JP6955672B2 - Electrolyte for lithium secondary battery - Google Patents

Description

The present invention relates to an electrolytic solution for a lithium secondary battery.

In recent years, lithium secondary batteries have been used as portable power sources for personal computers, mobile terminals, etc., vehicle drive power sources for electric vehicles (EV), hybrid vehicles (HV), plug-in hybrid vehicles (PHV), power storage power sources, etc. It is preferably used.

With the widespread use of lithium secondary batteries, further improvement in performance is desired. In a large lithium secondary battery used as a power source for driving a vehicle, a power source for storing electric power, or the like, an organic solvent is generally used as a solvent for an electrolytic solution. However, organic solvents are highly volatile and flammable and are classified as flammable substances. Therefore, there is room for improvement in safety in a lithium secondary battery that uses an organic solvent as an electrolytic solution.
Therefore, in recent years, in order to improve the safety of the lithium secondary battery, it has been proposed to include a flame-retardant ionic liquid containing a quaternary ammonium organic cation (normal temperature molten salt) in the electrolytic solution (for example). , Patent Documents 1 to 3).

JP-A-2007-141489 Japanese Unexamined Patent Publication No. 2009-070636 Japanese Unexamined Patent Publication No. 2010-287380

However, in the electrolytic solution for a lithium secondary battery containing the above ionic liquid, it tends to be difficult to increase the conductivity as compared with the conventional electrolytic solution even when the contained ionic liquid has a relatively low viscosity. When the viscosity is high, the impregnation property tends to be insufficient for the polyolefin separator generally used for lithium secondary batteries. Therefore, in the conventional electrolytic solution containing an ionic liquid, the output characteristics and cycle characteristics when used in a lithium secondary battery can be obtained with a conventional organic solvent (carbonate) while ensuring good impregnation property with respect to a polyolefin separator. It has not been able to be higher than that of a secondary battery equipped with an electrolytic solution using (class).

Therefore, an object of the present invention is to provide an electrolytic solution which has good impregnation property with respect to a polyolefin separator while using an ionic liquid, has high conductivity, and gives a high cycle characteristic to a lithium secondary battery. do.

Electrolytic solution for a lithium secondary battery disclosed herein, a saturated aliphatic cyclic quaternary ammonium organic cation, and an ionic liquid composed of the N (SO _{2 F)} 2 anions, and carbonate, the following formula ( It contains a light metal salt represented by I) and at least one additive selected from the group consisting of a phosphorus compound represented by the following formula (II). The ratio of the ionic liquid to the total of the ionic liquid and the carbonates is 5% by volume or more and less than 30% by volume.

(In formula (I), R11 is a -C (= O) -R21-C (= O) -group (R21 represents an alkylene group, a halogenated alkylene group, an arylene group, or a halogenated arylene group). Or -C (= O) -C (= O) -group, R12 represents a halogen atom, an alkyl group, an alkyl halide group, an aryl group, or an aryl halide group, and X11 and X12 are oxygen, respectively. Represents an atom or sulfur atom, M11 represents a transition metal element or a Group 13 element, a Group 14 element, or a Group 15 element in the periodic table, and M21 represents a Group 1 element, a Group 2 element, or Al in the periodic table. , A is an integer of 1 to 4, b is an integer of 0 to 8, c, d, e, and f are each of an integer of 1 to 3, and in equation (II), M ^{n +} is the period. In the table, they represent Group 1 element ions, Group 2 element ions, Al ions, or onium ions, A1 and A2 represent oxygen atoms or sulfur atoms, respectively, and X1 and X2 represent halogen atoms and carbon numbers 1 to 6, respectively. Represents a saturated or unsaturated hydrocarbon group of, or a saturated or unsaturated hydrocarbon group having 1 to 6 carbon atoms with a hetero atom, or bonds with each other to form a ring structure, where n is a valence. be.)

According to such a configuration, by using an ionic liquid of a combination of a specific cation and a specific anion and setting the amount to a specific ratio, a sufficient potential window is secured while keeping the viscosity of the electrolytic solution low. be able to. Furthermore, by combining with a specific additive, the cycle characteristics of the lithium secondary battery can be further enhanced. Therefore, according to such a configuration, an electrolytic solution which has good impregnation property with respect to a polyolefin separator while using an ionic liquid, has high conductivity, and gives a high cycle characteristic to a lithium secondary battery is provided. can do.

