JP2011060559A - Electrode for lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for lithium ion secondary battery, suppressed in increase in resistance caused due to a charge and discharge cycle. <P>SOLUTION: The electrode includes a collector 3, and an active substance layer 4 formed on a surface of the collector and containing an active substance 5, a not-shown conductive assistant and a binder. The binder contains: a thermosetting resin 6a (for example, polyimide), and a thermoplastic resin 7 (for example, polyvinylidene fluoride) or a thermosetting resin having an uncured part. The sheet resistance value of the electrode in charging is not more than 2×10<SP>4</SP>mΩ cm<SP>2</SP>. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrode for a lithium ion secondary battery. In particular, the present invention relates to an improvement for suppressing an increase in resistance due to repeated charge and discharge.

  In recent years, in order to cope with global warming, reduction of the amount of carbon dioxide is eagerly desired. In the automobile industry, there is a great expectation for reducing carbon dioxide emissions by introducing electric vehicles (EV) and hybrid electric vehicles (HEV), and the development of secondary batteries for motor drive that holds the key to commercialization of these is thriving. Has been done.

  As a secondary battery for driving a motor, it is required to exhibit extremely high output characteristics and high energy density as compared with a consumer lithium ion secondary battery used in a mobile phone, a notebook personal computer, or the like. Therefore, lithium ion secondary batteries having the highest theoretical energy among all the batteries are attracting attention, and are currently being developed rapidly.

  Lithium ion secondary batteries generally have a positive electrode in which a positive electrode active material or the like is applied to the surface of the current collector using a binder, and a negative electrode active material or the like applied to the surface of the current collector using a binder. The negative electrode is connected via an electrolyte layer containing an electrolyte and is housed in a battery case.

  As an active material of such a lithium ion secondary battery, for example, a negative electrode active material, a carbon / graphite negative electrode material, or an alloy negative electrode such as silicon (Si) or tin (Sn) that can be alloyed with lithium. Materials can be used. In particular, alloy-based negative electrode materials can achieve a higher energy density than carbon / graphite-based negative electrode materials, and are expected as candidates for vehicle batteries.

  The alloy-based negative electrode material has a large expansion / contraction associated with insertion / extraction of lithium ions. For example, the volume expansion when lithium ions are stored is about 1.2 times that of graphite, whereas silicon-based negative electrode The material reaches about 4 times. When the negative electrode active material expands and contracts greatly in this way, the contact between the active materials is weakened during repeated charging and discharging, or the adhesion with the current collector is reduced due to deformation of the active material layer (electrode layer). To do. As a result, it has been pointed out that the active material layer is cracked or pulverized, or peeled off from the current collector. In addition, the electrode which has such a problem may become a cause which reduces the cycling characteristics of a lithium ion secondary battery.

  Therefore, Patent Document 1 proposes a non-aqueous electrolyte secondary battery using a polyimide resin having an elastic modulus of 500 to 3000 MPa as a binder for the active material. According to this document, a non-aqueous electrolyte secondary battery having excellent cycle life characteristics is provided because it has excellent binding force between an electrode material containing an active material and a binder and a current collector, or binding force of an electrode material bulk. I can do it.

JP 2000-21412 A

  However, since the polyimide resin used as a binder in Patent Document 1 has a large binding force and low flexibility, local deformation occurs in the active material layer due to expansion and contraction of the active material. For this reason, by repeating charge and discharge, the contact between the active material and the conductive assistant, the contact between the active materials, or the contact between the active material layer and the current collector is weakened, and the conductive network in the electrode can be cut. As a result, there has been a problem that the resistance of the electrode increases while charging and discharging are repeated.

  This invention is made | formed in view of the said situation, and it aims at providing the electrode for lithium ion secondary batteries which can suppress a raise of resistance.

The electrode for a lithium ion secondary battery of the present invention has a current collector and an active material layer formed on the current collector surface and containing an active material, a conductive additive, and a binder. The binder includes a thermosetting resin and a thermoplastic resin, or a thermosetting resin having an uncured portion. And the sheet resistance value at the time of charge of this electrode is 2 * 10 < ⁴ > m (ohm) * cm < ² > or less.

  According to the present invention, the binder is provided with flexibility by having a thermoplastic resin in the binder containing the thermosetting resin or having the uncured portion of the thermosetting resin as the binder. In addition, local deformation of the active material layer when the active material expands and contracts can be suppressed. As a result, the deterioration of the contact between the constituent members in the active material layer or the contact between the active material layer and the current collector is prevented, and the conductive network in the active material layer is maintained. Therefore, it is possible to suppress an increase in electrode resistance due to repeated charge and discharge.

It is sectional drawing which represented typically the electrode for lithium ion secondary batteries concerning one Embodiment of this invention. FIG. 1A shows the state of the electrode when the active material is contracted. FIG. 1B shows the state of the electrode when the active material is expanded. It is sectional drawing which represented typically the electrode for lithium ion secondary batteries concerning other one Embodiment of this invention. FIG. 2A shows the state of the electrode when the active material is contracted. FIG. 2B shows the state of the electrode when the active material is expanded. It is sectional drawing which represented typically the whole structure of the laminated type lithium ion secondary battery which is not a bipolar type which concerns on this form. It is a graph showing the sheet resistance value with respect to the load of the negative electrode produced in Example 4 and Comparative Examples 4 and 5.

Hereinafter, preferred embodiments of the present invention will be described. The present embodiment relates to an electrode for a lithium ion secondary battery, which has a current collector and an active material layer formed on the current collector surface and containing an active material, a conductive additive, and a binder. The binder includes a thermosetting resin and a thermoplastic resin, or a thermosetting resin having an uncured portion. And the sheet resistance value at the time of charge of this electrode is 2 * 10 < ⁴ > m (ohm) * cm < ² > or less.

