WO2013076996A1 - リチウムイオン二次電池用負極およびその製造方法、ならびにリチウムイオン二次電池 - Google Patents
リチウムイオン二次電池用負極およびその製造方法、ならびにリチウムイオン二次電池 Download PDFInfo
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- WO2013076996A1 WO2013076996A1 PCT/JP2012/007542 JP2012007542W WO2013076996A1 WO 2013076996 A1 WO2013076996 A1 WO 2013076996A1 JP 2012007542 W JP2012007542 W JP 2012007542W WO 2013076996 A1 WO2013076996 A1 WO 2013076996A1
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- electrode mixture
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- ion secondary
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/133—Electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/139—Processes of manufacture
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/021—Physical characteristics, e.g. porosity, surface area
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
- Y02P70/50—Manufacturing or production processes characterised by the final manufactured product
Definitions
- the present invention relates to a negative electrode of a lithium ion secondary battery, and more specifically to an improvement of a negative electrode mixture layer containing negative electrode active material particles of a negative electrode for a lithium ion secondary battery.
- the lithium ion secondary battery includes a positive electrode and a negative electrode that occlude and release lithium ions, a separator interposed therebetween, and a nonaqueous electrolyte.
- a negative electrode contains negative electrode core material sheets, such as copper foil, and the negative mix layer supported by this surface.
- Such a negative electrode is formed by, for example, applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of a negative electrode core sheet, drying, and rolling with a rolling roll. The By this rolling, the negative electrode mixture layer is densified.
- the negative electrode mixture slurry is prepared, for example, by dispersing negative electrode active material particles and a binder in a dispersion medium such as water.
- Patent Document 1 discloses forming a groove or a cup-shaped recess by pressing a mold against the surface of an active material layer. According to such a method, when the lithium ion secondary battery is continuously used at a high temperature, the unevenness of the surface is deformed by the softening of the separator, and the space for holding the non-aqueous electrolyte is reduced. It is disclosed that the problem that the high output characteristic is lowered can be improved.
- Patent Document 2 discloses forming a groove extending to the peripheral edge on the surface of the active material layer in order to improve the permeability of the non-aqueous electrolyte into the electrode group during production.
- Patent Document 3 discloses that the surface of the active material layer is formed on the surface of the active material layer for the purpose of solving the problem that the strength of the electrode is lowered when a groove is provided in order to improve the permeability of the nonaqueous electrolyte into the electrode group. Disclose the formation of grid-like grooves.
- JP 2008-10253 A Japanese Patent Laid-Open No. 11-154508 JP 2007-31328 A
- the negative electrode active material is densified in the negative electrode mixture layer rolled with a rolling roll, the negative electrode capacity can be increased to some extent. However, when the filling density of the negative electrode active material is increased, the permeability of the non-aqueous electrolyte is likely to decrease.
- recesses and grooves are formed on the surface of the negative electrode mixture layer. Such recesses and grooves can ensure the retention of the nonaqueous electrolyte on the surface of the negative electrode mixture layer to some extent, but are insufficient to improve the penetration into the inside. Moreover, when forming a recessed part and a groove
- the negative electrode active material when the negative electrode active material is densely filled in the negative electrode mixture layer, the stress applied to the negative electrode active material due to rolling, pressing, or the like is hardly relaxed.
- the negative electrode active material in a state where a large stress is applied is further subjected to stress associated with insertion and extraction of lithium ions during charging and discharging of the battery. As a result, the negative electrode active material particles are cracked, crushed, or the negative electrode mixture layer is easily dropped, so that the charge / discharge characteristics are easily impaired.
- An object of the present invention is to provide a negative electrode for a lithium ion secondary battery that can improve the permeability of the nonaqueous electrolyte to the negative electrode mixture layer and improve the input / output characteristics.
- One aspect of the present invention includes a negative electrode core sheet and a negative electrode mixture layer supported by the negative electrode core sheet, and the negative electrode mixture layer includes negative electrode active material particles and a binder, and the negative electrode mixture layer Has a plurality of pores scattered on the surface and inside, the average maximum diameter R of the pores is 45 to 125 ⁇ m, and the number density of the pores per 1 cm 2 of the surface of the negative electrode mixture layer or in the surface direction
- the present invention relates to a negative electrode for a lithium ion secondary battery having 8 to 17 per 1 cm 2 in cross section.
- Another aspect of the present invention includes a step of preparing a negative electrode mixture slurry containing negative electrode active material particles, a binder, and a dispersion medium containing at least water, and the negative electrode mixture slurry as a negative electrode core sheet.
- the binder comprises a sodium salt of carboxymethylcellulose, and the sodium salt of carboxymethylcellulose has a degree of etherification of 0.23 to 0.7 and an average degree of polymerization of 20 to 1600
- the present invention relates to a method for producing a negative electrode for a battery.
- Still another aspect of the present invention relates to a lithium ion secondary battery including a positive electrode, the negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte.
- a negative electrode capable of improving the permeability of the nonaqueous electrolyte to the negative electrode mixture layer and improving the input / output characteristics (or discharge load characteristics) of the lithium ion secondary battery, and the lithium ion secondary battery are provided. Can be provided.
- FIG. 1 is a schematic cross-sectional view schematically showing the structure of a negative electrode for a lithium ion secondary battery according to an embodiment of the present invention.
- FIG. 2 is a partially cutaway perspective view schematically showing the configuration of a prismatic lithium ion secondary battery according to an embodiment of the present invention.
- the negative electrode for a lithium ion secondary battery of the present invention includes a negative electrode core sheet and a negative electrode mixture layer supported by the negative electrode core sheet, and the negative electrode mixture layer includes negative electrode active material particles and a binder. .
- the negative electrode mixture layer has a plurality of pores scattered on the surface and inside, the average maximum diameter R of the pores is 45 to 125 ⁇ m, and the number density of the pores is the surface of the negative electrode mixture layer Alternatively, there are 8 to 17 pieces per 1 cm 2 of the cross section in the plane direction.
- the presence of such voids can improve the permeability of the non-aqueous electrolyte to the negative electrode mixture layer, so that the input / output characteristics (or discharge load characteristics) of the battery can be improved.
- hole can also store a nonaqueous electrolyte, the retainability (liquid retention property) of the nonaqueous electrolyte in the surface and inside of a negative mix layer can be improved significantly. Thereby, since the movement of lithium in the negative electrode is facilitated, the charge / discharge characteristics of the battery are improved.
- the packing density of the negative electrode active material in the negative electrode mixture layer is high (that is, when the capacity of the battery is increased)
- the high permeability of the nonaqueous electrolyte can be maintained, and high input / output characteristics can be obtained. Can be maintained.
- the void formed on the surface of the negative electrode mixture layer is also referred to as a recess having an opening on the surface of the negative electrode mixture layer.
- hole formed in the inside of a negative mix layer is also called a void. That is, the void includes a plurality of concave portions scattered on the surface of the negative electrode mixture layer and a plurality of voids scattered inside the negative electrode mixture layer.
- the pores are formed in the step of drying the coating film of the negative electrode mixture slurry. That is, in the present invention, it is possible to form homogeneous holes (specifically, recesses and voids) without applying pressure as in Patent Document 1. Therefore, the stress applied to the negative electrode active material particles is significantly reduced by the formation of the holes.
- the stress associated with the volume change of the negative electrode active material particles during charge / discharge can be effectively relieved. can do. Therefore, cracking and crushing of the negative electrode active material particles and peeling and dropping of the negative electrode mixture layer are suppressed. That is, the strength and durability of the negative electrode mixture layer can be increased.
- FIG. 1 is a schematic cross-sectional view schematically showing a cross-section of a negative electrode 10 for a lithium ion secondary battery.
- the negative electrode 10 includes a negative electrode core material sheet 1 and a negative electrode mixture layer 2 formed on the surface of the negative electrode core material sheet 1.
- the negative electrode mixture layer 2 includes negative electrode active material particles 3 and a binder. Holes (recesses) 6 are formed on the surface of the negative electrode mixture layer 2, and holes (voids) 7 are formed inside the negative electrode mixture layer 2.
- the binder includes sodium salt of carboxymethyl cellulose (CMC-Na) 4 and rubber particles 5.
- the surface of the negative electrode mixture layer 2 is uniformly provided with scattered recesses 6, and the negative electrode mixture layer 2 is uniformly provided with scattered voids 7.
- Such voids such as the recesses 6 and the voids 7 improve the permeability of the nonaqueous electrolyte and store the nonaqueous electrolyte.
- the average maximum diameter R of the pores is 45 ⁇ m or more, preferably 50 ⁇ m or more, more preferably 60 ⁇ m or more (for example, 65 ⁇ m or more).
- the average maximum diameter R is 125 ⁇ m or less (for example, 105 ⁇ m or less), preferably 100 ⁇ m or less (for example, 90 ⁇ m or less), more preferably 80 ⁇ m or less (for example, 70 ⁇ m or less). These lower limit value and upper limit value can be appropriately selected and combined.
- the average maximum diameter R may be 45 to 125 ⁇ m, preferably 50 to 100 ⁇ m, more preferably 50 to 70 ⁇ m.
- the average maximum diameter R When the average maximum diameter R is less than 45 ⁇ m, the permeability or liquid retention of the nonaqueous electrolyte becomes insufficient, and the input / output characteristics are deteriorated. When the average maximum diameter R exceeds 125 ⁇ m, the capacity tends to decrease due to a decrease in the area occupied by the negative electrode active material particles. In addition, the strength of the negative electrode mixture layer tends to decrease.
- the average maximum diameter R is evaluated by the average maximum diameter R 1 of the recess openings on the surface of the negative electrode mixture layer or the average maximum diameter of voids (average maximum diameter R 2 of the cross section of the void in the surface direction of the negative electrode mixture layer). can do. Any one of the average maximum diameter R 1 of the opening of the recess and the average maximum diameter R 2 of the void may be in the range of the average maximum diameter R, and both are in the range of the average maximum diameter R. It is more preferable.
- the number density of the holes is 8 or more, preferably 9 or more, more preferably 10 or more per 1 cm 2 of the surface of the negative electrode mixture layer or 1 cm 2 of the cross section in the plane direction.
