JP5713198B2 - Method for manufacturing lithium secondary battery - Google Patents

Description

  The present invention relates to a method for manufacturing a lithium secondary battery (typically a lithium ion battery). In detail, it is related with the manufacturing method of the negative electrode of this battery.

  Lithium ion batteries and other lithium secondary batteries are widely used for consumer use (such as power supplies for personal computers and portable terminals) because they are smaller, lighter, and have higher energy density than existing batteries. Moreover, since the output density is high, it is preferably used as a high output power source for driving a vehicle such as a hybrid vehicle (HV).

This type of lithium secondary battery (typically a lithium ion battery) has a configuration in which an electrode body composed of a positive electrode and a negative electrode and an electrolyte (typically an electrolyte solution) are contained in a battery case. The electrodes (positive electrode and negative electrode) are electrode mixture layers (mainly composed of active materials capable of reversibly occluding and releasing charge carriers (typically lithium ions) on corresponding positive and negative current collectors) Specifically, a positive electrode mixture layer and a negative electrode mixture layer) are respectively formed. For example, in the case of a negative electrode of a lithium secondary battery, a slurry (including a paste and ink) composition (negative electrode) prepared by mixing a negative electrode active material and a binder (binder) in an appropriate solvent. By applying (typically coating) the negative electrode current collector to the negative electrode current collector, a negative electrode mixture layer is formed.
The binder is used to bind the negative electrode active material particles in the negative electrode mixture layer and between the active material and the current collector to maintain the shape of the electrode. Typically, for example, a cellulose polymer such as carboxymethyl cellulose (CMC) and a rubber material such as styrene butadiene rubber (SBR) are known.

  When CMC is used as the binder, those having a relatively large molecular weight (for example, 100,000 to 400,000) are preferably used. This is because the higher the molecular weight (degree of polymerization), the higher the viscosity, the dispersion stability of the negative electrode mixture slurry (that is, the negative electrode active material particles are less likely to settle), and the addition of the slurry onto the current collector. This is because the adhesion (adhesive strength) between the formed negative electrode mixture layer and the current collector can be increased. However, CMC having a relatively large molecular weight has a high viscosity, making it difficult to handle during coating, or covering the surface of the negative electrode active material particles widely with CMC, so that lithium ions are occluded and released during battery reaction. May be hindered and the battery performance may be greatly reduced (typically, an increase in IV resistance). As a conventional technique for solving this type of problem, Patent Document 1 discloses that the negative electrode mixture layer contains a metal salt of carboxymethyl cellulose and a synthetic rubber latex adhesive, and the average molecular weight of CMC is 185,000. A method using a mixture of the following and 350,000 or less is introduced.

JP 2008-171575 A

By the way, in recent years, a CMC having a higher molecular weight (for example, a weight average molecular weight of 500,000 or more, typically 1 million or more) is used in a battery having a high output density (for example, a power source for driving a vehicle). It has become like this. By using such a high molecular weight CMC, the adhesion (adhesive strength) between the negative electrode mixture layer and the negative electrode current collector can be further improved, and the battery performance can be improved. Further, for example, in a lithium secondary battery used for a vehicle driving power source or the like, it is required to reduce the amount of CMC in order to realize a higher output density. However, such a problem is caused by using a high molecular weight CMC. Can be dealt with.
On the other hand, CMC having a large molecular weight also has the above-mentioned problems (that is, difficulty in handling at the time of coating, deterioration in battery performance, etc.), and therefore, detailed examination is required for use. However, the technology disclosed in Patent Document 1 is intended for consumer use (so-called portable power supply), and high-power density batteries (for example, vehicle drive power supplies) using CMC having a very large molecular weight are examined ( Is not made).

  The present invention has been made in view of the above points, and the object of the present invention is to increase the adhesion between the negative electrode current collector and the negative electrode mixture layer and improve battery performance (for example, to reduce battery resistance). Another object is to provide a lithium secondary battery with improved cycle characteristics. Another object of the present invention is to provide a suitable method for producing such a lithium secondary battery.

In order to achieve the above object, the present invention provides a method for producing a lithium secondary battery including a negative electrode having a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The manufacturing method disclosed herein includes a step of preparing a slurry-like composition (negative electrode mixture slurry) by dispersing a negative electrode active material and a binder containing at least carboxymethyl cellulose (CMC) in a solvent, and the above negative electrode mixture The method includes a step of applying slurry on the negative electrode current collector to form the negative electrode mixture layer, and a step of constructing a lithium secondary battery using the negative electrode on which the negative electrode mixture layer is formed. Here, in this production method, as the carboxymethyl cellulose (CMC), a low molecular region CMC having a weight average molecular weight of less than 1 million and a high molecular region CMC having a weight average molecular weight of 3 million or more are used. Use so that the weight ratio (A: B) of the molecular region CMC (A) to the polymer region CMC (B) is in the ratio of 25:75 to 75:25 (more preferably 45:55 to 65:35). It is characterized by doing.
The method for manufacturing a lithium secondary battery disclosed herein uses CMC in which the weight ratio between the low molecular region CMC and the high molecular region CMC satisfies the above range. That is, the negative electrode mixture layer produced using CMC satisfying the above range has good adhesion between the negative electrode active material and the current collector, and the degree of coating of the negative electrode active material by CMC is kept low. Therefore, in the lithium secondary battery provided with such a negative electrode composite material layer, the battery performance can be improved (for example, the IV resistance can be reduced and the durability can be improved). It can be suitably used for a power source or the like.

As a preferred embodiment of the production method disclosed herein, in the negative electrode mixture slurry, a synthetic rubber material is used as a binder in addition to the CMC.
The negative electrode composite layer produced by using the CMC (that is, the low molecular region CMC and the high molecular region CMC) and a synthetic rubber material (typically styrene butadiene rubber (SBR)) is used as a negative electrode active material and a current collector. Therefore, in the lithium secondary battery provided with such a negative electrode mixture layer, battery performance can be further improved (for example, reduction in IV resistance and improvement in durability).

As a preferred embodiment of the production method disclosed herein, in the step of preparing the negative electrode mixture slurry, the low molecular region CMC is first added to the solvent containing the negative electrode active material, and then the high molecular region CMC. Is added.
First, the low molecular region CMC is added to the solvent containing the negative electrode active material, so that the low molecular CMC preferentially covers the negative electrode active material. Thereafter, the object of the present invention can be further realized by adding the polymer region CMC. Therefore, in the lithium secondary battery provided with such a negative electrode composite material layer, battery performance can be further improved (for example, reduction of IV resistance and improvement of durability).

As a preferred embodiment of the production method disclosed herein, in the step of preparing the negative electrode mixture slurry, the mixing ratio of the low molecular region CMC region and the high molecular region CMC region is such that the viscosity of the slurry composition is 300 mPa. -Deciding so that it may become in the range of s-2000mPa * s is mentioned.
When the viscosity of the negative electrode mixture slurry is in the above range, the dispersion stability is excellent, and the negative electrode active material is difficult to settle. Further, such a negative electrode mixture slurry has good coatability and can form a negative electrode mixture layer with high accuracy. Therefore, in the lithium secondary battery provided with such a negative electrode mixture layer, battery performance can be improved (for example, reduction of IV resistance and improvement of durability).

