JP5287352B2 - Electrolyte for lithium ion battery - Google Patents

Description

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

A conventional lithium ion battery uses lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), a solid solution thereof, lithium manganate (LiMn ₂ O ₄ ) or the like as a positive electrode, and consists of carbon such as graphite as a negative electrode. A negative electrode material is used. An electrolytic solution in which a liquid organic compound such as ethylene carbonate or propylene carbonate is dissolved in a solvent and a lithium salt as a solute is used.

In order to further increase the energy density of such a lithium ion battery, a search for a new positive electrode active material is underway. For example, _Li 2 NiPO ₄ F in Patent Documents 1 and _2, LiNiPO _4, LiCoPO 4 and _Li 2 CoPO ₄ F has been proposed as a high positive electrode active material energy density. If these positive electrode active materials having a large energy density are used in a lithium ion battery, theoretically, a lithium ion battery having a large charge capacity should be obtained.

  However, since the charging reaction of such a positive electrode active material occurs at an extremely high potential, the conventional lithium ion battery electrolyte using an organic solvent such as cyclic carbonate or chain carbonate is used after the solvent is oxidized and decomposed. There was a problem that it was impossible. For this reason, there is a problem that the capacity that can be actually taken out is less than half of the theoretical capacity (Non-Patent Document 1).

Patent No. 3624205 Patent No. 3631202

Journal of Power Sources 146 (2005) 565-569

The inventor conducted intensive studies to solve the above-described conventional problems, and found that an organic solvent having a nitrile group hardly decomposes even at a high positive potential and has a wide potential window. And the electrolyte solution for lithium ion batteries in which 90 weight% or more of nitrile compounds are contained with respect to the weight of the organic solvent was developed, and the patent application has already been filed (Japanese Patent Application No. 2007-333829). According to this electrolyte solution for lithium ion batteries, because it has a wide potential window, particularly at high positive _{potential, Li 2 CoPO 4 F, Li} 2 NiPO 4 F, a high positive electrode material having the redox potential such LiCoPO 4, LiNiPO ₄ Can be used. For this reason, it can be set as a battery with a large electromotive force.

  However, since this lithium ion battery electrolyte contains a nitrile compound having a high viscosity as the main solvent component, the specific conductivity is small and it is difficult to extract a large current. For this reason, there has been a demand for an electrolyte solution for lithium ion batteries that not only has a wide potential window but also has a low viscosity and a high specific conductivity.

  The present invention has been made in view of the above-described conventional situation, and a positive electrode active material that has a low viscosity, a high specific conductivity, hardly decomposes even at a high potential, and reaches a positive potential region where charge and discharge are high. Providing an electrolyte for a lithium ion battery that can be used as a substance is an issue to be solved.

  In order to solve the above-described conventional problems, the present inventor has further conducted earnest test research on an electrolytic solution for a lithium ion battery using an organic solvent having a nitrile group. As a result, even when various nitrile compounds are mixed with at least one of cyclic carbonate, cyclic ester, and chain carbonate, the potential window is kept wide, and the lithium ion battery has low viscosity and high specific conductivity. The surprising fact that it can be used as an electrolytic solution has been found and the present invention has been completed.

  That is, the electrolyte for a lithium ion battery of the present invention is an electrolyte for a lithium ion battery in which a lithium salt is dissolved in an organic solvent, and the organic solvent includes a nitrile at both ends of the chain saturated hydrocarbon compound. A chain saturated hydrocarbon dinitrile compound having a group bonded thereto, a chain ether nitrile compound having a nitrile group bonded to at least one terminal of the chain ether compound, and at least one nitrile compound of a cyanoacetate, a cyclic carbonate, a cyclic It contains at least one of an ester and a chain carbonate.

  In the electrolyte solution for a lithium ion battery of the present invention, a chain saturated hydrocarbon dinitrile compound in which a nitrile group is bonded to both ends of a chain saturated hydrocarbon compound as an organic solvent, a nitrile group at least at one end of the chain ether compound. Is mixed with at least one of cyclic carbonate, cyclic ester and chain carbonate in a high viscosity nitrile compound called chain ether nitrile compound and cyanoacetate bonded to each other, resulting in low viscosity and specific conductivity. growing.

The reason why the electrolyte solution for lithium ion batteries of the present invention has a wide potential window even though it contains at least one of cyclic carbonate, cyclic ester and chain carbonate, which does not have a wide potential window. Although it is not necessarily clear, it is considered as follows. That is, among the organic solvents used in the electrolyte solution for lithium ion batteries of the present invention, the nitrile compound serves to widen the potential window. In addition, chain carbonate is presumed to play a role of increasing specific conductivity in order to lower the viscosity. Cyclic carbonates and cyclic esters dissolve many lithium salts and form a protective film called SEI on the carbon negative electrode, thereby improving the reduction resistance and allowing the passage of Li ions. Can be granted. Therefore, it is considered that it is possible to exert an effect on the enlargement of the negative and positive potential windows.
From the above, it is preferable to use a chain carbonate and a cyclic carbonate and / or a cyclic ester in combination. More preferably, it is a combined use of a chain carbonate and a cyclic carbonate. Specifically, dimethyl carbonate and ethylene carbonate are used in combination. The blending ratio of both is not particularly limited, but the same amount is preferable.

