JP2011204578A - Electrode for lithium secondary battery, and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for a lithium secondary battery, superior in binding strength between active material particles and binding strength between an electrode mixture layer and a collector while suppressing excessive particle breakage in the process of manufacturing the electrode.SOLUTION: The electrode for the lithium secondary battery includes the sheet-like collector, and the electrode mixture layer deposited on the surface of the collector. The electrode mixture layer includes an active material and particular binding agent. The particular binding agent is formed of a resin including styrene units and butadiene units. 50-90% of the resin is kept in a rubbery state in a temperature region of 0-40°C.

Description

  The present invention mainly relates to an electrode for a lithium secondary battery, and more particularly to improvement of a particulate binder contained in an electrode mixture layer.

  With the improvement in performance of portable devices and the development of electric vehicles and hybrid vehicles, there is a strong demand for higher performance of lithium secondary batteries. For example, research for achieving both high capacity of an electrode for a lithium secondary battery (hereinafter also simply referred to as an electrode) and improvement of electrode strength is active. A general electrode includes a sheet-like current collector and an electrode mixture layer attached to the surface thereof.

  The electrode mixture layer includes an electrode active material and a binder. Specifically, an electrode mixture layer is formed by preparing a slurry containing an electrode active material, a binder and a liquid dispersion medium, applying the slurry to the surface of the current collector, drying and rolling. . Thereafter, the electrode mixture layer is cut (slit) into a predetermined shape together with the current collector. The process from rolling to cutting is performed at room temperature.

  The binder has a function of binding the particles of the active material and binding the active material and the current collector. Among the binders, the particulate binder is regarded as promising because it can reduce the amount used without excessively covering the surface of the active material. For example, a particulate binder having rubber elasticity containing a styrene unit and a butadiene unit has been developed. The particulate binder is required to maintain binding properties in various temperature ranges from a low temperature of 0 ° C. or lower to a high temperature of 50 ° C. or higher (see Patent Documents 1 and 2).

JP 2001-76731 A JP 2001-15116 A

  Since the electrode repeats expansion and contraction due to the charge / discharge cycle of the battery, if the flexibility of the binder is insufficient, the active material may fall off or the electrode may break. Therefore, conventional particulate binders have been developed with an emphasis on improving charge / discharge cycle characteristics in the battery use temperature range (from a low temperature of 0 ° C. or lower to a high temperature of 50 ° C. or higher). Specifically, from the viewpoint of ensuring flexibility, conventionally, a resin that is almost completely in a rubber state at around 25 ° C. has been used.

On the other hand, from the viewpoint of suppressing the falling off of the active material, it is required for the binder to maintain the strength in the environmental temperature region (15 to 25 ° C.) in which the rolling and cutting operations are performed. Specifically, as the capacity of the electrode is increased, the rolling force when forming the electrode mixture layer is increasing. Accordingly, the stress applied to the active material particles is increasing both when the electrode mixture layer is cut together with the current collector and when the rolling is performed. Conventional particulate binders cannot cope with such stress, and as a result, excessive stress is applied to the active material particles, causing the electrode mixture layer to fall off and cracking many particles. . Therefore, it is considered that a binder that is too rubbery at around 25 ° C. is not suitable for the high capacity of the electrode. Therefore, a binder having almost no rubbery part at 25 ° C. is also used.
However, in the vicinity of 25 ° C., a binder having an excessively large or excessively small ratio in a rubbery state may greatly change the physical properties in the operating temperature range of the battery. When the physical properties of the binder change greatly, the electrode characteristics also change greatly, so that the charge / discharge characteristics become unstable.

The present invention comprises a sheet-like current collector and an electrode mixture layer attached to the surface of the current collector, and the electrode mixture layer includes an electrode active material and a particulate binder, The lithium secondary battery, wherein the particulate binder is made of a resin containing a styrene unit and a butadiene unit, and the resin is in a temperature range of 0 ° C. to 40 ° C. and a portion of 50% to 90% is in a rubber state. The present invention relates to an electrode.
The present invention also relates to a lithium secondary battery comprising a positive electrode, a negative electrode, and a lithium ion conductive electrolyte layer interposed between the positive electrode and the negative electrode, wherein the negative electrode is the electrode described above.

  According to the present invention, excessive particle cracking can be suppressed in the manufacturing process of the electrode for the lithium secondary battery, and the binding strength between the active material particles and the binding between the electrode mixture layer and the current collector. An electrode having excellent strength can be obtained. In addition, according to the present invention, it is possible to obtain an electrode whose characteristics do not change greatly in the operating temperature range of the battery.

It is a figure which shows the relationship between enthalpy (H) of one example of resin which comprises a particulate-form binder, and temperature (T). It is a perspective view which shows typically the structure of an example of the lithium secondary battery which concerns on one Embodiment of this invention. 2 is a DSC curve of the particulate binder according to Example 1. 2 is a DSC curve of a particulate binder according to Example 2. 3 is a DSC curve of a particulate binder according to Example 3. 2 is a DSC curve of a particulate binder according to Comparative Example 1. 3 is a DSC curve of a particulate binder according to Comparative Example 2.

  The electrode for a lithium secondary battery of the present invention includes a sheet-shaped current collector and an electrode mixture layer attached to the surface of the current collector. In the present invention, the electrode mixture layer includes a case where the electrode mixture layer is formed on only one side of the current collector and a case where the electrode mixture layer is formed on both sides of the current collector. A solid active material and a particulate binder may be included as essential components, and a conductive agent, a thickener, and the like may be included as optional components. The particulate binder plays a role of binding between the active material particles and binding the electrode mixture layer to the current collector.

