WO2021205750A1

WO2021205750A1 - Electrolytic solution for electrochemical device, and electrochemical device

Info

Publication number: WO2021205750A1
Application number: PCT/JP2021/005681
Authority: WO
Inventors: 続木武男; 加納幸司
Original assignee: 太陽誘電株式会社
Priority date: 2020-04-07
Filing date: 2021-02-16
Publication date: 2021-10-14
Also published as: JP2021166244A

Abstract

This electrolytic solution for electrochemical devices comprises a solvent and an electrolyte dissolved therein, the electrolytic solution being characterized in that the solvent comprises a cyclic carbonate and a chain carbonate and the electrolyte comprises an imide-based lithium salt and a non-imide-based lithium salt and that an additive comprising lithium difluorophosphate, lithium oxalatophosphate, and lithium oxalatoborate has been added to the electrolytic solution and the concentration of the additive in the electrolytic solution is 0.5-2.0 wt%, the ratio between the weight of the lithium difluorophosphate and the total weight of the lithium oxalatophosphate and the lithium oxalatoborate being 1:9 to 3:7 and the ratio between the weight of the lithium oxalatophosphate and the weight of the lithium oxalatoborate being 1:1 to 3:1.　

Description

Electrolytes for electrochemical devices and electrochemical devices

The present invention relates to an electrolytic solution for an electrochemical device and an electrochemical device.

Electrochemical devices such as electric double-layer capacitors and lithium-ion capacitors that use non-aqueous electrolytes can have a high withstand voltage because the electrolysis voltage of the non-aqueous solvent is high, and can store a large amount of energy. ..

In recent years, electrochemical devices have been required to reduce internal resistance at low temperatures and ensure reliability at high temperatures. Regarding the low temperature characteristics, it is considered that the internal resistance increases due to the difficulty of dissociation of the electrolyte in the electrolytic solution and the increase in the viscosity of the non-aqueous electrolytic solution.

As for the high-temperature reliability, PF ₆ is an electrolyte ^- or anions decomposition products such as hydrogen fluoride and decomposing occurs such non-aqueous electrolyte solution reductive decomposition to a high resistance film on the negative electrode near It is thought that various characteristics of the cell are deteriorated due to the formation.

In order to solve the above problem, for example, Patent Document 1 uses an imide-based lithium salt having an imide structure and includes a polymer having a RED (Relative Energy Difference) value larger than 1 based on the Hansen solubility parameter. A lithium ion capacitor using a binder has been proposed.

Further, Patent Document 2 proposes a lithium ion secondary battery in which a plurality of additives are added to an electrolytic solution obtained by adding an imide-based lithium salt and LiPF _{6 to a non-aqueous organic solvent.}

Then, Patent Document 3 proposes a lithium ion capacitor in which a specific additive is added to an electrolytic solution obtained by adding either _{LiPF 6} or LiBF ₄ and LiFSI to a mixed solvent of chain carbonate and cyclic carbonate. Has been done.

Further, in Patent Document 4, by adding an imide-based lithium salt, an alkylsulfonic acid lithium salt, a difluorophosphate lithium salt, a lithium oxalate borate, and a vinylene carbonate to the electrolytic solution, the electric resistance after high temperature storage is increased. A suppressed lithium-ion secondary battery has been proposed.

Japanese Unexamined Patent Publication No. 2017-17299 Special Table 2016-503571A WO2016 / 006632 JP-A-2019-175578

In Patent Document 1, LiFSI is used as the imide-based lithium salt, and a binder containing a polymer such that the RED value based on the Hansen solubility parameter is larger than 1 is used to float the lithium ion capacitor at a high temperature of about 85 ° C. It is stated that the reliability will be good. However, although the low temperature characteristics have been discussed based on the presence or absence of electrolyte precipitation and the value of ionic conductivity, no specific cell evaluation has been performed.