- the number density is 17 or less, preferably 16 or less, and more preferably 15 or less. These lower limit value and upper limit value can be appropriately selected and combined.
- the number density of the holes may be 8 to 17, preferably 10 to 15.
- the non-aqueous electrolyte When the number density of the holes is less than 8, the non-aqueous electrolyte has insufficient permeability or liquid retention, and input / output characteristics are deteriorated. In addition, when the number density exceeds 17, the capacity is likely to decrease due to a decrease in the area occupied by the negative electrode active material particles. In addition, the strength of the negative electrode mixture layer tends to decrease.
- the number density of holes on the surface of the negative electrode mixture layer is the number density of recesses on the surface of the negative electrode mixture layer.
- the number density of holes in the cross section (cross section in the plane direction) of the negative electrode mixture layer is the sum of the number densities of both the recesses and voids, but the cross section of the negative electrode mixture layer at a position close to the negative electrode core material.
- the number density of vacancies in may be regarded as the number density of voids.
- the average maximum length L of pores in the thickness direction of the negative electrode mixture layer is, for example, 45 ⁇ m or more, preferably 50 ⁇ m or more (for example, 60 ⁇ m or more), more preferably 65 ⁇ m or more (for example, 70 ⁇ m or more).
- the average maximum length L is, for example, 125 ⁇ m or less (for example, 105 ⁇ m or less), preferably 100 ⁇ m or less (for example, 90 ⁇ m or less), and more preferably 80 ⁇ m or less. These lower limit value and upper limit value can be appropriately selected and combined.
- the average maximum length L may preferably be 45 to 125 ⁇ m (for example, 50 to 100 ⁇ m), more preferably 60 to 90 ⁇ m, or 70 to 80 ⁇ m.
- the average maximum length L When the average maximum length L is less than 45 ⁇ m, the permeability or liquid retention of the nonaqueous electrolyte becomes insufficient. When the average maximum length L exceeds 125 ⁇ m, the capacity tends to decrease due to a decrease in the area occupied by the negative electrode active material particles. In addition, the strength of the negative electrode mixture layer tends to decrease.
- the average maximum length L of pores in the thickness direction of the negative electrode mixture layer can be evaluated by the average maximum depth H of the recesses and / or the average maximum length L 2 of the voids. Either the average maximum length L 2 average of the maximum depth H and voids recesses may meet the range of the average maximum length L of the above, both are an average maximum length L of the The range may be satisfied.
- the average maximum length L of pores (average maximum depth H of recesses, average maximum length L 2 of voids, etc.) is determined from an electron microscope (SEM) image of a cross section in the thickness direction of the negative electrode mixture layer. Can do. More specifically, in the SEM photographed image, a plurality of (for example, 200) holes (recesses and / or voids) are arbitrarily selected, and the maximum length in the thickness direction of the negative electrode mixture layer is measured. By calculating the average value, the average maximum length L of the holes can be obtained.
- SEM electron microscope
- the average maximum diameter R (average maximum diameter R 1 , R 2 ) of the pores can be determined from the SEM image of the cross section in the surface or the plane direction of the negative electrode mixture layer. More specifically, the average maximum diameter R is determined by arbitrarily selecting a plurality of (for example, 200) holes in the SEM image of the cross section in the surface or plane direction of the negative electrode mixture layer. It can be determined by measuring (the maximum diameter of the opening of the recess or the maximum diameter of the holes in the cross section) and calculating the average value.
- the number density of holes is obtained by selecting an arbitrary 1 cm ⁇ 1 cm region in the SEM image of the cross section in the surface or plane direction of the negative electrode mixture layer and counting the number of holes existing in this region. Can do.
- the SEM image of the cross section of the negative electrode mixture layer can be taken by exposing the cross section by solidifying the negative electrode mixture layer with a curable resin such as an epoxy resin and chemically polishing the exposed cross section.
- the magnification of the SEM is not particularly limited as long as the magnification is sufficient to confirm the pores, and may be, for example, about 100 times.
- negative electrode active material particles As the negative electrode active material constituting the negative electrode active material particles, carbonaceous materials conventionally used as negative electrode active materials for negative electrodes for lithium ion secondary batteries and alloy-based negative electrode active materials containing silicon and tin are particularly limited. Used without. Among these, carbonaceous materials having a graphite structure, specifically, graphite (natural graphite, artificial graphite, etc.), graphitized mesophase carbon and the like are particularly preferably used.
- a negative electrode active material can be used individually by 1 type or in combination of 2 or more types.
- the carbonaceous material having a graphite structure is such that a diffraction image measured by a wide-angle X-ray diffraction method has a peak attributed to the (101) plane and a peak attributed to the (100) plane. Particularly preferred from the viewpoint of graphitization degree.
- the ratio of the peak intensity I (101) attributed to the (101) plane and the peak intensity I (100) attributed to the (100) plane is 0.01 ⁇ I (101) / It is preferable to satisfy I (100) ⁇ 0.25, and 0.08 ⁇ I (101) / I (100) ⁇ 0.20 is preferable from the viewpoint of battery capacity.
- the average particle diameter R a of the negative electrode active material particles for example, 5 ⁇ 40 [mu] m, preferably 14 ⁇ 25 [mu] m, more preferably 16 ⁇ 23 .mu.m.
- the average maximum length of pores L e.g., the average maximum depth H of the concave portion
- the ratio R a / L of the average particle diameter R a of the negative electrode active material particles for example, 0.1 to 0.55
- it is 0.2 to 0.5.
- the average particle diameter means a median diameter (D 50V ) in a volume-based particle size distribution.
- the average particle size can be determined using, for example, a laser diffraction / scattering particle size distribution measuring device (for example, LA-920 manufactured by Horiba, Ltd., Microtrack manufactured by Nikkiso Co., Ltd.).
- the average sphericity of the negative electrode active material particles such as graphite particles is, for example, 85% or more, preferably 90 to 95%, and more preferably 91 to 94%.
- the average sphericity is represented by 4 ⁇ S / L a 2 (where S is the area of the orthographic projection image of the negative electrode active material particles, L a is the perimeter of the orthographic projection image) ⁇ 100 (%).
- S is the area of the orthographic projection image of the negative electrode active material particles
- L a is the perimeter of the orthographic projection image
- the average value of the sphericity of any 100 negative electrode active material particles is preferably in the above range.
- the specific surface area of the negative electrode active material particles is preferably 3 to 7 m 2 / g, and more preferably 3.5 to 6.5 m 2 / g.
- the specific surface area can be measured by the BET method.
- the negative electrode mixture layer is easily slipped when forming the negative electrode mixture layer, so that it is easy to fill with high density. At the same time, the binding strength of the negative electrode mixture layer tends to increase.
- fluorine resins such as polyvinylidene fluoride (PVDF); acrylic resins such as polymethyl acrylate and ethylene-methyl methacrylate copolymer; rubbery materials; carboxymethyl cellulose (CMC), carboxyethyl cellulose (CEC) And cellulose ether compounds (including salts of cellulose ether) such as hydroxyethylcellulose and hydroxypropylmethylcellulose.
- PVDF polyvinylidene fluoride
- acrylic resins such as polymethyl acrylate and ethylene-methyl methacrylate copolymer
- rubbery materials such as carboxymethyl cellulose (CMC), carboxyethyl cellulose (CEC)
- CMC carboxymethyl cellulose
- CEC carboxyethyl cellulose
- cellulose ether compounds including salts of cellulose ether
- the negative electrode mixture layer is formed by drying and rolling the coating film of the negative electrode mixture slurry.
- pores are formed in the coating film drying step.
- a binder containing at least a swellable component in the dispersion medium contained in the slurry it is preferable to use a binder containing at least a swellable component in the dispersion medium contained in the slurry. Such a component is swollen by the dispersion medium in the coating film of the negative electrode mixture slurry, but shrinks by releasing the dispersion medium as the coating film is dried. Therefore, moderately sized holes are efficiently formed in the contracted portion.
- Such a component can be appropriately selected from the above binders depending on the type of the dispersion medium.
- a fluorine resin such as PVDF, an acrylic resin, or the like can be used.
- a dispersion medium containing water a water-swellable component (binder), for example, A cellulose ether compound or the like is preferable. Since the cellulose ether compound is suitable for adjusting the viscosity of the negative electrode mixture slurry, in addition to being easy to obtain high binding properties, it is easy to adjust the applicability of the negative electrode mixture slurry.
- the binder preferably contains a binder such as a rubber-like material in addition to the swellable component in the dispersion medium as described above.
- the rubbery material is preferably used in combination with a dispersion medium containing at least water. Therefore, the binder preferably includes at least a water-swellable component and a rubber-like material.
- carboxyalkyl celluloses such as CMC and CEC
- alkali metal salts of carboxyalkyl cellulose such as sodium salt of CMC and sodium salt of CEC are preferable.
- the carboxyalkyl group of the carboxyalkyl cellulose is preferably a carboxy C 1-4 alkyl group, and more preferably a carboxy C 1-3 alkyl group.
- alkali metal forming the alkali metal salt sodium, potassium and the like are preferable. Of these, CMC and sodium salt of CMC (CMC-Na) are preferable, and CMC-Na is particularly preferable.
- Carboxyalkylcelluloses such as CMC and CMC-Na and salts thereof have a high affinity for water. It is preferable to adjust the degree of etherification and the average degree of polymerization in order to ensure binding properties and to effectively form pores.
- the salt of carboxyalkyl cellulose may be simply called carboxyalkyl cellulose.
- the degree of etherification of carboxyalkyl cellulose is, for example, 0.23 or more, preferably 0.25 or more, and more preferably 0.3 or more. Further, the degree of etherification is, for example, 0.7 or less, preferably 0.6 or less, and more preferably 0.55 or less. These lower limit value and upper limit value can be appropriately selected and combined. The degree of etherification may be, for example, 0.23 to 0.7, 0.25 to 0.7, or 0.3 to 0.7.
