JP2005302333A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance charge/discharge cycle characteristics of a nonaqueous electrolyte secondary battery having a nonaqueous electrolyte containing γ-butyrolactone. <P>SOLUTION: The nonaqueous electrolyte secondary battery is equipped with a positive electrode, a negative electrode, and the nonaqueous electrolyte containing γ-butyrolactone, and the negative electrode satisfies the relation I<SB>A</SB>/I<SB>B</SB>≤0.4 of reduction peak intensity I<SB>A</SB>(mA) appearing between 0.5 V and 0.7 V vs. lithium potential in potential sweep in a first cycle and a reduction peak intensity I<SB>B</SB>(mA) appearing between 0.3 V and 0.5 V vs. lithium potential when potential sweep is conducted at a sweep rate of 0.5 mV/s or less in the reduction direction in cyclic voltammetry (CV) of a triode cell. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  In recent years, electronic devices such as mobile communication devices, notebook computers, palmtop computers, integrated video cameras, portable CD (MD) players, cordless telephones, etc. have been reduced in size and weight. As a power source, a particularly small and large capacity battery is required.

  Examples of batteries that are widely used as power sources for these electronic devices include primary batteries such as alkaline manganese batteries, and secondary batteries such as nickel cadmium batteries and nickel metal hydride batteries. Among these, non-aqueous electrolyte secondary batteries that use lithium composite oxide for the positive electrode and carbonaceous materials that can occlude and release lithium ions for the negative electrode are small and light, have high unit cell voltage, and have high energy density. Is attracting attention.

  In recent years, as a container for housing the electrode group including the positive electrode and the negative electrode as described above, an outer can made of a metal plate such as aluminum or a laminate film obtained by bonding a metal foil such as aluminum and a resin into a bag shape. Or what was shape | molded in cup shape etc. is used. In particular, the container made of a laminate film has made it possible to further reduce the weight and size of the nonaqueous electrolyte secondary battery.

  However, in a non-aqueous electrolyte secondary battery using a container made of such a metal plate or laminate film, since the container is highly flexible, gas is generated due to a decomposition reaction of the non-aqueous electrolyte in the battery at the first charge. There is a problem that the thickness of the battery increases or the container is deformed. An increase in the thickness of the battery and deformation of the container not only deteriorate the appearance but also adversely affect the battery performance.

To solve this problem, γ-butyrolactone (GBL), which is unlikely to cause gas generation due to a decomposition reaction at the time of initial charge, and also difficult to cause an exothermic reaction even in situations such as overcharge, is replaced with ethylene carbonate ( A mixture of cyclic carbonates such as EC) is used as a non-aqueous solvent, and the use of a non-aqueous electrolyte using lithium tetrafluoroborate (LiBF ₄ ), which hardly causes an exothermic reaction, is being studied as an electrolyte. .

  For example, Patent Document 1 relates to a lithium secondary battery in which a lithium transition metal oxide is used for a positive electrode and a graphite-based material is used for a negative electrode. In an electrolyte, γ-butyrolactone is a main component, and at least as a subcomponent. It is described that an organic electrolytic solution in which a lithium salt is dissolved in a solvent containing ethylene carbonate is used.

  Although γ-butyrolactone can suppress gas generation during the initial charge, it has a problem that it reacts with the negative electrode during the charge / discharge cycle to form a by-product and deteriorates the charge / discharge cycle characteristics.

  In Patent Document 2, by adding an additive and VC to the electrolytic solution, the potential sweep at a rate of 10 mV / sec causes 0.6 V to 0.3 V due to a reduction reaction on the basal surface on the HOPG electrode. If both the reduction peak appearing between the two and the reduction peak appearing between 0.9V and 0.6V due to the reduction reaction at the edge surface on the HOPG electrode are reduced, the reductive decomposition reaction of the electrolyte solution at the negative electrode It is described that the self-discharge rate during storage at high temperatures is reduced.

However, when an electrolytic solution containing γ-butyrolactone was used, excellent charge / discharge cycle characteristics could not be obtained even when the above-described negative electrode having two smaller peaks was used.
JP 11-31525 A Japanese Patent Laid-Open No. 2002-158035

  An object of the present invention is to improve charge / discharge cycle characteristics of a nonaqueous electrolyte secondary battery including a nonaqueous electrolyte containing γ-butyrolactone.

A nonaqueous electrolyte secondary battery according to the present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte containing γ-butyrolactone,
When the potential sweep was performed in the reduction direction at a sweep rate of 0.5 mV / sec or less in cyclic voltammetry (CV) in a three-electrode cell, the negative electrode was zero as a potential against lithium in the potential sweep in the first cycle. The reduction peak intensity I _A (mA) appearing between 5 V and 0.7 V and the reduction peak intensity I _B (mA) appearing between 0.3 V and 0.5 V with respect to the lithium potential are expressed by the following formula (1). It is characterized by satisfaction.

