JP2012069454A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery in which lithium titanate is used for a negative electrode active material, and 4 V-class active material is used for a positive electrode, and a LiPF-containing nonaqueous electrolyte is used as the electrolyte of the battery, and which realizes a high conductivity and offers an excellent reliability even in storage under a harsh environment such as a humid one.SOLUTION: The nonaqueous electrolyte secondary battery comprises: lithium titanate for a negative electrode; 4 V-class active material for a positive electrode; and LiPFas an electrolyte. As to the negative electrode, vapor-phase growing carbon fiber is used as a conducting agent, and polyacrylic acid is used as a binding agent, which allows a high conductivity and an excellent reliability to be achieved even in storage under a humid environment. With the addition amount of the polyacrylic acid, it is preferable to add not less than 1 to not more than 7 mass% of polyacrylic acid with respect to the whole negative electrode. Further, it is preferable that the vapor-phase growing carbon fiber has an average fiber length of 1 to 5 μm inclusive.

Description

  The present invention relates to an improvement in long-term reliability of a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery excellent in reliability in a humid environment.

   Due to the rapid development of technology in the electronics field in recent years, electronic devices have been miniaturized, and as a power source for these devices, there is an increasing demand for batteries that are small and light and have a high energy density. Among them, secondary batteries that can be charged and discharged are particularly attracting attention because of their low environmental impact.

In order to meet the above-mentioned demand, the positive electrode active material is a transition metal oxide containing 4 V class lithium such as LiCoO ₂ , LiNiO ₂ or LiMn ₂ O ₄ (an oxide having an open circuit voltage of 4 V or more when metallic lithium is used as a counter electrode). ) And LiPF ₆ having both high oxidation resistance capable of withstanding the high potential of the positive electrode and high ion conductivity as an electrolyte, a high battery voltage and high energy density can be realized, and these were used. Secondary batteries are used as the main power source for a wide range of electronic devices.

  On the other hand, as the negative electrode active material, a material using metallic lithium has attracted attention and has been extensively studied. However, when metallic lithium is used, the shape of the lithium negative electrode is impaired by repeated charge and discharge, and sufficient charge / discharge cycle life cannot be obtained, or lithium dendrite penetrates the separator, causing internal short circuit of the battery. There was a problem such as.

  As a means to solve this problem, improvements have been made by using lithium alloys such as lithium-aluminum alloys, lithium-lead alloys, lithium-tin alloys, and various carbon materials and transition metal oxides for the negative electrode. I came.

In particular, since lithium titanate represented by the chemical formula Li ₄ Ti ₅ O ₁₂ has excellent stability in a wide range of potential regions, it not only dramatically improves the charge / discharge cycle life, but also has over-discharge resistance and resistance. It is also excellent in battery reliability such as overcharge characteristics, and has attracted widespread attention.

  However, since lithium titanate has a high specific resistance, it has a problem that the internal resistance of the battery is increased. Particularly, when used for a small battery, the electrode area is reduced and the reaction area is reduced. There is a problem that the internal resistance is remarkably increased and sufficient output characteristics cannot be obtained.

  In general, an electrode of a battery is prepared by mixing an active material, a carbon material such as graphite or carbon black as a conductive agent, and a binder for maintaining a conductive path between the conductive agent and the active material. . However, when a large amount of carbon material or binder is mixed, the ratio of the active material inevitably decreases, leading to a decrease in energy density. In addition, since the internal resistance and reliability of the battery are greatly affected by the type and mixing ratio of the carbon material and binder, various studies are conducted on the carbon material and binder, including the type and mixing ratio. Has been made.

For example, Patent Document 1 discloses that by using a vapor-grown carbon fiber having high crystallinity and low bulk density, it is possible to suppress overcharge deterioration and obtain excellent output characteristics. Patent Document 2 discloses that both good cycle characteristics and high output characteristics can be secured by using vapor grown carbon fibers and a small amount of polyacrylic acid added as a part of the binder. ing.