  Hereinafter, the present embodiment will be described with reference to the drawings. However, the technical scope of the present invention should be determined based on the description of the scope of claims, and is not limited to the following embodiments. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

<Electrode>
FIG. 1 is a cross-sectional view schematically showing an electrode for a lithium ion secondary battery according to an embodiment of the present invention. FIG. 1A shows the state of the electrode when the active material is contracted. FIG. 1B shows the state of the electrode when the active material is expanded.

  The electrode (1A, 1B) in FIG. 1 is formed by forming an active material layer 4 on one surface of a current collector 3 made of copper foil. The active material layer 4 includes silicon monoxide (SiO) 5 as a negative electrode active material, polyimide 6a as a thermosetting resin, polyvinylidene fluoride (PVDF) 7 as a thermoplastic resin, and acetylene black ( (Not shown) are almost uniformly dispersed in the electrode layer while coexisting. Here, the polyimide 6a and the polyvinylidene fluoride 7 have a role as a binder for binding the constituent members of the active material layers or the constituent members of the active material layer and the current collector.

  Among the binders, the polyimide 6a is in contact with each member on the surface, and thus has a strong binding force. For this reason, even when the silicon monoxide 5 expands and contracts, the bond between adjacent silicon monoxides 5, the contact between the silicon monoxide 5 and the conductive assistant, or the silicon monoxide 5 or the conductive assistant Contact with the current collector 3 can be maintained. However, when only polyimide is used as a binder as in the prior art, since the binding force is strong, local deformation of the active material layer may increase. In addition, there is a problem that the conductive auxiliary agent is unevenly distributed on the polyimide side and the conductive network in the active material layer is cut.

  However, in this embodiment, as shown in FIGS. 1A and 1B, polyvinylidene fluoride 7 is used as a binder in addition to polyimide 6a. Since the polyvinylidene fluoride 7 is in contact with each member by a line, it is more flexible than the polyimide 6a and can follow the expansion / contraction of the silicon monoxide 5 due to charge / discharge. Thereby, the deformation of the active material layer 4 can be prevented. Further, since the conductive additive is uniformly dispersed in the electrolyte layer 4 together with the polyvinylidene fluoride 7, the conductive network is maintained even if the conductive additive is small. As a result, it is possible to suppress an increase in electrode resistance due to repeated charge and discharge.

  FIG. 2 is a cross-sectional view schematically showing an electrode for a lithium ion secondary battery according to another embodiment of the present invention. FIG. 2A shows the state of the electrode when the active material is contracted. FIG. 2B shows the state of the electrode when the active material is expanded.

  The electrode (2A, 2B) in FIG. 2 is formed by forming an active material layer 4 on one surface of a current collector 3 made of copper foil. In the active material layer 4, silicon monoxide (SiO) 5 as a negative electrode active material, polyimide 6b having an uncured portion as a binder, and acetylene black (not shown) as a conductive assistant are uniformly dispersed. It becomes.

  As shown in FIG. 2, in this embodiment, the polyimide 6b as the binder has an uncured portion, so that the binder is given flexibility. Therefore, as shown in FIG. 2A and FIG. 2B, the polyimide 6b can follow the expansion / contraction of the silicon monoxide 5 due to charge / discharge, and the deformation of the active material layer 4 is prevented as in the embodiment of FIG. . In addition, the conductive network is maintained even with a small amount of conductive aid. Therefore, it is possible to suppress an increase in electrode resistance due to repeated charge and discharge. Hereinafter, although each member which comprises the electrode of this form is demonstrated, the technical scope of this invention is not restrict | limited only to the following form.

[Current collector]
The current collector is made of a conductive material, and an active material layer is disposed on one side or both sides thereof. There is no particular limitation on the material constituting the current collector, and for example, a conductive resin in which a conductive filler is added to a metal or a conductive polymer material or a non-conductive polymer material may be employed.

  Examples of the metal include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, a clad material of nickel and aluminum, a clad material of copper and aluminum, or a plating material of a combination of these metals can be preferably used. Moreover, the foil by which aluminum is coat | covered on the metal surface may be sufficient. Of these, aluminum, stainless steel, and copper are preferable from the viewpoints of electronic conductivity and battery operating potential.

  Examples of the conductive polymer material include polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylene vinylene, polyacrylonitrile, and polyoxadiazole. Since such a conductive polymer material has sufficient conductivity without adding a conductive filler, it is advantageous in terms of facilitating the manufacturing process or reducing the weight of the current collector.

  Examples of non-conductive polymer materials include polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE)), polypropylene (PP), polyethylene terephthalate (PET), polyether nitrile (PEN), polyimide ( PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), Examples include polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), and polystyrene (PS). Such a non-conductive polymer material may have excellent potential resistance or solvent resistance.

  A conductive filler may be added to the conductive polymer material or the non-conductive polymer material as necessary. In particular, when the resin used as the base material of the current collector is made of only a non-conductive polymer, a conductive filler is inevitably necessary to impart conductivity to the resin. The conductive filler can be used without particular limitation as long as it is a substance having conductivity. For example, metals, conductive carbon, etc. are mentioned as a material excellent in electroconductivity, electric potential resistance, or lithium ion barrier | blocking property. The metal is not particularly limited, but at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, Sb, and K, or these metals It is preferable to contain an alloy or metal oxide containing. The conductive carbon is not particularly limited, but is at least selected from the group consisting of acetylene black, vulcan, black pearl, carbon nanofiber, ketjen black, carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene. It is preferable that 1 type is included. The amount of the conductive filler added is not particularly limited as long as it is an amount capable of imparting sufficient conductivity to the current collector, and is generally about 5 to 35% by mass.

  The size of the current collector is determined according to the intended use of the battery. For example, if it is used for a large battery that requires a high energy density, a current collector having a large area is used. Although there is no restriction | limiting in particular also about the thickness of a collector, Usually, it is about 1-100 micrometers.