In Patent Document 2, a group consisting of lithium difluorooxalate phosphate, trimethylsilylpropyl phosphate, 1,3-propensulton, and ethylene sulfate is added to an electrolytic solution obtained by adding an imide-based lithium salt and LiPF _{6 to a non-aqueous organic solvent.} It is described that the addition of one or more types improves the output characteristics at low temperature (-30 ° C) and high temperature (60 ° C). However, the high temperature side is evaluated only up to 60 ° C., and it is unclear whether it can withstand a high temperature such as 85 ° C.

In Patent Document 3, a mixed solvent prepared by mixing either ethylene carbonate (EC) or propylene carbonate (PC) with any one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) is used. I'm using it. Then, one of LiPF ₆ and LiBF ₄ and LiFSI are added to this mixed solvent as an electrolyte to prepare an electrolytic solution. Further, any compound of a chain ether, a fluorinated chain ether, and a propionic acid ester is added to this electrolytic solution, or a sulton compound, a cyclic phosphazene, a fluorocyclic carbonate, a cyclic carbonate, and a cyclic carboxylic acid. It is described in Patent Document 3 that any compound of ester and cyclic acid anhydride is added. It is described in Patent Document 3 that this improves the output characteristics of the lithium ion capacitor at −30 ° C. and suppresses the generation of gas when the lithium ion capacitor is stored at 60 ° C. However, the high temperature side is evaluated only up to 60 ° C., and it is unclear whether it can withstand a high temperature such as 85 ° C.

In Patent Document 4, Lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium (oxalate) borate, and vinylene carbonate are added to an electrolytic solution obtained by adding _{LiPF 6} and an imide-based lithium salt to a non-aqueous organic solvent. , A lithium ion secondary battery capable of reducing the rate of increase in resistance at −20 ° C. even after storage at 80 ° C. for 48 hours is disclosed. However, the time for storing at a high temperature is only 48 hours, and it is unclear whether the characteristics of the lithium ion secondary battery deteriorate even if the lithium ion secondary battery is stored at a high temperature for a long period of time such as 1000 hours.

The present invention has been made in view of the above problems, and provides an electrolytic solution for an electrochemical device capable of improving both low temperature characteristics and high temperature reliability of an electrochemical device, and an electrochemical device including the same. The purpose is to do.

The electrolytic solution for an electrochemical device according to the present invention is an electrolytic solution in which an electrolyte is dissolved in a solvent, the solvent contains a cyclic carbonate and a chain carbonate, and the electrolyte is an imide-based lithium salt and a non-imide-based electrolyte. An additive containing a lithium salt, a lithium difluorophosphate, a lithium oxalateric acid salt, and a lithium oxalatoboate salt is added to the electrolytic solution, and the concentration of the additive in the electrolytic solution is 0.5 wt% or more 2 It is 0.0 wt% or less, and the ratio of the weight of the lithium difluorophosphate to the combined weight of the lithium oxalatrate salt and the lithium oxalateborate salt is 1: 9 to 3: 7, and the lithium oxalaterate is obtained. The ratio of the weight of the salt to the weight of the lithium oxalate borate salt is 1: 1 to 3: 1.

In the electrolytic solution for an electrochemical device, the lithium oxalatric acid salt may be lithium difluorobis (oxalat) or lithium tetrafluoro (oxalat) phosphate.

In the electrolytic solution for an electrochemical device, the lithium oxalate borate salt may be lithium bis (oxalate) borate or lithium difluoro (oxalate) borate.

In the electrolytic solution for an electrochemical device, the cyclic carbonate may be propylene carbonate or ethylene carbonate, and the chain carbonate may be ethyl methyl carbonate or diethyl carbonate.

In the electrolytic solution for an electrochemical device, the imide-based lithium salt may be lithium bisfluorosulfonylimide, and the non-imide-based lithium salt may be lithium hexafluorophosphate.

The electrochemical device according to the present invention includes a power storage element in which a positive electrode and a negative electrode are laminated via a separator, and is used for any of the above electrochemical devices on the active material of the positive electrode and the active material of the negative electrode, or the separator. It is characterized in that it is impregnated with an electrolytic solution.

