WO2012073642A1 - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

Provided is a nonaqueous electrolyte secondary battery which has improved large current discharge characteristics as well as improved cycle characteristics. A battery element (10) of a nonaqueous electrolyte secondary battery is provided with a positive electrode (11), a negative electrode (12) and a nonaqueous electrolyte solution. The negative electrode (12) contains a spinel-type lithium titanium complex oxide. The nonaqueous electrolyte solution contains a nonaqueous solvent in which propylene carbonate and γ-butyrolactone are mixed at a volume ratio of from 6:4 to 4:6.

Description

Non-aqueous electrolyte secondary battery

The present invention generally relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery having improved cycle characteristics as well as discharge rate characteristics.

Conventionally, in a non-aqueous electrolyte secondary battery, generally, for example, a non-aqueous electrolyte obtained by dissolving a lithium salt such as lithium hexafluorophosphate as an electrolyte in a non-aqueous solvent such as ethylene carbonate or dimethyl carbonate. , A lithium transition metal composite oxide as a positive electrode active material, and a carbon material as a negative electrode active material. In such a non-aqueous electrolyte secondary battery, it has been proposed to use γ-butyrolactone as a thermally stable non-aqueous solvent having a high boiling point.

However, as described in Japanese Patent Application Laid-Open No. 2004-296181 (hereinafter referred to as Patent Document 1), in a lithium secondary battery using γ-butyrolactone as a solvent for a nonaqueous electrolytic solution, a natural negative electrode active material is used. When graphite is used, there are problems that charging / discharging becomes difficult and the capacity decreases during storage in a discharged state. This problem is solved by using a spinel type lithium titanium composite oxide such as lithium titanate as the negative electrode active material.

Therefore, for example, as described in JP-A-2006-318797 (hereinafter referred to as Patent Document 2), spinel type lithium titanate is used as the negative electrode active material, and propylene carbonate, ethylene carbonate is used as the non-aqueous solvent. And non-aqueous electrolyte secondary batteries using a mixed solvent in which two or more of the group consisting of γ-butyrolactone are mixed have been proposed. Patent Document 2 describes an example in which a non-aqueous electrolyte secondary battery was produced using a mixed solvent of ethylene carbonate and γ-butyrolactone (volume ratio 1: 2).

On the other hand, in Patent Document 1, it is preferable that γ-butyrolactone is used as a main solvent in the nonaqueous electrolytic solution, and specifically, it is preferable that 90% or more of γ-butyrolactone is contained in the entire mixed solvent. Are listed.

JP 2004-296181 A JP 2006-318797 A

However, as a result of various studies on the composition of a mixed solvent comprising a combination of non-aqueous solvents as disclosed in Patent Document 2, the inventors have found that the content ratio of γ-butyrolactone in the mixed solvent of propylene carbonate and γ-butyrolactone It has been found that the cycle characteristics deteriorate while the large current discharge characteristics are improved when the value is increased. The present invention has been made to solve the above problems.

Therefore, an object of the present invention is to provide a non-aqueous electrolyte secondary battery capable of improving cycle characteristics as well as large current discharge characteristics.

The non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode includes a spinel type lithium titanium composite oxide. The non-aqueous electrolyte contains a non-aqueous solvent in which propylene carbonate and γ-butyrolactone are mixed at a volume ratio of 6: 4 to 4: 6.

In the non-aqueous electrolyte secondary battery of the present invention, it is preferable that the non-aqueous electrolyte contains trioctyl phosphate.

In the non-aqueous electrolyte secondary battery of the present invention, the negative electrode includes a spinel-type lithium titanium composite oxide, and the non-aqueous electrolyte includes propylene carbonate and γ-butyrolactone in a volume ratio of 6: 4 to 4: 6. Since the non-aqueous solvent mixed in (1) is included, it is possible to improve cycle characteristics as well as large current discharge characteristics.

It is a perspective view which fractures | ruptures and shows a part of external appearance of the nonaqueous electrolyte secondary battery produced in the Example of this invention. It is sectional drawing which shows schematically the structure of the battery element accommodated in the outer packaging member of the non-aqueous-electrolyte secondary battery shown by FIG. It is a figure which shows the relationship between the discharge rate and discharge capacity in the nonaqueous electrolyte secondary battery produced in Example A of this invention. It is a figure which shows the relationship between the discharge rate and discharge capacity maintenance factor in the non-aqueous-electrolyte secondary battery produced in Example A of this invention. It is a figure which shows the relationship between the cycle number and charge capacity in the nonaqueous electrolyte secondary battery produced in Example A of this invention. It is a figure which shows the relationship between the cycle number in a nonaqueous electrolyte secondary battery produced in Example A of this invention, and a charge capacity maintenance factor. It is a figure which shows the relationship between the discharge rate and discharge capacity in the non-aqueous-electrolyte secondary battery produced in Example B of this invention. It is a figure which shows the relationship between the discharge rate and discharge capacity maintenance factor in the non-aqueous-electrolyte secondary battery produced in Example B of this invention.

