JP2011034893A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery excellent in battery characteristics such as preservation characteristics, output characteristics and others. <P>SOLUTION: In the nonaqueous electrolyte secondary battery including a positive electrode containing a positive electrode active material occluding and emitting lithium ions, a negative electrode containing a negative electrode active material occluding and emitting lithium ions, and a nonaqueous electrolyte containing a nonaqueous solvent, the nonaqueous electrolyte contains lithium salt having an oxalato complex as anions and acetonitrile and the content of the acetonitrile is 0.6-1.0 mass% relative to the content of lithium salt having the oxalato complex as anions. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention includes a positive electrode including a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode including a negative electrode active material capable of occluding and releasing lithium ions, and a nonaqueous electrolyte including a nonaqueous solvent. The present invention relates to a non-aqueous electrolyte secondary battery, and particularly to improvement of battery characteristics such as storage characteristics and output characteristics.

In recent years, as a battery used in portable electronic / communication equipment such as a small video camera, a mobile phone, and a laptop computer, a carbon material or an alloy that can occlude / release lithium ions is used as a negative electrode active material, and lithium cobalt oxide (LiCoO _2). ), Non-aqueous electrolyte secondary batteries using lithium transition metal oxides such as lithium manganate (LiMn ₂ O ₄ ) and lithium nickelate (LiNiO ₂ ) as a positive electrode active material are small, light, high voltage, and high capacity It has come to be put into practical use as a battery that can be charged and discharged. In addition, the use of such a non-aqueous electrolyte secondary battery as a power source for a hybrid vehicle or an electric vehicle has been studied.

  For such a non-aqueous electrolyte secondary battery, it is required to improve cycle characteristics and storage characteristics, and as a method for improving cycle characteristics and storage characteristics, the non-aqueous electrolyte in the non-aqueous electrolyte secondary battery, A technique for including a lithium salt having an oxalato complex as an anion has been proposed.

However, in a non-aqueous electrolyte secondary battery in which a lithium salt containing an oxalate complex as an anion is contained in the non-aqueous electrolyte, the IV resistance increases. Therefore, this type of non-aqueous electrolyte secondary battery is used in a hybrid vehicle or an electric vehicle. There was a problem that sufficient output / regeneration characteristics could not be obtained when used as a power source.
Here, the IV resistance is a value obtained by measuring each voltage value when a battery is charged or discharged at a plurality of current values for a certain period of time, plotting each current value and voltage value, and tilting the voltage value with respect to the current value. It is a value obtained from The value of the IV resistance is an index for knowing how much current can flow through the battery.

JP 2006-196250 A

  In order to solve this problem, Patent Document 1 proposes that a non-aqueous electrolyte containing a lithium salt having an oxalato complex as an anion further contains a film forming agent such as vinylene carbonate.

  However, in the said patent document 1, the effect which suppresses the fall of the output characteristic by the increase in IV resistance which arises when the non-aqueous electrolyte contains the lithium salt which uses an oxalato complex as an anion is inadequate. Improvement is desired.

  The present invention has been made to solve the above-described problems, and aims to improve the output characteristics of a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte containing a lithium salt having an oxalato complex as an anion. .

  The present invention includes a positive electrode including a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode including a negative electrode active material capable of occluding and releasing lithium ions, and a nonaqueous electrolyte including a nonaqueous solvent. In the non-aqueous electrolyte secondary battery, the non-aqueous electrolyte contains a lithium salt having an oxalato complex as an anion and acetonitrile, and the content of acetonitrile is relative to the content of a lithium salt having the oxalato complex as an anion. 0.6 mass% to 1.0 mass%.

  As a result of various investigations, the present inventors have found that a nonaqueous electrolyte containing a lithium salt having an oxalato complex as an anion further contains a specific amount of acetonitrile, so that the nonaqueous electrolyte has an oxalato complex as an anion. It has been found that an increase in IV resistance that occurs when a lithium salt is contained can be suppressed.

  In the present invention, the content of acetonitrile in the non-aqueous electrolyte is preferably 0.6% by mass to 1.0% by mass with respect to the content of lithium salt having an oxalato complex as an anion.