According to the present invention, it is possible to provide an electrolytic solution for an electrochemical device capable of improving both low temperature characteristics and high temperature reliability of the electrochemical device, and an electrochemical device provided with the electrolytic solution.

It is an exploded view of a lithium ion capacitor. It is sectional drawing in the stacking direction of the positive electrode, the negative electrode and a separator of a lithium ion capacitor. It is an exploded view of a lithium ion capacitor. It is an external view of a lithium ion capacitor.

Hereinafter, embodiments will be described with reference to the drawings.
(Embodiment)
First, a lithium ion capacitor will be described as an example of an electrochemical device. FIG. 1 is an exploded view of the lithium ion capacitor 100. As illustrated in FIG. 1, the lithium ion capacitor 100 includes a power storage element 50 having a structure in which a positive electrode 10 and a negative electrode 20 are wound around a separator 30. The power storage element 50 has a substantially cylindrical shape. A drawer terminal 41 is connected to the positive electrode 10. The extraction terminal 42 is connected to the negative electrode 20.

FIG. 2 is a cross-sectional view of the positive electrode 10, the negative electrode 20, and the separator 30 in the stacking direction. As illustrated in FIG. 2, the positive electrode 10 has a structure in which the positive electrode layer 12 is laminated on one surface of the positive electrode current collector 11. The separator 30 is laminated on the positive electrode layer 12 of the positive electrode 10. The negative electrode 20 is laminated on the separator 30. The negative electrode 20 has a structure in which the negative electrode layer 22 is laminated on the surface of the negative electrode current collector 21 on the positive electrode 10 side. The separator 30 is laminated on the negative electrode current collector 21 of the negative electrode 20. In the power storage element 50, the laminated units of the positive electrode 10, the separator 30, the negative electrode 20, and the separator 30 are wound. The positive electrode layer 12 may be provided on both sides of the positive electrode current collector 11. The negative electrode layer 22 may be provided on both sides of the negative electrode current collector 21.

As illustrated in FIG. 3, a drawer terminal 41 and a drawer terminal 42 are inserted into two through holes of a substantially cylindrical sealing rubber 60 having a diameter substantially the same as that of the power storage element 50. Further, the power storage element 50 is housed in a bottomed substantially cylindrical container 70.

As illustrated in FIG. 4, the sealing rubber 60 is crimped around the opening of the container 70. As a result, the hermeticity of the power storage element 50 is maintained. The non-aqueous electrolyte solution is sealed in the container 70 and impregnated with the active material of the positive electrode 10 and the active material of the negative electrode 20, or the separator 30.

(Positive electrode)
The positive electrode current collector 11 is a metal foil, for example, an aluminum foil or the like. This aluminum foil may be a perforated foil. The positive electrode layer 12 may have a known material and structure used for the electrode layer of an electric double layer capacitor or a redox capacitor, and may include, for example, polyacene (PAS), polyaniline (PAN), activated carbon, carbon black, graphite, and the like. It contains an active material such as carbon nanotubes, and also contains other components such as a conductive auxiliary agent and a binder used for an electrode layer such as an electric double layer capacitor, if necessary.

(Negative electrode)
The negative electrode current collector 21 is a metal foil, for example, a copper foil or the like. This copper foil may be a perforated foil. The negative electrode layer 22 contains an active material such as graphitized carbon, graphite, tin oxide, or silicon oxide, and is a conductive auxiliary agent such as carbon black or metal powder, polytetrafluoroethylene (PTFE), or polyfluoridene fluoride. Binders such as vinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR) are also contained as required.

(Separator)
By providing the separator 30 between the positive electrode 10 and the negative electrode 20, for example, a short circuit due to contact between these two electrodes is prevented. The separator 30 forms a conductive path between the electrodes by holding the non-aqueous electrolyte solution in the pores. As the material of the separator 30, for example, porous cellulose, polypropylene, polyethylene, a fluororesin, or the like can be used.

