JP5652666B2 - Method for manufacturing electrode for secondary battery - Google Patents

Description

  The present invention relates to a method for manufacturing an electrode for a secondary battery. In detail, it is related with the preparation process of the composition for forming the electrode active material layer of this electrode.

  In recent years, secondary batteries such as lithium secondary batteries and nickel metal hydride batteries have become increasingly important as power sources mounted on vehicles using electricity as a drive source, or power sources mounted on personal computers, portable terminals, and other electrical products. . In particular, a lithium secondary battery that is lightweight and obtains a high energy density is preferably used as a high-output power source for mounting on a vehicle.

  A typical electrode (a positive electrode and a negative electrode) of this type of secondary battery has an electrode active material layer (specifically, an electrode active material layer (specifically, an electrode active material capable of reversibly occluding and releasing chemical species as charge carriers). , A positive electrode active material layer and a negative electrode active material layer) are provided with an electrode having a structure formed on an electrode current collector. Such an electrode active material layer is a paste or slurry composition prepared by adding an appropriate solvent together with a binder and a thickener (hereinafter referred to as “electrode active material layer forming paste”, “positive electrode active material layer”). It may be abbreviated as “formation paste” or “negative electrode active material layer formation paste”) to the electrode current collector (typically coating and drying). Patent documents 1 and 2 are mentioned as conventional technology about the manufacturing method of the above-mentioned electrode.

JP 2006-092760 A JP 2009-099441 A

Conventionally, when forming a negative electrode active material layer forming paste, a two-stage kneading method is used in which a negative electrode active material and a thickener are kneaded (solid kneaded) with a small amount of water and then diluted with water. Yes. However, when water is used for dilution, when the negative electrode active material layer forming paste applied on the negative electrode current collector is dried, the binder in the vicinity of the current collector moves to the surface of the paste coating due to the convection of the paint. Segregation (migration) occurs on the surface of the paste coating. As a result, there is a problem in that the adhesion (bonding strength) between the negative electrode active material layer and the current collector is reduced, and the negative electrode active material layer is peeled off while being repeatedly charged and discharged.
Patent Document 1 describes a technique in which a thickener aqueous solution is added to graphite (negative electrode active material) and kneaded, and then the kneaded product is diluted with the thickener aqueous solution. According to such a technique, since the thickener aqueous solution is used at the time of dilution, the binder migration (segregation) as described above is suppressed, but since the thickener aqueous solution is added at the time of solidification, it absorbs moisture. Thus, the swollen thickener may cover the surface of the negative electrode active material widely. If the swollen thickener covers the surface of the negative electrode active material widely, it is not preferable because the thickener hinders the transfer of electrons and increases the reaction resistance of the electrode.

  The present invention has been made in view of the above points, and its main purpose is a production method capable of producing an electrode for a secondary battery having low battery resistance and having an electrode active material layer that is hardly peeled off and having excellent durability. Is to provide. Another object is to provide a secondary battery including such an electrode as a negative electrode.

  In order to achieve the above object, the present invention provides a method for producing an electrode for a secondary battery in which an electrode active material layer is formed on the surface of an electrode current collector. The manufacturing method disclosed here includes the following steps: (1). A powder mixing step of mixing the thickener powder and the electrode active material powder, (2). A kneading step of kneading (typically solid kneading) the powder mixture obtained in the powder mixing step and water, (3). A thickener aqueous solution mixing step of obtaining a paste for forming an electrode active material layer by mixing the kneaded material obtained in the kneading step and the thickener aqueous solution, (4). Applying the electrode active material layer forming paste to an electrode current collector to form an electrode active material layer on the current collector.

  In the present specification, “secondary battery” generally refers to a battery that can be repeatedly charged, and involves a chemical reaction such as a lithium secondary battery (typically a lithium ion battery), a nickel metal hydride battery, or a nickel cadmium battery. This term includes so-called chemical batteries and so-called physical batteries such as electric double layer capacitors. In the present specification, the “electrode active material” refers to an active material capable of reversibly occluding and releasing (typically inserting and removing) a chemical species that serves as a charge carrier in a secondary battery. The “thixotropy index (TI value)” is an index represented by the ratio (ηb / ηa) of the viscosity ηa at the rotational speed a and the viscosity ηb at the rotational speed b, and is maintained at a liquid temperature of 25 ° C. here. The values when the rotational speed a is 20 rpm and the rotational speed b is 2 rpm are shown using the B-type viscometer. The larger the TI value of the electrode active material layer forming paste, the more the convection of the paint during coating drying is suppressed, and the excessive migration (the binder in the vicinity of the current collector moves to the surface layer portion of the paste coating and segregates. ) Is better controlled.

  According to the method for producing an electrode of the present invention, after mixing a powdery thickener and an electrode active material, water is added and kneaded, so when the thickener aqueous solution is added at the time of solidification Such a thickening agent that has absorbed and swelled moisture can be prevented from covering the surface of the negative electrode active material widely, thereby preventing an increase in reaction resistance. Furthermore, since the thickener aqueous solution is used in the thickener aqueous solution mixing step, the TI value of the electrode active material layer forming paste becomes larger than when diluted with water, and excessive migration (binder near the current collector) Is better segregated by moving to the surface layer portion of the paste coating and segregating. Therefore, an electrode active material layer having high adhesion (bonding strength) to the electrode current collector can be formed. Therefore, according to the present invention, it is possible to provide a secondary battery that has lower battery resistance, is less likely to peel off the electrode active material layer, and has excellent durability (for example, a high capacity retention rate after cycle durability).

  In a preferable aspect of the electrode manufacturing method disclosed herein, the amount A of the thickener added in the powder mixing step with respect to 100 parts by mass of the electrode active material is based on 100 parts by mass of the electrode active material. More than the amount B of the thickener added in the thickener aqueous solution mixing step. For example, the ratio (A / B) between the amount A of the thickener charged in the powder mixing step and the amount B of the thickener charged in the thickener aqueous solution mixing step is approximately 1 <A. The range of / B ≦ 10 is appropriate, preferably 2 ≦ A / B ≦ 8, and particularly preferably 2 ≦ A / B ≦ 5. According to this configuration, since the ratio between the amount of the thickener added in the powder mixing step and the amount of the thickener added in the thickener aqueous solution mixing step is in an appropriate balance, It can be demonstrated better.

