JP2015146249A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery superior in cycle characteristics by preventing the delamination of negative electrode mixture layers.SOLUTION: A nonaqueous electrolyte secondary battery comprises: a positive electrode; a negative electrode; and a nonaqueous electrolyte. The negative electrode includes a collector, a first active material layer including a first carbon particle group, an intermediate layer including fibrillated cellulose, and a second active material layer including a second carbon particle group in this order. The first carbon particle group is 12-50 μm in D50, and the second carbon particle group is 1-18 μm in D50. The intermediate layer is 1.0-5.0 μm in thickness, and the fibrillated cellulose is 0.3-1 μm in d50.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

Various attempts have been made to improve the lithium ion (Li ⁺ ) acceptability of the negative electrode in the nonaqueous electrolyte secondary battery. For example, in International Publication No. 2011/114433 (Patent Document 1), the negative electrode active material layer provided on the negative electrode current collector has a two-layer structure, and the negative electrode active material contained in the surface side active material layer A lithium secondary battery in which the average specific surface area is larger than the average specific surface area of the negative electrode active material contained in the current collector-side active material layer is disclosed.

International Publication No. 2011/114433

Because of the high negative active material Li ⁺ acceptance large specific surface area, by the Li ⁺ acceptance (opposite side of the anode current collector) surface layer of the anode active material layer are arranged a high negative electrode active material, a non-aqueous The Li ⁺ acceptability of the negative electrode of the electrolyte secondary battery can be improved. However, if the entire negative electrode active material layer is composed of a negative electrode active material having a large specific surface area, the reactivity of the negative electrode active material layer becomes too high during high temperature storage, causing problems such as gas generation. Decreases. For this reason, by combining the active material layer having a small specific surface area and the active material layer having a large specific surface area, it is possible to maintain the high temperature storage characteristics while improving the Li ⁺ acceptability.

However, when a negative electrode active material layer having a two-layer structure as disclosed in Patent Document 1 is used, Li ⁺ acceptability and high-temperature storage characteristics are excellent, but two types of negative electrode active materials having different specific surface areas are Li ⁺ The amount of expansion / contraction associated with insertion / removal is different. Due to this, if the charge / discharge cycle is repeated, peeling may occur at the interface between the two types of negative electrode active material layers. When such delamination occurs, a part of the negative electrode active material layer on the surface layer side is electrically isolated, and the negative electrode active material cannot contribute to charging / discharging at that part, so a significant capacity loss of the lithium secondary battery Is caused.

  The present invention has been made in view of the problems as described above, and an object thereof is to provide a non-aqueous electrolyte secondary battery excellent in cycle characteristics by preventing delamination of the negative electrode mixture layer. There is to do.

The nonaqueous electrolyte secondary battery of the present invention comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte,
The negative electrode includes a current collector, a first active material layer including a first carbon particle group, an intermediate layer including fibrillated cellulose, and a second active material layer including a second carbon particle group in this order. ,
D50 of the first carbon particle group is 12 to 50 μm,
D50 of the second carbon particle group is 1 to 18 μm,
The intermediate layer has a thickness of 1.0 to 5.0 μm,
The fibrillated cellulose has a d50 of 0.3 to 1.

  As described above, in the non-aqueous electrolyte secondary battery of the present invention, in the negative electrode, an intermediate layer containing fibrillated cellulose between the first active material layer (current collector side) and the second active material layer (surface layer side). Is included. For this reason, even when the charge / discharge cycle is repeated due to the anchor effect (immobilization effect due to the incorporation of fine fibers of fibrillated cellulose in the gap between the first active material layer and the second active material layer), the first active material layer And the second active material layer are prevented from peeling. Thereby, a nonaqueous electrolyte secondary battery having excellent cycle characteristics can be obtained.

  The non-aqueous electrolyte secondary battery of the present invention includes an intermediate layer containing fibrillated cellulose between the first active material layer (current collector side) and the second active material layer (surface layer side). Separation between the first active material layer and the second active material layer is suppressed, and a nonaqueous electrolyte secondary battery excellent in cycle characteristics can be provided.

In the nonaqueous electrolyte secondary battery of the present invention, the specific surface area of the second carbon particle group included in the second active material layer (surface layer side) is the first active material layer (current collector side) included in the first active material layer. By being larger than the specific surface area of the carbon particle group, the Li ⁺ acceptability of the negative electrode is improved and high-rate charging is possible.

It is a schematic sectional drawing which shows the structure of the negative electrode in one Embodiment of this invention. It is a schematic diagram for demonstrating the structure of the fibrillated cellulose in one Embodiment of this invention. It is a schematic sectional drawing for demonstrating preparation of the negative electrode of an Example and a comparative example. It is a schematic sectional drawing for demonstrating the assembly of the secondary battery of an Example and a comparative example. It is a schematic sectional drawing which shows an example of the conventional negative electrode.