- the gelled product In the carboxyalkyl cellulose having a degree of etherification in the above range, a relatively large number of hydroxyl groups remain in the molecular chain, so that the number of pseudo-crosslinking points between the molecular chains due to hydrogen bonding tends to increase. Therefore, gelation easily occurs and the ratio of the gelled product increases. In the gelled region, the gelled product swells with water in the dispersion medium in the negative electrode mixture slurry, but it is difficult to dissolve in water because the molecular chains of the carboxyalkyl cellulose are firmly fixed. Therefore, in the coating film of the negative electrode mixture slurry, the gelled product exists in a state in which the shape is maintained, and the gelled product releases the dispersion medium such as water by drying and shrinks, thereby making the pores more effective. Can be formed.
- the degree of etherification of the carboxyalkyl cellulose is in the above range, it is possible to suppress the viscosity of the negative electrode mixture slurry from becoming too high, and it is easy to suppress a decrease in coatability. Therefore, it is easy to improve the binding property of the negative electrode mixture layer.
- the degree of etherification is an index indicating the amount of etherified by substitution of the hydroxyl group of cellulose contained in the cellulose ether compound.
- carboxyalkyl cellulose is specifically substituted with a carboxyalkyl group such as carboxymethyl group, sodium carboxymethyl group (—CH 2 COONa), or a salt thereof among ether groups contained in a cellulose ether compound. It is an indicator of the amount of ether groups formed.
- Cellulose has three hydroxyl groups per anhydroglucose unit. When one hydroxyl group is etherified in all anhydroglucose units, the degree of etherification is 1. Accordingly, the degree of etherification of the cellulose compound is in the range of 0-3.
- the average degree of polymerization of the carboxyalkyl cellulose is, for example, 20 or more, preferably 80 or more (for example, 100 or more), and more preferably 500 or more (for example, 700 or more or 1000 or more).
- an average degree of polymerization is 1600 or less, for example, Preferably it is 1500 or less (for example, 1300 or less), More preferably, it is 1200 or less. These lower limit value and upper limit value can be appropriately selected and combined.
- the average degree of polymerization may be, for example, 20 to 1600, 20 to 1200, or 500 to 1200.
- the carboxyalkyl cellulose having a degree of etherification as described above generally has a lower degree of etherification than that used in the negative electrode mixture slurry. If the degree of etherification is low, the interaction between the molecular chains of the carboxyalkyl cellulose tends to be strong, so that the viscosity of the negative electrode mixture slurry tends to be high, and this tends to lower the coatability. Thus, adjusting the average degree of polymerization to the above range is advantageous because it can more effectively suppress an increase in the viscosity of the negative electrode mixture slurry and suppress a decrease in coatability. Further, when the average degree of polymerization is in the above range, it is more effective to enhance the binding property.
- the carboxyalkyl cellulose having a degree of etherification and an average degree of polymerization in the above ranges partially dissolves in water and partially gels and swells.
- the solubility of carboxyalkyl cellulose in water is preferably 90 to 97%.
- the solubility indicates the proportion of the dissolved component in the total mass of the carboxyalkyl cellulose. When the solubility is in such a range, it is easy to suppress a decrease in the coating property of the negative electrode mixture slurry, and it is easy to form pores.
- a component that is swellable with respect to a dispersion medium such as carboxyalkyl cellulose is preferably used in the form of particles.
- the average particle diameter of such particles is, for example, 30 ⁇ m or more, preferably 35 ⁇ m or more, more preferably 40 ⁇ m or more (for example, 50 ⁇ m or more).
- the average particle size is, for example, 100 ⁇ m or less, preferably 90 ⁇ m or less, more preferably 85 ⁇ m or less (for example, 80 ⁇ m or less). These lower limit value and upper limit value can be appropriately selected and combined.
- the average particle diameter may be, for example, 30 to 100 ⁇ m, 30 to 90 ⁇ m, or 40 to 80 ⁇ m. When the average particle size is in such a range, it is easy to adjust the quantitative balance between the dissolved component and the gelled product.
- carboxyalkyl cellulose a commercially available product may be used, or a product produced by a known method may be used. A method for producing carboxyalkyl cellulose will be described below by taking the case of CMC-Na as an example.
- CMC-Na is produced by allowing sodium monochloroacetate and sodium hydroxide to act on pulp containing cellulose. According to such a production method, CMC-Na is synthesized by reacting a hydroxyl group in cellulose with sodium monochloroacetate. In the obtained CMC-Na, sodium chloride, sodium carbonate, sodium glucoseate and the like mixed during production are contained as impurities. Such impurities are preferably removed using a sulfuric acid purification method, a methanol purification method, a water medium method, or the like.
- the CMC-Na from which impurities have been removed is dried and then pulverized to a predetermined particle size.
- a known pulverization method can be employed, and for example, a jet mill pulverizer, a valverizer, an impact pulverizer, a hammer mill pulverizer, or the like may be used. Of these, a jet mill grinder is preferable.
- the crushed CMC-Na may be classified as necessary.
- the classification method include gravity classification, centrifugal classification, and inertia classification.
- centrifugal classification using a cyclone classifier is preferable from the viewpoint of easily obtaining a powder having a small average particle diameter.
- rubbery materials include conjugated diene rubbers such as polybutadiene and polyisoprene; copolymer rubbers of styrene such as styrene butadiene rubber (SBR) and conjugated dienes (1,3-butadiene, isoprene, etc.); Examples thereof include copolymer rubbers of acrylonitrile and conjugated dienes such as acrylonitrile-butadiene rubber (NBR); acrylic rubbers; or modified products thereof. These rubber-like materials can be used alone or in combination of two or more.
- the modified body may contain other monomer (copolymerizable monomer) units that can be copolymerized in addition to the main constituent monomers (for example, styrene and butadiene in SBR) units of the rubber-like material, It may be a hydride or may have other functional groups introduced therein.
- copolymerizable monomers include acrylonitrile; polymerizable unsaturated carboxylic acids such as acrylic acid and methacrylic acid; and esters of polymerizable unsaturated carboxylic acids (methyl acrylate, methyl methacrylate, ethyl acrylate, 2-ethylhexyl acrylate, butyl acrylate). Etc.).
- the copolymerizable monomer can use copolymerizable monomers other than the main constituent monomer of each rubber-like material according to the kind of rubber-like material.
- copolymer rubber of styrene and conjugated diene is preferable.
- SBR usually has a glass transition point in the range of ⁇ 30 to + 40 ° C., has excellent binding properties in the operating temperature range of the battery, and is stable at the negative electrode potential. Particularly preferred as a dressing.
- the content of styrene units is, for example, 30 to 70 mol%, preferably 40 to 65 mol%.
- the rubbery material is preferably used in the form of particles.
- the particles of rubbery material (hereinafter also simply referred to as “rubber particles”) can be mixed into the negative electrode mixture slurry in the form of an aqueous latex, emulsion, or suspension.
- the component swellable in the dispersion medium such as CMC-Na adheres to the surface of the negative electrode active material particles.
- the rubber particles have high binding properties, and bind the negative electrode active material particles to which the above components are adhered, and the negative electrode active material particles and the negative electrode core material sheet by point contact.
- the average particle diameter of the rubber particles is, for example, 50 to 200 nm, preferably 50 to 180 nm (for example, 50 to 150 nm), and more preferably 100 to 170 nm (for example, 100 to 120 nm).
- the average particle diameter of the rubber particles is within such a range, the binding property in the negative electrode mixture layer can be improved more effectively.
- the amount of the binder is, for example, 1 to 7 parts by mass, preferably 1.5 to 5 parts by mass, and more preferably 1.7 to 4 parts by mass with respect to 100 parts by mass of the negative electrode active material particles.
- the amount of the binder is within such a range, it is easy to increase the battery capacity and holes are easily formed. Moreover, the increase in internal resistance can be suppressed. Therefore, it is possible to more effectively suppress the deterioration of input / output characteristics.
- the amount of such component is, for example, 0.8 to 3 parts by mass with respect to 100 parts by mass of the negative electrode active material particles.
- the amount is preferably 0.8 to 2.8 parts by mass (for example, 0.9 to 2.7 parts by mass), more preferably 0.8 to 1.5 parts by mass, or 1 to 2 parts by mass.
- the amount of the rubber-like material is, for example, 0.5 to 1.5 parts by mass, preferably 0.6 to 1. 2 parts by mass.
- the amount of the rubber-like material is within such a range, in addition to easily increasing the binding strength, it is easy to suppress the negative electrode active material particles from being excessively covered with the rubber-like material, thereby increasing the internal resistance. It can be effectively suppressed.
- the negative electrode mixture layer may further contain a conductive agent and / or a thickener as necessary.
- a conductive agent include carbon black; conductive fibers such as carbon fibers; and carbon fluoride.
- the thickener include poly C 2-4 alkylene glycol such as polyethylene glycol.
- the apparent density of the negative electrode mixture layer is not particularly limited.
- the negative electrode mixture layer has pores of an appropriate size in an appropriate distribution state, even when the apparent density of the negative electrode mixture layer is high, the nonaqueous electrolyte has a high permeability and a high liquid Since the turnability can be maintained, input / output characteristics (or discharge load characteristics) can be improved.
- the apparent density of the negative electrode mixture layer is, for example, 1.5 to 1.8 g. / Cm 3 , preferably 1.55 to 1.75 g / cm 3 .
- the apparent density of the negative electrode mixture layer excluding the pores can be calculated by dividing the weight of the negative electrode mixture layer by the volume of the non-porous portion of the negative electrode mixture layer. Specifically, the apparent density of the negative electrode mixture layer excluding the voids can be calculated as follows, for example.
- R 2 is the average maximum diameter of the voids
- L 2 is the average maximum length of the voids
- n is the number density of voids in the cross section in the plane direction of the negative electrode mixture layer
- R 1 is the concave portion
- H is the average maximum depth of the recesses
- N is the number density of the recesses on the surface of the negative electrode mixture layer.
- the variation in the apparent density in the region excluding the pores of the negative electrode mixture layer is, for example, less than 1%, preferably less than 0.7% with respect to the average value.