  In a preferred embodiment of the electrode manufacturing method disclosed herein, the thixotropy index (TI value) of the electrode active material layer forming paste is generally 3.0 or more, preferably 3.5 or more, particularly preferably. Is 4.0 or more. If the TI value of the electrode active material layer forming paste is less than 3.0, convection of the paint during drying becomes active, and binder migration (segregation) may be promoted. On the other hand, if the TI value of the paste becomes too large, the difference in viscosity due to the difference in shear rate becomes large. In the electrode manufacturing process, there are processes that affect the viscosity, and the shear rates are different from each other. If the difference in viscosity due to the difference in shear rate becomes large, clogging due to piping, filters, etc. may occur, which is not preferable. From such a viewpoint, approximately 10 or less is preferable.

  In a preferred embodiment of the electrode manufacturing method disclosed herein, the electrode active material is carbon particles capable of reversibly occluding and releasing charge carriers. By suppressing the coating of the swelling thickener on the surface of the carbon particles, the output characteristics of the secondary battery including the electrode can be improved. According to the production method of the present invention, since a powdery thickener is added at the time of solidification, the thickener that has absorbed water and swelled as when an aqueous solution of the thickener is added at the time of solidification. It is possible to prevent the surfaces of the carbon particles from being widely bonded to each other. Therefore, the trouble by the coating of the swelling thickener does not occur, and an electrode for a secondary battery excellent in output characteristics can be manufactured.

  Preferably, the thickener is a cellulose derivative (eg, carboxymethylcellulose (CMC)). A cellulose derivative preferable as a thickener is a polymer that dissolves in water, and has an appropriate viscosity when dissolved in water and kneaded.

  Moreover, according to this invention, the secondary battery provided with the electrode manufactured by one of the methods disclosed here as a negative electrode is provided. Since such a secondary battery includes an electrode having the above-described performance in the negative electrode, it can exhibit excellent performance. For example, it may satisfy at least one (preferably all) of good cycle characteristics, excellent output characteristics, high durability, and good production stability.

  Furthermore, the secondary battery having such a configuration is suitable as a battery (typically a battery for use as a driving power source) mounted on a vehicle such as an automobile. Therefore, according to the present invention, there is provided a vehicle including any of the secondary batteries disclosed herein (which may be in the form of an assembled battery in which a plurality of batteries are connected). In particular, a vehicle (for example, a plug-in hybrid vehicle (PHV) or an electric vehicle (EV) that can be charged with a household power source) provided with the secondary battery as a power source is provided.

It is a figure which shows typically the secondary battery which concerns on one Embodiment of this invention. It is a figure for demonstrating the winding electrode body which concerns on one Embodiment of this invention. It is sectional drawing which shows the III-III cross section of FIG. It is a figure which shows the manufacture flow of the electrode which concerns on one Embodiment of this invention. It is a figure which shows typically the tensile testing machine used by the tensile test. It is a side view which shows typically the vehicle (automobile) carrying a secondary battery.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  As a preferred embodiment of a method for producing an electrode for a secondary battery disclosed herein, a lithium secondary battery (typically a lithium secondary battery) in which an electrode active material layer is formed on the surface of an electrode current collector. The method for producing a negative electrode for an ion battery will be described in detail by way of example, but it is not intended to limit the application target of the present invention to such an electrode or battery. In addition, in the following drawings, the same code | symbol is attached | subjected to the member and site | part which show | plays the same effect | action, and the overlapping description may be abbreviate | omitted or simplified. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships.

  FIG. 1 is a diagram schematically showing a rectangular lithium secondary battery according to an embodiment. As shown in the drawing, the lithium secondary battery 100 according to the present embodiment includes a case 40 made of metal (a resin or a laminate film is also suitable). The case (outer container) 40 includes a flat rectangular parallelepiped case main body 42 whose upper end is opened, and a lid body 44 that closes the opening. On the upper surface of the case 40 (that is, the lid body 44), a positive electrode terminal 92 that is electrically connected to the positive electrode 50 of the wound electrode body 80 and a negative electrode terminal 94 that is electrically connected to the negative electrode 60 of the electrode body 80 are provided. ing. In the case 40, for example, a long sheet-like positive electrode (positive electrode sheet) 50 and a long sheet-like negative electrode (negative electrode sheet) 60 are laminated together with a total of two long sheet-like separators (separator sheets) 70. A flat wound electrode body 80 produced by winding and then crushing the resulting wound body from the side direction and kidnapping is housed.

  As shown in FIGS. 2 and 3, the positive electrode sheet 50 and the negative electrode sheet 60 are configured such that an electrode active material layer mainly composed of an electrode active material is provided on both surfaces of a long sheet-like electrode current collector, respectively. Have At one end in the width direction of these electrode sheets 50, 60, an electrode active material layer non-formed portion in which the electrode active material layer is not provided on any surface is formed. In the above lamination, the positive electrode sheet 50 and the negative electrode active material layer non-formed portion of the positive electrode sheet 50 and the negative electrode active material layer non-formed portion of the negative electrode sheet 60 protrude from both sides of the separator sheet 70 in the width direction. The negative electrode sheet 60 is overlaid with a slight shift in the width direction. As a result, in the lateral direction with respect to the winding direction of the wound electrode body 80, the electrode active material layer non-forming portions of the positive electrode sheet 50 and the negative electrode sheet 60 are respectively wound core portions (that is, the positive electrode active material layer forming portion of the positive electrode sheet 50). And the negative electrode active material layer forming part of the negative electrode sheet 60 and the two separator sheets 70 are closely wound). A positive electrode lead terminal 96 and a negative electrode lead terminal 98 are attached to the protruding portion (that is, the portion where the positive electrode active material layer is not formed) 82 and the negative electrode side protruding portion (that is, the portion where the negative electrode active material layer is not formed) 84, respectively. Are electrically connected to the positive electrode terminal 92 and the negative electrode terminal 94, respectively.

