Electrolyte for electrochemical device and electrochemical device

Abstract

The present invention suppresses decreases in electrochemical device performance that accompany electrolyte solution decomposition and degradation during charging. This electrolyte solution for an electrochemical device contains a solvent, an ionic material, and additives. The additives include α-methyl-γ-butyrolactone and δ-valerolactone.

Description

[Technical Field]

[0001] The present invention relates to a liquid electrolyte for an electrochemical device and an electrochemical device. [Background technology]

[0002] An electrochemical device includes a positive electrode, a negative electrode, and an electrolyte. An electric double layer capacitor, which is an example of an electrochemical device, has a longer life, is capable of rapid charging, and has superior output characteristics compared to secondary batteries, and is therefore widely used as a backup power source, etc.

[0003] The electrolytic solution contains a solvent and an ionic substance. For example, γ-butyrolactone is used as the solvent for the electrolytic solution to improve the heat resistance of the electrolytic solution (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6357639 specification Summary of the Invention

[0005] During charging of an electrochemical device, the solvent in the electrolyte may come into contact with the negative electrode active material and undergo reductive decomposition, resulting in deterioration of the electrolyte. This deterioration of the electrolyte may result in a decrease in the performance of the electrochemical device.

[0006] In view of the above, one aspect of the present invention relates to a liquid electrolyte for electrochemical device, comprising a solvent, an ionic substance, and an additive, wherein the additive comprises α-methyl-γ-butyrolactone and δ-valerolactone.

[0007] Another aspect of the present invention is a method for detecting a magnetic field comprising: a pair of electrodes; For the above electrochemical devices Electrolyte and El , relating to electrochemical devices.

[0008] According to the present invention, the deterioration of the performance of an electrochemical device due to the decomposition and deterioration of the electrolyte solution during charging is suppressed. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a partially cutaway perspective view of an electrochemical device according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Electrolytes for electrochemical devices] A liquid electrolyte for electrochemical devices according to one embodiment of the present invention includes a solvent, an ionic substance, and an additive, and the additive includes α-methyl-γ-butyrolactone (hereinafter referred to as AMGBL) and δ-valerolactone (hereinafter referred to as DVL).

[0011] By including AMGBL and DVL in the electrolyte, a high-quality coating derived from AMGBL and DVL is formed on the surface of the negative electrode active material during charging. During charging, the formation of this coating takes precedence over the reductive decomposition of the solvent, such as γ-butyrolactone, on the negative electrode side. The preferential formation of this coating suppresses the reductive decomposition of the solvent in the electrolyte due to contact with the negative electrode active material, thereby suppressing the degradation of the performance of the electrochemical device associated with this decomposition. For example, a decrease in the capacity of the electrochemical device and an increase in internal resistance are suppressed.

[0012] When AMGBL and DVL are used together, the reductive decomposition of the solvent in the electrolyte during charging is significantly suppressed. While the exact reason for this is unclear, it is speculated that the interaction between AMGBL and DVL, among other factors, results in the formation of a thin, dense coating. For example, the methyl group in AMGBL causes the formation of a sparse AMGBL-derived coating on the surface of the negative electrode active material. In addition, a coating derived from DVL, which has a different reductive decomposition voltage from AMGBL, is formed in stages. It is speculated that the sparse AMGBL-derived coating is easily filled by the DVL-derived coating. Furthermore, the coatings derived from AMGBL and DVL have low resistance. This allows for the production of electrochemical devices with low internal resistance and high capacity.

[0013] From the viewpoint of suppressing decomposition of the solvent in the electrolyte solution during charging, the total content of AMGBL and DVL in the electrolyte solution (mass ratio to the total amount of the electrolyte solution) may be 3.5% by mass or less, 0.1% by mass or more and 3.5% by mass or less, 0.1% by mass or more and 3.1% by mass or less, 0.1% by mass or more and 1.5% by mass or less, or 0.15% by mass or more and 1.5% by mass or less.