JP 2006-278282 A JP 2006-32296 A

  As described in these patent documents, a vapor-grown carbon fiber has a fiber structure, so that a sufficient conductive network can be constructed, and an electrode having excellent conductivity can be obtained even when lithium titanate is used. is there. However, when the batteries disclosed in these patent documents are used for a long time in a severe environment that may occur in actual use, there is a problem that sufficient output characteristics and reliability cannot be secured.

  Non-aqueous electrolyte secondary batteries used for main power applications of equipment are required to have high output characteristics and reliability over a long period of time. Moreover, in actual use, it is often used not only in a high temperature environment but also in a humid environment, so it is important to take measures against humidity.

A non-aqueous electrolyte battery, unlike an aqueous battery, has a very weak surface against moisture, and has a problem that battery characteristics deteriorate early due to the ingress of moisture from the outside. When LiPF ₆ is used as an electrolyte, when moisture enters the battery, LiPF ₆ reacts with the entered moisture to dissociate fluorine ions. It is considered that this fluorine ion degrades the binder in the electrode, causes collapse of the electrode structure, disruption of the conductive network, and the like, thereby reducing the output characteristics and reliability of the battery.

  In particular, in an electrode using vapor-grown carbon fibers as a conductive agent, the collapse of the electrode structure and the disruption of the conductive network are significantly increased. Graphite generally used as a conductive agent has a structure in which layered sheets of hexagonal carbon atoms called graphene are laid out in a plane. On the other hand, the vapor growth carbon fiber has a cylindrical structure in which a graphene sheet is wound along the center of the fiber, and has stronger mechanical characteristics than graphite. For this reason, the vapor-grown carbon fiber is in a state where a large amount of stress is accumulated in the pressure-filled electrode, and when the binding force is reduced due to the deterioration of the binder, the influence of the stress becomes stronger, and the electrode structure collapses. This is thought to be due to the disruption of the conductive network.

The present invention has been made in view of the above problems, and uses a lithium titanate as a negative electrode active material, a non-aqueous electrolytic solution containing a 4V-class lithium-containing transition metal oxide as a positive electrode active material, and LiPF ₆ as an electrolyte. Non-aqueous electrolyte secondary battery using a non-aqueous electrolyte secondary battery that exhibits high reliability while maintaining high conductivity even when stored in harsh environments such as humid environments An object is to provide a battery.

In order to achieve the above object, the non-aqueous electrolyte secondary battery of the present invention is a 4V class in which the open circuit voltage when a negative electrode using lithium titanate as a negative electrode active material and metallic lithium as a counter electrode is 4V or more. A non-aqueous electrolyte contained in a battery case composed of a battery case, a sealing plate and a gasket, together with a non-aqueous electrolyte containing LiPF ₆ as an electrolyte, facing a positive electrode made of a lithium-containing transition metal oxide via a separator In the secondary battery, the negative electrode is characterized by using vapor grown carbon fiber as a conductive agent and polyacrylic acid as a binder.

More preferably, the polyacrylic acid is mixed in a proportion of 1% by mass or more and 7% by mass or less with respect to the whole negative electrode, and the vapor-grown carbon fiber has an average fiber length (50 % Integrated diameter) is 1 μm or more and 5 μm or less.

  According to the present invention, in a non-aqueous electrolyte secondary battery, battery characteristics deterioration due to external moisture ingress in a humid environment is greatly suppressed, and excellent non-aqueous characteristics that combine high output characteristics, high reliability, and high energy density. An electrolyte secondary battery can be provided.

Sectional drawing of the nonaqueous electrolyte secondary battery in one embodiment of this invention

According to a first aspect of the present invention, there is provided a negative electrode using lithium titanate as a negative electrode active material, and a positive electrode made of a transition metal oxide containing 4V class lithium having an open circuit voltage of 4 V or more when metallic lithium is used as a counter electrode. In a non-aqueous electrolyte secondary battery that is disposed opposite to a separator and contains a non-aqueous electrolyte containing LiPF ₆ as an electrolyte, and a battery case including a battery case, a sealing plate, and a gasket, the negative electrode serves as a conductive agent. A non-aqueous electrolyte secondary battery characterized by using vapor-grown carbon fiber and polyacrylic acid as a binder.