When the power storage element 50 and the non-aqueous electrolytic solution are sealed in the container 70, the lithium metal sheet is electrically connected to the negative electrode 20. As a result, lithium in the lithium metal sheet is dissolved in the non-aqueous electrolyte solution, and lithium ions are pre-doped into the negative electrode layer 22 of the negative electrode 20. As a result, the potential of the negative electrode 20 becomes lower than the potential of the positive electrode 10 by, for example, about 3 V in the state before charging.

Further, in the present embodiment, the lithium ion capacitor 100 has a structure in which the power storage element 50 having a wound structure is enclosed in a cylindrical container 70, but the present invention is not limited thereto. For example, the power storage element 50 may have a laminated structure. Further, the container 70 in this case may be a square can or the like.

(Non-aqueous electrolyte)
A non-aqueous electrolyte solution is prepared by dissolving an electrolyte in a non-aqueous solvent as described below and adding an additive to the electrolyte.

(Non-aqueous solvent)
Cyclic carbonate and chain carbonate are used as the non-aqueous solvent. The cyclic carbonate is, for example, a cyclic carbonate ester such as propylene carbonate (PC) or ethylene carbonate (EC). Since the cyclic carbonic acid ester has a high dielectric constant, it has a property of dissolving a lithium salt well. Further, the non-aqueous electrolytic solution using the cyclic carbonate as a non-aqueous solvent has high ionic conductivity. Therefore, when cyclic carbonate is used as a non-aqueous solvent, the initial characteristics of the lithium ion capacitor 100 are improved. Further, when the cyclic carbonate is used as a non-aqueous solvent, sufficient electrochemical stability during operation of the lithium ion capacitor 100 is realized after the film is formed on the negative electrode 20.

On the other hand, the chain carbonate is, for example, ethyl methyl carbonate (EMC) or diethyl carbonate (DEC), which are chain carbonates.

In the present embodiment, the ratio of the cyclic carbonate to the chain carbonate in the non-aqueous solvent is 40:60 to 20:80 in volume ratio. The lower limit of the ratio of the chain carbonate is set to 60 because if the amount of the chain carbonate is less than this, the low temperature characteristics are extremely deteriorated. Further, the upper limit of the ratio of the chain carbonate is set to 80 because if the amount of the chain carbonate is larger than this, the high temperature reliability is extremely deteriorated. Further, the ratio of the cyclic carbonate to the chain carbonate in the non-aqueous solvent is preferably 35:65 to 25:75 in volume ratio.

(Electrolytes)
As the electrolyte, a mixture of an imide-based lithium salt and a non-imide-based lithium salt is used.

Of these, the imide-based lithium salt is, for example, LiFSI (lithium bisfluorosulfonylimide). LiFSI improves the capacity and DCR of the lithium ion capacitor 100 at low temperatures.

On the other hand, the non-imide-based lithium salt is, for example, LiPF ₆ (lithium hexafluorophosphate). Since LiPF ₆ has a high degree of dissociation among general-purpose lithium salts, it realizes good initial characteristics (capacity and DCR) of the lithium ion capacitor 100.

In the present embodiment, the molar ratio of the imide-based lithium salt to the non-imide-based lithium salt in the electrolyte is set to 40:60 to 99.9: 0.1. The reason why the lower limit of the molar ratio of the imide-based lithium salt is set to 40 is that if the amount of the imide-based lithium salt is less than this, the low temperature characteristics are extremely deteriorated. Further, the lower limit of the molar ratio of the non-imide lithium salt is set to 0.1 because the high temperature reliability deteriorates in the electrolyte composed of only the imide-based lithium salt. Further, the molar ratio of the imide-based lithium salt to the non-imide-based lithium salt in the electrolyte is preferably 60:40 to 99.5: 0.5, preferably 70:30 to 99.0: 1.0. More preferred.

The concentration of the electrolyte in the non-aqueous solvent is preferably 0.7 mol / L to 1.5 mol / L. The lower limit of the electrolyte concentration is set to 0.7 mol / L because if the electrolyte concentration is lower than this, the internal resistance increases due to the decrease in the number of ions that act effectively. Further, the upper limit of the concentration of the electrolyte is set to 1.5 mol / L because if the concentration of the electrolyte is higher than this, the internal resistance increases due to the increase in the viscosity of the non-aqueous electrolyte solution.