[0014] When the total content of AMGBL and DVL in the electrolyte is 3.1% by mass or less, a coating derived from AMGBL and DVL is likely to be formed with an appropriate thickness on the surface of the negative electrode active material, the resistance of the negative electrode is likely to be reduced, and capacity loss due to increased negative electrode resistance is likely to be suppressed.When the total content of AMGBL and DVL in the electrolyte is 0.1% by mass or more, the effect of suppressing decomposition of the solvent in the electrolyte during charging is likely to be obtained.

[0015] From the viewpoint of suppressing capacity reduction due to decomposition and degradation of the electrolyte solution during charging, the content of AMGBL in the electrolyte solution may be 0.01% to 3.5% by mass, 0.05% to 3% by mass, 0.05% to 1% by mass, or 0.05% to 0.5% by mass. From the same viewpoint as above, the content of DVL in the electrolyte solution may be 0.01% to 3.5% by mass, 0.05% to 3% by mass, 0.05% to 1% by mass, or 0.05% to 0.5% by mass.

[0016] In the electrolyte, the mass ratio of DVL to AMGBL (DVL / AMGBL) may be 0.03 or more and 70 or less, 0.03 or more and 60 or less, or 0.1 or more and 10 or less.

[0017] When preparing the electrolyte solution (before the first charge of the electrochemical device), the contents of AMGBL and DVL in the electrolyte solution and their total contents may be within the above ranges. During charging of the electrochemical device, some of the AMGBL and DVL in the electrolyte solution may be consumed to form a coating on the surface of the negative electrode active material. After the first charge of the electrochemical device, the contents of AMGBL and DVL in the electrolyte solution and their total contents may be smaller than the above ranges, for example, a trace amount close to the detection limit. The presence of AMGBL and DVL in the electrolyte solution in the electrochemical device will result in a corresponding improvement in the performance of the electrochemical device. The contents of AMGBL and DVL in the electrolyte solution and their total contents may be determined by gas chromatography-mass spectrometry (GC / MS) or the like.

[0018] The solvent contained in the electrolytic solution may be, for example, a non-aqueous solvent. Examples of the non-aqueous solvent include cyclic carbonate compounds, lactone compounds other than AMGBL and DVL, sulfoxide compounds, sulfone compounds, amide compounds, chain carbonate compounds, chain ether compounds, cyclic ether compounds, and chain carboxylic acid ester compounds. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0019] From the viewpoint of improving the electrical conductivity of the electrolyte solution, the non-aqueous solvent may contain at least one solvent having a high relative dielectric constant selected from the group consisting of cyclic carbonate compounds, lactone compounds, sulfoxide compounds, sulfone compounds, and amide compounds.

[0020] Furthermore, from the viewpoint of improving the mobility of ions in the electrolyte solution, the non-aqueous solvent may contain at least one low-viscosity solvent selected from the group consisting of chain carbonate compounds, chain ether compounds, cyclic ether compounds, and chain carboxylic acid ester compounds.

[0021] Examples of cyclic carbonate compounds include ethylene carbonate, 1,2-propylene carbonate, 1,3-propylene carbonate (PC), 1,2-butylene carbonate, 1,3-butylene carbonate, fluoroethylene carbonate, etc. Examples of chain carbonate compounds include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, etc.

[0022] Examples of lactone compounds other than AMGBL and DVL include γ-butyrolactone (GBL), γ-valerolactone, γ-caprolactone, δ-hexanolactone, δ-octanolactone, etc. Examples of nitrile compounds include acetonitrile (AN), propionitrile, etc. Examples of amide compounds include N-methylacetamide, N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, etc.

[0023] Sulfoxide compounds include, for example, dimethyl sulfoxide, diethyl sulfoxide, diphenyl sulfoxide, thiophene, etc. Sulfone compounds include methyl sulfone, diethyl sulfone, diphenyl sulfone, sulfolane, 3-methyl sulfolane, 2,4-dimethyl sulfolane, sulfolene, 3-methyl sulfolene, 3-ethyl sulfolene, etc.

[0024] Examples of cyclic ether compounds include tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, etc. Examples of chain ether compounds include 1,2-dimethoxyethane, ethoxymethoxyethane, 1,2-diethoxyethane, ethylene glycol bis(trifluoroethyl) ether, ethylene glycol bis(trifluoromethyl) ether, etc.