It is sectional drawing which shows typically the internal structure of the lithium secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the winding electrode body of the lithium secondary battery which concerns on one Embodiment of this invention.

Hereinafter, embodiments according to the present invention will be described. It should be noted that matters other than those specifically mentioned in the present specification and necessary for carrying out the present invention (for example, general constitution and manufacture of an electrolytic solution for a lithium secondary battery which does not characterize the present invention). The process) can be grasped as a design matter of a person skilled in the art based on the prior art in the art. The present invention can be carried out based on the contents disclosed in the present specification and common general technical knowledge in the art.

In the present specification, the "secondary battery" generally refers to a power storage device capable of being repeatedly charged and discharged, and is a term including a so-called storage battery and a power storage element such as an electric double layer capacitor.
Further, in the present specification, the "lithium secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and realizes charge / discharge by the transfer of charges accompanying the lithium ions between the positive and negative electrodes.

Electrolytic solution for lithium secondary battery according to this embodiment includes a saturated aliphatic cyclic quaternary ammonium organic cation, and an ionic liquid composed of the N (SO _{2 F)} 2 anions, and carbonate, the following formula ( It contains a light metal salt represented by I) and at least one additive selected from the group consisting of a phosphorus compound represented by the following formula (II). The ratio of the ionic liquid to the total of the ionic liquid and the carbonates is 5% by volume or more and less than 30% by volume.

(In formula (I), R11 is a -C (= O) -R21-C (= O) -group (R21 represents an alkylene group, a halogenated alkylene group, an arylene group, or a halogenated arylene group). Or -C (= O) -C (= O) -group, R12 represents a halogen atom, an alkyl group, an alkyl halide group, an aryl group, or an aryl halide group, and X11 and X12 are oxygen, respectively. Represents an atom or sulfur atom, M11 represents a transition metal element or a Group 13 element, a Group 14 element, or a Group 15 element in the periodic table, and M21 represents a Group 1 element, a Group 2 element, or Al in the periodic table. , A is an integer of 1 to 4, b is an integer of 0 to 8, c, d, e, and f are each of an integer of 1 to 3, and in equation (II), M ^{n +} is the period. In the table, they represent Group 1 element ions, Group 2 element ions, Al ions, or onium ions, A1 and A2 represent oxygen atoms or sulfur atoms, respectively, and X1 and X2 represent halogen atoms and carbon numbers 1 to 6, respectively. Represents a saturated or unsaturated hydrocarbon group of, or a saturated or unsaturated hydrocarbon group having 1 to 6 carbon atoms with a hetero atom, or bonds with each other to form a ring structure, where n is a valence. be.)

The ionic liquid is a component that imparts conductivity to the electrolytic solution. Further, since the ionic liquid is a flame-retardant solvent, the safety of the lithium secondary battery using the electrolytic solution according to the present embodiment can be enhanced.
The ionic liquid contains saturated aliphatic cyclic quaternary ammonium organic cations as cations. Examples of saturated aliphatic cyclic quaternary ammonium organic cations are N, N-dimethylpyrrolidinium cation, N-methyl-N-ethylpyrrolidinium cation, N-methyl-propylpyrrolidinium cation, N-methyl- Examples thereof include a butylpyrrolidinium cation, an N-butyl-methylpyrrolidinium cation, and the like.
Ionic liquids, as an anion, _{N (SO 2 F)} 2 anions _{(i.e., N (SO 2 F) 2} -) containing.

Examples of the carbonates include chain carbonates such as dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate and methylpropyl carbonate; and cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate.
The carbonates are preferably at least one selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, and ethylene carbonate.

If the ratio of the ionic liquid to the total of the ionic liquid and the carbonates is too small, the conductivity of the electrolytic solution tends to decrease. Therefore, the ratio of the ionic liquid is 5% by volume or more, preferably 10% by volume or more. On the other hand, if the proportion of the ionic liquid is too large, the impregnation property of the polyolefin separator tends to deteriorate, and the cycle characteristics of the battery tend to deteriorate. Therefore, the ratio of the ionic liquid is less than 30% by volume, preferably 25% by volume or less, and more preferably 20% by volume or less.