- the variation in the apparent density is obtained by calculating the apparent density as described above for an arbitrary plurality of locations (for example, 20 locations) of the negative electrode mixture layer to obtain an average value, and the maximum or minimum of the calculated apparent density values. And the difference between the average value and the average value.
- the thickness (average thickness T) of the negative electrode mixture layer is not particularly limited, and is, for example, about 30 to 200 ⁇ m, preferably about 50 to 150 ⁇ m.
- the ratio L / T of the average maximum length L of the pores (for example, the average maximum depth H of the recesses) to the average thickness T of the negative electrode mixture layer is, for example, 0.4 or more, preferably 0.5 or more. More preferably, it is 0.55 or more.
- the ratio L / T is 1 or less, preferably 0.9 or less, more preferably 0.8 or less (for example, 0.75 or less). These lower limit value and upper limit value can be appropriately selected and combined.
- the ratio L / T may be, for example, 0.4 to 1, or 0.4 to 0.8. When the ratio L / T is in such a range, it is more effective for improving the permeability of the electrolyte.
- the negative electrode core sheet those conventionally used as a negative electrode core sheet for lithium ion secondary batteries can be used without any particular limitation.
- the material of the negative electrode core sheet include copper, copper alloy, stainless steel, nickel and the like. Of these, copper or a copper alloy is preferable. The content of components other than copper in the copper alloy is, for example, 0.2 mol% or less.
- the negative electrode core sheet may be a non-porous conductive substrate or a porous conductive substrate having a plurality of through holes.
- a non-porous conductive substrate a metal foil, a metal sheet, or the like can be used.
- the porous conductive substrate include a metal foil having a communication hole (perforation), a mesh body, a punching sheet, and an expanded metal.
- a copper foil, particularly an electrolytic copper foil or the like is preferable.
- the thickness of the negative electrode core sheet is, for example, about 5 to 30 ⁇ m, preferably about 5 to 15 ⁇ m.
- the negative electrode mixture layer may be formed on one surface of the negative electrode core material sheet, or may be formed on both surfaces.
- the negative electrode of the present invention is formed by applying the negative electrode mixture slurry to the surface of the negative electrode core sheet, drying the coating film, and rolling the dried coating film. More specifically, the negative electrode is (I) preparing a negative electrode mixture slurry containing negative electrode active material particles, a binder, and a dispersion medium; (Ii) applying a negative electrode mixture slurry to the negative electrode core sheet to form a coating film; (Iii) a step of drying the coating film by heating; (Iv) By rolling the dried coating film, it can manufacture by passing through the process of forming the negative mix layer supported by the negative electrode core material sheet.
- the pores of the negative electrode mixture layer are formed by a process of drying the coating film.
- the size, shape, number density, etc. of the pores can be adjusted for drying conditions, binder type, physical properties, amount, average particle size, average particle size of negative electrode active material particles, and / or type of dispersion medium, etc. It can be adjusted by selecting. Therefore, it is not necessary to separately provide a step of forming a recess or the like as in the conventional method, and the negative electrode can be easily manufactured.
- a binder containing a component that swells in a dispersion medium such as CMC-Na In order to efficiently form pores on and inside the negative electrode mixture layer, it is preferable to use a binder containing a component that swells in a dispersion medium such as CMC-Na.
- CMC-Na having the above etherification degree and average polymerization degree is used as such a component, it is possible to form pores of an appropriate size and to easily control the number density of the pores.
- a binder containing such CMC-Na and a rubber-like material it is preferable to use a binder containing such CMC-Na and a rubber-like material.
- the negative electrode mixture slurry can be prepared by mixing negative electrode active material particles, a binder, and a dispersion medium.
- the negative electrode active material particles and / or the binder may be used after being previously dispersed in at least a part of the dispersion medium.
- a conventional mixer or kneader can be used for mixing.
- the dispersion medium examples include water, alcohols such as ethanol, ethers such as tetrahydrofuran (THF), ketones such as acetone, N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof.
- the binder uses a component that swells in water such as CMC-Na
- the dispersion medium preferably contains at least water, and water and a water-soluble organic solvent (for example, C 1-4 alkanol such as ethanol). THF, acetone, NMP, etc.) may be used.
- the content of water in the dispersion medium is, for example, 50% by mass or more, preferably 70% by mass or more, and more preferably 80% by mass or more.
- the negative electrode mixture layer contains other components such as a conductive agent and a thickener, these components may be added in the step of preparing the negative electrode mixture slurry.
- the coating film is formed by applying the negative electrode mixture slurry to the surface of the negative electrode core sheet.
- a known coating method using various coaters such as a die coater can be employed.
- the coating amount of the negative electrode mixture slurry (the thickness of the coating film) can be appropriately adjusted according to the desired thickness of the negative electrode mixture layer, the packing density of the negative electrode active material, and the like.
- step (iii) the coating film formed in step (ii) is dried to remove the dispersion medium contained in the coating film.
- pores are formed on the surface and inside of the negative electrode mixture layer.
- a binder containing a component that swells in a dispersion medium such as CMC-Na is used, pores can be efficiently formed particularly on the surface and inside of the negative electrode mixture layer.
- the coating film may be naturally dried, but is preferably dried by heating and / or under reduced pressure.
- the heating temperature can be appropriately selected according to the type of dispersion medium, the atmospheric pressure during drying, and the like.
- the heating temperature is, for example, 40 to 250 ° C., preferably 50 to 200 ° C., more preferably 60 to 150 ° C.
- Drying may be performed in the air or in an inert gas atmosphere.
- the inert gas include helium, neon, argon, krypton, xenon, and nitrogen.
- the drying time can be appropriately selected according to the type of the dispersion medium, the drying temperature, the atmospheric pressure, etc. For example, it may be 1 to 20 hours, preferably 1.5 to 12 hours, more preferably about 2 to 10 hours. Good.
- step (iv) the dried coating film obtained in step (iii) is rolled together with the negative electrode core sheet.
- a rolling roll etc. can be used for rolling.
- the linear pressure of the rolling roll can be appropriately set so that the thickness of the negative electrode mixture layer, the packing density of the negative electrode active material, and the like are in a desired range while retaining the pores.
- the linear pressure is, for example, 0.2 to 1 kN / cm, preferably 0.3 to 0.8 kN / cm, and more preferably 0.4 to 0.7 kN / cm.
- the number of rolling is, for example, 1 to 5 times, preferably 1 to 3 times.
- the negative electrode of the present invention has high nonaqueous electrolyte permeability and can improve input / output characteristics (or discharge load characteristics) and the like. Therefore, it is suitable for using as a negative electrode of a lithium ion secondary battery.
- the lithium ion secondary battery includes a positive electrode, the negative electrode, a separator interposed therebetween, and a nonaqueous electrolyte.
- the configuration of the lithium ion secondary battery will be described in more detail with reference to the drawings.
- FIG. 2 is a perspective view schematically showing a lithium ion secondary battery according to an embodiment of the present invention. 2, in order to show the structure of the principal part of the battery 21, the part is notched and shown.
- the battery 21 is a rectangular battery in which a flat electrode group 10 and a nonaqueous electrolyte (not shown) are accommodated in a rectangular battery case 11.
- the electrode group 10 is formed by winding the negative electrode and the positive electrode in a spiral shape with a separator interposed therebetween, and pressing the negative electrode and the positive electrode so as to be sandwiched from the side surface, thereby forming a flat shape. ing.
- One end of the positive electrode lead 14 is welded to the end of the positive electrode core material sheet constituting the positive electrode 13, and the other end of the positive electrode lead 14 is welded to the sealing plate 12 serving as the positive electrode terminal.
- One end of the negative electrode lead 15 is welded to the end of the negative electrode core sheet that constitutes the negative electrode, and the other end of the negative electrode lead 15 is welded to the negative electrode terminal 13.
- a gasket 16 is disposed between the sealing plate 12 and the negative electrode terminal 13 to insulate them.
- a frame 18 made of an insulating material such as polypropylene is usually disposed between the sealing plate 12 and the electrode group 10 to insulate the negative electrode lead 15 from the sealing plate 12.
- the sealing plate 12 is joined to the open end of the rectangular battery case 11 to seal the rectangular battery case 11.
- a liquid injection hole 17 a is formed in the sealing plate 12, and the liquid injection hole 17 a is closed by the plug 17 after the nonaqueous electrolyte is injected into the rectangular battery case 11.
- the positive electrode includes, for example, a positive electrode core material sheet and a positive electrode mixture layer supported by the positive electrode core material sheet.
- the material of the positive electrode core material sheet include known materials such as stainless steel, aluminum, aluminum alloy, and titanium. Of these, aluminum or an aluminum alloy is preferable.
- the positive electrode core material sheet may be a nonporous or porous conductive substrate similar to that exemplified for the negative electrode core material sheet. As the positive electrode core sheet, an aluminum foil or the like is preferable.
- the thickness of the positive electrode core material sheet is, for example, 5 to 30 ⁇ m, preferably 7 to 20 ⁇ m.
- the positive electrode mixture layer may be formed on one surface of the positive electrode core material sheet, or may be formed on both surfaces.
- the thickness of the positive electrode mixture layer can be selected from the same range as the thickness of the negative electrode mixture layer.
- the positive electrode mixture layer includes particles of a positive electrode active material and a binder as essential components, and a conductive agent as an optional component.
- positive electrode active material known positive electrode active materials for lithium secondary batteries can be used, and among them, lithium-containing composite oxides, olivine type lithium phosphate and the like are preferable.
- the lithium-containing composite oxide is a metal oxide containing lithium and a transition metal element or an oxide in which a part of the transition metal element in the metal oxide is substituted with a different element.
- the transition metal element include Sc, Y, Mn, Fe, Co, Ni, Cu, and Cr. Among these transition metal elements, Mn, Co, Ni and the like are preferable.
- the different elements include Na, Mg, Zn, Al, Pb, Sb, and B. Of these different elements, Mg, Al, and the like are preferable.
- the transition metal element and the different element can be used singly or in combination of two or more.
