JP2014143064A - Secondary battery and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery in which a negative electrode active material is prevented from being cracked during a paste kneading process, and thereby deterioration in battery performance is suppressed, and a suitable method for manufacturing the secondary battery.SOLUTION: A secondary battery 100 provided by the present invention includes a positive electrode 50 and a negative electrode 60. The negative electrode 60 includes a negative electrode collector and a negative electrode active material layer held by the negative electrode collector. The negative electrode active material layer includes a negative electrode active material and expanded graphite as a conductive material. The average particle size D1 of the expanded graphite is smaller than the average particle size D2 of the negative electrode active material (D1<D2).

Description

  The present invention relates to a secondary battery and a method for manufacturing the same.

  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. Patent document 1 is mentioned as a prior art regarding a lithium secondary battery.

  One typical configuration of a lithium secondary battery includes an electrode having a configuration in which a material (electrode active material) capable of reversibly occluding and releasing lithium ions is held by a conductive member (electrode current collector). A typical example of the electrode active material (negative electrode active material) used for the negative electrode is a graphite material. A typical example of the electrode current collector (negative electrode current collector) used for the negative electrode is a sheet-like or foil-like member mainly composed of copper or a copper alloy. Such a negative electrode for a battery is, for example, a paste-like or ink-like composition containing a negative electrode active material, a thickener, and a solvent (hereinafter, such a suspension-like composition is simply referred to as “paste”). That is, it is manufactured by preparing a paste for forming an active material layer, applying it to a current collector and drying it.

JP-A-11-312525

  By the way, when the paste is prepared, since the thickener is not sufficiently dissolved, the paste may contain a lump of undissolved material or agglomerates (dama). Since the generation of such agglomerates adversely affects battery performance, it has been attempted to disperse such agglomerates and uniformly disperse the thickener in the solvent. The present inventor uses a biaxial kneader having a high shear force to promote dispersion of the thickener, and kneads the negative electrode active material and the thickener under a high shear force. Are considering. However, when the negative electrode active material and the thickener are kneaded under a high shear force, the negative electrode active material is likely to be cracked or cracked. If the negative electrode active material is cracked or cracked, it may cause an increase in irreversible capacity or a decrease in storage capacity.

  The present invention has been made in view of the above points, and its main object is to provide a secondary battery in which degradation of battery performance is suppressed by preventing cracking of the negative electrode active material during paste kneading, and the secondary battery. It is providing the suitable manufacturing method of these.

  In order to achieve the above object, the present invention provides a method for producing a secondary battery including a positive electrode and a negative electrode. In this manufacturing method, a paste raw material containing a negative electrode active material, expanded graphite as a conductive material, a thickener, and a solvent is put into a kneader and kneaded to obtain a paste for forming a negative electrode active material layer. Including that. Here, the average particle diameter D1 of the expanded graphite charged into the kneader is smaller than the average particle diameter D2 of the negative electrode active material (D1 <D2). In addition, the method includes coating the negative electrode active material layer forming paste on a negative electrode current collector to obtain a negative electrode having a negative electrode active material layer formed on the negative electrode current collector. Furthermore, it includes constructing a secondary battery using the obtained negative electrode.

  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. The term “expanded graphite” refers to graphite in which the interlayer of the graphite crystal structure is expanded. Typically, after intercalating a chemical solution such as concentrated sulfuric acid between the layers of the graphite crystal structure, the chemical solution is gasified, It refers to the one in which the graphite crystal structure is expanded by the pressure at that time.

  According to the production method of the present invention, since expanded graphite having a particle size smaller than that of the negative electrode active material is added and kneaded, cracking and cracking of the negative electrode active material that may occur during paste kneading are suppressed, and the viscosity is further increased. Dispersion (typically dissolution) of the agent is facilitated. Therefore, it is avoided that the thickener and the agglomerates of the thickener are generated, and the deterioration of the battery performance due to this is prevented. A secondary battery constructed using such a paste can have, for example, a high post-cycle capacity retention rate and excellent high-temperature storage characteristics.

  In a preferred embodiment of the secondary battery manufacturing method disclosed herein, a biaxial kneader is used as the kneader. By using a twin-screw kneader, the dispersion of the thickener can be further promoted, and the generation of an undissolved mass or aggregate of the thickener can be better suppressed.

  In a preferred embodiment of the secondary battery manufacturing method disclosed herein, the solid content concentration of the negative electrode active material layer forming paste is 50% by mass to 70% by mass. Within such a solid content concentration range, a strong shearing force is applied to the powder of the thickener and the negative electrode active material, and the agglomerates of each powder are dispersed in a short time to form a uniform kneaded product. Can be produced.

  In a preferred embodiment of the secondary battery manufacturing method disclosed herein, the proportion of the expanded graphite is 1% by mass when the total mass of the solid content of the negative electrode active material layer forming paste is 100% by mass. To 10% by mass (preferably 3% to 8% by mass). When the proportion of the expanded graphite is less than 1% by mass, the performance improvement effect by using the expanded graphite may be reduced. On the other hand, when the proportion of the expanded graphite is more than 10% by mass, the expanded graphite may reaggregate and the electrode reaction may become non-uniform.

  In a preferred embodiment of the secondary battery manufacturing method disclosed herein, when the paste raw material is kneaded, first, the negative electrode active material, expanded graphite, thickener, and solvent are put into a kneader and kneaded. Then, the obtained kneaded product is diluted with a solvent to obtain a negative electrode active material layer forming paste from the kneaded product. A negative electrode active material layer forming paste in which a negative electrode active material, expanded graphite, and a thickener are uniformly dispersed by using a two-step kneading method of kneading with a small amount of solvent and then diluting with a solvent. Can be obtained.