  The particulate binder is made of a resin containing a styrene unit and a butadiene unit. Since such a resin is stable at the electrode potential of the lithium secondary battery and has an appropriate viscosity and elasticity, it is suitable as a binder that plays the above role.

  In the present invention, as a resin constituting the particulate binder (hereinafter also referred to as a binder resin), a resin having a rubber state of 50% to 90% in a temperature range of 0 ° C. to 40 ° C. is used. . The reason will be described with reference to FIG.

  FIG. 1 shows the relationship between enthalpy (H) and temperature (T) of an example of a binder resin used in the present invention. The binder resin in FIG. 1 has two glass transition points (Tg1 = −20 ° C., Tg2 = 60 ° C.), but the glass transition point does not need to appear clearly.

  In the temperature range below −20 ° C. (A range in the figure), most of the binder resin is in the glass state. Since the binder resin in the glass state is hard and brittle, the electrode mixture layer becomes poor in flexibility.

  Moreover, in the temperature region (C region in the figure) exceeding 60 ° C., most of the binder resin is in a rubber state. Since the rubber binder resin has low strength, the binding strength between the active material particles (mixture strength) and the binding strength between the electrode mixture layer and the current collector (peeling strength) are low.

  On the other hand, in the temperature region (B region in the figure) from the end of the first transition to the start of the second transition, the glassy part and the rubbery part coexist in the binder resin. is doing. In this coexistence state, the balance between the flexibility and rigidity of the electrode mixture layer can be controlled by the ratio of the glass state portion and the rubber state portion.

  Specifically, when a portion of 50% to 90% of the binder resin is in a rubber state in a temperature range of 0 ° C. to 40 ° C., necessary and sufficient rigidity is obtained, and the electrode mixture layer in the manufacturing process is obtained. Even when a strong pressure or stress is applied, excessive particle cracking can be suppressed. In addition, the mixture strength and peel strength are extremely high. At the same time, necessary and sufficient flexibility is also obtained, so that even if the electrode repeatedly expands and contracts due to the charge / discharge cycle, the active material is prevented from falling off or the electrode is disconnected.

  The temperature range of 0 ° C. to 40 ° C. is a general operating temperature range of the battery, and is also an atmospheric temperature range in which most of the manufacturing process of the electrode mixture layer is performed. In the temperature range of 0 ° C. to 40 ° C., 70% to 90% of the binder resin is preferably in a rubber state. In this state, even when extremely strong pressure or stress is applied to the electrode mixture layer in the manufacturing process, it is easy to obtain an electrode having excellent mixture strength and peel strength.

  The ratio of the rubber-like portion can be determined as follows using a differential scanning calorimeter (DSC). For example, Q2000 manufactured by TA Instruments is used as the measuring device.

A measurement sample will be described.
When the binder resin is in an emulsion or dispersion state, the binder resin is recovered from the liquid and used as a measurement sample. For example, moisture may be removed by drying to recover the binder resin, or the binder resin may be separated or recovered by the flocculant. A 5 mg sample is sealed in an aluminum sample pan.

The measurement conditions are as follows.
Measurement start temperature: -60 ° C
Measurement end temperature: 200 ° C
Temperature increase rate: 10 ° C / min Temperature decrease rate: 10 ° C / min Atmosphere in the sample holder: Nitrogen gas atmosphere at a flow rate of 50 ml / min

The measurement procedure is as follows.
First, a sample set in a measuring device at room temperature is (1) lowered to a measurement start temperature, and (2) measured to a measurement end temperature at a predetermined temperature rise rate. Thereafter, (3) the temperature is again lowered to the measurement start temperature, and (4) the measurement is performed to the measurement end temperature at a predetermined temperature rise rate. Thereby, a DSC curve can be obtained. The above (1) to (3) are pre-measurement heat treatments (JIS-K7121).

  Next, a DSC curve is drawn on a plane in which the vertical axis represents heat flow and the horizontal axis represents temperature. Then, the heat difference ΔH between the baseline near −50 ° C. and the baseline near + 90 ° C. is obtained. Here, ΔH is defined as the amount of heat necessary to completely bring the binder resin, which is entirely in a glass state, into a rubber state.

  In the same manner as described above, a heat amount change Δh1 from −50 ° C. to 0 ° C. and a heat amount change Δh2 from −50 ° C. to 40 ° C. are obtained. Here, the percentage value represented by 100 × (Δh1 / ΔH) is defined as the ratio of the rubber-like portion at 0 ° C. Further, the percentage value represented by 100 × (Δh2 / ΔH) is defined as the ratio of the rubber-like portion at 40 ° C. When the values of 100 × (Δh1 / ΔH) and 100 × (Δh2 / ΔH) are both in the range of 50 to 90%, the binder resin is in the temperature range of 0 ° C. to 40 ° C. and 50% to 90%. % Is judged to be rubber. Further, it is determined that the portion of the binder resin of 100 × (Δh1 / ΔH)% to 100 × (Δh2 / ΔH)% is in a rubber state.