- lithium-containing composite oxide for example, Li x CoO 2, Li x NiO 2, Li x MnO 2, Li x Co m Ni 1-m O 2, Li x Co m M 1-m O n, Li x Ni 1-m M m O n, Li x Mn 2 O 4, etc. Li x Mn 2-m M m O 4 and the like.
- M represents at least one element selected from the group consisting of Sc, Y, Mn, Fe, Co, Ni, Cu, Cr, Na, Mg, Zn, Al, Pb, Sb and B.
- x, m, and n are 0 ⁇ x ⁇ 1.2, 0 ⁇ m ⁇ 0.9, and 2.0 ⁇ n ⁇ 2.3, respectively.
- lithium-containing composite oxides those containing Co and / or Ni are preferable.
- Li x CoO 2 and Li x NiO 2 are preferable.
- Li x Ni y M 1 z M 2 1- (y + z) O 2 + d (1) is also preferred.
- M 1 corresponds to the element M, and among these, at least one element selected from the group consisting of Co and Mn is preferable.
- M 2 corresponds to the element M, and among these, at least one element selected from the group consisting of Al, Cr, Fe, Mg, and Zn is preferable.
- x, y and d are 0.98 ⁇ x ⁇ 1.1, 0 ⁇ z ⁇ 0.7, 0.9 ⁇ (y + z) ⁇ 1 and ⁇ 0.01 ⁇ d ⁇ 0. Satisfies 01.
- olivine type lithium phosphate examples include LiZPO 4 and Li 2 ZPO 4 F.
- Z is at least one element selected from the group consisting of Co, Ni, Mn and Fe.
- the molar ratio of lithium is a value immediately after the synthesis of the positive electrode active material, and increases or decreases due to charge / discharge.
- a positive electrode active material can be used individually by 1 type or in combination of 2 or more types.
- binder examples include fluorine resins such as polytetrafluoroethylene and polyvinylidene fluoride; acrylic resins such as polymethyl acrylate; rubber-like materials such as SBR and acrylic rubber; or a mixture thereof.
- Examples of the conductive agent include those similar to those exemplified for the negative electrode mixture layer, and graphite (natural graphite, artificial graphite, etc.) and the like.
- the positive electrode mixture layer may contain a thickener and a known additive as necessary.
- Examples of the thickener include cellulose ether compounds such as CMC and CMC-Na, as well as those exemplified for the negative electrode mixture layer.
- the separator a resin-made microporous film, nonwoven fabric or woven fabric can be used.
- the resin constituting the separator include polyolefins such as polyethylene and polypropylene; polyamides; polyamideimides; polyimides and the like.
- the thickness of the separator is, for example, 5 to 50 ⁇ m, preferably 10 to 30 ⁇ m.
- Non-aqueous electrolyte includes a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent.
- Non-aqueous solvents include cyclic carbonates such as propylene carbonate and ethylene carbonate (EC); chain carbonates such as diethyl carbonate, ethyl methyl carbonate (MEC), and dimethyl carbonate; cyclic esters such as ⁇ -butyrolactone and ⁇ -valerolactone. Examples thereof include carboxylic acid esters.
- EC propylene carbonate and ethylene carbonate
- MEC ethyl methyl carbonate
- cyclic esters such as ⁇ -butyrolactone and ⁇ -valerolactone. Examples thereof include carboxylic acid esters.
- lithium salt LiPF 6, LiBF 4, LiSbF 6, LiAsF 6, LiClO 4, LiAlCl 4, LiSCN, LiCF 3 CO 2, LiCF 3 SO 3, LiN (SO 2 CF 3) 2, LiN (SO 2 C 2 F 5 ) 2 , LiC (SO 2 CF 3 ) 3 and the like.
- a lithium salt can be used individually by 1 type or in combination of 2 or more types.
- the concentration of the lithium salt in the nonaqueous electrolyte is, for example, 0.5 to 2 mol / L.
- Nonaqueous electrolytes include known additives such as vinylene carbonate compounds such as vinylene carbonate (VC); cyclic carbonate compounds having a vinyl group such as vinyl ethylene carbonate and divinyl ethylene carbonate; and fragrances such as cyclohexylbenzene, biphenyl, and diphenyl ether. Group compounds may be added.
- vinylene carbonate compounds such as vinylene carbonate (VC)
- cyclic carbonate compounds having a vinyl group such as vinyl ethylene carbonate and divinyl ethylene carbonate
- fragrances such as cyclohexylbenzene, biphenyl, and diphenyl ether.
- Group compounds may be added.
- the shape of the lithium ion secondary battery is not limited to a square shape, and may be a cylindrical shape, a coin shape, a flat shape, or the like.
- the lithium ion secondary battery may be a laminate type.
- Battery case materials include stainless steel plates, aluminum, aluminum alloys (alloys containing a trace amount of metals such as manganese and copper), and laminate films with a metal layer such as an aluminum layer sandwiched between resin films. it can.
- the electrode group is not limited to a wound type depending on the shape of the battery, but may be a laminate of a positive electrode, a negative electrode, and a separator interposed therebetween, or a zigzag fold.
- the shape of the electrode group is not limited to a flat shape depending on the shape of the battery or the battery case, and may be a cylindrical shape.
- Example 1 Production of CMC-Na powder Pulp powder was obtained by crushing pulp pieces using a crusher (a twin-screw crusher TIGER SHRED manufactured by Fujitex Co., Ltd.). Then, 191 g of the obtained pulp powder and 5,730 mL of a 2-propanol aqueous solution having a concentration of 88% by mass were stirred and mixed at 45 ° C. to prepare a slurry. The obtained slurry was naturally cooled to room temperature, and then 13.7 g of sodium hydroxide was added to obtain alkali cellulose. Then, the alkali cellulose slurry was ice-cooled to 10 ° C. or lower. Next, 31.4 g of monochloroacetic acid was added to the cooled alkali cellulose slurry and stirred for 5 minutes, then cooled to 5 ° C. and allowed to stand for 2 hours.
- a crusher a twin-screw crusher TIGER SHRED manufactured by Fujitex Co., Ltd.
- the obtained slurry was put in a flask, then boiled in a hot water bath and reacted at the boiling point for 80 minutes. And 5 mass% hydrochloric acid was added excessively in the flask, it stirred for 5 minutes, and the slurry was cooled to room temperature. The resulting slurry was washed with an aqueous methanol solution (methanol concentration 80% by mass), filtered, and chloride ions were removed by repeating this washing and filtration a total of 5 times. The residue was dried at 80 ° C. for 3 hours to obtain a solid of CMC-Na.
- the obtained CMC-Na solid was pulverized using a jet mill pulverizer (100AFG manufactured by Hosokawa Micron Co., Ltd.) and then passed through a sieve having a mesh size of 120 ⁇ m to obtain a maximum particle size of 120 ⁇ m (average particle size).
- a CMC-Na powder having a diameter of 60 ⁇ m was obtained.
- the resulting CMC-Na had a degree of etherification of 0.4 and an average degree of polymerization of 1200.
- the negative electrode mixture slurry obtained in (2) above was applied to both surfaces of an electrolytic copper foil (thickness 10 ⁇ m) as a negative electrode core material sheet using a die coater. And the coating film was dried at 60 degreeC for 10 hours. The dried coating film was rolled together with the negative electrode core sheet with a rolling roller at a linear pressure of 490 N / cm to form a negative electrode mixture layer. The obtained negative electrode was cut into a predetermined shape. The obtained negative electrode mixture layer had a thickness of 120 ⁇ m, and an apparent density in a region excluding vacancies was 1.6 g / cm 3 .
- a positive electrode mixture slurry was prepared by mixing 100 parts by mass of LiCoO 2 as a positive electrode active material, 4 parts by mass of PVDF as a binder, and an appropriate amount of NMP.
- the obtained positive electrode mixture slurry was applied to both surfaces of an aluminum foil (thickness 15 ⁇ m) as a positive electrode core sheet, dried and rolled to form a positive electrode mixture layer.
- the obtained positive electrode was cut into a predetermined shape. In the obtained positive electrode, the thickness of the positive electrode mixture layer was 100 ⁇ m.
- Nonaqueous Electrolyte was prepared by dissolving LiPF 6 at a concentration of 1 mol / L in a nonaqueous solvent containing EC and MEC at a volume ratio of 30:70. And VC was added to the nonaqueous electrolyte as an additive. The concentration of VC in the non-aqueous electrolyte was 3% by mass.
- Battery assembly A square lithium ion secondary battery as shown in FIG. 2 was produced by the following procedure. Winding with a separator (polyethylene microporous film, Celgard A089, thickness 20 ⁇ m) interposed between the negative electrode and the positive electrode obtained in (3) and (4) above, and further from the side A flat electrode group having a substantially oval cross section was created by pressing. One end of the positive electrode lead was welded to the positive electrode, and one end of the negative electrode lead was welded to the negative electrode.
- a separator polyethylene microporous film, Celgard A089, thickness 20 ⁇ m
- the obtained electrode group was accommodated in a square battery case made of aluminum.
- the battery case has a bottom portion and a side wall, and an upper portion is open.
- the main flat part of the side wall was rectangular and the thickness was 80 ⁇ m.
- An insulating frame for preventing a short-circuit between the sealing plate, the battery case, and the negative electrode lead was disposed on the electrode group.
- the other end of the positive electrode lead drawn out from the electrode group was welded to a substantially rectangular sealing plate.
- a negative electrode terminal is disposed in the central portion of the sealing plate, and a gasket that insulates both is disposed between the negative electrode terminal and the sealing plate.
- the other end of the negative electrode lead pulled out from the electrode group was welded to the negative electrode terminal.
- the sealing plate was placed in the opening of the battery case, and the periphery of the sealing plate and the periphery of the opening of the battery case were sealed by laser welding.
- a non-aqueous electrolyte injection port was formed on the sealing plate, and 2.5 g of the non-aqueous electrolyte was injected into the battery case from the injection port.
- the square-shaped lithium ion secondary battery was obtained by plugging a liquid-injection port with a stopper by welding.
- the obtained prismatic lithium ion secondary battery had a height of 50 mm, a width of 34 mm, an inner space thickness of 5.2 mm, and a design capacity of 850 mAh.