[0025] Examples of the chain carboxylic acid ester compound include methyl formate, ethyl formate, methyl acetate, ethyl acetate, methyl propionate, and ethyl propionate. Examples of the non-aqueous solvent that can be used include polyhydric alcohol compounds such as ethylene glycol and propylene glycol, ketone compounds such as methyl ethyl ketone, and formaldehyde.

[0026] Among these, the solvent preferably contains at least one selected from the group consisting of GBL, AN, and PC, and more preferably contains GBL. In this case, a coating derived from AMGBL and DVL is likely to be preferentially formed during charging, and the effect of including AMGBL and DVL in the electrolyte solution is significantly achieved. While the detailed reason for this is unknown, it is presumed that this is due to the fact that GBL, like AMGBL and DVL, is a lactone. Furthermore, GBL has excellent chemical and thermal stability. AN has low viscosity, which is advantageous in terms of improving the mobility of ions in the electrolyte solution.

[0027] The proportion of GBL in the solvent is preferably 80% by volume or more, more preferably 90% by volume or more. The proportion of AN in the solvent is preferably 80% by volume or more, more preferably 90% by volume or more. The proportion of PC in the solvent is preferably 70% by volume or more, more preferably 90% by volume or more.

[0028] The solvent may be a mixed solvent of AN and PC. In this case, the proportion of AN in the mixed solvent (total of AN and PC) is preferably 80% by volume or more and 95% by volume or less. The solvent may be a mixed solvent of GBL and PC. In this case, the proportion of GBL in the mixed solvent (total of GBL and PC) is preferably 80% by volume or more and 95% by volume or less. The solvent may be a mixed solvent of AN and GBL. In this case, the proportion of AN in the mixed solvent (total of AN and GBL) is preferably 80% by volume or more and 95% by volume or less. Among the above mixed solvents, a mixed solvent of AN and GBL is preferred from the viewpoint of suppressing an increase in internal resistance due to decomposition and deterioration of the electrolyte during charging.

[0029] The ionic substance is dissolved in a solvent and contains a cation and an anion. The ionic substance may contain, for example, a low-melting-point compound (ionic liquid) that can exist as a liquid at around room temperature. The cation and the anion may each be used alone or in combination of two or more.

[0030] Examples of the cation include organic cations such as quaternary ammonium cations and quaternary phosphonium cations. Examples of the quaternary ammonium cation include cations derived from aliphatic amines, alicyclic amines, and aromatic amines. Specific examples include diethyldimethylammonium (DEDMA) cation, triethylmethylammonium (TEMA) cation, tetraethylammonium (TEA) cation, tetramethylammonium cation, trimethylethylammonium cation, and trimethylpropylammonium cation.

[0031] The quaternary ammonium cation may also include a cation derived from a cyclic amine (a cation having a nitrogen-containing heterocycle), such as a cation having a skeleton of imidazole, pyridine, pyrrolidine, or piperidine.

[0032] Examples of cations having an imidazole skeleton (imidazoliums) include 1,3-dimethylimidazolium cation, 1-ethyl-3-methylimidazolium cation, 1,3-diethylimidazolium cation, 1,2,3-trimethylimidazolium cation, 1,2,3,4-tetramethylimidazolium cation, etc. Examples of cations having a pyridine skeleton (pyridiniums) include 1-methylpyridinium cation, 1-ethylpyridinium cation, 1-butylpyridinium cation, etc.

[0033] Examples of cations having a pyrrolidine skeleton (pyrrolidiniums) include 1,1-dimethylpyrrolidinium cation, 1-ethyl-1-methylpyrrolidinium cation, 1,1-diethylpyrrolidinium cation, etc. Examples of cations having a piperidine skeleton (piperidiniums) include 1,1-dimethylpiperidinium cation, 1-ethyl-1-methylpiperidinium cation, 1,1-diethylpiperidinium cation, etc.