(First additive)
In order to reduce the internal resistance of the lithium ion capacitor 100, lithium difluorophosphate (LiDFP (LiPO ₂ F ₂ )) is added as a first additive to the non-aqueous electrolytic solution.

It is preferable to set a lower limit on the concentration of the first additive in order to sufficiently obtain the effect of the first additive. On the other hand, if the concentration of the first additive in the electrolytic solution is too high, the float reliability of the lithium ion capacitor 100 at a high temperature may decrease. Therefore, it is preferable to set an upper limit on the concentration of the first additive in the electrolytic solution. In the present embodiment, the concentration of the first additive in the electrolytic solution is 0.1 wt% to 0.4 wt%.

(Second additive)
In order to reduce the internal resistance of the lithium ion capacitor 100 and increase the float reliability at high temperature, a lithium oxalateric acid salt is added to the non-aqueous electrolytic solution as a second additive. Such lithium oxalatrate salts include lithium difluorobis (oxalate) phosphate (LiDFBOP (LiP (C ₂ O ₄ ) ₂ F ₂ )) or lithium tetrafluoro (oxalate) phosphate (LiTFOP (LiP (C ₂ O)). ₄ ) _{There are F 4} )).

In order to obtain the full effect of the second additive, it is preferable to set a lower limit on the concentration of the second additive. On the other hand, if the concentration of the second additive in the electrolytic solution is too high, the internal resistance of the lithium ion capacitor 100 may increase, or the float reliability at high temperatures may decrease. Therefore, it is preferable to set an upper limit on the concentration of the second additive in the electrolytic solution. In the present embodiment, the concentration of the second additive in the electrolytic solution is 0.2 wt% to 1.2 wt%.

(Third additive)
In order to improve the float reliability of the lithium ion capacitor 100 at high temperature, a lithium oxalateborate salt is added to the non-aqueous electrolytic solution as a third additive. Such lithium oxalate oxalate salts include lithium bis (oxalate) oxalate (LiBOB (LiB (C ₂ O ₄ ) ₂ )) or lithium difluoro (oxalate) oxalate (LiDFOB (LiB (C ₂ O ₄ ) F _2). )).

It is preferable to set a lower limit on the concentration of the third additive in order to sufficiently obtain the effect of the third additive. On the other hand, if the concentration of the third additive in the electrolytic solution is too high, the internal resistance of the lithium ion capacitor 100 may increase. Therefore, it is preferable to set an upper limit on the concentration of the third additive in the electrolytic solution. In the present embodiment, the concentration of the third additive in the electrolytic solution is 0.2 wt% to 0.4 wt%.

Further, in the present embodiment, the combined concentration of the first additive, the second additive, and the third additive in the non-aqueous electrolytic solution is 0.5 wt% or more and 2.0 wt% or less. Further, the ratio of the weight of the first additive to the combined weight of the second additive and the third additive is set to 1: 9 to 3: 7. Then, the ratio of the weight of the second additive to the weight of the third additive is set to 1: 1 to 3: 1. As a result, the internal resistance of the lithium ion capacitor 100 at low temperature can be reduced, the float reliability at high temperature can be improved, and both the low temperature characteristics and high temperature reliability of the lithium ion capacitor 100 can be improved.

In the present embodiment, attention was paid to the electrolytic solution of the lithium ion capacitor as an electrochemical device, but the present invention is not limited to this. For example, the non-aqueous electrolytic solution according to the present embodiment can also be used as an electrolytic solution for other electrochemical devices such as an electric double layer capacitor.

A lithium ion capacitor was prepared according to the above embodiment, and its characteristics were investigated. Table 1 is a diagram showing test conditions for each of Examples and Comparative Examples.