  Hereinafter, although the embodiment concerning the present invention is described in detail, the present invention is not limited to these. FIG. 1 is a schematic cross-sectional view showing the configuration of the negative electrode of the present embodiment.

<Nonaqueous electrolyte secondary battery>
The nonaqueous electrolyte secondary battery of this embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. Referring to FIG. 1, this negative electrode includes a current collector 10, a first active material layer 21 including a first carbon particle group, an intermediate layer including fibrillated cellulose, and a second carbon layer including a second carbon particle group. The active material layer 22 is included in this order.

  FIG. 5 is a schematic cross-sectional view showing the configuration of the negative electrode in the prior art. Referring to FIG. 5, in the conventional negative electrode, when the charge / discharge cycle is repeated, in the interface region 25 due to the difference in expansion / contraction amount between the first active material layer 21 and the second active material layer 22. In some cases, delamination occurred.

  On the other hand, in this embodiment, as shown in FIG. 1, an intermediate layer 23 containing fibrillated cellulose is included between the first active material layer 21 and the second active material layer 22. Thereby, the anchor effect is expressed between the first active material layer 21 and the second active material layer 22 and the intermediate layer 23, and the peel strength is improved.

<Negative electrode>
The negative electrode of the present embodiment includes a current collector 10 and a negative electrode mixture layer 2 formed on the current collector 10. The negative electrode mixture layer 2 has a laminated structure including a first active material layer 21 (current collector side), an intermediate layer, and a second active material layer 22 (surface layer side).

The first active material layer 21 includes a first carbon particle group, and the second active material layer 22 includes a first carbon particle group. The particle size of the second carbon particle group constituting the second active material layer 22 is smaller than the particle size of the first carbon particle group constituting the first active material layer 21. In this case, the specific surface area of the second carbon particle group constituting the second active material layer 22 is generally larger than the specific surface area of the first carbon particle group constituting the first active material layer 21. Thereby, the Li ⁺ acceptability of the negative electrode is improved, and high rate charging is possible. The “specific surface area” means an average specific surface area determined based on the adsorption isotherm of BET.

  The negative electrode of this embodiment may contain other layers as long as it has these configurations. For example, another layer may be provided on the second active material layer 22, or another layer may be provided between the first active material layer 21 and the current collector 10. The shape of the negative electrode of the present embodiment is not particularly limited, but is typically a sheet-like member.

In the negative electrode of the present embodiment, the mass ratio between the first active material layer 21 and the second active material layer 22 is not particularly limited, but preferably the first active material layer: second active material layer = 1: 9-9: 1. Also, the density of the negative electrode mixture layer (the mass of the negative electrode mixture layer / the volume of the negative electrode mixture layer) is not particularly limited, but is preferably 0.5 to 2.5 g from the viewpoint of Li ⁺ acceptability and cycle durability. / Cm ³ or so.

(First active material layer)
The first active material layer 21 is provided on the current collector 10. The first active material layer 21 includes a first carbon particle group. The 1st active material layer may contain other components, such as a binder mentioned below and a thickener, as needed.

D50 of the first carbon particle group is 12 to 50 μm. The specific surface area of the first active material layer 21 is not particularly limited, but is preferably less than 4.00 m ² / g from the viewpoint of storage characteristics.

  The specific surface area (average specific surface area based on the adsorption isotherm of BET) can be measured by a conventionally known physical adsorption type specific surface area measuring device. An example of such an apparatus is “ASAP2010” manufactured by Micromeritics.

(Second active material layer)
The 2nd active material layer 22 is provided on the 1st active material layer 21 through the below-mentioned intermediate | middle layer, and comprises the upper layer of 2 layer structure. The second active material layer 22 includes a second carbon particle group. The 2nd active material layer may contain other components, such as a binder mentioned below and a thickener, as needed.

The second active material layer (second carbon particle group) has a larger specific surface area than the first active material layer (first carbon particle group). Here, from the viewpoint of improving the high rate charging characteristics, the specific surface area of the second carbon particle group is preferably 4.00 m ² / g or more, more preferably 5.00 m ² / g or more, and still more preferably. 5.50 m ² / g or more.

(Carbon particle group)
The first carbon particle group and the second carbon particle group have a function as a negative electrode active material. That is, it is a carbon material that can occlude and release Li ⁺ electrochemically. As such a carbon material, for example, natural graphite, artificial graphite, coke, or a mixture thereof can be used. The shape of the carbon particles is not particularly limited, and may be spherical particles or scaly particles.

  The first carbon particle group and the second carbon particle group can be produced by appropriately classifying and mixing carbon materials available on the market.