  In a preferred aspect of the secondary battery manufacturing method disclosed herein, the average particle diameter D1 of the expanded graphite is 0.01 ≦ (D1 / D2) ≦ the average particle diameter D2 of the negative electrode active material. 0.1, preferably 0.01 ≦ (D1 / D2) ≦ 0.05. By setting the particle size ratio (D1 / D2) of the expanded graphite to the negative electrode active material as described above, cracking and cracking of the negative electrode active material that may occur during kneading can be prevented more effectively.

  In a preferred embodiment of the secondary battery manufacturing method disclosed herein, the expanded graphite has an average particle diameter D1 of 0.5 μm or less, and the negative electrode active material has an average particle diameter D2 of 10 μm or more. By setting such a particle size difference, cracking and cracking of the negative electrode active material that may occur during kneading can be prevented better.

  In another aspect of the present invention, a secondary battery including a positive electrode and a negative electrode is provided. The negative electrode includes a negative electrode current collector and a negative electrode active material layer held by the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material and expanded graphite as a conductive material. The average particle diameter D1 of the expanded graphite is smaller than the average particle diameter D2 of the negative electrode active material (D1 <D2). Such a secondary battery can have, for example, a high capacity retention rate after durability and excellent high-temperature storage characteristics.

  In a preferred aspect of the secondary battery manufacturing method disclosed herein, the average particle diameter D1 of the expanded graphite is 0.01 ≦ (D1 / D2) ≦ the average particle diameter D2 of the negative electrode active material. 0.1. The average particle diameter D1 of the expanded graphite may be 0.01 ≦ (D1 / D2) ≦ 0.05 with respect to the average particle diameter D2 of the negative electrode active material. The expanded graphite may have an average particle diameter D1 of 0.5 μm or less, and the negative electrode active material may have an average particle diameter D2 of 10 μm or more. Furthermore, when the total mass of the negative electrode active material layer is 100% by mass, the proportion of the expanded graphite may be 1% by mass to 10% by mass. Such a secondary battery may have, for example, a higher capacity retention rate after endurance and better high-temperature storage characteristics.

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 secondary battery which concerns on one Embodiment of this invention. It is a figure which shows a biaxial kneader typically. It is a figure which shows typically the VI-VI cross section of FIG. 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.

<Negative electrode sheet>
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 negative electrode active material layer 64 containing a negative electrode active material, a conductive material, a thickener, and a binder on the surface (both surfaces here) of the negative electrode current collector 62. It has a retained structure. 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, graphite has a small particle size and a large surface area per unit volume, and thus can be a negative electrode active material suitable for rapid charge / discharge (for example, high power discharge).

<Average particle size of negative electrode active material>
The average particle size of the negative electrode active material disclosed herein can be selected as appropriate according to the battery configuration, but is usually substantially equal to particles having an average particle size in the range of about 1 μm to 50 μm. The use of the composed negative electrode active material is appropriate, preferably about 5 μm to 40 μm, more preferably about 10 μm to 30 μm, still more preferably about 10 μm to 20 μm, and even about 10 μm to 15 μm. Good. When the particle size of the negative electrode active material is too large, the specific surface area of the negative electrode active material becomes small, and the battery performance may tend to decrease. On the other hand, if the particle size of the negative electrode active material is too small, there is a problem that the bulkiness is increased and the productivity is lowered, or side reactions in the battery are increased and the life is deteriorated. It is preferable to use a negative electrode active material of about 1 μm or more (preferably 10 μm or more). Here, the average particle diameter of the negative electrode active material refers to a volume-based D50 diameter measured using a general laser diffraction particle size distribution measuring apparatus.

<Conductive material>
As the conductive material, expanded graphite is used. Here, expanded graphite is graphite obtained by rapidly heating a graphite intercalation compound to expand the graphite interlayer, and in the present embodiment, the powder is used. Here, the expanded graphite typically has a particle size smaller than that of the negative electrode active material. For example, the average particle diameter D1 of the expanded graphite is preferably 0.01 ≦ (D1 / D2) ≦ 0.1, more preferably 0.01 ≦ with respect to the average particle diameter D2 of the negative electrode active material. (D1 / D2) ≦ 0.05. By setting the particle size ratio of the expanded graphite to the negative electrode active material as described above, cracks and cracks in the negative electrode active material that can occur during the preparation of the negative electrode active material layer forming paste described later can be more effectively suppressed. For example, expanded graphite having an average particle diameter D1 smaller than the average particle diameter D2 of the negative electrode active material and in the range of about 0.01 μm to 15 μm is suitable, preferably about 0.05 μm to 1 μm. More preferably, it is about 0.08 μm to 0.9 μm, more preferably about 0.1 μm to 0.9 μm, and may be about 0.1 μm to 0.5 μm. .

