JP5398321B2 - Non-aqueous electrolyte for secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte for a secondary battery and a non-aqueous electrolyte secondary battery using such a non-aqueous electrolyte for a secondary battery. In particular, it improves the non-aqueous electrolyte for secondary batteries, improves the load characteristics in non-aqueous electrolyte secondary batteries, suppresses battery capacity from decreasing even under high temperature conditions, and has good battery characteristics over a long period of time It has a feature in that it can be obtained.

  In recent years, as a new secondary battery with high output and high energy density, a non-aqueous electrolyte containing a lithium salt of an electrolyte in a non-aqueous solvent is used, and lithium ions are moved between the positive and negative electrodes for charging and discharging. Non-aqueous electrolyte secondary batteries made to be used are widely used.

In order to obtain good charge / discharge characteristics in such a non-aqueous electrolyte secondary battery, conventionally, as the non-aqueous electrolyte, a cyclic carbonate such as ethylene carbonate is used as a non-aqueous solvent. And a mixed solvent in which chain carbonates such as diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate are mixed, and an electrolyte composed of a lithium salt such as LiPF ₆ or LiBF ₄ is used in this mixed solvent. Yes.

  However, in order to evaluate the durability of the non-aqueous electrolyte secondary battery using the non-aqueous electrolyte as described above, the non-aqueous electrolyte secondary battery is charged and stored under high temperature conditions in a charged state. When the test was performed, there was a problem that the above-described non-aqueous electrolyte caused a side reaction with the positive electrode and the negative electrode, resulting in a decrease in battery capacity.

  In recent years, mobile devices such as mobile phones, notebook PCs, and PDAs have been remarkably reduced in size and weight, and power consumption has increased with the increase in functionality. There is a demand for higher capacity of nonaqueous electrolyte secondary batteries.

  However, in order to increase the capacity of the non-aqueous electrolyte secondary battery, when the application amount and packing density of the electrode material in the positive electrode and the negative electrode are increased, the above non-aqueous electrolyte does not sufficiently penetrate into these electrodes. There is also a problem that good load characteristics cannot be obtained.

  Conventionally, as shown in Patent Document 1, lithium perfluoroalkanesulfonate is used as an electrolyte, and a mixed solvent of propylene carbonate, 1,2-dimethoxyethane, and an aliphatic nitrile is used as a non-aqueous solvent. There have been proposed those in which the electrical conductivity of the non-aqueous electrolyte is increased to improve the discharge characteristics of the battery.

  However, when a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte as described above is allowed to stand under high temperature conditions, the aliphatic nitrile contained in the non-aqueous solvent reacts with the positive electrode and negative electrode to decompose. However, there has been a problem that storage characteristics and the like under high temperature conditions are greatly reduced.

  In Patent Document 2, a non-aqueous electrolyte contains a fluorinated nitrile compound, and a lithium ion permeable and stable coating is efficiently generated on the electrode surface from the initial charging, Proposals have been made to improve the charge / discharge efficiency and storage characteristics by suppressing the decomposition of the non-aqueous electrolyte.

  However, in Patent Document 2, if the amount of the fluorinated nitrile compound in the non-aqueous electrolyte is 0.01 to 10% by weight and the amount of the fluorinated nitrile compound is too large, the battery It has an adverse effect on characteristics.

  Moreover, in patent document 3, the non-aqueous electrolyte secondary solution which uses 70-95 volume% of ester solvents containing cyclic ester and the nonaqueous solvent containing 5-30 volume% of nitrile solvents is used for nonaqueous electrolyte solution. A battery that suppresses high-temperature swelling in a battery has been proposed.

  And in this patent document 3, when the quantity of a nitrile-type solvent becomes larger than said range, it is said that a problem will arise in battery performance.

  Here, the nitrile solvent generally has a low viscosity and a high dielectric constant, which contributes to improving the load characteristics in the non-aqueous electrolyte secondary battery, but has a drawback of high reactivity with the electrode. .

  Therefore, if the amount of the nitrile solvent in the non-aqueous electrolyte is increased, when the non-aqueous electrolyte secondary battery is allowed to stand under high temperature conditions, the above nitrile solvent reacts with the electrode to preserve the storage characteristics. Therefore, it is considered that the amount of the nitrile solvent in the non-aqueous electrolyte is reduced as shown in Patent Document 2 and Patent Document 3 described above.