(Example 1)
Activated carbon was used as the active material for the positive electrode 10. A slurry was prepared using carboxymethyl cellulose and styrene-butadiene rubber as a binder, and the prepared slurry was applied onto a perforated aluminum foil to prepare a sheet. Graphitized carbon was used as the active material for the negative electrode 20. A slurry was prepared using carboxymethyl cellulose and styrene-butadiene rubber as a binder, and the prepared slurry was applied onto a copper foil having been subjected to perforation processing to prepare a sheet. A cellulosic separator 30 is sandwiched between these

electrodes

10 and 20, the extraction terminal 41 is attached to the positive electrode current collector 11 by ultrasonic welding, the extraction terminal 42 is attached to the negative electrode current collector 21, and then these are wound. The power storage element 50 was fixed with a polyimide adhesive tape. After attaching the sealing rubber 60 to the produced power storage element 50 and vacuum-drying it at about 180 ° C., a lithium foil was attached to the negative electrode 20 and the power storage element 50 was placed in the container 70.

Then, a non-aqueous electrolyte solution was prepared by dissolving an electrolyte in which _{LiFSI and LiPF 6} were mixed in a molar ratio of 7: 3 in a non-aqueous solvent in which PC and EMC were mixed in a volume ratio of 3: 7. bottom. The concentration of the electrolyte in the non-aqueous electrolyte solution was 1.0 mol / L. _{Further, lithium difluorophosphate (LiPO 2} F ₂ ) was added to the non-aqueous electrolyte solution as a first additive at a concentration of 0.1 wt%, and lithium difluorobis (oxalate) phosphate (LiP (C)) was added as a second additive. ₂ O ₄ ) ₂ F ₂ ) was added at a concentration of 0.6 wt%, and lithium bis (oxalate) borate (LiB (C ₂ O ₄ ) ₂ ) was added at a concentration of 0.3 wt% as a third additive. bottom. Then, after injecting this non-aqueous electrolytic solution into the container 70, the portion of the sealing rubber 60 was crimped to prepare a lithium ion capacitor 100.

(Example 2)
In Example 2, the concentration of the second additive was 0.5 wt%, and the concentration of the third additive was 0.4 wt%. Other conditions were the same as in Example 1.

(Example 3)
In Example 3, the concentration of the first additive was 0.2 wt%, the concentration of the second additive was 0.6 wt%, and the concentration of the third additive was 0.2 wt%. Other conditions were the same as in Example 1.

(Example 4)
In Example 4, the concentration of the second additive was 0.5 wt%, and the concentration of the third additive was 0.3 wt%. Other conditions were the same as in Example 3.

(Example 5)
In Example 5, the concentration of the second additive was 0.4 wt%, and the concentration of the third additive was 0.4 wt%. Other conditions were the same as in Example 3.

(Example 6)
In Example 6, the concentration of the first additive was 0.3 wt%, the concentration of the second additive was 0.5 wt%, and the concentration of the third additive was 0.2 wt%. Other conditions were the same as in Example 1.

(Example 7)
In Example 7, the concentration of the second additive was 0.4 wt%, and the concentration of the third additive was 0.3 wt%. Other conditions were the same as in Example 6.

(Example 8)
In Example 8, the concentration of the first additive was 0.1 wt%, the concentration of the second additive was 0.2 wt%, and the concentration of the third additive was 0.2 wt%. Other conditions were the same as in Example 1.

(Example 9)
In Example 9, the concentration of the first additive was 0.2 wt%, the concentration of the second additive was 0.9 wt%, and the concentration of the third additive was 0.4 wt%. Other conditions were the same as in Example 1.

(Example 10)
In Example 10, the concentration of the first additive was 0.4 wt%, the concentration of the second additive was 1.2 wt%, and the concentration of the third additive was 0.4 wt%. Other conditions were the same as in Example 1.

(Comparative Example 1)
In Comparative Example 1, the concentration of the second additive was 0.7 wt%, and the concentration of the third additive was 0.2 wt%. Other conditions were the same as in Example 1.

(Comparative Example 2)
In Comparative Example 2, the concentration of the second additive was 0.4 wt%, and the concentration of the third additive was 0.5 wt%. Other conditions were the same as in Example 1.