  The lithium secondary battery obtained by the production method disclosed herein can reduce the resistance of the negative electrode and improve the battery performance as compared with the conventional one. In particular, vehicles (typically electric motors such as plug-in hybrid vehicles (PHV), hybrid vehicles (HV), electric vehicles (EV), fuel cell vehicles (FCV)) are capable of exhibiting such high performance. Can be suitably used as a power source (drive power source) for a motor mounted on the motor.

It is sectional drawing which shows typically the structure of the negative electrode of the lithium secondary battery containing only high molecular weight CMC. It is sectional drawing which shows typically the structure of the negative electrode of the lithium secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the lithium secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the winding electrode body of the lithium secondary battery which concerns on one Embodiment of this invention. It is a side view which shows typically the vehicle (automobile) provided with the lithium secondary battery which concerns on one Embodiment of this invention. It is a graph which shows transition of the adhesive force which concerns on one Example of this invention, and battery resistance.

In this specification, the battery is the same as various chemical batteries (for example, lithium ion batteries) such as electric double layer capacitors in addition to so-called chemical batteries such as lithium secondary batteries, nickel hydride batteries, nickel cadmium batteries, and lead storage batteries. Power storage elements (physical batteries), pseudo-capacitance capacitors, redox capacitors, hybrid capacitors combining these, and the like that can be used in the same industrial field. In addition, the “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged / discharged by movement of charges accompanying the lithium ions between the positive and negative electrodes. A secondary battery generally referred to as a lithium ion battery (or a lithium ion secondary battery), a lithium polymer battery, or the like is a typical example included in the lithium secondary battery in this specification. The “active material” refers to a substance (compound) that participates in power storage on the positive electrode side or the negative electrode side. That is, it refers to a substance that is involved in the insertion and extraction of electrons during battery charge / discharge.
In addition, unless otherwise indicated, the weight average molecular weight (Mw) of CMC in this specification refers to the value measured by GPC (Gel Permeation Chromatography) -RI (Refractive Index; differential refractive index detector).

  Hereinafter, preferred embodiments of the lithium secondary battery disclosed herein will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for implementation can be grasped as design matters of those skilled in the art based on the prior art in this field. The lithium secondary battery having such a structure can be implemented based on the contents disclosed in the present specification and common technical knowledge in the field.

  The manufacturing method of the lithium secondary battery disclosed here will be described in detail. As described above, the production method disclosed herein is a method characterized by the production process of a slurry-like (including paste-like and ink-like) composition (negative-electrode mixture slurry) used in the production of a negative electrode. is there. First, the preferable aspect of the manufacturing method of the negative electrode for lithium secondary batteries using this negative electrode compound slurry is demonstrated in detail.

  The method for producing a negative electrode for a lithium secondary battery disclosed herein prepares a slurry-like composition (negative electrode mixture slurry) by dispersing a negative electrode active material and a binder containing at least carboxymethyl cellulose (CMC) in a solvent. And a step (preparation step) of applying the prepared negative electrode mixture slurry onto the negative electrode current collector to form the negative electrode mixture layer.

<Preparation process>
In the adjustment step in the production method disclosed herein, first, a solvent, a negative electrode active material, and a binder containing at least carboxymethylcellulose (CMC) are prepared, and the negative electrode active material and the binder are dissolved or dispersed in the solvent. A slurry-like composition (negative electrode mixture slurry) is prepared. Through this process, the negative electrode active material and the binder can be uniformly dispersed in the solution.

  As the negative electrode active material used here, one or more of materials conventionally used in lithium secondary batteries can be used without any particular limitation. Examples thereof include carbon materials (carbon powder), oxides such as lithium titanate (LTO), tin (Sn), silicon (Si) and lithium alloys, and the like. The carbon material includes at least a graphite structure (layered structure), graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), or a combination thereof Etc. can be used.

The average particle diameter of the negative electrode active material may be about 6 to 30 μm, for example. Moreover, the specific surface area of this active material may be about 2-5 m < ² > / g, for example. If the specific surface area is too small, a sufficient current density may not be obtained during charging and discharging. If the specific surface area is too large, the battery capacity may decrease due to an increase in irreversible capacity. As the specific surface area, a value (BET specific surface area) measured by a nitrogen adsorption method is employed.
The oil absorption amount of the negative electrode active material may be about 45 (mL / 100 g) to 63 (mL / 100 g). The oil absorption can be an index indicating the affinity of the negative electrode active material for CMC. If the amount of oil absorption is too small, the viscosity of the negative electrode mixture slurry decreases, and the dispersion stability may decrease (for example, the negative electrode active material tends to settle). On the other hand, when the oil absorption is too large, the viscosity of the negative electrode mixture slurry becomes too high, and the coatability and dispersion stability may be lowered (for example, the viscosity is likely to increase). As the absorption amount (mL / 100 g), a value measured according to JIS K6217-4 is adopted.

In the production method disclosed herein, at least carboxymethyl cellulose (CMC) is used as the binder. CMC (typically an alkali metal salt such as a sodium salt) is a derivative of cellulose and is generally used as a binder or a thickener (viscosity adjusting agent) in the production of a lithium secondary battery.
The CMC used here is a low molecular region CMC having a weight average molecular weight (Mw) of less than 1 million (typically 500,000 to less than 1 million), and a weight average molecular weight (Mw) of 3 million or more (typical). 3 to 5 million or less, for example, 3 to 4 million, for example. Therefore, as long as the range of the Mw value is satisfied, two or three or more kinds of CMCs that are usually used when manufacturing a lithium secondary battery can be used without any particular limitation.

The weight average molecular weight (Mw) of CMC can be measured, for example, under the following conditions. That is, CMC (powder) to be measured is dissolved in an eluent, adjusted to an appropriate concentration, and molecular weight distribution is obtained by performing GPC (Gel Permeation Chromatography) -RI (Refractive Index; differential refractive index detector). . From this molecular weight distribution, the weight average molecular weight (Mw) can be determined using the following formula (1).
Measuring device: Model “HLC-8320” (manufactured by Tosoh Corporation)
Column (stationary phase); TSKguard-column PWXL and TSKgelGMPWXL
Eluent: 0.1M sodium chloride aqueous solution Molecular weight standard substance: Monodispersed pullulan (manufactured by Showa Denko)
Mw = Σ (M _i × W _i ) / W = Σ (M _i × H _i ) / ΣH _i (1)
Where M _i ; molecular weight of the i-th eluted component
W _i ; weight of the i-th eluted component
W: Total weight of ingredients
H _i ; peak height of the i-th eluted component

When obtaining the weight average molecular weight (Mw) of CMC contained as one component in the electrode mixture layer, first, CMC to be measured is separated from other materials. For example, the CMC is extracted by immersing the electrode in water or the like. At this time, CMC can be efficiently extracted by applying ultrasonic waves or the like. Then, after the extract is centrifuged to remove the active material, an eluent is prepared using the obtained supernatant (that is, a solution in which the extracted CMC is dissolved (water, etc.)). Similarly, the Mw value may be measured. Alternatively, solid CMC can be obtained by collecting water in which CMC is dissolved and evaporating to dryness. What is necessary is just to measure Mw value similarly to the above about this CMC sample.
Further, when starting from the state of the battery, for example, the electrode is taken out from the container, the electrolytic solution is washed and removed, and the electrode having CMC to be measured is separated from other battery components. CMC can be extracted by immersing this electrode in water or the like.