  The specific surface area of the active material layer is generally inversely proportional to the particle size of the carbon particle group. Therefore, the second carbon particle group constituting the second active material layer having a large specific surface area has a particle size distribution having a smaller particle diameter than the first carbon particle group constituting the first active material layer. Specifically, D50 of the first carbon particle group is 12 to 50 μm, and D50 of the second carbon particle group is 1 to 18 μm. Here, “D50” means a particle size in which the cumulative number is 50% of the total number of particles when the particle size distribution of a certain particle group is expressed as a cumulative distribution.

  When D50 of the first carbon particle group constituting the first active material layer is smaller than this, the reaction area of the negative electrode itself is large, and a side reaction between the negative electrode and the electrolytic solution is likely to occur. The battery characteristics may be greatly deteriorated during the test. In addition, when the D50 of the second carbon particle group constituting the second active material layer is larger than this, the reaction area of the electrode is small, the electrode itself is poor in reactivity, the resistance of the negative electrode is increased, and the battery characteristics are improved. It may deteriorate. That is, as described above, appropriately controlling the particle size distribution of the first carbon particle group constituting the first active material layer and the particle size distribution of the second carbon particle group constituting the second active material layer. Thus, a nonaqueous electrolyte secondary battery having excellent cycle durability and battery characteristics can be obtained.

  The particle size distribution of the carbon particle group can be measured by a conventionally known particle size distribution measuring apparatus. For example, a laser diffraction / scattering particle size distribution measuring device “MT3000II” manufactured by Nikkiso Co., Ltd. can be used. The particle size distribution measured by the laser diffraction / scattering method is essentially a volume-based particle size distribution, but in the present embodiment, these measurement results are arithmetically converted and used based on the number. As a method for directly measuring the number-based particle size distribution, an electrical detection band method or the like can be used. As a particle size distribution measuring apparatus based on the electrical detection band method, for example, “Multisizer 3” manufactured by Beckman Coulter, Inc. can be used.

(Middle layer)
The intermediate layer is a layer placed between the first active material layer and the second active material layer, and contains fibrillated cellulose.

  The “fibrillated cellulose” is a particulate material made of cellulose and having a large number of fine fibers (fibrils). The fibrillated cellulose can be obtained by performing a treatment (fibrillation treatment) for finely dividing a polymer fiber made of cellulose. The fibrillation treatment can be performed using, for example, mechanical shearing force. The surface of the fibrillated cellulose thus fibrillated is loosened and a large number of fine fibers (fibrils) are generated.

  Referring to FIG. 2, fibrillated cellulose 24 has a large number of beard-like fine fibers (fibrils) 24a on its surface. In FIG. 2, only one fine fiber 24a is shown for simplification.

  In the present embodiment, the anchor effect is exhibited by the fine fibers of fibrillated cellulose, which is a constituent material of the intermediate layer, and the adhesion between the active material layers having different specific surface areas (as compared to cellulose not subjected to fibrillation treatment) is enhanced. be able to.

  The d50 of fine fibers (fibrils) on the surface of the fibrillated cellulose is preferably 0.3 to 1.0 μm. Here, “d50” is a fiber diameter in which the cumulative number is 50% of the total number of fibers when the distribution of the fiber diameter d (see FIG. 2) based on the number of particles (fibrillated cellulose) group is expressed as a cumulative distribution. d. The fiber diameter d can be grasped by, for example, image analysis using a scanning electron microscope (SEM). Specifically, for example, the fiber diameter d of 100 fine fibers can be measured by image analysis using SEM, and d50 can be obtained from the distribution of the fiber diameters d.

  When d50 is larger than this range, the fine fibers cannot enter the voids of the active material layer, and the anchor effect is not sufficiently exhibited. Therefore, the active material layer (first active material layer and second active material layer) and There exists a tendency for adhesiveness with an intermediate | middle layer to fall. On the other hand, if d50 is smaller than this range, the fine fibers are too thin with respect to the voids of the active material layer, and thus the bonding force between the active material layer (the first active material layer and the second active material layer) and the intermediate layer. Tend to run out.

  Although not particularly limited, the fiber diameter (thickness) of the polymer fiber backbone portion 24b (see FIG. 2) is suitably about 3 μm to 20 μm, for example, 3 μm to 10 μm (for example, More preferably, it is about 5 μm. In addition, the fiber length of the backbone portion 24b is suitably about 0.1 μm to 5 μm, and more preferably 0.3 μm to 2 μm, for example. For example, the fiber diameter (thickness) of the polymer fiber core portion 24b is about 3 μm to 10 μm, and the fiber length is about 1 μm to 5 μm.

