JP2010129449A - Nonaqueous electrolyte for secondary battery and nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain excellent charge preservation characteristics under a high-temperature environment. <P>SOLUTION: The non-aqueous electrolyte solution contains a fluorinated cyclic carbonic acid ester as a solvent and lithium salt as an electrolyte. As the lithium salt, it contains lithium bis-fluorosulfonyl imide as expressed by a structural formula: (F-O<SB>2</SB>S-N-SO<SB>2</SB>-F)Li. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte for a secondary battery and a non-aqueous electrolyte secondary battery using the same.

  In recent years, mobile information terminals such as mobile phones, notebook personal computers, and PDAs have been rapidly reduced in size and weight, and a secondary battery used as a driving power source is desired to have a higher capacity. As a secondary battery that meets this requirement, a non-aqueous electrolyte secondary battery having a high battery voltage and a high energy density has been attracting attention. In particular, a lithium ion secondary battery using a lithium-containing transition metal oxide as a positive electrode active material and a graphite-based carbon material as a negative electrode active material is generally used. However, it is difficult to say that current lithium ion secondary batteries completely satisfy the long-term driving demands of mobile information terminals, and there is a demand for higher capacity.

  As one means to meet this demand, there is a method of increasing the charging voltage and increasing the charging potential in a lithium ion secondary battery using lithium cobalt oxide or nickel manganese cobalt composite oxide as a positive electrode active material. is there. However, this method increases the oxidation reaction of the electrolyte solution on the surface of the positive electrode, and causes deterioration of cycle characteristics and battery characteristics in charge storage.

Currently, electrolytes used in lithium ion secondary batteries include cyclic carbonates such as ethylene carbonate (EC) and chain chains such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). An electrolytic solution in which a lithium salt such as LiPF ₆ or LiBF ₄ is dissolved in a solvent in which a carbonate ester is mixed is common. These electrolytes have succeeded in extracting good charge / discharge characteristics when the battery voltage is 4.2 V (when graphite is used for the negative electrode, the positive electrode potential is about 4.3 V with respect to metal lithium).

  However, when the charging voltage of these non-aqueous electrolytes is increased, decomposition of the electrolyte solution is promoted due to a side reaction between the electrode and the electrolyte solution, causing problems such as a decrease in battery capacity. This problem appears remarkably when a charge storage test is performed in which the battery is left at high temperature in a charged state in order to accelerate evaluation of the durability of the secondary battery.

  Under such circumstances, it is desired to develop a non-aqueous electrolyte secondary battery having excellent high-temperature durability during high-voltage charging.

  As described above, the conventional electrolytic solution has a problem that the battery deteriorates due to a side reaction between the electrode and the electrolytic solution when stored in a high temperature environment in a charged state. In particular, when the positive electrode potential is charged to 4.40 V or more on the basis of metallic lithium, it is significantly deteriorated.

  In Patent Document 1, the thermal stability of the battery is improved by using 4-fluoroethylene carbonate (FEC) as an electrolytic solution. Thus, 4-fluoroethylene carbonate (FEC) is a substance necessary for improving battery safety. However, when a graphite-based active material is used as the negative electrode active material, 4-fluoroethylene carbonate (FEC) is reduced and decomposed on the negative electrode, resulting in a problem that the remaining / recovery capacity during charge storage is reduced.

  In Patent Document 2, it is proposed to use a fluorinated cyclic ester and lithium bistrifluoromethylsulfonylimide (LiTFSI) as a non-aqueous electrolyte for a secondary battery to improve high-temperature cycle characteristics. As will be described later, the present invention includes a fluorinated cyclic carbonate as a solvent and uses lithium bisfluorosulfonylimide (LiFSI) as an electrolyte to improve charge storage characteristics in a high temperature environment. Document 2 does not disclose any charge storage characteristics or use of lithium bisfluorosulfonylimide (LiFSI).

  Patent Document 3 describes that by using an electrolytic solution in which lithium bisfluorosulfonylimide (LiFSI) is dissolved in a γ-butyrolactone-based electrolytic solution, the remaining / recovery capacity after charge storage is improved. However, there is no description about using a fluorinated cyclic ester as a solvent. In Patent Document 3, storage characteristics at a battery voltage of 4.2 V (a positive electrode potential of about 4.3 V based on metallic lithium) are evaluated, and a battery voltage of 4.4 V (a positive electrode potential of about 4.5 V based on metallic lithium is evaluated). ) Is not disclosed at all.