Examples of the chain saturated hydrocarbon dinitrile compound in which nitrile groups are bonded to both ends of the chain saturated hydrocarbon compound include succinonitrile NC (CH ₂ ) ₂ CN, glutaronitrile NC (CH ₂ ) ₃ CN, adiponitrile In addition to linear dinitrile compounds such as NC (CH ₂ ) ₄ CN, sebaconitrile NC (CH ₂ ) ₈ CN, dodecanedinitrile NC (CH ₂ ) ₁₀ CN, 2-methylglutaronitrile NCCH (CH ₃ ) _CH 2 _CH 2 CN may have a branched as such. These chain saturated hydrocarbon dinitrile compounds are not particularly limited in carbon number, but are preferably 7 to 20. More preferably, it is 10-12.

Examples of the chain ether nitrile compound in which a nitrile group is bonded to at least one end of the chain ether compound include oxydipropionitrile NCCH ₂ CH ₂ —O—CH ₂ CH ₂ CN and 3-methoxypropionitrile. _{CH 3 -O-CH 2 CH 2} CN , and the like. These chain ether nitrile compounds are not particularly limited in carbon number, but are preferably 20 or less.

  Furthermore, examples of the cyanoacetate include methyl cyanoacetate, ethyl cyanoacetate, propyl cyanoacetate, and butyl cyanoacetate. These cyanoacetic acid esters are not particularly limited in carbon number, but are preferably 20 or less.

Examples of the lithium salt added to the electrolyte for the lithium ion battery of the present invention include LiPF ₆ (lithium hexafluorophosphate), LiBF ₄ (lithium tetrafluoroborate), LiTFSI (lithium bis (trifluoromethanesulfonyl) imide. ), LiTFS (lithium trifluoromethanesulfonate), LiBETI (lithium bis (pentafluoroethanesulfonyl) imide), and the like can be used. These lithium salts dissolve in the nitrile organic solvent used in the electrolyte for lithium ion batteries of the present invention and have a sufficient potential window that does not decompose even at high potentials. Among these, it is preferable that at least one of LiPF ₆ (lithium hexafluorophosphate), LiBF ₄ (lithium tetrafluoroborate) and LiTFSI (lithium bis (trifluoromethanesulfonyl) imide) is contained. LiPF ₆ (lithium hexafluorophosphate) and LiBF ₄ (lithium tetrafluoroborate) are less corrosive to aluminum, which is often used as an electrode substrate for a positive electrode of a lithium ion battery. Furthermore, they are also more effective in expanding the potential window than LiTFS (lithium trifluoromethanesulfonate) and LiBETI (lithium bis (pentafluoroethanesulfonyl) imide). LiTFSI (lithium bis (trifluoromethanesulfonyl) imide) has a particularly high decomposition temperature, improved heat resistance, high solubility, and high specific conductivity.

  The concentration of the lithium salt is 0.01 mol / L or more, and is preferably lower than the saturated state. When the concentration of the lithium salt is less than 0.01 mol / L, since the number of dissociated Li ions is small, the Li ion conductivity becomes extremely small and Li ion conduction cannot be ensured. Therefore, there is a possibility that the overvoltage is increased and the potential of the original electrolyte is greatly shifted. On the other hand, when the concentration of the lithium salt is saturated, there is a possibility that the dissolved lithium salt precipitates due to a change in temperature and deforms the electrode or the like.

  Moreover, it is preferable that the density | concentration of the nitrile compound contained in the electrolyte solution for lithium ion batteries of this invention is 1 volume% or more and less than 100 volume%. When the concentration of the nitrile compound is less than 1% by volume, the effect of expanding the potential window is reduced. More preferably, it is 5 volume% or more and less than 90 volume%. When the concentration of the nitrile compound is 90% by volume or more, the solubility of the electrolyte is lowered and the viscosity is also increased. Therefore, the conductivity is lowered, and the internal resistance of the battery is increased. Most preferred is 30% by volume or more and less than 70% by volume.

When the lithium ion battery electrolyte of the present invention is used as an electrolyte of a lithium ion battery, a high potential redox positive electrode in which the potential for charging is present in a region exceeding 5.2 V (vs. Li / Li ⁺ ). An active material can be used. For this reason, it can be set as a battery with a large electromotive force and high energy density.