[0034] The quaternary ammonium cation may include a cation having a spiro skeleton in which the spiro atom is a nitrogen atom (two rings are connected by sharing one nitrogen atom). Specific examples include the spiro-(1,1')-bipyrrolidinium (SBP) cation and the spiro-(1,1')-bipiperidinium cation. The quaternary ammonium cation may also include a cation having a 1,4-diazabicyclo[2.2.2]octane (DABCO) skeleton. Specific examples include the N-methyl-1,4-diazabicyclo[2.2.2]octane ammonium cation.

[0035] Quaternary phosphonium cations include, for example, tetraalkylphosphonium cations, etc. Tetraalkylphosphonium cations include, for example, tetramethylphosphonium cation, tetraethylphosphonium cation, etc.

[0036] The cation may also include inorganic cations such as metal ions. Examples of metal ions include alkali metal ions and alkaline earth metal ions. Examples of alkali metal ions include lithium ions, sodium ions, and potassium ions. Examples of alkaline earth metal ions include magnesium ions and calcium ions.

[0037] The anion is, for example, BF4 - , PF6 - , AsF6 - , SbF6 - , N(FSO2)2 - , F - , Cl - , Br - , I - , NO3 - , NO2 - , ClO4 - , AlCl4 - , AlF4 - , TaF6 - , NbF6 - , SiF6 - , C.N. - Among these, from the viewpoint of easily improving the withstand voltage characteristics and high ionic conductivity, the anion preferably contains an anion of a fluorine-containing acid, such as BF4 - and / or PF6 - More preferably, it contains BF4 - It is more preferred that the composition contains:

[0038] The anion is N(RfSO3)2 - , C(RfSO2)3 - , RfSO3 - , CH3BF3 -It is also possible to include an organic anion such as the above. Rf is a fluoroalkyl group having 1 to 12 carbon atoms. Rf includes a trifluoromethyl group (CF3), a pentafluoroethyl group (C2F5), and the like.

[0039] The ionic substance preferably contains a salt in which at least one of the cation and the anion contains an organic substance (organic salt). From the viewpoint of improving the electrical conductivity of the electrolyte, the organic salt preferably contains a quaternary ammonium salt. The quaternary ammonium salt is a salt containing a quaternary ammonium cation and an anion of a fluorine-containing acid (particularly BF4 - ) and. Specific examples of such quaternary ammonium salts include diethyldimethylammonium tetrafluoroborate (DEDMABF4), triethylmethylammonium tetrafluoroborate (TEMABF4), tetraethylammonium tetrafluoroborate (TEABF4), spiro-(1,1')-bipyrrolidinium tetrafluoroborate (SBPBF4), N-methyl-1,4-diazabicyclo[2.2.2]octanammonium tetrafluoroborate, 1-ethyl-1-methylpyrrolidinium tetrafluoroborate (EMPyBF4), 1,1-dimethylpyrrolidinium tetrafluoroborate (DMPyBF4), and the like. Among these, from the viewpoint of excellent oxidation resistance and reduction resistance, DEDMABF4, EMPyBF4, and DMPyBF4 are preferred as the quaternary ammonium salt, and DEDMABF4 and EMPyBF4 are more preferred.

[0040] The concentration of the ionic substance in the electrolyte is, for example, 0.5 mol / L or more and 2.0 mol / L or less. When the concentration of the ionic substance in the electrolyte is within the above range, an electrochemical device with a large capacity and a small internal resistance is easily obtained.

[0041] [Electrochemical Devices] An electrochemical device according to one embodiment of the present invention includes a pair of electrodes and an electrolyte, the electrolyte being the above-described electrolyte solution for electrochemical devices. One of the pair of electrodes is a positive electrode, and the other of the pair of electrodes is a negative electrode. Examples of the electrochemical device include an electric double layer capacitor and a lithium ion capacitor.

[0042] At least one of the pair of electrodes may include an active layer and a current collector supporting the active layer. The active layer includes an active material capable of adsorbing and desorbing ions, and the active material includes a carbon material. When the electrochemical device is an electric double layer capacitor, each of the pair of electrodes (positive electrode and negative electrode) may include an active layer and a current collector supporting the active layer. When the electrochemical device is a lithium ion capacitor, one of the pair of electrodes (positive electrode) may include an active layer and a current collector supporting the active layer. In this case, the other of the pair of electrodes (negative electrode) may be a negative electrode used in a lithium ion secondary battery. The negative electrode used in a lithium ion secondary battery includes a negative electrode active material (e.g., graphite) capable of absorbing and releasing lithium ions.