Regarding additives, in the above formula (I), oxygen atoms are preferable as X11 and X12. As M11, a boron atom or a phosphorus atom is preferable. A halogen atom is preferable as R12. A lithium atom is preferable as M21. b is preferably an integer of 0 to 2.
Examples of the light metal salt represented by the above formula (I) used as an additive include bis [oxorat-O, O'] lithium borate (lithium bis (oxalat) borate) and difluoro [oxorat-O, O'. ] Lithium borate (lithium difluorooxalatoborate), difluorobis [oxorat-O, O'] lithium phosphate, difluoro [3,3,3-trifluoro-2-oxide-2-trifluoromethylpropionato (2) -)-O, O'] Lithium borate, bis [3,3,3-trifluoro-2-oxide-2-trifluoromethylpropionato (2-)-O, O'] Lithium borate, tetrafluoro [Oxorat-O, O'] Lithium phosphate and the like can be mentioned. Among them, lithium bis (oxalat) borate (LiBOB) and lithium difluorooxalat borate (LiDFOB) are preferable, and lithium bis (oxalat) borate is more preferable.

In the above formula (II), lithium ion is preferable as ^{Mn +.} Oxygen atoms are preferable as A1 and A2. As X1 and X2, a halogen atom and an alkyl group having 1 to 4 carbon atoms are preferable, and a fluorine atom is more preferable.
Examples of the phosphorus compound represented by the above formula (II) used as an additive include lithium dimethyl phosphate, lithium diethyl phosphate, lithium dipropyl phosphate, lithium dibutyl phosphate, lithium dipentyl phosphate, lithium ethylene phosphate, and binaphthyl. Lithium phosphate, sodium dimethyl phosphate, sodium diethyl phosphate, sodium dipropyl phosphate, sodium dibutyl phosphate, sodium dipentyl phosphate, sodium ethylene phosphate, sodium binaphthyl phosphate, magnesium dimethyl phosphate, magnesium diethyl phosphate, magnesium dipropyl phosphate , Magnesium dibutyl phosphate, magnesium dipentyl phosphate, magnesium ethylene phosphate, magnesium binaphthyl phosphate, triethylmethylammonium dimethyl phosphate, triethylmethylammonium diethyl phosphate, triethylmethylammonium dipropyl phosphate, triethylmethylammonium dibutyl phosphate, triethyl Methylammonium dipentyl phosphate, triethylmethylammonium ethylene phosphate, triethylmethylammonium binaphthyl phosphate, lithium diiodrine, sodium diiodrine, potassium diiodrine, lithium dibromophosphate, sodium dibromophosphate, potassium dibromophosphate, dichlorophosphate Examples thereof include lithium, sodium dichlorophosphate, potassium dichlorophosphate, lithium difluorophosphate, sodium difluorophosphate, potassium difluorophosphate, etc. Among them, lithium difluorophosphate and sodium difluorophosphate are preferable, and lithium difluorophosphate is preferable. Is more preferable.

If the content of the additive in the electrolytic solution is too small, the effect of the additive may not be sufficiently exhibited. Therefore, the content of the additive in the electrolytic solution is preferably more than 0.05% by mass, preferably 0.1% by mass or more. On the other hand, if the content of the above additive in the electrolytic solution is too large, it may not be sufficiently dissolved. Therefore, the content of the additive in the electrolytic solution is preferably 5% by mass or less, more preferably 3% by mass or less, and most preferably 2% by mass or less.

The electrolytic solution according to this embodiment usually further contains a supporting salt. Examples of supporting salts include _{lithium salts such as LiPF 6} , LiBF ₄ , LiClO ₄ , lithium bis (fluorosulfonyl) imide (LiFSI), lithium bis (trifluoromethane) sulfonimide (LiTFSI), and among them, LiPF. ₆ and lithium bis (fluorosulfonyl) imide are preferred.
The concentration of the supporting salt in the electrolytic solution is preferably 0.5 mol / L or more and 5 mol / L or less, and more preferably 0.7 mol / L or more and 2.5 mol / L or less.