As the CMC used here, CMC having a weight ratio of the low molecular region CMC to the high molecular region CMC of 25:75 to 75:25 (more preferably 45:55 to 65:35) is used. . Generally, when the weight average molecular weight (Mw) of the CMC used is too small, the viscosity of the negative electrode mixture slurry becomes insufficient and dispersion becomes unstable. For this reason, there is a possibility that the negative electrode active material slurry may cause aggregation or precipitation of the negative electrode active material. In addition, when applying the negative electrode mixture slurry to the current collector (coating), the slurry may sag, or a dilatancy phenomenon may occur due to the negative electrode active material (typically particulate), resulting in poor coating such as streaking. May occur. Furthermore, when the negative electrode mixture layer is formed using the negative electrode mixture slurry, the adhesiveness (adhesive strength) with the current collector is insufficient, and there is a possibility that the mixture layer is cracked or peeled off. Conversely, when the molecular weight of CMC used is large, the dispersion stability of the negative electrode mixture slurry as described above and the adhesion between the negative electrode mixture layer and the current collector are excellent, but CMC is the surface of the negative electrode active material particles. Is widely coated to form a film. In such a case, since the occlusion and release of lithium ions accompanying the battery reaction are hindered, the battery performance may be greatly reduced (typically, the IV resistance is increased).
However, in the manufacturing method disclosed herein, the negative electrode mixture layer produced using CMC satisfying the above range has good adhesion between the negative electrode active material and the current collector, and the negative electrode active material is coated with CMC. Is kept to a minimum. Therefore, in the lithium secondary battery provided with such a negative electrode mixture layer, battery performance can be improved (for example, reduction of IV resistance and improvement of durability).

  FIG. 1 shows a schematic diagram of a negative electrode mixture layer using only high molecular weight CMC. Moreover, the schematic diagram of the negative mix layer manufactured with the manufacturing method disclosed here is shown in FIG. FIG. 1 and FIG. 2 are enlarged schematic views of a bonding portion between the negative electrode current collector 22 and the negative electrode mixture layer 24 provided in the negative electrode 20 shown in FIG. 3 (or FIG. 4) described later. The negative electrode mixture layer 24 includes a negative electrode active material 26, a binder (CMC), and a conductive material (not shown). As shown in FIG. 1, when only high molecular weight CMC is used, the negative electrode active material particles 26 included in the negative electrode mixture layer 24 and the active material 26 and the current collector 22 are firmly bound. However, the surface of the negative electrode active material particles 26 is widely covered with a binder (CMC) 28, and the battery performance may be deteriorated. On the other hand, as shown in FIG. 2, when the low molecular region CMC 29 and the polymer region CMC 28 are used by the manufacturing method disclosed herein, the negative electrode active material 26 and the negative electrode current collector 22 are separated by the polymer region CMC 28. Since the low-molecular region CMC 29 binds strongly between the negative electrode active material particles 26 and is strongly bonded, the coating of the negative electrode active material particles 26 with the CMC is minimized.

  In the production method disclosed herein, when the amount of CMC included in the overlapping portion of the molecular weight distribution of the low molecular region CMC and the molecular weight distribution of the high molecular region CMC is small (for example, the low molecular region and the low molecular region In case of less than 10% of the total mass of each polymer region), in the lithium secondary battery having such a configuration, the low molecular region and the polymer region are clearly separated and the molecular weight distribution hardly overlaps. The features of the invention disclosed herein can be realized more clearly and are more preferable.

  In addition, the weight average molecular weight (Mw) of CMC used conventionally is 500,000 or less (for example, 2000 to 400,000, typically 50,000 to 400,000), and was used in the invention disclosed herein. The weight average molecular weight (Mw) of CMC is much larger than that of the prior art (specifically, CMC having a weight average molecular weight (Mw) of 900,000 to 3,300,000 was used as shown in the examples described later). It is.

  The degree of etherification of CMC used here (also referred to as the degree of substitution (DS)) is not particularly limited, but if it is too low, the solubility in water is low, and it is uniformly dispersed when using an aqueous solvent. Difficult to do. Moreover, the function as a binder also falls (that is, adhesiveness (adhesion strength) is insufficient). Also, CMC having a high degree of etherification is preferable because the viscosity is less affected by pH (that is, the viscosity is less dependent on pH) than CMC having a low value. More specifically, the degree of viscosity reduction is low in a high pH range (particularly under strong alkaline conditions of pH 12 or higher). Therefore, the viscosity of the aqueous active material composition can be more appropriately controlled by using such a cellulose derivative. For this reason, the degree of etherification of CMC can be, for example, 0.6 to 2.6, and preferably 0.65 to 1.60.

The “degree of etherification” refers to the number (average value) of OH groups substituted with carboxymethyl groups among the three hydroxyl groups (OH groups) on the glucose ring constituting cellulose. Therefore, the value is theoretically a value between 0 and 3.
The degree of etherification of CMC can be measured, for example, as follows. That is, 1.0 g of CMC (powder) to be measured is precisely weighed, wrapped in filter paper, put into a magnetic crucible, and incinerated at 600 ° C. in air using heating means such as an electric furnace. The sodium compound contained in the ashing residue is dissolved in water, and titrated with a 0.1N sulfuric acid solution using phenolphthalein as an indicator. From the result, the ether degree can be calculated using the following formula (2).
Degree of etherification = (162 × A × f) / (1000−80 × A × f) (2)
However, A: Amount of 0.1N sulfuric acid solution required for neutralization (mL)
f; 0.1N sulfuric acid titer

The negative electrode mixture slurry disclosed herein may have a composition containing a solid component (arbitrary component) other than the negative electrode active material and the CMC as necessary, as long as the effects of the present invention are not significantly impaired. Examples of such materials include binders other than CMC, various polymer materials that can function as thickeners, conductive materials, and the like.
As such a binder, one kind or two or more kinds of various polymer materials usually used for producing a lithium secondary battery can be used without particular limitation. For example, when the negative electrode mixture layer is formed using an aqueous slurry, a polymer material that dissolves or disperses in water can be preferably used. Examples of such polymer materials include cellulosic polymers (typically sodium salts), fluorine resins, vinyl acetate copolymers, synthetic rubber materials (latex), and the like. More specifically, hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), methylcellulose (MC), cellulose acetate phthalate (CAP), hydroxypropylmethylcellulose (HPMC), hydroxypropylmethylcellulose phthalate (HPMCP), polyvinyl alcohol ( PVA), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), styrene butadiene rubber (SBR), butadiene rubber (BR), nitrile butadiene rubber (NBR), acrylic acid modified SBR, Examples thereof include carboxy-modified SBR. Alternatively, when a negative electrode mixture layer is formed using a solvent-based slurry (a solvent-based composition in which the main component of the dispersion medium is an organic solvent), PVdF, polyvinylidene chloride (PVdC), polyethylene oxide (PEO) A polymer material that is dispersed or dissolved in an organic solvent such as the above can be preferably used. In particular, styrene butadiene rubber (SBR), which is a synthetic rubber material, is preferably used.