[0043] The active layer contains a carbon material as an active material as an essential component, and may contain a binder, a conductive agent, and the like as optional components. Examples of carbon materials that can be used include activated carbon, carbon nanotubes, graphite, and graphene. Of these, activated carbon is preferred. Examples of raw materials for activated carbon include wood, coconut shells, pulp waste liquid, coal or coal-based pitch obtained by thermal decomposition thereof, heavy oil or petroleum-based pitch obtained by thermal decomposition thereof, phenolic resin, petroleum coke, and coal coke. The activated carbon is preferably one that has been activated.

[0044] Examples of binders that can be used include resin materials such as polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR). Examples of conductive agents that can be used include carbon black such as acetylene black.

[0045] The electrode can be obtained by, for example, applying a slurry containing a carbon material, a binder and / or a conductive agent, and a dispersion medium to the surface of a current collector, drying the coating, and rolling it to form an active layer. The current collector can be made of a metal foil such as aluminum foil.

[0046] It is desirable to interpose a separator between the pair of electrodes. The separator has ion permeability and serves to physically separate the pair of electrodes to prevent short circuits. Examples of separators that can be used include cellulose-based nonwoven fabrics, glass fiber mats, and microporous films of polyolefins such as polyethylene.

[0047] An electrochemical device according to an embodiment of the present invention will now be described with reference to Fig. 1. Fig. 1 is a partially cutaway perspective view of an electrochemical device according to an embodiment of the present invention. Note that the present invention is not limited to the electrochemical device shown in Fig. 1.

[0048] The electrochemical device 10 in FIG. 1 is an electric double layer capacitor and includes a wound-type capacitor element 1. The capacitor element 1 is configured by winding a first electrode 2 and a second electrode 3, each in sheet form, with a separator 4 interposed therebetween. The first electrode 2 and the second electrode 3 have a first current collector and a second current collector, respectively, made of metal, with a first active layer and a second active layer supported on the surfaces thereof, and exhibit capacitance by adsorbing and desorbing ions. The current collectors may be made of, for example, aluminum foil. The surfaces of the current collectors may be roughened by etching or other techniques. The separator 4 may be made of, for example, a nonwoven fabric primarily composed of cellulose. A first lead wire 5a and a second lead wire 5b are connected to the first electrode 2 and the second electrode 3, respectively, as lead members. The capacitor element 1 is housed in a cylindrical outer case 6 together with an electrolyte (not shown). The outer case 6 may be made of a metal such as aluminum, stainless steel, copper, iron, or brass. The opening of the exterior case 6 is sealed with a sealing member 7. The lead wires 5a and 5b are led out to the outside so as to pass through the sealing member 7. The sealing member 7 is made of a rubber material such as butyl rubber, for example.

[0049] In the above embodiment, a wound type capacitor has been described, but the scope of application of the present invention is not limited to the above, and it can also be applied to capacitors of other structures, such as stacked type or coin type capacitors.

[0050] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0051] Example 1 In this example, a wound-type electric double layer capacitor (Φ (diameter) A specific method for producing the electrochemical device will be described below.

[0052] (Electrode preparation) A slurry was prepared by dispersing 88 parts by mass of activated carbon (carbon material), 6 parts by mass of polytetrafluoroethylene (binder), and 6 parts by mass of acetylene black (conductive agent) in water. The resulting slurry was applied to an Al foil (thickness: 30 μm), and the coating was dried at 110°C and rolled to form an active layer (thickness: 40 μm), resulting in an electrode.