  However, if the amount of the nitrile solvent in the non-aqueous electrolyte is reduced in this way, it is still difficult to improve the load characteristics in the non-aqueous electrolyte secondary battery and sufficiently increase the battery capacity. There was a problem.

JP-A-5-144470 JP 2003-7336 A JP 2005-72003 A

  This invention makes it a subject to solve the above problems in the nonaqueous electrolyte secondary battery using a nonaqueous electrolyte.

  That is, in the present invention, in a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte containing a nitrile solvent as a non-aqueous solvent, in order to improve the load characteristics in the non-aqueous electrolyte secondary battery, Even when the amount of the nitrile solvent in the solvent is increased, the non-aqueous electrolyte can be prevented from reacting with the electrode, and good load characteristics can be obtained, and the battery capacity can be reduced even under high temperature conditions. It is an object of the present invention to be suppressed and to obtain good storage characteristics.

In the present invention, in order to solve the above problems, in the non-aqueous electrolyte for a secondary battery in which a lithium salt of an electrolyte is contained in a non-aqueous solvent, the non-aqueous solvent includes at least the following chemical formula (1 ) And a fluorinated cyclic carbonate, and the non-aqueous solvent contains the fluorinated nitrile in an amount of 40 to 80 % by volume, and the fluorinated cyclic carbonate is 5 It was made to be contained in the range of ˜40% by volume.

CF ₃ -R-CN (1)
In the formula, R represents an alkyl group having 1 or 2 carbon atoms, and a part of hydrogen may be substituted with fluorine.

  Here, when the non-aqueous solvent contains the fluorinated nitrile and the fluorinated cyclic carbonate as described above, the fluorinated nitrile and the fluorinated cyclic carbonate are specifically decomposed mutually on the surface of the electrode. It is considered that a good film is formed on the surface of the electrode, and further decomposition of the fluorinated nitrile on the surface of the electrode is suppressed.

For this reason, in order to improve the load characteristics in the non-aqueous electrolyte secondary battery, the fluorinated nitrile may be contained in the non-aqueous solvent in the range of 40 to 80 % by volume. Further decomposition of nitrile on the surface of the electrode is suppressed, and good storage characteristics can be obtained even under high temperature conditions.

In the present invention, as shown in the chemical formula (1), the fluorinated nitrile in which the terminal carbons are all fluorinated is used because the terminal carbon atoms are not similar to CH ₂ F— or CHF ₂ —. In the case of a fluorinated nitrile in which all of the carbon is not fluorinated and hydrogen remains, the viscosity of the non-aqueous electrolyte increases and the load characteristics of the non-aqueous electrolyte secondary battery cannot be sufficiently improved. It is. In addition, in the case of a nitrile compound in which the terminal carbon is not fluorinated at all, the load characteristics are improved. However, like a fluorinated nitrile, the fluorinated cyclic carbonate and the electrode surface are specifically decomposed to each other. This is because a good film is not formed on the surface of the electrode, and the storage characteristics under high temperature conditions deteriorate.

In the above fluorinated nitrile, when the carbon number of R in the formula increases, the viscosity of the non-aqueous electrolyte also increases in this case, and the load characteristics in the non-aqueous electrolyte secondary battery are sufficiently improved. Can not be made. For this reason, as R, an alkyl group having 1 or 2 carbon atoms or a part of hydrogen substituted with fluorine as described above is used. Preferably, as the fluorinated nitrile, 3, 3 is used. , 3-trifluoropropionitrile CF ₃ CH ₂ CN is used.

Further, if the amount of the fluorinated nitrile in the non-aqueous solvent is small, it becomes difficult to sufficiently improve the load characteristics in the non-aqueous electrolyte secondary battery, while the amount is too large, The amount of the fluorinated cyclic carbonate is relatively lowered, and a good film is not formed on the surface of the electrode. For this reason, in this invention, the quantity of said fluorinated nitrile in a non-aqueous solvent is made to be the range of 40-80 volume% .

  On the other hand, if the amount of the fluorinated cyclic carbonate in the non-aqueous solvent is small, a good film cannot be formed on the surface of the electrode as described above. On the other hand, if the amount is too large, the viscosity of the non-aqueous electrolyte increases. As a result, the load characteristics in the non-aqueous electrolyte secondary battery cannot be sufficiently improved. For this reason, in the present invention, the fluorinated cyclic carbonate is contained in the range of 5 to 40% by volume in the non-aqueous solvent as described above.