  Hereinafter, each component of the negative electrode of the lithium secondary battery 100 according to the present embodiment will be described. The negative electrode sheet 60 for a lithium secondary battery disclosed here has a configuration in which a negative electrode active material layer 64 is formed on the surface of a negative electrode current collector 62. As a constituent material of the negative electrode current collector 62 constituting the negative electrode, a conductive member made of a highly conductive metal is preferably used. For example, copper or an alloy containing copper as a main component can be given. In addition, the negative electrode current collector 62 is preferably applied to the negative electrode current collector 62 in the form of a sheet or foil in a battery including the wound electrode body 80 according to the present embodiment.

<Negative electrode active material>
As the negative electrode active material constituting the negative electrode active material layer 64, one or more of materials conventionally used in lithium secondary batteries can be used without particular limitation. For example, a suitable negative electrode active material includes a carbon material capable of reversibly occluding and releasing charge carriers (here, lithium). A particulate carbon material (carbon particles) containing a graphite structure (layered structure) at least partially is preferably used. Any carbon material of a so-called graphitic material (graphite), non-graphitizable carbon material (hard carbon), easily graphitized carbon material (soft carbon), or a combination of these materials is preferably used. obtain. In particular, the graphite particles can be a negative electrode active material more suitable for rapid charge / discharge (for example, high-power discharge) because the particle size is small and the surface area per unit volume is large.

  In addition to the negative electrode active material, the negative electrode active material layer 64 contains a material that can be used as a constituent component of the negative electrode active material layer in a general lithium secondary battery. Examples of such a material include a polymer material dispersed in water that can function as a binder (binder) for the negative electrode active material, and a polymer material that dissolves in water that can function as a thickener for the paste for forming the negative electrode active material layer. It is done.

<Thickener>
Examples of the polymer material that can be dissolved in water and function as a thickener include, for example, carboxymethylcellulose (CMC; typically sodium salt), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), methylcellulose (MC), Examples thereof include cellulose derivatives such as cellulose acetate phthalate (CAP), hydroxypropylmethylcellulose (HPMC) and hydroxypropylmethylcellulose phthalate (HPMCP), and polyvinyl alcohol (PVA). Of these, cellulose derivatives (for example, CMC) can be particularly preferably used. In addition, such a polymer material may be used individually by 1 type, and may be used in combination of 2 or more type.

<Binder>
Examples of polymer materials dispersed in water that can function as a binder include vinyl acetate copolymers, styrene butadiene block copolymers (SBR), acrylic acid-modified SBR resins (SBR latex), rubbers such as gum arabic, Or, polyethylene oxide (PEO), polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoro Examples thereof include fluorine resins such as ethylene copolymer (ETFE). Among these, a styrene butadiene block copolymer (SBR) can be preferably used. In addition, such a polymer material may be used individually by 1 type, and may be used in combination of 2 or more type.

  Although not particularly limited, the ratio of the negative electrode active material to the entire negative electrode active material layer can be about 80% by mass or more (for example, 80% by mass to 99% by mass), and about 90% by mass or more (for example, 90% by mass). Mass% to 99 mass%, more preferably 95 mass% to 99 mass%). In the negative electrode active material layer having a composition containing a thickener, the proportion of the thickener in the negative electrode active material layer can be, for example, 0.3% by mass to 10% by mass, and is usually about 1% by mass. It is preferable to set it as -5 mass%. Moreover, the negative electrode active material layer of the composition containing a binder can make the ratio of the binder to this negative electrode active material layer into 0.3 mass%-15 mass%, for example, is about 1 mass%-10 mass%. It is preferable.

  The electrode active material layer forming paste used in the electrode manufacturing method disclosed herein (in this embodiment, the negative electrode active material layer forming paste) is the above-described negative electrode active material, thickener, and as necessary. The binder may be an aqueous composition in which the binder is dispersed in water. Also, the negative electrode active material layer 64 disclosed herein may be formed by applying (coating and drying) such a negative electrode active material layer forming paste onto the negative electrode current collector 62.

  Next, referring to the flowchart shown in FIG. 4, taking as an example a method for producing a negative electrode for a lithium secondary battery (lithium ion battery), as an example of a preferred embodiment of a method for producing an electrode for a secondary battery according to the present invention. However, it will be explained.

  As shown in FIG. 4, in the electrode manufacturing method disclosed herein, a powder mixing step (step S10) for mixing the thickener powder and the electrode active material powder (here, the negative electrode active material powder), and the powder The powder mixture obtained in the mixing step and water are kneaded (typically kneaded) (step S20), and the kneaded product obtained in the kneading step and the thickener aqueous solution are mixed to form an electrode. A thickener aqueous solution mixing step (step S30) for obtaining an active material layer forming paste (here, negative electrode active material layer forming paste) and an electrode active material layer forming paste as an electrode current collector (here, negative electrode current collector) And the step of forming an electrode active material layer on the current collector (step S50). In this embodiment, the paste for electrode active material layer formation is finally completed by mixing the binder after the thickener aqueous solution mixing step (step S40). Hereinafter, each process will be described in detail.