  The binder resin used in the present invention includes a condition that a styrene unit and a butadiene unit are included, and a 50% to 90% portion is in a rubber state in a temperature range of 0 ° C. to 40 ° C. (hereinafter referred to as application conditions). As long as it is satisfied, there is no particular limitation. For example, a random copolymer, a block copolymer, a graft copolymer, a compound of a plurality of polymer components, and the like may be used. The compound may also include various rubbers and thermoplastic elastomers. Examples of the rubber include styrene butadiene rubber, high styrene rubber, ethylene propylene rubber, butyl rubber, chloroprene rubber, butadiene rubber, isoprene rubber, acrylic rubber, urethane rubber, acrylonitrile butadiene rubber, acrylonitrile rubber, fluorine rubber, and silicone rubber. Examples of the thermoplastic elastomer include olefin-based, styrene-based, vinyl chloride-based, urethane-based, polyester-based, polyamide-based, and fluorine-based thermoplastic elastomers. These may be used alone or in combination of two or more.

  In a preferred embodiment of the present invention, the binder resin has a glass transition point (Tg1) of 0 ° C. or lower and a glass transition point (Tg2) of 40 ° C. or higher. For example, the binder resin includes a first polymer component having a glass transition point (Tg1) of 0 ° C. or lower and a second polymer component having a glass transition point (Tg2) of 40 ° C. or higher. By compounding these two types of polymer components, it becomes easy to obtain a binder resin that satisfies the above application conditions. The composite resin draws a DSC curve reflecting the two glass transition points. Moreover, the area | region B of FIG. 1 becomes an area | region between these two glass transition points. Therefore, it is easy to control application conditions by the ratio of the first polymer component and the second polymer component.

  Since the binder resin includes a styrene unit and a butadiene unit, at least one of the first polymer component and the second polymer component includes a styrene unit and a butadiene unit. However, from the viewpoint of the balance between viscosity and elasticity, it is desirable that the first polymer component having a glass transition point Tg1 of at least 0 ° C. or less contains styrene units and butadiene units.

  The glass transition point Tg1 of 0 ° C. or lower is preferably −30 ° C. or higher. If Tg1 is too low, the peel strength at low temperatures may decrease.

  The glass transition point Tg2 of 40 ° C. or higher is preferably + 60 ° C. or lower. If Tg2 is too high, the flexibility of the electrode mixture layer may decrease.

  The binder resin may have a core-shell type structure. In this case, one of the first polymer component and the second polymer component constitutes a particulate core, and the other constitutes a shell covering the core. By adopting a core-shell type structure, the properties of the particulate binder are stabilized, and good rigidity and flexibility are easily exhibited.

  In a further preferred embodiment of the present invention (hereinafter referred to as embodiment A), 70 wt% to 100 wt%, further 90 wt% to 100 wt% of the first polymer component consists of styrene units and butadiene units, 70 wt% to 100 wt%, further 90 wt% to 100 wt% of the second polymer component is composed of styrene units and butadiene units. The polymer component having such a composition is extremely stable at the electrode potential of the lithium secondary battery and has appropriate viscosity and elasticity.

  In another more preferred embodiment of the present invention (hereinafter referred to as embodiment B), 70 wt% to 100 wt%, further 90 wt% to 100 wt% of the first polymer component is composed of styrene units and butadiene units. The second polymer component is 70 wt% to 100 wt%, more preferably 90 wt% to 100 wt%, at least one selected from the group consisting of styrene units, acrylate units and methacrylate units Of ester units. The polymer component having such a composition is extremely stable at the electrode potential of the lithium secondary battery and has appropriate viscosity and elasticity.

  In the more preferred embodiments A and B described above, the remaining amount of the first polymer component and the second polymer component is less than 30% by weight, but for example, a fluorinated vinyl monomer unit, an acrylonitrile unit, a urethane monomer or Oligomer units, polysiloxane chains, polyester chains and the like. These may be used alone or in combination of two or more.

  In Embodiments A and B, the first polymer component is preferably 5 to 60 mol%, more preferably 30 to 50 mol%, of styrene units in the total of styrene units and butadiene units. If it is this range, the glass transition point Tg1 below 0 degreeC can be controlled to -20 degreeC vicinity. A polymer component having a Tg1 of around −20 ° C. is extremely suitable as the first polymer component of the binder resin that satisfies the above application conditions.

  In Embodiment A, the second polymer component is preferably 65 to 95 mol%, more preferably 65 to 80 mol%, of styrene units in the total of styrene units and butadiene units. If it is this range, 40 degreeC or more glass transition point Tg2 can be controlled to 60 degreeC vicinity. In Embodiment B, the second polymer component is preferably 50 to 90 mol%, more preferably 55 to 70 mol%, of styrene units in the total of units selected from styrene units and ester units. If it is this range, 40 degreeC or more glass transition point Tg2 can be controlled to 60 degreeC vicinity. A polymer component having a Tg2 of around 60 ° C. is extremely suitable as the second polymer component of the binder resin that satisfies the above application conditions.

  The content of the first polymer component and the second polymer component in the total of the first polymer component and the second polymer component is 50% to 90% in the temperature range of 0 ° C. to 40 ° C. What is necessary is just to select suitably so that it may become. For example, it is preferable that the content of the first polymer component is 70 to 90% by weight and the balance is the second polymer component.

  The volume-based average particle diameter of the particulate binder used in the present invention is preferably 50 to 200 nm, and more preferably 90 to 150 nm. When the particle size is within this range, the surface of the active material particles is not excessively coated, and efficiently adheres to the surface of the active material. Therefore, the amount of binder used can be reduced. Therefore, it is advantageous for increasing the capacity of the electrode.

  The volume-based average particle diameter of the particulate binder can be measured by, for example, a commercially available laser diffraction particle size distribution measuring apparatus.