The electrolytic solution according to the present embodiment further contains, for example, a gas generator such as biphenyl (BP) and cyclohexylbenzene (CHB); a dispersant; a thickener and the like, as long as the effects of the present invention are not significantly impaired. You may be.

The conventional electrolytic solution for a lithium secondary battery using an ionic liquid has a high viscosity, and the mobility of lithium ions cannot be sufficiently improved. Moreover, even if the viscosity could be suppressed low, a sufficient potential window could not be secured. Therefore, a lithium secondary battery provided with an electrolytic solution using a conventional ionic liquid has output characteristics and cycle characteristics equal to or higher than those of a lithium secondary battery provided with a non-aqueous electrolytic solution using a conventional organic solvent (carbonates). Could not be compatible.
On the other hand, in the electrolytic solution for the lithium secondary battery according to the present embodiment, the viscosity of the electrolytic solution is lowered by using an ionic liquid of a combination of a specific cation and a specific anion and setting the amount to a specific ratio. It is possible to secure a sufficient potential window while suppressing it. As a result, it is possible to improve the impregnation property of the polyolefin separator and to improve the conductivity as compared with the non-aqueous electrolytic solution using a conventional organic solvent (carbonates). As a result, the resistance of the lithium secondary battery can be reduced, and a lithium secondary battery having excellent output characteristics can be provided. In addition to this, by containing a specific additive in the electrolytic solution, the cycle characteristics of the lithium secondary battery can be changed to that of the lithium secondary battery provided with a non-aqueous electrolytic solution using a conventional organic solvent (carbonates). It can be increased to the same level or higher.
That is, the electrolytic solution for the lithium secondary battery according to the present embodiment has good impregnation property with the polyolefin separator and has high conductivity even though it uses an ionic liquid. Then, a high output characteristic can be given to the lithium secondary battery, and a high cycle characteristic can be given to the lithium secondary battery.
The electrolytic solution for the lithium secondary battery according to the present embodiment can be used for the lithium secondary battery according to a known method.

Next, an outline of a configuration example of the lithium secondary battery using the electrolytic solution for the lithium secondary battery according to the present embodiment will be described below with reference to the drawings. In the following drawings, members and parts that perform the same action are described with the same reference numerals. Further, the dimensional relations (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relations.

The lithium secondary battery 100 shown in FIG. 1 is a sealed battery constructed by housing a flat wound electrode body 20 and an electrolytic solution 80 in a flat square battery case (that is, an outer container) 30. be. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. There is. Further, the battery case 30 is provided with an injection port (not shown) for injecting the electrolytic solution 80. The positive electrode terminal 42 is electrically connected to the positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to the negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight metal material having good thermal conductivity such as aluminum is used.

In the wound electrode body 20, as shown in FIGS. 1 and 2, a positive electrode active material layer 54 is formed along the longitudinal direction on one side or both sides (here, both sides) of the elongated positive electrode current collector 52. The positive electrode sheet 50 and the negative electrode sheet 60 in which the negative electrode active material layer 64 is formed along the longitudinal direction on one side or both sides (here, both sides) of the long negative electrode current collector 62 are formed in two long shapes. It has a form of being overlapped with each other via a separator sheet 70 and wound in the longitudinal direction. The positive electrode active material layer non-formed portion 52a (that is, the positive electrode activity) formed so as to protrude outward from both ends of the winding electrode body 20 in the winding axis direction (that is, the sheet width direction orthogonal to the longitudinal direction). A portion where the positive electrode current collector 52 is exposed without forming the material layer 54) and a portion 62a where the negative electrode active material layer is not formed (that is, a portion where the negative electrode current collector 62 is exposed without forming the negative electrode active material layer 64). A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are joined to each of the above.

As the positive electrode sheet 50 and the negative electrode sheet 60, the same ones used in the conventional lithium secondary battery can be used without particular limitation. A typical aspect is shown below.