<Thickener>
The thickener used for the negative electrode active material layer 64 may be a polymer material that can function as a thickener for a negative electrode active material layer forming paste described later. For example, when the negative electrode active material layer forming paste is an aqueous solvent (solution in which the dispersion medium is mainly an aqueous solvent) composition, a polymer that is dispersed or dissolved in the aqueous solvent can be used. Examples of the polymer material that can be dissolved in an aqueous solvent and function as a thickener include, for example, carboxymethylcellulose (CMC; typically sodium salt), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), and methylcellulose (MC). And cellulose derivatives such as cellulose acetate phthalate (CAP), hydroxypropylmethylcellulose (HPMC), 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>
The binder used for the negative electrode active material layer 64 is for bonding the negative electrode active material and the conductive material, and the material constituting the binder itself is used for a conventionally known negative electrode for a lithium secondary battery. Similar materials may be used. For example, vinyl acetate copolymer, styrene butadiene block copolymer (SBR), acrylic acid-modified SBR resin (SBR latex), rubber such as gum arabic, polyethylene oxide (PEO), polytetrafluoroethylene (PTFE) ), Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), etc. Is done. 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%). Further, when the total mass of the negative electrode active material layer is 100% by mass, the proportion of the expanded graphite can be 0.5% by mass or more (for example, 0.5% by mass to 15% by mass). It is preferable that it is 1 mass% or more (for example, 1 mass%-15 mass%, More preferably, it is 1 mass%-10 mass%, Most preferably, it is 1 mass%-5 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 usually about 1% by mass to 5%. It is preferable to set it as the 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 negative electrode active material layer forming paste used in the battery manufacturing method disclosed herein has the above-described negative electrode active material, expanded graphite, thickener, and, if necessary, a binder dispersed in water. It can be an aqueous composition. 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, as an example of a preferred embodiment of a method for producing a negative electrode for a secondary battery according to the present invention, a method for producing a negative electrode for a lithium secondary battery (lithium ion secondary battery) is taken as an example, and the flowchart shown in FIG. Will be described with reference to FIG.

  As shown in FIG. 4, in the negative electrode manufacturing method disclosed herein, a paste raw material containing a negative electrode active material, expanded graphite as a conductive material, a thickener, and a solvent is put into a kneader and kneaded. A kneading step (step S10), a dilution step (step S20) in which the kneaded product obtained in this kneading step is further diluted with a solvent to obtain a negative electrode active material layer forming paste, and the negative electrode active material thus obtained Applying a layer forming paste to the negative electrode current collector to form a negative electrode active material layer on the current collector (step S40). In this embodiment, after the dilution step, a binder is further mixed to adjust the viscosity, and finally a negative electrode active material layer forming paste is obtained (step S30).

  As the solvent used in the negative electrode active material layer forming paste, use of an aqueous solvent is possible from various viewpoints such as reduction of environmental load, reduction of material cost, simplification of equipment, reduction of waste, and improvement of handling property. preferable. As the aqueous solvent, water or a mixed solvent mainly composed of water is preferably used. As a solvent component other than water constituting such a mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. For example, it is preferable to use an aqueous solvent in which 80% by mass or more (more preferably 90% by mass or more, more preferably 95% by mass or more) of the aqueous solvent is water. A particularly preferred example is an aqueous solvent substantially consisting of water.

  Although it does not specifically limit as an apparatus used for the said kneading | mixing process and a dilution process, For example, a biaxial kneader is used suitably. 5 and 6 are diagrams showing the structure of a general twin-screw kneader 10. As shown in FIG. 5, the twin-screw kneader 10 performs kneading in the kneading section 12 and performs dilution in the dilution kneading section 14. That is, a negative electrode active material, expanded graphite, and a thickening agent, which are paste raw materials, are input into the chamber 13 from the input port 11, and a solvent (here, water) is added into the chamber 13 from an addition port (not shown). The paste raw material is kneaded by rotating the paddle 15 (see FIG. 6) in the kneading section 12 in the chamber 13. Further, an addition port (not shown) is provided at a position spaced a predetermined distance from the charging port 11, and a solvent (water here) is added for dilution from the addition port, whereby the kneaded material obtained by the kneading is After being diluted and kneaded in the dilution kneading section 14 in the chamber 13, it is discharged from the discharge port 19.

  As shown in FIG. 6, the kneading unit 12 includes a pair of shafts 16 extending in parallel with each other, a pair of paddles 15 combined with each shaft 16, and a barrel 18 containing the paddles 15. Yes. The pair of paddles 15 are incorporated in a pair of shafts 16 extending in parallel to each other, and are rotated at the same speed and in the same direction. Further, the pair of paddles 15 rotate so that one tip rubs the other, so that the paste raw material is conveyed forward in any paddle 15 without adhering. Further, a predetermined minute gap is formed between the tip of each paddle 15 and the barrel 18. Then, by rotating each paddle 15 in the same direction, the process of kneading while conveying the paste raw material between the paddle 15 and the barrel 18 and between the paddle 15 and the paddle 15 can be performed continuously.

  The rotation speed of the paddle 15 in the kneading section 12 (kneading step) is preferably in the range of 50 rpm to 400 rpm, preferably 100 rpm to 300 rpm, and more preferably 200 rpm to 300 rpm. Further, the preferable kneading time in the kneading process may be a time until the active material, the thickener and the expanded graphite are uniformly dispersed, and may vary depending on the apparatus configuration and kneading conditions. It is suitable to make it within the range of 80 minutes, Preferably it is 6 minutes-60 minutes, More preferably, it is 5 minutes-20 minutes. Within such a range of rotation speed and kneading time, since the negative electrode active material and the thickener are kneaded with a strong force (for example, shearing force), the dispersion of the thickener proceeds, and the active material and the thickener. Can be dispersed to form a uniform state. Further, the content of the solvent at the time of kneading is not particularly limited, but is generally 20% by mass to 50% by mass of the entire kneaded product, and preferably 30% by mass to 40% by mass. Within such a solvent content range, a strong shearing force is applied to each powder, and agglomerates of each powder can be dispersed in a short time to produce a uniform kneaded product.