  The particulate binder is preferably synthesized by solution polymerization or emulsion polymerization. In particular, when styrene and butadiene are used as monomers, emulsion polymerization is suitable. By emulsion polymerization, an aqueous emulsion of a particulate binder having an average particle size of 50 to 200 nm is obtained. In the production of an electrode for a lithium secondary battery, the use of an aqueous emulsion is advantageous in terms of the production process.

  When solution polymerization or emulsion polymerization is not suitable, or when a compound is used, a plurality of polymer components are melt-kneaded. Alternatively, a plurality of polymer components are dissolved and mixed in a solvent, and then the solvent is removed. The obtained block resin can be mechanically pulverized by a method such as wind pulverization or freeze pulverization to adjust the average particle size, thereby obtaining a particulate binder. The obtained particulate binder is put into water together with a surfactant, and dispersed using a homogenizer or a disper mixer.

Next, other elements of the lithium secondary battery electrode will be described.
The particulate active material that is an essential component of the electrode mixture layer is a material capable of occluding (charging) and releasing (discharging) lithium ions. Examples of such a material include a carbon material and an alloy-based negative electrode active material, and a carbon material is preferable. Examples of the carbon material include natural graphite, artificial graphite, coke, graphitized carbon, and amorphous carbon. Among these, carbon particles having a graphite region (hereinafter referred to as graphite particles) are preferable. An active material may be used individually by 1 type, and may be used in combination of multiple types.

  Graphite particles are a general term for carbon particles including a region having a graphite structure. Thus, the graphite particles include natural graphite, artificial graphite, graphitized mesophase carbon particles, and the like. The diffraction image of the graphite particles measured by the wide angle X-ray diffraction method has a peak attributed to the (101) plane and a peak attributed to the (100) plane. Here, the ratio of the peak intensity I (101) attributed to the (101) plane and the peak intensity I (100) attributed to the (100) plane is 0.01 <I (101) / I. It is preferable to satisfy (100) <0.25, and it is more preferable to satisfy 0.08 <I (101) / I (100) <0.20. The peak intensity means the peak height.

  The average particle size of the graphite particles is preferably 14 to 25 μm, and more preferably 16 to 23 μm. When the average particle diameter is within the above range, the slipping property of the graphite particles in the negative electrode mixture layer is improved, the filling state of the graphite particles is improved, and it is advantageous for improving the adhesive strength between the graphite particles. The average particle diameter means the median diameter (D50) in the volume particle size distribution of the graphite particles. The volume particle size distribution of the graphite particles can be measured by, for example, a commercially available laser diffraction type particle size distribution measuring apparatus.

The average circularity of the graphite particles is preferably 0.90 to 0.95, and more preferably 0.91 to 0.94. When the average circularity is included in the above range, the slipping property of the graphite particles in the negative electrode mixture layer is improved, which is advantageous in improving the filling properties of the graphite particles and the adhesion strength between the graphite particles. The average circularity is represented by 4πS / L ² (where S is the area of the orthographic image of graphite particles, and L is the perimeter of the orthographic image). For example, the average circularity of 100 arbitrary graphite particles is preferably in the above range.

The specific surface area S of the graphite particles is preferably 3~7m ² / g, more preferably 3.5~6.5m ² / g. When the specific surface area is included in the above range, the slipperiness of the graphite particles in the negative electrode mixture layer is improved, which is advantageous for improving the adhesive strength between the graphite particles. Moreover, the suitable quantity of the 1st water-soluble polymer which coat | covers the surface of a graphite particle can be decreased. The specific surface area is measured by the BET method.

  The electrode mixture layer may contain a conductive agent, a thickener and the like as optional components. The conductive agent increases the electron conductivity between the active material particles and improves the current collecting property. The thickener adjusts the viscosity of the slurry containing the active material, the particulate binder, and the liquid dispersion medium, which is prepared when the electrode mixture layer is formed.

  Examples of the conductive agent include carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, and carbon fiber. A conductive agent may be used individually by 1 type, and may be used in combination of multiple types.

  A polysaccharide compound is preferably used for the thickener. The polysaccharide compound refers to a saccharide in which a plurality of monosaccharide molecules are polymerized by glycosidic bonds. Examples thereof include starch, amylose, amylopectin, glycogen, cellulose, and derivatives thereof. Among these, cellulose derivatives are preferable from the viewpoint of heat resistance, cost, etc., and carboxymethyl cellulose is most preferable.

  The amount of the particulate binder contained in the electrode mixture layer is preferably 0.4 to 1.5 parts by weight, more preferably 0.6 to 1.2 parts by weight, per 100 parts by weight of the graphite particles. When the amount of the particulate binder is less than 0.4 parts by weight, the binding property between the active material particles may be insufficient, and the electrode plate strength may be lowered. When the amount of the particulate binder exceeds 1.5 parts by weight, the negative electrode capacity may decrease.

  The amount of the thickener contained in the electrode mixture layer is preferably 0.6 to 3 parts by weight, more preferably 0.6 to 1.5 parts by weight, per 100 parts by weight of the active material particles. If the amount of the thickener is less than 0.6 parts by weight, the electrode plate strength may decrease. When the amount of the thickener exceeds 3 parts by weight, the negative electrode capacity may decrease.

The present invention is suitable when the density of the electrode mixture layer is 1.5 g / cm ³ or more. In order to obtain such an electrode mixture layer, it is necessary to increase the rolling force in the electrode manufacturing process. Accordingly, when the electrode mixture layer is cut together with the current collector, the stress applied to the active material particles also increases. According to the present invention, even in such a case, an electrode that can suppress excessive particle cracking and has excellent binding strength between active material particles and binding strength between an electrode mixture layer and a current collector. Is obtained.