[Active material layer]
The active material layer includes an active material, a conductive additive, and a binder, and may further include other additives as necessary. The active material generates electrical energy by reversibly inserting and desorbing lithium ions. The active material layer may be formed on one surface of the current collector, or may be formed on both surfaces of the current collector.

(Positive electrode active material)
The positive electrode active material layer includes a positive electrode active material. The positive electrode active material has a composition that occludes lithium ions during discharging and releases lithium ions during charging. A preferable example is a lithium-transition metal composite oxide that is a composite oxide of a transition metal and lithium. Specifically, Li · Co-based composite oxide such as LiCoO _2, Li · Ni-based composite oxide such as LiNiO _2, Li · Mn-based composite oxide such as spinel LiMn ₂ O _4, Li · such LiFeO ₂ Fe-based composite oxides and those obtained by replacing some of these transition metals with other elements can be used. These lithium-transition metal composite oxides are excellent in reactivity and cycle characteristics, and are low-cost materials. Therefore, it is possible to form a battery having excellent output characteristics by using these materials for electrodes. In addition, examples of the positive electrode active material include transition metal oxides such as LiFePO ₄ and lithium phosphate compounds and sulfuric acid compounds; transition metal oxides such as V ₂ O ₅ , MnO ₂ , TiS ₂ , MoS ₂ , and MoO ₃ , and sulfides. Materials; PbO ₂ , AgO, NiOOH, etc. can also be used. The positive electrode active material may be used alone or in the form of a mixture of two or more.

  The average particle diameter of the positive electrode active material is not particularly limited, but is preferably 1 to 100 μm, more preferably 1 to 20 μm, from the viewpoint of increasing the capacity, reactivity, and cycle durability of the positive electrode active material. Within such a range, the secondary battery can suppress an increase in the internal resistance of the battery during charging and discharging under high output conditions, and can extract a sufficient current. When the positive electrode active material is secondary particles, it can be said that the average particle diameter of the primary particles constituting the secondary particles is preferably in the range of 10 nm to 1 μm. It is not limited to. However, it goes without saying that, depending on the manufacturing method, the positive electrode active material may not be a secondary particle formed by aggregation, agglomeration, or the like. As the particle diameter of the positive electrode active material and the particle diameter of the primary particles, a median diameter obtained by using a laser diffraction method can be used. The shape of the positive electrode active material varies depending on the type and manufacturing method, and examples thereof include a spherical shape (powdered shape), a plate shape, a needle shape, a column shape, and a square shape, but are not limited thereto. Any shape can be used without any problems. Preferably, an optimal shape that can improve battery characteristics such as charge / discharge characteristics is appropriately selected.

(Negative electrode active material)
The negative electrode active material layer includes a negative electrode active material. The negative electrode active material has a composition capable of releasing lithium ions during discharging and occluding lithium ions during charging. The negative electrode active material is not particularly limited as long as it can reversibly occlude and release lithium. Examples of the negative electrode active material include metals such as Si and Sn, TiO, Ti ₂ O ₃ , TiO ₂ , or Metal oxides such as SiO ₂ , SiO, SnO ₂ , complex oxides of lithium and transition metals such as Li _4/3 Ti _5/3 O ₄ or Li ₇ MnN, Li—Pb alloys, Li—Al alloys , Li, or carbon-based materials such as natural graphite, artificial graphite, carbon black, activated carbon, carbon fiber, coke, soft carbon, or hard carbon. The negative electrode active material may be used alone or in the form of a mixture of two or more.

  The negative electrode active material preferably contains an element that forms an alloy with lithium. By using an element that forms an alloy with lithium, a battery having a high energy density and a high capacity and excellent output characteristics can be obtained. The element alloying with lithium is not limited to the following, but specifically, Si, Ge, Sn, Pb, Al, In, Zn, H, Ca, Sr, Ba, Ru, Rh, Ir, Pd, Pt, Ag, Au, Cd, Hg, Ga, Tl, C, N, Sb, Bi, O, S, Se, Te, Cl and the like can be mentioned.

  Among the negative electrode active materials, it is preferable to include a carbon-based material, or at least one element selected from the group consisting of Si, Ge, Sn, Pb, Al, In, and Zn. More preferably, it contains an element of Si or Sn. Although these negative electrode active materials can constitute a battery having excellent capacity and energy density, the volume change due to expansion / contraction of the active material during charging / discharging is large, and deformation of the active material layer and disconnection of the conductive network are likely to occur. It also has the problem of. Therefore, by using these negative electrode active materials for the electrode of the present invention, the effect of the present invention of suppressing an increase in resistance becomes more remarkable.

  Note that the particle diameter and shape of the negative electrode active material are not particularly limited, and can take the same form as the above-described positive electrode active material, and thus detailed description thereof is omitted here.

  The active material is preferably in the form of a surface coated with a carbon material. According to such a form, a strong conductive network can be constructed by the binder between the carbon materials that are conductive on the surface of the active material or between the carbon material and the conductive auxiliary. Thereby, even when an active material having large expansion / contraction is used, a conductive path in the electrode can be secured, and an increase in resistance can be suppressed even when charging and discharging are repeated. More preferably, from the viewpoint of securing a conductive network of the electrode and improving the energy density of the electrode, a form in which a material to be alloyed with high-capacity lithium is coated with a conductive carbon material is used. In this case, the coating amount of the carbon material may be an amount that provides good electrical contact between the active materials or between the active material and the conductive additive, depending on the particle size of the active material (particles). Preferably, it is about 2 to 20% by mass with respect to the total mass of the coated active material. In the present invention, the term “coating” includes not only the form in which the entire surface of the active material is covered with the carbon material but also the form in which the carbon material is present (attached) on a part of the surface of the active material. Shall be.