  In a negative electrode using lithium titanate as a negative electrode active material, when graphite, carbon black or the like that is widely used as a conductive agent is used, a large amount of addition is required to obtain sufficient electrode conductivity. In addition, lithium titanate has a very high specific resistance compared to carbon materials such as graphite and coke that are widely used as a negative electrode active material, and the conductivity of the material itself is low. This greatly depends on the conductive network constructed by the adhesive, and is significantly affected by the collapse of the electrode structure. Therefore, in the case of using a fluorine resin such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC) or the like that is widely used as a binder. In the storage in a humid environment, the electrode structure collapses due to the expansion and cracking of the electrode, which greatly increases the internal resistance of the battery.

  As a result of various studies, we have used vapor-grown carbon fiber as the conductive agent and polyacrylic acid as the binder, so that even when water enters the battery, the electrode structure can be collapsed with a small amount. It has been found that the separation of the conductive network can be greatly suppressed, and sufficient conductivity of the electrode can be secured even in an electrode using lithium titanate as an active material.

  Polyacrylic acid contains a large amount of carboxyl groups (—COOH) which are polar groups in the molecule. This carboxyl group forms a hydrogen bond with water molecules that have entered the inside of the battery and suppresses the generation of fluorine ions, so that the deterioration of polyacrylic acid can be suppressed. Thereby, it is considered that a conductive network formed in a three-dimensional network by vapor-grown carbon fibers can be secured for a long time even in a humid environment, and excellent conductivity of the electrode can be maintained.

  According to a second aspect of the present invention, there is provided the non-aqueous electrolyte according to the first aspect, wherein the content of polyacrylic acid is 1% by mass or more and 7% by mass or less based on the whole negative electrode. It is a secondary battery. If the ratio is 1% by mass or more and 7% by mass or less, a binding force sufficient to maintain the electrode shape and a sufficient carboxyl group to form hydrogen bonds with water molecules that have entered the battery can be secured. Moreover, it is more preferable because sufficient conductivity and energy density of the electrode can be maintained.

According to a third aspect of the present invention, there is provided the non-aqueous electrolysis according to the first aspect, wherein vapor-grown carbon fibers having an average fiber length (50% integrated diameter) of 1 μm to 5 μm are used. It is a liquid secondary battery. By using vapor-grown carbon fibers with a fiber length average (50% integrated diameter) of 1 μm or more and 5 μm or less on a number basis, a sufficient conductive network with a three-dimensional network structure can be formed, and residual stress in the electrode can be reduced. This can be reduced and is more preferable.

  The fiber length can be measured by the following method. First, using a flow type particle image analyzer, the observed image is captured as image data into a CCD camera. The fiber length of the obtained image data is calculated using an image analyzer. The number of measurement is 1000 or more, and the fiber length average (50% integrated diameter) on the basis of the number can be obtained.

  Embodiments of the present invention will be described below. The following embodiment is an example embodying the present invention, and does not limit the technical scope of the present invention.

  FIG. 1 is a cross-sectional view of a flat nonaqueous electrolyte secondary battery as an example of the nonaqueous electrolyte secondary battery of the present invention.

  In this battery, a power generation element composed of a positive electrode 1 and a negative electrode 2, a separator 3 and a non-aqueous electrolyte (not shown), a battery case 4 also serving as a positive electrode terminal, a sealing plate 5 also serving as a negative electrode terminal, and a battery case 4 and a sealing plate 5 It is stored and sealed by a gasket 6 that insulates. A current collector is disposed on the inner surfaces of the battery case 4 and the sealing plate 5.

  The positive electrode 1 is an electrode molded into a pellet shape, and examples of the positive electrode active material include lithium cobalt oxide containing lithium, lithium nickelate, and spinel-type lithium-containing transition metal oxides such as lithium manganate. . Further, magnesium, aluminum, or the like may be added to these oxides.

  The negative electrode 2 is also an electrode molded into a pellet, and lithium titanate is used as the negative electrode active material, vapor grown carbon fiber (VGCF) is used as the conductive agent, and polyacrylic acid is used as the binder. Furthermore, graphite or carbon black as a conductive agent, fluorine resin as a binder, or SBR or CMC may be mixed.