<Preparation of thickener and aqueous solution of thickener>
In this embodiment, first, a thickener (for example, CMC) powder made of a polymer material that dissolves in water and a thickener aqueous solution in which the thickener powder is dissolved in water are prepared (for example, prepared). The thickener aqueous solution is prepared by adding water to the thickener powder and kneading. It is preferable that the thickener aqueous solution is sufficiently kneaded until the thickener aggregates disappear and uniform viscosity is obtained in the entire solution. Although the apparatus used for the said kneading | mixing is not specifically limited, For example, a planetary mixer, a homogenizer, a disper, a ball mill, a kneader, a mixer etc. are mentioned. In particular, the planetary mixer is preferably used because the blade shaft can be planetarily moved in the tank and kneaded strongly. If necessary, an aqueous thickener solution may be passed through the filter. By passing the thickener aqueous solution through the filter, undissolved components and mixed foreign matters having a size that can cause coating defects (for example, a diameter of 500 μm or more, preferably 300 μm or more, particularly preferably 100 μm or more) can be removed. The concentration of the thickener aqueous solution is determined by the molecular weight and the degree of etherification of the thickener and is not particularly limited, but is generally 0.3% by mass to 2% by mass, preferably 0.5% by mass to 1% by mass. It is. By using the thickener aqueous solution in such a concentration range, the final paste viscosity and TI value can be suitably adjusted in the thickener aqueous solution mixing step.

<Powder mixing process>
After preparing the thickener powder and the thickener aqueous solution in this way, next, in step S10, the prepared thickener powder and the electrode active material (here, the negative electrode active material) powder are mixed with each other. A mixing step is performed. Although the apparatus used for this powder mixing is not specifically limited, For example, a planetary mixer, a disper, a ball mill, a kneader, a mixer etc. are mentioned. In particular, the use of a planetary mixer is preferred. For example, a negative electrode active material powder (for example, carbon powder) and a thickener powder (for example, CMC powder) may be charged into a planetary mixer and mixed in a powder state to prepare a powder mixture. In this case, not all the thickeners to be added to the paste are added, but the thickener added in the form of powder in the powder mixing step before the kneading step, and the post-kneading step. In the thickener aqueous solution mixing step, the thickener is charged separately in a liquid form dissolved in water.

  In such a powder mixing step, it is preferable that the negative electrode active material and the thickener are sufficiently mixed until they are in a uniformly mixed state. For example, the stirring blade of the planetary mixer may be rotated at a relatively low rotational speed (for example, about 15 rpm) for about 5 to 10 minutes. From the viewpoint of enhancing the uniform mixing property, the average particle diameter (D50) of the negative electrode active material powder based on the laser scattering method is generally 5 μm to 30 μm, preferably 8 μm to 25 μm.

<Kneading process>
Next, in step S20, a step of kneading (typically kneading) the powder mixture obtained in the powder mixing step and a small amount of water is performed. In this kneading step, since the viscosity of the kneaded product is relatively high, the negative electrode active material and the thickener powder are rubbed together with a strong shearing force. Therefore, the agglomerates (dama) of each powder can be unwound and dispersed uniformly. In addition, after mixing the powdery thickener and the negative electrode active material, it is configured to add water and knead, so that it absorbs moisture, such as when a thickener aqueous solution is added during kneading. It is possible to prevent the swollen thickener from widely covering the surface of the negative electrode active material. If the thickener that has absorbed water and swelled covers the surface of the negative electrode active material widely, the thickener may interfere with the exchange of electrons and may increase the reaction resistance of the negative electrode.
On the other hand, as described above, in the manufacturing method according to the present embodiment, since the coating of the swelling thickener on the negative electrode active material surface can be suppressed, there is no problem due to the coating of the swelling thickener, and the reaction resistance is low. Lower and higher output can be realized.

  In such a kneading step, it is desirable to appropriately adjust the kneading conditions when kneading so that a strong shearing force is applied to the powder during kneading. For example, when a solvent (water) is gradually added to the powder, the kneading torque increases, reaches a peak, and decreases. Based on this knowledge, the kneading torque may be appropriately adjusted so that an appropriate shearing force is applied to the powder during kneading. The kneading time can be, for example, about 30 minutes to 2 hours. As an example, after roughing by rotating a stirring blade of a planetary mixer at a relatively low rotation speed (for example, about 15 rpm) for about 10 to 20 minutes, a higher rotation speed (for example, about 35 rpm) is set to 30. It is good to perform the kneading to rotate about minutes to 2 hours. Thereby, since the active material and the thickener are kneaded with a strong force (for example, shearing force), the dispersion of the thickener progresses, and the active material and the thickener are dispersed to form a uniform state. Further, the water content during kneading is not particularly limited, but is generally 20% by mass to 50% by mass, and preferably 30% by mass to 40% by mass of the entire kneaded product. Within such a range of water content, a strong shearing force is applied to the powder, and agglomerates of the powder are dispersed in a short time to produce a uniform kneaded product.

<Thickener aqueous solution mixing step>
Next, in step S30, a step of obtaining an electrode active material layer forming paste (here, a negative electrode active material layer forming paste) by mixing the kneaded material obtained in the kneading step and the thickener aqueous solution is performed. For example, the thickener aqueous solution may be charged into a planetary mixer and rotated for about 10 to 20 minutes while maintaining the rotation speed (for example, about 35 rpm) in the kneading step. Thereby, the viscosity and solid content rate of the final paste are adjusted.

  In this thickener aqueous solution mixing step, since the kneaded material is diluted with the thickener aqueous solution, the TI value of the paste becomes larger than when diluted with water. For example, using a commercially available B-type viscometer, the TI value when measured by rotating the rotor at 2 rpm and 20 rpm after adjusting the liquid temperature to 25 ° C. is generally 3.0 or more, preferably It is 3.5 or more, more preferably 4.0 or more, and particularly preferably 5.0 or more. When the TI value of the paste is 3.0 or more, the migration (the binder in the vicinity of the current collector moves to the surface layer portion of the paste application and segregates) when the paste is applied and dried is better. It is possible to obtain an electrode that is suppressed and the electrode active material layer is hardly peeled off. The upper limit value of the TI value is not particularly limited, but when the TI value of the paste becomes too large, the difference in viscosity due to the difference in shear rate becomes large. In the electrode manufacturing process, there are processes that affect the viscosity, and the shear rates are different from each other. If the difference in viscosity due to the difference in shear rate becomes large, clogging due to piping, filters, etc. may occur, which is not preferable. From such a viewpoint, it is generally 10 or less, preferably 7 or less.