  In the kneading part 12 (kneading step), since the paste raw material stays between the paddle 15 and the barrel 18 without any gap and the viscosity of the kneaded material is relatively high, the powder of each material has a strong shearing force. Are rubbed together. Therefore, the agglomerates (dama) of each powder can be unwound and dispersed uniformly. In the present embodiment, expanded graphite having a particle diameter smaller than that of the negative electrode active material is added and kneaded. Therefore, the expanded graphite exerts an action as a lubricant, thereby adding to the negative electrode active material. Although the stress is relaxed and rubbed with a strong shearing force, cracking and cracking of the negative electrode active material are suppressed. If the negative electrode active material is cracked or cracked, side reactions in the battery increase, which may increase the irreversible capacity or decrease the storage capacity.

  On the other hand, as described above, in the manufacturing method according to this embodiment, the expanded graphite having a particle size smaller than that of the negative electrode active material exerts an action as a lubricant, thereby suitably suppressing cracking and cracking of the negative electrode active material. Therefore, there is no problem due to cracking or cracking of the negative electrode active material. Furthermore, since there are many edge surfaces having oxygen-containing functional groups on the surface of the expanded graphite, the expanded graphite shows a role of a medium that promotes dispersion with respect to the thickener. As a result, the dispersion of the thickener is further promoted, the generation of the undissolved mass of the thickener and the generation of agglomerates can be avoided, and the deterioration of the battery performance due to this can be suitably prevented.

  The cross-sectional shape of the chamber 13 in the dilution kneading unit 14 (dilution step) may be the same as or different from that of the kneading unit 12. Further, the rotational speed of the paddle 15 in the dilution kneading unit 14 (dilution step) may be the same as or different from that of the kneading unit 12. A preferable kneading time in the dilution kneading unit 14 (dilution step) may be a time until the kneaded product and the solvent are uniformly mixed.

  In the preferable technique of the negative electrode manufacturing method disclosed herein, the amount A of the solvent (here, water) added in the kneading step with respect to 100 parts by mass of the negative electrode active material is the dilution step with respect to 100 parts by mass of the negative electrode active material. The amount of the solvent (in this case, water) added in step B is greater than B. For example, the ratio (A / B) between the amount A of the solvent added in the kneading step and the amount B of the solvent added in the dilution step is approximately 1 ≦ A / B ≦ 15 (for example, 1.23 ≦ A). /B≦10.8) is appropriate, and preferably 1.4 ≦ A / B ≦ 5 (for example, 1.42 ≦ A / B ≦ 4.6). According to such a configuration, since the ratio of the amount of water added in the kneading step and the amount of the solvent added in the dilution step is in an appropriate balance, the above-described effects can be exhibited better.

  After the kneaded material is diluted, the final negative electrode active material layer forming paste is completed by mixing the binder in step S30 of FIG. Thereby, the viscosity and solid content rate of the final paste are adjusted.

  Thereafter, in step S40, a process for forming a negative electrode active material layer on the current collector by applying a negative electrode active material layer forming paste to the negative electrode 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 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 thus obtained has a negative electrode active material layer 64 containing a negative electrode active material, expanded graphite, and a thickener, as a negative electrode current collector. 62 is held. The negative electrode active material layer 64 is formed by using a biaxial kneader during paste kneading and adding expanded graphite having a particle size smaller than that of the negative electrode active material to the paste as a conductive material and lubricant. is there. Therefore, in the obtained negative electrode active material layer 64, cracking of the negative electrode active material is suppressed, and the formation of the undissolved mass and aggregate of the thickener is better suppressed. Further, the expanded graphite can be uniformly dispersed. The lithium secondary battery 100 including the negative electrode active material layer 64 may have, for example, a high capacity retention rate after cycling and excellent high-temperature storage characteristics.

  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.

<Positive electrode sheet>
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.

<Separator>
As illustrated in FIG. 2, the separator 70 is a member that separates the positive electrode sheet 50 and the negative electrode sheet 60. In this example, the separator 70 is formed of a strip-shaped sheet material having a predetermined width and having a plurality of minute holes. As the separator 70, for example, a single layer structure separator or a multilayer structure separator made of a porous polyolefin resin can be used. In this example, as shown in FIG. 2, the width of the negative electrode active material layer 64 is slightly wider than the width of the positive electrode active material layer 54. Furthermore, the width of the separator 70 is slightly wider than the width of the negative electrode active material layer 64.

  In the example illustrated in FIG. 2, the separator 70 is configured by a sheet-like member. The separator 70 may be a member that insulates the positive electrode active material layer 54 and the negative electrode active material layer 64 and allows the electrolyte to move. Therefore, it is not limited to a sheet-like member. For example, the separator 70 may be formed of a porous layer of insulating particles formed on the surface of the positive electrode active material layer 54 or the negative electrode active material layer 64 instead of the sheet-like member. Here, as the particles having insulating properties, inorganic fillers having insulating properties (for example, fillers such as metal oxides (for example, alumina) and metal hydroxides), or resin particles having insulating properties (for example, polyethylene, Or particles such as polypropylene). A porous layer of such insulating particles may be formed on the surface of the separator 70 (sheet-like member). 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 placed (injected) into the case main body 42.

<Electrolyte>
As the electrolytic solution (non-aqueous electrolytic solution), the same non-aqueous electrolytic solution that has been conventionally used in lithium secondary batteries 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. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,3-dioxolane, and the like. One kind or two or more kinds selected from the group can be used. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) _{3 and the} like. Lithium salts can be used. As an example, a nonaqueous electrolytic solution in which LiPF ₆ is contained in a mixed solvent of ethylene carbonate and diethyl carbonate (for example, a mass ratio of 1: 1) at a concentration of about 1 mol / L can be given.