As a suitable aspect in the manufacturing method disclosed here, in addition to CMC, a synthetic rubber material is used as the binder in the slurry composition.
SBR is a copolymer of styrene and butadiene, and is widely used as a binder in general lithium secondary batteries. As SBR used here, 1 type, or 2 or more types can be used without specifically limiting among SBR normally used when manufacturing a lithium secondary battery. Moreover, SBR in which monomers other than styrene and butadiene are copolymerized may be used. For example, SBR in which the total amount of styrene and butadiene accounts for 50% by mass or more (typically 75% by mass or more, for example, 90% by mass or more) of the total amount of monomers is preferably used. Since SBR used here can be used in the form of an aqueous emulsion (latex) dispersed in an aqueous solvent (typically water), SBR having a carboxyl group introduced into the polymer can be suitably used. Alternatively, a monomer other than styrene and butadiene is not substantially copolymerized (the content of monomers other than styrene and butadiene is 5% by mass or less of the total amount of monomers, and further 1% by mass or less). Also good.
As in the manufacturing method disclosed herein, the negative electrode mixture layer produced by using CMC and a synthetic rubber material (typically styrene butadiene rubber (SBR)) in combination includes a negative electrode active material and a current collector. Adhesiveness becomes stronger, and therefore, in a lithium secondary battery provided with such a negative electrode mixture layer, battery performance can be improved (for example, reduction of IV resistance and improvement of durability).

  As the dispersion solvent used for the preparation of the negative electrode mixture slurry used here, as long as CMC can be suitably dissolved, one of various dispersion solvents usually used in the production of lithium secondary batteries conventionally or Two or more kinds can be used without any particular limitation. As such a dispersion solvent, for example, an aqueous solvent (water or a mixed solvent mainly composed of water) is preferably used. Examples of the solvent component other than water constituting the mixed solvent include organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water. For example, it is preferable to use an aqueous solvent in which 50% by mass or more (more preferably 80% by mass or more, more preferably 90% by mass or more) of the aqueous solvent is water. A particularly preferred example is an aqueous solvent substantially consisting of water. Alternatively, the solvent is not limited to an aqueous solvent, and may be a non-aqueous solvent (the dispersion medium of the active material is mainly an organic solvent). As the non-aqueous solvent, for example, N-methylpyrrolidone (NMP) can be used.

  Although not particularly limited, the solid concentration (hereinafter referred to as NV) of the negative electrode mixture slurry used here is approximately 30% by mass or more (typically 30% by mass to 90% by mass, preferably 40%). Mass% to 60 mass%). Further, the proportion of the negative electrode active material in the slurry solid content is preferably about 50 mass% or more (typically 70 mass% to 99.5 mass%), and is preferably about 80 mass% to 99 mass%. Is more preferable. Moreover, the ratio of the binder in slurry solid content can be 0.1 mass-10 mass%, for example, and should be about 0.5 mass%-6 mass% (for example, 1 mass%-3 mass%). Is preferred. Moreover, when adding a thickener in slurry solid content, the ratio can be made into 10 mass% or less, for example, about 0.5 mass%-6 mass% (for example, 1 mass%-5 mass%). ) Is preferable.

If the amount of the binder added is too small, cracking or the like may occur in the negative electrode mixture layer when the negative electrode mixture slurry is applied to the current collector and dried. In the subsequent production process (for example, cutting to a predetermined width), the negative electrode mixture layer may be peeled off from the current collector. On the other hand, if the binder is added excessively, it may excessively adhere to the surface of the negative electrode active material, obstruct lithium ion occlusion and release (that is, electrochemical reaction), and decrease battery performance.
In particular, in a battery having a high output density such as a power source for vehicles, it is required to secure a sufficient binding property while reducing the amount of binder added as much as possible (that is, increasing the ratio of the negative electrode active material). Yes.

The viscosity of the negative electrode mixture slurry used here may be in the range of about 300 mPa · s to 2000 mPa · s (preferably 500 mPa · s to 1500 mPa · s, more preferably 600 mPa · s to 1000 mPa · s). The specific method for measuring the viscosity will be described in detail in Examples described later. Here, the value measured by an E-type viscometer (manufactured by Toki Sangyo) is adopted. Such a viscosity can be prepared by the above-described molecular weight distribution of CMC (that is, the blending ratio of the low molecular region CMC and the high molecular region CMC).
When the viscosity of the slurry is too low, the negative electrode active material aggregates and settles with time, and the dispersion stability of the negative electrode slurry (that is, the difficulty of precipitation of the negative electrode active material) decreases. For this reason, it can be difficult to accurately form a negative electrode mixture layer having a uniform composition on the negative electrode current collector. If the viscosity is too high, it may be difficult to handle the slurry, or it may be difficult to apply the slurry to the negative electrode current collector with a constant (high accuracy) basis weight.
When the viscosity of the negative electrode mixture slurry is in the above range, the dispersion stability is excellent, and the negative electrode active material is difficult to settle. Further, such a negative electrode mixture slurry has good coatability and can form a negative electrode mixture layer with high accuracy. Therefore, in the lithium secondary battery provided with such a negative electrode mixture layer, battery performance can be improved (for example, reduction of IV resistance and improvement of durability).

The negative electrode mixture slurry can be prepared, for example, by the following procedure. First, the low molecular region CMC is dispersed by adding a smaller amount of water (for example, ion-exchanged water) than the final NV to prepare a highly viscous CMC dispersion slurry. Next, the negative electrode active material is added to the CMC dispersion slurry, mixed and kneaded. Further, the polymer region CMC is added to the slurry and kneaded, and water (for example, ion exchange water) is added to adjust the viscosity. Then, for example, by depressurizing the container containing the slurry, bubbles in the slurry whose viscosity has been adjusted are removed, and a target negative electrode mixture slurry is obtained.
As described above, by first adding the low molecular region CMC to the solvent containing the negative electrode active material, the low molecular CMC preferentially covers the negative electrode active material. Thereafter, by adding the polymer CMC, the features of the present invention can be realized more clearly.

<Granting process>
Next, in the application step in the production method disclosed herein, an appropriate amount of the prepared negative electrode mixture slurry is applied to one or both surfaces of the negative electrode current collector, and dried to form a negative electrode mixture layer (negative electrode active layer). A material layer).
The method for applying the negative electrode slurry onto the negative electrode current collector is not particularly limited, and a conventionally known method can be appropriately employed. As a preferred method, a coating method using a slit die is exemplified. When the negative electrode slurry is applied to the negative electrode current collector, the coating amount on one side of the current collector can be 4 mg / cm ² or less. According to the technique disclosed here, the NV of the negative electrode slurry is set to be relatively high (40% by mass or more, for example, approximately 40% by mass to 60% by mass) and has a low basis weight of 4 mg / cm ² or less. However, it is possible to form a thin negative electrode mixture layer that is less likely to cause streaking or formation of lumps during coating, and that is thin and has little density variation.