In the non-aqueous electrolyte secondary battery in which the negative electrode includes a spinel-type lithium titanium composite oxide, the present inventors have proposed a non-aqueous electrolyte for improving both large current discharge characteristics and cycle characteristics, that is, a non-aqueous electrolyte. Various studies were made on the composition of the aqueous solvent. As a result, it has been found that if the non-aqueous solvent is a mixture of propylene carbonate and γ-butyrolactone in a volume ratio of 6: 4 to 4: 6, both the large current discharge characteristics and the cycle characteristics are improved. It was. The present invention has been made based on such knowledge of the present inventors.

That is, the non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator. The negative electrode includes a spinel type lithium titanium composite oxide. The non-aqueous electrolyte contains a non-aqueous solvent in which propylene carbonate and γ-butyrolactone are mixed at a volume ratio of 6: 4 to 4: 6.

Since the non-aqueous electrolyte is configured as described above, it is possible to improve the discharge rate characteristics, particularly the high rate characteristics, that is, the large current discharge characteristics, and the cycle characteristics. Specifically, for example, the cycle characteristics at 60 ° C. can be improved together with the discharge rate characteristics at 25 ° C., particularly the high rate characteristics.

If the volume ratio of γ-butyrolactone is less than 40% by volume, the large current discharge characteristics deteriorate. On the other hand, when the volume ratio of γ-butyrolactone exceeds 60% by volume, cycle characteristics deteriorate.

In the non-aqueous electrolyte secondary battery of the present invention, it is preferable that the non-aqueous electrolyte contains trioctyl phosphate. In this case, the liquid content per unit time with respect to the electrolyte solution of a separator can be improved. Thereby, it is possible to further improve the discharge rate characteristic, particularly the high rate characteristic, that is, the large current discharge characteristic. Specifically, for example, the discharge rate characteristic at 25 ° C., particularly the high rate characteristic can be further improved.

In the nonaqueous electrolyte secondary battery of the present invention, the negative electrode includes a spinel type lithium titanium composite oxide as a negative electrode active material. When the negative electrode active material is a carbon-based material, γ-butyrolactone may decompose, but when the negative electrode active material contains a spinel-type lithium titanium composite oxide, γ-butyrolactone does not decompose. . For this reason, the lifetime of a battery can be improved.

In one embodiment of the present invention, the positive electrode and the negative electrode of the non-aqueous electrolyte secondary battery are alternately stacked via a separator. The structure of the battery element may be composed of a stack of a plurality of strip-shaped positive electrodes, a plurality of strip-shaped separators and a plurality of strip-shaped negative electrodes, a stack of so-called single-wafer structures. It may be configured by folding and interposing a strip-shaped positive electrode and a strip-shaped negative electrode alternately. Moreover, as a structure of the battery element, a winding type structure in which a long positive electrode, a long separator, and a long negative electrode are wound may be employed. In the following examples, a wound structure is adopted as the structure of the battery element.

In the positive electrode, a positive electrode mixture layer including a positive electrode active material, a conductive agent, and a binder is formed on both surfaces of the positive electrode current collector. As an example, the positive electrode current collector is made of aluminum. The positive electrode active materials are lithium cobalt oxide composite oxide (LCO), lithium manganate composite oxide (LMO), lithium nickelate composite oxide (LNO), lithium-nickel-manganese-cobalt composite oxide (LNMCO), lithium -Manganese-nickel composite oxide (LMNO), lithium-manganese-cobalt composite oxide (LMCO), lithium-nickel-cobalt composite oxide (LNCO), or the like can be used. Further, the positive electrode active material may be appropriately selected from the above materials and mixed. The positive electrode active material may be an olivine-based material such as LiFePO ₄ . Carbon or the like is used as a conductive agent for the positive electrode. Polyvinylidene fluoride (PVDF) or polyamideimide (PAI) is used as the binder for binding the positive electrode active material and the conductive agent.