Examples of the positive electrode current collector 52 constituting the positive electrode sheet 50 include aluminum foil and the like. Examples of the positive electrode active material contained in the positive electrode active material layer 54 include lithium transition metal oxides (eg, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O). ₄ , LiNi _0.5 Mn _1.5 O _4, etc.), lithium transition metal phosphate compounds (eg, LiFePO _4, etc.) and the like. The positive electrode active material layer 54 may contain components other than the active material, such as a conductive material and a binder. As the conductive material, for example, carbon black such as acetylene black (AB) or other carbon material (eg, graphite or the like) can be preferably used. As the binder, for example, polyvinylidene fluoride (PVDF) or the like can be used.

Examples of the negative electrode current collector 62 constituting the negative electrode sheet 60 include copper foil and the like. As the negative electrode active material contained in the negative electrode active material layer 64, a carbon material such as graphite, hard carbon, or soft carbon can be used. The negative electrode active material layer 64 may contain components other than the active material, such as a binder and a thickener. As the binder, for example, styrene butadiene rubber (SBR) or the like can be used. As the thickener, for example, carboxymethyl cellulose (CMC) or the like can be used.

As the separator 70, a porous sheet (film) made of polyolefin such as polyethylene (PE) or polypropylene (PP) is preferably used. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat resistant layer (HRL) may be provided on the surface of the separator 70.
The air permeability of the separator 70 obtained by the Garley test method is preferably 350 seconds / 100 cc or less.

As the electrolytic solution 80, the electrolytic solution for the lithium secondary battery according to the present embodiment described above is used. Note that FIG. 1 does not strictly show the amount of the electrolytic solution 80 injected into the battery case 30.

The lithium secondary battery 100 configured as described above can be used for various purposes. Suitable applications include drive power supplies mounted on vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs). The lithium secondary battery 100 may also be used in the form of an assembled battery, which typically consists of a plurality of batteries connected in series and / or in parallel.

As an example, a square lithium secondary battery 100 including a flat wound electrode body 20 has been described. However, the lithium secondary battery can also be configured as a lithium secondary battery including a laminated electrode body. Further, the lithium secondary battery can also be configured as a cylindrical lithium secondary battery, a laminated lithium secondary battery, or the like.

Hereinafter, examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in such examples.

<Preparation of electrolyte>
The ionic liquids and carbonates shown in Table 1 were mixed as solvents in the volume ratios shown in Table 1. The supporting salts shown in Table 1 were dissolved therein at the concentrations shown in Table 1 to obtain the electrolytic solutions of Examples 1 to 25. The conductivity of this electrolyte was measured at 25 ° C. A conductivity meter "Seven Multi" manufactured by METTLER TORDO was used for measuring the conductivity. The results are shown in Table 1.

Ionic liquid: Cationic EMIm: 1-ethyl-3-methylimidazolium BMIm: 1-butyl-3-methylimidazolium PYR13: N-methyl-N-propylpyrrolidinium S222: Tetraethylsulfonium P4441: Tributylmethylphosphonium. Ionic liquid: Anion FSI: Bis (fluorosulfonyl) imide TFSI: Bis (trifluoromethanesulfonyl) imide / supporting salt LiPF ₆ : Lithium hexafluorophosphate LiFSI: Lithium bis (fluorosulfonyl) imide carbonates DMC: Dimethyl carbonate EC : Ethylene carbonate EMC: Ethylmethyl carbonate

In Table 1, the volume ratios of the solvents are listed in the order in which the solvents are described.
From the results in Table 1, it can be seen that the conductivity can be increased by using the ionic liquid and carbonates in combination. It can also be seen that the conductivity varies greatly depending on the type of ionic liquid. In particular, it can be seen that the ionic liquid in which the quaternary ammonium cation and the N (SO ₂ F) ₂ anion are combined has considerably higher conductivity than the conventional electrolytic solution using only carbonates.