[0053] (Preparation of Electrolyte) An electrolyte solution was prepared by dissolving diethyldimethylammonium tetrafluoroborate (DEDMABF4) in gamma-butyrolactone (GBL) and adding additives. The additives used were alpha-methyl-gamma-butyrolactone (AMGBL) and delta-valerolactone (DVL). The concentration of DEDMABF4 in the electrolyte solution was 1.0 mol / L. The content of AMGBL in the electrolyte solution was 0.05% by mass. The content of DVL in the electrolyte solution was 0.05% by mass.

[0054] (Fabrication of electrochemical devices) A pair of electrodes was prepared, and lead wires were connected to each electrode. The electrodes were wound with a cellulose nonwoven fabric separator to form a capacitor element. The capacitor element was then housed in a specified outer case together with an electrolyte solution and sealed with a sealing material to complete the electrochemical device (electric double layer capacitor) A1. After that, the device was subjected to an aging treatment at 60°C for 16 hours while applying the rated voltage.

[0055] The electrochemical device A1 obtained above was evaluated as follows. [evaluation] (Measurement of capacitance and electrical resistance of electrochemical devices before storage) The battery was charged at a constant current of 1.5 A in a -30°C environment until the voltage reached 2.8 V, and then held at 2.8 V for 7 minutes. It was then discharged at a constant current of 1.35 A in a -30°C environment until the voltage reached 0 V.

[0056] During the above discharge, the time t (sec) required for the voltage to drop from 2.24 V to 1.12 V was measured. 2.24 V is a voltage equivalent to 80% of 2.8 V (the voltage when fully charged), and 1.12 V is a voltage equivalent to 40% of 2.8 V. Using the measured time t, the capacitance C1 (F) before storage was calculated using the following formula (1). Capacitance C1=Id×t / V (1) In the formula (1), Id is the current value during discharge (1.35 A), and V is the value obtained by subtracting 1.12 V from 2.24 V (1.12 V).

[0057] Using the discharge curve obtained from the above discharge (vertical axis: discharge voltage, horizontal axis: discharge time), a linear approximation was made for the discharge curve from 0.5 to 2 seconds after the start of discharge, and the voltage Vs at the intercept of the approximation was calculated. ΔV was calculated by subtracting the voltage Vs from the voltage V0 at the start of discharge (0 seconds after the start of discharge), giving the value (V0 - Vs). The electrical resistance R1 (Ω) before storage was calculated using ΔV (V) and the current value Id (1.35 A) during discharge according to the following formula (2): Electrical resistance R1=ΔV / Id (2)

[0058] (Storage test for electrochemical devices) The device was charged at a constant current of 1.5 A in a 60°C environment until the voltage reached 2.8 V, and then maintained at 2.8 V for 1,250 hours. The electrochemical device was then stored with a voltage of 2.8 V applied. The device was then discharged at a constant current of 1.35 A in a 60°C environment until the voltage reached 0 V.

[0059] (Measurement of capacitance and electrical resistance of electrochemical devices after storage) Thereafter, the electrochemical device was charged and discharged in an environment of -30°C using the same method as in the measurement of the capacitance and electrical resistance before storage described above, and the capacitance C2 (F) and electrical resistance R2 (Ω) after storage were determined.

[0060] (Measurement of capacity retention rate and resistance increase rate) The capacitance C1 before storage and the capacitance C2 after storage obtained above were used to calculate the capacity retention rate according to the following formula (3). Capacity retention rate (%) = (Capacitance C2 / Capacitance C1) × 100 (3)

[0061] Using the electrical resistance R1 before storage and the electrical resistance R2 after storage obtained above, the resistance increase rate was calculated according to the following formula (4). Resistance increase rate (%) = (electrical resistance R2 / electrical resistance R1) × 100 (4)

[0062] Comparative Example 1 An electrochemical device B1 of Comparative Example 1 was fabricated and evaluated in the same manner as the electrochemical device A1 of Example 1, except that the electrolyte solution did not contain any additives (AMGBL and DVL).

[0063] Comparative Example 2 An electrochemical device B2 of Comparative Example 2 was fabricated and evaluated in the same manner as the electrochemical device A1 of Example 1, except that DVL was not added to the electrolyte solution.