  For the separator 3, it is preferable to use conventionally used polyethylene, polypropylene, engineering plastics such as cellulose and polyphenylene sulfide.

  For the gasket 6, it is preferable to use conventionally used polypropylene or engineering plastics such as polyphenylene sulfide.

As a solute constituting the non-aqueous electrolyte, LiPF ₆ alone or a mixture of a plurality of components such as LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ may be used. it can. In addition, as a solvent constituting the non-aqueous electrolyte, propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, sulfolane, dimethoxyethane, diethoxyethane, tetrahydrofuran, dioxolane, γ-butyrolactone, or the like Multiple components can be used, but are not limited to these.

  The current collector 7 is obtained by applying a conductive carbon paint to the inner surfaces of the battery case 4 and the sealing plate 5.

(Examination of conductive agent type and binder type)
The positive electrode 1 is obtained by mixing LiCoO ₂ with carbon black as a conductive agent and fluororesin powder as a binder in a mass ratio of 90: 5: 5, and forming into a pellet shape having a diameter of 10 mm and a thickness of 0.5 mm. What was dried at 200 ° C. for 24 hours was used.

  The negative electrode 2 was prepared by mixing vapor-grown carbon fiber (VGCF) and polyacrylic acid with lithium titanate in a mass ratio of 92: 5: 3, and forming into a pellet shape having a diameter of 10 mm and a thickness of 0.5 mm. What was heat-processed at 24 degreeC for 24 hours was used.

The separator 3 was made of a polypropylene nonwoven fabric, the battery case 4 and the sealing plate 5 were made of stainless steel, and the gasket 6 was made of a polypropylene gasket. As the current collector 7, a conductive carbon coating was applied to the inner surfaces of the battery case 4 and the sealing plate 5 and then dried at 150 ° C. for 6 hours in order to remove moisture from the coating film. In addition, a battery using a battery in which LiPF ₆ was dissolved at a rate of 1 mol / l as an electrolyte in a solvent in which ethylene carbonate and methyl ethyl carbonate were mixed at a rate of 1: 3 as an electrolyte was used as a battery A1. It was.
A battery produced in the same manner as the battery A1 except that vapor-grown carbon fiber (VGCF) and graphite powder were used in a ratio of 4: 1 as a conductive agent was designated as a battery A2.

  A battery produced in the same manner as the battery A1 except that vapor-grown carbon fiber (VGCF) and graphite powder were used in a ratio of 1: 4 as a conductive agent was designated as a battery A3.

  A battery produced in the same manner as battery A1 except that graphite powder was used as the conductive agent was designated as battery B1.

  A battery produced in the same manner as the battery A1 except that acetylene black was used as the conductive agent was designated as a battery B2.

  A battery produced in the same manner as the battery A1 except that ketjen black was used as the conductive agent was designated as a battery B3.

  A battery produced in the same manner as the battery A1 except that PVDF was used as a binder was designated as a battery B4.

  A battery produced in the same manner as the battery A1 except that SBR was used as a binder was designated as a battery B5.

  All of these batteries have a diameter of 16 mm and a thickness of 1.6 mm.

  About these nonaqueous electrolyte secondary batteries, after assembling, they were charged with a constant voltage of 2.6 V for 24 hours (protection resistance 50Ω). About these batteries, the internal resistance of the battery after 30-day preservation | save in 70 degreeC90% high temperature and humidity environment was confirmed. In this experiment, the internal resistance of each battery was measured before starting the test for 50 batteries. For each battery, the volume of the negative electrode was measured before and after the test, and the expansion coefficient was calculated by the ratio (%) of the volume of the negative electrode after the test to the volume of the negative electrode before the test. These results are shown in (Table 1).

  From (Table 1), the batteries A1 to A3 using vapor grown carbon fiber (VGCF) and polyacrylic acid can significantly suppress the increase in battery internal resistance when stored in a humid environment compared to the batteries B1 to B5. ing. This is because even when moisture enters the battery, the binding force of the polyacrylic acid is maintained, the negative electrode can be prevented from expanding, and the conductivity of the electrode is improved by the conductive network of vapor grown carbon fiber (VGCF). This is because it can be secured. In the batteries B1 to B3, the binding force of polyacrylic acid is maintained, and the expansion of the negative electrode can be suppressed to about 105%, but the conductive network of the conductive agent is insufficient, so that the conductivity of the electrode can be secured. It is thought that it was gone.