Patent Document 4 describes that a fluorinated cyclic ester is used for a non-aqueous electrolyte for a secondary battery to improve high voltage charging characteristics. However, it only describes the improvement of the maintenance rate of the charge / discharge cycle, and does not describe any improvement of the remaining / recovery capacity during high-temperature storage. Further, there is no disclosure about lithium bisfluorosulfonylimide (LiFSI).
Special table 2007-504628 JP 2006-294375 A JP 2004-165151 A JP 2008-108689 A

  The objective of this invention is providing the nonaqueous electrolyte for lithium secondary batteries which can acquire the favorable charging / discharging storage characteristic in a high temperature environment, and a nonaqueous electrolyte secondary battery using the same.

The non-aqueous electrolyte for a secondary battery according to the present invention is a non-aqueous electrolyte for a secondary battery that contains a fluorinated cyclic carbonate as a solvent and a lithium salt as an electrolyte, and has a structural formula (FO) as the lithium salt. is characterized by containing a lithium bis fluoro sulfonyl Louis bromide represented by _{2 S-N-SO 2 -F} ) Li.

According to the present invention, lithium bisfluorosulfonylimide (LiFSI) represented by the structural formula (F—O ₂ S—N—SO ₂ —F) Li is used as the lithium salt, and fluorinated cyclic carbonate is used as the solvent. Thus, excellent charge storage characteristics in a high temperature environment can be obtained.

  In particular, when the potential of the positive electrode at full charge is 4.25 V or higher, more preferably 4.40 V or higher, based on metallic lithium, good charge storage characteristics can be obtained. The upper limit of the positive electrode potential at full charge is not particularly limited, but is generally 4.80 V or less.

  Examples of the fluorinated cyclic carbonate include 4-fluoroethylene carbonate (FEC) and derivatives thereof.

  Reasons for improving the charge storage characteristics in a high-temperature environment by adding lithium bisfluorosulfonylimide (LiFSI) to an electrolytic solution containing a fluorinated cyclic carbonate such as 4-fluoroethylene carbonate (FEC) as a solvent The details of are unknown, but it is considered that lithium bisfluorosulfonylimide (LiFSI) suppresses the reaction between a fluorinated cyclic carbonate such as 4-fluoroethylene carbonate (FEC) and the negative electrode. For this reason, by using an electrolytic solution in which a fluorinated cyclic carbonate such as 4-fluoroethylene carbonate (FEC) and lithium bisfluorosulfonylimide (LiFSI) coexist, the residual capacity and the return capacity are specifically reduced. Can be suppressed. Such a phenomenon cannot be confirmed when ethylene carbonate (EC) and lithium bisfluorosulfonylimide (LiFSI) coexist.

  In this invention, it is preferable that content with respect to the whole solvent of fluorinated cyclic carbonates, such as 4-fluoroethylene carbonate (FEC), exists in the range of 5-30 volume%. If it is less than 5% by volume, the effect of the present invention that good charge storage characteristics can be obtained may not be sufficiently obtained. On the other hand, if it exceeds 30% by volume, a large amount of gas may be generated in the electrolytic solution, which is disadvantageous in terms of cost.

  In the present invention, lithium bisfluorosulfonylimide (LiFSI) is preferably contained in the nonaqueous electrolytic solution in a range of 0.01 to 0.5 mol / liter. If it is less than 0.01 mol / liter, the effect of improving the charge storage characteristics may not be sufficiently obtained. On the other hand, if it exceeds 0.5 mol / liter, it is disadvantageous in terms of cost.

Other lithium salts contained in the non-aqueous electrolyte of the present invention include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , CH ₃ SO ₃ Li, LiCF ₃ SO ₃ , LiCl, LiBr, LiB (C ₆ H ₅ ) ₄ , LiC (CF ₃ SO ₂ ) ₃ , LiB (C ₂ O ₄ ) ₂ and mixtures thereof. Among these, it is particularly desirable to use LiPF ₆ .

  In the present invention, other solvents used for the non-aqueous electrolyte include cyclic carbonates such as ethylene carbonate (EC), lactams such as 2-methyl-1-pyrrolidone, and cyclic carbamic acids such as 3-methyl-2-oxazolidinone. Esters, cyclic sulfones such as tetramethylene sulfone, chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, or methyl propyl carbonate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl isobutyrate, or Chain carboxylic acid esters such as methyl trimethylacetate, ketones such as pinacholine, 1,2-dimethoxyethane, tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,3-dioxane, or 1,4 Ethers such as dioxane, chain amides such as N, N-dimethylformamide, N, N-dimethylacetamide, or chain carbamic acid esters such as methyl N, N-dimethylcarbamate and methyl N, N-dimethylcarbamate Etc. Among these, it is particularly desirable to use a chain carbonate ester.