  As a suitable example of the negative electrode active material layer forming paste disclosed herein, a negative electrode active when measured by rotating a rotor at 20 rpm after adjusting the liquid temperature to 25 ° C. using a commercially available B-type viscometer. The viscosity of the material layer forming paste is 5000 to 10000 mPa · s, and the viscosity of the negative electrode active material layer forming paste when measured by rotating the rotor at 2 rpm is in the range of 30000 to 55000 mPa · s. The viscosity of the negative electrode active material layer forming paste when measured by rotating the rotor at 20 rpm is 6000 to 10,000 mPa · s, and the negative electrode active material layer forming when measured by rotating the rotor at 2 rpm The viscosity of the paste for use is in the range of 25000-55000 mPa · s, and the negative electrode active when measured by rotating the rotor at 20 rpm. The viscosity of the paste for forming a porous layer is 9000 to 10000 mPa · s, and the viscosity of the paste for forming a negative electrode active material layer when measured by rotating the rotor at 2 rpm is in the range of 50000 to 55000 mPa · s. And the like. By having such a viscosity within the predetermined range, a negative electrode active material layer forming paste that satisfies both of the migration suppressing effect and the good coatability that could not be obtained conventionally can be obtained.

  In the preferable technique of the electrode manufacturing method disclosed herein, the amount A of the thickener added in the powder mixing step with respect to 100 parts by mass of the electrode active material is equal to the thickener with respect to 100 parts by mass of the electrode active material. The amount is larger than the amount B of the thickener added in the aqueous solution mixing step. For example, the ratio (A / B) between the amount A of the thickener added in the powder mixing step and the amount B of the thickener added in the thickener aqueous solution mixing step is approximately 1 <A / The range of B ≦ 10 is appropriate, preferably 2 ≦ A / B ≦ 8, and particularly preferably 2 ≦ A / B ≦ 5. According to this configuration, since the ratio between the amount of the thickener added in the powder mixing step and the amount of the thickener added in the thickener aqueous solution mixing step is in an appropriate balance, It can be demonstrated better. For example, it is possible to manufacture a negative electrode for a secondary battery having a lower battery resistance, an electrode active material layer that is less likely to be peeled off, and an excellent durability (for example, a high capacity retention rate after cycle durability).

  In step S40, after the kneaded material is diluted, the final paste for forming the negative electrode active material layer is completed by mixing the binder.

  Thereafter, in step S50, a process for applying a negative electrode active material layer forming paste to the negative electrode current collector to form a negative electrode active material layer on the current collector is performed. The negative electrode active material layer can be formed by applying the prepared paste for forming a negative electrode active material layer on the surface of the negative electrode current collector, volatilizing water, which is a solvent contained in the paste, and drying it. The negative electrode active material layer formed on the surface of the negative electrode current collector using the negative electrode active material layer forming paste having a TI value of 3.0 or more as described above is difficult to peel. Therefore, according to the manufacturing method disclosed herein, it is possible to manufacture an electrode for a secondary battery with good cycle characteristics in which a decrease in capacity retention rate due to tearing or peeling of the electrode active material layer is suppressed.

  The operation of applying such an active material layer forming paste to the negative electrode current collector can be performed in the same manner as in the case of producing a conventional negative electrode for a lithium secondary battery. For example, by applying a predetermined amount of the active material layer forming paste to the negative electrode current collector to a uniform thickness using a suitable coating device (die coater, slit coater, comma coater, etc.) Can be manufactured. Then, the solvent (water) in the negative electrode active material layer forming paste is removed by volatilizing the solvent (water) in the negative electrode active material layer forming paste in a drying furnace. The negative electrode active material layer is formed by removing the solvent (water) from the negative electrode active material layer forming paste. Thus, a negative electrode (negative electrode sheet) in which a negative electrode active material layer is formed on a negative electrode current collector can be obtained. In addition, after drying, the thickness and density of a negative electrode active material layer can be suitably adjusted by performing an appropriate press process (for example, roll press process) as needed.

  As shown in FIGS. 2 and 3, the negative electrode 60 for a lithium secondary battery obtained in this way has a negative electrode active material layer 64 containing a negative electrode active material and a thickener held by a negative electrode current collector 62. Have a configuration. This negative electrode active material layer 64 is obtained by applying a negative electrode active material layer forming paste using a powdery thickener at the time of solidification and a thickener aqueous solution at the time of dilution onto the negative electrode current collector 62. It is formed. Therefore, in the obtained negative electrode active material layer 64, excessive migration (the binder in the vicinity of the current collector moves to the surface layer and segregates) is better suppressed, and the binder can be preferably dispersed. For example, when the negative electrode active material layer 64 is divided into two at the center in the vertical direction (thickness direction), the degree of binder uneven distribution (binder amount on the upper layer side / binder amount on the lower layer side) is 2.0 or less (for example, 0.8. 8 to 2.0, preferably 1.5 or less, for example 1.0 to 1.5). Accordingly, an appropriate amount of binder is disposed on the surface of the current collector 62, and the bonding strength between the negative electrode active material layer 64 and the current collector 62 can be increased. The distribution of the binder amount in the active material layer can be examined, for example, by observation of a distribution cross section by an electron beam microanalyzer (EPMA) of a binder dyed with Br or Os (osmium) when the binder is SBR.