In a further preferred embodiment of the present invention, the active material is graphite particles, the amount of the particulate binder is 0.4 to 1.5 parts by weight per 100 parts by weight of the active material, and the density of the electrode mixture layer Is 1.5 g / cm ³ or more, and the binding strength between the graphite particles is 150 N / cm ² or more. In order to satisfy all of the above conditions, it is necessary that the particulate binder has a good balance between rigidity and flexibility in the manufacturing process of the electrode. Since the particulate binder satisfying the above application conditions has a good balance between rigidity and flexibility, excessive particle cracking can be suppressed. Therefore, it becomes easy to make the binding strength between the graphite particles 150 N / cm ² or more.

  A metal foil is preferably used for the current collector carrying the electrode mixture layer. In the case of the negative electrode, the metal foil is preferably a copper foil or a copper alloy foil. Of these, copper foil (which may contain components other than copper of 0.2 mol% or less) is preferable, and electrolytic copper foil is particularly preferable.

Next, a lithium secondary battery according to an embodiment of the present invention will be described.
The lithium secondary battery includes a positive electrode, a negative electrode, and a lithium ion conductive electrolyte layer interposed therebetween, and the negative electrode is the electrode described above.

  FIG. 2 is a perspective view schematically showing a configuration of an example of the lithium secondary battery 1 according to the embodiment. 2, in order to show the structure of the principal part of the battery 1, the part is notched and shown. The battery 1 is a square battery in which a flat electrode group 10 and a nonaqueous electrolyte are accommodated in a square battery case 11.

  Electrode group 10 includes a positive electrode, a negative electrode, and a separator (all not shown). The positive electrode lead 12 connects the positive electrode current collector of the positive electrode and the sealing plate 14 having a function as a positive electrode terminal. The negative electrode lead 13 connects the negative electrode current collector of the negative electrode and the negative electrode terminal 15. The gasket 16 insulates the sealing plate 14 from the negative electrode terminal 15. The sealing plate 14 is joined to the open end of the rectangular battery case 11 and seals the rectangular battery case 11. A liquid injection hole (not shown) is formed in the sealing plate 14. The liquid injection hole is closed by a plug 17 after the nonaqueous electrolyte is injected into the rectangular battery case 11. The electrode group 10 can be produced by interposing a separator between the positive electrode and the negative electrode, winding them, and press-molding the wound body into a flat shape.

For the positive electrode, for example, a positive electrode mixture slurry containing a positive electrode active material, a conductive agent such as carbon black, and a binder such as polyvinylidene fluoride is applied to a positive electrode core material such as an aluminum foil, dried, and rolled. Can be obtained. As the positive electrode active material, a lithium-containing transition metal composite oxide is preferable. Typical examples thereof include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , Li _x Ni _y M _z Me _{1- (y + z)} O _{2 + d, and the} like. Among these, lithium nickel-containing composite oxides are preferable because high capacity can be secured.

Specific lithium nickel-containing composite oxides include, for example, the general formula (1):
_{Li x Ni y M z Me 1-} (y + z) O 2 + d (1)
(M is at least one element selected from the group consisting of Co and Mn, Me is at least one element selected from the group consisting of Al, Cr, Fe, Mg, and Zn; 98 ≦ x ≦ 1.10, 0.3 ≦ w ≦ 1.0, 0 ≦ z ≦ 0.7, 0.9 ≦ (y + z) ≦ 1.0 and −0.01 ≦ d ≦ 0.01 ).

  The separator has a pore structure, and the pores are impregnated with a nonaqueous electrolyte. These constitute a lithium ion conductive electrolyte layer. The separator is preferably a microporous film made of polyethylene or polypropylene. The thickness of the separator is, for example, 10 to 30 μm.

The non-aqueous electrolyte has lithium ion conductivity, includes a solute (supporting salt) and a non-aqueous solvent, and further includes various additives as necessary. Solutes include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ and the like. Solutes can be used alone or in combination of two or more. The solute is preferably added at a ratio of 0.5 to 2 moles per liter of the nonaqueous solvent. Nonaqueous solvents include cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of the cyclic carbonate include propylene carbonate and ethylene carbonate. Examples of chain carbonic acid esters include diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate. Examples of the cyclic carboxylic acid ester include γ-butyrolactone and γ-valerolactone. A non-aqueous solvent can be used individually by 1 type or in combination of 2 or more types.

Next, the present invention will be specifically described based on examples and comparative examples. However, the present invention is not limited to the following examples.
First, particulate binders A to E having the properties shown in Table 1 were synthesized. As a DSC curve and Tg measuring apparatus, Q2000 manufactured by TA Instruments was used.

In Table 1, each symbol is as follows.
St: styrene unit Bt: butadiene unit Ba: butyl acrylate unit <Δh1>: 100 × (Δh1 / ΔH)
<Δh2>: 100 × (Δh2 / ΔH)
St / Bt and St / Ba indicate the molar ratio of monomers.