When the content of acetonitrile in the non-aqueous electrolyte is 0.6% by mass to 1.0% by mass with respect to the content of lithium salt having an oxalato complex as an anion, a lithium salt having an oxalato complex as an anion is contained. It is considered that the effect of suppressing the increase in IV resistance generated when using a non-aqueous electrolyte is great.
Therefore, according to the present invention, it is possible to suppress an increase in IV resistance that occurs when a non-aqueous electrolyte containing a lithium salt having an oxalato complex as an anion is used, and battery characteristics such as storage characteristics and output characteristics can be suppressed. An excellent nonaqueous electrolyte secondary battery can be obtained.

This is presumed to be as follows. When the non-aqueous electrolyte contains a lithium salt having an oxalato complex as an anion, the lithium salt having an oxalato complex as an anion is reduced and decomposed on the negative electrode surface by charge and discharge, and is stable on the negative electrode surface and excellent in lithium ion permeability. It is thought that a film is formed. It is considered that this coating suppresses the reaction between the nonaqueous electrolyte and the negative electrode and improves the cycle characteristics, storage characteristics, and the like. However, the problem is that the IV resistance increases due to the resistance of the coating formed on the negative electrode surface.
According to the present invention, it is considered that the acetonitrile contained in the non-aqueous electrolyte is taken into the film formed on the negative electrode surface, whereby the film resistance can be reduced and the increase in IV resistance can be suppressed.

  In the present invention, the content of the lithium salt having an oxalato complex as an anion in the non-aqueous electrolyte is preferably 0.08 to 0.14 mol / liter with respect to the non-aqueous solvent.

  When the content of the lithium salt having an oxalato complex as an anion in the non-aqueous electrolyte is 0.08 to 0.14 mol / liter with respect to the non-aqueous solvent, a good film is sufficiently formed on the negative electrode surface. Therefore, excellent cycle characteristics and storage characteristics can be obtained.

In the present invention, LiB (C ₂ O ₄ ) ₂ , LiBF ₂ (C ₂ O ₄ ), LiPF ₂ (C ₂ O ₄ ) and the like can be used as the lithium salt having an oxalato complex as an anion. LiB (C ₂ O ₄ ) ₂ is preferable.

When lithium-bis (oxalato) borate (LiB (C ₂ O ₄ ) ₂ ) is used, it is formed on the surface of the negative electrode as compared with the case of using LiBF ₂ (C ₂ O ₄ ) or LiPF ₂ (C ₂ O ₄ ). The coating film is stable and has a great effect of improving cycle characteristics and storage characteristics.

  In the present invention, the non-aqueous electrolyte preferably contains another lithium salt different from the lithium salt having an oxalato complex as an anion.

As another lithium salt different from the lithium salt having an oxalato complex as an anion, those generally used in non-aqueous electrolyte secondary batteries can be used. 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 ₂ 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. As the addition amount of the other lithium salt different from the lithium salt having an oxalato complex as an anion, it is preferable that the concentration range is 0.8 to 1.4M with respect to the nonaqueous solvent, and 1.0 to 1.2M. It is more preferable to set the concentration range.

  In the present invention, the nonaqueous electrolyte preferably contains vinylene carbonate (VC).

  When vinylene carbonate is contained in the nonaqueous electrolyte, the coating film formed on the negative electrode surface is further improved, and the battery characteristics are improved. The content of vinylene carbonate in the nonaqueous electrolyte is preferably in the range of 0.1 to 5.0 mass%, and in the range of 0.5 to 3.0 mass%, based on the total mass of the nonaqueous solvent. More preferably, it is more preferable to set it as the range of 1.0-2.0 mass%.

Examples of the positive electrode active material that can be used in the present invention include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiNi _1-x Mn _x O ₂ (0 <x <1), LiNi _1-x Co _x. O ₂ (0 <x <1), LiNi _x Mn _y Co _z O ₂ (0 <x, y, z <1, x + y + z = 1) and other olivines such as LiFePO ₄ Examples thereof include a phosphoric acid compound having a structure. As the negative electrode active material can be used in the present invention, graphite, non-graphitizable carbon and graphitizable carbon material such as carbon, it includes LiTiO ₂ and TiO ₂ or the like.

  In the present invention, the positive electrode active material capable of occluding and releasing lithium ions is a lithium transition metal oxide, and the negative electrode active material capable of occluding and releasing lithium ions is preferably a carbon material, and graphite is particularly preferred. .

  In addition, what is generally used in the nonaqueous electrolyte secondary battery can be used as the nonaqueous solvent in the present invention. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate, and chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate can be used. In particular, it is preferable to use a mixed solvent of a cyclic carbonate and a chain carbonate.