On the other hand, in the negative electrode, a negative electrode mixture layer including a negative electrode active material and a binder is formed on both surfaces of a negative electrode current collector. As an example, the negative electrode current collector is made of aluminum, and the negative electrode active material is a spinel-type lithium titanium composite oxide that is a noble material with a lithium storage / release potential of 1.0 V (vs Li / Li ⁺ ) or more, for example, It consists of lithium titanate represented by Li ₄ Ti ₅ O ₁₂ . The negative electrode may contain carbon that acts as a conductive agent. Polyvinylidene fluoride (PVDF) or polyamideimide (PAI) is used as a binder for binding the negative electrode active material.

The nonaqueous electrolytic solution is prepared by dissolving an electrolyte in a nonaqueous solvent. As the electrolyte, for example, a solution obtained by dissolving LiPF ₆ in a nonaqueous solvent at a concentration of 1.0 mol / L is used. As an electrolyte other than LiPF ₆ , lithium salts such as LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiAlCl ₄ , LiSiF ₆ are used. Can be mentioned. Among these, it is preferable from the viewpoint of oxidation stability that LiPF ₆ or LiBF ₄ is used as the electrolyte. Such an electrolyte is preferably used by being dissolved in a non-aqueous solvent at a concentration of 0.1 mol / L to 3.0 mol / L, and is preferably dissolved at a concentration of 0.5 mol / L to 2.0 mol / L. More preferably, it is used. As the non-aqueous solvent, a mixed solvent in which propylene carbonate and γ-butyrolactone are mixed at a volume ratio of 6: 4 to 4: 6 is used. It is preferable to prepare the nonaqueous electrolytic solution so that the nonaqueous electrolytic solution contains about 0.1 to 1.0% by mass of trioctyl phosphate, for example.

The separator uses a porous film containing polypropylene or polyethylene. For example, a porous film made of polypropylene having a porosity of about 40% to 80% is preferably used as the separator. By increasing the porosity of the separator, the liquid content per unit time of the separator with respect to the electrolyte can be further increased. Thereby, it is possible to further improve the discharge rate characteristic, particularly the high rate characteristic, that is, the large current discharge characteristic. Specifically, for example, the discharge rate characteristic at 25 ° C., particularly the high rate characteristic can be further improved.

Next, specific examples of the present invention will be described. In addition, the Example shown below is an example and this invention is not limited to the following Example.

Using the positive electrode, the negative electrode, and the non-aqueous electrolyte prepared as described below, the non-aqueous electrolyte secondary batteries of Examples and Comparative Examples were manufactured by changing the composition of the non-aqueous electrolyte.

[Example A]

(Preparation of positive electrode)

Lithium-nickel-manganese-cobalt composite oxide (LNMCO) represented by the composition formula LiNi _0.33 Mn _0.33 Co _0.33 O ₂ as a positive electrode active material, carbon as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder ) In a mass ratio of 88: 6: 6 and kneaded with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry. This positive electrode mixture slurry was applied to both surfaces of an aluminum foil as a positive electrode current collector, dried, and then rolled with a rolling roller to attach a positive electrode terminal to produce a positive electrode.

The basis weight of the positive electrode mixture per unit area at this time was 13.3 mg / cm ² and the packing density was 3.0 g / cc. The unit capacity of this positive electrode is 1 mol / L LiPF ₆ as the electrolyte of the electrolytic solution, and a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7 as the solvent. Measurement was performed in a voltage range of 2.5 to 4.3 V using lithium metal. As a result, a unit capacity of 150 mAh per 1 g was obtained.

(Preparation of negative electrode)

A lithium titanate represented by the composition formula Li ₄ Ti ₅ O ₁₂ as a negative electrode active material, carbon as a conductive agent, and PVDF as a binder were blended in a mass ratio of 93: 3: 4. A negative electrode mixture slurry was prepared by kneading with NMP. This negative electrode mixture slurry was applied to both sides of an aluminum foil as a negative electrode current collector and dried, and then a negative electrode terminal was attached to the one rolled by a rolling roller to produce a negative electrode.

At this time, the basis weight of the negative electrode mixture per unit area was 14.6 mg / cm ² , and the packing density was 2.4 g / cc. The unit capacity of this positive electrode is 1 mol / L LiPF ₆ as the electrolyte of the electrolytic solution, and a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7 as the solvent. Measurement was performed in a voltage range of 1.0 to 2.0 V using lithium metal. As a result, a unit capacity of 165 mAh per 1 g was obtained.