  Here, various fluorinated cyclic carbonates can be used as the fluorinated cyclic carbonate, but 4-fluoroethylene carbonate is preferably used.

  When 4-fluoroethylene carbonate is used as the fluorinated cyclic carbonate, if the amount of 4-fluoroethylene carbonate is small, a sufficient film is not formed on the electrode as described above, and the fluorinated nitrile is decomposed. When the water electrolyte secondary battery is left in a charged state under a high temperature condition, the storage characteristics deteriorate. On the other hand, if the amount of 4-fluoroethylene carbonate is excessive, the viscosity of the non-aqueous electrolyte increases and the load is increased. Characteristics are degraded. For this reason, it is more preferable to make the quantity of 4-fluoroethylene carbonate in a non-aqueous solvent into the range of 10-30 volume%.

  In the non-aqueous electrolyte for a secondary battery, other non-aqueous solvent can be added to the non-aqueous solvent in addition to the fluorinated nitrile and the fluorinated cyclic carbonate. Here, as such other non-aqueous solvents, for example, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl acetate, methyl propionate, ethyl acetate, and the like can be used.

  Further, in order to increase the electrical conductivity of the non-aqueous electrolyte, it is possible to mix ethylene carbonate, propylene carbonate, γ-butyrolactone, etc., which are high dielectric constant solvents.

  In the non-aqueous electrolyte for a secondary battery of the present invention, a cyclic carbonate having a carbon double bond C = C can be added as a film forming agent for forming a suitable film on the negative electrode. .

  Here, as the cyclic carbonate having such a carbon double bond C = C, for example, vinylene carbonate, 4,5-dimethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-ethyl-5-methyl Vinylene carbonate, vinyl ethylene carbonate, divinyl ethylene carbonate, and the like can be used. In particular, vinylene carbonate and vinyl ethylene carbonate are preferably used from the viewpoint of forming a good film on the negative electrode.

Moreover, in the non-aqueous electrolyte for secondary batteries of the present invention, as an electrolyte composed of a lithium salt dissolved in the above non-aqueous solvent, a lithium salt generally used in non-aqueous electrolyte secondary batteries can be used. it can. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiClO ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiB (C ₂ O ₄ ) ₂ , LiBF ₂ (C ₂ O ₄ ), etc. In particular, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiB (C ₂ O ₄ ) ₂ are preferably used.

  Further, the non-aqueous electrolyte secondary battery of the present invention is a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. A water electrolyte is used, and the positive electrode active material in the positive electrode and the negative electrode active material in the negative electrode are not particularly limited.

  Here, the positive electrode active material used for the positive electrode of the non-aqueous electrolyte secondary battery is not particularly limited as long as it is a material that can occlude and release lithium and has a noble potential, and is generally used. A known positive electrode active material can be used. For example, lithium transition metal composite oxides having a layered structure, a spinel structure, or an olivine structure can be used singly or in combination, and in particular, to obtain a high energy density non-aqueous electrolyte secondary battery. It is preferable to use a lithium transition metal complex oxide having a layered structure.

  Examples of the lithium transition metal composite oxide having such a layered structure include lithium / cobalt composite oxide, lithium / cobalt / nickel / manganese composite oxide, and lithium / cobalt / nickel / aluminum composite oxide. It is preferable to use a lithium transition metal composite oxide. In particular, from the viewpoint of the stability of the crystal structure, it is preferable to use lithium cobalt oxide in which Al or Mg is dissolved in the crystal and Zr is fixed to the particle surface.

  The negative electrode active material used for the negative electrode of the non-aqueous electrolyte secondary battery is not particularly limited as long as it is a material capable of occluding and releasing lithium, and a commonly used negative electrode active material is used. be able to. For example, lithium metal such as lithium metal, lithium-aluminum alloy, lithium-lead alloy, lithium-silicon alloy, lithium-tin alloy, carbon materials such as graphite, coke, organic fired body, lithium / titanium composite oxide, lithium Metal oxides that have a lower potential than the positive electrode active material can be used, such as vanadium composite oxides, and in particular, graphite-based carbon materials that are less reversible and have excellent reversibility due to the volume change associated with insertion and extraction of lithium are used. It is preferable.