  In the batteries B4 to B5, it is considered that PVDF and SBR are deteriorated by fluorine ions generated by moisture that has penetrated into the inside of the battery and the expansion of the negative electrode has led to an increase in the internal resistance of the battery.

  From this, in the negative electrode using lithium titanate as an active material, by mixing vapor grown carbon fiber (VGCF) and polyacrylic acid, even when water enters the battery, which is inevitable in actual use, It can be seen that excellent reliability can be realized without impairing the conductivity of the electrode.

(Examination of compounding ratio of polyacrylic acid)
A battery produced in the same manner as the battery A1 except that the negative electrode 2 was prepared by mixing lithium titanate with vapor grown carbon fiber (VGCF) and polyacrylic acid in a mass ratio of 94: 5: 1. The battery was A4.

  A battery produced in the same manner as the battery A1 except that the negative electrode 2 was prepared by mixing lithium titanate with vapor grown carbon fiber (VGCF) and polyacrylic acid in a mass ratio of 90: 5: 5. The battery was A5.

  A battery produced in the same manner as the battery A1 except that the negative electrode 2 was prepared by mixing lithium titanate with vapor grown carbon fiber (VGCF) and polyacrylic acid in a mass ratio of 88: 5: 7. Battery A6 was obtained.

The negative electrode 2 was the same as the battery A1, except that lithium titanate was mixed with vapor grown carbon fiber (VGCF) and polyacrylic acid in a mass ratio of 94.5: 5: 0.5. The produced battery was designated as battery B6.

  A battery produced in the same manner as the battery A1, except that the negative electrode 2 was prepared by mixing lithium titanate with vapor grown carbon fiber (VGCF) and polyacrylic acid in a mass ratio of 87: 5: 8. Battery B7 was obtained.

  A battery produced in the same manner as the battery A1 except that the negative electrode 2 was prepared by mixing lithium titanate with vapor grown carbon fiber (VGCF) and polyacrylic acid in a mass ratio of 85: 5: 10. Battery B8 was obtained.

  About these nonaqueous electrolyte secondary batteries, after assembling, they were charged with a constant voltage of 2.6 V for 24 hours (protection resistance 50Ω). About these batteries, the internal resistance of the battery after 30-day preservation | save in 70 degreeC90% high temperature and humidity environment was confirmed. In this experiment, the internal resistance of each battery was measured before starting the test for 50 batteries. For each battery, the volume of the negative electrode was measured before and after the test, and the expansion coefficient was calculated by the ratio (%) of the volume of the negative electrode after the test to the volume of the negative electrode before the test. These results are shown in (Table 2).

  From (Table 2), the batteries A4 to A6 using polyacrylic acid in a proportion of 1% by mass or more and 7% by mass or less based on the whole of the negative electrode showed the expansion of the negative electrode and the internal resistance of the battery when stored in a humid environment. The rise is largely suppressed. On the other hand, in the battery B6 in which the blending ratio of polyacrylic acid is less than 1% by mass with respect to the whole negative electrode, the negative electrode is greatly expanded and the internal resistance of the battery is increased. Further, in the batteries B7 and B8, in which the blending ratio of polyacrylic acid is larger than 7% by mass with respect to the whole negative electrode, the expansion of the negative electrode can be suppressed to almost zero, but the internal resistance of the battery is increased before and after the test. confirmed. This is considered to be because the permeability of the electrolyte solution into the electrode decreased due to an increase in insulation due to an increase in the amount of polyacrylic acid as an insulator and an increase in binding force.

  From this, it can be seen that it is more preferable that polyacrylic acid is used in a proportion of 1% by mass or more and 7% by mass or less with respect to the whole negative electrode.