(Preparation of non-aqueous electrolyte)

As a non-aqueous solvent, a mixed solvent in which propylene carbonate (PC) and γ-butyrolactone (GBL) are mixed at a volume ratio of 8: 2, 6: 4, 4: 6, 2: 8 is used. As a non-aqueous electrolyte solution, LiPF ₆ was dissolved to a concentration of 1 mol / L. Thus, the non-aqueous electrolyte of Comparative Example 1 (volume ratio PC: GBL = 8: 2), the non-aqueous electrolyte of Example 1 (volume ratio PC: GBL = 6: 4), and the non-aqueous electrolyte of Example 2 were used. A water electrolyte (volume ratio PC: GBL = 4: 6) and a non-aqueous electrolyte (volume ratio PC: GBL = 2: 8) of Comparative Example 2 were prepared.

(Battery production)

As shown in FIG. 2, a separator 13 having a porosity of 80% made of a lithium ion permeable polypropylene (PP) microporous membrane is interposed between the positive electrode 11 and the negative electrode 12 produced as described above. Then, the battery element (power generation element) 10 was produced by winding in a flat shape. As shown in FIG. 1, the battery element 10 was accommodated in an outer packaging material 20 made of a laminate film containing aluminum as an intermediate layer. A positive electrode terminal 30 is attached to the positive electrode and a negative electrode terminal 40 is attached to the negative electrode so as to extend from the inside of the outer packaging material 20 to the outside. Then, after injecting the non-aqueous electrolyte prepared above into the outer packaging material 20, the opening of the outer packaging material 20 is sealed, so that the battery capacity is 250 mAh and Examples 1 and 2 and Comparative Example 1 2 non-aqueous electrolyte secondary batteries were produced.

(Measurement of large current discharge characteristics)

After charging each battery until the voltage reached 2.75 V at a room temperature (25 ° C.) with a charging current of 250 mA, the charging current was further attenuated while the voltage was maintained at 2.75 V, and the charging current was 12.5 mA. Each battery was fully charged until Then, the discharge capacity when each battery was discharged until the voltage became 1.25 V with a discharge current of 250 mA was measured. This discharge capacity was defined as 1C discharge capacity.

Further, the batteries were fully charged under the same charging conditions as those for the 1C discharge capacity measurement, and the discharge capacity was measured when each battery was discharged until the voltage became 1.25 V with a discharge current of 750 mA. This discharge capacity was defined as 3C discharge capacity.

In the same manner as described above, the battery was fully charged under the same charging conditions as those for the 1C discharge capacity measurement, and the discharge capacity when the discharge current was sequentially increased was measured. When the discharge current is 1250 mA, the discharge capacity is 5 C discharge capacity, when the discharge current is 2500 mA, the discharge capacity is 10 C discharge capacity, when the discharge current is 3750 mA, the discharge capacity is 15 C discharge capacity, and when the discharge current is 50000 A The discharge capacity was set to 20C discharge capacity. FIG. 3 shows the relationship between the discharge rate and the discharge capacity of the nonaqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2 thus obtained.

Moreover, the ratio (= {(each discharge capacity) / (1C discharge capacity)} × 100 [%]) of each discharge capacity of 3C, 5C, 10C, 15C, and 20C to the 1C discharge capacity was calculated. This ratio was defined as the discharge capacity maintenance rate. FIG. 4 shows the relationship between the discharge rates and discharge capacity retention rates of the non-aqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2 calculated in this way.

(Measurement of cycle characteristics)

After each battery was charged until the voltage reached 2.75 V at a temperature of 60 ° C. with a charging current of 5 C, the charging current was attenuated while the voltage was maintained at 2.75 V, and the charging current was reduced to 12.5 mA. Each battery was fully charged until Then, each battery was discharged until the voltage became 1.25V by setting the discharge current to 1C. Under this condition, charging / discharging was repeated 100 cycles. FIG. 5 shows the change in the charge capacity with respect to the number of cycles, and FIG. 6 shows the change in the charge capacity maintenance rate.

(Evaluation results)

From the results shown in FIGS. 3 and 4, the nonaqueous electrolyte secondary of Examples 1 and 2 was obtained by using a nonaqueous electrolyte containing a mixed solvent having a volume ratio of PC: GBL = 6: 4 to 4: 6. The discharge rate characteristics of the battery at 25 ° C., particularly the high rate characteristics, specifically the 20C discharge capacity retention rate, of Comparative Example 1 using a non-aqueous electrolyte containing a mixed solvent with a volume ratio of PC: GBL = 8: 2 Compared with a non-aqueous electrolyte secondary battery, it can be increased, that is, the large current discharge characteristics can be enhanced.