  The intermediate layer has a thickness of 1.0 to 5.0 μm. When the thickness is less than 1.0 μm, the expansion / contraction difference between the first active material layer and the second active material layer cannot be absorbed, and there is a tendency that separation between both layers tends to occur. On the other hand, when the thickness is greater than 5.0 μm, the resistance of the negative electrode increases, and the battery characteristics tend to deteriorate.

The intermediate layer containing such fibrillated cellulose is excellent in Li ⁺ permeability and has high electrolyte retention, and thus hardly inhibits Li migration. For this reason, the effect by a multilayer negative electrode can be show | played, without causing the deterioration of the battery characteristic resulting from the raise of interface resistance.

  Examples of cellulose include carboxymethylcellulose (CMC), methylcellulose (MC), hydroxypropylmethylcellulose (HPMC), and hydroxyethylmethylcellulose (HEMC). CMC is preferable in terms of easy fibrillation treatment, solubility in water, and rheological properties after dissolution (difficult to string).

  In addition, in the range where the effect of the present invention is exhibited, fibrillated polymer fibers such as polyvinyl alcohol (PVA) and ethylene-vinyl alcohol copolymer (EVOH) can be used instead of cellulose.

  Examples of the method for fibrillating cellulose include a method of pulverizing cellulose aggregates (blocks) using a jet mill, a rotary mill, an attritor (for example, a mass collider), a refiner, a high-pressure homogenizer, or the like.

  By applying mechanical shearing force and compressive force to the polymer fiber using these, the polymer fiber is torn mainly in a direction parallel to the fiber axis, and is loosened to develop fine fibers (fibrils) 24a on the surface. The fine fiber (fibril) 24a exhibits an anchor effect and improves the peel strength of the negative electrode. The conditions for the pulverization vary depending on each machine. For example, when a jet mill is used, conditions of a pulverization pressure of 0.015 MPa to 3.5 MPa and a pulverization frequency of 1 to 5 times can be employed. Such pulverization may be carried out dry or wet.

(Current collector)
As the current collector 10, a metal foil having high conductivity and high chemical and electrochemical stability can be used. As such a metal foil, for example, a copper (Cu) foil or a copper alloy foil is preferable. When using copper foil, it is preferable that the thickness of foil is about 5-20 micrometers from a viewpoint of intensity | strength and electroconductivity.

(Production method of negative electrode)
The negative electrode of this embodiment can be produced as follows. That is, after the first paste containing the first carbon particle group is applied and dried on the current collector 10 to form the first active material layer 21, the first active material layer 21 containing the fibrillated cellulose is added to the first paste. 3 pastes are applied and dried to form the intermediate layer 23, and further, the second paste containing the second carbon particle group is applied to the intermediate layer 23 and dried to form the second active material layer 22. Thus, the negative electrode mixture layer 2 including the first active material layer 21, the intermediate layer 23, and the second active material layer 22 is obtained, and the negative electrode mixture layer 2 can be further compressed to a predetermined thickness. .

  In addition, as a coating method, a conventionally well-known method can be employ | adopted, for example, the gravure method and the die-coating method can be used. Moreover, after apply | coating a 1st paste, a 3rd paste, and a 2nd paste in order on a collector, you may dry and form the negative mix layer 2.

  The first paste and the second paste can be formed, for example, by kneading a carbon particle group, a binder, and a solvent (for example, water). As the binder, for example, styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyethylene oxide (PEO), acrylic resin, or the like can be used. Further, from the viewpoint of dispersion stability of the carbon particle group, a thickener may be used in combination. As the thickener, for example, carboxymethylcellulose (CMC) and its derivatives, polyvinyl alcohol (PVA), and the like can be used.

  In addition, although the content rate of the carbon particle group in the negative electrode composite material layer 2 is not specifically limited, For example, it is about 90-99 mass%, Preferably it is 95 mass% or more and 99 mass% or less.

  The third paste can be formed, for example, by kneading a water-soluble salt (such as Na salt) of fibrillated cellulose and a solvent (such as water). In addition, you may use the thickener etc. similar to the above together.

  In the non-aqueous electrolyte secondary battery of this embodiment, as long as the negative electrode has the above-described configuration, other configurations such as the positive electrode and the non-aqueous electrolyte are not particularly limited, and are appropriately changed according to the required specifications of the battery. It shall be possible. Hereinafter, an example of another configuration will be described.

<Positive electrode>
For the positive electrode, for example, a positive electrode mixture paste obtained by kneading a positive electrode active material, a conductive additive, a binder, and an organic solvent (for example, N-methyl-2-pyrrolidone (NMP)) After coating and drying to form a positive electrode mixture layer on the positive electrode current collector, it can be produced by compressing the positive electrode mixture layer to a predetermined thickness.

Examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiNi _a Co _b O ₂ (a + b = 1, 0 <a <1, 0 <b <1), LiMnO ₂ , LiMn ₂ O ₄ , LiNi _a Co _b Mn _{c O 2 (a + b +} c = 1,0 <a <1,0 <b <1,0 <c <1), it is possible to use a lithium-containing transition metal oxides such as LiFePO _4. The content rate of the positive electrode active material in the positive electrode mixture is, for example, about 90 to 99% by mass.

Moreover, as a conductive support material, carbon materials, such as acetylene black (AB), can be used, for example, As a binder, polyvinylidene fluoride (PVdF) etc. can be used, for example. As the positive electrode current collector, for example, an aluminum (Al) foil or an aluminum alloy foil can be used. The density of the positive electrode mixture layer (mass of the positive electrode mixture layer ÷ volume of the positive electrode mixture layer) is, for example, about 2.0 to 4.0 g / cm ³ .

<Nonaqueous electrolyte>
The nonaqueous electrolyte is typically a liquid electrolyte in which a solute (lithium salt) is dissolved in an aprotic solvent. Here, examples of the aprotic solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (GBL) and vinylene carbonate (VC), and dimethyl carbonate. Chain carbonates such as (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) can be used. These aprotic solvents can be used in an appropriate combination of two or more from the viewpoint of electrical conductivity and electrochemical stability. In particular, it is preferable to use a mixture of a cyclic carbonate and a chain carbonate, and the volume ratio of the cyclic carbonate to the chain carbonate is preferably about 1: 9 to 5: 5. If a specific example is given, 3 types, EC, EMC, and DEC, for example can be mixed and used. In addition, the nonaqueous electrolyte of this embodiment can also be made into a gel form and a solid form.

Examples of solute lithium salts include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), and bis (trifluoromethane). Sulfonyl) imidolithium (Li (CF ₃ SO ₂ ) ₂ N), lithium trifluoromethanesulfonate (Li (CF ₃ SO ₃ )), or the like can be used. Moreover, you may use 2 or more types together about these solutes. The concentration of the solute in the nonaqueous electrolyte is not particularly limited, but is preferably about 0.5 to 2.0 mol / L from the viewpoint of discharge characteristics and storage characteristics.

<Separator>
The separator is intended to transmit Li ⁺ and prevent electrical contact between the positive electrode and the negative electrode. As such a separator, a microporous film made of a polyolefin-based material is preferable from the viewpoint of mechanical strength and chemical stability. Here, as the polyolefin-based material, polyethylene (PE), polypropylene (PP), or the like can be used, and these can also be used in combination.

<Battery structure>
Although it does not restrict | limit especially as a battery structure, For example, the following structures can be illustrated. First, a current collector lead tab is welded to a sheet-like positive electrode and a negative electrode, and wound so that the positive electrode and the negative electrode face each other with a sheet-like separator interposed therebetween. Configure. Next, the electrode group is inserted into an exterior case having a predetermined shape, and each current collector lead tab and each electrode terminal are electrically connected to each other provided on the sealing body (lid). And after joining an exterior case and a sealing body by a predetermined means, liquid nonaqueous electrolyte is inject | poured from the injection hole provided in the sealing body, and a battery is comprised by sealing this injection hole. Can do.

  Here, examples of the shape of the exterior case include a cylindrical shape and a rectangular shape. The material of the outer case (and the sealing body) may be appropriately selected from various metals or alloy materials in consideration of withstand voltage and strength. For example, aluminum (Al) and its alloy, iron (Fe), stainless steel, etc. can be used.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example, this invention is not limited to these.

<Example 1>
(Production of fibrillated CMC)
Aggregates (lumps) of CMC (BSH-6, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) and PJM jet mill machine (manufactured by Nippon Pneumatic Industry Co., Ltd .; crushing pressure 0.9 MPa, once crushing) and super mass collider machine A fibrillation treatment CMC (fibrillation CMC) was produced by using a pulverization process (made by Masuko Sangyo Co., Ltd .; rotational speed 3000 rpm, clearance between grinding wheels 5 μm). At that time, the shear force and compressive force applied to the CMC are appropriately changed, pulverized, classified by an MDS-1 type airflow classifier (manufactured by Nippon Pneumatic Industry Co., Ltd.), and fibrillated CMC having a d50 of 0.5 μm. Got.

(Preparation of negative electrode)
Mass of natural graphite (first carbon particle group: D50 = 20 μm, BET specific surface area = 3 m ² / g), SBR (binder) and CMC (thickener: BSH-6, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) A slurry (first paste for forming the first active material layer of the negative electrode) in which each component was dispersed was prepared by mixing in a ratio of 100: 1: 1 and kneading in water (solvent). .