By using the lithium ion battery electrolyte of the present invention, a high potential redox positive electrode active material that exists in a region where the potential for charging exceeds 5.2 V (vs. Li / Li ⁺ ) is used as the positive electrode active material. Therefore, the electromotive force and energy density of the battery can be extremely increased. Such high potential redox positive electrode active material, for _{example, Li 2 CoPO 4 F, Li} 2 NiPO 4 F, LiCoPO 4, LiNiPO 4 , and the like. These positive electrode active materials have high energy density and can be a lithium ion battery having a large capacity. For example, Li ₂ CoPO ₄ F is predicted to have a theoretical energy density that is at least twice that of LiCoO ₂ as a positive electrode active material. If the potential density can be sufficiently exhibited, a lithium-ion battery having a large capacity can be produced. be able to. In addition, since the potential at which Li ₂ CoPO ₄ F is oxidized extends to a high potential region, a battery with high electromotive force can be obtained. Furthermore, Li ₂ CoPO ₄ F is excellent in thermal stability, and it is known from the thermal analysis results that it does not show an exothermic reaction even at a high temperature of 400 ° C., and it is possible to prevent the battery temperature from rising.

  Examples in which the lithium ion battery electrolyte of the present invention is embodied will be described in more detail below.

Example 1
In Example 1, a solvent in which adiponitrile, ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 50:25:25 was used as the organic solvent, and LiPF ₆ ( Lithium hexafluorophosphate) was dissolved at 0.05 mol / L to obtain an electrolyte for a lithium ion battery.

(Comparative Example 1)
In Comparative Example 1, a mixed solvent of 50% by volume of ethylene carbonate and 50% by volume of dimethyl carbonate was used as the organic solvent, and LiPF ₆ was dissolved as a lithium salt at a concentration of 1 mol / L. did.

(Examples 2-9)
In Examples 2 to 9, the solvent was various nitrile: ethylene carbonate: dimethyl carbonate = 50: 25: 25 (volume ratio), and the electrolyte was LiPF ₆ (lithium hexafluorophosphate) at 1 mol / L in this mixed solvent. The so-dissolved solution was used as an electrolyte solution for a lithium ion battery (however, in Example 6 in which the nitrile compound was oxydipropionitrile, LiPF ₆ was 0.5 mol / L).
The types of nitriles used in each example are as follows.
Example 1 Adiponitrile NC (CH ₂ ) ₄ CN
Example 2 Succinonitrile NC (CH ₂ ) ₂ CN
Example 3 Sevacononitrile NC (CH ₂ ) ₈ CN
Example 4 Dodecanedinitrile NC (CH ₂ ) ₁₀ CN
Example 5 2-Methylglutaronitrile NCCH (CH ₃ ) CH ₂ CH ₂ CN
Example 6-oxy dipropionate nitrile _{NCCH 2 CH 2 -O-CH 2} CH 2 CN
EXAMPLE 7 3-methoxy propionitrile _{CH 3 -O-CH 2 CH 2} CN
Example 8 Methyl cyanoacetate NCCH ₂ COOCH ₃
Example 9 cyanoacetate butyl _{NCCH 2 COO (CH 2) 3} CH 3

-Evaluation-
(Measurement of potential-current curve)
With respect to the electrolyte solutions for lithium ion batteries of Examples 1 to 9 and Comparative Example 1 prepared as described above, potential-current curves were measured. A potentiogalvanostat was used for measurement, glassy carbon was used for the working electrode, and a platinum wire was used for the counter electrode. As the reference electrode, (Ag / Ag +) or (Li / Li +) was used. In the measurement, the positive side and the negative side were scanned several times, and then the potential was swept from the natural potential in the positive direction or the negative direction at a rate of 5 mV / sec, and the potential-current curve was measured. The results are shown in FIG. 1, FIG. 2 and FIG.

As a result, as shown in FIG. 1, the potential window of the electrolyte solution of Example 1 was 6.9 V with respect to the Li potential (Li / Li ⁺ ) (the judgment criterion of the potential window was 50 μA / cm ^2, and so on). It became. On the other hand, the potential window of Comparative Example 1 using a mixed solvent of ethylene carbonate and dimethyl carbonate is 5.2 V as shown in FIG. 3, and the potential window of the electrolytic solution of Example 1 is Comparative Example 1. Compared to the electrolyte solution of, it was found that it spreads greatly on the positive side. From this result, when the electrolytic solution of Example 1 is used, a high potential redox positive electrode active material that exists in a region where the potential for charging exceeds 5.2 V can be used as the positive electrode active material of the lithium ion battery. Thus, a lithium ion battery having high electromotive force and energy density and large capacity can be obtained. For example, in the electrolytic solution of Comparative Example 1, the organic solvent undergoes electrolysis even at the redox potential of Li ₂ CoPO ₄ F or Li ₂ NiPO ₄ F, and these positive electrode oxidizing substances cannot be used. If the electrolytic solution of Example 1 is used, not only Li ₂ CoPO ₄ F or Li ₂ NiPO ₄ F can be used as the positive electrode active material, but also LiCoPO ₄ , LiNiPO _4, etc. can be used.