In the present invention, the non-aqueous electrolyte in the non-aqueous electrolyte secondary battery includes the fluorinated nitrile represented by the chemical formula (1) in the range of 40 to 80 % by volume in the non-aqueous solvent. Since the fluorinated cyclic carbonate contained 5 to 40% by volume is used, the above fluorinated nitrile improves the load characteristics in the non-aqueous electrolyte secondary battery, and the fluorinated nitrile. And fluorinated cyclic carbonate are specifically decomposed on the surface of the electrode to form a good coating on the surface of the electrode, and further decomposition of the fluorinated nitrile on the surface of the electrode is suppressed. Become so.

  As a result, in the non-aqueous electrolyte secondary battery using such a non-aqueous electrolyte, the load characteristics are improved by the fluorinated nitrile and the fluorinated nitrile is continuously decomposed even under high temperature conditions. Therefore, the battery capacity is prevented from decreasing under high temperature conditions, and good storage characteristics can be obtained.

It is a schematic sectional drawing of the nonaqueous electrolyte secondary battery produced in the Example and comparative example of this invention.

  Next, the non-aqueous electrolyte for a secondary battery and the non-aqueous electrolyte secondary battery according to the present invention will be specifically described with examples, and in the non-aqueous electrolyte secondary battery according to this example, A comparative example will clarify that load characteristics are improved and that battery capacity is prevented from decreasing under high-temperature conditions and storage characteristics are improved. The non-aqueous electrolyte for secondary battery and the non-aqueous electrolyte secondary battery of the present invention are not limited to those shown in the following examples, and can be appropriately modified and implemented without departing from the scope of the invention. Is.

Example 1
In Example 1, a nonaqueous electrolyte secondary battery having a cylindrical shape and a design capacity of 2300 mAh as shown in FIG. 1 was prepared using a positive electrode, a negative electrode, and a nonaqueous electrolyte prepared as described below.

[Production of positive electrode]
In producing the positive electrode, as the positive electrode active material, 1.0 mol% of Al and Mg were respectively dissolved in lithium cobalt oxide LiCoO ₂ and 0.05 mol% of Zr was added to the particle surface of the solid solution. Using.

  Then, the positive electrode active material, the carbon of the conductive agent, and the polyvinylidene fluoride of the binder are in a weight ratio of 95: 2.5: 2.5, and these are converted into N-methyl-2- A positive electrode mixture slurry was prepared by kneading in a pyrrolidone solution. And this positive electrode mixture slurry was apply | coated on both surfaces of the positive electrode electrical power collector which consists of aluminum foil, and after drying this, it rolled and produced the positive electrode.

[Production of negative electrode]
In producing the negative electrode, the negative electrode active material graphite, the binder styrene-butadiene rubber, and the thickener carboxymethyl cellulose in a weight ratio of 97.5: 1.5: 1, These were kneaded in an aqueous solution to prepare a negative electrode mixture slurry. And this negative electrode mixture slurry was apply | coated on both surfaces of the negative electrode collector which consists of copper foils, and after drying this, it rolled and produced the negative electrode.

[Preparation of non-aqueous electrolyte]
In preparing the non-aqueous electrolyte, as the non-aqueous solvent, 4-fluoroethylene carbonate (FEC), which is a fluorinated cyclic carbonate, and 3,3,3-, which is the fluorinated nitrile shown in the above chemical formula (1), are used. Using a mixed solvent in which trifluoropropionitrile CF ₃ CH ₂ CN was mixed at a volume ratio of 2: 8, lithium hexafluorophosphate LiPF ₆ was dissolved as an electrolyte in the mixed solvent at a ratio of 1 mol / l, A non-aqueous electrolyte was prepared.

Here, a method for synthesizing the 3,3,3-trifluoropropionitrile CF ₃ CH ₂ CN used as the fluorinated nitrile is shown below.

  Into a 1 liter eggplant type flask equipped with a nitrogen-sealed Dimroth condenser and a thermometer, 114 g (0.90 mol) of 3,3,3-trifluoropropionamide, 600 ml of xylene and 64 g of diphosphorus pentoxide (0 .45 mol / 0.5 eq) was added and heated to 130-145 ° C. with stirring and refluxed for 1 hour. Then, after cooling this to room temperature, the distillation apparatus was attached to the flask and it distilled at normal pressure, and the fraction of 92-97 degreeC was collected. Subsequently, after drying with molecular sieve 4A, the product was purified by distillation again to obtain 57 g (0.52 mol, yield 58%) of 3,3,3-trifluoropropionitrile.