(Binder)
The binder is added for the purpose of maintaining the electrode structure by binding the constituent members contained in the active material or the active material layer and the current collector.

  In one mode of the present invention (the mode shown in FIG. 1), the binder essentially includes a thermosetting resin and a thermoplastic resin. A thermosetting resin that causes a curing reaction by heating generally has a strong binding force. On the other hand, a thermoplastic resin is melted by heating and is generally more flexible than a thermosetting resin. Therefore, by combining a thermosetting resin and a thermoplastic resin into a binder, a binder having excellent binding force and flexibility can be obtained. Therefore, the active material layer containing such a binder can effectively suppress local deformation of the active material layer even when the active material expands and contracts due to charge and discharge. Thereby, the deterioration of the contact between the constituent members in the active material layer or the contact between the active material layer and the current collector is prevented, and the conductive network in the active material layer layer can be maintained. Therefore, it is possible to suppress an increase in electrode resistance due to repeated charge and discharge.

  The thermosetting resin used as the binder can be used without particular limitation as long as it is a resin that causes a curing reaction by heating. In the present specification, the term “thermosetting resin” is a concept including not only a resin in which a curing reaction is completed but also a resin including an uncured portion in which a curing reaction is not completed. Further, the “uncured portion” refers to a precursor portion containing a functional group that can cause a curing reaction when heated. The precursor may be a monomer or a prepolymer (an intermediate product in which the polymerization or condensation reaction of the monomer is stopped at an appropriate place) in which the curing reaction is not completely completed.

  Examples of the thermosetting resin include polyimide (PI), phenol resin (PF), epoxy resin (EP), melamine resin (MF), urea resin (UF), unsaturated polyester resin (UP), alkyd resin, and Examples thereof include polyurethane (PUR). Among these, it is more preferable to use polyimide (PI) from the viewpoint of having a high melting point and excellent heat resistance and having a strong binding force. These thermosetting resins may be used individually by 1 type, and may be used in combination of 2 or more type.

The thermoplastic resin that can be used as the binder can be used without particular limitation as long as it is a resin that has a property of being melted by heating. Specifically, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl acetate, polyvinyl chloride (PVC), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyether nitrile (PEN) ), Polyethylene (PE), polypropylene (PP), and polyacrylonitrile (PAN). Among these, it is preferable to use polyvinylidene fluoride (PVDF) from the viewpoint that uniform dispersion is possible and that a slurry can be easily prepared during production. These thermoplastic resins may be used individually by 1 type, and may be used in combination of 2 or more type.

  In addition to the above thermoplastic resin and thermosetting resin, rubber materials such as styrene butadiene rubber (SBR) and fiber materials such as cellulose resin may be appropriately added.

  When the binder contains a thermosetting resin and a thermoplastic resin as in the present embodiment, the content of each resin is the type of resin used, the size of the electrode, or the material of the constituent member included in the electrode. It is possible to adjust appropriately. As a preferable form, the mass ratio of the thermoplastic resin is preferably 0.3 to 2.5 and more preferably 0.5 to 2.0 with respect to the total mass 1 of the thermosetting resin. Preferably, it is 0.5-1.5. By setting the contents of the thermoplastic resin and the thermosetting resin in such a range, it is possible to obtain a binder having both excellent binding force and flexibility, and forming a good conductive network in the active material layer. can do.

  Moreover, in the other form (form shown in FIG. 2) of this invention, a binder essentially contains the thermosetting resin containing an uncured part. Thus, a softness | flexibility can be provided to the thermosetting resin which has a strong binding force, when a thermosetting resin contains an uncured part.

  As the thermoplastic resin including an uncured portion, a resin similar to the above-described thermoplastic resin can be used. Preferably, polyimide (PI) is preferably used because the curing reaction can be easily controlled by heating conditions.

(Conductive aid)
A conductive support agent means the additive mix | blended in order to improve the electroconductivity of an active material layer. Examples of the conductive aid include carbon blacks such as acetylene black and ketjen black, vapor grown carbon fibers such as carbon nanofibers and carbon nanotubes, activated carbon, and carbon materials such as graphite. The average particle diameter of the electron conductive agent is generally 1 to 500 nm, preferably 5 to 200 nm, and more preferably 10 to 100 nm.

  The content of the conductive auxiliary agent is preferably 5% by mass or less with respect to the total mass of the active material, the conductive auxiliary agent, and the binder contained in the active material layer. Since the electrode of this embodiment is formed using the binder having flexibility as described above, it is possible to prevent uneven distribution of the conductive additive in the active material layer. Therefore, in the conventional electrode, it was necessary to use about 10% by mass of a conductive additive. In the present invention, even when the conductive auxiliary is 5% by mass or less, the resistance of the electrode is kept low. Is possible.

  Moreover, it is preferable that the electrode of this form is disperse | distributed so that a conductive support agent may occupy 10-20 area% with respect to the cross-sectional area of an electrode. When the conductive auxiliary agent occupies such an area, even when the active material expands, a good conductive network can be maintained. In addition, these areas can be calculated | required by analyzing the enlarged image of the cross section of the electrode using a scanning electron microscope (SEM).

As described above, the electrode of this embodiment is a contact between constituent members included in the active material layer, or active, even when charging and discharging are repeated with a binder having excellent binding force and flexibility. Good contact between the material layer and the current collector is maintained. Therefore, the conductive network in the electrode is maintained, and an increase in electrode resistance can be suppressed. Specifically, the sheet resistance value during charging of the electrode of this embodiment is preferably 2 × 10 ⁴ mΩ · cm ² or less. In addition, the sheet resistance value of an electrode can be measured by the 4 terminal method used in the below-mentioned Example.

<Method for producing electrode>
Hereinafter, as a preferable manufacturing method of the electrode of this embodiment, a manufacturing method in the case where polyimide is included as a thermosetting resin will be described. The electrode manufacturing method according to the present embodiment includes a step of preparing an active material slurry containing an active material, a conductive auxiliary agent, and a polyamic acid, and applying the active material slurry to one surface of a current collector and heating. And imidating the polyamic acid.