I _A / I _B ≦ 0.4 (1)

  ADVANTAGE OF THE INVENTION According to this invention, the charge / discharge cycle characteristic of the nonaqueous electrolyte secondary battery provided with the nonaqueous electrolyte containing (gamma) -butyrolactone can be improved.

  When the potential sweep is performed in the reduction direction at a large speed of 10 mV / sec as in Patent Document 1 described above, the sweep speed exceeds the lithium ion occlusion speed of the negative electrode, so that the initial state before the lithium ions are occluded is obtained. The reaction between the near negative electrode and the nonaqueous electrolyte is the main. The reaction between the negative electrode and γ-butyrolactone (GBL) during the charge / discharge cycle is more likely to occur in a state where lithium ions are occluded in the negative electrode. Therefore, when a non-aqueous electrolyte containing GBL is used, Patent Literature Even if the negative electrode having two small peaks described in 1 is used, a long-life secondary battery cannot be obtained.

  In the cyclic voltammetry (CV), the present inventors obtained the intensity ratio of two reduction peaks obtained when the potential sweep was performed in the reduction direction at a low speed of 0.5 mV / second or less, and the charge / discharge cycle. A correlation was found between the reactivity of the negative electrode to GBL and the present invention was achieved.

  In the triode cell used for cyclic voltammetry (CV) measurement, the negative electrode is a working electrode, and the reference electrode and the counter electrode are metallic lithium. Moreover, what has the composition similar to a nonaqueous electrolyte is used for the lithium ion conductive solvent which immerses these three electrodes.

  Here, the reason why the sweep speed is regulated to 0.5 mV / second or less is for the reason described below.

When the potential sweep is performed at a rate exceeding 0.5 mV / second, the potential change becomes faster than the rate at which lithium ions are occluded in the negative electrode. Therefore, the negative electrode in a state where lithium ions are not sufficiently occluded and the nonaqueous electrolyte Since a reaction is observed, the reaction with GBL is not suppressed even when a negative electrode whose reduction peak intensities I _A and I _B satisfy the above formula (1) is used for the secondary battery. For this reason, the sweep speed is set to 0.5 mV / second or less.

  The reason why the reduction peak in the first cycle of the potential sweep is observed is that the reduction peak in the first cycle appears with the highest intensity. In the triode cell used for CV measurement, the lithium ion conductive solvent is in excess with respect to the size of the negative electrode surface, so film formation and side reactions that can occur on the negative electrode by the potential sweep in the first cycle are almost completed. To do. For this reason, a peak appears with the highest intensity in the potential sweep in the first cycle, and the peak intensity tends to be weak after the second cycle. In an actual cell, since the nonaqueous electrolyte is in an appropriate amount relative to the size of the negative electrode surface, the side reaction that can occur on the negative electrode surface is considered to proceed gradually.

Reduction peak appearing between the 0.3V~0.5V versus lithium potential (hereinafter, referred to as a reduction peak B, and indicates the intensity I _B) is for less dependence on the type of the negative electrode and a nonaqueous electrolyte On the other hand, a reduction peak appearing between 0.5 V and 0.7 V (hereinafter referred to as reduction peak A, and its intensity is expressed as I _A ) is likely to vary depending on the type of the negative electrode or nonaqueous electrolyte. When a negative electrode having a small difference in peak intensity such that the two peak intensity ratios I _A / I _B exceed 0.4 is used, a significant amount of side reaction product is generated by reacting with GBL. Life is shortened. The reduction peak A appearing between the 0.5V~0.7V made smaller, by the peak intensity ratio I _A / I _B uses a negative electrode of 0.4 or less, the reaction of the non-aqueous electrolyte containing GBL It is suppressed, deterioration of cycle characteristics due to formation of side reaction products can be prevented, and cycle life can be improved. Moreover, if this negative electrode is used, the distortion of the electrode group resulting from expansion | swelling of the electrode itself in a secondary battery can also be suppressed.

Although the peak intensity ratio I _A / I _B is less likely to occur reaction with small negative electrode as GBL, its lower limit is preferably 0.05.

It is due to reasons explained below to define the lower limit of the peak intensity ratio I _A / I _B to the number.

When the peak intensity ratio I _A / I _B is less than 0.05, the energy density is because there may be reduced. That is, as a means for the peak intensity ratio I _A / I _B below 0.05, lowering the electrode density of the negative electrode, there is a method to increase the vinylene carbonate (VC) amount of the nonaqueous electrolyte. If the electrode density is reduced, the capacity itself is lowered, and this may reduce the energy density. On the other hand, when the amount of VC added is increased, the film on the electrode surface becomes too thick, which may lead to a decrease in capacity.

  Hereinafter, the constituent components of the negative electrode will be described.

  The negative electrode includes a current collector and a negative electrode layer that is supported on one or both surfaces of the current collector and contains a negative electrode active material.