  And in producing a non-aqueous electrolyte secondary battery, as shown in FIG. 1, a lithium ion permeable polyethylene-made separator 3 is provided between the positive electrode 1 and the negative electrode 2 produced as described above. After interposing a microporous film and winding them in a spiral shape and accommodating them in the battery can 4, the non-aqueous electrolyte solution is injected into the battery can 4 and sealed, and the positive electrode 1 is connected to the positive electrode. The tab 1a is connected to the positive electrode external terminal 5a attached to the positive electrode lid 5, and the negative electrode 2 is connected to the battery can 4 by the negative electrode tab 2a. The battery can 4 and the positive electrode lid 5 are electrically connected by the insulating packing 6. Separated.

(Example 2)
In Example 2, in the preparation of the non-aqueous electrolyte in Example 1 above, 4-fluoroethylene carbonate (FEC), which is a fluorinated cyclic carbonate, and fluorine shown in the above chemical formula (1) are used as the non-aqueous solvent. Nitrile 3,3,3-trifluoropropionitrile CF ₃ CH ₂ CN and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 2: 4: 4, and other than that, A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 above.

(Comparative Example 1)
In Comparative Example 1, a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 7 was used as a non-aqueous solvent, and hexafluorophosphorus was used as an electrolyte in the mixed solvent. Lithium acid LiPF ₆ was dissolved at a rate of 1 mol / l, and a nonaqueous electrolytic solution to which vinylene carbonate (VC) was added at a rate of 2% by weight was used. A non-aqueous electrolyte secondary battery was produced in the same manner as in the case.

(Comparative Example 2)
In Comparative Example 2, in the preparation of the non-aqueous electrolyte in Example 1 above, 4-fluoroethylene carbonate (FEC), which is a fluorinated cyclic carbonate, and non-fluorinated propionitrile CH ₃ are used as non-aqueous solvents. A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 except that a mixed solvent in which CH ₂ CN was mixed at a volume ratio of 2: 8 was used.

(Comparative Example 3)
In Comparative Example 3, in the preparation of the non-aqueous electrolyte in Example 1 described above, fluorinated cyclic carbonate 4-fluoroethylene carbonate (FEC) and non-fluorinated propionitrile CH ₃ were used as the non-aqueous solvent. Using a mixed solvent in which CH ₂ CN and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 2: 4: 4, other than that, the non-aqueous electrolyte 2 was used in the same manner as in Example 1 above. A secondary battery was produced.

(Comparative Example 4)
In Comparative Example 4, the preparation of the non-aqueous electrolyte in Example 1 described above was shown as non-fluorinated cyclic carbonate ethylene carbonate (EC) as the non-aqueous solvent and the above chemical formula (1). A mixed solvent obtained by mixing fluorinated nitrile 3,3,3-trifluoropropionitrile CF ₃ CH ₂ CN in a volume ratio of 2: 8 was used, and other than that, the same as in the case of Example 1 above. Thus, a non-aqueous electrolyte secondary battery was produced.

  Next, the non-aqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 4 manufactured as described above were charged to 4.2 V at a constant current of 460 mA at 25 ° C., respectively. Furthermore, after charging at a constant voltage of 4.2 V until the current value reaches 46 mA, the battery is discharged at a constant current of 460 mA until it reaches 2.75 V, and the initial discharge capacity of each non-aqueous electrolyte secondary battery Was measured.

  Then, assuming that the initial discharge capacity in the nonaqueous electrolyte secondary battery of Comparative Example 1 was 100, the initial discharge capacity of each nonaqueous electrolyte secondary battery was calculated, and the results are shown in Table 1 below. In the table, 4-fluoroethylene carbonate is indicated as FEC, ethylene carbonate as EC, ethyl methyl carbonate as EMC, and vinylene carbonate as VC.

  As a result, in each of the nonaqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3, the initial charge / discharge could be performed and the initial discharge capacity was almost the same.

On the other hand, as a non-aqueous solvent, ethylene carbonate (EC), which is a non-fluorinated cyclic carbonate, and 3,3,3-trifluoropropionitrile CF ₃ CH ₂ CN, which is a fluorinated nitrile, In the non-aqueous electrolyte secondary battery of Comparative Example 4 in which the ratio of the fluorinated nitrile in the non-aqueous solvent was increased to 80% by volume, the current value did not decay during constant voltage charging, and abnormal charging occurred. The discharge could not be performed. This is because when the amount of the fluorinated nitrile is increased without containing the fluorinated cyclic carbonate in the non-aqueous solvent, the decomposition of the fluorinated nitrile proceeds on the surface of the electrode, which is described in Patent Document 2 and Patent Document 3 described above. It is considered that this is because the fluorinated nitrile acted as a material that adversely affects the battery characteristics.