[0064] Comparative Example 3 An electrochemical device B3 of Comparative Example 3 was fabricated and evaluated in the same manner as the electrochemical device A1 of Example 1, except that the electrolyte solution did not contain AMGBL.

[0065] Table 1 shows the evaluation results of the electrochemical devices A1 and B1 to B3.

[0066] [Table 1]

[0067] Electrochemical device A1 exhibited a high capacity retention rate and a low resistance increase rate, significantly improving reliability. By incorporating both AMGBL and DVL into the electrolyte solution, electrochemical device A1 significantly suppressed decomposition and degradation of the electrolyte solution during storage in a charged state with an applied voltage of 2.8 V. Consequently, the decrease in capacitance and the increase in electrical resistance of electrochemical device A1 were suppressed after storage.

[0068] In electrochemical devices B1 to B3, since the electrolyte did not contain both AMGBL and DVL, a small capacity retention rate and a large resistance increase rate were obtained, resulting in a decrease in reliability.

[0069] Examples 2 to 13 Electrochemical devices A2 to A13 of Examples 2 to 13 were fabricated and evaluated in the same manner as electrochemical device A1 of Example 1, except that the AMGBL content and DVL content in the electrolyte solution were set to the values ​​shown in Table 2.

[0070] The evaluation results of electrochemical devices A2 to A13 are shown in Table 2. Table 2 also shows the evaluation results of electrochemical device A1.

[0071] [Table 2]

[0072] All of the electrochemical devices A2 to A13 exhibited a large capacity retention rate and a small resistance increase rate.

[0073] Examples 14 to 21 1-Ethyl-1-methylpyrrolidinium tetrafluoroborate (EMPyBF4) was used instead of DEDMABF4. The AMGBL content and DVL content in the electrolyte were set to the values ​​shown in Table 3. Except for the above, electrochemical devices A14 to A21 of Examples 14 to 21 were fabricated and evaluated in the same manner as electrochemical device A1 of Example 1. The evaluation results of electrochemical devices A14 to A21 are shown in Table 3.

[0074] [Table 3]

[0075] All of the electrochemical devices A14 to A21 exhibited a large capacity retention rate and a small resistance increase rate.

[0076] Examples 22 to 26 Electrochemical devices A22 to A26 of Examples 22 to 26 were fabricated and evaluated in the same manner as electrochemical device A3 of Example 3, except that the ionic substances shown in Table 4 were used instead of DEDMABF4. In Table 4, TEABF4 is tetraethylammonium tetrafluoroborate, TEMABF4 is triethylmethylammonium tetrafluoroborate, SBPBF4 is spiro-(1,1')-bipyrrolidinium tetrafluoroborate, and DMPyBF4 is 1,1-dimethylpyrrolidinium tetrafluoroborate.

[0077] The evaluation results of electrochemical devices A22 to A26 are shown in Table 4. Table 4 also shows the evaluation results of electrochemical device A3.

[0078] [Table 4]

[0079] All of the electrochemical devices A22 to A26 exhibited a large capacity retention rate and a small resistance increase rate.

[0080] Examples 27 to 29 Electrochemical devices A27 to A29 of Examples 27 to 29 were fabricated and evaluated in the same manner as electrochemical devices A3, A22 and A23 of Examples 3, 22 and 23, except that acetonitrile (AN) was used instead of GBL.

[0081] Comparative Example 4 Electrochemical device B4 of Comparative Example 4 was fabricated and evaluated in the same manner as electrochemical device A3 of Example 3, except that AN was used instead of GBL and no additives (AMGBL and DVL) were added to the electrolyte solution.

[0082] Examples 30 to 32 Electrochemical devices A30 to A32 of Examples 30 to 32 were fabricated and evaluated in the same manner as electrochemical devices A3, A22 and A23 of Examples 3, 22 and 23, except that propylene carbonate (PC) was used instead of GBL.

[0083] Comparative Example 5 Electrochemical device B5 of Comparative Example 5 was fabricated and evaluated in the same manner as electrochemical device A3 of Example 3, except that PC was used instead of GBL and no additives (AMGBL and DVL) were added to the electrolyte solution.

[0084] Table 5 shows the evaluation results of electrochemical devices A27 to A32 and B4 to B5.

