JP2007250440A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably obtain superior output characteristics under a low temperature condition even in the case that the amount of a lithium salt having oxalate complex as an anion is changed in a nonaqueous electrolyte secondary battery in which the lithium salt having an oxalate complex as the anion has been added to a nonaqueous electrolytic solution. <P>SOLUTION: In the nonaqueous electrolyte secondary battery provided with a positive electrode 1 and a negative electrode 2 capable of storing and releasing lithium and with the nonaqueous electrolytic solution having lithium ion conductivity in which a solvent is dissolved into a nonaqueous solvent, the lithium salt having the oxalate complex as the anion is added to the nonaqueous electrolytic solution, and fibrous carbon is contained in the positive electrode as a conductive agent. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and a non-aqueous electrolyte having lithium ion conductivity in which a solute is dissolved in a non-aqueous solvent. It is characterized in that excellent output characteristics can be stably obtained under low temperature conditions.

  Conventionally, as a new secondary battery with high output and high energy density, a non-aqueous electrolyte secondary battery that uses a non-aqueous electrolyte and moves lithium ions between the positive electrode and the negative electrode to charge and discharge is used. Is being used.

  In recent years, the use of such non-aqueous electrolyte secondary batteries as power sources for electric vehicles and the like has been studied.

  Here, when the nonaqueous electrolyte secondary battery is used for a power source of an electric vehicle or the like, it is required to improve storage durability under high temperature conditions and output characteristics under low temperature conditions.

Therefore, in recent years, as disclosed in Patent Document 1, oxalate such as LiBF ₂ (C ₂ O ₄ ) and LiPF ₂ (C ₂ O ₄ ) ₂ is used as a non-aqueous electrolyte in a non-aqueous electrolyte secondary battery. It has been proposed to add a lithium salt having a complex as an anion to improve storage durability under high temperature conditions and output characteristics under low temperature conditions.

Here, as shown in Patent Document 1, when a lithium salt having an oxalato complex such as LiBF ₂ (C ₂ O ₄ ) or LiPF ₂ (C ₂ O ₄ ) ₂ as an anion is added to the nonaqueous electrolytic solution As shown in FIG. 4, the output characteristics under low temperature conditions are greatly changed by the change in the amount of LiPF ₂ (C ₂ O ₄ ) ₂ added, and the non-aqueous electrolyte is excellent in output characteristics under low temperature conditions. In obtaining a secondary battery, the amount of LiPF ₂ (C ₂ O ₄ ) ₂ to be added must be set appropriately, which complicates quality control in the production of nonaqueous electrolyte secondary batteries. It was.

Further, when this non-aqueous electrolyte secondary battery is charged and discharged, the LiPF ₂ (C ₂ O ₄ ) ₂ in the non-aqueous electrolyte is consumed and LiPF ₂ (C ₂ O in the non-aqueous electrolyte is consumed. ₄ ) There was a problem that the amount of ₂ changed, and the output characteristics under low temperature conditions changed accordingly, and stable low temperature output characteristics could not be obtained.

  The present invention provides a nonaqueous electrolyte secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and a nonaqueous electrolyte solution having lithium ion conductivity in which a solute is dissolved in a nonaqueous solvent. In particular, when a lithium salt having an oxalato complex as an anion is added to a non-aqueous electrolyte, the amount of the lithium salt having the oxalato complex as an anion changes. However, it is an object to stably obtain excellent output characteristics under low temperature conditions.

  In order to solve the above-described problems, the present invention includes a positive electrode and a negative electrode capable of occluding and releasing lithium, and a non-aqueous electrolyte having lithium ion conductivity in which a solute is dissolved in a non-aqueous solvent. In the nonaqueous electrolyte secondary battery, a lithium salt having an oxalato complex as an anion was added to the nonaqueous electrolyte solution, and fibrous carbon was included as a conductive agent in the positive electrode.

Here, as the fibrous carbon included in the positive electrode, it is preferable to use carbon having a fiber diameter in the range of 50 to 300 nm and a fiber length in the range of 5 to 100 μm in order to sufficiently reduce the resistance in the positive electrode. The amount of fibrous carbon is preferably 1 to 10% by weight in the positive electrode mixture, and the specific surface area is preferably 30 m ² / g or less. The fibrous carbon may be used by mixing with acetylene black.