  The negative electrode active material is capable of occluding and releasing lithium ions, for example, graphite materials such as graphite, coke, carbon fiber, spherical carbon, pyrolytic vapor carbonaceous material, resin fired body, composite graphite material, or Carbonaceous materials: Thermosetting resins, isotropic pitches, mesophase pitch-based carbon, mesophase pitch-based carbon fibers, mesophase microspheres, etc. (especially mesophase pitch-based carbon fibers are preferred because their capacity and charge / discharge cycle characteristics increase) Examples thereof include a graphite material or a carbonaceous material obtained by performing a heat treatment at 500 to 3000 ° C. As the negative electrode active material, one or more kinds of materials selected from these graphite materials and carbonaceous materials can be used.

  Here, the composite graphite material is obtained by forming a carbon material layer having lower crystallinity than the graphite particles on at least a part of the surface of the graphite particles.

  In order to avoid a decrease in the amount of occluded / released lithium ions, the proportion of the carbon material layer in the composite graphite material is preferably 30% by weight or less.

Plane spacing d ₀₀₂ of the composite graphite material is preferably less than 0.336 nm. The lower limit of the surface spacing d ₀₀₂ is spacing of (002) plane in complete graphite crystal d _002, i.e. it is preferable to 0.3354 nm.

  The composite graphite material may have a peak or a peak at diffraction angles 2θ of 42.8 ° to 44.0 ° and 45.5 ° to 46.6 ° in powder X-ray diffraction measurement using CuKα rays. Although it can be used, it is desirable that a peak appears from the viewpoint of suppressing the decomposition reaction of the solvent.

The specific surface area of the composite graphite material by the BET method is desirably in the range of 0.5 to 10 m ² / g.

The average particle size (D ₅₀ ) of the composite graphite material is preferably in the range of 3 to 50 μm.

  As the negative electrode active material, it is preferable to use a mesophase pitch-based carbon fiber (MCF) or a composite graphite material among the above-described graphite materials and carbonaceous materials.

  Examples of the non-aqueous electrolyte when the negative electrode is used in a secondary battery include those described below.

  The non-aqueous electrolyte includes a non-aqueous solvent containing GBL and an electrolyte (for example, lithium salt) dissolved in the non-aqueous solvent. The non-aqueous electrolyte can be in a liquid or gel form.

  The weight ratio of GBL in the non-aqueous solvent is preferably in the range of 40 to 80% by weight since a significant improvement in charge / discharge cycle life is observed.

  It is desirable that the non-aqueous solvent further contains a cyclic carbonate. Examples of the cyclic carbonate include ethylene carbonate (EC) and propylene carbonate (PC). The type of cyclic carbonate can be one type or two or more types. In particular, the non-aqueous solvent preferably contains EC. This is because the reduction peak B appearing between 0.3 V and 0.5 V with respect to the lithium potential is considered to be derived from EC. The weight ratio of EC in the non-aqueous solvent is preferably in the range of 20 to 50% by weight.

  The non-aqueous solvent may contain a solvent other than GBL and cyclic carbonate as an auxiliary component.

  As an auxiliary component, for example, vinylene carbonate (VC), vinyl ethylene carbonate (VEC), phenyl ethylene carbonate (phEC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), γ-valerolactone (VL), methyl propionate (MP), ethyl propionate (EP), 2-methylfuran (2Me-F), furan (F), thiophene (TIOP), catechol carbonate (CATC), ethylene sulfite (ES) , 12-crown-4 (Crown), tetraethylene glycol dimethyl ether (Ether), trioctyl phosphate (TOP) and the like.

  The kind of subcomponent can be made into one kind or two kinds or more.

  The weight ratio of the minor component in the non-aqueous solvent is desirably in the range of 10% by weight or less.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenide (LiAsF ₆ ), and trifluoro Lithium such as lithium metasulfonate (LiCF ₃ SO ₃ ), bistrifluoromethylsulfonylimide lithium (LiN (CF ₃ SO ₂ ) ₂ ), bispentafluoroethylsulfonylimide lithium (LiN (C ₂ F ₅ SO ₂ ) ₂ ) Mention may be made of salts. The type of electrolyte used can be one type or two or more types.

Among them, LiBF ₄ is preferable because it has a low reactivity with the positive electrode when the temperature of the secondary battery rises, and thus can quickly terminate the decomposition reaction of the nonaqueous electrolyte after the current interruption. Further, a mixed salt containing a lithium salt made of at least one of LiN (CF ₃ SO ₂ ) ₂ and LiN (C ₂ F ₅ SO ₂ ) ₂ and a lithium salt made of LiBF ₄ , or LiBF ₄ and LiPF _When a mixed salt containing ₆ is used, the cycle life at a high temperature can be further improved.

  The concentration of the electrolyte in the non-aqueous electrolyte is preferably 0.5 to 2.5 mol / L.