- the physical properties (I) of the CMC-Na powder, the negative electrode mixture layer (II), and the battery evaluation (III) were performed according to the following procedures.
- (I) Physical properties of CMC-Na powder The etherification degree, average polymerization degree, and average particle diameter of CMC-Na powder were measured by the following methods. ⁇ Measurement of degree of etherification> The degree of etherification can be determined by boiling the incinerated sample with sulfuric acid, adding a phenolphthalein indicator, and back titrating excess acid with potassium hydroxide. This will be described in more detail below.
- the alkalinity A of CMC-Na was measured. Specifically, 1 g of CMC-Na powder was dissolved in 200 ml of water in a flask. 5 mL of 0.05 mol / mL sulfuric acid was added to this solution and boiled for 10 minutes. After boiling, it was cooled to room temperature. And the phenolphthalein indicator was added to the obtained solution, and it titrated with 0.1 mol / mL potassium hydroxide aqueous solution.
- the degree of etherification DS was measured by the following procedure.
- CMC-Na powder 0.5g was incinerated.
- the resulting ash, 250 mL of water, and 35 mL of 0.05 mol / mL sulfuric acid were added to a beaker and boiled for 30 minutes.
- the resulting solution was cooled to room temperature and titrated with a 0.1 mol / mL aqueous potassium hydroxide solution.
- DS 162W / (10000-80W)
- W (af′ ⁇ bf) / M a ⁇ A
- a Volume of sulfuric acid (mL)
- f ′ sulfuric acid titer (g / mL)
- b Volume of potassium hydroxide required for titration (mL) f, A, and M a are the same as above
- Negative electrode mixture layer The following procedure is used to determine the size of pores (recesses and voids), the number density of pores, the apparent density and variation of the negative electrode mixture layer, the binding strength, and the permeability of the nonaqueous electrolyte. It was evaluated with. ⁇ Average maximum depth of recess H> The negative electrode was embedded with an epoxy resin, cut so that a cross section in the thickness direction of the negative electrode mixture layer was exposed, and then the exposed cross section was chemically polished. An image of the polished surface was taken using a SEM (JSM-7800F, JEOL Ltd., acceleration voltage 10 kV) (magnification 100 times).
- ⁇ Average maximum diameter of holes R 1 and R 2 , number density of holes> An image of a cross section in the surface and plane direction of the negative electrode mixture layer was taken using a SEM (JSM-7800F manufactured by JEOL Ltd., acceleration voltage 10 kV) (magnification 100 times). In the photographed image of the surface, 200 vacancies (recess) arbitrarily selected maximum diameter of the opening of each recess is measured by calculating the average value, to obtain an average maximum diameter R 1. In cross-section captured image, arbitrarily selecting the 200 holes, the maximum diameter of the holes was determined by calculating the average value, to obtain an average maximum diameter R 2. Further, in the photographed image of the surface, an arbitrary 1 cm ⁇ 1 cm size area was selected, and the number density was obtained by counting the number of holes present in this area.
- the binding strength between the negative electrode active material particles in the negative electrode mixture layer was determined by the following procedure. First, a test piece of 2 cm long ⁇ 3 cm wide was cut out from the negative electrode. The negative electrode mixture layer was peeled off only from one surface of the test piece. The other surface of the test piece in which the negative electrode mixture layer remained was attached to the adhesive surface of a double-sided tape (No. 515, manufactured by Nitto Denko Corporation) attached on a glass plate. Subsequently, the negative electrode core material was peeled off from the test piece to expose the negative electrode mixture layer. In this way, a measurement sample in which the negative electrode mixture layer adhered to one side of the double-sided tape was produced.
- a double-sided tape No. 515, manufactured by Nitto Denko Corporation
- the adhesion surface peeled off from the glass plate of the measurement sample was attached to the tip of a probe (tip diameter: 0.2 cm) of a tacking tester (trade name: TAC-II, manufactured by Reska Co., Ltd.).
- a peeling test was performed by pressing the measurement probe against the negative electrode mixture layer of the measurement sample and pulling it off under the following conditions, and the maximum load at which peeling occurred between the active material particles was measured.
- the maximum load measured by dividing the cross-sectional area of the probe (0.031cm 2), was determined binding strength between the active material particles (N / cm 2). After the measurement of the binding strength, the peeling surface on the probe side of the negative electrode sample subjected to the measurement was observed to confirm that peeling occurred between the active material particles.
- Measurement probe indentation speed 30 mm / min
- Measurement probe indentation time 10 seconds
- Measurement probe indentation load 3.9
- Measurement probe pulling speed 600 mm / min
- nonaqueous electrolyte 5 ⁇ L of nonaqueous electrolyte was dropped on the surface of the negative electrode mixture layer of the negative electrode, and the time until the nonaqueous electrolyte completely penetrated into the negative electrode mixture layer was measured.
- a solution obtained by dissolving LiPF 6 at a concentration of 1 mol / L in a non-aqueous solvent containing EC and MEC at a volume ratio of 30:70 was used.
- Constant current charging Charging current value 850 mA / end-of-charge voltage 4.2 V
- Constant voltage charging Charging voltage value 4.2V / end-of-charge current 100mA
- Constant current discharge discharge current value 425 mA / discharge end voltage 3.0 V
- Constant current charging Charging current value 850 mA / end-of-charge voltage 4.2 V
- Constant voltage charging Charging voltage value 4.2V / end-of-charge current 100mA
- Constant current discharge discharge current value 1700 mA / discharge end voltage 3.0 V
- Example 2 to 4 and Comparative Example 1 In the production of CMC-Na powder (1), the sieve openings through which the coarse particles after pulverization pass are 80 ⁇ m (Example 2), 160 ⁇ m (Example 3), 200 ⁇ m (Example 4) or 50 ⁇ m (comparison).
- a CMC-Na powder was produced in the same manner as in Example 1 except that the procedure was changed to Example 1).
- a negative electrode and a battery were produced in the same manner as in Example 1 except that the obtained CMC-Na powder was used, and the same evaluation as in Example 1 was performed.
- Example 5 to 6 and Comparative Examples 2 to 3 CMC-Na powder was produced in the same manner as in Example 1 except that in the production of CMC-Na powder (1), the addition amounts of sodium hydroxide and monochloroacetic acid were changed. A negative electrode and a battery were produced in the same manner as in Example 1 except that the obtained CMC-Na powder was used, and the same evaluation as in Example 1 was performed.
- the amount of sodium hydroxide added was 12.7 g (Example 5), 8.6 g (Example 6), 29.1 g (Comparative Example 2), or 6.9 g (Comparative Example 3).
- the amount added was 39.1 g (Example 5), 19.6 g (Example 6), 66.8 g (Comparative Example 2), or 15.7 g (Comparative Example 3).
- Example 7 In the preparation of the negative electrode mixture slurry (2), the amount of the CMC-Na powder with respect to 100 parts by mass of natural graphite was 0.9 parts by mass (Example 7), 2.7 parts by mass (Example 8), 0.5 A negative electrode and a battery were produced in the same manner as in Example 1 except that the amount was changed to part by mass (Comparative Example 4) or 3.5 parts by mass (Comparative Example 5), and the same evaluation as in Example 1 was performed.
- Example 9 to 10 In the production of CMC-Na powder (1), the CMC-Na powder was prepared in the same manner as in Example 1 except that the reaction time at the boiling point was 100 minutes (Example 9) or 7 minutes (Example 10). Produced. A negative electrode and a battery were produced in the same manner as in Example 1 except that the obtained CMC-Na powder was used, and the same evaluation as in Example 1 was performed. Tables 1 and 2 show the results of Examples and Comparative Examples.
- Comparative Example 1 in which the average maximum diameter of pores (average maximum diameter R 1 , R 2 ) is less than 45 ⁇ m, although the binding strength is high, the nonaqueous electrolyte permeability is low and the discharge load characteristics are low. Decreased. Further, in Comparative Examples 2 to 4 in which the number density of pores was less than 8 or more than 17, the permeability of the non-aqueous electrolyte was lowered and the discharge load characteristics were lowered. In particular, in Comparative Example 3, the binding strength of the negative electrode mixture layer was greatly reduced.
- Comparative Example 5 in which the number density of the pores exceeds 17, the electrolyte permeability is high and the amount of the binder is relatively large. Therefore, even if the number density of the pores is large, the binding strength is high. The decline is not so noticeable. However, in Comparative Example 5, the discharge load characteristics deteriorated because the resistance during discharge increased due to the relatively large amount of the binder. For the same reason, the discharge characteristics and the battery capacity are likely to be reduced.
- the negative electrode of the present invention has high permeability to the non-aqueous electrolyte, when used in a lithium ion secondary battery, the input / output characteristics of the battery can be improved. Since the lithium ion secondary battery using such a negative electrode is excellent in output characteristics, it can be used in various applications, for example, a power source for driving a motor in a hybrid electric vehicle (particularly for a plug-in hybrid vehicle), a mobile phone, and a notebook personal computer. It can be used as a driving power source in various portable electronic devices such as a video camcorder, and a large power source in a household power storage device.
