the present invention, for each component not specifically mentioned, the prior art in electrochemical devices can be referred to as appropriate. For example, the configuration of the current collector for each of the positive and negative electrodes, the packaging of the electrodes, and a means for taking out the generated current are not particularly limited in the present invention.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the embodiments.

[Lithium dope]
Graphite with an average particle size of 10 μm was prepared. The graphite was dried at a temperature of 100 ° C. or higher for 12 hours. In the electrolytic solution from which moisture was removed, 10 g of dried graphite and 0.015 g of lithium were mixed and stirred. Thereby, lithium was doped in the graphite. The electrolyte used was a 1 mol / L lithium hexafluorophosphate solution, and the solvent was a 1: 2 (volume ratio) mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). After all the lithium had disappeared, it was kept in a sealed state for 12 hours. Thereafter, the graphite was recovered by filtration through a filter, and this was washed with ethanol and then with pure water. Washing with pure water was repeated until the pH of the washing solution was 9 or less. After completion of washing with pure, the mixture was filtered through a filter and dried at 100 ° C. under reduced pressure for 12 hours. As a result, dry graphite doped with lithium was obtained. It was confirmed by the above measurement method that a part of lithium used as a raw material was doped in graphite. It was confirmed that 10 g of graphite was doped with 0.01 g of lithium. The dope amount was 0.1 wt%.

[Positive voltage]
Lithium-doped graphite (100 parts by weight) obtained by the above treatment and PTFE (10 parts by weight) were kneaded to form a sheet and placed on an aluminum foil (working electrode). Separately, lithium was attached to a nickel net to create a reference lithium electrode (Li electrode). The working electrode and the Li electrode were immersed in an electrolytic solution having the same composition as that used for lithium doping, and the voltage was measured. As a result, it was confirmed that the working electrode voltage was 1.8V.

[Manufacture and evaluation of electrical devices]
Positive electrode: activated carbon (90 parts by weight), carboxymethyl cellulose (CMC) (3 parts by weight), styrene butadiene rubber (SBR) (3 parts by weight), acetylene black (AB) (3 parts by weight), lithium prepared above Doped graphite (1 part by weight) was mixed in pure water to obtain a slurry. The slurry was applied to an aluminum foil and dried to form an electrode having a thickness of 0.1 mm. An aluminum ribbon for current collection was welded to this electrode (positive electrode).

  Negative electrode: Graphite (95 parts by weight), CMC (2 parts by weight), and SBR (3 parts by weight) were mixed in pure water to obtain a slurry. The slurry was applied to a copper foil and dried to form a plate-like body having a thickness of 0.03 mm. A negative electrode was formed by sticking a lithium foil to the plate-like body after extending it with a polypropylene roller. The weight of the lithium foil was 0.08 g per 1 g of graphite, and the lithium foil occupied 20% or more of the negative electrode area. A nickel ribbon for current collection was welded to the negative electrode.

An element was configured by interposing an electrolytic paper as a separator between the positive electrode and the negative electrode. Three sides of the prepared element were heat-sealed, and an electrolyte solution was injected from the remaining one side and sealed. An electric device of Example 1 was obtained by fixing the sealed element with a PP plate having a thickness of 1 mm. The solvent of the electrolyte is a 1: 2 (volume ratio) mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), and lithium hexafluorophosphate (LiPF ₆ ) is dissolved at a concentration of 1 mol / L in this solvent. I let you.
For other examples and comparative examples, the amount of carbon material (graphite) used for the positive electrode and the amount of lithium doped into the carbon material (wt%) were changed as shown in Table 1 to obtain each electric device. . The oxygen concentration on the negative electrode surface was determined by the measurement method described above. The manufacturing conditions are summarized in Table 1. For example, in Example 1, as described above, by mixing graphite doped with lithium and activated carbon so that the voltage becomes 1.8 V, an electrode potential of 2.5 V as shown in Table 1 is obtained. It was.

[Evaluation]
About each electronic device, the resistance value and battery capacity immediately after preparation were measured, and these were evaluated as initial characteristics. Each electronic device was left at room temperature for 2 weeks. Thereafter, overdischarge was performed at each voltage, and the swelling of the electric device was visually evaluated. Moreover, the output of 25C was measured. The measurement results are summarized in Table 2. In "Initial characteristics" and "Output of 25C" in Table 2, the results in Example 1 are 100%, and the results of other examples and comparative examples are described as relative values. The “output” is a current value that can be discharged per unit time.

DESCRIPTION OF SYMBOLS 1 Activated carbon 2 Lithium 3 Carbon material 4 Binder 5 Aluminum foil 10 Positive electrode 20 Negative electrode 30 Separator

Claims (2)

A positive electrode comprising a polarizable electrode containing a conductive auxiliary comprising a carbon material having a graphite structure and lithium doped in the carbon material;
A negative electrode having a carbon material from which lithium ions can be removed and inserted, and an ion conductive medium interposed between the positive electrode and the negative electrode,
The oxygen concentration on the surface of the positive electrode in the analysis of a minute range of 50 μm or less by a photoelectron spectrometer (XPS) is 3 mol% or more,
An electrochemical device in which the conductive auxiliary agent accounts for 0.2 to 3 wt% of the total weight of the positive electrode. Doped electrochemical device of claim 1, wherein the lithium account for at least 0.1 wt% of the total weight conductive auxiliary agent.

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