From the results shown in FIG. 5 and FIG. 6, the non-aqueous electrolytes of Examples 1 and 2 were obtained by using a non-aqueous electrolyte containing a mixed solvent having a volume ratio of PC: GBL = 6: 4 to 4: 6. The decrease of the charge capacity and the charge capacity maintenance rate with the increase in the number of cycles at 60 ° C. of the secondary battery is shown in Comparative Example 2 using a non-aqueous electrolyte containing a mixed solvent with a volume ratio of PC: GBL = 2: 8. Compared with a non-aqueous electrolyte secondary battery, it can be suppressed, that is, cycle characteristics can be improved.

Therefore, by using a non-aqueous electrolyte containing a mixed solvent with a volume ratio of PC: GBL = 6: 4 to 4: 6, the cycle characteristics as well as the large current discharge characteristics of the non-aqueous electrolyte secondary battery can be improved. it can.

[Example B]

Two kinds of separators 13 having porosity of 40% and 80% were used, and as a non-aqueous solvent, a mixed solvent in which propylene carbonate (PC) and γ-butyrolactone (GBL) were mixed at a volume ratio of 4: 6 was used. Example 3 with a battery capacity of 250 mAh (with a porosity of 40), except that the non-aqueous electrolyte was prepared so as to contain 0.5% by mass of trioctyl phosphate. % Separator) and Example 5 (using a separator with a porosity of 80%) non-aqueous electrolyte secondary batteries were fabricated. A separator having a three-layer structure of polypropylene (PP) / polyethylene (PE) / polypropylene (PP) is used as a separator having a porosity of 40%, and a separator made of polypropylene (PP) is used as a separator having a porosity of 80%. The single layer structure was used.

(Measurement of AC resistance value)

1 hour after injecting the non-aqueous electrolyte, the AC resistance value of each battery produced above was measured. The measurement results are shown in Table 1. For comparison, using a non-aqueous electrolyte not containing trioctyl phosphate, Example 4 using a separator with a porosity of 40% and Example 6 using a separator with a porosity of 80%. The measurement result of AC resistance value is also shown about a nonaqueous electrolyte secondary battery.

(Measurement of large current discharge characteristics)

Regarding the non-aqueous electrolyte secondary batteries of Examples 5 and 6 (using a separator with a porosity of 80%), the relationship between the discharge rate and the discharge capacity measured in the same manner as in Example A above is shown. FIG. 7 shows the relationship between the discharge rate and the discharge capacity retention rate measured in the same manner as in FIG. 7 and Example A above.

(Evaluation results)

From the results shown in Table 1, in the non-aqueous electrolyte secondary battery of Example 3 using a separator with a porosity of 40%, the AC resistance value is non-existent by adding trioctyl phosphate to the non-aqueous electrolyte. One hour after the injection of the water electrolyte, the non-aqueous electrolyte was reduced to about 1/500 compared to the non-aqueous electrolyte secondary battery of Example 4 that did not contain trioctyl phosphate. It can be seen that the liquid content per unit time can be greatly improved. In addition, in the nonaqueous electrolyte secondary battery of Example 5 using a separator with a porosity of 80%, by adding trioctyl phosphate to the nonaqueous electrolyte, the AC resistance value can be obtained by injecting the nonaqueous electrolyte. One hour later, the non-aqueous electrolyte decreased to about 1/6 compared with the non-aqueous electrolyte secondary battery of Example 6 that did not contain trioctyl phosphate. It can be seen that the liquidity can be improved.

Further, from the results shown in FIG. 7 and FIG. 8, in the non-aqueous electrolyte secondary battery of Example 5 using a separator with a porosity of 80%, by adding trioctyl phosphate to the non-aqueous electrolyte, 25 Increase discharge rate characteristics at ° C., particularly high rate characteristics, specifically 20C discharge capacity retention rate, as compared to the non-aqueous electrolyte secondary battery of Example 6 in which the non-aqueous electrolyte does not contain trioctyl phosphate. In other words, the large current discharge characteristics can be improved.

It should be considered that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is shown not by the above embodiments and examples but by the claims, and is intended to include all modifications and variations within the meaning and scope equivalent to the claims.

It is possible to provide a non-aqueous electrolyte secondary battery capable of improving cycle characteristics as well as large current discharge characteristics.

1: non-aqueous electrolyte secondary battery, 10: battery element, 11: positive electrode, 12: negative electrode, 13: separator.

Claims (2)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte,
The negative electrode includes a spinel type lithium titanium composite oxide,
The non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte includes a non-aqueous solvent in which propylene carbonate and γ-butyrolactone are mixed at a volume ratio of 6: 4 to 4: 6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte includes trioctyl phosphate.

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