Hereinafter, a method for synthesizing each particulate binder will be described.
<Binder X1>
Using a homomixer, 52 g of styrene, 50 g of ion-exchanged water, and 3 g of an anionic surfactant (manufactured by Daiichi Kogyo Seiyaku Co., Ltd .: Neogen RK) are mixed, and a monomer emulsion (hereinafter, emulsion a) is mixed. Got.
On the other hand, 90 g of ion-exchanged water was charged into an autoclave equipped with a dropping device and a stirrer whose interior space was replaced with nitrogen, and the temperature was raised to 70 ° C. while stirring. Thereafter, an aqueous solution consisting of 10 g of ion-exchanged water and 0.5 g of potassium persulfate was added. Next, while continuing stirring and maintaining the temperature at 70 ° C., 81.6 g of the emulsion a and 44 g of butadiene obtained by liquefaction under pressure were dropped over 4 hours to synthesize the polymer component A. Subsequently, 20.4 g of emulsion a and 4 g of butadiene obtained by liquefaction under pressure were added dropwise over 1 hour to synthesize polymer component B to obtain an emulsion of particulate binder X1.

<Binder X2>
In an autoclave equipped with a dropping device and a stirrer whose interior space was replaced with nitrogen, 90 g of ion-exchanged water was charged, and the temperature was raised to 70 ° C. while stirring. Thereafter, an aqueous solution consisting of 10 g of ion-exchanged water and 0.5 g of potassium persulfate was added. Next, while continuing stirring and maintaining the temperature at 70 ° C., 20.4 g of emulsion a and 4 g of butadiene obtained by liquefaction under pressure were dropped over 1 hour to synthesize polymer component B. Subsequently, 81.6 g of the emulsified product a and 44 g of butadiene which was liquefied under pressure were dropped over 4 hours to synthesize the polymer component A to obtain an emulsion of the particulate binder X2.

<Binder X3>
Using a homomixer, 36 g of styrene, 50 g of ion-exchanged water and 3 g of an anionic surfactant (manufactured by Daiichi Kogyo Seiyaku Co., Ltd .: Neogen RK) are mixed, and an emulsion of monomers (hereinafter, emulsion b1) Got.
In addition, using a homomixer, 14 g of styrene, 6 g of butyl acrylate, 20 g of ion-exchanged water, and 1 g of an anionic surfactant (Daiichi Kogyo Seiyaku Co., Ltd .: Neogen RK) are mixed to obtain an emulsion of monomers. (Hereinafter, emulsion b2) was obtained.
On the other hand, 70 g of ion-exchanged water was charged into an autoclave equipped with a dropping device and a stirrer whose interior space was replaced with nitrogen, and the temperature was raised to 70 ° C. while stirring. Thereafter, an aqueous solution consisting of 10 g of ion-exchanged water and 0.5 g of potassium persulfate was added. Next, while continuing stirring and maintaining the temperature at 70 ° C., 86 g of emulsion b1 and 44 g of butadiene obtained by liquefaction under pressure were dropped over 4 hours to synthesize polymer component A. Subsequently, 40 g of the emulsion b2 was dropped over 2 hours to synthesize the polymer component C to obtain an emulsion of the particulate binder X3.

<Binder Y1>
Using a homomixer, 45 g of styrene, 50 g of ion-exchanged water, and 3 g of an anionic surfactant (manufactured by Daiichi Kogyo Seiyaku Co., Ltd .: Neogen RK) are mixed together to give a monomer emulsion (hereinafter, emulsion c). Got.
On the other hand, 90 g of ion-exchanged water was charged into an autoclave equipped with a dropping device and a stirrer whose interior space was replaced with nitrogen, and the temperature was raised to 70 ° C. while stirring. Thereafter, an aqueous solution consisting of 10 g of ion-exchanged water and 0.5 g of potassium persulfate was added. Next, while stirring and maintaining the temperature at 70 ° C., 95 g of emulsion c and 55 g of butadiene obtained by liquefaction under pressure are dropped over 4 hours, and particulate binder Y1 composed of polymer component A is added. An emulsion was obtained.

<Binder Y2>
Using a homomixer, 60 g of styrene, 50 g of ion-exchanged water, and 3 g of an anionic surfactant (manufactured by Daiichi Kogyo Seiyaku Co., Ltd .: Neogen RK) are mixed, and a monomer emulsion (hereinafter, emulsion d) is mixed. Got.
On the other hand, 90 g of ion-exchanged water was charged into an autoclave equipped with a dropping device and a stirrer whose interior space was replaced with nitrogen, and the temperature was raised to 70 ° C. while stirring. Thereafter, an aqueous solution consisting of 10 g of ion-exchanged water and 0.5 g of potassium persulfate was added. Next, while continuing stirring and maintaining the temperature at 70 ° C., 110 g of the emulsion d and 40 g of butadiene obtained by liquefying the emulsion are dropped over 5 hours, and the particulate binder Y2 composed of the polymer component D is added. An emulsion was obtained.

  The DSC curves for X1, X2, X3, Y1 and Y2 of the particulate binder are shown in FIGS.

Example 1
A negative electrode for a lithium secondary battery was produced by the following procedure.
Natural graphite particles (average particle size 20 μm, BET specific surface area 4.8 m ² / g) 100 parts by weight, carboxymethyl cellulose (CMC) 1 g, particulate binder X1 emulsion (solid content concentration 40% by weight) 2.5 weights Part and an appropriate amount of water were mixed with a double-arm kneader to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to both sides of a copper foil having a thickness of 10 μm, and after drying, the coating film was rolled at a linear pressure of 2000 N (204 kgf) / cm to produce a negative electrode plate. The total thickness of the negative electrode mixture layers on both sides and the negative electrode core material was 150 μm. Thereafter, the negative electrode plate was cut into a predetermined size to obtain a strip-shaped negative electrode.