  Moreover, as a lithium salt which uses the oxalato complex added to said non-aqueous electrolyte as an anion, it is preferable to use what is shown in the following chemical formula 1, but what is shown in the following chemical formula 2 can also be used similarly. .

式中、Ｘは遷移金属、周期律表の１３，１４又は１５族の元素、Ｍはハロゲン元素であり、ｎは０〜４の整数、ｍは１〜３の整数である。

In the formula, X is a transition metal, an element of Group 13, 14 or 15 of the periodic table, M is a halogen element, n is an integer of 0 to 4, and m is an integer of 1 to 3.

As the solute used in the non-aqueous electrolyte described above, there can be used those which are generally used in a nonaqueous electrolyte secondary battery, for _{example, LiPF 6, LiBF 4, LiCF} 3 SO 3, 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 , LiAsF ₆ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ , a mixture thereof, or the like can be used.

  Here, the reason why the output characteristics under low temperature conditions are stabilized by adding the oxalato complex to the non-aqueous electrolyte solution is not clear, but the oxalato complex is not only the negative electrode but also directly or indirectly to the positive electrode. It is considered that when the electronic conductivity of the positive electrode is improved, especially when fibrous carbon is included as a conductive agent, the output characteristics under low temperature conditions are stabilized.

In addition, when adding a lithium salt having the oxalato complex as an anion to the non-aqueous electrolyte, if the addition amount is small, the output characteristics under low temperature conditions cannot be sufficiently improved, while the addition amount However, if the amount added is too large, the viscosity of the non-aqueous electrolyte will increase and the lithium ion conductivity will decrease and gas generation will increase. For example, when LiPF ₆ is used as the solute of the non-aqueous electrolyte, a lithium salt having the oxalato complex as an anion is in the range of 0.001 to 0.1 mol / l with respect to the non-aqueous electrolyte. Is preferably added.

  In addition, as the non-aqueous solvent in the non-aqueous electrolyte, a known non-aqueous solvent that is generally used can be used. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, dimethyl carbonate, methyl Chain carbonates such as ethyl carbonate and diethyl carbonate can be used, and it is preferable to use a mixed solvent in which a cyclic carbonate and a chain carbonate are mixed from the viewpoint of the stability and ionic conductivity of the non-aqueous electrolyte. It is preferable to contain vinylene carbonate.

  Furthermore, when vinylene carbonate is contained in the non-aqueous electrolyte, a film made of lithium salt and vinylene carbonate having the above oxalato complex as an anion is formed on the surface of the negative electrode. The storage durability below is also improved. When the amount of vinylene carbonate contained in the non-aqueous electrolyte increases, the film formed on the surface of the negative electrode by this vinylene carbonate becomes too thick, and the low temperature based on the film of lithium salt having the above oxalato complex as an anion is low. Since the effect of improving the output characteristics is reduced, the ratio of vinylene carbonate to the nonaqueous electrolytic solution is preferably in the range of 0.5 to 2 wt%.

  Further, in the nonaqueous electrolyte secondary battery of the present invention, a publicly known negative electrode active material can be used as the negative electrode active material used for the negative electrode, and the energy density and battery voltage in the nonaqueous electrolyte secondary battery can be used. In order to increase the thickness, it is preferable to use carbon material graphite.

  In the non-aqueous electrolyte secondary battery according to the present invention, the lithium salt having an oxalato complex as an anion is added to the non-aqueous electrolyte as described above, and fibrous carbon is included as a conductive agent in the positive electrode. Even if the amount of lithium salt having the above oxalate complex as an anion in the non-aqueous electrolyte is changed, the output characteristics under low temperature conditions are improved even when the resistance at the positive electrode is sufficiently reduced by the fibrous carbon. The change is prevented, and excellent output characteristics can be stably obtained under low temperature conditions.

  Hereinafter, the non-aqueous electrolyte secondary battery according to the present invention will be specifically described with reference to examples. It will be clarified by a comparative example that excellent output characteristics can be stably obtained even under a low temperature condition even when the amount of lithium salt to be changed is changed. The nonaqueous electrolyte secondary battery in the present invention is not limited to those shown in the following examples, and can be implemented with appropriate modifications within the scope not changing the gist thereof.