  Next, each of the nonaqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3 was charged to 4.2 V at a constant current of 2300 mA at 25 ° C., and then 4.2 V. The battery was charged at a constant voltage until the current value reached 46 mA at a constant voltage of.

Each of the non-aqueous electrolyte secondary batteries charged in this way has a discharge capacity Q _0.2C and 4600 mA (2C) of discharge capacity when discharged to 2.75 V at a current of 460 mA (0.2 C). a discharge capacity _{Q 2C} when was discharged at a current to 2.75V were measured.

Then, as the load characteristics, determine the volume ratio of the discharge capacity _{Q 2C} in 2C to the discharge capacity _{Q 0.2 C} at 0.2 C (%) by the following equation. The results are shown in Table 2 below.

Capacity ratio (%) = (Q _2C / Q _0.2C ) × 100

  As a result, each of the nonaqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 2 and 3 in which a nitrile solvent composed of a fluorinated nitrile or a non-fluorinated nitrile was contained in the nonaqueous solvent was Compared with the non-aqueous electrolyte secondary battery of Comparative Example 1 which did not contain the nitrile solvent, the capacity ratio was high, and the load characteristics were improved. This is considered to be because a non-aqueous solvent contains a nitrile solvent having a low viscosity and a high dielectric constant.

Moreover, about each nonaqueous electrolyte secondary battery of said Examples 1, 2 and Comparative Examples 1-3, it charged until it became 4.2V with a constant current of 460 mA at 25 degreeC, respectively, and also 4.2V After charging at a constant voltage until the current value reached 46 mA, the battery was discharged at a constant current of 460 mA until it reached 2.75 V, and the discharge capacity D ₁ before storage was measured.

Next, each non-aqueous electrolyte secondary battery is charged at 25 ° C. with a constant current of 2300 mA until it reaches 4.2 V, and further with a constant voltage of 4.2 V, a constant voltage until the current value reaches 46 mA. In this state, each non-aqueous electrolyte secondary battery was stored in a thermostatic bath at 60 ° C. for 10 days, and each non-aqueous electrolyte secondary battery after storage was stored at a constant temperature of 460 mA at 25 ° C. The remaining capacity D ₂ after storage was determined by discharging until the current reached 2.75V.

Then, based on the discharge capacity D ₁ before storage and the remaining capacity D ₂ after storage measured as described above, the nonaqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 3 are expressed by the following equations. The residual capacity rate (%) after storage was determined, and the results are shown in Table 3 below.

Capacity remaining rate (%) = (D ₂ / D ₁ ) × 100

As a result, a mixed solvent of 4-fluoroethylene carbonate (FEC), which is a fluorinated cyclic carbonate, and CF ₃ CH ₂ CN, which is a fluorinated nitrile represented by the above chemical formula (1), was used as a non-aqueous solvent. The non-aqueous electrolyte secondary batteries of Examples 1 and 2 were compared with the non-aqueous electrolyte secondary batteries of Comparative Examples 2 and 3 using a non-fluorinated nitrile as a nitrile solvent. The non-aqueous solvent in which vinylene carbonate (VC) was added to a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), which has been widely used in the past, was used. Compared to the nonaqueous electrolyte secondary battery of Comparative Example 1, the capacity remaining rate after storage was improved.

  This is because when the nitrile compound in which the terminal carbon is not fluorinated at all is used as in the non-aqueous electrolyte secondary batteries of Comparative Examples 2 and 3, the non-aqueous solutions of Examples 1 and 2 using fluorinated nitriles. Unlike the electrolyte secondary battery, the fluorinated cyclic carbonate and the surface of the electrode are not specifically decomposed to form a good film on the surface of the electrode. It is considered that the storage characteristics under high temperature conditions are reduced as compared with the non-aqueous electrolyte secondary battery.

(Comparative Example 5)
In Comparative Example 5, in the preparation of the non-aqueous electrolyte in Example 1 above, as the non-aqueous solvent, fluorinated cyclic carbonate 4-fluoroethylene carbonate (FEC), propylene carbonate (PC), and ethyl methyl carbonate A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 except that a mixed solvent in which (EMC) was mixed at a volume ratio of 20: 5: 75 was used.