  In the present invention, additives such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and propane sultone (PS) that form a film on the positive electrode and the negative electrode may be included.

  A non-aqueous electrolyte secondary battery of the present invention is a non-aqueous electrolyte secondary battery comprising a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte, and the non-aqueous electrolyte solution Is the non-aqueous electrolyte of the present invention.

  In the non-aqueous electrolyte secondary battery of the present invention, since the non-aqueous electrolyte of the present invention is used, the charge storage characteristics in a high temperature environment can be improved. In the nonaqueous electrolyte secondary battery of the present invention, the potential of the positive electrode when fully charged is preferably 4.25 V or more, more preferably 4.40 V or more, based on metallic lithium. By setting the potential of the positive electrode at full charge to 4.25 V or more, more preferably 4.40 V or more on the basis of metallic lithium, the charge / discharge capacity of the secondary battery can be increased, the charge / discharge capacity is high, and It can be set as the secondary battery excellent in the charge storage characteristic in a high temperature environment.

  Examples of the positive electrode active material that can be used in the present invention include lithium-containing transition metal composite oxides such as lithium cobaltate, lithium nickelate, and nickel manganese lithium cobaltate. Furthermore, materials obtained by adding different elements such as Al, Zr, Ti, Mg, Mo, Fe, Cr, V, and Nb to these lithium-containing transition metal composite oxides can be used. The addition amount of the different element is preferably about 0.01 to 5 mol% with respect to the transition metal in the lithium-containing transition metal composite oxide. In particular, lithium cobalt oxide in which Al and / or Mg is solid-solved and a compound containing Zr is attached to the surface is preferably used. In the present invention, two or more kinds of positive electrode active materials may be mixed and used.

  The negative electrode active material that can be used in the present invention is not particularly limited, but an active material made of a carbon material such as graphite can be preferably used.

  According to the present invention, by using a combination of a fluorinated cyclic carbonate such as 4-fluoroethylene carbonate (FEC) and lithium bisfluorosulfonylimide (LiFSI), a non-aqueous electrolyte secondary battery after being charged and stored. The remaining capacity and the return capacity can be improved. Therefore, according to the present invention, good charge storage characteristics in a high temperature environment can be obtained.

  Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments, and changes in design as appropriate based on ordinary knowledge of those skilled in the art without departing from the spirit of the present invention. It should be understood that improvements and the like can be made.

Example 1
[Production of positive electrode]
As a positive electrode active material, lithium cobalt oxide in which Al and Mg were each solid-dissolved in 1.0 mol% and a compound containing Zr was adhered to the surface so that Zr was 0.05 mol% was prepared. . Specifically, Li ₂ CO ₃ , Co ₃ O ₄ , ZrO ₂ , MgO, Al (OH) ₃ were mixed so as to have a predetermined molar ratio, then heat-treated in an air atmosphere and pulverized. .

  After the lithium cobaltate, carbon as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed at a weight ratio of 95: 2.5: 2.5, N-methyl- It added and kneaded in 2-pyrrolidone (NMP) solution, and the positive electrode slurry was produced. The prepared slurry was applied to both surfaces of an aluminum foil as a current collector so as to have a predetermined coating amount, dried, and then rolled to a predetermined packing density to prepare a positive electrode.

(Production of negative electrode)
Graphite as a negative electrode active material, styrene butadiene (SBR) rubber as a binder, and carboxymethyl cellulose (CMC) as a thickener are mixed so as to have a weight ratio of 98: 1: 1, and then an aqueous solution. The mixture was kneaded in to prepare a negative electrode slurry. The prepared negative electrode slurry was applied on both sides of a copper foil as a current collector so as to have a predetermined coating amount, dried and then rolled to a predetermined packing density to prepare a negative electrode.

[Preparation of non-aqueous electrolyte]
As a solvent, 4-fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 20:80, and LiPF ₆ was added to this mixed solvent as a lithium salt at 1 mol / liter, lithium bis. Fluorosulfonylimide (LiFSI) was dissolved at 0.1 mol / liter to prepare a non-aqueous electrolyte.

[Production of battery]
The produced positive electrode and negative electrode were wound up so as to face each other through a polyethylene separator, and a wound body was produced. The wound body was inserted into an outer package made of a laminate material produced by laminating polyethylene terephthalate, aluminum, etc., and each electrode tab protruded from the end.

  Next, in a glove box under an inert gas atmosphere, the nonaqueous electrolyte solution is injected so as to be impregnated into the wound body in the outer package, and after the injection, the opening is sealed and the nonaqueous electrolyte secondary solution is filled. A battery was produced.