Moreover, as shown in FIG. 2, also in the electrolyte solution of Examples 2-9, it turned out that an electric potential window spreads in the positive direction as compared with the electrolyte solution of Comparative Example 1 similarly to Example 1. . From these results, the chain saturated hydrocarbon dinitrile compound (Examples 1 to 5) in which nitrile groups were bonded to both ends of the chain saturated hydrocarbon compound, ethylene carbonate and dimethyl carbonate, the end of the chain ether compound By adding a chain ether nitrile compound (Examples 6 and 7) having at least one nitrile group bonded thereto and at least one nitrile compound (Examples 8 and 9) among cyanoacetic acid esters, the solvent is decomposed to a high potential. It was found that it can exist stably without doing. In particular, the potential window greatly widened in Examples 1 to 5 using a chain saturated hydrocarbon dinitrile compound in which a nitrile group was bonded to both ends of a chain saturated hydrocarbon compound as a nitrile compound, which has branches. Also in Example 5, it was found that the potential window greatly spreads in the positive direction. Also, in Example 6 using oxydipropionitrile NCCH ₂ CH ₂ —O—CH ₂ CH ₂ CN, it was found that the potential window greatly expanded in the positive direction.

(Examples 10 to 17)
In Examples 10 to 17, the solvent was various nitrile: ethylene carbonate: diethyl carbonate = 50: 25: 25 (volume ratio), and the electrolyte was LiPF ₆ (lithium hexafluorophosphate) at 1 mol / L in this mixed solvent. What was dissolved in this way was used as the electrolyte solution for lithium ion batteries (however, in Example 10 in which the nitrile compound was glutaronitrile, LiPF ₆ (lithium hexafluorophosphate) was 0.5 mol / L. ).
The types of nitriles used in each example are as follows.
Example 10 Glutaronitrile NC (CH ₂ ) ₃ CN
Example 11 sebaconitrile NC _{(CH 2)} 8 CN
Example 12 Dodecanedinitrile NC (CH ₂ ) ₁₀ CN
Example 13 2-methylglutaronitrile _{NCCH (CH 3) CH 2 CH} 2 CN
Example 14 oxy dipropionate nitrile _{NCCH 2 CH 2 -O-CH 2} CH 2 CN
Example 15 3-Methoxy-propionitrile _{CH 3 -O-CH 2 CH 2} CN
Example 16 methyl cyanoacetate NCCH ₂ COOCH ₃
Example 17 cyanoacetate butyl _{NCCH 2 COO (CH 2) 3} CH 3

(Comparative Example 2)
In Comparative Example 2, a mixed solvent of ethylene carbonate: diethyl carbonate = 50: 50 (volume ratio) was used as the organic solvent, and LiPF ₆ was dissolved as a lithium salt in an amount of 1 mol / L to perform electrolysis for a lithium ion battery. Liquid.

-Evaluation-
(Measurement of potential-current curve)
About the electrolyte solution of Examples 10-17 and the comparative example 2, it carried out similarly to the above-mentioned method, and measured the electric potential-current curve. The results are shown in FIG.
From this figure, it was found that in the electrolytic solutions of Examples 10 to 17, the potential window spreads in the positive direction as compared with the electrolytic solution of Comparative Example 2. From this result, even when ethylene carbonate and diethyl carbonate were used as solvents, both of the chain saturated hydrocarbon compounds were the same as when ethylene carbonate and dimethyl carbonate were used as solvents (that is, in the case of Examples 1 to 9). Add at least one nitrile compound among a chain saturated hydrocarbon dinitrile compound having a nitrile group bonded to the terminal, a chain ether nitrile compound having a nitrile group bonded to at least one terminal of the chain ether compound, and a cyanoacetate ester. It was found that the solvent exists stably up to a high potential. In particular, the potential window greatly widened in Examples 10 to 13 using a chain saturated hydrocarbon dinitrile compound in which a nitrile group was bonded to both ends of a chain saturated hydrocarbon compound as a nitrile compound, which has branches. Also in Example 13, it was found that the potential window greatly expanded in the positive direction. Also, in Example 14 and Example 15 using a chain ether nitrile compound in which a nitrile group was bonded to at least one end of the chain ether compound, it was found that the potential window greatly expanded in the positive direction.

(Examples 18 to 25)
In Examples 18 to 25, the solvent was various nitrile: γ-butyrolactone: dimethyl carbonate = 50: 25: 25 (volume ratio), and the electrolyte was LiPF ₆ (lithium hexafluorophosphate) at 1 mol / L in this mixed solvent. What was dissolved so that it might become was set as the electrolyte solution for lithium ion batteries. In the example using adiponitrile as the nitrile, LiPF ₆ was 0.5 mol / L.
The types of nitriles used in each example are as follows.
Example 18 Glutaronitrile NC (CH ₂ ) ₃ CN
Example 19 adiponitrile NC _{(CH 2)} 4 CN
Example 20 sebaconitrile NC _{(CH 2)} 8 CN
Example 21 dodecane dinitrile _{NC (CH 2)} 10 CN
Example 22 2-methylglutaronitrile _{NCCH (CH 3) CH 2 CH} 2 CN
Example 23 oxy dipropionate nitrile _{NCCH 2 CH 2 -O-CH 2} CH 2 CN
Example 24 methyl cyanoacetate NCCH ₂ COOCH ₃
Example 25 cyanoacetate butyl _{NCCH 2 COO (CH 2) 3} CH 3

(Comparative Example 3)
In Comparative Example 3, a mixed solvent of γ-butyrolactone: dimethyl carbonate = 50: 50 (volume ratio) was used as the organic solvent, and LiPF ₆ was dissolved as a lithium salt at 1 mol / L so as to be used for a lithium ion battery. An electrolyte was used.