[0085] [Table 5] Electrochemical devices A27 to A32 all exhibited a large capacity retention rate and a small resistance increase rate. Electrochemical devices B4 and B5 did not contain additives (AMGBL and DVL) in the electrolyte solution, and therefore exhibited a small capacity retention rate and a large resistance increase rate.

[0086] Example 33 An electrochemical device A33 of Example 33 was fabricated and evaluated in the same manner as the electrochemical device A3 of Example 3, except that a mixed solvent of AN and PC (volume ratio 95:5) was used instead of GBL.

[0087] Example 34 An electrochemical device A34 of Example 34 was produced and evaluated in the same manner as the electrochemical device A3 of Example 3, except that a mixed solvent of GBL and PC (volume ratio 95:5) was used instead of GBL.

[0088] Example 35 An electrochemical device A35 of Example 35 was fabricated and evaluated in the same manner as the electrochemical device A3 of Example 3, except that a mixed solvent of AN and GBL (volume ratio 95:5) was used instead of GBL.

[0089] Table 6 shows the evaluation results of electrochemical devices A33 to A35.

[0090] [Table 6]

[0091] All of the electrochemical devices A33 to A35 exhibited a large capacity retention rate and a small resistance increase rate.

[0092] Comparative Examples 6 to 8 Electrochemical devices B6 to B8 of Comparative Examples 6 to 8 were fabricated and evaluated in the same manner as electrochemical device A3 of Example 3, except that the compounds shown in Table 7 were used instead of AMGBL together with DVL as additives.

[0093] Comparative Example 9 An electrochemical device B9 of Comparative Example 9 was fabricated and evaluated in the same manner as electrochemical device A3 of Example 3, except that γ-valerolactone was used instead of DVL together with AMGBL as an additive.

[0094] The evaluation results of electrochemical devices B6 to B9 are shown in Table 7. The evaluation results of electrochemical device A3 are also shown in Table 7. The numbers in parentheses in the additive column in Table 7 indicate the content in the electrolyte solution.

[0095] [Table 7]

[0096] In electrochemical devices B6 to B9, the electrolyte did not contain any additives (AMGBL and DVL), and therefore a small capacity retention rate and a large resistance increase rate were obtained. [Industrial Applicability]

[0097] The electrolyte solution according to the present invention is suitable for use in electrochemical devices that require high reliability. [Explanation of symbols]

[0098] 1: capacitor element, 2: first electrode, 3: second electrode, 4: separator, 5a: first lead wire, 5b: second lead wire, 6: outer case, 7: sealing member, 10: electrochemical device

Claims

1. The composition includes a solvent, an ionic substance, α-methyl-γ-butyrolactone, and δ-valerolactone, The electrolyte solution for electrochemical device has a total content of the α-methyl-γ-butyrolactone and the δ-valerolactone of 3.6 mass % or less.

2. 2. The liquid electrolyte for electrochemical devices according to claim 1, wherein the total content of the α-methyl-γ-butyrolactone and the δ-valerolactone in the liquid electrolyte for electrochemical devices is 3.5 mass % or less.

3. 3. The liquid electrolyte for electrochemical device according to claim 1, wherein the solvent comprises at least one selected from the group consisting of γ-butyrolactone, acetonitrile, and propylene carbonate.

4. 4. The liquid electrolyte for electrochemical device according to claim 1, wherein the ionic substance includes a quaternary ammonium salt.

5. 5. The liquid electrolyte for electrochemical device according to claim 4, wherein the quaternary ammonium salt comprises at least one selected from the group consisting of diethyldimethylammonium tetrafluoroborate and 1-ethyl-1-methylpyrrolidinium tetrafluoroborate.

6. An electrochemical device comprising a pair of electrodes and the liquid electrolyte for electrochemical devices according to any one of claims 1 to 5.

7. 7. The electrochemical device according to claim 6, wherein at least one of the pair of electrodes comprises an active layer containing a carbon material and a current collector supporting the active layer.

8. The electrochemical device of claim 7 , wherein the carbon material comprises activated carbon.

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