  In addition, the obtained negative electrode 60 for a lithium secondary battery is not charged all the thickeners to be added to the paste at once, but is charged in powder form in the powder mixing step prior to the kneading step. And a thickener added in a liquid form dissolved in water in a thickener aqueous solution mixing step after the kneading step. Therefore, the thickener contained in the negative electrode active material layer disclosed herein is a bimodal having two peaks, a first peak having a relatively low molecular weight and a second peak having a relatively high molecular weight. It can have a molecular weight distribution. For example, when the thickening agent is carboxymethyl cellulose (CMC), the peak has a mass average molecular weight of the top of the first peak having a relatively low molecular weight in the range of 500,000 to 1,000,000, and the peak Among them, the mass average molecular weight of the top of the second peak having a relatively large molecular weight is in the range of 2500000 to 5000000. Among these, the first peak having a relatively low molecular weight corresponds to the molecular weight of CMC charged in liquid form in the thickener aqueous solution mixing step, and the second peak having a relatively high molecular weight is the powder mixing. This corresponds to the molecular weight of CMC charged in powder form in the process. The molecular weight can be measured using, for example, a gel permeation chromatograph GPC (equipment No. GPC-MALLS2) (detector: differential refractive index detector RI (manufactured by Waters, RI-2414, sensitivity × 512), Column: TSKgel GMPWXL × 2 (manufactured by Tosoh Corporation, S / N E0027, M0110, φ7.8 mm × 30 cm), solvent: 0.1M sodium chloride aqueous solution, flow rate: 0.7 mL / min, column temperature 23 ° C., standard sample: monodisperse Pullulan (made by Showa Denko)).

  Other materials and members themselves constituting the lithium secondary battery 100 including the negative electrode 60 manufactured by the method disclosed herein may be the same as those conventionally provided in the same type of battery, and are not particularly limited. Hereinafter, other components will be described, but the present invention is not intended to be limited to such embodiments.

  For example, the positive electrode (positive electrode sheet) 50 may have a configuration in which a positive electrode active material layer 54 is formed on a sheet-like positive electrode current collector 52 (for example, an aluminum foil), as shown in FIGS. The positive electrode active material layer 54 is a composition in which a positive electrode active material and various additives such as a conductive material, a binder, and a thickener added as necessary are mixed in an appropriate solvent (positive electrode active material layer formation). The paste is applied to the positive electrode current collector 52, and the solvent is dried and compression molded.

As the positive electrode active material, a material capable of occluding and releasing lithium is used, and one or more of materials conventionally used in lithium secondary batteries (for example, an oxide having a layered structure or an oxide having a spinel structure) are used. It is used without particular limitation. Examples thereof include lithium-containing transition metal oxides such as lithium nickel composite oxide, lithium cobalt composite oxide, and lithium manganese composite oxide. An olivine-type lithium phosphate represented by the general formula LiMPO ₄ (M is at least one element of Co, Ni, Mn, and Fe; for example, LiFePO ₄ and LiMnPO ₄ ) may be used as the positive electrode active material. .

  As the conductive material, a conductive powder material such as carbon powder or carbon fiber is preferably used. As the carbon powder, various carbon blacks (for example, acetylene black, furnace black, ketjen black), graphite powder, and the like can be used. In addition, conductive fibers such as carbon fibers and metal fibers, metal powders such as copper and nickel, and organic conductive materials such as polyphenylene derivatives can be contained alone or as a mixture thereof. In addition, even if only 1 type is used among these, 2 or more types may be used together.

  Moreover, as a suitable example of the separator sheet 70 used between the positive and negative electrode sheets 50 and 60, one made of a porous polyolefin-based resin can be cited. For example, a porous separator sheet made of synthetic resin (for example, made of polyolefin such as polyethylene) can be suitably used. When a solid electrolyte or a gel electrolyte is used as the electrolyte, a separator may not be necessary (that is, in this case, the electrolyte itself can function as a separator).

The wound electrode body 80 having such a configuration is accommodated in the case main body 42 (FIG. 1), and an appropriate nonaqueous electrolytic solution is disposed (injected) in the case main body 42. As the non-aqueous electrolyte accommodated in the case main body 42 together with the wound electrode body 80, the same non-aqueous electrolyte as used in a conventional lithium ion battery can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. For example, a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3: 4: 3 contains LiPF ₆ as a supporting salt at a concentration of about 1 mol / liter. A nonaqueous electrolyte solution can be used.

  The non-aqueous electrolyte is accommodated in the case main body 42 (FIG. 1) together with the wound electrode body 80, and the opening of the case main body 42 is sealed with the lid body 44 (FIG. 1). Construction (assembly) of the secondary battery 100 is completed. In addition, the sealing process of the case main body 42 and the arrangement | positioning (injection) process of electrolyte solution can be performed like the method currently performed by manufacture of the conventional lithium secondary battery. Moreover, there is no restriction | limiting in particular about the structure of the said battery case 40, a magnitude | size, material (For example, it can be metal or a product made from a laminate film), etc.

  The secondary battery constructed in this way is excellent because it is constructed using an electrode (for example, a negative electrode) having an electrode active material layer with low battery resistance and good adhesion as described above. It shows battery performance. For example, by constructing a battery using the above electrode, a secondary satisfying at least one (preferably all) of good cycle characteristics, excellent output characteristics, high durability, and good production stability. A battery can be provided.

  Hereinafter, although the test example regarding this invention is demonstrated, it is not intending to limit this invention to what is shown to this specific example.

<Test Example 1>
An electrode for a secondary battery according to the present invention was prepared, and a tensile test of an electrode active material layer in the electrode was performed to evaluate peel strength. A specific method will be described below.

[Preparation of negative electrode for lithium secondary battery]
Example 1
For the purpose of preparing a paste for forming a negative electrode active material layer, 100 parts by mass of carbon powder (negative electrode active material) and 0.7 parts by mass of CMC powder (thickening agent) were mixed with a planetary mixer (trade name Hibis Disper Mix, Primes Co., Ltd.) The powder was kneaded between the powders at a rotation speed of 15 rpm for 5 minutes (powder mixing step). Subsequently, pure water was added so that the solid content rate was about 62% by mass, kneaded for 10 minutes at a rotation speed of 15 rpm, and further kneaded for 60 minutes at a rotation speed of 35 rpm (kneading step). The kneaded product was charged with a CMC solution (a solution obtained by dissolving 0.3 parts by mass of CMC in pure water) and kneaded for 10 minutes at a rotation speed of 35 rpm (thickening agent aqueous solution mixing step). Thereafter, 1 part by mass of SBR (binder) was mixed to obtain a target negative electrode active material layer forming paste. The final proportion of solid content in the negative electrode active material layer forming paste was adjusted to 54 mass%. The viscosity and TI value of the obtained negative electrode active material layer forming paste were measured by adjusting the liquid temperature to 25 ° C. using a commercially available B-type viscometer and rotating the rotor at 2 rpm and 20 rpm, respectively. The TI value was defined as the viscosity ratio between the rotational speed of 2 rpm and the rotational speed of 20 rpm (viscosity at the rotational speed of 2 rpm / viscosity at the rotational speed of 20 rpm). The results are shown in Table 1.
Further, the obtained negative electrode active material layer forming paste was applied in a strip shape on both sides of a long sheet-like copper foil (negative electrode current collector) and dried to thereby form a negative electrode active material layer on both sides of the negative electrode current collector A negative electrode sheet provided with was prepared. The coating amount of the negative electrode active material layer forming paste was adjusted so that the both surfaces were combined to be about 10 mg / cm ² (solid content basis).