  In addition, the ratio of the positive electrode capacity and the negative electrode capacity was adjusted so that the end-of-charge voltage was 4.2 V (the positive electrode potential was about 4.3 V with respect to metallic lithium), and the nonaqueous electrolyte secondary battery T1 was manufactured.

(Example 2)
Except for preparing the battery by adjusting the ratio of the positive electrode capacity and the negative electrode capacity so that the end-of-charge voltage is 4.4 V (positive electrode potential is about 4.5 V with respect to metallic lithium), the same procedure as in Example 1 is performed. Invention battery T2 was prepared.

(Comparative Example 1)
As a solvent, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 20:80, and LiPF ₆ is added as a lithium salt to this mixed solvent so as to be 1 mol / liter. A non-aqueous electrolyte was prepared. A comparative battery R1 was produced in the same manner as in Example 1 except that this nonaqueous electrolytic solution was used.

(Comparative Example 2)
In the same manner as in Comparative Example 1, a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was prepared, to this mixed solvent, the LiPF ₆ 1 mol / liter, lithium bis trifluoromethylsulfonyl Louis bromide (LiTFSI) A nonaqueous electrolytic solution was prepared by adding 0.1 mol / liter. A comparative battery R2 was produced in the same manner as in Comparative Example 1 except that this nonaqueous electrolytic solution was used.

(Comparative Example 3)
In the same manner as in Comparative Example 1, a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was prepared. To this mixed solvent, 1 mol / liter of LiPF ₆ and 0 of lithium bisfluorosulfonylimide (LiFSI) were added. A non-aqueous electrolyte was prepared by adding 1 mol / liter. A comparative battery R3 was produced in the same manner as in Comparative Example 1 except that this nonaqueous electrolytic solution was used.

(Comparative Example 4)
As a mixed solvent, instead of a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), 4-fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 20:80. A comparative battery R4 was produced in the same manner as in Comparative Example 1 except that the mixed solvent was used.

(Comparative Example 5)
Non-aqueous solution prepared by using the mixed solvent used in Comparative Example 4 and adding LiPF ₆ to 1 mol / liter and lithium bistrifluoromethylsulfonylimide (LiTFSI) to this mixed solvent to 0.1 mol / liter. A comparative battery R5 was produced in the same manner as in Comparative Example 4 except that the electrolytic solution was used.

(Comparative Example 6)
A comparative battery R6 was produced in the same manner as in Comparative Example 3, except that the ratio of the positive electrode capacity and the negative electrode capacity was adjusted so that the end-of-charge voltage was 4.4V (positive electrode potential of about 4.5V with respect to metallic lithium). .

(Comparative Example 7)
A comparative battery R7 was produced in the same manner as in Comparative Example 4 except that the ratio of the positive electrode capacity and the negative electrode capacity was adjusted so that the end-of-charge voltage was 4.4V (positive electrode potential of about 4.5V based on metallic lithium). .

[Evaluation of Charging and Storage Characteristics at Battery Voltage 4.2V (Metal Lithium Reference 4.3V)]
The charge storage characteristics of the present invention battery T1 and the comparative batteries R1 to R5 were evaluated. First, the battery is charged at a constant current (1 C) -constant voltage (0.02 C cut) at 25 ° C. to a battery voltage of 4.2 V and discharged to a battery voltage of 2.75 V at a constant current (1 C). the discharge capacity D ₁ was measured.

Next, after charging to a battery voltage of 4.2 V with a constant current (1C) -constant voltage (0.02 C cut), each battery was stored in a thermostatic bath at 60 ° C. for 10 days. The battery after the storage test, by discharging until the battery voltage 2.75V with a constant current (1C), to determine the remaining capacity D ₂ after storage.

From before storage discharge capacity D ₁ and after the storage remaining capacity D _2, the following formula was calculated residual capacity ratio after storage (%).

Capacity remaining rate after storage (%) = (Remaining capacity D ₂ after storage / Discharge capacity D ₁ before storage) × 100
The measurement results are shown in Table 1.

As shown in Table 1, in the case of an electrolytic solution using ethylene carbonate (EC) as a solvent, a comparative battery R1 containing only LiPF ₆ as a lithium salt, a comparative battery containing lithium bistrifluoromethylsulfonylimide (LiTFSI) and LiPF ₆ Comparative battery R3 containing R2, lithium bisfluorosulfonylimide (LiFSI) and LiPF ₆ did not have a large difference in capacity remaining rate after storage.