(Comparative Example 3)
In Comparative Example 3, the concentration of the second additive was 0.7 wt%, and the concentration of the third additive was 0.1 wt%. Other conditions were the same as in Example 3.

(Comparative Example 4)
In Comparative Example 4, the concentration of the second additive was 0.3 wt%, and the concentration of the third additive was 0.5 wt%. Other conditions were the same as in Example 3.

(Comparative Example 5)
In Comparative Example 5, the concentration of the second additive was 0.6 wt%, and the concentration of the third additive was 0.1 wt%. Other conditions were the same as in Example 6.

(Comparative Example 6)
In Comparative Example 6, the concentration of the second additive was 0.3 wt%, and the concentration of the third additive was 0.4 wt%. Other conditions were the same as in Example 6.

(Comparative Example 7)
In Comparative Example 7, the concentration of the first additive was 0.4 wt%, the concentration of the second additive was 0.3 wt%, and the concentration of the third additive was 0.3 wt%. Other conditions were the same as in Example 1.

(Comparative Example 8)
In Comparative Example 8, the concentration of the first additive was 0.05 wt%, the concentration of the second additive was 0.1 wt%, and the concentration of the third additive was 0.1 wt%. Other conditions were the same as in Example 1.

(Comparative Example 9)
In Comparative Example 9, the concentration of the first additive was 0.5 wt%, the concentration of the second additive was 1.5 wt%, and the concentration of the third additive was 0.5 wt%. Other conditions were the same as in Example 1.

(Comparative Example 10)
In Comparative Example 10, the concentration of the second additive was 0.5 wt%, and the concentration of the third additive was 0.5 wt%. The first additive was not added to the electrolytic solution. Other conditions were the same as in Example 1.

(Comparative Example 11)
In Comparative Example 11, the concentration of the first additive was 0.3 wt%, and the concentration of the third additive was 0.7 wt%. No second additive was added to the electrolytic solution. Other conditions were the same as in Example 1.

(Comparative Example 12)
In Comparative Example 12, the concentration of the first additive was 0.3 wt%, and the concentration of the second additive was 0.7 wt%. The third additive was not added to the electrolytic solution. Other conditions were the same as in Example 1.

(Evaluation method)
Lithium ion capacitors 100 of Examples 1 to 10 and Comparative Examples 1 to 12 were produced. Then, as an initial characteristic, DCR (internal resistance) at room temperature (25 ° C.) was measured.

The low temperature characteristics were evaluated based on the rate of change of this value from 25 ° C. by measuring the DCR at −40 ° C. after leaving the cell at −40 ° C. for 2 hours.

Further, in order to evaluate the high temperature reliability, a float test was conducted in which the battery was continuously charged at a voltage of 3.5 V for 1000 hours in a constant temperature bath at 85 ° C. After the float test, the cell was allowed to cool to room temperature (25 ° C.), DCR was measured, and the rate of change of the value before and after the test was calculated. Table 2 shows the test results of each of the examples and comparative examples.

(Low temperature characteristics)
The criteria for judging the quality of the low temperature characteristics at −40 ° C. was that the resistance increase rate was within 1500%, and if this criterion was not met, it was judged to be defective. The rate of increase in resistance is the rate of increase in internal resistance when the temperature is 25 ° C. as a reference.

As is clear from the results of Examples 1 to 7 and Comparative Examples 1 to 7, 10 to 12, it is the third additive lithium bis (oxalate) borate that has the greatest effect on the low temperature characteristics, and the amount thereof added. It was confirmed that the low temperature characteristics tended to deteriorate as the amount increased. Further, as the amount of the first additive lithium difluorophosphate added increased, the low temperature characteristics tended to improve.

On the other hand, according to the results of Examples 8 to 10 and Comparative Examples 8 to 9, when the total concentration of each of the first additive, the second additive, and the third additive is higher than 2.0 wt%, the temperature is low. It was confirmed that the criteria for the characteristics were not met.