Example 1
In Example 1, a negative electrode, a positive electrode, and a non-aqueous electrolyte produced as described below were used, and a non-aqueous electrolyte 2 having a cylindrical 18650 size as shown in FIG. 1 and a discharge capacity of 1.4 Ah was used. A secondary battery was produced.

[Production of positive electrode]
In producing the positive electrode, a powder of lithium / nickel / cobalt / manganese composite oxide having a layered structure represented by LiNi _0.4 Co _0.3 Mn _0.3 O ₂ is used as the positive electrode active material, and the conductive agent has a fiber diameter Vapor-grown fibrous carbon having a thickness of 150 nm and a fiber length in the range of 5 to 20 μm was used.

  Then, the positive electrode active material, the conductive agent, and the polyvinylidene fluoride as the binder are in a weight ratio of 90: 5: 5, and N-methyl-2-pyrrolidone is added thereto and kneaded to mix the positive electrode. The agent slurry was prepared, this positive electrode mixture slurry was applied to both surfaces of a positive electrode current collector made of aluminum foil, dried, and then rolled with a rolling roller to produce a positive electrode.

[Production of negative electrode]
In producing the negative electrode, graphite powder is used as the negative electrode active material, and the graphite powder, the binder styrene-butadiene rubber, and the thickening agent carboxymethylcellulose are in a weight ratio of 98: 1: 1. Thus, the above graphite powder, styrene-butadiene rubber and carboxymethyl cellulose aqueous solution were kneaded to prepare a negative electrode mixture slurry, and this negative electrode mixture slurry was applied to both surfaces of a current collector made of copper foil. After drying, this was rolled with a rolling roller to produce a negative electrode.

[Preparation of non-aqueous electrolyte]
In preparing the non-aqueous electrolyte, LiPF ₆ was used as a solute in a mixed solvent in which ethylene carbonate of cyclic carbonate and ethylmethyl carbonate and dimethyl carbonate of chain carbonate were mixed at a volume ratio of 3: 3: 4. It is dissolved at a concentration of 1 mol / l, 1 wt% of vinylene carbonate is added to the non-aqueous electrolyte, and LiBF ₂ (C ₂ O, which is a lithium salt having an oxalato complex as an anion to the non-aqueous electrolyte. ₄ ) was added at a rate of 0.001 mol / l.

[Production of battery]
In producing the battery, as shown in FIG. 1, a lithium ion-permeable polyethylene microporous film is interposed as a separator 3 between the positive electrode 1 and the negative electrode 2 produced as described above. Is spirally wound and accommodated in the battery can 4, the nonaqueous electrolyte is poured into the battery can 4 and sealed, and the positive electrode 1 is connected to the positive electrode lid via the positive electrode lead 5. 6 and the negative electrode 2 is connected to the battery can 4 via the negative electrode lead 7, and the battery can 4 and the positive electrode lid 6 are electrically separated by the insulating packing 8. I let you.

(Examples 2 to 5)
In Examples 2 to 5, LiBF ₂ (C ₂ O ₄ ), which is a lithium salt having an oxalato complex as an anion in the non-aqueous electrolyte in the preparation of the non-aqueous electrolyte in Example 1 above, is used. The nonaqueous electrolyte secondary batteries of Examples 2 to 5 were produced in the same manner as in Example 1 except that only the ratio of addition was changed.

Here, the ratio of adding LiBF ₂ (C ₂ O ₄ ), which is a lithium salt having an oxalato complex as an anion, to the nonaqueous electrolytic solution is 0.005 mol / l in Example 2, and is 0.00 in Example 3. 01 mol / l, 0.05 mol / l in Example 4, and 0.1 mol / l in Example 5.

(Comparative Example 1)
In Comparative Example 1, LiBF ₂ (C ₂ O ₄ ), which is a lithium salt having the above oxalato complex as an anion, was not added in the preparation of the non-aqueous electrolyte in Example 1 above, and otherwise, A non-aqueous electrolyte secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 above.

  And the output characteristic in -30 degreeC was measured about each nonaqueous electrolyte secondary battery of Examples 1-5 and Comparative Example 1 produced as mentioned above.