-Evaluation-
(Measurement of potential-current curve)
For the electrolytic solutions of Examples 18 to 25 and Comparative Example 3, potential-current curves were measured in the same manner as described above. The results are shown in FIG.
From this figure, it was found that also in the electrolyte solutions of Examples 18 to 25, the potential window greatly expanded in the positive direction as compared with the electrolyte solution of Comparative Example 3. From this result, even when dimethyl carbonate and cyclic ester γ-butyrolactone are used as a solvent instead of cyclic carbonate ethylene carbonate, a chain formula in which nitrile groups are bonded to both ends of the chain saturated hydrocarbon compound. It was found that by adding the saturated hydrocarbon dinitrile compound, the solvent exists stably to a high potential. Moreover, it turned out that a potential window spreads in the positive direction greatly not only in Examples 18 to 21 which are linear molecules among chain-type saturated hydrocarbon dinitrile compounds but also in Example 22 having branches. Further, in Example 23 using a chain ether nitrile compound in which a nitrile group is bonded to both ends of the chain ether compound, and in Examples 24 and 25 using cyanoacetate, the potential window is greatly widened in the positive direction. I understood that.

(Examples 26 to 31)
In Examples 26 to 31, the solvent was various nitriles: dimethyl carbonate = 50: 50 (volume ratio), and the electrolyte was LiPF ₆ (lithium hexafluorophosphate) 1 mol / L (Examples 30 and 31). In this case, an electrolyte solution for a lithium ion battery was dissolved so as to be 0.1 mol / L. The types of nitriles used in each example are as follows.
Example 26 glutaronitrile NC _{(CH 2)} 3 CN
Example 27 sebaconitrile NC _{(CH 2)} 8 CN
Example 28 Dodecanedinitrile NC (CH ₂ ) ₁₀ CN
Example 29 2-methylglutaronitrile _{NCCH (CH 3) CH 2 CH} 2 CN
Example 30 oxy dipropionate nitrile _{NCCH 2 CH 2 -O-CH 2} CH 2 CN
Example 31 methyl cyanoacetate NCCH ₂ COOCH ₃

(Comparative Example 4)
In Comparative Example 4, LiPF ₆ as a lithium salt was dissolved in dimethyl carbonate as an organic solvent so as to be 1 mol / L to obtain an electrolytic solution for a lithium ion battery.

-Evaluation-
(Measurement of potential-current curve)
About the electrolyte solution of Examples 26-31 and the comparative example 4, the electric potential-current curve was measured like the above-mentioned method. The results are shown in FIG.
From this figure, in the electrolytic solutions of Examples 26 to 31 in which dimethyl carbonate alone was added in addition to the nitrile compound as a solvent, the potential window was positive in comparison with the electrolytic solution of Comparative Example 3 in which dimethyl carbonate was the sole solvent. I understood that it spreads. It was also found that the solvent was stably present up to a high potential by adding a chain saturated hydrocarbon dinitrile compound having nitrile groups bonded to both ends of the chain saturated hydrocarbon compound (Examples 26 to 29). Furthermore, it was found that, among the chain saturated hydrocarbon dinitrile compounds, not only in Examples 26 to 28 which are linear molecules, but also in Example 29 having branches, the potential window greatly spreads in the positive direction. Furthermore, in Example 30 using a chain ether nitrile compound in which a nitrile group was bonded to both ends of the chain ether compound, the potential window was greatly expanded, and in Example 31 using cyanoacetate, the potential window was expanded.

(Examples 32 and 33)
In Examples 32 and 33, the solvent was various nitrile: propylene carbonate = 50: 50 (volume ratio), and the electrolyte was dissolved in this mixed solvent so that LiPF ₆ (lithium hexafluorophosphate) was 1 mol / L. This was used as an electrolyte for lithium ion batteries. The types of nitriles used in each example are as follows.
Example 32 sebaconitrile NC _{(CH 2)} 8 CN
Example 33 dodecane dinitrile _{NC (CH 2)} 10 CN

(Comparative Example 5)
In Comparative Example 5, an electrolyte solution for a lithium ion battery was prepared by dissolving LiPF ₆ as a lithium salt in propylene carbonate as an organic solvent so as to be 1 mol / L.

-Evaluation-
(Measurement of potential-current curve)
With respect to the electrolytic solutions of Examples 32 and 33 and Comparative Example 5, potential-current curves were measured in the same manner as described above. The results are shown in FIG.
From this figure, in the electrolytic solutions of Examples 32 and 33 in which propylene carbonate alone was added in addition to the nitrile compound as the solvent, the potential window was positive in comparison with the electrolytic solution of Comparative Example 5 in which propylene carbonate was the sole solvent. It was found that it spreads greatly. It was also found that the solvent was stably present up to a high potential by adding a chain saturated hydrocarbon dinitrile compound having nitrile groups bonded to both ends of the chain saturated hydrocarbon compound.