  An example of a method for preparing an active material slurry in the case of an electrode containing polyimide, which is a thermosetting resin, and polyvinylidene fluoride, which is a thermoplastic resin, as a binder is as follows. First, a polyamic acid solution in which polyamic acid (polyimide precursor) is dissolved in N-methyl-2-pyrrolidone (NMP) as a viscosity adjusting solvent is prepared. Separately, a PVDF solution in which polyvinylidene fluoride (PVDF) is dissolved in N-methyl-2-pyrrolidone is prepared. Further, a conductive aid is added to the PVDF solution and sufficiently dispersed. When the conductive assistant is previously dispersed in the PVDF solution in this manner, the PVDF is melted and the conductive assistant and PVDF are uniformly dispersed in the active material layer in the heating step described later. Then, stirring is performed while adding the PVDF solution in which the conductive auxiliary agent is dispersed in the polyamic acid solution, thereby preparing a mixed liquid containing the conductive auxiliary agent and the binder. And an active material slurry is obtained by mixing, adding an active material little by little to this mixed solution. Thus, the lithium ion secondary battery electrode is manufactured by preparing a mixed solution containing the conductive additive and the binder in advance, and then adding and mixing the active material to the mixed solution. By such embodiment, the electrode for secondary batteries which has a thermoplastic resin in the binder containing a thermosetting resin can be manufactured. Such an electrode gives flexibility to the binder and can suppress local deformation of the active material layer when the active material expands and contracts. Therefore, the conductive auxiliary agent and the binder can be reliably bound to the active material, and a good conductive network can be constructed.

  Next, a method for preparing an active material slurry in the case where a thermosetting resin having an uncured part is included as a binder will be described. First, a polyamic acid solution in which polyamic acid (polyimide precursor) is dissolved in N-methyl-2-pyrrolidone (NMP) as a viscosity adjusting solvent is prepared. Further, a conductive additive is added to the polyamic acid solution and sufficiently dispersed. And an active material slurry is obtained by mixing, adding an active material little by little to the polyamic acid solution in which the conductive support agent was disperse | distributed.

  The viscosity adjusting solvent is not particularly limited, and examples thereof include dimethylformamide, dimethylacetamide, and methylformamide in addition to N-methyl-2-pyrrolidone (NMP). Further, the order of mixing and dispersing the components such as the active material, the conductive auxiliary agent, and the binder is not particularly limited. All these components may be mixed and dispersed at the same time, or may be mixed and dispersed stepwise for each component as described above.

  Next, the active material slurry prepared above is applied to one surface of the current collector. There is no restriction | limiting in particular in the method to apply | coat, Conventionally well-known methods, such as a die-coater method, a screen printing method, and a doctor blade method, can be employ | adopted suitably.

  Thereafter, the current collector coated with the active material slurry is heated to be dried for the purpose of removing the solvent to obtain an electrode. Thereafter, the polyamic acid contained as a binder is imidized by further heating. The heating means is not particularly limited, and conventionally known knowledge about the production of the electrode can be appropriately referred to. For example, the heat processing by use of a vacuum dryer is illustrated.

  The heating temperature for imidization is not particularly limited as long as the polyamic acid can be imidized, and can be appropriately set depending on the material of the binder, the amount of slurry applied, etc. It is 400 degreeC, More preferably, it is 150-350 degreeC. The heating time for imidization may be appropriately set according to the binder material, the amount of slurry applied, etc., but is preferably 1 minute to 20 hours, more preferably about 5 minutes to 10 hours. is there.

  In addition, before the heating for imidizing the polyamic acid, a step of drying the active material slurry applied to the current collector at a temperature of about 120 ° C. in order to remove the solvent component contained in the active material slurry. It may be performed separately.

  In particular, as in the embodiment shown in FIG. 2 described above, when preparing an active material slurry containing a thermosetting resin having an uncured portion as a binder and manufacturing an electrode for a lithium ion secondary battery, the heating temperature is It is 150-350 degreeC, and it is preferable that it is 200-300 degreeC. The heating time is 3 to 10 hours, and more preferably 4 to 8 hours. By taking such embodiment, the electrode for secondary batteries in which the thermosetting resin which is a binder has an uncured part can be manufactured. Such an electrode gives flexibility to the binder and can suppress local deformation of the active material layer when the active material expands and contracts. As a result, the deterioration of the contact between the constituent members in the active material layer or the contact between the active material layer and the current collector is prevented, and the conductive network in the active material layer is maintained. In the form shown in FIG. 2, when the heating temperature is less than 150 ° C. or the heating time is less than 3 hours, the ratio of the uncured portion becomes excessively large, and a desired sheet resistance value may not be obtained. On the other hand, when the heating temperature is 350 ° C. or higher and the heating time is 10 hours or longer, the proportion of the uncured portion becomes excessively small, and there is a possibility that the polyimide cannot follow the expansion / contraction of the active material. For this reason, the contact between the constituent members of the active material layer may deteriorate, and a desired sheet resistance value may not be obtained.

  When a thermoplastic resin such as PVDF is included as a binder, the thermoplastic resin can be melted and uniformly dispersed in the active material layer by the heating step.

Then, if necessary, an electrode having an active material layer formed on the current collector is obtained by performing a press treatment. The pressing means is not particularly limited, and conventionally known means can be appropriately employed. If an example of a press means is given, a calendar roll, a flat plate press, etc. will be mentioned.

<Secondary battery>
FIG. 3 is a schematic cross-sectional view schematically showing the entire structure of a laminated lithium ion secondary battery (hereinafter also simply referred to as “lithium ion battery”) according to this embodiment.