  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 similarly to 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 has a negative electrode active material in which cracking of the negative electrode active material during kneading of the paste is prevented as described above, and the negative electrode active material, thickener and expanded graphite are uniformly dispersed. Since it is constructed using a negative electrode provided with a material layer, it exhibits excellent battery performance. For example, by constructing a battery using the negative electrode, the battery satisfies at least one (preferably all) of good cycle characteristics, excellent high-temperature storage characteristics, high durability, and good production stability. A secondary 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.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not intended to be limited to those shown in the examples.

<Negative electrode sheet>
<Sample 1>
For the purpose of preparing a negative electrode active material layer forming paste, 100 parts by mass of graphite powder as a negative electrode active material, 5 parts by mass of expanded graphite powder as a conductive material, and 1 part by mass of CMC powder as a thickener Put into a twin-screw kneader (trade name: KRC Kneader T2 (trademark), manufactured by Kurimoto Seiko Co., Ltd.), add pure water so that the solid content is about 65.2% by mass, and then rotate at 250 rpm And kneading for 10 minutes (kneading step). Subsequently, pure water was further added so that the solid content rate was about 55.4% by mass, and the mixture was diluted and kneaded at a rotation speed of 250 rpm for 10 minutes (dilution step). Thereafter, 1 part by mass of SBR as a binder was mixed to obtain a target negative electrode active material layer forming paste. As the expanded graphite powder, commercially available expanded graphite (trade name BSP-3000, manufactured by Chuetsu Graphite Manufacturing Co., Ltd.) was used. In this example, the average particle size of the expanded graphite powder was 5 μm, and the average particle size of the negative electrode active material powder was 10 μm.

The obtained paste for forming a negative electrode active material layer is applied to both sides of a long sheet-like copper foil (negative electrode current collector: thickness 14 μm) in a strip shape and dried, whereby the negative electrode active material is coated on both sides of the negative electrode current collector. A negative electrode sheet provided with a material layer was produced. The coating amount of the negative electrode active material layer forming paste was adjusted so that the total amount of both surfaces was about 140 mg / cm ² (solid content basis). The total thickness of the negative electrode sheet was 154 μm, the length was 4700 mm, and the coating width of the negative electrode active material layer was 100 mm.

<Sample 2>
A negative electrode sheet was produced in the same manner as in Sample 1 except that the average particle size of the expanded graphite powder was changed to 0.1 μm.

<Sample 3>
A negative electrode sheet was produced in the same manner as in Sample 1 except that the average particle size of the expanded graphite powder was changed to 0.9 μm.

<Sample 4>
A negative electrode sheet was produced in the same manner as Sample 1 except that the average particle size of the expanded graphite powder was changed to 0.08 μm.

<Sample 5>
A negative electrode sheet was produced in the same manner as in Sample 1 except that the average particle size of the expanded graphite powder was changed to 1 μm.

<Sample 6>
A negative electrode sheet was produced in the same manner as in Sample 1 except that the average particle size of the expanded graphite powder was changed to 12 μm.

<Sample 7>
A negative electrode sheet was produced in the same manner as in Sample 1 except that the average particle size of the expanded graphite powder was changed to 12 μm.

<Sample 8>
A negative electrode sheet was prepared in the same manner as Sample 1 except that the expanded graphite powder was not added.

<Sample 9>
A negative electrode sheet was produced in the same manner as in Sample 1, except that the expanded graphite powder was not added and that the paste was kneaded with a planetary mixer (rotation speed: 25 rpm, kneading time: 240 minutes).

<Sample 10>
A negative electrode sheet was produced in the same manner as Sample 1 except that ordinary graphite powder (average particle size: 0.5 μm) was added instead of the expanded graphite powder.

<Sample 11>
A negative electrode sheet was produced in the same manner as Sample 1 except that stainless steel (SUS) powder (average particle size: 0.5 μm) was added instead of the expanded graphite powder.

<Sample 12>
A negative electrode sheet was produced in the same manner as in Sample 1 except that polytetrafluoroethylene (PTFE) powder (average particle size: 0.5 μm) was added instead of the expanded graphite powder.

<Sample 13>
A negative electrode sheet was produced in the same manner as Sample 1, except that the amount of expanded graphite powder added was changed to 1 part by mass.

<Sample 14>
A negative electrode sheet was produced in the same manner as Sample 1, except that the amount of expanded graphite powder added was changed to 10 parts by mass.

<Sample 15>
A negative electrode sheet was produced in the same manner as Sample 1, except that the amount of expanded graphite powder added was changed to 0.6 parts by mass.

<Sample 16>
A negative electrode sheet was produced in the same manner as in Sample 1 except that the amount of expanded graphite powder added was changed to 12 parts by mass.

<Sample 17>
A negative electrode sheet was produced in the same manner as Sample 1 except that the number of rotations of the biaxial kneader in the kneading step was changed to 100 rpm.

<Sample 18>
A negative electrode sheet was produced in the same manner as Sample 1 except that the number of rotations of the biaxial kneader in the kneading step was changed to 300 rpm.

<Sample 19>
A negative electrode sheet was produced in the same manner as Sample 1 except that the number of rotations of the biaxial kneader in the kneading step was changed to 50 rpm.

<Sample 20>
A negative electrode sheet was produced in the same manner as Sample 1 except that the number of rotations of the biaxial kneader in the kneading step was changed to 400 rpm.