  Here, examples of the material for the negative electrode current collector include copper, nickel, titanium, and stainless steel. The form is not particularly limited, and a rod-like body, a plate-like body, a foil-like body, a net-like body, or the like can be used. In a battery including a wound electrode body to be described later, a foil-like body made of copper or an alloy (copper alloy) containing copper as a main component is used. The thickness of the foil-shaped current collector is not particularly limited, but about 5 μm to 200 μm (more preferably 8 μm to 50 μm) can be preferably used in consideration of the capacity density of the battery and the strength of the current collector.

Drying of the applied negative electrode mixture slurry can be performed under heating as necessary. In a preferred embodiment, the drying temperature is about 200 ° C. or lower (typically 100 ° C. or higher and lower than 200 ° C.), and the drying time is about 1 minute to 10 minutes.
By the way, when the negative electrode mixture slurry is dried, the binder (typically CMC) contained in the slurry moves as the solvent evaporates, and may be unevenly distributed (migrated) on the surface of the negative electrode mixture layer. . As a result, the binder in the vicinity of the current collector is reduced, and the adhesion (adhesive strength) between the negative electrode mixture layer and the negative electrode current collector is reduced. Due to such a decrease in adhesion, the negative electrode composite material layer may be cracked or peeled off in the subsequent production process. This tendency becomes more prominent when the drying speed is increased to improve productivity. However, according to the manufacturing method disclosed herein, unlike the conventional case, a very high molecular weight CMC is used as the binder, so even if convection occurs during drying of the slurry, CMC is difficult to float on the surface layer. As a result, the amount of CMC in the vicinity of the current collector is secured, and the adhesion between the composite layer and the current collector can be improved. That is, according to the manufacturing method disclosed herein, since the segregation of the binder due to the migration phenomenon can be eliminated or alleviated, an electrode having a negative electrode mixture layer having high adhesion to the current collector can be manufactured. it can.

After drying the negative electrode mixture slurry, the thickness and density of the negative electrode mixture layer are appropriately subjected to press treatment (for example, various conventionally known press methods such as a roll press method and a flat plate press method can be employed). Can be prepared. The negative electrode mixture layer density after drying and rolling can be, for example, 2.0 g / cm ³ or less (typically 1.0 g / cm ³ or more and 1.5 g / cm ³ or less).

  As the positive electrode of the lithium secondary battery disclosed here, a granular positive electrode active material is dispersed in a suitable dispersion medium together with a conductive material, a binder, and the like, and kneaded to include a slurry (including paste and ink). The composition (positive electrode mixture slurry) is prepared. A method in which an appropriate amount of this positive electrode mixture slurry is applied to one or both surfaces of the positive electrode current collector to form a positive electrode mixture layer and then dried can be preferably employed. This drying can be performed under heating as necessary.

As the positive electrode active material used here, one or more of materials conventionally used in lithium secondary batteries can be used without any particular limitation. For example, an oxide containing lithium and a transition metal element as constituent metal elements such as lithium nickel oxide (eg LiNiO ₂ ), lithium cobalt oxide (eg LiCoO ₂ ), lithium manganese oxide (eg LiMn ₂ O ₄ ) Examples thereof include lithium transition metal oxides), phosphates including lithium and transition metal elements such as lithium manganese phosphate (LiMnPO ₄ ) and lithium iron phosphate (LiFePO ₄ ) as constituent metal elements. Among them, a positive electrode active material (typically substantially lithium nickel cobalt, which is mainly composed of lithium nickel cobalt manganese composite oxide (also referred to as NCM, for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ). A positive electrode active material made of a manganese composite oxide can be preferably used because of its excellent thermal stability and high energy density.

  Here, the lithium nickel cobalt manganese composite oxide is an oxide having Li, Ni, Co, and Mn as constituent metal elements, and at least one other metal element (Li, Ni, Co, and Mn) in addition to Li, Ni, Co, and Mn. The meaning also includes oxides containing transition metal elements and / or typical metal elements other than Ni, Co, and Mn. Such metal elements are, for example, one or more elements of Al, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce. It can be. The same applies to lithium nickel oxide, lithium cobalt oxide, and lithium manganese oxide. As such a lithium transition metal oxide (typically in particulate form), for example, a lithium transition metal oxide powder prepared by a conventionally known method can be used as it is. For example, lithium transition metal oxide powder substantially composed of secondary particles having an average particle diameter in the range of about 1 μm to 25 μm can be preferably used as the positive electrode active material.

  As the conductive material used here, one kind or two or more kinds of substances conventionally used in lithium secondary batteries can be used without particular limitation. For example, carbon materials such as carbon powder and carbon fiber are preferably used. Alternatively, conductive metal powder such as nickel powder may be used. As the carbon powder, various carbon blacks (for example, acetylene black (AB), furnace black, ketjen black (KB)), graphite powder, and the like can be used. Among these, a preferable carbon powder is acetylene black (AB).

  For the positive electrode mixture layer of the lithium secondary battery disclosed herein, an appropriate material can be selected from the polymer materials exemplified as the binder for the negative electrode mixture layer. Examples thereof include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), and the like.

  As a solvent used here, 1 type, or 2 or more types can be used without specifically limiting among the solvents used at the time of preparation of a lithium secondary battery conventionally. Such a solvent is roughly classified into an aqueous solvent and an organic solvent, and examples of the organic solvent include amide, alcohol, ketone, ester, amine, ether, nitrile, cyclic ether, and aromatic hydrocarbon. More specifically, N-methyl-2-pyrrolidone (NMP), N, N-dimethylformamide (DMF), N, N-dimethylacetamide, 2-propanol, ethanol, methanol, acetone, methyl ethyl ketone, methyl propenoate, Cyclohexanone, methyl acetate, ethyl acetate, methyl acrylate, diethyl triamine, N, N-dimethylaminopropylamine, acetonitrile, ethylene oxide, tetrahydrofuran (THF), dioxane, benzene, toluene, ethylbenzene, xylene dimethyl sulfoxide (DMSO), dichloromethane, Examples include trichloromethane and dichloroethane. In particular, N-methyl-2-pyrrolidone (NMP) is preferably used.

Although not particularly limited, NV of the slurry for forming the positive electrode mixture layer is approximately 40% by mass to 70% by mass (preferably 45% by mass to 65% by mass, more preferably 50% by mass to 60% by mass). can do. Although not particularly limited, the ratio of the positive electrode active material to the entire positive electrode mixture layer is typically about 50% by mass or more (typically 70% to 99% by mass), and is about It is preferable that it is 80 mass%-99 mass%. Moreover, the ratio of the binder to the whole positive electrode composite material layer can be, for example, about 0.1% by mass to 10% by mass, and preferably about 1% by mass to 5% by mass. The proportion of the conductive material in the entire positive electrode mixture layer can be, for example, about 5% by mass to 15% by mass, and preferably about 2% by mass to 8% by mass.
In addition, various polymer materials that can function as thickeners (for example, carboxymethylcellulose (CMC)), dispersants, and other various additives (for example, inorganic compounds that generate gas during overcharge (for example, phosphates) And carbonates)) and the like may be appropriately contained.