On the other hand, in the case of the electrolyte solution containing 4-fluoroethylene carbonate (FEC), the comparative battery R5 containing lithium bistrifluoromethylsulfonylimide (LiTFSI) and LiPF ₆ is stored after the comparison battery R4 containing only LiPF ₆ only. The capacity remaining rate of was poor. On the other hand, according to the present invention, the battery T1 of the present invention containing lithium bisfluorosulfonylimide (LiFSI) together with LiPF ₆ as the lithium salt has a capacity remaining rate after storage of about 1% as compared with the comparative battery R4. It has improved.

  From the above results, when using an electrolytic solution containing 4-fluoroethylene carbonate (FEC) as a solvent, by using lithium bisfluorosulfonylimide (LiFSI) as a lithium salt, the positive electrode potential is 4.25 V based on metallic lithium. In the charge storage characteristic test charged as described above, it was confirmed that excellent charge storage characteristics were exhibited.

[Evaluation of Charging and Storage Characteristics at Battery Voltage 4.4V (Metal Lithium Reference 4.5V)]
The charge storage characteristics of the present invention battery T2 and the comparative batteries R6 to R7 were evaluated. First, the battery is charged at a constant current (1C) -constant voltage (0.02C cut) at a temperature of 4.4V and discharged at a constant current (1C) to a battery voltage of 2.75V. the discharge capacity D ₁ was measured.

Next, after charging to a battery voltage of 4.4 V with a constant current (1C) -constant voltage (0.02 C cut), each battery was stored in a thermostatic bath at 60 ° C. for 10 days. The battery after the storage test, by discharging until the battery voltage 2.75V with a constant current (1C), to determine the remaining capacity D ₂ after storage.

From before storage discharge capacity D ₁ and after the storage remaining capacity D _2, the following formula was calculated residual capacity ratio after storage (%).

Capacity remaining rate after storage (%) = (Remaining capacity D ₂ after storage / Discharge capacity D ₁ before storage) × 100
The measurement results are shown in Table 2.

  As shown in Table 2, when an electrolytic solution containing ethylene carbonate (EC) was used, the capacity remaining rate of the comparative battery R7 containing lithium bisfluorosulfonylimide (LiFSI) was not high. . Moreover, even if it is the electrolyte solution containing 4-fluoroethylene carbonate (FEC), the high capacity | capacitance residual rate is not obtained even in the comparison battery R7 which does not contain lithium bisfluoro sulfonylimide (LiFSI).

  However, in accordance with the present invention, the battery T2 of the present invention using a combination of 4-fluoroethylene carbonate (FEC) and lithium bisfluorosulfonylimide (LiFSI) after storage is about 10% higher than the comparative battery R7. The capacity remaining rate is obtained.

  From the above results, according to the present invention, when using an electrolytic solution containing 4-fluoroethylene carbonate (FEC), the capacity remaining rate after storage is improved by adding lithium bisfluorosulfonylimide (LiFSI). Can do. Further, as is clear from the comparison with the results shown in Table 1, it can be seen that further excellent charge storage characteristics can be obtained by charging the positive electrode potential to 4.40 V or more on the basis of metallic lithium.

Claims (9)

A non-aqueous electrolyte for a secondary battery containing a fluorinated cyclic carbonate as a solvent and a lithium salt as an electrolyte,
A nonaqueous electrolytic solution for a secondary battery, comprising lithium bisfluorosulfonylimide represented by the structural formula (F—O ₂ S—N—SO ₂ —F) Li as the lithium salt.   The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the fluorinated cyclic carbonate is 4-fluoroethylene carbonate and / or a derivative thereof.   The nonaqueous electrolytic solution for a secondary battery according to claim 2, wherein the 4-fluoroethylene carbonate and a derivative thereof are contained in an amount of 5 to 30% by volume based on the entire solvent.   4. The secondary battery according to claim 1, wherein the lithium bisfluorosulfonylimide is contained in a non-aqueous electrolyte solution in an amount of 0.01 to 0.5 mol / liter. Non-aqueous electrolyte. A non-aqueous electrolyte secondary battery comprising a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte,
The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte is the non-aqueous electrolyte according to claim 1.   The nonaqueous electrolyte secondary battery according to claim 5, wherein the potential of the positive electrode when fully charged is 4.25 V or more based on metallic lithium.   The non-aqueous electrolyte secondary battery according to claim 5, wherein the potential of the positive electrode when fully charged is 4.40 V or more based on metallic lithium.   8. The lithium cobalt oxide in which Al and / or Mg is solid-dissolved and a compound containing Zr is attached to the surface is used as the positive electrode active material. Non-aqueous electrolyte secondary battery.   The non-aqueous electrolyte secondary battery according to claim 5, wherein the negative electrode active material is graphite.