  As shown in FIG. 3, the lithium ion battery 10 of the present embodiment has a structure in which a substantially rectangular power generation element 17 in which a charge / discharge reaction actually proceeds is sealed inside a laminate film 22 that is a battery exterior material. Have. Specifically, the power generation element 17 is housed and sealed by using a polymer-metal composite laminate film as a battery exterior material and joining all of its peripheral parts by thermal fusion.

  The power generation element 17 includes a positive electrode in which the positive electrode active material layer 12 is disposed on both surfaces of the positive electrode current collector 11 (only one side for the lowermost layer and the uppermost layer of the power generation element), an electrolyte layer 13, and a negative electrode current collector 14. The negative electrode in which the negative electrode active material layer 15 is disposed on both sides of the negative electrode is laminated. Specifically, the positive electrode, the electrolyte layer 13 and the negative electrode are laminated in this order so that one positive electrode active material layer 12 and the negative electrode active material layer 15 adjacent thereto face each other with the electrolyte layer 13 therebetween. Yes.

  As a result, the adjacent positive electrode, electrolyte layer 13 and negative electrode constitute one single cell layer 16. Therefore, it can be said that the lithium ion battery 10 of the present embodiment has a configuration in which a plurality of single battery layers 16 are stacked and electrically connected in parallel. Further, a seal portion (insulating layer) for insulating between the adjacent positive electrode current collector 11 and negative electrode current collector 14 may be provided on the outer periphery of the unit cell layer 16. The positive electrode active material layer 12 is disposed on only one side of the outermost positive electrode current collector 11 a located in both outermost layers of the power generation element 17. Note that the arrangement of the positive electrode and the negative electrode is reversed from that in FIG. 3 so that the outermost negative electrode current collector is positioned in both outermost layers of the power generation element 17, and the negative electrode is formed only on one side of the outermost negative electrode current collector. An active material layer may be arranged.

  The positive electrode current collector 11 and the negative electrode current collector 14 are attached with a positive electrode current collector plate 18 and a negative electrode current collector plate 19 that are electrically connected to the respective electrodes (positive electrode and negative electrode), and are sandwiched between end portions of the laminate film 22. Thus, it has a structure led out of the laminate film 22. The positive electrode current collector 18 and the negative electrode current collector 19 are connected to the positive electrode current collector 11 and the negative electrode current collector 14 of each electrode by ultrasonic welding or resistance via the positive electrode terminal lead 20 and the negative electrode terminal lead 21 as necessary. It may be attached by welding or the like. However, the positive electrode current collector 11 may be extended to form the positive electrode current collector plate 18 and may be led out from the laminate film 22. Similarly, the negative electrode current collector 14 may be extended to form a negative electrode current collector plate 19, which may be similarly derived from the battery exterior material 22.

  As another form of the secondary battery, a bipolar electrode in which a positive electrode active material layer is formed on one surface of a current collector and a negative electrode active material layer is formed on the other surface is interposed through an electrolyte layer. Bipolar secondary batteries stacked in this manner can be given. The lithium ion secondary battery 10 and the bipolar secondary battery are basically the same except that the electrical connection form (electrode structure) in both batteries is different.

  Hereinafter, although the member which comprises the lithium ion secondary battery of this form is demonstrated easily, it is not restrict | limited only to the following form, A conventionally well-known form may be employ | adopted similarly.

[Electrolyte layer]
The electrolyte layer functions as a spatial partition (spacer) between the positive electrode active material layer and the negative electrode active material layer. In addition, it also has a function of holding an electrolyte that is a lithium ion transfer medium between the positive and negative electrodes during charging and discharging.

  There is no restriction | limiting in particular in the electrolyte which comprises an electrolyte layer, Polymer electrolytes, such as a liquid electrolyte and a polymer gel electrolyte and a polymer solid electrolyte, can be used suitably.

The liquid electrolyte has a form in which a lithium salt as a supporting salt is dissolved in an organic solvent as a plasticizer. Examples of the organic solvent used as the plasticizer include carbonates such as ethylene carbonate (EC) and propylene carbonate (PC). As the supporting salt (lithium _{salt), LiN (SO 2 C 2} F 5) 2, LiN (SO 2 CF 3) 2, LiPF 6, LiBF 4, LiClO 4, electrodes such as LiAsF 6, LiSO ₃ CF ₃ A compound that can be added to the active material layer can be similarly used.

  On the other hand, the polymer electrolyte is classified into a gel electrolyte containing an electrolytic solution and a polymer solid electrolyte containing no electrolytic solution.

  The gel electrolyte has a configuration in which the above liquid electrolyte is injected into a matrix polymer having lithium ion conductivity. Examples of the matrix polymer having lithium ion conductivity include polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof. In such a matrix polymer, an electrolyte salt such as a lithium salt can be well dissolved.

  In addition, when an electrolyte layer is comprised from a liquid electrolyte or a gel electrolyte, you may use a separator for an electrolyte layer. Specific examples of the separator include a microporous film made of a polyolefin such as polyethylene or polypropylene, a hydrocarbon such as polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), glass fiber, or the like.

  The polymer solid electrolyte has a structure in which a supporting salt (lithium salt) is dissolved in the matrix polymer, and does not include an organic solvent that is a plasticizer. Therefore, when the electrolyte layer is composed of a polymer solid electrolyte, there is no fear of liquid leakage from the battery, and the battery reliability can be improved.

  A matrix polymer of a polymer gel electrolyte or a polymer solid electrolyte can exhibit excellent mechanical strength by forming a crosslinked structure. In order to form a crosslinked structure, thermal polymerization, ultraviolet polymerization, radiation polymerization, electron beam polymerization, etc. are performed on a polymerizable polymer (for example, PEO or PPO) for forming a polymer electrolyte, using an appropriate polymerization initiator. A polymerization treatment may be performed. In addition, the said electrolyte may be contained in the active material layer of an electrode.