Similarly, natural graphite (second carbon particle group: D50 = 10 μm, BET specific surface area = 8 m ² / g), SBR (binder) and CMC (thickener: BSH-6, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) ) Are mixed so as to have a mass ratio of 100: 1: 1, and are kneaded in water (solvent), whereby a slurry in which each component is dispersed (second paste for forming the second active material layer of the negative electrode) ) Was prepared.

  Further, an aqueous solution containing 1 wt% of the fibrillated CMC Na salt having a d50 of 0.5 μm is prepared, and a slurry in which the fibrillated CMC is dispersed (a third paste for forming an intermediate layer of the negative electrode) is prepared. did.

Next, referring to FIG. 3, first, the first paste 21a is applied to both main surfaces of a copper foil (negative electrode current collector 10, thickness 14 μm) so that the coating weight on one side is 7 mg / cm ² (after drying). ) Was applied. Thereafter, before the first paste 21a solidified, the third paste 23a was applied on the first paste 21a so that the thickness after coating was 3 μm. Finally, before the third paste 23a was solidified, the third paste 23a was applied on the third paste 23a so that the thickness after coating was 3 μm. Such application was performed using a coating apparatus 6 having three dispensers corresponding to each paste so that the first paste 21a, the third paste 23a, and the second paste 22a can be sequentially applied (see FIG. 3). ). In FIG. 3, only the paste applied to one side of the negative electrode current collector 10 is shown for simplification.

  Then, each paste was dried with a 100 degreeC hot-air drying furnace, and the sheet-like negative electrode (Thickness: 150 micrometers, the width | variety of a negative mix layer: 100mm, length: 4700mm) was produced. The negative electrode has an uncoated portion where the negative electrode current collector is exposed on one side in the width direction.

  In addition, the number-based particle size distribution of the carbon particle group made of natural graphite particles is measured by a particle size distribution measuring device “MT3000II” manufactured by Nikkiso Co., Ltd. The specific surface area of each carbon particle group is measured by a specific surface area measuring device “ASAP2010” manufactured by Micromeritics.

(Preparation of positive electrode)
Lithium cobaltate (LiCoO ₂ ) as a positive electrode active material, AB (conductive aid), and an NMP solution of PVdF (binder) are mixed so as to have a mass ratio of 90: 4: 6 and kneaded. As a result, a paste for positive electrode mixture was obtained.

Next, the positive electrode mixture paste was applied to both surfaces of an Al foil (thickness 20 μm) as a positive electrode current collector so that the coating mass on one side was 30 mg / cm ² , dried, and the Al A positive electrode mixture layer was formed on the foil. Subsequently, the positive electrode mixture layer and the Al foil were rolled using a roll mill to obtain a sheet-like positive electrode (thickness: 170 μm, positive electrode mixture layer width: 94 mm, length: 4500 mm). This positive electrode has an uncoated portion where the positive electrode current collector is exposed on one side in the width direction.

(Preparation of non-aqueous electrolyte)
EC, DMC, and DEC were mixed at a volume ratio of EC: DMC: DEC = 3: 4: 3 to obtain an aprotic solvent. Next, by dissolving 1.0 M (1.0 mol / L) LiPF ₆ , 1 wt% CHB (cyclohexylbenzene), and 1 wt% BP (biphenyl) as solutes in the aprotic solvent. A non-aqueous electrolyte was prepared.

(Production of secondary battery)
Next, a secondary battery was manufactured using the material manufactured as described above.

Referring to FIG. 4, a sheet-like separator, the positive electrode, and the negative electrode are wound in an elliptical column shape so that the positive electrode and the negative electrode face each other with the separator interposed therebetween, and then 4 kN / cm ² by a flat plate at room temperature. The flat electrode group (tabless electrode group) 3 was constituted by pressurizing at a pressure of 2 minutes. As a separator, a heat resistance layer (HRL) layer made of alumina (Al ₂ O ₃ ), which is an insulating metal oxide, is formed on both surfaces of a base material composed of a three-layer structure of PP / PE / PP. A separator having a thickness of 25 μm and a width of 110 mm was used.

  Next, the electrode group 3 is inserted into a rectangular aluminum outer case 5, and the exposed ends of the negative electrode current collector 10 and the positive electrode current collector 40 are connected to the negative electrode current collector terminal plate 10a and the positive electrode current collector terminal. The negative electrode current collector terminal plate 10a and the positive electrode current collector terminal plate 40a are electrically connected to the plate 40a, and each of the negative electrode current collector terminal plate 10a and the positive electrode current collector terminal plate 40a is provided on a sealing body (lid) that is an upper part of the outer case 5 41 was electrically connected.

  Next, 125 g of the nonaqueous electrolyte was injected into the outer case 5 from the injection hole provided in the sealing body, and the injection hole was then sealed with a sealing cap. The sealing body includes a gas discharge valve as a CID (Current Interrupt Device).