FIG. 1 is a diagram showing IV resistance (mΩ) during discharge of the nonaqueous electrolyte secondary battery in Example 1 and Comparative Example 1.

  Hereinafter, the present invention will be described in detail using examples and comparative examples. However, the following examples show examples of non-aqueous electrolyte secondary batteries for embodying the technical idea of the present invention, and are not intended to specify the present invention to these examples. The present invention can be equally applied to various modifications without departing from the technical idea shown in the claims.

First, a method for producing a positive electrode plate and a negative electrode plate common to the examples and the comparative examples will be described.
[Production of negative electrode plate]
Artificial graphite as a negative electrode active material and an N-methylpyrrolidone (NMP) solution in which polyimide (PI) as a binder was dissolved were kneaded to prepare a negative electrode slurry. Here, it mixed so that mass ratio of negative electrode active material: polyimide (PI) might be set to 97: 3. Next, the prepared negative electrode slurry was applied on a copper foil (thickness: 10 μm) as a negative electrode core, and then dried to remove NMP used as a solvent at the time of slurry preparation to form a negative electrode active material mixture layer. . Then, it rolled until it became a predetermined packing density (1.1 g / cc) using the rolling roller, the negative electrode current collection tab was attached, and the negative electrode plate was produced.

[Production of positive electrode plate]
Li ₂ CO ₃ and (Ni _0.35 Co _0.35 Mn _0.3 ) ₃ O ₄ have a molar ratio of Li and (Ni _0.35 Co _0.35 Mn _0.3 ) of 1: 1. It mixed so that it might become. Next, this mixture was fired at 900 ° C. for 20 hours in an air atmosphere to obtain a lithium transition metal oxide represented by LiNi _0.35 Co _0.35 Mn _0.3 O ₂ , and used as a positive electrode active material. . The positive electrode active material obtained as described above, exfoliated graphite and carbon black as a conductive agent, NMP solution of polyvinylidene fluoride (PVdF) as a binder, lithium transition metal oxide: exfoliated graphite: carbon black : Polyvinylidene fluoride (PVdF) was kneaded so as to have a mass ratio of 88: 7: 2: 3 to prepare a positive electrode slurry. The prepared positive electrode slurry was applied as a positive electrode core onto an aluminum alloy foil (thickness: 15 μm), and then dried to remove NMP used as a solvent during slurry preparation to form a positive electrode active material mixture layer. Then, it rolled until it became a predetermined packing density (2.6g / cc) using the rolling roll, and after cut | disconnecting to the predetermined dimension, the positive electrode current collection tab was attached and the positive electrode plate was produced.

[Example 1]
1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) is used as an electrolyte salt with respect to a non-aqueous solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 3: 7. Dissolved in liters.
In the solution thus obtained, vinylene carbonate (VC) is 1% by mass relative to the mass of the non-aqueous solvent, and lithium-bis (oxalato) borate (LiB (C ₂ O ₄ ) ₂ ) is used as the non-aqueous solvent. A non-aqueous electrolyte was prepared by adding 0.1 mol / liter and acetonitrile to 0.8% by mass with respect to the mass of LiB (C ₂ O ₄ ) ₂ .

[Production of non-aqueous electrolyte secondary battery]
Each of the positive electrode plate produced as described above and the negative electrode plate produced as described above was laminated with a separator made of a polypropylene microporous film interposed therebetween, and then wound in a spiral shape. Thus, a spiral electrode group was obtained. Next, after inserting the spiral electrode group into a bottomed cylindrical metal outer can, the upper outer periphery of the metal outer can was drawn to form a drawn portion. Thereafter, a negative electrode current collecting tab extending from the negative electrode plate was welded to the inner bottom surface of the metal outer can. On the other hand, the positive electrode current collection tab extended from a positive electrode plate was welded to the bottom face of the positive electrode cover of the sealing body, and the ring-shaped insulating gasket was arrange | positioned in the outer peripheral part of the sealing body. In addition, a cap-shaped positive terminal is disposed on the upper part of the positive cover, and a valve plate for sealing an exhaust hole formed at the center of the positive cover and the cap is pressed into the cap-shaped positive terminal. A valve body made of a spring is arranged. Next, the nonaqueous electrolyte prepared as described above was poured into a metal outer can. Then, after placing a sealing body with a ring-shaped insulating gasket disposed on the outer peripheral portion on the narrowed portion formed on the upper outer periphery of the metallic outer can, the upper end portion of the metallic outer can is placed on the sealing body side. The non-aqueous electrolyte secondary battery of Example 1 was produced by caulking and sealing.