<Sample 21>
A negative electrode sheet was produced in the same manner as Sample 1 except that the kneading time of the biaxial kneader in the kneading step was changed to 6 minutes.

<Sample 22>
A negative electrode sheet was produced in the same manner as Sample 1 except that the kneading time of the biaxial kneader in the kneading step was changed to 60 minutes.

<Sample 23>
A negative electrode sheet was produced in the same manner as in Sample 1, except that the kneading time of the biaxial kneader in the kneading step was changed to 5 minutes.

<Sample 24>
A negative electrode sheet was produced in the same manner as Sample 1 except that the kneading time of the biaxial kneader in the kneading step was changed to 70 minutes.

<Measurement of pinhole>
The surface of the negative electrode active material layer of each of the negative electrode sheets obtained in Samples 1 to 22 was observed with an SEM (Scanning Electron Microscope), and an aggregate formed per 1000 cm ^{2 of} the negative electrode sheet and a diameter of 0. The number of pinholes of 5 mm or more was measured.

  Moreover, the lithium ion secondary battery for a test was constructed | assembled using the negative electrode sheet obtained by the said samples 1-11, and the performance was evaluated. The test lithium ion secondary battery was constructed as follows.

<Preparation of positive electrode>
100 parts by mass of nickel cobalt lithium manganate (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) powder (average particle diameter (D50 diameter): 5.7 μm) as a positive electrode active material, and acetylene as a conductive material A paste for forming a positive electrode active material layer is prepared by adding 5 parts by mass of black (AB) and 2 parts by mass of polyvinylidene fluoride (PVdF) as a binder to N-methylpyrrolidone (NMP). A positive electrode sheet having a positive electrode active material layer provided on both sides of a positive electrode current collector was prepared by applying a belt-like shape on both sides of the aluminum foil (positive electrode current collector, thickness 15 μm) and drying. The coating amount of the positive electrode active material layer forming paste was adjusted so as to be about 30 mg / cm ² (solid content basis) for both surfaces. The total thickness of the positive electrode sheet was 170 μm, the length was 4500 mm, and the coating width of the positive electrode active material layer was 94 mm.

<Lithium ion secondary battery>
A porous layer made of alumina particles is formed on the surface of a sheet-like member (thickness 20 μm) having a three-layer structure made of two separators (polypropylene (PP) / polyethylene (PE) / polypropylene (PP). A wound electrode body was produced by winding through the wire. 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 3: 4: 3 contains 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. In this way, a lithium secondary battery according to Sample 1 was constructed.

<Measurement of battery capacity>
Each of the test lithium ion secondary batteries according to each sample obtained as described above was charged to a voltage of 4.1 V at a current value of 1 C under a temperature condition of 25 ° C. After 5 minutes of rest, the charged battery was discharged to a voltage of 3.0 V at a current value of 1 C at 25 ° C. Then, after a pause of 5 minutes, the battery was charged at a current value of 1 C to a voltage of 4.1 V, and then charged by a constant voltage method until the current value was reduced to 0.1 C. The charged battery was discharged at 25 ° C. at a current value of 1 C to a voltage of 3.0 V, and then discharged by a constant voltage method until the current value was reduced to 0.1 C. The discharge capacity at this time was defined as battery capacity (battery rated capacity). Here, the battery capacity (battery rated capacity) of the test lithium ion secondary battery was 24 Ah.

<Capacity retention after cycle>
A charge / discharge pattern that repeats CC charge / discharge at 2C was applied to each of the test lithium ion secondary batteries, and a cycle test was performed. Specifically, in an environment of about 50 ° C., a charging / discharging cycle in which charging is performed up to 4.1 V by constant current charging at 2 C and discharging is performed up to 3.0 V by constant current discharging at 2 C is repeated 1000 times continuously. It was. And the capacity | capacitance maintenance factor was computed from the capacity | capacitance after a charging / discharging cycle test, and a rated capacity. Here, the capacity after the charge / discharge cycle test was performed in the same manner as in “Measurement of Rated Capacity”. The capacity retention rate was determined by “battery capacity after charge / discharge cycle test / rated capacity” × 100. Here, the above test was performed on five batteries of each sample, and the average value was calculated.

<Capacity maintenance ratio after 0 ° C pulse cycle>
For each of the test lithium ion secondary batteries, after adjusting to a charged state (SOC 50%) of approximately 50% of the rated capacity, CC discharge at 20C for 10 seconds and 20 seconds at 20C under the temperature condition of 0 ° C. The charge and discharge cycle for performing the CC charging was continuously repeated 50000 times. Then, the battery capacity after 0 ° C. pulse cycle is measured under the same conditions as the rated capacity. From [(battery capacity after 0 ° C. pulse cycle) / (rated capacity)] × 100 (%), The capacity maintenance rate was obtained.

<High temperature storage test>
Each of the test lithium ion secondary batteries was adjusted to a charged state (SOC 100%) of approximately 100% of the rated capacity, and then housed in a constant temperature bath at 60 ° C. and subjected to high temperature aging for 100 days. The battery capacity after high-temperature storage was measured under the same conditions as the rated capacity, and the capacity retention rate after high-temperature storage was determined from [(battery capacity after high-temperature storage) / (rated capacity)] × 100 (%). . Here, the above test was performed on 50 batteries of each sample, and the average value was calculated.

<Evaluation of each sample>
The results of the above test are shown in Table 1 and Table 2 for each sample. Table 1 summarizes the configuration of each sample. Table 2 is a table summarizing the evaluation for each sample.