  As described above, a square nonaqueous electrolyte secondary battery having a battery rated capacity of 24.0 Ah was produced.

<Examples 2-5 and Comparative Examples 2-5>
Example 2 was the same as Example 1 except that d50 of fibrillated CMC and the thickness after application of the third paste were as shown in Table 2 in “Preparation of negative electrode” in Example 1. To 5 and Comparative Examples 2 to 5 were fabricated.

<Comparative Example 1>
A secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 except that the third paste was not applied (the thickness of the third paste was 0 μm) in “Production of Negative Electrode” of Example 1. did.

<Comparative Example 6>
A secondary battery of Comparative Example 6 was produced in the same manner as in Example 1 except that D50 of natural graphite (carbon particle group) used for the first paste in “Production of negative electrode” in Example 1 was 10 μm. .

<Comparative Example 7>
A secondary battery of Comparative Example 7 was produced in the same manner as in Example 1 except that D50 of natural graphite (carbon particle group) used for the second paste in Example 1 “Production of negative electrode” was 20 μm. .

<Evaluation test>
For the secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 7, the following (1) measurement of battery resistance, (2) cycle durability test, (3) storage durability test and (4) storage durability test An overcharge test was performed. In the following description, “constant current” is described as “CC” (Constant Current), and “constant voltage” is described as “CV” (Constant Voltage). Further, “It” is used as a unit of the current value, and 1 It is assumed that the current value for discharging the rated capacity of the battery in one hour.

(1) Measurement of battery resistance Ten secondary batteries in each of Examples and Comparative Examples were adjusted to a charging rate of 60% (25 ° C.). With respect to the battery after adjustment, the IV resistance was measured by a discharge pulse with a period of 10 seconds at a current value of 10 It. The average value of the measured resistance values is shown in the “battery resistance” column of Table 1.

(2) Cycle durability test For each of the five secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 7, charging and discharging under the following conditions are defined as one cycle in a thermostat set to 50 ° C. The charge / discharge cycle was executed 1000 times, and the percentage (capacity maintenance rate) of the battery capacity after 1000 cycles to the initial capacity was calculated. The average value of the capacity maintenance ratio is shown in the “cycle capacity maintenance ratio” column of Table 1. The current value 2It during charging assumes high rate charging.

(Charge / discharge cycle conditions)
Charging conditions: CC charging (current value: 2 It), cut-off voltage: 4.1V.
Discharge conditions: CC discharge (current value: 2 It), cut-off voltage: 3.0V.

(Capacity measurement conditions)
The initial capacity and the capacity after 1000 cycles were measured under the following conditions. The ambient environment temperature at the time of measuring the capacity was 25 ° C. (room temperature).
(I) Charging conditions The battery was charged by CC-CV charging. That is, the battery was charged at a constant current of 1 It until the battery voltage reached 4.1 V (CC charging), and after reaching 4.1 V, charging was performed while maintaining the voltage of 4.1 V while the current was attenuated ( CV charging). The charging was terminated when the current decayed to 0.1 IT (cut-off current).
(Ii) Discharge condition After leaving the battery for 5 minutes after the end of charge, CC-CV discharge was performed. That is, discharging was performed until the battery voltage was attenuated to 3.0 V at a current value of 1 It (CC discharge), and after being attenuated to 3.0 V, discharging was performed while maintaining the voltage of 3.0 V while attenuating the current. (CV discharge). The discharge was terminated when the current was attenuated to 0.1 IT (cut-off current). The sum of the discharge capacity of CC discharge and the discharge capacity of CV discharge was defined as the battery capacity.

(3) Storage endurance test Each of the 50 secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 7 was adjusted to a charging rate of 100% (25 ° C). The battery after adjustment was stored at 60 ° C. for 100 days, and then the battery capacity was measured and the capacity retention rate was calculated in the same manner as in the cycle durability test. The average value of the capacity maintenance ratio is shown in the “Capacity maintenance ratio after storage” column of Table 1.

(4) Overcharge test after storage endurance test Among the secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 7 after the storage endurance test, an overcharge test was performed on 10 batteries each. The overcharge test conditions were a charging current 24A (corresponding to 1IT), a charging upper limit voltage 20V, and an environmental temperature 25 ° C. About the secondary battery after an overcharge test, it was confirmed by measuring battery voltage whether CID act | operated. The number of batteries in which the CID is activated among the 10 secondary batteries is shown in the “post-storage overcharge test (number of CID activation)” column of Table 1.