  Here, as a material of the positive electrode current collector, a member mainly composed of a metal having good conductivity such as aluminum, nickel, titanium, stainless steel, or the like can be used. The shape of the current collector is not particularly limited because it may vary depending on the shape of the battery constructed using the obtained electrode, and a rod-like body, a plate-like body, a foil-like body, a net-like body, or the like may be used. it can. In a battery provided with a wound electrode body to be described later, a foil-like body made of an alloy (aluminum alloy) mainly containing aluminum is used. The thickness of the foil-shaped current collector is not particularly limited, but about 5 μm to 200 μm (more preferably 10 μm to 30 μm) can be preferably used in consideration of the capacity density of the battery and the strength of the current collector.

After drying of the positive electrode mixture slurry, the thickness and density of the positive electrode mixture layer are appropriately subjected to press treatment (for example, various conventionally known press methods such as a roll press method and a flat plate press method can be employed). Can be prepared. The density of the positive electrode mixture layer after drying and rolling can be set to, for example, 2.0 g / cm ^{3 to} 4.2 g / cm ³ (typically 2.5 g / cm ^{3 to} 3.2 g / cm ³ ).

An electrode body in which the positive electrode and the negative electrode are laminated is prepared and accommodated in an appropriate battery case together with the electrolytic solution to construct a lithium secondary battery. In the typical configuration of the lithium secondary battery disclosed herein, a separator is interposed between the positive electrode and the negative electrode.
As a battery case, the material and shape which are used for the conventional lithium secondary battery can be used. Examples of the material include relatively light metal materials such as aluminum and steel, and resin materials such as PPS and polyimide resin. The shape (outer shape of the container) is not particularly limited, and may be, for example, a cylindrical shape, a rectangular shape, a rectangular parallelepiped shape, a coin shape, a bag shape, or the like. The case may be provided with a safety mechanism such as a current interruption mechanism (a mechanism capable of interrupting current in response to an increase in internal pressure when the battery is overcharged).

As the electrolytic solution used here, one kind or two or more kinds similar to the nonaqueous electrolytic solution used in the conventional lithium secondary battery can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which an electrolyte (lithium salt) is contained in a suitable nonaqueous solvent.
As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be used. For example, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, Examples include 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, and γ-butyrolactone. Of these, nonaqueous solvents mainly composed of carbonates are preferably used. For example, the nonaqueous solvent contains one or more carbonates, and the total volume of these carbonates is 60% by volume or more (more preferably 75% by volume or more, and further preferably 90% by volume) of the total volume of the nonaqueous solvent. The non-aqueous electrolyte solution occupying the above and substantially 100% by volume) is preferably used. Further, it may be a solid (gel) electrolytic solution in which a polymer is added to the liquid electrolytic solution.
Examples of the electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiClO _{4 and the} like are exemplified. Of these, LiPF ₆ is preferably used. The concentration of the electrolyte is not particularly limited, but if the concentration of the electrolyte is too low, the amount of lithium ions contained in the electrolytic solution tends to be insufficient, and the ionic conductivity tends to decrease. On the other hand, if the concentration of the supporting electrolyte is too high, the viscosity of the non-aqueous electrolyte solution becomes too high, and the ionic conductivity tends to decrease. For this reason, a nonaqueous electrolytic solution containing an electrolyte at a concentration of about 0.1 mol / L to 5 mol / L (preferably about 0.8 mol / L to 1.5 mol / L) is preferably used.

  As the separator used here, various porous sheets similar to those conventionally used for lithium secondary batteries can be used. For example, a porous resin sheet (film, nonwoven fabric, etc.) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide or the like can be mentioned. Such a porous resin sheet may have a single layer structure, or may have a two or more layers structure (for example, a three-layer structure in which a PP layer is laminated on both sides of a PE layer). Although it is not particularly limited, preferred porous sheets (typically porous resin sheets) used as the separator base material have an average pore diameter of about 0.001 μm to 30 μm and a thickness of 5 μm to 100 μm. Examples of the porous resin sheet are (more preferably 10 μm to 30 μm). The porosity (porosity) of the porous sheet may be, for example, about 20% to 90% by volume (preferably 30% to 80% by volume). Note that in a lithium secondary battery (lithium polymer battery) using a solid electrolyte, the electrolyte may also serve as a separator.

  Although not intended to be particularly limited, as a schematic configuration of the lithium secondary battery according to one embodiment of the present invention, a flatly wound electrode body (winding electrode body), a non-aqueous electrolyte solution, As an example, a lithium secondary battery (unit cell) in a form in which the battery is housed in a flat box-shaped (cuboid) container is shown in FIGS. In the following drawings, members / parts having the same action are denoted by the same reference numerals, and redundant description may be omitted or simplified. The dimensional relationship (length, width, thickness, etc.) in each figure does not reflect the actual dimensional relationship.

  FIG. 3 shows a lithium secondary battery (unit cell) 100. The lithium secondary battery 100 includes a wound electrode body 80 and a battery case 50. FIG. 4 is a view showing a wound electrode body 80.

As schematically shown in FIG. 3, a lithium secondary battery 100 according to an embodiment of the present invention includes a long positive electrode sheet 10 and a long negative electrode sheet 20 having long separators 40A and 40B. An electrode body (winding electrode body) 80 wound in a flat shape is accommodated in a flat box-shaped (cuboid shape) battery case 50 together with a non-aqueous electrolyte (not shown).
The battery case 50 includes a flat cuboid case main body 52 having an open upper end, and a lid 54 that closes the opening. On the upper surface (that is, the lid 54) of the battery case 50, there are a positive electrode terminal 70 that is electrically connected to the positive electrode 10 of the wound electrode body 80 and a negative electrode terminal 72 that is electrically connected to the negative electrode 20 of the electrode body 80. Is provided.

FIG. 4 is a diagram schematically showing a long sheet structure (electrode sheet) in a stage before assembling the wound electrode body 80. Of the positive electrode sheet 10 in which the positive electrode mixture layer 14 is formed along the longitudinal direction on one or both surfaces (typically both surfaces) of the long positive electrode current collector 12, and the long negative electrode current collector 22. The negative electrode sheet 20 on which the negative electrode mixture layer 24 is formed along the longitudinal direction on one side or both sides (typically both sides) is superposed together with the two long separators 40A and 40B in the longitudinal direction. Turn to produce a wound electrode body. A flat wound electrode body 80 is obtained by squashing and winding the wound electrode body from the side surface direction.
The positive electrode sheet 10 is formed such that the positive electrode mixture layer 14 is not provided (or removed) at one end along the longitudinal direction, and the positive electrode current collector 12 is exposed. Similarly, the wound negative electrode sheet 20 is not provided with (or removed from) the negative electrode mixture layer 24 at one end along the longitudinal direction, so that the negative electrode current collector 22 is exposed. Is formed. A positive electrode current collector plate is attached to the exposed end portion 74 of the positive electrode current collector 12, and a negative electrode current collector plate is attached to the exposed end portion 76 of the negative electrode current collector 22, respectively. 72 are electrically connected to each other.