  As shown in Table 1 and Table 2, in Samples 1 to 7, expanded graphite is added as a conductive material to the negative electrode active material layer. About these samples 1-7, the capacity | capacitance maintenance factor after the high-temperature storage test preserve | saved at 60 degreeC for 100 days is all favorable as 80% or more, Compared with the sample 8 which does not add the expanded graphite, High durability performance was shown. In Samples 1 to 6, expanded graphite having an average particle diameter D1 smaller than the negative electrode active material (average particle diameter D2: 10 μm) is added (D1 <D2). In these samples 1 to 6, the capacity maintenance ratio after the 0 ° C. pulse cycle was 79% or more, and a much better result was obtained, and the low temperature cycle durability was superior to that of sample 7 (D1> D2). . Thus, by adding expanded graphite as a conductive material and making the average particle diameter D1 of the expanded graphite smaller than the average particle diameter D2 of the negative electrode active material, the lithium ion secondary battery has a remarkable capacity maintenance rate. To improve.

  The inventor has found that the capacity retention ratio after durability can be increased by adding expanded graphite as a conductive material and making the average particle diameter D1 of the expanded graphite smaller than the average particle diameter D2 of the negative electrode active material. It was. About this phenomenon, this inventor guesses as follows. That is, by adding expanded graphite as a conductive material and making the average particle diameter D1 of the expanded graphite smaller than the average particle diameter D2 of the negative electrode active material, the expanded graphite exhibits an effect as a lubricant. Further, cracking and cracking of the negative electrode active material that can occur during paste kneading are suppressed. For this reason, problems due to cracking of the negative electrode active material (increase in side reactions in the battery) are unlikely to occur, so that a decrease in battery capacity is suppressed and the post-endurance capacity retention rate is improved.

  In Samples 1 to 3, the average particle diameter D1 of the expanded graphite is 0.01 ≦ D1 / D2 <0.1 with respect to the average particle diameter D2 of the negative electrode active material. In Samples 1 to 3, the number of aggregates and pinholes could be further reduced as compared with Sample 4 (D1 / D2 ≦ 0.08). In sample 4, since the average particle diameter D1 of the expanded graphite is too small, the cohesiveness of the expanded graphite is increased, and it is considered that the surface quality is deteriorated. From this result, it is preferable that the average particle diameter D1 of the expanded graphite is 0.08 <D1 / D2 and 0.01 ≦ D1 / D2 with respect to the average particle diameter D2 of the negative electrode active material. Particularly preferred. In Samples 1 to 3, the post-cycle capacity retention rate and the high temperature storage capacity retention rate could be further increased than those of Samples 5 and 6 (0.1 ≦ D1 / D2). From this result, the average particle diameter D1 of the expanded graphite is preferably D1 / D2 <0.1 and D1 / D2 ≦ 0.05 with respect to the average particle diameter D2 of the negative electrode active material. Particularly preferred.

  In sample 10, graphite powder having an average particle size of 0.5 μm is added. In Sample 10, the capacity retention rate after cycling and the capacity retention rate after high storage were both lower than those in Sample 1 to which expanded graphite was added. When graphite powder is added as in Sample 10, the graphite powder is poor in lubricity compared to expanded graphite, so it is understood that the negative electrode active material cracked during paste kneading, leading to a decrease in battery capacity. . In sample 11, SUS powder having an average particle size of 0.5 μm is added. In sample 11, the 0 ° C. pulse capacity retention rate was lower than in sample 1 to which expanded graphite was added. When the metal powder is added as in the sample 11, it is understood that the dissolution and precipitation of the metal occurs due to charge / discharge, and the battery characteristics are deteriorated. Furthermore, in the sample 12, PTFE powder having an average particle size of 0.5 μm is added. In Sample 12, both the capacity retention after cycle and the capacity retention after high storage were lower than those in Sample 1 to which expanded graphite was added. When the fluorine resin is added as in the sample 12, the fluorine resin does not have conductivity, so that the battery resistance is increased and the battery characteristics are deteriorated.

  In Samples 1, 13, and 14, the amount of expanded graphite added is 1 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material. In Samples 1, 13, and 14, the capacity retention rate after cycling and the capacity retention rate after high storage should be higher than those of Sample 15 (addition amount 0.6 parts by mass) and Sample 16 (addition amount 12 parts by mass). I was able to. In Sample 15, since the amount of expanded graphite added was too small, the effect of suppressing cracking of the negative electrode active material tended to decrease, and it is understood that the battery characteristics were deteriorated compared to Samples 1, 13, and 14. In addition, in Sample 16, the amount of expanded graphite added was excessive, so that the expanded graphite re-agglomerated, and the uniform dispersibility in the negative electrode active material layer tended to decrease, and the battery characteristics compared to Samples 1, 13, and 14 Is understood to have deteriorated. From this result, it is preferable that the addition amount of expanded graphite is in the range of 1 part by mass to 10 parts by mass, and particularly preferably 3 parts by mass to 8 parts by mass with respect to 100 parts by mass of the negative electrode active material. .

  In Samples 1, 17 and 18, the rotational speed of the biaxial kneader in the kneading step is set to 100 rpm to 300 rpm. In Samples 1, 17 and 18, the capacity retention ratio after cycle and the capacity retention ratio after high storage could be further increased as compared with Sample 19 (rotation speed 50 rpm) and Sample 20 (rotation speed 400 rpm). In Sample 19, since the rotational speed of the twin-screw kneader was too small, the dispersion of the thickener was insufficient, and the uniform dispersibility in the negative electrode active material layer tended to decrease, and the battery compared to Samples 1, 17 and 18 It is understood that the characteristics deteriorated. Further, in sample 20, since the rotational speed of the biaxial kneader was too large, the negative electrode active material was cracked, and it is understood that the battery characteristics were deteriorated as compared to samples 1, 17, and 18. From this result, it is preferable to set the rotation speed of the twin-screw kneader within a range of 100 rpm to 300 rpm, and particularly preferably 200 rpm to 300 rpm.