(Examples 34 to 36)
In Examples 34 to 36, the solvent was various nitriles: γ-butyrolactone = 50: 50 (volume ratio), and the electrolyte was dissolved in this mixed solvent so that LiPF ₆ (lithium hexafluorophosphate) was 1 mol / L. This was used as an electrolyte for a lithium ion battery. The types of nitriles used in each example are as follows.
Example 34 glutaronitrile NC _{(CH 2)} 3 CN
Example 35 sebaconitrile NC _{(CH 2)} 8 CN
Example 36 dodecane dinitrile _{NC (CH 2)} 10 CN

(Comparative Example 6)
In Comparative Example 6, an electrolyte solution for a lithium ion battery was prepared by dissolving LiPF ₆ as a lithium salt in γ-butyrolactone as an organic solvent so as to be 0.1 mol / L.

-Evaluation-
(Measurement of potential-current curve)
For the electrolytic solutions of Examples 34 to 36 and Comparative Example 6, potential-current curves were measured in the same manner as described above. The results are shown in FIG.
From this figure, in the electrolytic solutions of Examples 34 to 36 in which γ-butyrolactone was added alone as a solvent in addition to the nitrile compound, the potential window was larger than that of the electrolytic solution of Comparative Example 5 in which γ-butyrolactone was the sole solvent. It turns out that it spreads greatly in the positive direction. It was also found that the solvent was stably present up to a high potential by adding a chain saturated hydrocarbon dinitrile compound having nitrile groups bonded to both ends of the chain saturated hydrocarbon compound.

(Examples 37 to 39)
In Examples 37 to 39, the solvent is various nitrile: ethylene carbonate: γ-butyrolactone = 50: 25: 25 (volume ratio), and the electrolyte is LiPF ₆ (lithium hexafluorophosphate) at 1 mol / L in this mixed solvent. What was melt | dissolved so that it might become was used as the electrolyte solution for lithium ion batteries. The types of nitriles used in each example are as follows.
Example 37 sebaconitrile NC _{(CH 2)} 8 CN
Example 38 2-methylglutaronitrile _{NCCH (CH 3) CH 2 CH} 2 CN
Example 39 oxy dipropionate nitrile _{NCCH 2 CH 2 -O-CH 2} CH 2 CN

(Comparative Example 7)
In Comparative Example 7, an electrolyte solution for a lithium ion battery was prepared by dissolving LiPF ₆ as a lithium salt at 1 mol / L in ethylene carbonate: γ-butyrolactone = 50: 50 (volume ratio) as an organic solvent.

-Evaluation-
(Measurement of potential-current curve)
About the electrolyte solution of Examples 37-39 and the comparative example 7, the electric potential-current curve was measured like the above-mentioned method. The results are shown in FIG.
From this figure, in the electrolytic solutions of Examples 37 to 39 in which ethylene carbonate, which is a cyclic carbonate, and γ-butyrolactone, which is a cyclic ester, were added as solvents in addition to the nitrile compound, the nitrile compound was not added. It was found that the potential window spreads greatly in the positive and negative directions as compared with the electrolyte. It was also found that the solvent was stably present up to a high potential by adding a chain saturated hydrocarbon dinitrile compound having nitrile groups bonded to both ends of the chain saturated hydrocarbon compound. Furthermore, it was found that, among the chain-type saturated hydrocarbon dinitrile compounds, not only in Example 37, which is a linear molecule, but also in Example 38 having branches, the potential window greatly spreads in the positive and negative directions. Further, in Example 39 using a chain ether nitrile compound in which a nitrile group was bonded to both ends of the chain ether compound, the potential window was greatly widened in the positive and negative directions.

(Examples 40 to 44)
In Examples 40 to 44, the solvent was sebacononitrile (adiponitrile in Example 44): ethylene carbonate: dimethyl carbonate = 50: 25: 25 (volume ratio), and various electrolytes were dissolved in this mixed solvent so as to be 1 mol / L. This was used as an electrolyte for a lithium ion battery. The types of electrolyte used in each example are as follows.
Example 40 LiPF ₆
Example 41 LiTFSI
Example 42 LiTFSI
Example 43 LiBF ₄
Example 44 LiBETI

(Example 45)
In Example 45, a solvent in which sebacononitrile, ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 50:25:25 was used as the organic solvent, and LiPF ₆ ( Lithium hexafluorophosphate) was dissolved at 0.05 mol / L and LiTFSI (lithium bis (trifluoromethanesulfonyl) imide) was dissolved at 1.0 mol / L to obtain an electrolyte for a lithium ion battery.

(Example 46)
In Example 46, a solvent in which butyl cyanoacetate, ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 50:25:25 as an organic solvent, and LiTFSI as a lithium salt was used. (Lithium bis (trifluoromethanesulfonyl) imide) was dissolved at 1.0 mol / L to obtain an electrolytic solution for a lithium ion battery.