  The lithium secondary battery disclosed herein can be used for various applications, but has a high output density and a high energy density, and has a good cycle characteristic. Therefore, for example, as shown in FIG. 5, the lithium secondary battery 100 disclosed herein can be suitably used as a power source (drive power source) for a motor mounted on a vehicle 1 such as an automobile. Although the kind of vehicle 1 is not specifically limited, Typically, a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), an electric vehicle (EV), and a fuel cell vehicle (FCV) are mentioned. Further, the lithium secondary battery 100 may be used alone (that is, a single battery), or may be used in the form of an assembled battery that is connected in series and / or in parallel.

Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to the specific examples. In the following description, “%” is based on mass unless otherwise specified.
In all the following examples, CMC (sodium salt) was used in powder form. The weight average molecular weight (Mw) based on GPC-RI of CMC used in this example is shown in Table 1 below.

[Construction of lithium secondary battery]
<Example 1>
CMC-A (Mw = 900,000, low molecular region CMC) and CMC-D (Mw = 3.3 million, high molecular region CMC) were dry-mixed at a ratio of 50%: 50%, and the mixed CMC was finally The amount of water (in this case, ion-exchanged water) is less than the target NV, and dispersed in a stirrer (model “TK Hibismix”, manufactured by Primix Co., Ltd.). A dispersion slurry was prepared. Next, the CMC dispersion slurry, natural graphite (powder) as the negative electrode active material, and styrene butadiene rubber (SBR) as the binder have a mass ratio (solid content ratio) of these materials of 1: 98: 1. The mixture was mixed, and water (ion exchange water) was further added to adjust the viscosity so that NV was 50%. Then, the slurry whose viscosity was adjusted was reduced in pressure to remove (defoam) bubbles in the slurry, thereby obtaining a target aqueous negative electrode mixture slurry.

[Viscosity measurement]
The viscosity immediately after completion of kneading of the obtained negative electrode mixture slurry (sometimes referred to as initial viscosity) was measured under the following conditions. The results are shown in the corresponding places in Table 2 below.
Measuring device: Model “E-type viscometer” (manufactured by Toki Sangyo Co., Ltd.)
Cone plate; 3 ° × R14
Measurement temperature: 20 ° C
Rotation speed: 20rpm

Next, this negative electrode mixture slurry is applied to the both sides of a long copper foil (negative electrode current collector) having a thickness of about 10 μm on one side using a slit die (coating speed 30 m / sec) (in terms of solid content) Was applied so that the dry weight of the negative electrode composite material layer was 4.0 mg / cm ² to form a negative electrode composite material layer. The obtained negative electrode was dried and then pressed to obtain a negative electrode sheet (Example 1) having a composite layer density of 1.2 g / cm ³ .

[Peel strength test]
The adhesion of the negative electrode mixture layer to the negative electrode current collector was evaluated by a 90 ° peel test defined in JIS-C6481-1995. Specifically, the surface of the negative electrode mixture layer side is fixed to the table with double-sided tape, and the negative electrode current collector is perpendicular to the surface of the negative electrode mixture layer (90 °) using an autograph manufactured by Shimadzu Corporation. The film was pulled in the direction of 50 mm / min and continuously peeled off by about 50 mm. And the minimum value of the load in the meantime was measured as peel strength [N / m], and the adhesion between the negative electrode current collector and the negative electrode mixture layer was evaluated. The results are shown in the corresponding places in Table 2 below.

Then, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM) powder as the positive electrode active material powder, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder A slurry-like composition for forming a positive electrode mixture layer by mixing these materials so that the mass ratio of these materials is 93: 4: 3 and diluting with N-methylpyrrolidone (NMP) so that NV is approximately 55%. (Positive electrode mixture slurry) was prepared. As PVdF, trade name “KF polymer # 7305” manufactured by Kureha Co., Ltd. was used. This positive electrode mixture slurry has a basis weight (applied amount in terms of solid content, that is, dry mass of the positive electrode mixture layer) on one side of both sides of a long aluminum foil (positive electrode current collector) having a thickness of about 15 μm. grant so that the 0 mg / cm ^2, drying the resulting positive electrode to form a positive-electrode mixture layer. Next, this positive electrode sheet was pressed so that the density of the positive electrode mixture layer was 2.5 g / cm ³ to produce a positive electrode sheet.

Then, the positive electrode sheet and the negative electrode sheet prepared above were overlapped and wound via two separators (here, a porous polyethylene sheet (PE) was used) to prepare an electrode body. Such an electrode body is mixed with a non-aqueous electrolyte (here, ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7) and LiPF ₆ as an electrolyte at a concentration of about 1 mol / L. A dissolved electrolyte solution was used.) And was housed in a cylindrical battery case. A lid was attached to the opening of the battery case, welded and joined to form a 18650 type (diameter 18 mm, height 65 mm) lithium secondary battery.

[Resistance measurement]
In order to evaluate the battery characteristics of the manufactured lithium secondary battery, the battery resistance was measured at 1 kHz and 10 mA using a battery tester manufactured by Hioki Electric at a temperature of 25 ° C. The results are shown in the corresponding places in Table 2 below.

<Example 2>
CMC-B (Mw = 1,500, low molecular region CMC) and CMC-D (Mw = 3.3 million, high molecular region CMC) were dry-mixed at a ratio of 50%: 50% during the preparation of the negative electrode mixture slurry. A negative electrode sheet (Example 2) was produced in the same manner as in Example 1 except that those were used. Using this negative electrode sheet, a 18650-type lithium secondary battery was constructed in the same manner as in Example 1.

<Example 3>
CMC-C (Mw = 2.4 million, low molecular region CMC) and CMC-D (Mw = 3.3 million, high molecular region CMC) were dry-mixed at a ratio of 50%: 50% during the preparation of the negative electrode mixture slurry. A negative electrode sheet (Example 3) was produced in the same manner as in Example 1 except that the material used was used. Using this negative electrode sheet, a 18650-type lithium secondary battery was constructed in the same manner as in Example 1.

  From Table 2, although the negative electrode mixture slurry viscosity and adhesive force hardly changed in Examples 1 to 3, Example 2 and Example 3 in which the Mw value of CMC used as the low molecular region CMC exceeded 1 million. Then, battery resistance increased and battery performance decreased. This result supports the technical significance of setting the molecular weight of the low molecular region CMC to 1 million or less.