  The amount of the nonaqueous electrolyte is preferably 0.2 to 0.6 g per 100 mAh of battery unit capacity.

  The negative electrode used in the present invention is, for example, a kneaded mesophase pitch-based carbon fiber (MCF) or composite graphite material and a binder in the presence of a solvent, and the resulting suspension is applied to a current collector and dried. After that, it can be produced by pressing once or multistage pressing 2 to 5 times. At this time, the type of the negative electrode active material according to the VC content in the non-aqueous electrolyte (for example, particle size distribution, ratio) The surface area, the degree of graphitization, etc.) and the electrode density of the negative electrode are important.

  A method for producing a negative electrode used in the present invention will be described more specifically.

  When the non-aqueous electrolyte to be used does not contain VC as a subcomponent, the negative electrode can be produced by using MCF as the negative electrode active material and adjusting the press pressure so as to obtain a low electrode density.

  On the other hand, when the non-aqueous electrolyte to be used contains VC as a subcomponent, MCF or composite graphite material can be used as the negative electrode active material. As for the electrode density, the tendency to be easily cracked when pressed is the same for both MCF and composite graphite materials. Good release characteristics. For this reason, when using a composite graphite material, it is possible to obtain a negative electrode having low reactivity to GBL by increasing the electrode density and increasing the amount of VC added. In the case of MCF, a negative electrode having low reactivity with respect to GBL can be obtained by increasing the electrode density to such an extent that it does not break and adjusting the amount of VC added according to this electrode density.

  When MCF is used as the negative electrode active material, the electrode density of the negative electrode is preferably in the range of 1.10 to 1.60 g / cc. When a composite graphite material is used as the negative electrode active material, the electrode density of the negative electrode is preferably in the range of 1.10 to 1.70 g / cc.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like. Can be used.

  The mixing ratio of the negative electrode active material and the binder is preferably in the range of 80 to 98% by weight of the negative electrode active material and 2 to 20% by weight of the binder.

  As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, copper, stainless steel, or nickel.

  Next, the positive electrode will be described.

  The positive electrode includes a current collector and a positive electrode layer supported on one or both surfaces of the current collector and containing a positive electrode active material, a binder, and a conductive agent.

Examples of the positive electrode active material include various oxides such as manganese dioxide, lithium manganese composite oxide, lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel cobalt oxide, lithium-containing iron oxide, and lithium. Examples thereof include vanadium oxide and chalcogen compounds such as titanium disulfide and molybdenum disulfide. Among them, when a lithium-containing cobalt oxide (for example, LiCoO ₂ ), a lithium-containing nickel cobalt oxide (for example, LiNi _0.8 Co _0.2 O ₂ ), or a lithium manganese composite oxide (for example, LiMn ₂ O ₄ , LiMnO ₂ ) is used. This is preferable because a high voltage can be obtained. Note that as the positive electrode active material, one kind of oxide or chalcogen compound may be used alone, or two or more kinds of oxide or chalcogen compound may be mixed and used. Moreover, you may use what mixed the oxide and the chalcogen compound as a positive electrode active material.

  Examples of the conductive agent include acetylene black, carbon black, and graphite.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyether sulfone, ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). Can be used.

  As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, aluminum, stainless steel, or nickel.

  The positive electrode can be produced, for example, by suspending a positive electrode active material, a conductive agent and a binder in an appropriate solvent, applying the suspension to a current collector, and drying to form a thin plate. .

  The positive electrode and the negative electrode can form an electrode group with a separator or the like interposed therebetween.

  As the separator, a microporous film, a woven fabric, a non-woven fabric, a laminate of the same material or different materials, or the like can be used. In particular, the microporous membrane causes a so-called shutdown phenomenon in which the resin constituting the separator plastically deforms and closes the fine pores when the temperature of the electrode group rises abnormally due to heat generated by overcharging or the like. This is preferable because the ion flow is cut off, further heat generation is prevented, and the overcharge state can be safely terminated. Examples of the material for forming the separator include polyethylene, polypropylene, ethylene-propylene copolymer, and ethylene-butene copolymer. As a material for forming the separator, one type or two or more types can be selected and used from the types described above.

  For example, (i) the positive electrode and the negative electrode are wound in a flat shape or a spiral shape with a separator interposed therebetween, or (ii) the positive electrode and the negative electrode are wound in a spiral shape with a separator interposed therebetween. Rotating and then compressing in the radial direction, or (iii) bending the positive electrode and the negative electrode one or more times with a separator interposed therebetween, or (iv) laminating the positive electrode and the negative electrode with a separator interposed therebetween Can be produced.

  The electrode group need not be pressed, but may be pressed to increase the integrated strength of the positive electrode, the negative electrode, and the separator. It is also possible to heat at the time of pressing.

  A thin nonaqueous electrolyte secondary battery which is an example of a nonaqueous electrolyte secondary battery according to the present invention will be described with reference to FIGS.