(High temperature reliability)
The criteria for judging the quality of high temperature reliability was that the resistance increase rate was within 200%, and if this criterion was not met, it was judged to be defective. The rate of increase in resistance is the rate of increase in internal resistance before and after the float test.

As is clear from the results of Examples 1 to 7 and Comparative Examples 1 to 7, 10 to 12, the higher the amount of the third additive lithium bis (oxalate) borate added, the better the high temperature reliability, but the higher the reliability. It was confirmed that if the amount added was too large, the high temperature reliability would rather deteriorate. Further, as the amount of lithium difluorophosphate added as the first additive increased, the high temperature reliability tended to deteriorate.

On the other hand, according to the results of Examples 8 to 10 and Comparative Examples 8 to 9, the total concentration of each of the first additive, the second additive, and the third additive is 0.5 wt% to 2.0 wt%. It was confirmed that the high temperature reliability deteriorated when the value was out of the range of. As shown in Comparative Example 11, if the electrolytic solution does not contain the second additive lithium difluorobis (oxalate) phosphate, not only the high temperature reliability is poor, but also the low temperature characteristics are deteriorated. Obtained.

Further, according to Table 2, in Examples 1 to 10 that satisfy the criteria for both low temperature characteristics and high temperature reliability, the sum of the concentrations of the first additive, the second additive, and the third additive, respectively. Is 0.5 wt% or more and 2.0 wt% or less, and the ratio of the weight of the first additive to the combined weight of the second additive and the third additive is 1: 9 to 3: 7. The ratio of the weight of the second additive to the weight of the third additive is 1: 1 to 3: 1.

Therefore, in order to achieve both low temperature characteristics and high temperature reliability, the total concentration of each of the first additive, the second additive, and the third additive should be 0.5 wt% or more and 2.0 wt% or less. The ratio of the weight of the 1 additive to the combined weight of the 2nd additive and the 3rd additive is 1: 9 to 3: 7, and the ratio of the weight of the 2nd additive to the weight of the 3rd additive. It was confirmed that it is effective to set the value to 1: 1 to 3: 1.

Claims

An electrolytic solution in which an electrolyte is dissolved in a solvent.
The solvent contains a cyclic carbonate and a chain carbonate.
The electrolyte contains an imide-based lithium salt and a non-imide-based lithium salt.
An additive containing lithium difluorophosphate, lithium oxalateric acid salt, and lithium oxalatoboate salt was added to the electrolytic solution, and the mixture was added.
The concentration of the additive in the electrolytic solution is 0.5 wt% or more and 2.0 wt% or less.
The ratio of the weight of the lithium difluorophosphate to the combined weight of the lithium oxalatrate salt and the lithium oxalatoboate salt is 1: 9 to 3: 7.
An electrolytic solution for an electrochemical device, wherein the ratio of the weight of the lithium oxalatric acid salt to the weight of the lithium oxalate borate salt is 1: 1 to 3: 1.
The electrolytic solution for an electrochemical device according to claim 1, wherein the lithium oxalatric acid salt is lithium difluorobis (oxalate) phosphate or lithium tetrafluoro (oxalate) phosphate.
The electrolytic solution for an electrochemical device according to claim 1 or 2, wherein the lithium oxalate borate salt is lithium bis (oxalate) borate or lithium difluoro (oxalate) borate.
The electrolysis for an electrochemical device according to any one of claims 1 to 3, wherein the cyclic carbonate is propylene carbonate or ethylene carbonate, and the chain carbonate is ethyl methyl carbonate or diethyl carbonate. liquid.
The electrolysis for an electrochemical device according to any one of claims 1 to 4, wherein the imide-based lithium salt is lithium bisfluorosulfonylimide, and the non-imide-based lithium salt is lithium hexafluorophosphate. liquid.
A power storage element in which a positive electrode and a negative electrode are laminated via a separator is provided.
An electrochemical device, wherein the active material of the positive electrode, the active material of the negative electrode, or the separator is impregnated with the electrolytic solution for an electrochemical device according to any one of claims 1 to 5.