(Comparative Example 6)
In Comparative Example 6, in the preparation of the non-aqueous electrolyte in Example 1 above, as the non-aqueous solvent, fluorinated cyclic carbonates 4-fluoroethylene carbonate (FEC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) was mixed in a volume ratio of 20: 5: 75, and lithium hexafluorophosphate LiPF ₆ was dissolved as an electrolyte in the mixed solvent at a ratio of 1 mol / l. A nonaqueous electrolytic solution to which 3,3,3-trifluoropropionitrile CF ₃ CH ₂ CN, which is a fluorinated nitrile represented by the above chemical formula (1), was added at a ratio of 1% by weight was used. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1. In addition, the ratio in said non-aqueous solvent of said fluorinated nitrile was about 1 volume%.

  And about each nonaqueous electrolyte secondary battery of Comparative Examples 5 and 6 produced in this way, the initial discharge capacity was measured in the same manner as the nonaqueous electrolyte secondary battery of Example 1 and the like, The initial discharge capacity of the non-aqueous electrolyte secondary battery of Comparative Example 5 was set to 100, and the initial discharge capacity of each non-aqueous electrolyte secondary battery was calculated. The results are shown in Table 4 below.

Further, each non-aqueous electrolyte secondary battery of Comparative Examples 5 and 6 was also discharged at 0.2 C as load characteristics in the same manner as the non-aqueous electrolyte secondary battery of Example 1 and the like. together determine the capacity ratio of the discharge capacity Q _2C in 2C to volume Q _{0.2 C} (%), obtains the residual capacity ratio (%) after storage at each of the non-aqueous electrolyte secondary batteries, these results below It is shown in Table 4.

As a result, Comparative Example 6 in which 3,3,3-trifluoropropionitrile CF ₃ CH ₂ CN, which is a fluorinated nitrile represented by the above chemical formula (1), was contained in only about 1% by volume in the non-aqueous solvent. The non-aqueous electrolyte secondary battery has the same capacity ratio as the non-aqueous electrolyte secondary battery of Comparative Example 5 that does not contain the fluorinated nitrile, and the load characteristics are not improved. On the contrary, the capacity remaining rate was slightly reduced, and the storage characteristics under high temperature conditions were not improved. For this reason, it is understood that when the non-aqueous solvent contains the fluorinated nitrile and the fluorinated cyclic carbonate represented by the chemical formula (1), a sufficient effect cannot be obtained if the amount of the fluorinated nitrile is small.

DESCRIPTION OF SYMBOLS 1 Positive electrode 1a Positive electrode tab 2 Negative electrode 2a Negative electrode tab 3 Separator 4 Battery can 5 Positive electrode cover 5a Positive electrode external terminal 6 Insulation packing

Claims (5)

In a non-aqueous electrolyte for a secondary battery in which an electrolyte lithium salt is contained in a non-aqueous solvent, the non-aqueous solvent includes at least a fluorinated nitrile represented by the following chemical formula (1) and a fluorinated cyclic carbonate. In the non-aqueous solvent, the fluorinated nitrile is contained in the range of 40 to 80 % by volume, and the fluorinated cyclic carbonate is contained in the range of 5 to 40% by volume. Non-aqueous electrolyte for secondary battery.
CF ₃ -R-CN (1)
In the formula, R represents an alkyl group having 1 or 2 carbon atoms, and a part of hydrogen may be substituted with fluorine. The non-aqueous electrolyte for secondary batteries according to claim 1, wherein the fluorinated nitrile is CF ₃ CH ₂ CN. The nonaqueous electrolytic solution for a secondary battery according to claim 1 or 2 , wherein the fluorinated cyclic carbonate is 4-fluoroethylene carbonate. 4. The secondary battery according to claim 3 , wherein the 4-fluoroethylene carbonate is contained in a non-aqueous solvent in a range of 10 to 30% by volume. Non-aqueous electrolyte for use. In the non-aqueous electrolyte secondary battery provided with the positive electrode, the negative electrode, and the non-aqueous electrolyte, the non-aqueous electrolyte for secondary batteries of any one of Claims 1-4 is used for the non-aqueous electrolyte. A non-aqueous electrolyte secondary battery characterized by

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