<Examples 4 to 9>
Next, in order to examine the blending ratio suitable for the invention, CMC-A (Mw = 900,000, low molecular region CMC) and CMC-D (Mw = 3.3 million, high molecular region CMC) were used. The blending ratio was changed as shown in Table 3 below, and the characteristics were evaluated in the same manner as in Example 1.
In Examples 4 to 9, the negative electrode sheet was prepared in the same manner as in Example 1 except that CMC-A and CMC-D were dry-mixed at the ratio shown in Table 3 below when the negative electrode mixture slurry was prepared. (Examples 4 to 9) were produced. Using this negative electrode sheet, a 18650-type lithium secondary battery was constructed in the same manner as in Example 1. The evaluation results are shown in the corresponding part of Table 3 below and in FIG.

  From Table 3, as the blending ratio of the polymer region CMC increases, the viscosity of the negative electrode mixture slurry increases, and in Example 4 in which the blending ratio of the polymer region CMC is 80%, the viscosity greatly increases. Along with this, the negative electrode mixture slurry dispersibility decreased, so that the battery resistance increased and the battery characteristics deteriorated. Moreover, as shown in Table 3 and FIG. 6, as the blending ratio of the low molecular region CMC increased, the adhesion decreased, and in Example 9 in which the blending ratio of the low molecular region CMC was 80%, the blending ratio rapidly decreased. Along with this, battery resistance increased and battery characteristics deteriorated. In addition, between Example 1 and Example 7 in which the blending ratio of the low molecular region CMC was 50% to 67%, the best results were obtained in both adhesion and battery resistance. As a result, the weight ratio of low molecular region CMC having a weight average molecular weight (Mw) of less than 1 million as carboxymethyl cellulose (CMC) to high molecular region CMC having a weight average molecular weight (Mw) of 3 million or more is 25. The technical significance of using CMC mixed in a ratio of 75:75 to 75:25 (more preferably 45:55 to 65:35) is supported.

<Example 10>
And in order to examine the manufacturing method which can exhibit the effect of this invention suitably, CMC-A (Mw = 900,000, low molecular region CMC) and CMC-D (Mw = 3.3 million, high molecular region CMC) are used. The characteristics were evaluated in the same manner as in Example 1 by changing the timing of the addition.
In Example 10, at the time of preparing the negative electrode mixture slurry, first, CMC-A (Mw = 900,000, low molecular region CMC) was added with a smaller amount of water (for example, ion-exchanged water) than the final target NV. And a high-viscosity CMC dispersion slurry was prepared. Next, natural graphite (powder) as a negative electrode active material and styrene butadiene rubber (SBR) as a binder were added to the CMC dispersion slurry, mixed and kneaded. Further, CMC-D (Mw = 3.3 million, polymer region CMC) of the same amount as CMC-A is added to the slurry and kneaded, and water (for example, ion exchange water) is added so that NV becomes 50%. The viscosity was adjusted. In addition, mass ratio (solid content rate) of these materials is the same as that of Example 1 (that is, CMC: negative electrode active material: SBR = 1: 98: 1 formed by mixing). Then, the slurry whose viscosity was adjusted was reduced in pressure to remove (defoam) bubbles in the slurry, thereby obtaining a target aqueous negative electrode mixture slurry. A negative electrode sheet (Example 10) was produced in the same manner as Example 1 using the negative electrode mixture slurry. Using this negative electrode sheet, a 18650-type lithium secondary battery was constructed in the same manner as in Example 1. The evaluation results are shown in the corresponding places in Table 4 below.

In Example 10 in which the low molecular region CMC was introduced first and the polymer region CMC was introduced later, the adhesion was clearly improved as compared with Example 1 in which the low molecular region CMC and the polymer region CMC were introduced all at once. . This is probably because the low-molecular region CMC introduced first was preferentially used to adhere the negative electrode active materials to each other. Since the amount of the binder adhering to the negative electrode active material is determined to some extent by the physical property value such as the oil absorption amount, for example, the amount of the polymer region CMC introduced later decreases due to electrostatic repulsion or the like, and the negative electrode active material It is thought that it was used to bind the active material and the current collector. Such a result supports the technical significance of introducing the low molecular region CMC first in the production method shown here.
As described above, according to the manufacturing method disclosed herein, by optimizing the weight average molecular weight (Mw) of CMC used as the binder and the timing of its addition, the inside of the negative electrode mixture layer (that is, between the negative electrode active materials) In addition, it was shown that a lithium secondary battery having excellent battery characteristics can be provided while ensuring adhesion between the negative electrode active material and the negative electrode current collector.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

1 Automobile (vehicle)
10 Positive electrode sheet (positive electrode)
12 Positive electrode current collector 14 Positive electrode mixture layer 20 Negative electrode sheet (negative electrode)
22 Negative electrode current collector 24 Negative electrode mixture layer 26 Negative electrode active material 28 Binder (polymer region CMC)
29 Binder (Low molecular region CMC)
40A, 40B Separator sheet 50 Battery case 52 Case body 54 Lid 70 Positive electrode terminal 72 Negative electrode terminal 80 Winding electrode body 90 Flat plate 100 Lithium secondary battery 110 Cooling plate 120 End plate 130 Restraint band 140 Connection member 150 Spacer member 155 Screw 200 Assembled battery

Claims (6)

A method for producing a lithium secondary battery comprising a negative electrode having a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector,
A step of preparing a slurry-like composition by dispersing a negative electrode active material and a binder containing at least carboxymethylcellulose (CMC) in a solvent;
Applying the prepared slurry-like composition onto the negative electrode current collector to form the negative electrode mixture layer; and constructing a lithium secondary battery using the negative electrode on which the negative electrode mixture layer is formed The step of:
Including
Here, as the carboxymethyl cellulose (CMC), a low molecular region CMC having a weight average molecular weight of 500,000 or more and less than 1,000,000 and a high molecular region CMC having a weight average molecular weight of 3 million or more are classified into the low molecular region CMC. A method for producing a lithium secondary battery, wherein the weight ratio (A: B) between (A) and the polymer region CMC (B) is in a ratio of 25:75 to 75:25 .   The method for manufacturing a lithium secondary battery according to claim 1, wherein a synthetic rubber material is used as the binder in the slurry composition in addition to the mixed CMC.   3. The method according to claim 1, wherein, in the step of preparing the slurry-like composition, the low molecular region CMC is first added to the solvent containing the negative electrode active material, and then the polymer region CMC is added. A method for producing a lithium secondary battery.   In the step of preparing the slurry composition, the mixing ratio of the low molecular region CMC and the polymer region CMC is such that the viscosity of the slurry composition is in the range of 300 mPa · s to 2000 mPa · s. The manufacturing method of the lithium secondary battery as described in any one of Claim 1 to 3 to determine. A lithium secondary battery comprising a negative electrode having a negative electrode current collector, and a negative electrode mixture layer formed on the negative electrode current collector,
The negative electrode mixture layer includes a negative electrode active material and carboxymethyl cellulose (CMC),
The CMC is a mixture of a low molecular region CMC having a weight average molecular weight of 500,000 to less than 1 million and a high molecular region CMC having a weight average molecular weight of 3 million or more in a weight ratio of 25:75 to 75:25. It is composed of a lithium secondary battery. A vehicle comprising the lithium secondary battery according to claim 5 .

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