(Example 47)
In Example 47, a solvent obtained by mixing butyl cyanoacetate, ethylene carbonate (EC), and dimethyl carbonate (DMC) in a volume ratio of 50:25:25 as an organic solvent, and LiBF as a lithium salt was used. ₄ (lithium tetrafluoroborate) was dissolved at 1.0 mol / L to obtain an electrolyte for a lithium ion battery.

(Comparative Examples 8 to 10)
In Comparative Examples 8 to 10, various lithium salts were added instead of LiPF ₆ which is the lithium salt in Comparative Example 1. That is, various lithium salts (LiTFSI in Comparative Example 8, LiBF ₄ in Comparative Example 9, LiBETI in Comparative Example 10) at 1 mol / L as an organic solvent, ethylene carbonate: dimethyl carbonate = 50: 50 (volume ratio). It was made to melt | dissolve and it was set as the electrolyte solution for lithium ion batteries.

-Evaluation-
(Measurement of potential-current curve)
With respect to the electrolytic solutions of Examples 40 to 44, Comparative Example 1 and Comparative Examples 8 to 10, potential-current curves were measured in the same manner as described above. The results are shown in FIG. In addition, Table 1 shows the value of the potential obtained from this figure when the current density is 50 μA / cm ² .
From FIG. 10 and Table 1, in the electrolytic solutions of Examples 40 to 44 in which ethylene carbonate as a cyclic carbonate and dimethyl carbonate as a chain carbonate were added as solvents in addition to a nitrile compound as a solvent, regardless of the type of electrolyte. It was found that the potential window greatly expanded in the positive direction as compared with the electrolytic solutions of Comparative Example 1 and Comparative Examples 8 to 10 in which no nitrile compound was added.
In addition, the potential window of the electrolytic solution of Example 45 is 6.6 V (see FIG. 11), the potential window of the electrolytic solution of Example 46 is 5.4 V (see FIG. 12), and the potential window of the electrolytic solution of Example 47 is It became 6.1V (refer FIG. 13), and it turned out that all have spread to the positive side.

Similarly, the lithium salt and LiPF _6, the electrolyte of the other examples and comparative examples, the potential - was determined from current curve, shown in Table 2 an electrode potential when a predetermined current density. From this table, a chain saturated hydrocarbon dinitrile compound having a nitrile group bonded to both ends of the chain saturated hydrocarbon compound, a chain ether nitrile compound having a nitrile group bonded to at least one of the ends of the chain ether compound, and cyano It can be seen that the potential window widens in the positive direction when at least one nitrile compound of acetic acid ester and at least one of cyclic carbonate, cyclic ester and chain carbonate are contained.

<Influence of nitrile addition amount>
In order to examine the influence of the addition amount of nitrile in the electrolytic solution of the present invention, a predetermined amount of sebacononitrile was added to a mixed solvent of ethylene carbonate: dimethyl carbonate = 1: 1 (volume ratio), and a potential-current curve was measured. Incidentally, the electrolyte was added LiPF ₆ as a 1M (However, in the case of sebaconitrile 100% by volume, it was 0.1M for dissolution of 1M was difficult). The results are shown in FIG. From this figure, it was found that the amount of sebacononitrile added had the effect of expanding the potential window even at 1% by volume, and the potential window expanded in the higher potential direction as the amount added increased. However, when 100% by volume of sebacononitrile is used, the solubility of LiPF ₆ that is an electrolyte is lowered and the viscosity is also increased. Therefore, there is a problem that the conductivity is lowered and the internal resistance of the battery is increased. For this reason, the preferable amount of sebacononitrile added as an electrolyte for a lithium battery is 1% by volume or more and less than 100% by volume, more preferably 5% by volume or more and less than 90% by volume, and most preferably 30% by volume or more and 70% by volume. %.

  As described above, in the potential-current curve for the electrolytic solution of the example, it was found that the potential window greatly expanded in the positive direction by adding the nitrile compound to the organic solvent. In the measurement of the potential-current curve of the above example, as described above, after scanning several times on the positive side and the negative side, the potential is swept at a rate of 5 mV / sec from the natural potential in the positive direction or the negative direction. And the potential-current curve is measured. In the potential scan several times before this measurement, the potential window spreads after the second time. Therefore, by sweeping the potential in the positive direction in the electrolytic solution of the present invention, an electrode having a wide potential window is formed. It turns out that it can manufacture.