  FIG. 1 is a perspective view showing a thin non-aqueous electrolyte secondary battery which is an example of a non-aqueous electrolyte secondary battery according to the present invention, and FIG. 2 shows the thin non-aqueous electrolyte secondary battery of FIG. It is the fragmentary sectional view cut | disconnected along.

  As shown in FIG. 1, an electrode group 2 is accommodated in a container body 1 having a rectangular cup shape. The electrode group 2 has a structure in which a laminate including a positive electrode 3, a negative electrode 4, and a separator 5 disposed between the positive electrode 3 and the negative electrode 4 is wound into a flat shape. The nonaqueous electrolyte is held in the electrode group 2. A lid plate 6 is integrated with the container body 1. The container body 1 and the cover plate 6 are each composed of a laminate film. The laminate film includes an external protective layer 7, an internal protective layer 8 containing a thermoplastic resin, and a metal layer 9 disposed between the external protective layer 7 and the internal protective layer 8. A lid 6 is fixed to the container body 1 by heat sealing using a thermoplastic resin of the inner protective layer 8, whereby the electrode group 2 is sealed in the container. A positive electrode tab 10 is connected to the positive electrode 3, and a negative electrode tab 11 is connected to the negative electrode 4, and each is pulled out of the container and serves as a positive electrode terminal and a negative electrode terminal.

  A prismatic nonaqueous electrolyte secondary battery as another example of the nonaqueous electrolyte secondary battery according to the present invention will be described with reference to FIG.

  As shown in FIG. 3, a bottomed rectangular tube-shaped outer can 21 made of metal, for example, aluminum also serves as a positive electrode terminal, for example, and an insulator 22 is disposed on the inner surface of the bottom. An electrode group 23 is accommodated in the outer can 21. The electrode group 23 is produced by winding the negative electrode 24, the separator 25, and the positive electrode 26 in a spiral shape so that the positive electrode 26 is located on the outermost periphery, and then press-molding it into a flat shape. . A positive electrode lead (not shown) is connected to the current collector of the positive electrode 26, and the other end of the positive electrode lead is connected to the outer can 21.

  A spacer 27 made of, for example, a synthetic resin having a lead extraction hole in the vicinity of the center is disposed on the electrode group 23 in the outer can 21. The metal lid 28 is airtightly joined to the upper end opening of the outer can 21 by, for example, laser welding. An extraction hole 29 for the negative electrode terminal 30 is opened near the center of the lid body 28. The lid 28 is provided with a safety valve mechanism (not shown) that breaks and releases the internal pressure when the internal pressure in the outer can 21 rises extremely. The negative electrode terminal 30 is hermetically sealed in the extraction hole 29 of the lid 28 via an insulating material 31 made of glass or resin. A negative electrode lead 32 is connected to the lower end surface of the negative electrode terminal 30, and the other end of the negative electrode lead 32 is connected to a current collector of the negative electrode 24 of the electrode group 23. The insulating sealing plate 33 is disposed on the upper surface of the lid body 28. The insulating outer tube 34 covers the side and bottom edges of the outer can 21 and the outer periphery of the insulating sealing plate 33.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

[Example]
Embodiments of the present invention will be described below with reference to the drawings.

(Example 1)
<Production of negative electrode>
In the particle size distribution measured by the laser diffraction method, the average particle size is 14 μm, the particle size is 6.8 to 24.3 μm, and 80% by volume is present, and the particles having a particle size of 6.8 μm or less are 10% or less. A mesophase pitch carbon fiber (MCF) pulverized moderately was prepared. This MCF had a specific surface area by the BET method of 2.43 m ² / g, and the (002) plane spacing (d ₀₀₂ ) by powder X-ray diffraction was 0.336 nm. In addition, the space _| interval d002 of (002) plane is the value calculated | required by the half-value-width midpoint method from the powder X-ray diffraction spectrum. At this time, scattering correction such as Lorentz scattering was not performed.

A slurry was prepared by adding 1.2 wt% CMC and 1.7 wt% SBR to 97.1 wt% of the MCF as a carbonaceous material and mixing them. The slurry was applied to both sides of a current collector made of copper foil having a thickness of 12 μm so as to be 78 g / m ² , dried, and pressed to support the negative electrode layer on both sides of the current collector. A negative electrode having the above structure was produced.

  The electrode density of the obtained negative electrode was measured as described below.

  A negative electrode having an active material coated on both sides was cut into a size of 5 cm × 5 cm, and the total weight and thickness of the electrode were measured. Next, the active material layer was peeled off from both surfaces of the electrode using acetone, and the weight and thickness of the current collector were measured. The density of the electrode was calculated by (total electrode weight−current collector weight) / ((electrode thickness−current collector thickness) × area). The results are shown in Table 1.

