JP2020136161A - Electrolyte solution for secondary battery - Google Patents

Abstract

To provide a nonaqueous electrolyte solution arranged for a secondary battery with an imidazolium salt added thereto, which enables the reduction in the resistance of a secondary battery and enables the enhancement in the durability under a high-temperature condition.SOLUTION: An electrolyte solution for a secondary battery, disclosed herein comprises an imidazolium salt as an additive agent. The imidazolium salt is a salt of a dialkyl imidazolium cation, and a phosphoric acid ester anion or phosphonic ester anion. In the nonaqueous electrolyte solution, the content of the imidazolium salt is over 0 mass% and 1.5 mass% or less.SELECTED DRAWING: None

Description

The present invention relates to a non-aqueous electrolyte solution for a secondary battery.

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

With the widespread use of secondary batteries such as lithium secondary batteries, further improvement in performance is desired. One of the measures to improve the performance of the secondary battery is to improve the non-aqueous electrolyte solution with additives. As a specific example, it is known that an imidazolium salt is used as an additive for a non-aqueous electrolytic solution.

For example, in Patent Document 1, a salt of an imidazolium cation substituted with an organic group having a polymerizable unsaturated bond and a polyoxyalkylene group and a halogen anion is used as an additive for a non-aqueous electrolytic solution. It is described that the output of the secondary battery can be increased and the deterioration of the charge / discharge cycle characteristics can be suppressed.
Further, in Patent Document 2, by using a salt of an imidazolium cation substituted with an alkyl group, an aryl group or the like and an anion such as a carboxylic acid as an additive for a non-aqueous electrolytic solution, the conductivity and thermal stability are obtained. It is stated that the sex etc. will be higher.

Japanese Unexamined Patent Publication No. 2010-118337 Japanese Unexamined Patent Publication No. 8-32149

However, as a result of diligent studies by the present inventors, in response to the increasing demand for higher performance of the secondary battery, the conventional non-aqueous electrolyte solution for the secondary battery containing the imidazolium salt as an additive has an initial resistance and a high temperature. It was found that the durability underneath was insufficient.

Therefore, the present invention is a non-aqueous electrolytic solution for a secondary battery to which an imidazolium salt is added, which can reduce the resistance of the secondary battery and improve the durability at a high temperature. It is an object of the present invention to provide a water electrolyte.

The non-aqueous electrolytic solution for a secondary battery disclosed herein contains an imidazolium salt as an additive. The imidazolium salt is a salt of a dialkyl imidazolium cation and a phosphate ester anion or a phosphonate ester anion. The content of the imidazolium salt in the non-aqueous electrolytic solution is more than 0% by mass and 1.5% by mass or less.
By using a non-aqueous electrolyte solution having such a configuration in a secondary battery, the resistance of the secondary battery can be reduced, and durability at high temperatures (specifically, capacity deterioration resistance and resistance increase suppression performance) can be improved. Can be improved.

It is sectional drawing which shows typically the internal structure of the lithium secondary battery using the non-aqueous electrolytic solution 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 using the non-aqueous electrolytic solution 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 a non-aqueous electrolyte solution for a secondary battery which does not characterize the present invention). Process) can be grasped as a design matter of a person skilled in the art based on the prior art in the field. 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 repeatedly charging and discharging, and is a term including a so-called storage battery and a power storage element such as an electric double layer capacitor.

The non-aqueous electrolytic solution for a secondary battery according to the present embodiment contains an imidazolium salt as an additive. The imidazolium salt is a salt of a dialkyl imidazolium cation and a phosphate ester anion or a phosphonate ester anion. The content of the imidazolium salt in the non-aqueous electrolytic solution is more than 0% by mass and 1.5% by mass or less.

It is difficult to achieve both an improvement in initial resistance (lower resistance) and durability, which means that the characteristics do not change significantly even after repeated charging and discharging at high temperatures, because the factors are different.
However, as a result of diligent studies by the present inventors, as the imidazolium salt used as an additive, a salt obtained by combining a dialkyl imidazolium cation and a phosphate ester anion or a phosphonate ester anion is used in a specific concentration range. By doing so, it has been found that the resistance of the secondary battery can be reduced and the durability at high temperatures can be improved.