<Manufacturing of lithium secondary battery for evaluation>
_{LiNi 1/3} Co _1/3 Mn _1/3 O ₂ (LNCM) as a positive electrode active material powder, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are used as LNCM: A slurry for forming a positive electrode active material layer was prepared by mixing with N-methylpyrrolidone (NMP) at a mass ratio of AB: PVdF = 87: 10: 3. A positive electrode sheet was prepared by applying this slurry to an aluminum foil and drying it.
As the negative electrode active material, a natural graphite-based carbon material (C) having an average particle diameter of 20 μm, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener are used. C: SBR: CMC = A slurry for forming a negative electrode active material layer was prepared by mixing with ion-exchanged water at a mass ratio of 98: 1: 1. A negative electrode sheet was prepared by applying this slurry to a copper foil and drying it.
Further, as a separator, a porous polyolefin membrane having a three-layer structure of PP / PE / PP with an air permeability of 300 seconds / 100 cc obtained by the Garley test method was prepared.
The prepared positive electrode sheet and the negative electrode sheet were opposed to each other via a separator to prepare an electrode body.
A current collector was attached to the prepared electrode body, and the current collector was housed in a laminate case together with the electrolytic solution and sealed. As the electrolytic solution, those shown in Table 2 were used. Specifically, the ionic liquids and carbonates shown in Table 2 are mixed at the volume ratios shown in Table 2, and the supporting salts shown in Table 2 are dissolved therein at the concentrations shown in Table 2, and further, Table 2 is added. An electrolytic solution prepared by dissolving the additives described in Table 2 at the concentrations shown in Table 2 was used.
In this way, evaluation lithium secondary batteries A1 to A4 and B1 to B9 were produced.

<Activation and initial characterization>
Each of the prepared lithium secondary batteries for evaluation was placed in a constant temperature bath at 25 ° C. and charged for the first time. For the first charge, each evaluation lithium secondary battery was constantly charged to 4.10 V with a current value of 0.3 C. Then, a constant current was discharged to 3.0 V with a current value of 0.3 C.
Further, each evaluation lithium secondary battery is subjected to constant current-constant voltage charging (constant current charging to 4.10 V with a current value of 0.2 C, and then constant voltage charging until the current value becomes 1/50 C). , Fully charged. Then, a constant current was discharged to 3.0 V with a current value of 0.2 C. The discharge capacity at this time was measured and used as the initial capacity.
Further, with the above initial capacity as SOC 100%, each evaluation lithium secondary battery was charged with a current value of 0.3 C in a constant temperature bath at 25 ° C. until the charging depth (SOC) reached 30%. Next, the battery was discharged at current values of 5C, 15C, 30C, and 45C for 10 seconds in a constant temperature bath at 25 ° C., and the battery voltage after discharging at each current value was measured. Each current value and each battery voltage were plotted to obtain the IV characteristic at the time of discharge, and the IV resistance (Ω) at the time of discharge was obtained as the initial resistance from the slope of the obtained straight line. The ratio of the initial resistance of the other evaluation lithium secondary batteries to the evaluation lithium secondary battery B1 when the initial resistance of the evaluation lithium secondary battery B1 was 1.00 was determined. The results are shown in Table 2.

<High temperature cycle characterization>
Subsequently, each evaluation lithium secondary battery was placed in a constant temperature bath at 60 ° C. For each evaluation lithium secondary battery, charging / discharging with constant current charging at 2C up to 4.1V and constant current discharging at 2C up to 3.0V was repeated for 200 cycles. After that, the discharge capacity was measured by the same method as described above, and the discharge capacity at this time was determined as the battery capacity after 200 cycles of charging and discharging. The capacity retention rate (%) was determined as (battery capacity / initial capacity after 200 cycles of charging / discharging) × 100. The results are shown in Table 2.

PYR14: N-Butyl-N-Methylpyrrolidinium LiBOB: Lithium bis (oxalate) borate LiPO ₂ F ₂ : Lithium difluorophosphate (Others are the same as in Table 1).