[Comparative Example 1]
A nonaqueous electrolyte secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 except that the amount of acetonitrile added was 0.3% by mass with respect to the mass of LiB (C ₂ O ₄ ) ₂ .

  The following tests were performed on the nonaqueous electrolyte secondary batteries of Example 1 and Comparative Example 1.

[IV resistance measurement]
With a charge current of 1A, until the depth of charge (SOC) reaches 50%, the battery is discharged for 10 seconds with a current of 1 / 3A, 1A, 3A and 5A, and each battery voltage is measured. The battery voltage was plotted against the value, and the slope was taken as the IV resistance during discharge. The IV resistance obtained in this way was used as the initial IV resistance.

[Preservation test]
The battery was charged at a charging current of 1 A until the depth of charge (SOC) reached 50%, stored at 70 ° C. for a predetermined period, and IV resistance was determined by the same method as described above. The storage period was 7 days, 14 days, 21 days, 28 days, and 42 days.

FIG. 1 shows the test results. FIG. 1 shows IV resistance values measured at the initial stage of the nonaqueous electrolyte secondary batteries of Example 1 and Comparative Example 1 and after discharging after storage for a predetermined period at 70 ° C. in a state of SOC 50%. Than 1, compared with the non-aqueous electrolyte secondary battery of Comparative Example 1 using _{LiB (C 2} O ₄₎ non-aqueous electrolyte was added 0.3 wt% with respect to ₂ mass acetonitrile, acetonitrile LiB (C ₂ O ₄ ) In the non-aqueous electrolyte secondary battery of Example 1 using the non-aqueous electrolyte added by 0.8 mass% with respect to the mass of ₂ , it is possible to suppress an increase in IV resistance after the initial stage and after storage. In particular, it can be seen that the effect of suppressing an increase in IV resistance after storage is great.

  From this, when the content of acetonitrile in the non-aqueous electrolyte is about 0.6% by mass to 1.0% by mass with respect to the mass of the lithium salt having the oxalato complex as an anion, the oxalato complex is used as an anion. Compared with the case of containing about 0.3% by mass with respect to the mass of the lithium salt, it is possible to suppress an increase in IV resistance that occurs when a non-aqueous electrolyte containing a lithium salt having an oxalato complex as an anion is used. It is thought that.

As described above, according to the present invention, it is possible to suppress an increase in IV resistance that occurs when a non-aqueous electrolyte containing a lithium salt having an oxalato complex as an anion is used,
A nonaqueous electrolyte secondary battery having excellent battery characteristics such as storage characteristics and output characteristics can be provided.

Claims (6)

A non-aqueous electrolyte comprising: a positive electrode including a positive electrode active material capable of occluding and releasing lithium ions; a negative electrode including a negative electrode active material capable of occluding and releasing lithium ions; and a non-aqueous electrolyte including a non-aqueous solvent. In the next battery,
The non-aqueous electrolyte contains a lithium salt having an oxalato complex as an anion and acetonitrile, and the content of acetonitrile is 0.6% by mass to 1.% by mass with respect to the content of the lithium salt having an oxalato complex as an anion. A non-aqueous electrolyte secondary battery characterized by being 0% by mass.   2. The non-aqueous electrolyte according to claim 1, wherein a content of the lithium salt having an oxalato complex as an anion in the non-aqueous electrolyte is 0.08 to 0.14 mol / liter with respect to the non-aqueous solvent. Water electrolyte secondary battery. 3. The non-aqueous electrolyte secondary according to claim 1, wherein the lithium salt having the oxalato complex as an anion is lithium-bis (oxalato) borate (LiB (C ₂ O ₄ ) ₂ ). battery.   4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte contains another lithium salt different from a lithium salt having an oxalato complex as an anion. 5.   The nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the nonaqueous electrolyte contains vinylene carbonate. The positive electrode active material capable of occluding and releasing lithium ions is a lithium transition metal oxide, and the negative electrode active material capable of occluding and releasing lithium ions is a carbon material. Item 6. The nonaqueous electrolyte secondary battery according to any one of Items 5 to 6.

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