When the present inventors actually produced and analyzed a secondary battery using the non-aqueous electrolytic solution for the secondary battery according to the present embodiment, it was confirmed that a film was formed on the electrode. From this, when the non-aqueous solvent of the non-aqueous electrolyte solution and the dialkylimidazolium cation are reduced and decomposed to form a film on the electrode, PO ₄ derived from the phosphate ester anion or the phosphonate ester anion is formed in the film. It is considered that ^3- etc. Are incorporated to form a mixed film. Then, this hybrid coating has a large effect of reducing the initial resistance and an effect of improving the durability at a high temperature (an effect of suppressing capacity deterioration and an effect of suppressing an increase in resistance when charging and discharging are repeated at a high temperature). It is thought that it will be done.

The dialkyl imidazolium cation preferably has two N atoms of the imidazolium cation substituted with an alkyl group, respectively.
Since a higher initial resistance reducing effect can be obtained, the alkyl group preferably has 1 to 6 carbon atoms, more preferably 1 to 4 carbon atoms, and even more preferably 1 or 2 carbon atoms.
The alkyl group may be linear or branched.
Specific examples of the alkyl group include methyl group, ethyl group, propyl group, isopropyl group, butyl group, iso-butyl group, tert-butyl group, pentyl group, 2-methylpentyl group, 3-methylpentyl group and hexyl. Group etc. can be mentioned.
The two alkyl groups that replace the two N atoms of the imidazolium cation may be the same or different.
As the dialkylimidazolium cation, 1-ethyl-3-methylimidazolium cation, 1,3-dimethylimidazolium cation, and 1-butyl-3-methylimidazolium cation are preferable.

As the phosphoric acid ester anion, a monoanion of a phosphoric acid dialkyl ester is preferable.
As the phosphonate ester anion, a phosphonate alkyl ester anion is preferable.
The number of carbon atoms of the alkyl group of these alkyl esters is preferably 1 to 6, more preferably 1 to 4, and even more preferably 1 or 2, because a higher initial resistance reducing effect can be obtained.
The alkyl group may be linear or branched.
Specific examples of the alkyl group include the same as the specific example of the alkyl group of the dialkylimidazolium cation. The two alkyl groups of the monoanion of the phosphoric acid dialkyl ester may be the same or different. ..
As the phosphate ester anion and the phosphonate ester anion, a dimethyl phosphate anion, a diethyl phosphate anion, a dibutyl phosphate anion, and a methyl phosphonate (methyl phosphonate) anion are preferable.

In the present embodiment, the imidazolium salt of the specific anion and the specific cation can be used alone or in combination of two or more.
Particularly preferably, the imidazolium salt is 1-ethyl-3-methylimidazolium dimethyl phosphate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1,3-dimethylimidazolium dimethyl phosphate, 1-butyl-3-methyl. It is at least one selected from the group consisting of imidazolium dibutyl phosphate and 1-ethyl-3-methylimidazolium methylphosphonate.

As described above, the content of the imidazolium salt in the non-aqueous electrolytic solution is more than 0% by mass and 1.5% by mass or less. If the content exceeds 1.5% by mass, it becomes difficult to sufficiently obtain the effects of the present invention (particularly, durability at high temperatures). The content is preferably 0.3% by mass or more and 1.2% by mass or less, and more preferably 0.5% by mass or more and 1.0% by mass or less.

The non-aqueous electrolyte solution for a secondary battery according to the present embodiment usually contains a non-aqueous solvent and an electrolyte salt.
As the non-aqueous solvent, a known non-aqueous solvent used in the non-aqueous electrolyte solution for a secondary battery can be used without particular limitation, and specifically, for example, carbonates, ethers, esters, nitriles, etc. Sulfones, lactones and the like can be used. Of these, carbonates are preferable. Examples of carbonates include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like. These can be used alone or in combination of two or more.

As the electrolyte salt, a known electrolyte salt used in the non-aqueous electrolyte solution for a secondary battery can be used without particular limitation, and specifically, for example, a lithium salt (particularly, a lithium salt containing a fluorine atom) is used. be able to. Examples of lithium salts include LiPF ₆ , LiBF ₄ , lithium bis (fluorosulfonyl) imide (LiFSI), lithium bis (trifluoromethane) sulfonimide (LiTFSI) and the like. These can be used alone or in combination of two or more. Since the resistance reducing effect is particularly high, the electrolyte salt preferably contains at least LiPF ₆ , and more preferably contains LiPF ₆ and LiFSI.

The concentration of the electrolyte salt is not particularly limited as long as the effects of the present invention can be obtained. From the viewpoint of appropriately exerting the function as an electrolyte salt, the concentration of the electrolyte salt in the non-aqueous electrolyte solution is preferably 0.5 mol / L or more and 3 mol / L or less, and more preferably 0.8 mol / L or more 1 It is 0.6 mol / L or less.