<Preparation of liquid nonaqueous electrolyte (nonaqueous electrolyte)>
A non-aqueous solvent is prepared by adding 1% by weight of vinylene carbonate (VC) to a mixture of ethylene carbonate (EC) and γ-butyrolactone (GBL) so that the weight ratio (EC: GBL) is 35:65. did. Lithium tetrafluoroborate (LiBF ₄ ) was dissolved in the obtained non-aqueous solvent so that its concentration was 1.5 mol / L to prepare a non-aqueous electrolyte.

<Preparation of positive electrode>
First, 90% by weight of lithium-containing cobalt oxide (Li _x CoO ₂ ; x is 0 <x ≦ 1) powder contains 5% by weight of acetylene black and 5% by weight of polyvinylidene fluoride (PVdF). Dimethylformamide (DMF) solution was added and mixed to prepare a slurry. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm, then dried and pressed to produce a positive electrode having a structure in which the positive electrode layer was supported on both sides of the current collector.

<Separator>
A separator made of a microporous polyethylene film having a thickness of 25 μm and a porosity of 45% was prepared.

<Assembly of secondary battery>
After the positive electrode lead made of a strip-shaped aluminum foil (thickness 100 μm) is ultrasonically welded to the positive electrode current collector, and the negative electrode lead made of a strip-shaped nickel foil (thickness 100 μm) is ultrasonically welded to the negative electrode current collector The positive electrode and the negative electrode were spirally wound through the separator between them to prepare an electrode group. The electrode group was pressed into a flat shape by pressing with a press.

  A laminate film having a thickness of 100 μm in which both surfaces of an aluminum foil were covered with polyethylene was formed into a rectangular cup shape by a press machine, and the electrode group was housed in the obtained container.

  Next, the electrode group in the container was vacuum dried at 80 ° C. for 12 hours to remove moisture contained in the electrode group and the laminate film.

  By injecting the non-aqueous electrolyte into the electrode group in the container so that the amount per 1Ah of battery capacity is 4.8 g and sealing with heat sealing, the structure shown in FIGS. A thin nonaqueous electrolyte secondary battery having a thickness of 3.6 mm, a width of 35 mm, a height of 62 mm, and a nominal capacity of 0.60 Ah was assembled.

(Example 2)
A thin nonaqueous electrolyte secondary battery was assembled in the same manner as in Example 1 except that VC was not added to the nonaqueous electrolyte.

(Examples 3 and 4)
A thin nonaqueous electrolyte secondary battery was assembled in the same manner as in Example 1 except that the electrode density of the negative electrode and the VC addition amount of the nonaqueous electrolyte were changed to the values shown in Table 1.

(Examples 5 and 6)
A carbon material layer is formed by subjecting natural graphite particles that have been spheroidized to surface coating treatment by plasma CVD, and a composite graphite material having an average particle diameter D ₅₀ of 30 μm and a specific surface area of 1.90 m ² / g by BET method is obtained. Produced. The proportion of the carbon material layer in the composite graphite material was 3% by weight. The obtained composite graphite material had a (002) plane spacing (d ₀₀₂ ) of 0.3356 nm by powder X-ray diffraction, and a diffraction angle 2θ of 43.35 ° and 46 in an X-ray diffraction measurement using CuKα rays. A peak appeared at 19 °. In addition, the space _| interval d002 of (002) plane is the value calculated | required by the half-value-width midpoint method from the powder X-ray diffraction spectrum. At this time, scattering correction such as Lorentz scattering was not performed.

  A thin nonaqueous electrolyte 2 was prepared in the same manner as in Example 1 except that the composite graphite material obtained as the negative electrode active material was used, and the electrode density of the negative electrode and the VC addition amount of the nonaqueous electrolyte were changed to the values shown in Table 1. The next battery was assembled.

(Comparative Examples 1-3)
A thin nonaqueous electrolyte secondary battery was assembled in the same manner as in Example 1 except that the electrode density of the negative electrode and the VC addition amount of the nonaqueous electrolyte were changed to the values shown in Table 1.

(Comparative Examples 4-6)
A composite graphite material of the same type as in Example 5 was used as the negative electrode active material, and the non-thin non-thickness was formed in the same manner as in Example 1 except that the electrode density of the negative electrode and the VC addition amount of the nonaqueous electrolyte were changed to the values shown in Table 1. A water electrolyte secondary battery was assembled.

  In addition, about the negative electrode of Examples 1-6 and Comparative Examples 1-6, CV measurement was performed so that it might demonstrate below before an assembly.

<CV measurement>
A part of the obtained negative electrode layer was cut out to prepare a tripolar cell, and the reduction peak intensity was measured by a cyclic voltammetry (CV) method. The method of manufacturing and measuring this tripolar cell is as follows.