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Abstract
Description
例えば、特許文献1は、活物質層の表面に金型を押し当てることにより、溝やコップ状の凹部を形成することを開示する。そして、このような方法によれば、リチウムイオン二次電池を高温で連続使用した場合に、セパレータの軟化によりその表面の凹凸が変形して非水電解液を保持する空間が減少し、それにより、高出力特性が低下するという問題を改善できることを開示する。
本発明のリチウムイオン二次電池用負極は、負極芯材シートと負極芯材シートに支持された負極合剤層とを含み、負極合剤層は、負極活物質粒子と結着剤とを含む。そして、負極合剤層は、表面および内部に点在する複数の空孔を有し、空孔の平均最大径Rが45~125μmであり、空孔の個数密度が、負極合剤層の表面または面方向における断面1cm2あたり8~17個である。
図1はリチウムイオン二次電池用負極10の断面の様子を模式的に示す模式断面図である。
凹部の開口の平均最大径R1およびボイドの平均最大径R2のいずれか一方が、上記の平均最大径Rの範囲にあればよく、双方ともが、上記の平均最大径Rの範囲にあることがより好ましい。
(負極活物質粒子)
負極活物質粒子を構成する負極活物質としては、従来からリチウムイオン二次電池用負極の負極活物質として用いられている炭素質材料や、ケイ素や錫を含む合金系負極活物質が、特に限定なく用いられる。これらの中でも、特に、黒鉛構造を有する炭素質材料、具体的には、例えば、黒鉛(天然黒鉛、人造黒鉛など)、黒鉛化メソフェーズカーボンなどが特に好ましく用いられる。負極活物質は、一種を単独でまたは二種以上を組み合わせて使用できる。
なお、本明細書中、カルボキシアルキルセルロースの塩を含めて、単に、カルボキシアルキルセルロースと称する場合がある。
ここで、R2はボイドの平均最大径であり、L2はボイドの平均最大長さであり、nは負極合剤層の面方向における断面におけるボイドの個数密度であり、R1は凹部の開口部の平均最大径であり、Hは凹部の平均最大深さであり、Nは負極合剤層の表面における凹部の個数密度である。
見かけ密度のばらつきは、負極合剤層の任意の複数箇所(例えば、20箇所)について、上述のように見かけ密度を算出してその平均値を求め、見かけ密度の算出値のうち、最大または最小の値と、平均値との差で評価することができる。
負極芯材シートの材質としては、銅、銅合金、ステンレス鋼、ニッケルなどが例示できる。これらのうち、銅または銅合金が好ましい。銅合金中の銅以外の成分の含有量は、例えば、0.2モル%以下である。
負極芯材シートとしては、銅箔、特に、電解銅箔などが好ましい。
負極合剤層は、負極芯材シートの一方の表面に形成してもよく、両方の表面に形成してもよい。
本発明の負極は、負極合剤スラリーを、負極芯材シートの表面に塗布し、塗膜を乾燥し、乾燥した塗膜を圧延することにより形成される。
より具体的に説明すると、負極は、
(i)負極活物質粒子と、結着剤と、分散媒とを含む負極合剤スラリーを調製する工程と、
(ii)負極合剤スラリーを、負極芯材シートに塗布して塗膜を形成する工程と、
(iii)塗膜を加熱により乾燥する工程と、
(iv)乾燥した塗膜を、圧延することにより、負極芯材シートに支持された負極合剤層を形成する工程と、を経ることにより製造できる。
負極合剤スラリーの粘度は、例えば、2000~100000cP(=2~100Pa・s)程度であることが好ましい。
負極合剤スラリーの塗布量(塗膜の厚み)は、所望する負極合剤層の厚み、負極活物質の充填密度などに応じて適宜調整できる。
加熱温度は、分散媒の種類、乾燥時の雰囲気圧力などに応じて適宜選択できる。水を含む分散媒を用いる場合、加熱温度は、例えば、40~250℃,好ましくは50~200℃、さらに好ましくは60~150℃である。
圧延の回数は、例えば、1~5回、好ましくは1~3回である。
リチウムイオン二次電池は、正極と、上記の負極と、これらの間に介在するセパレータと、非水電解質とを備える。
リチウムイオン二次電池の構成について、図面を参照しながら、より詳細に説明する。
(正極)
正極は、例えば、正極芯材シートと、正極芯材シートに支持された正極合剤層とを含む。
正極芯材シートの材質としては、公知のもの、例えば、ステンレス鋼、アルミニウム、アルミニウム合金、チタンなどが例示できる。これらのうち、アルミニウムまたはアルミニウム合金などが好ましい。正極芯材シートは、負極芯材シートについて例示したものと同様の、無孔性または多孔性の導電性基板であってもよい。正極芯材シートとしては、アルミニウム箔などが好ましい。
正極合剤層は、正極芯材シートの一方の表面に形成してもよく、両方の表面に形成してもよい。正極合剤層の厚みは、負極合剤層の厚みと同様の範囲から選択できる。
正極合剤層は、必須成分として、正極活物質の粒子および結着剤を含み、任意成分として、導電剤を含む。
また、リチウム含有複合酸化物としては、LixNiyM1 zM2 1-(y+z)O2+d (1)も好ましい。式(1)において、M1は、上記元素Mに相当し、中でも、CoおよびMnよりなる群から選ばれる少なくとも1種の元素であることが好ましい。M2は、上記元素Mに相当し、中でも、Al、Cr、Fe、Mg、およびZnよりなる群から選ばれる少なくとも1種の元素であることが好ましい。式(1)において、x、yおよびdは、0.98≦x≦1.1、0≦z≦0.7、0.9≦(y+z)≦1および-0.01≦d≦0.01を満たす。
上記した各組成式において、リチウムのモル比は正極活物質合成直後の値であり、充放電により増減する。正極活物質は一種を単独でまたは二種以上を組み合わせて使用できる。
正極合剤層は、必要に応じて、増粘剤、公知の添加剤を含んでもよい。増粘剤としては、負極合剤層について例示したものと同様のものの他、CMC、CMC-Naなどのセルロースエーテル化合物などが例示できる。
セパレータとしては、樹脂製の、微多孔フィルム、不織布または織布などが使用できる。セパレータを構成する樹脂としては、例えば、ポリエチレン、ポリプロピレンなどのポリオレフィン;ポリアミド;ポリアミドイミド;ポリイミドなどが例示できる。
セパレータの厚みは、例えば、5~50μm、好ましくは10~30μmである。
非水電解質は、非水溶媒と、非水溶媒に溶解したリチウム塩とを含む。
非水溶媒としては、プロピレンカーボネート、エチレンカーボネート(EC)などの環状炭酸エステル;ジエチルカーボネート、エチルメチルカーボネート(MEC)、ジメチルカーボネートなどの鎖状炭酸エステル;γ-ブチロラクトン、γ-バレロラクトンなどの環状カルボン酸エステルなどが例示できる。これらの非水溶媒は、一種を単独でまたは二種以上を組み合わせて使用できる。
非水電解質中のリチウム塩の濃度は、例えば、0.5~2mol/Lである。
リチウムイオン二次電池の形状は、角型に限らず、円筒型、コイン型、偏平型などであってもよい。また、リチウムイオン二次電池は、ラミネート型であってもよい。
電池ケースの材料としては、ステンレス鋼鈑、アルミニウム、アルミニウム合金(マンガン、銅などの金属を微量含有する合金など)などの他、アルミニウム層などの金属層を樹脂フィルムで挟んだラミネートフィルムなどが使用できる。
(1)CMC-Na粉末の製造
破砕機((株)フジテックス製の二軸破砕機TIGER SHRED)を用いて、パルプ片を破砕することによりパルプ粉を得た。そして、得られたパルプ粉191gと濃度88質量%の2-プロパノール水溶液5,730mLを45℃で攪拌混合してスラリーを調製した。得られたスラリーを室温まで自然冷却した後、水酸化ナトリウムを13.7g添加することによりアルカリセルロースを得た。そして、アルカリセルロースのスラリーを10℃以下まで氷冷した。次に、冷却されたアルカリセルロースのスラリーにモノクロル酢酸を31.4g添加して5分間撹拌した後、5℃に冷却して2時間静置した。
得られたCMC-Na粉末を適量の水と混合した。次に、負極活物質としての天然黒鉛(平均粒子径20μm)と、CMC-Na粉末を含む混合液(固形分濃度1質量%)と、SBR(平均粒子径150nm、ガラス転移点25℃)を含む水分散液(JSR(株)製、SBR含有量48質量%)とを混合することにより、負極合剤スラリーを調製した。なお、天然黒鉛100質量部に対して、CMC-Na粉末およびSBRの量は、それぞれ、1質量部とした。
上記(2)で得られた負極合剤スラリーを、負極芯材シートとしての電解銅箔(厚み10μm)の両面に、ダイコーターを用いて塗布した。そして、塗膜を、60℃で10時間乾燥した。乾燥された塗膜を、負極芯材シートとともに、圧延ローラにて、線圧490N/cmで圧延することにより負極合剤層を形成した。得られた負極を、所定の形状に裁断した。
得られた負極合剤層は、厚みが120μmであり、空孔を除いた領域における見かけ密度が1.6g/cm3であった。
正極活物質であるLiCoO2 100質量部と、結着剤であるPVDF4質量部と、適量のNMPとを混合することにより、正極合剤スラリーを調製した。得られた正極合剤スラリーを、正極芯材シートであるアルミニウム箔(厚み15μm)の両面に塗布し、乾燥および圧延することにより正極合剤層を形成した。得られた正極を、所定の形状に裁断した。得られた正極において、正極合剤層の厚みは100μmであった。
ECとMECとを体積比30:70で含む非水溶媒に、1mol/Lの濃度でLiPF6を溶解させることにより、非水電解質を調製した。そして、非水電解質に、添加剤としてVCを添加した。非水電解質中のVCの濃度は、3質量%とした。
次のような手順で、図2に示すような角型リチウムイオン二次電池を作製した。
上記(3)および(4)で得られた負極および正極の間に、セパレータ(ポリエチレン製微多孔質フィルム、セルガード(株)製A089、厚み20μm)を介在させた状態で捲回し、さらに側面からプレスすることにより断面が略楕円形である扁平状の電極群を作成した。なお、正極には、正極リードの一端を溶接し、負極には、負極リードの一端を溶接した。
(I)CMC-Na粉末の物性
CMC-Na粉末のエーテル化度、平均重合度、平均粒子径の測定は、次の方法で行った。
<エーテル化度の測定>
エーテル化度は、灰化したサンプルを硫酸にて煮沸し、フェノールフタレイン指示薬を加え、過剰の酸を水酸化カリウムで逆滴定することにより求めることができる。以下に、より詳細に説明する。