[Current collector]
In a lithium ion secondary battery, for the purpose of extracting current outside the battery, a current collector plate (a positive current collector plate and a negative current collector plate) electrically connected to a current collector is external to the laminate film. Has been taken out.

  The material which comprises a current collector plate is not specifically limited, The well-known highly electroconductive material conventionally used as a current collector plate for lithium ion secondary batteries can be used. As a constituent material of the current collector plate, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. Note that the same material may be used for the positive electrode current collector plate and the negative electrode current collector plate, or different materials may be used.

[Positive terminal and negative terminal lead]
In the lithium ion battery 10 shown in FIG. 3, the current collector is electrically connected to the current collector plate via the positive terminal lead 20 and the negative terminal lead 21.

  As a material of the positive electrode and the negative electrode terminal lead, a lead used in a known lithium ion battery can be used. In addition, the parts removed from the battery exterior material should be heat-insulating so that they do not affect products (for example, automobile parts, especially electronic devices) by touching peripheral devices or wiring and causing leakage. It is preferable to coat with a heat shrinkable tube or the like.

[Exterior material]
A conventionally known metal can case can be used as the exterior material. In addition, the power generation element 17 may be packed using a laminate film 22 as shown in FIG. The laminate film can be configured as a three-layer structure in which, for example, polypropylene, aluminum, and nylon are laminated in this order.

  By using the above-described electrode of the present embodiment for the lithium ion secondary battery, a battery having excellent cycle characteristics can be manufactured.

  The effect of this invention is demonstrated using a following example and a comparative example. However, the technical scope of the present invention is not limited only to the following examples.

1. Effect of resin type and compounding ratio contained in binder <Preparation of negative electrode>
[Example 1]
85% by mass of silicon monoxide (SiO, manufactured by High-Purity Chemical Laboratory Co., Ltd.) as a negative electrode active material, 5% by mass of acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder ) And a polyimide precursor polyamic acid solution (U-Vanice-A manufactured by Ube Industries Co., Ltd.) at a ratio of 1: 1 (in terms of solid content) and 10% by mass (in terms of solid content), and slurry An appropriate amount of N-methyl-2-pyrrolidone (NMP) was mixed as a viscosity adjusting solvent to prepare a negative electrode active material slurry.

  The order of mixing each component is as follows. First, a binder was dissolved in NMP, and acetylene black was mixed in small portions with a homogenizer to prepare a carbon ink. While stirring the mixture, the active material was added, and finally an appropriate amount of NMP was added to adjust the slurry concentration to obtain a negative electrode active material slurry.

  The negative electrode active material slurry thus obtained was applied to one surface of a rolled copper foil (thickness: 15 μm) as a negative electrode current collector and then dried. This coating / drying step was performed using a self-propelled coater heated to 120 ° C. And this laminated body was heat-processed at 200 degreeC for 5 hours using the vacuum dryer, and the polyamic acid in a negative electrode active material layer was imidized. Then, it shape | molded so that the thickness of a negative electrode active material layer might be set to 40 micrometers by roll press, and the negative electrode was completed.

[Example 2]
A negative electrode was produced in the same manner as in Example 1, except that only 10% by mass (solid content conversion) of a polyamic acid solution was used as the binder.

[Example 3]
Except for using 10% by mass (in terms of solid content) of a mixed solution in which PVDF and a polyamic acid solution were mixed at a ratio of 7:10 (in terms of solid content) as a binder, the same method as in Example 1 was used. A negative electrode was produced.

[Comparative Example 1]
A negative electrode was produced in the same manner as in Example 1 except that only 10% by mass of PVDF was used as the binder and no heat treatment (imidation step) at 200 ° C. was performed.

[Comparative Example 2]
A negative electrode was produced in the same manner as in Example 1 except that the heat treatment (imidation step) at 200 ° C. was not performed.

[Comparative Example 3]
A negative electrode was produced in the same manner as in Example 1 except that only 10% by mass of PVDF was used as the binder.

<Production of coin cell for evaluation>
The negative electrode produced above was cut into a disk shape with a diameter of 16 mm to obtain a negative electrode for lamination. As the positive electrode, a metal lithium punched into a disk shape with a diameter of 16 mm was used. This negative electrode is laminated through a separator (resin separator, material: PE, thickness: 20 μm) so as to face the positive electrode, placed in a coin cell container, injected with an electrolytic solution, and covered with an upper lid. A coin cell was produced. As the electrolytic solution, a solution in which 1.0 M LiPF ₆ was dissolved in a mixed solvent (volume ratio 1: 1) of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used.

<Charge / discharge cycle test>
About each coin cell for evaluation produced by said method, it charged for 3 hours by the constant current constant voltage system (CCCV, electric current: 0.5C, voltage: 0.02V) in 25 degreeC atmosphere, and was made to rest for 5 minutes. Thereafter, the battery was discharged to a cell voltage of 2.5 V with a constant current (CC, current: 0.5 C), and rested for 5 minutes after the discharge. This charge / discharge process was defined as one cycle, and 40 charge / discharge cycle tests were conducted to determine the ratio of the discharge capacity after 40 cycles to the discharge capacity after 1 cycle (= capacity maintenance rate [%]). The results are shown in Table 1.

<Measurement of sheet resistance>
The electrode produced above is cut into a disk shape with a diameter of 16 mm, and a metal jig with a terminal (material: gold foil, specific resistance value: 0.1 to 1.0 mΩcm) is used using a high power jack capable of uniaxial pressing. Then, an electrode specimen cut into a disk shape was sandwiched. And the sheet resistance value at the time of charge and discharge was measured by 4 terminal method, applying a load of 100-1000 kg / cm < ² > at room temperature. Among the obtained results, the value at the time of 100 kg / cm ² load was defined as the sheet resistance value. In addition, as a measuring apparatus, TER-2000SS manufactured by ULVAC-RIKO was used. The results are shown in Table 1.