  The working electrode is obtained by attaching a nickel lead electrode to a negative electrode layer cut out in a 2 × 2 cm square. The counter electrode is obtained by attaching a stainless steel lead electrode to metallic lithium having the same size as the cut-out negative electrode layer. The working electrode and the counter electrode thus produced were fixed to face each other with a glass filter interposed therebetween. Furthermore, the reference electrode produced by the same method as the counter electrode was fixed at a position where neither the working electrode nor the counter electrode was in contact. The working electrode, the counter electrode, and the reference electrode thus produced are placed in a glass or resin cell, and a lithium ion conductive solvent having the same composition as the liquid non-aqueous electrolyte is added to the entire laminate including the electrode pressing plate. An amount sufficient to immerse, 20 mL in this case, was injected and sealed to form the triode cell shown in FIG.

  As shown in FIG. 4, a working electrode in which a nickel lead electrode 43 is attached to a negative electrode layer 42 is accommodated in an outer case 41 made of glass or resin. On the negative electrode layer 42 of the working electrode, a measurement separator 44 made of a glass filter is laminated. On the measurement separator 44, a counter electrode in which a stainless steel lead electrode 46 is attached to a metal lithium 45 is laminated. The electrode laminate having the configuration described above is sandwiched between two electrode pressing plates 47a and 47b made of, for example, an acrylic plate. One electrode pressing plate 47 a is stacked on the lead electrode 46, and the other electrode pressing plate 47 b is stacked on the lead electrode 43. The electrode pressing plates 47a and 47b are fixed by bolts 48 and nuts 49, whereby the counter electrode, the separator, and the working electrode can be fixed in contact with each other. The lithium ion conductive solvent 50 is accommodated in the outer case 41.

  The reference electrode 51 made of lithium foil is immersed in the lithium ion conductive solvent 50. The lead electrodes 43 and 46 and the reference electrode 51 are electrically connected to a current / voltage detector (not shown) by leads.

  The thus configured tripolar cell is placed in a constant temperature bath at 25 ° C., the potential measuring terminal of a CV measuring device (eg Solartoron 1286 manufactured by Solartron) is connected to the working electrode and the reference electrode, and the current measuring terminal is connected to the working electrode. And connect to the counter electrode. In this state, a potential sweep was performed from the natural potential in the reduction direction at a sweep rate of 0.4 mV / sec. An example of the result is shown in FIG. In FIG. 5, the vertical axis indicates the current value (mA), and the horizontal axis indicates the potential (V).

As is clear from FIG. 5, a reduction peak A indicated by an arrow A appears between 0.5 V and 0.7 V in the reduction direction with respect to lithium potential, and between 0.3 V and 0.5 V. A reduction peak B indicated by an arrow B in the figure appeared. The resulting peak intensity I _A of the reduction peak A to peak intensity I _B of the reduction peak B were measured, Table 1 shows the result of obtaining the peak intensity ratio I _A / I _B.

When the peak is broken like the reduction peak A, the larger peak intensity is I _A. The same applies to the reduction peak B.

  Although not observable in FIG. 5, the peak detected when the VC film is formed on the negative electrode appears in the vicinity of 1 V. Therefore, it can be seen that the reduction peak A is not a peak due to the VC film. From this, it is considered that the reduction peak A is a peak derived from a side reaction product formed slightly by reacting the negative electrode with GBL. On the other hand, the reduction peak B is considered to be a peak derived from EC which is a component of the nonaqueous solvent.

  The secondary battery of Examples 1 to 6 and Comparative Examples 1 to 6 after assembly was subjected to the following treatment as the initial charge / discharge process. First, after injecting, the solution was allowed to stand at room temperature for 6 hours, and then charged with constant current / constant voltage to 4.2 V at 0.2 C at room temperature for 15 hours. Then, it discharged to 3.0V at 0.2C at room temperature.

  Here, 1C is a current value necessary for discharging the nominal capacity (Ah) in one hour. Therefore, 0.2 C is a current value necessary for discharging the nominal capacity (Ah) in 5 hours.

  The secondary battery after the first charge / discharge was subjected to a charge / discharge cycle test as described below.

<Charge / discharge cycle test>
About the obtained secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 6, charging / discharging at a charge / discharge rate of 1 C, a charge end voltage of 4.2 V, and a discharge end voltage of 3.0 V was performed in an environment at a temperature of 20 ° C. The 500th discharge capacity when it was repeated 500 times was measured. The discharge capacity at this time is expressed with the first discharge capacity as 100%, and the result is shown in the following Table 1 as the discharge capacity retention rate.

As is evident from Table 1, the secondary batteries of Examples 1 to 6 in which the peak intensity ratio I _A / I _B is used a negative electrode is 0.4 or less, the discharge capacity of the charge and discharge 500 times th, 1 st It exceeded 80% of the discharge capacity, and was excellent in charge / discharge cycle characteristics. From these results, it was confirmed that the secondary batteries of Examples 1 to 6 were suitable for use as a power source for portable devices used while being repeatedly charged and discharged.