(Example 2)
A negative electrode sheet in the same manner as in Example 1, except that the CMC charged in the powder mixing step was 0.8 parts by mass and the CMC charged in the thickener aqueous solution mixing step was 0.2 parts by mass. Was made.

Example 3
A negative electrode sheet in the same manner as in Example 1 except that the CMC charged in the powder mixing step was 0.9 parts by mass and the CMC charged in the thickener aqueous solution mixing step was 0.1 parts by mass. Was made.

Example 4
A negative electrode sheet in the same manner as in Example 1 except that the CMC charged in the powder mixing step was 0.7 parts by mass and the CMC charged in the thickener aqueous solution mixing step was 0.1 parts by mass. Was made.

(Comparative Example 1)
A negative electrode sheet was produced in the same manner as in Example 1 except that 1.0 part by mass of CMC charged in the powder mixing step was used and that pure water was used in the thickener aqueous solution mixing step.

(Comparative Example 2)
A CMC solution was prepared by charging 0.8 parts by mass of CMC powder (thickener) and pure water into a planetary mixer (trade name Hibis Disper Mix, manufactured by Primics Co., Ltd.) and kneading. 100 parts by mass of carbon powder (negative electrode active material) was added to this CMC solution, kneaded for 10 minutes at a rotation speed of 15 rpm, and further kneaded for 60 minutes at a rotation speed of 35 rpm (kneading step). CMC solution (using 0.2 parts by mass of CMC powder dissolved in pure water) was added to the kneaded product, and kneaded for 10 minutes at a rotation speed of 35 rpm (thickening agent aqueous solution mixing step). Thereafter, 1 part by mass of SBR (binder) was mixed to obtain a target negative electrode active material layer forming paste. The final solid content in the paste was adjusted to 54 mass%. Thereafter, a negative electrode sheet was produced in the same manner as in Example 1.

[Binder uneven distribution]
When the cross sections of the negative electrode sheets according to Examples 1 to 4 and Comparative Examples 1 and 2 thus obtained were analyzed by an electron beam microanalyzer (EPMA), and the negative electrode active material layer cross section was divided into half in the thickness direction The binder uneven distribution degree (binder concentration on the upper layer side / binder concentration on the lower layer side) was examined. The ratio of the binder concentration between the upper layer side and the lower layer side was calculated from the Br element detection intensity ratio when the binder (SBR) was dyed with Br element. Table 1 shows the degree of binder uneven distribution in each example.

[Tensile test]
Further, the bonding strength between the current collectors of the various obtained negative electrode sheets and the negative electrode active material layer was evaluated by a 90 ° peel test using a tensile tester. Specifically, a test piece was prepared by peeling off the negative electrode active material layer on one side of the negative electrode sheet. Then, as shown in FIG. 5, the test piece 60 was placed on the measurement table 68, and the negative electrode current collector 62 was fixed to the measurement table 68 so that the negative electrode active material layer 64 was on the upper side in the vertical direction. Then, one side of a double-sided tape (φ10 mm) 66 is affixed to the lower end of the tension jig 65, the other side is affixed to the negative electrode active material layer 64, and the jig 65 is attached to the surface of the negative electrode current collector 62. Then, it was pulled in a direction that was vertical (peeling angle was 90 ± 5 °) and peeled off at a speed of 0.5 mm per second. And the average value of the load while the negative electrode active material layer 64 peeled from the negative electrode collector 62 was measured as peeling strength [N / m]. The results are shown in Table 1.

  As is clear from Table 1, the negative electrode sheet according to Comparative Example 1 using pure water at the time of dilution caused migration at the time of drying. Therefore, the uneven distribution of the binder exceeded 2.0, and the binder was not uniformly distributed. It was a thing. The peel strength was less than 3.5 N / m, and the bondability between the negative electrode active material layer and the current collector was poor. On the other hand, the negative electrode sheets according to Examples 1 to 4 using the CMC solution at the time of dilution had good peel strength of 4.7 N / m or more, and exhibited extremely high bonding strength. From this result, it was confirmed that the bonding strength between the negative electrode active material layer and the current collector was improved by using the CMC solution at the time of dilution. As is clear from Table 1, the peel strength tended to increase as the TI value of the paste increased. In the case of the negative electrode sheet used here, an extremely high peel strength of 1.5 N / m or more could be achieved, particularly by setting the TI value of the paste to 3.7 or more. From the viewpoint of improving the peel strength, the TI value of the paste is generally about 3.0 or more (Examples 1 to 4), preferably 4.0 or more (Examples 1 to 3), and particularly preferably. It is 5.0 or more (Example 1).

<Test Example 2>
A lithium secondary battery was produced using the various negative electrode sheets produced in Test Example 1, and the performance was evaluated. The lithium secondary battery was produced as follows.