具体的には、フラスコ中でCMC-Na粉末1gを水200mlに溶解させた。この溶液に0.05mol/mL濃度の硫酸を5mL加えて10分間煮沸した。煮沸後、室温まで冷却した。そして、得られた溶液にフェノールフタレイン指示薬を加え、0.1mol/mLの水酸化カリウム水溶液で滴定した。
A=(e-da)f/Ma・・・(a)
e:空試験の滴定の際に要した水酸化カリウム水溶液の体積(mL)
da:CMC-Naと硫酸の溶液の滴定に要した水酸化カリウム水溶液の体積(mL)
f:水酸化カリウム水溶液の力価(g/mL)
Ma:CMC-Na粉末の質量
CMC-Na粉末0.5gを灰化した。得られた灰分と、水250mLと、0.05mol/mLの硫酸35mLとを、そしてビーカーに加え、30分間煮沸した。得られた溶液を室温まで冷却した後、0.1mol/mLの水酸化カリウム水溶液で滴定した。
DS=162W/(10000-80W) ・・・(b)
W=(af'-bf)/Ma-A ・・・(c)
a:硫酸の体積(mL)
f’:硫酸の力価(g/mL)
b:滴定に要した水酸化カリウムの体積(mL)
f、A、およびMaは上記に同じ
毛細管粘度計((株)草野化学製、キャノンフェンスケ型)にて、極限粘度ηを求めた。得られた値を下記式(d)に代入することにより、質量平均分子量Mmを算出した。そして、質量平均分子量Mmを、平均重合度に換算した。
η=6.46×10-16×Mm ・・・(d)
乾燥状態のCMC-Na粉末を、エタノールに分散させた分散液を用いて、レーザー回折/散乱式の粒度分布測定装置(日機装(株)製のマイクロトラック)により体積基準の粒度分布における累積体積50%における粒子径(平均粒子径D50v)を求めた。
空孔(凹部およびボイド)の寸法、空孔の個数密度、負極合剤層の見かけ密度およびそのばらつき、結着強度、ならびに非水電解質の浸透性を、以下の手順で評価した。
<凹部の平均最大深さH>
負極を、エポキシ樹脂で包埋して、負極合剤層の厚み方向の断面が露出するようにカットし、次いで露出した断面を化学研磨した。研磨面の画像を、SEM(日本電子(株)製JSM-7800F、加速電圧10kV)を用いて撮影した(倍率100倍)。
負極合剤層の表面および面方向における断面の画像を、SEM(日本電子(株)製JSM-7800F、加速電圧10kV)を用いて撮影した(倍率100倍)。
表面の撮影画像において、200個の空孔(凹部)を任意に選択し、各凹部の開口部の最大径を測定し、平均値を算出することにより、平均最大径R1を求めた。
断面の撮影画像において、200個の空孔を任意に選択し、各空孔の最大径を測定し、平均値を算出することにより、平均最大径R2を求めた。
また、表面の撮影画像において、任意の1cm×1cmのサイズの領域を選択し、この領域に存在する空孔の個数を計数することにより、個数密度を求めた。
空孔を除いた負極合剤層の見かけ密度およびそのばらつき既述の方法により測定した。なお、見かけ密度の平均値は、負極合剤層の任意の20箇所について測定した見かけ密度から算出し、この平均値に基づいて、見かけ密度のばらつきを求めた。
以下の手順で、負極合剤層における負極活物質粒子間の結着強度を求めた。
まず、負極から、縦2cm×横3cmの試験片を切り出した。試験片の一方の面からのみ、負極合剤層を剥がした。負極合剤層が残っている試験片の他方の面を、ガラス板上に貼り付けた両面テープ(品番:No.515、日東電工(株)製)の接着面に貼り付けた。次いで、試験片から負極芯材を剥離して負極合剤層を露出させた。このようにして、両面テープの片面に負極合剤層が付着した測定用試料を作製した。
なお、結着強度の測定後には、測定に供した負極試料の測定子側の剥離面を観察し、活物質粒子間で剥離が起こっていることを確認した。
測定プローブの押し込み速度:30mm/分
測定プローブの押し込み時間:10秒
測定プローブの押し込み荷重:3.9N
測定プローブの引き離し速度:600mm/分
負極合剤層の非水電解質に対する浸透性の指標として、負極合剤層の電解質の吸液性を下記の手順で評価した。なお、非水電解質に対する浸透性は、負極合剤層中での非水電解質の移動の容易さの目安となる。
角型リチウムイオン二次電池を、20℃の環境下、以下の条件で、2回充放電した。なお、2サイクル目の放電容量を1サイクル目の放電容量で除した値を放電負荷特性として求めた。
定電流充電:充電電流値850mA/充電終止電圧4.2V
定電圧充電:充電電圧値4.2V/充電終止電流100mA
定電流放電:放電電流値425mA/放電終止電圧3.0V
定電流充電:充電電流値850mA/充電終止電圧4.2V
定電圧充電:充電電圧値4.2V/充電終止電流100mA
定電流放電:放電電流値1700mA/放電終止電圧3.0V
CMC-Na粉末の製造(1)において、粉砕した後の粗粒を通過させる篩の目開きを、80μm(実施例2)、160μm(実施例3)、200μm(実施例4)または50μm(比較例1)に変更する以外は、実施例1と同様にしてCMC-Na粉末を作製した。得られたCMC-Na粉末を用いる以外は、実施例1と同様にして負極および電池を作製し、実施例1と同様の評価を行った。
CMC-Na粉末の製造(1)において、水酸化ナトリウムおよびモノクロル酢酸の添加量を変更する以外は、実施例1と同様にしてCMC-Na粉末を作製した。得られたCMC-Na粉末を用いる以外は、実施例1と同様にして負極および電池を作製し、実施例1と同様の評価を行った。
水酸化ナトリウムの添加量は、12.7g(実施例5)、8.6g(実施例6)、29.1g(比較例2)、または6.9g(比較例3)であり、モノクロル酢酸の添加量は、39.1g(実施例5)、19.6g(実施例6)、66.8g(比較例2)、または15.7g(比較例3)であった。
負極合剤スラリーの調製(2)において、天然黒鉛100質量部に対するCMC-Na粉末の量を、0.9質量部(実施例7)、2.7質量部(実施例8)、0.5質量部(比較例4)、または3.5質量部(比較例5)に変更する以外は、実施例1と同様にして負極および電池を作製し、実施例1と同様の評価を行った。
CMC-Na粉末の製造(1)において、沸点で反応させる時間を、100分間(実施例9)または7分間(実施例10)にする以外は、実施例1と同様にしてCMC-Na粉末を作製した。得られたCMC-Na粉末を用いる以外は、実施例1と同様にして負極および電池を作製し、実施例1と同様の評価を行った。
実施例および比較例の結果を、表1および表2に示す。
2 負極合剤層
3 負極活物質粒子
4 カルボキシメチルセルロースのナトリウム塩
5 ゴム粒子
6 凹部
7 ボイド
11 角形電池ケース
12 封口板
13 負極端子
14 正極リード
15 負極リード
16 ガスケット
17 封栓
17a 注液孔
18 絶縁性枠体
21 リチウムイオン二次電池
Claims (12)
- 負極芯材シートと前記負極芯材シートに支持された負極合剤層とを含み、
前記負極合剤層は、負極活物質粒子と結着剤とを含み、
前記負極合剤層は、表面および内部に点在する複数の空孔を有し、
前記空孔の平均最大径Rが45~125μmであり、
前記空孔の個数密度が、前記負極合剤層の表面1cm2あたりまたは面方向における断面1cm2あたり8~17個である、リチウムイオン二次電池用負極。 - 前記空孔が、前記負極合剤層の表面に点在する複数の凹部と、前記負極合剤層の内部に点在する複数のボイドとを含み、
前記凹部の平均最大深さHが45~125μmである、請求項1記載のリチウムイオン二次電池用負極。 - 前記結着剤が、カルボキシメチルセルロースのナトリウム塩を含み、
前記カルボキシメチルセルロースのナトリウム塩が、0.23~0.7のエーテル化度と、20~1600の平均重合度を有する、請求項1または2記載のリチウムイオン二次電池用負極。 - 前記負極合剤層が、前記負極活物質粒子100質量部に対して、前記カルボキシメチルセルロースのナトリウム塩を0.8~3質量部含む、請求項3記載のリチウムイオン二次電池用負極。
- 前記カルボキシメチルセルロースのナトリウム塩が、平均粒子径30~100μmの粒子状である、請求項3または4記載のリチウムイオン二次電池用負極。
- 前記結着剤が、さらにゴム状材料を含む、請求項3~5のいずれか1項記載のリチウムイオン二次電池用負極。
- 前記負極合剤層中の前記ゴム状材料の含有量が、前記負極活物質粒子100質量部に対して、0.5~1.5質量部である、請求項6記載のリチウムイオン二次電池用負極。
- 前記負極合剤層の前記空孔を除いた領域における、見かけ密度のばらつきが、平均値に対して1%未満である、請求項1~7のいずれか1項記載のリチウムイオン二次電池用負極。
- 前記負極活物質粒子を構成する負極活物質が、黒鉛構造を有する炭素質材料であり、
前記空孔を除いた前記負極合剤層の見かけ密度が、1.5~1.8g/cm3である、請求項1~8のいずれか1項に記載のリチウムイオン二次電池用負極。 - 負極活物質粒子と、結着剤と、少なくとも水を含む分散媒とを含む負極合剤スラリーを調製する工程と、
前記負極合剤スラリーを、負極芯材シートに塗布して塗膜を形成する工程と、
前記塗膜を乾燥する工程と、
乾燥した前記塗膜を、圧延することにより、前記負極芯材シートに支持された負極合剤層を形成する工程と、を含み、
前記結着剤が、カルボキシメチルセルロースのナトリウム塩を含み、
前記カルボキシメチルセルロースのナトリウム塩が、0.23~0.7のエーテル化度、および20~1600の平均重合度を有する、リチウムイオン二次電池用負極の製造方法。 - 前記結着剤が、さらにゴム状材料を含む、請求項10記載のリチウムイオン二次電池用負極の製造方法。
- 正極と、負極と、前記正極および前記負極の間に介在するセパレータと、非水電解質とを備え、
前記負極が、請求項1~9のいずれか1項記載のリチウムイオン二次電池用負極である、リチウムイオン二次電池。
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US9362550B2 (en) | 2016-06-07 |
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CN104025342A (zh) | 2014-09-03 |
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