(Examination of fiber length of vapor grown carbon fiber)
A battery produced in the same manner as the battery A1 was used as the battery A7, except that the vapor-grown carbon fiber (VGCF) used had a fiber length average (50% integrated diameter) of 3.2 μm on a number basis. .

  A battery produced in the same manner as the battery A1 was used as the battery A8, except that a vapor-grown carbon fiber (VGCF) having a fiber length average (50% integrated diameter) of 1.1 μm on a number basis was used. .

  A battery produced in the same manner as the battery A1 was used as the battery A9, except that the vapor-grown carbon fiber (VGCF) used had a fiber length average (50% integrated diameter) of 4.8 μm based on the number. .

  A battery produced in the same manner as the battery A1 was used as the battery B9, except that the vapor-grown carbon fiber (VGCF) used had a fiber length average (50% integrated diameter) of 0.8 μm based on the number. .

  A battery produced in the same manner as the battery A1 was used as the battery B10, except that the vapor-grown carbon fiber (VGCF) used had a fiber length average (50% integrated diameter) of 5.5 μm based on the number. .

  About these nonaqueous electrolyte secondary batteries, after assembling, they were charged with a constant voltage of 2.6 V for 24 hours (protection resistance 50Ω). About these batteries, the internal resistance of the battery after 30-day preservation | save in 70 degreeC90% high temperature and humidity environment was confirmed. In this experiment, the internal resistance of each battery was measured before starting the test for 50 batteries. For each battery, the volume of the negative electrode was measured before and after the test, and the expansion coefficient was calculated by the ratio (%) of the volume of the negative electrode after the test to the volume of the negative electrode before the test. These results are shown in (Table 3).

  From Table 3, batteries A7 to A9 using vapor-grown carbon fibers (VGCF) having a fiber length average (50% integrated diameter) of 1 μm or more and 5 μm or less on the basis of the number of expansion of the negative electrode and the inside of the battery The increase in resistance can be largely suppressed. On the other hand, in the battery B9 in which the fiber length average (50% integrated diameter) on the number basis is less than 1 μm and the battery B10 in which the fiber length average (50% integrated diameter) on the number basis is greater than 5 μm, the battery internal resistance increases. Was confirmed. In Battery B9, since the fiber length is short, it is considered that this is because a sufficient conductive network cannot be maintained against the expansion of the negative electrode. In the battery B10, since the fiber length is long, a large amount of vapor grown carbon fiber (VGCF) stress remains in the electrode, which is considered to be caused by the large expansion of the negative electrode with a decrease in the binding force.

  From this, it can be seen that it is more preferable to use a vapor-grown carbon fiber (VGCF) having an average fiber length (50% integrated diameter) of 1 μm or more and 5 μm or less on a number basis.

In this embodiment, a flat nonaqueous electrolyte secondary battery is shown as an example. However, the present invention is not limited to the flat shape, and can be applied to batteries having various shapes such as a cylindrical shape and a rectangular shape. However, since the flat battery whose sealing portion is a caulking seal is very difficult to suppress moisture ingress from the outside, the effect of the present invention is great.

  The non-aqueous electrolyte secondary battery of the present invention is useful as a main power source or backup power source for electronic devices and the like.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Battery case 5 Sealing plate 6 Gasket 7 Current collector

Claims (3)

A negative electrode using lithium titanate as a negative electrode active material and a positive electrode made of a transition metal oxide containing 4 V class lithium containing 4 V or more of an open circuit voltage when metallic lithium is used as a counter electrode are arranged opposite to each other with a separator interposed therebetween. In the non-aqueous electrolyte secondary battery housed in a battery container comprising a battery case, a sealing plate and a gasket together with a non-aqueous electrolyte containing LiPF ₆ as the negative electrode, the negative electrode binds vapor-grown carbon fiber as a conductive agent A non-aqueous electrolyte secondary battery using polyacrylic acid as an agent. The non-aqueous electrolyte secondary battery according to claim 1, wherein polyacrylic acid as the binder is used in a proportion of 1% by mass or more and 7% by mass or less with respect to the whole negative electrode. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the vapor-grown carbon fiber of the conductive agent is a vapor-grown carbon fiber having a fiber length average (50% integrated diameter) of 1 μm to 5 μm. .