Example 2
A negative electrode was produced in the same manner as in Example 1 except that the particulate binder X2 was used instead of the particulate binder X1.

Example 3
A negative electrode was produced in the same manner as in Example 1 except that the particulate binder X3 was used instead of the particulate binder X1.

<< Comparative Example 1 >>
A negative electrode was produced in the same manner as in Example 1 except that the particulate binder Y1 was used instead of the particulate binder X1.

<< Comparative Example 2 >>
A negative electrode was produced in the same manner as in Example 1 except that the particulate binder Y2 was used instead of the particulate binder X1.

About the negative electrode of Examples 1-3 and Comparative Examples 1-2, the following binding strength was evaluated.
[Measurement method of binding strength between graphite particles]
A 2 cm × 3 cm negative electrode piece was cut out from the negative electrode having the negative electrode mixture layer formed on both sides. The negative electrode mixture layer was peeled off from one surface of the negative electrode piece, and the negative electrode mixture layer on the other surface was left as it was. The other surface of the negative electrode piece was attached to an adhesive layer of a double-sided tape (product number: No. 515, manufactured by Nitto Denko Corporation) attached on a glass plate. Subsequently, the negative electrode core material was peeled from the negative electrode piece to expose the negative electrode mixture layer. Thus, a measurement sample in which the negative electrode mixture layer adhered to one side of the double-sided tape was produced.

The double-sided tape side of this measurement sample was attached to the tip of a probe (tip diameter: 0.2 cm) of a tacking tester (trade name: TAC-II, manufactured by Resuka Co., Ltd.). Next, the measurement probe was pressed against the negative electrode mixture layer under the following conditions and separated to perform a peel test. In this peeling test, the maximum load at which peeling occurs between graphite particles was measured. The value obtained by dividing the obtained maximum load by the cross-sectional area of the probe (0.031 cm ² ) was defined as the binding strength (N) between the graphite particles.
The results are shown in Table 2.

<Test conditions>
Measurement probe indentation speed 30 mm / min Measurement probe indentation time 10 seconds Measurement probe indentation load 3.9 N
Measuring probe pulling speed 600mm / min

[Measurement method of binding strength between negative electrode core material and negative electrode mixture layer]
The negative electrode was cut to prepare a test piece having a width of 15 mm and a length of 80 mm. A double-sided tape (No. 151 manufactured by Nitto Denko Corporation) was attached to the negative electrode mixture layer on one side of the test piece and fixed to a smooth stainless steel (SUS) substrate. The SUS board | substrate with which the test piece was fixed was installed so that it might become horizontal. Next, one end of the negative electrode core material in the longitudinal direction of the test piece is attached to a tensile tester (trade name: Tensilon Universal Tester RTC1210, It was fixed to a movable jig of A & D). And the movable jig was moved so that the negative mix layer and core material of a test piece may peel at a speed | rate of 20 mm / min. At that time, the tensile direction was always maintained at 90 ° with respect to the SUS substrate on which the test piece was fixed. The numerical value of the stable tensile strength when 30 mm or more of the test piece was peeled was read and used as the binding strength between the negative electrode core material and the negative electrode mixture layer.

Next, the charge / discharge characteristics of the lithium secondary battery produced using the negative electrode were evaluated.
The lithium secondary battery was produced by the following procedure.
(1) Preparation of positive electrode LiNi _0.82 Co _0.15 Al _0.03 O ₂ (positive electrode active material) 100 parts by weight, acetylene black (conductive agent) 1 part by weight, polyvinylidene fluoride (binder) 1 part by weight and N-methyl-2 -Pyrrolidone 25 weight part was mixed with the double arm type kneader, and the positive mix slurry was prepared. The positive electrode mixture slurry was applied to both sides of a 15 μm thick strip-shaped aluminum foil (positive electrode core material), dried, and then the coated film was rolled to prepare a positive electrode plate. The total thickness of the positive electrode mixture layers on both sides and the positive electrode core material was 120 μm. Thereafter, the positive electrode plate was cut into a predetermined size to obtain a strip-shaped positive electrode.

(2) Preparation of nonaqueous electrolyte 1 part by weight of vinylene carbonate was added to 99 parts by weight of a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 3 to obtain a mixed solvent. Thereafter, LiPF ₆ was dissolved in a mixed solvent so as to have a concentration of 1.0 mol / L to prepare a nonaqueous electrolyte.

(3) Production of electrode group An electrode group was produced using the positive electrode, the negative electrode and the non-aqueous electrolyte.
Specifically, one end of the aluminum lead was connected to the positive electrode core material. One end of the nickel lead was connected to the negative electrode core material. These were wound by interposing a separator having a thickness of 16 μm (polyethylene microporous film, trade name: Hypore, manufactured by Asahi Kasei Co., Ltd.) between the positive electrode and the negative electrode. The obtained wound body was pressed under an environment of 25 ° C. to produce a flat electrode group. The press pressure was 0.5 MPa.

(4) Battery assembly The obtained electrode group was inserted into a stainless steel prismatic battery case. A resin frame was attached to the upper part of the electrode group. The resin frame body separates the electrode group from the stainless steel sealing plate and prevents the aluminum lead or nickel lead from contacting the battery case. The other end of the aluminum lead was connected to the lower surface of the sealing plate. The other end of the nickel lead was connected to a stainless steel negative electrode terminal. The negative electrode terminal was disposed on the sealing plate via a polypropylene gasket. A sealing plate was placed in the opening of the battery case and welded. A predetermined amount of nonaqueous electrolyte was injected into the battery case from the injection hole of the sealing plate. Thereafter, the liquid injection port was closed with a plug to produce the nonaqueous electrolyte secondary battery of the present invention.