  Here, in measuring the output characteristics at −30 ° C., each of the nonaqueous electrolyte secondary batteries of Examples 1 to 5 and Comparative Example 1 was charged with a charging current of 1.4 A at room temperature of 25 ° C., respectively. After charging until the depth of charge (SOC) reaches 50%, the battery is cooled to −30 ° C., and the battery surface temperature is confirmed to be −30 ° C., and then 0.24A, 0.70A, Charging and discharging were carried out for 10 seconds at a current of 2.4 A and 4.2 A, and the battery voltage at the time of discharging was measured.

  For each non-aqueous electrolyte secondary battery, the current value and the measured battery voltage are plotted to determine the IV characteristics at the time of discharge, and the IV resistance at the time of discharge is determined from the slope of the obtained straight line. An extrapolated current value at the lower limit voltage of 3.0 V is calculated, an output represented by the product of the IV resistance and the extrapolated current value is obtained, and an output in the nonaqueous electrolyte secondary battery of Comparative Example 1 is 1 As the output of each non-aqueous electrolyte secondary battery of Examples 1 to 5, the results are shown in Table 1 and FIG.

As a result, LiBF ₂ (C ₂ O ₄ ), which is a lithium salt having an oxalato complex as an anion, was added in a range of 0.001 to 0.1 mol / l with respect to the non-aqueous electrolyte in the preparation of the non-aqueous electrolyte. In each of the nonaqueous electrolyte secondary batteries of Examples 1 to 5, the nonaqueous electrolyte secondary battery of Comparative Example 1 in which LiBF ₂ (C ₂ O ₄ ), which is a lithium salt having an oxalato complex as an anion, was not added. Compared with the secondary battery, the output characteristics at −30 ° C. were greatly improved.

In each of the nonaqueous electrolyte secondary batteries of Examples 1 to 5, LiBF ₂ (C ₂ O ₄ ), which is a lithium salt having an oxalato complex as an anion as described above, is added to the nonaqueous electrolyte solution in an amount of 0.0. In addition to being added in the range of 001 to 0.1 mol / l, since the vapor-grown fibrous carbon was included in the positive electrode as a conductive agent, the amount of LiBF ₂ (C ₂ O ₄ ) added to the non-aqueous electrolyte changed. However, unlike the nonaqueous electrolyte secondary battery disclosed in Patent Document 1, the output characteristics under low temperature conditions are not greatly changed, and excellent output characteristics can be stably obtained under low temperature conditions. I understood.

It is a schematic sectional drawing of the nonaqueous electrolyte secondary battery produced in Examples 1-5 and Comparative Example 1 of this invention. It is the figure which showed the output characteristic on the low temperature conditions of the nonaqueous electrolyte secondary battery in said Examples 1-5 and Comparative Example 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Battery can 5 Positive electrode lead 6 Positive electrode lid 7 Negative electrode lead 8 Insulation packing 9 Positive electrode external terminal

Claims (6)

  In the nonaqueous electrolyte secondary battery comprising a positive electrode and a negative electrode capable of occluding and releasing lithium, and a nonaqueous electrolyte having lithium ion conductivity in which a solute is dissolved in a nonaqueous solvent, the nonaqueous electrolyte described above A non-aqueous electrolyte secondary battery comprising: a lithium salt having an oxalato complex as an anion and a fibrous carbon as a conductive agent in the positive electrode.   2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the fibrous carbon has a fiber diameter in the range of 50 to 300 nm and a fiber length in the range of 5 to 100 μm. Electrolyte secondary battery. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium salt having the oxalato complex as an anion added to the non-aqueous electrolyte is represented by the following chemical formula 1: A non-aqueous electrolyte secondary battery.

The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein LiPF ₆ is used as a solute of the non-aqueous electrolyte and a lithium salt having the oxalato complex as an anion is used as the non-aqueous electrolyte secondary battery. A non-aqueous electrolyte secondary battery, which is added in a range of 0.001 to 0.1 mol / l with respect to an aqueous electrolyte.   5. The non-aqueous electrolyte secondary battery according to claim 1, wherein vinylene carbonate is contained in the non-aqueous solvent in the non-aqueous electrolyte. battery.   6. The non-aqueous electrolyte secondary battery according to claim 5, wherein the ratio of vinylene carbonate to the non-aqueous electrolyte is in the range of 0.5 to 2 wt%.

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