Further, from Example 1 and Example 2, when MCF was used as the negative electrode active material and the electrode density of the negative electrode was 1.20 g / cc, Example 1 in which VC was added to the non-aqueous electrolyte but it is possible to reduce the peak intensity ratio I _a / I _B than in example 2 was not added to VC, it was confirmed that the increase of the discharge capacity maintenance rate. This is presumably because a dense protective film was formed on the negative electrode surface due to the addition of VC, thereby reducing the reactivity of the negative electrode with GBL.

By comparing Example 3 and Example 4, even when MCF was used as the negative electrode active material, the amount of VC added to the non-aqueous electrolyte was increased when the electrode density of the negative electrode was increased to 1.50 g / cc. even increased, showed no significant change in the peak intensity ratio I _a / I _B, it was confirmed that the increase in the amount of VC added is not unconditionally results in decreased GBL reactivity of the negative electrode.

On the other hand, secondary batteries of Comparative Examples 1 to 6 in which the peak intensity ratio I _A / I _B is using the anode in excess of 0.4, the discharge capacity retention ratio is below 78%, second Examples 1-6 following Compared with the battery, the charge / discharge cycle characteristics were deteriorated.

  This is because, in the secondary battery of Comparative Example 1, although MCF having low reactivity with the non-aqueous electrolyte was used as the negative electrode active material, the negative electrode had a higher electrode density than that of VC without addition. This is thought to be due to the increased reactivity to GBL.

  In the secondary batteries of Comparative Examples 2 and 3, the MCF used as the negative electrode active material was cracked during pressing, and the characteristics of the active material itself were deteriorated. Therefore, it is considered that the charge / discharge cycle characteristics were deteriorated.

  From the results of Comparative Examples 4 and 5, it can be seen that, when a composite graphite material is used as the negative electrode active material, a negative electrode having a high reactivity with GBL can be obtained when VC is not added even if the density is low.

  In the secondary battery of Comparative Example 6, although VC was added to the non-aqueous electrolyte, the amount of VC added was small relative to the electrode density of the negative electrode, and thus the reaction between the negative electrode and GBL could not be sufficiently suppressed. Therefore, it is considered that the charge / discharge cycle characteristics deteriorated.

In comparison between Comparative Example 3 and Comparative Example 6, although Comparative Example 6 using the composite graphite material as the negative electrode active material was used in Comparative Example 3 using MCF, despite the same electrode density and VC addition amount. increases the peak intensity ratio I _a / I _B as compared with the discharge capacity maintenance rate decreased. This is presumably because the composite graphite material is more susceptible to cracking during pressing when the electrode density is increased, and thus the reactivity with the non-aqueous electrolyte is higher than that of MCF.

The perspective view which shows the thin nonaqueous electrolyte secondary battery which is an example of the nonaqueous electrolyte secondary battery which concerns on this invention. The fragmentary sectional view which cut | disconnected the thin nonaqueous electrolyte secondary battery of FIG. 1 along the II-II line. The partial notch perspective view which shows the square nonaqueous electrolyte secondary battery which is an example of the nonaqueous electrolyte secondary battery which concerns on this invention. The block diagram which shows the outline of the tripolar cell for performing CV measurement to the negative electrode used for the nonaqueous electrolyte secondary battery of Examples 1-6 and Comparative Examples 1-6. The characteristic line figure which shows an example of the CV measurement result of a negative electrode.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Container main body, 2,23 ... Electrode group, 3,26 ... Positive electrode, 4,24 ... Negative electrode, 5,25 ... Separator, 6 ... Cover plate, 7 ... External protective layer, 8 ... Internal protective layer, 9 ... Metal Layers 10, positive electrode tabs 11, negative electrode tabs 21, exterior cans 22, insulators 27, spacers 28, lid bodies 29, extraction holes, 30 negative electrode terminals, 31 insulating materials, 32 negative electrode leads 33 ... Insulating sealing plate, 34 ... Exterior tube, 41 ... Exterior case, 42 ... Negative electrode layer, 43, 46 ... Lead electrode, 44 ... Measuring separator, 45 ... Metallic lithium, 47a, 47b ... Electrode holding plate, 48 ... Bolt, 49 ... nut, 50 ... lithium ion conductive solvent, 51 ... reference electrode.

Claims (1)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte containing γ-butyrolactone,
When the potential sweep was performed in the reduction direction at a sweep rate of 0.5 mV / sec or less in cyclic voltammetry (CV) in a three-electrode cell, the negative electrode was zero as a potential against lithium in the potential sweep in the first cycle. The reduction peak intensity I _A (mA) appearing between 5 V and 0.7 V and the reduction peak intensity I _B (mA) appearing between 0.3 V and 0.5 V with respect to the lithium potential are expressed by the following formula (1). A nonaqueous electrolyte secondary battery characterized by being satisfactory.
I _A / I _B ≦ 0.4 (1)

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