<Consideration of results>
As shown in Table 1, in Comparative Example 1 in which no intermediate layer exists at the interface between the first active material layer and the second active material layer, the cycle capacity retention rate and the capacity retention rate after storage are higher than those in the Examples. It is low, the number of CID operations in the overcharge test after storage is small, and it can be seen that the battery characteristics are inferior. This is presumed to be caused by the fact that the expansion / contraction difference between the first active material layer and the second active material layer cannot be absorbed, and peeling occurs at the interface between the two layers during cycling and storage. . When peeling occurs at this interface, the second active material layer having a large reaction area on the surface layer side is not used, and the battery capacity decreases. Moreover, it is estimated that Li was deposited on the second active material layer due to interfacial peeling, thereby inhibiting gas generation during overcharging and causing CID non-operation.

  In Comparative Example 2, since the fiber diameter of the fibrillated CMC is thin, the bonding force between the first active material layer and the second active material layer is insufficient, and interfacial peeling occurs, and the characteristics are higher than those of the example. Presumed to have fallen. On the other hand, in Comparative Example 3, since the fiber diameter of the fibrillated CMC is large, the fibers of the fibrillated CMC cannot sufficiently enter the gap between the first active material layer and the second active material layer, and interface peeling occurs. Thus, it is presumed that the characteristics are lower than those of the examples.

  In Comparative Example 4, since the intermediate layer was thin, the expansion / contraction difference between the first active material layer and the second active material layer could not be absorbed, interface peeling occurred, and the characteristics were deteriorated. Guessed. On the other hand, in Comparative Example 5, since the intermediate layer is thick, the resistance of the negative electrode is increased, and it is presumed that the characteristics are deteriorated as compared with the example.

  In Comparative Example 6, since the particle size of the first carbon particle group constituting the first active material layer is smaller than that of the example, the reaction area of the negative electrode itself is large, and a side reaction between the negative electrode and the electrolytic solution easily occurs. Thus, it is presumed that the battery characteristics are greatly deteriorated during the storage durability test and the cycle durability test, and the characteristics are deteriorated as compared with the examples. On the other hand, in Comparative Example 7, since the particle size of the second carbon particle group constituting the second active material layer is larger than that of the Example, the reaction area of the electrode is small, the electrode itself is poor in reactivity, and the resistance of the negative electrode It is presumed that the characteristics were lower than those of the examples due to the increase.

  On the other hand, in the said Example, using the negative electrode containing the intermediate | middle layer containing a fibrillated cellulose between a 1st active material layer and a 2nd active material layer, distribution of the fiber diameter of fibrillated cellulose, intermediate | middle By appropriately controlling the thickness of the layer, the particle size distribution of the first carbon particle group constituting the first active material layer, and the particle size distribution of the second carbon particle group constituting the second active material layer Further, separation between active material layers having different specific surface areas is suppressed, and a nonaqueous electrolyte secondary battery having excellent cycle durability can be obtained.

In the secondary battery of the example, the specific surface area of the second carbon particle group constituting the second active material layer is larger than the specific surface area of the first carbon particle group constituting the first active material layer (second carbon). (The particle size of the particle group is smaller than the particle size of the first carbon particle group) By improving the reactivity on the surface layer side of the negative electrode, Li precipitation is suppressed, Li ⁺ acceptability is improved, and high rate charging is possible It is.

  Note that a negative electrode including a current collector, a first active material layer including a first carbon particle group having a small specific surface area, and a second active material layer including a second carbon particle group having a large specific surface area in this order is used. Even in the conventional non-aqueous electrolyte secondary battery, high rate charging was possible. However, after the cycle endurance test, peeling occurs at the interface between the first active material layer and the second active material layer, so that the second active material layer having a large reaction area on the surface layer side is not used, and the capacity of the battery is increased. There was a problem of lowering.

  As described above, the embodiments and examples of the present invention have been described. However, it should be considered that the embodiments and examples disclosed this time are illustrative and not restrictive in all respects. . The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 10 Current collector (negative electrode current collector), 10a Negative electrode current collector terminal plate, 11 Negative electrode terminal, 2 Negative electrode mixture layer, 21 1st active material layer, 21a 1st paste, 22 2nd active material layer, 22a 2nd Paste, 23 intermediate layer, 23a third paste, 24 fibrillated cellulose, 24a fine fiber (fibril), 25 interface region, 3 tabless electrode group, 40 positive electrode current collector, 40a positive electrode current collector terminal plate, 41 positive electrode terminal, 5 exterior case, 6 coating device.

Claims (1)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte,
The negative electrode includes a current collector, a first active material layer including a first carbon particle group, an intermediate layer including fibrillated cellulose, and a second active material layer including a second carbon particle group in this order. ,
D50 of the first carbon particle group is 12 to 50 μm,
D50 of the second carbon particle group is 1 to 18 μm,
The intermediate layer has a thickness of 1.0 to 5.0 μm,
D50 of the fibrillated cellulose is 0.3-1 μm,
Non-aqueous electrolyte secondary battery.

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