[Lithium secondary battery]
These materials include nickel cobalt lithium manganate (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) powder as a positive electrode active material, polyvinylidene fluoride (PVDF) as a binder, and acetylene black as a conductive material. The positive electrode active material layer forming paste was prepared by mixing with N-methylpyrrolidone (NMP) so that the mass ratio of the mixture was 90: 8: 2, and this was used as a long sheet-like aluminum foil (positive electrode current collector) A positive electrode sheet in which a positive electrode active material layer was provided on both surfaces of a positive electrode current collector was produced by applying a belt-like shape on both surfaces and drying. The coating amount of the positive electrode active material layer forming paste was adjusted to be about 16 mg / cm ² (solid content basis) for both surfaces.

A wound electrode body was prepared by winding the positive electrode sheet and the negative electrode sheet through two separators. The wound electrode body thus obtained was housed in a battery container (here, a square shape was used) together with a nonaqueous electrolyte (nonaqueous electrolyte solution), and the opening of the battery container was hermetically sealed. As a non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1: 1: 1 is about 1 mol / liter of LiPF ₆ as a supporting salt. The non-aqueous electrolyte solution contained at a concentration of was used. In this way, a test lithium secondary battery was assembled. The theoretical capacity of this lithium secondary battery is 4 Ah. In this way, lithium secondary batteries according to Examples 1 to 4 and Comparative Examples 1 and 2 were constructed.

[Initial -30 ° C reaction resistance]
Each of the lithium secondary batteries thus obtained was CC charged at a constant current of 4 A to 3.7 V in a 25 ° C. environment, and further CV charged until the current value reached 0.2 A. The product was cooled to −30 ° C. and AC impedance measurement was performed. And reaction resistance (m (ohm)) was read from the Cole-Cole plot of the obtained impedance. The AC impedance measurement conditions were an AC applied voltage of 5 mV and a frequency range of 0.01 Hz to 1000000 Hz. The results are shown in Table 2.

[60 ° C cycle capacity maintenance rate]
Further, each of the obtained lithium secondary batteries was CC charged to 4.1 V with a constant current of 16 A in an environment of 25 ° C., and after 10 minutes of rest, it was 3.0 V with a constant current of 16 A. The charging / discharging cycle which performed CC discharge until it was stopped for 10 minutes was repeated 2000 times continuously. And, from the initial capacity before the charge / discharge cycle test and the discharge capacity after the charge / discharge cycle test, the capacity retention rate after the charge / discharge cycle test (= [discharge capacity after the charge / discharge cycle test / before the charge / discharge cycle test) Initial capacity] × 100).

As is clear from Table 2, the lithium secondary battery according to Comparative Example 2 using the CMC solution at the time of solidification tends to increase the initial reaction resistance because the surface of the negative electrode active material is widely covered with the swollen CMC. It was. On the other hand, the lithium secondary battery according to Comparative Example 1 using CMC powder at the time of solidification and using pure water at the time of dilution is not much different from Examples 1 to 4, but after the 60 ° C. cycle test. The capacity retention rate was less than 60% and lacked durability. On the other hand, the batteries according to Examples 1 to 4 using CMC powder at the time of solidification and using the CMC solution at the time of dilution have a lower reaction resistance than the batteries of Comparative Examples 1 and 2, and the cycle of 60 ° C. The capacity retention rate after the test was as good as 70% or more, and extremely high durability performance was exhibited.
From the above results, the lithium secondary battery formed using the negative electrode active material layer forming paste prepared in Examples 1 to 4 has lower battery resistance, and the negative electrode active material layer is less likely to be peeled off. It was confirmed that it was excellent.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment and Example are only illustrations and what changed and changed the above-mentioned specific example is contained in the invention disclosed here. For example, the type of battery is not limited to the above-described lithium secondary battery, and may be batteries having various contents with different electrode body constituent materials and electrolytes. Further, the size and other configurations of the battery can be appropriately changed depending on the application (typically for in-vehicle use).

  The electrode for a secondary battery according to the present invention includes an electrode active material layer that is difficult to peel off from the electrode current collector and has excellent adhesiveness. With such characteristics, the secondary battery including the electrode for the secondary battery according to the present invention can be suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile. Accordingly, as shown in FIG. 6, a vehicle 1 (typically, which includes such a secondary battery 100 (which may be in the form of an assembled battery formed by connecting a plurality of the batteries 100 in series) as a power source. Automobiles, in particular, automobiles equipped with electric motors such as hybrid cars and electric cars).

DESCRIPTION OF SYMBOLS 1 Vehicle 40 Battery case 42 Case main body 44 Cover body 50 Positive electrode sheet 52 Positive electrode current collector 54 Positive electrode active material layer 60 Negative electrode sheet 62 Negative electrode current collector 65 Tensile jig 68 Measuring stand 70 Separator sheet 80 Winding electrode body 92 Positive electrode terminal 94 Negative electrode terminal 96 Positive electrode lead terminal 98 Negative electrode lead terminal 100 Secondary battery

Claims (6)

A method for producing an electrode for a secondary battery in which an electrode active material layer is formed on the surface of an electrode current collector, the following steps:
A powder mixing step of mixing the thickener powder and the electrode active material powder;
A kneading step of kneading the powder mixture obtained in the powder mixing step with water;
A thickener aqueous solution mixing step of obtaining a paste for forming an electrode active material layer by mixing the kneaded material obtained in the kneading step and the thickener aqueous solution; and
Applying the electrode active material layer forming paste to an electrode current collector to form an electrode active material layer on the current collector;
An electrode manufacturing method comprising:   The amount of the thickener added in the powder mixing step with respect to 100 parts by mass of the electrode active material is equal to the thickener added in the thickener aqueous solution mixing step with respect to 100 parts by mass of the electrode active material. The electrode manufacturing method according to claim 1, wherein the amount is larger than the amount of the electrode.   The electrode manufacturing method according to claim 1 or 2, wherein a thixotropy index of the electrode active material layer forming paste is 3.0 or more.   The electrode manufacturing method according to claim 1, wherein the electrode active material is carbon particles capable of reversibly occluding and releasing charge carriers.   The said thickener is an electrode manufacturing method as described in any one of Claims 1-4 which is a cellulose derivative. A secondary battery comprising, as a negative electrode, an electrode obtained by the production method according to claim 1.

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