The non-aqueous electrolyte solution for a secondary battery according to the present embodiment is, for example, a gas generating agent such as biphenyl (BP) or cyclohexylbenzene (CHB); a film forming agent; as long as the effects of the present invention are not significantly impaired. Dispersant; may contain various additives such as thickeners.

The non-aqueous electrolyte solution for a secondary battery according to the present embodiment can be used for a secondary battery according to a known method. By using the non-aqueous electrolyte solution for a secondary battery according to the present embodiment for a secondary battery, the resistance of the secondary battery can be reduced and the durability under high temperature (specifically, capacity deterioration resistance and resistance increase). Suppression performance) can be improved. A lithium secondary battery is preferable as the secondary battery.

Therefore, the configuration of the lithium secondary battery including the non-aqueous electrolytic solution for the secondary battery according to the present embodiment will be described below with reference to the drawings. However, the configuration of the lithium secondary battery is not limited to the examples described below. In the following drawings, members / parts that perform the same action are described with the same reference numerals. Moreover, 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 closed type constructed by housing a flat wound electrode body 20 and a non-aqueous electrolytic solution 80 in a flat square battery case (that is, an outer container) 30. It is a battery. 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 non-aqueous 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 active portion) 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 other.

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 an 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.

Examples of the separator 70 include a porous sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. 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.

As the non-aqueous electrolyte solution 80, the non-aqueous electrolyte solution for a secondary battery according to the present embodiment described above is used. Note that FIG. 1 does not strictly show the amount of the non-aqueous 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. Further, according to a known method, it can be configured as a non-aqueous electrolyte secondary battery other than the lithium secondary battery.

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 non-aqueous electrolyte solution>
As a non-aqueous solvent, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 was prepared. Lithium salt (LiPF ₆ ) was added to this mixed solvent so as to have a concentration of 1.0 mol / L, and the imidazolium salt shown in Table 1 was added so as to have the content shown in Table 1. Non-aqueous electrolyte solutions of each Example and each Comparative Example were prepared.

<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. This slurry was applied to an aluminum foil, dried, and then roll-pressed until the density of the positive electrode active material layer reached 2.3 g / cm ³ , to prepare a positive electrode sheet.
As the negative electrode active material, a natural graphite-based material having an average particle diameter (D50) of 10 μm, a specific surface area of 4.8 m ² / g, C ₀ = 0.67 nm, and L _c = 27 nm was prepared. This natural graphite-based material (C), styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener are ionized at a mass ratio of C: SBR: CMC = 98: 1: 1. A slurry for forming a negative electrode active material layer was prepared by mixing with exchanged water. This slurry was applied to a copper foil, dried, and then roll-pressed to prepare a negative electrode sheet.
Further, as a separator sheet, a porous polyolefin sheet having a three-layer structure of PP / PE / PP was prepared.
The prepared positive electrode sheet and the negative electrode sheet were opposed to each other via a separator sheet to prepare an electrode body.
A current collector was attached to the prepared electrode body, and this was housed in a laminate case together with the non-aqueous electrolytic solution of each Example and each Comparative Example. By sealing the laminated case, a lithium secondary battery for evaluation was obtained.

<Conditioning>
Each lithium secondary battery produced above was placed in an environment of 25 ° C. Each lithium secondary battery is constantly charged to 4.1 V with a current value of 1 / 3C, then paused for 10 minutes, then discharged to 3.0 V with a current value of 1/3 C, and then discharged for 10 minutes. By resting, conditioning treatment was applied.

<Initial battery resistance measurement>
Each conditioned lithium secondary battery was adjusted to SOC 60%. This was placed in a temperature environment of 25 ° C. and charged for 10 seconds. The charging current rates were 1C, 3C, 5C, and 10C, and the voltage after charging at each current rate was measured. The IV resistance was calculated from the current rate and the amount of voltage change, and the average value was taken as the initial battery resistance. The ratio of the initial resistances of the other batteries when the initial resistance of the lithium secondary battery of Comparative Example 1 was set to "1.00" was calculated. The results are shown in Table 1.