  In Samples 1, 21, and 22, the kneading time of the biaxial kneader in the kneading step is set to 6 to 60 minutes. In Samples 1, 21, and 22, the capacity retention ratio after cycling and the capacity retention ratio after high storage could be further increased as compared with Sample 23 (kneading time 5 minutes) and Sample 24 (kneading time 70 minutes). In Sample 21, since the kneading time of the biaxial kneader was too short, the thickener was not sufficiently dispersed, and the uniform dispersibility in the negative electrode active material layer tended to decrease. It is understood that the characteristics deteriorated. In Sample 24, the kneading time of the biaxial kneader was too long, so the negative electrode active material was cracked, and it was understood that the battery characteristics were deteriorated compared to Samples 1, 21, and 22. From this result, the kneading time of the biaxial kneader is preferably set in the range of 6 minutes to 60 minutes, and particularly preferably 6 minutes to 15 minutes.

  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 secondary battery according to the present invention can prevent the negative electrode active material from cracking when the paste is kneaded, thereby suppressing battery performance deterioration. Due to such characteristics, 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. 7, a vehicle 1 (typically including 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 (typically). Automobiles, in particular, automobiles equipped with electric motors such as hybrid cars and electric cars).

DESCRIPTION OF SYMBOLS 1 Vehicle 10 Biaxial kneader 12 Kneading part 14 Dilution kneading part 15 Paddle 16 Shaft 18 Barrel 19 Discharge port 40 Battery case 42 Case main body 44 Lid 50 Positive electrode sheet 52 Positive electrode current collector 54 Positive electrode active material layer 60 Negative electrode sheet 62 Negative electrode Current collector 64 Negative electrode active material layer 70 Separator sheet 80 Winding electrode body 100 Secondary battery

Claims (13)

A secondary battery comprising a positive electrode and a negative electrode,
The negative electrode is
A negative electrode current collector;
A negative electrode active material layer held by the negative electrode current collector,
The negative electrode active material layer is
Including a negative electrode active material and expanded graphite as a conductive material,
A secondary battery in which an average particle diameter D1 of the expanded graphite is smaller than an average particle diameter D2 of the negative electrode active material (D1 <D2).   The secondary battery according to claim 1, wherein an average particle diameter D1 of the expanded graphite is 0.01 ≦ (D1 / D2) ≦ 0.1 with respect to an average particle diameter D2 of the negative electrode active material.   The secondary particle according to claim 1 or 2, wherein an average particle diameter D1 of the expanded graphite is 0.01 ≦ (D1 / D2) ≦ 0.05 with respect to an average particle diameter D2 of the negative electrode active material. battery. The expanded graphite has an average particle diameter D1 of 0.5 μm or less,
The secondary battery as described in any one of Claims 1-3 whose average particle diameter D2 of the said negative electrode active material is 10 micrometers or more.   The secondary as described in any one of Claims 1-4 whose ratio which the said expanded graphite accounts is 1 mass%-10 mass% when the total mass of the said negative electrode active material layer is 100 mass%. battery. A method for producing a secondary battery comprising a positive electrode and a negative electrode,
Obtaining a paste for forming a negative electrode active material layer by putting a paste raw material containing a negative electrode active material, expanded graphite as a conductive material, a thickener, and a solvent into a kneader and kneading;
Here, the average particle diameter D1 of the expanded graphite charged into the kneader is smaller than the average particle diameter D2 of the negative electrode active material (D1 <D2);
Coating the negative electrode active material layer forming paste on a negative electrode current collector to obtain a negative electrode having a negative electrode active material layer formed on the negative electrode current collector; and a secondary battery using the obtained negative electrode Constructing;
A method for manufacturing a secondary battery.   The battery manufacturing method according to claim 6, wherein a twin-screw kneader is used as the kneader.   The battery manufacturing method of Claim 6 or 7 whose solid content concentration of the said paste for negative electrode active material layer formation is 50 mass%-70 mass%.   The proportion of the expanded graphite is 1% by mass to 10% by mass when the total mass of the solid content of the negative electrode active material layer forming paste is 100% by mass. The battery manufacturing method as described in one. When kneading the paste raw material,
First, a negative electrode active material, expanded graphite, a thickener, and a solvent are put into a kneader and kneaded. Then, the obtained kneaded material is diluted with a solvent to form a negative electrode active material layer from the kneaded material. The battery manufacturing method according to any one of claims 6 to 9, wherein a paste is obtained.   The average particle diameter D1 of the expanded graphite is 0.01 ≦ (D1 / D2) ≦ 0.1 with respect to the average particle diameter D2 of the negative electrode active material. The battery manufacturing method as described in any one of.   The average particle diameter D1 of the expanded graphite is 0.01 ≦ (D1 / D2) ≦ 0.05 with respect to the average particle diameter D2 of the negative electrode active material. The battery manufacturing method as described in any one of. The expanded graphite has an average particle diameter D1 of 0.5 μm or less,
The battery manufacturing method as described in any one of Claims 6-12 whose average particle diameter D2 of the said negative electrode active material is 10 micrometers or more.

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