In the lithium secondary battery B1, an electrolytic solution using a conventionally common carbonate (that is, a mixed solvent of EC, DMC, and EMC) as a solvent was used. Further, in the lithium secondary battery B2, an electrolytic solution using DMC as a solvent and LiFSI as a supporting salt was used.
From a comparison with this, the lithium secondary batteries A1 to A4 using the electrolytic solution corresponding to the electrolytic solution for the lithium secondary battery according to the present embodiment have an initial resistance as compared with those using the conventional electrolytic solution. It was equal to or less than the same, and the cycle characteristics were equal to or higher than those using the conventional electrolytic solution. Further, in the lithium secondary batteries A1 to A4, the impregnation property of the electrolytic solution with respect to the separator was good.
On the other hand, the lithium secondary battery B3 containing no additive had low cycle characteristics.
In the lithium secondary battery B4 using bis (trifluoromethanesulfonyl) imide as an anion of the ionic liquid, the initial resistance was slightly high and the cycle characteristics were slightly low.
In the lithium secondary battery B5 using an aromatic cyclic quaternary ammonium organic cation as the cation of the ionic liquid, the initial resistance was high and the cycle characteristics were low.
The lithium secondary battery B6, which uses bis (trifluoromethanesulfonyl) imide as the anion of the ionic liquid and an aromatic cyclic quaternary ammonium organic cation as the cation of the ionic liquid, has a higher initial resistance and further cycle characteristics. It was low.
In the lithium secondary battery B7 using a sulfonium cation as the cation of the ionic liquid, the initial resistance was high and the cycle characteristics were low.
In the lithium secondary battery B8, which uses only an ionic liquid without using carbonates as a solvent and does not contain additives, the impregnation property of the electrolytic solution into the separator is poor, and the characteristics of the battery cannot be evaluated.
Even in the lithium secondary battery B9 using 30% by volume of an ionic liquid and containing no additive, the impregnation property of the electrolytic solution into the separator was poor, and the characteristics of the battery could not be evaluated.

Summarizing the results of Tables 1 and 2, the electrolytic solution for the lithium secondary battery according to the present embodiment has good impregnation property with respect to the polyolefin separator while using an ionic liquid, and has high conductivity. Moreover, it can be seen that it gives a high cycle characteristic to the lithium secondary battery.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of claims. The techniques described in the claims include various modifications and modifications of the specific examples illustrated above.

20 Winding electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector plate 44 Negative electrode terminal 44a Negative electrode current collector plate 50 Positive electrode sheet (positive electrode)
52 Positive electrode current collector 52a Positive electrode active material layer non-formed portion 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-formed portion 64 Negative electrode active material layer 70 Separator sheet (separator)
80 Electrolyte 100 Lithium secondary battery

Claims (1)

A saturated aliphatic cyclic quaternary ammonium organic cation, and an ionic liquid composed of the N (SO _{2 F)} 2 anions,
With carbonates
At least one additive selected from the group consisting of a light metal salt represented by the following formula (I) and a phosphorus compound represented by the following formula (II).
Lithium bis (fluorosulfonyl) imide and
Contains,
Of the ionic liquid, a percentage of the total of the ionic liquid and the carbonates are state, and are less than 5 vol% to 30 vol%,
The carbonates are dimethyl carbonates.
Electrolyte for lithium secondary batteries.

(In formula (I), R11 is a -C (= O) -R21-C (= O) -group (R21 represents an alkylene group, a halogenated alkylene group, an arylene group, or a halogenated arylene group). Or -C (= O) -C (= O) -group, R12 represents a halogen atom, an alkyl group, an alkyl halide group, an aryl group, or an aryl halide group, and X11 and X12 are oxygen, respectively. Represents an atom or sulfur atom, M11 represents a transition metal element or a Group 13 element, a Group 14 element, or a Group 15 element in the periodic table, and M21 represents a Group 1 element, a Group 2 element, or Al in the periodic table. , A is an integer of 1 to 4, b is an integer of 0 to 8, and c, d, e, and f are integers of 1 to 3, respectively.
In formula (II), M ^{n +} represents a Group 1 element ion, a Group 2 element ion, an Al ion, or an onium ion in the periodic table, A1 and A2 represent an oxygen atom or a sulfur atom, respectively, and X1 and X2 are. Each represents a halogen atom, a saturated or unsaturated hydrocarbon group having 1 to 6 carbon atoms, or a saturated or unsaturated hydrocarbon group having 1 to 6 carbon atoms having a hetero atom, or is bonded to each other. It forms a ring structure and n is a valence. )

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