From the above, the following matters are disclosed.
(1) an immersion step of immersing the electrode in an organic solvent containing a nitrile compound;
And a positive voltage applying step for applying a potential higher than the potential that can be applied when the electrode is immersed in a liquid containing only the organic solvent that does not contain the nitrile compound after the immersion step. Electrode processing method.
(2) The method for treating an electrode according to (1), wherein the high potential is more than 5.2 V, preferably 6.0 V or more with respect to the (Li / Li ⁺ ) reference electrode.
(3) The nitrile compound is a chain saturated hydrocarbon dinitrile compound in which a nitrile group is bonded to both ends of a chain saturated hydrocarbon compound, or a chain ether nitrile in which a nitrile group is bonded to at least one end of a chain ether compound. At least one of a compound and a cyanoacetic acid ester,
The electrode processing method according to (2), wherein the electrode is made of carbon.
(4) A chain saturated hydrocarbon dinitrile compound having a nitrile group bonded to both ends of a chain saturated hydrocarbon compound, a chain ether nitrile compound having a nitrile group bonded to at least one terminal of the chain ether compound, and cyanoacetic acid An immersion step of immersing the electrode in an organic solvent containing a nitrile compound that is at least one of the esters;
And a positive voltage applying step of applying a positive voltage to the electrode after the immersion step.

<Measurement of battery characteristics>
In order to evaluate the performance of the electrolyte solution for lithium ion batteries of the present invention as a battery, a potential-current curve was measured using a cathode for a lithium battery and a cathode for a lithium battery.
That is, the electrolyte solution for lithium ion batteries of Example 41 (that is, EC: DMC: sebacononitrile = 25: 25: 50 by volume ratio, LiTFSI (lithium bis (trifluoromethanesulfonyl) imide) as a lithium salt was 1.0 mol / L), and a lithium battery cathode and a lithium battery anode were used as working electrodes, and a potential-current curve of lithium occlusion / release was measured. The cathode for a lithium battery using _Li 4 _Ti 5 O1 _2, as a positive electrode for a lithium battery using LiCoO ₂ and LiCoPO _4. A potentiogalvanostat was used for the measurement. Further, (Ag / Ag ⁺ ) was used as the reference electrode. In the measurement, after scanning several times on the positive side and the negative side, the potential was swept from the natural potential in the positive direction or in the negative direction at a speed of 0.5 mV / sec, and the potential-current curve was measured.

As a result, as shown in FIG. 15, Li ₄ Ti ₅ O ₁₂ as the cathode for the lithium battery, LiCoO ₂ and LiCoPO ₄ as the cathode for the lithium battery, both of lithium (0) and lithium ions A nearly reversible current associated with redox was observed. From this result, it was found that by using the lithium ion battery electrolyte of Example 4, smooth charge / discharge between lithium (0) and lithium ions was possible.

  The present invention is not limited to the description of the embodiment of the invention. Various modifications may be included in the present invention as long as those skilled in the art can easily conceive without departing from the description of the scope of claims.

2 is a potential-current curve of Example 1 and Comparative Example 1. FIG. It is the electric potential-current curve of Examples 2-9 and Comparative Example 1. 6 is a potential-current curve of Comparative Example 1. It is an electric potential-current curve of Examples 10-17 and Comparative Example 2. It is an electric potential-current curve of Examples 18-25 and Comparative Example 3. It is an electric potential-current curve of Examples 26-31 and Comparative Example 4. It is an electric potential-current curve of Examples 32 and 33 and Comparative Example 5. It is an electric potential-current curve of Examples 34-36 and Comparative Example 6. It is an electric potential-current curve of Examples 37-39 and Comparative Example 7. 4 is a potential-current curve of Examples 41 to 45 and Comparative Example 1. FIG. 10 is a potential-current curve of Example 45 and Comparative Example 8. FIG. It is an electric potential-current curve of Example 46 and Comparative Example 8. It is an electric potential-current curve of Example 47 and Comparative Example 1. It is a potential-current curve in a mixed solvent in which ethylene carbonate: dimethyl carbonate = 1: 1 and a predetermined amount of sebacononitrile was added. 42 is a potential-current curve of lithium occlusion / release using the lithium ion battery electrolyte of Example 41. FIG.

Claims (3)

Of chain saturated hydrocarbon dinitrile compounds having nitrile groups bonded to both ends of the chain saturated hydrocarbon compound, chain ether nitrile compounds having nitrile groups bonded to at least one of the ends of the chain ether compound, and cyanoacetates An electrolyte for a lithium ion battery, comprising: an organic solvent containing at least 50% by volume of at least one nitrile compound; and a lithium salt dissolved in the organic solvent,
As the lithium salt, at least one of LiPF ₆ , LiBF ₄ , and LiBETI is dissolved as a whole in an amount of 1 mol / L or more, or as the lithium salt, at least one of LiPF ₆ , LiBF ₄ , and LiBETI and LiTFSI are dissolved. The concentration of LiTFSI is 1 mol / L or more,
The electrolyte solution for a lithium ion battery, wherein the organic solvent contains at least one of a cyclic carbonate, a cyclic ester, and a chain carbonate. The electrolyte for a lithium ion battery according to claim 1, for use in a region where the potential for charging exceeds 5.2 V (vs. Li / Li ⁺ ).   The nitrile compound includes at least one of succinonitrile, glutaronitrile, adiponitrile, sebaconitrile, dodecandinitrile, 2-methylglutaronitrile, methyl cyanoacetate, ethyl cyanoacetate, propyl cyanoacetate and butyl cyanoacetate. The electrolyte solution for lithium ion batteries according to claim 1.

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