[Battery capacity evaluation]
For the batteries of Examples 1 to 3 and Comparative Examples 1 and 2, the charge / discharge cycle was repeated 3 times under the following conditions, and the discharge capacity at the third time was obtained to obtain the battery capacity. The results are shown in Table 3.
Constant current charging: 200mA, end voltage 4.2V
Constant voltage charge: end current 20 mA, rest time 20 minutes Constant current discharge: current 200 mA, end voltage 2.5 V, rest time 20 minutes

[Charge / discharge cycle characteristics evaluation]
About the battery of Examples 1-3 and Comparative Examples 1-2, the following charging / discharging cycle was repeated 500 times at 45 degreeC.
Constant current charging: Charging current value 500 mA / end-of-charge voltage 4.2 V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
Constant current discharge: discharge current value 500 mA / discharge end voltage 3 V
And according to the following formula | equation, the capacity | capacitance maintenance factor (%) was calculated | required. The results are shown in Table 3.
Capacity retention rate (%) = (500th discharge capacity / first discharge capacity) × 100

  The binding strength between the graphite particles of Comparative Example 2 and the binding strength between the core material and the mixture layer are the same as those in the example, but the physical properties of the resin binder greatly change around 30 ° C. There was a significant decrease in the maintenance rate. This is considered to be because the characteristics of the particulate binder changed greatly when the temperature of the battery changed in the vicinity of 20 to 40 ° C. during the charge / discharge cycle.

  The negative electrode of the present invention and the nonaqueous electrolyte secondary battery including the negative electrode are useful as a power source for portable electronic devices. Examples of portable electronic devices include personal computers, mobile phones, mobile devices, personal digital assistants (PDAs), portable game devices, and video cameras. Further, the negative electrode of the present invention and the non-aqueous electrolyte secondary battery comprising the same are a main power source or auxiliary power source for driving an electric motor in a hybrid electric vehicle, a fuel cell vehicle, a plug-in HEV, and the like, an electric tool, a vacuum cleaner, and a robot. It is also expected to be used as a drive power source.

DESCRIPTION OF SYMBOLS 1 Nonaqueous electrolyte secondary battery 10 Electrode group 11 Battery case 12 Positive electrode lead 13 Negative electrode lead 14 Sealing plate 15 Negative electrode terminal 16 Gasket 17 Sealing

Claims (13)

A sheet-like current collector, and an electrode mixture layer attached to the surface of the current collector,
The electrode mixture layer includes an active material and a particulate binder,
The particulate binder is made of a resin containing a styrene unit and a butadiene unit,
The resin is an electrode for a lithium secondary battery in which a temperature range of 0 ° C. to 40 ° C. and a portion of 50% to 90% is in a rubber state.   The electrode for a lithium secondary battery according to claim 1, wherein the resin has a glass transition point of 0 ° C. or lower and a glass transition point of 40 ° C. or higher. The resin includes a first polymer component and a second polymer component,
The first polymer component has a glass transition point of 0 ° C. or less,
The lithium secondary battery electrode according to claim 2, wherein the second polymer component has a glass transition point of 40 ° C. or higher. 70% by weight or more of the first polymer component consists of styrene units and butadiene units, and
The electrode for a lithium secondary battery according to claim 3, wherein 70% by weight or more of the second polymer component comprises a styrene unit and a butadiene unit. 70% by weight or more of the first polymer component is composed of styrene units and butadiene units, and 70% by weight or more of the second polymer component is composed of styrene units, acrylate units and methacrylate units. The electrode for a lithium secondary battery according to claim 3, comprising at least one ester unit selected from the group.   The lithium according to claim 5, wherein the ester unit is at least one selected from the group consisting of an ethyl acrylate unit, a butyl acrylate unit, a 2-ethylhexyl acrylate unit, a hexyl methacrylate unit, and a 2-ethylhexyl methacrylate unit. Secondary battery electrode.   6. The electrode for a lithium secondary battery according to claim 4, wherein in the first polymer component, 5 to 60 mol% in the total of the styrene unit and the butadiene unit is a styrene unit.   5. The lithium secondary battery electrode according to claim 4, wherein in the second polymer component, 65 to 95 mol% of the total of styrene units and butadiene units is styrene units.   The electrode for a lithium secondary battery according to claim 5 or 6, wherein in the second polymer component, 50 to 90 mol% of the total of styrene units and ester units is a styrene unit.   The electrode for lithium secondary batteries according to any one of claims 1 to 9, wherein an average particle diameter of the particulate binder based on volume is 50 to 200 nm.   The lithium secondary battery electrode according to claim 3, wherein one of the first polymer component and the second polymer component constitutes a particulate core, and the other constitutes a shell covering the core. . The active material is carbon particles having a graphite region;
The amount of the particulate binder is 0.4 to 1.5 parts by weight per 100 parts by weight of the active material,
The density of the electrode mixture layer is 1.5 g / cm ³ or more,
The electrode for lithium secondary batteries according to any one of claims 1 to 11, wherein a binding strength between the carbon particles is 150 N / cm ² or more.   A lithium secondary battery comprising a positive electrode, a negative electrode, and a lithium ion conductive electrolyte layer interposed between the positive electrode and the negative electrode, wherein the negative electrode is the electrode according to claim 1.

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