From the results in Table 1, the sheet resistance value during charging was 2.0 × 10 ⁴ mΩ · cm ² or less in each of Examples 1 to 3, and the resistance of the electrode was significantly reduced compared to the comparative example. It has been shown.

  In addition, the discharge capacity retention ratio after 40 cycles was extremely high at 70.0% or more in Examples 1 to 3, indicating that the cycle characteristics were excellent.

  In Comparative Example 3, the electrode was greatly distorted by the charge / discharge cycle test, and the active material layer was peeled off from the current collector during the test.

2. Influence of polyimide heating conditions <Preparation of negative electrode>
[Example 4]
10% by mass of 90% by mass of graphite as a negative electrode active material and a conductive additive, and polyamic acid solution (U-Varnish-A manufactured by Ube Industries, Ltd., equivalent to 10% by mass of polyimide) as a binder as a binder. % And an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjusting solvent were mixed to prepare a negative electrode active material slurry.

  The order of mixing each component is as follows. First, a negative electrode active material slurry was obtained by dissolving a binder in NMP and mixing graphite in small portions with a homogenizer.

  The negative electrode active material slurry thus obtained was applied to one surface of a rolled copper foil (thickness: 15 μm) as a negative electrode current collector and then dried. This coating / drying step was performed using a self-propelled coater heated to 120 ° C. And this negative electrode was completed by heat-processing this laminated body at 200 degreeC for 5 hours using a vacuum dryer, and imidating the polyamic acid in a negative electrode active material layer.

[Comparative Example 4]
A negative electrode was produced in the same manner as in Example 4 except that heat treatment using a vacuum dryer was not performed.

[Comparative Example 5]
A negative electrode was produced in the same manner as in Example 4 except that the heat treatment was performed at 200 ° C. for 2 hours using a vacuum dryer.

<Measurement of sheet resistance>
The electrode produced above is cut into a disk shape with a diameter of 16 mm, and a high-power jack capable of uniaxial pressurization is connected to a metal jig (material: gold foil, specific resistance value: 0.1 to 1.0 mΩcm) with a terminal. An electrode specimen cut into a disk shape was sandwiched. And the sheet resistance value at the time of charge in each load in 0-200 kg / cm < ² > was measured by the 2 terminal method at room temperature. In addition, as a measuring apparatus, TER-2000SS manufactured by ULVAC-RIKO was used. The results are shown in FIG.

From the graph of FIG. 4, Example 4 which performed the heat processing for 5 hours over the whole range of 0-2000 kg / cm < ² > is the comparative example 4 which performed the heat processing for 2 hours, or the comparative example which did not perform the heat processing A sheet resistance value lower than 5 was exhibited. This result was considered to be due to the formation of a good conductive network in the active material layer of Example 4. In Comparative Examples 4 and 5, the sheet resistance value decreased as the load increased. This means that the electrodes of Comparative Examples 4 and 5 need to be pressed to form a conductive network.

1A, 1B, 2A, 2B electrodes,
3 Current collector,
4 active material layer,
5 SiO,
6a polyimide,
6b polyimide with uncured parts,
7 Polyvinylidene fluoride,
10 Lithium ion battery,
11 positive electrode current collector,
11a Outermost layer positive electrode current collector,
12 positive electrode active material layer,
13 electrolyte layer,
14 negative electrode current collector,
15 negative electrode active material layer,
16 cell layer,
17 Power generation elements,
18 positive current collector,
19 Negative current collector,
20 positive terminal lead,
21 negative terminal lead,
22 Laminated film.

Claims (11)

A current collector,
An active material layer formed on the surface of the current collector and including an active material, a conductive additive, and a binder, and an electrode for a lithium ion secondary battery,
The binder includes a thermosetting resin and a thermoplastic resin,
The electrode for lithium ion secondary batteries whose sheet resistance value at the time of charge of this electrode is 2 * 10 < ⁴ > m (ohm) * cm < ² > or less. A current collector,
An active material layer formed on the surface of the current collector and including an active material, a conductive additive, and a binder, and an electrode for a lithium ion secondary battery,
The binder includes a thermosetting resin having an uncured portion;
The electrode for lithium ion secondary batteries whose sheet resistance value at the time of charge of this electrode is 2 * 10 < ⁴ > m (ohm) * cm < ² > or less.   The electrode for a lithium ion secondary battery according to claim 1, wherein a mass ratio of the thermoplastic resin is 0.3 to 2.5 with respect to a total mass 1 of the thermosetting resin.   The electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the thermosetting resin contains polyimide.   The electrode for lithium ion secondary batteries of any one of Claim 1, 3, or 4 in which the said thermoplastic resin contains a polyvinylidene fluoride.   The lithium according to any one of claims 1 to 5, wherein a content of the conductive auxiliary is 5% by mass or less based on a total mass of the active material, the conductive auxiliary, and the binder. Ion secondary battery electrode.   The electrode for lithium ion secondary batteries of any one of Claims 1-6 in which the said active material contains the negative electrode active material which can occlude / release lithium ion. An electrode for a lithium ion secondary battery according to any one of claims 1 to 7,
A lithium ion secondary battery having a power generation element formed by laminating an electrolyte layer. Preparing an active material slurry containing an active material, a conductive additive, and a polyamic acid;
Applying the active material slurry to one surface of a current collector, and imidating the polyamic acid by heating at a heating temperature of 150 to 350 ° C. for a heating time of 3 to 10 hours. The manufacturing method of the electrode for secondary batteries.   The method for producing an electrode for a lithium ion secondary battery according to claim 9, wherein the active material slurry further contains a thermoplastic resin.   The method for producing an electrode for a lithium ion secondary battery according to claim 10, wherein the thermoplastic resin contains polyvinylidene fluoride.