<High temperature cycle characterization>
Each conditioned lithium secondary battery was placed in an environment of 25 ° C. This was charged with constant current-constant voltage (cut current: 1 / 50C) to 4.1V with a current value of 1 / 5C, paused for 10 minutes, and then discharged to 3.0V with a current value of 1 / 5C. .. The discharge capacity at this time was measured and used as the initial capacity.
Subsequently, each lithium secondary battery was placed in an environment of 70 ° C. Charging and discharging with constant current charging at 2C up to 4.1V and constant current discharging at 2C up to 3.0V were 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. As an index of capacity deterioration, the capacity ratio represented by (battery capacity after 200 cycles of charging / discharging / initial capacity) was obtained. The results are shown in Table 1. The larger the capacity ratio, the better the capacity deterioration resistance.
Further, the battery resistance was measured by the same method as described above, and the battery resistance at this time was determined as the battery resistance after 200 cycles of charging and discharging. As an index of the increase in resistance, the resistance ratio represented by (battery resistance / initial resistance after 200 cycles of charging / discharging) was obtained. The results are shown in Table 1. The smaller the resistance ratio, the better the resistance increase suppressing performance.

(The abbreviations for the types of imidazolium salts in the table are as follows.)
- imidazolium salt cationic species EMI of: 1-ethyl-3-methylimidazolium DMI: 1,3-dimethyl-imidazolium BMI: anionic species of 1-butyl-3-methylimidazolium imidazolium salt PF6 @: PF ₆ ^-
TFSI: Bis (trifluoromethanesulfonyl) imide anion

The comparative example 1 without addition of imidazolium salts, cationic species is 1-ethyl-3-methylimidazolium, anionic species PF ₆ ^- Comparative Example 2, and cations added imidazolium salt is In Comparative Example 3 in which the species was 1-ethyl-3-methylimidazolium and the anion species was bis (trifluoromethanesulfonyl) imide anion, no improvement in initial resistance and high temperature cycle characteristics was observed. Rather, the initial resistance and high temperature cycle characteristics deteriorated.
On the other hand, in Example 1 in which an imidazolium salt in which the cation species was 1-ethyl-3-methylimidazolium and the anion species was a phosphate ester was added, improvement in initial resistance and high-temperature cycle characteristics was observed. .. Similarly, improvements in initial resistance and high-temperature cycle characteristics were observed in Examples 5 to 7 in which the carbon number of the alkyl group of the cation species and the carbon number of the anion species were changed. Similarly, in Example 8 in which the anion species was changed to a phosphonate ester, improvement in initial resistance and high temperature cycle characteristics was observed.
Therefore, by adding an imidazolium salt of a combination of a specific cation species and a specific anion species (hereinafter referred to as "specific imidazolium salt"" to the non-aqueous electrolytic solution, the initial resistance improving effect (initial resistance reduction). It was confirmed that the effect) and the effect of improving the high temperature cycle characteristics (the effect of improving the durability at high temperature) can be obtained.
This, PO ₄ ^3- and the like derived from phosphoric acid ester anion or phosphonate ester anions are incorporated in the coating to be formed on the electrode, as a result, the hybrid film is formed by the hybrid coating, the initial resistance reducing It is considered that the effect and the effect of improving the high temperature cycle characteristics can be obtained.

From the comparison of Examples 1 to 4, the effect when the addition amount of a specific imidazolium salt is changed can be confirmed. In Examples 2 and 3 in which the amount of the specific imidazolium salt added was increased to 1.0% by mass and 1.5% by mass, the effect of improving the initial resistance and the high temperature cycle characteristics was also observed. In Example 4 in which the amount of the specific imidazolium salt added was reduced to 0.1% by mass, the effect of reducing the initial resistance was slightly smaller, but the initial resistance and high temperature were still higher than those in Comparative Examples 1 to 3. The cycle characteristics were greatly improved.
From this, it was confirmed that the effect of improving the initial resistance and the high temperature cycle characteristics can be obtained when the amount of the specific imidazolium salt added is in the range of 1.5% by mass or less.

From the above, it can be seen that the resistance of the secondary battery can be reduced by the non-aqueous electrolyte solution for the secondary battery according to the present embodiment described above. It is also found that the high temperature cycle characteristics can be improved.

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 Non-aqueous electrolyte 100 Lithium secondary battery

Claims (1)

A non-aqueous electrolyte solution for a secondary battery containing an imidazolium salt as an additive.
The imidazolium salt is a salt of a dialkyl imidazolium cation and a phosphate ester anion or a phosphonate ester anion.
The content of the imidazolium salt in the non-aqueous electrolytic solution is more than 0% by mass and 1.5% by mass or less.
Non-aqueous electrolyte for secondary batteries.

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