WO2011036759A1 - リチウム二次電池及びその製造方法 - Google Patents
リチウム二次電池及びその製造方法 Download PDFInfo
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- WO2011036759A1 WO2011036759A1 PCT/JP2009/066600 JP2009066600W WO2011036759A1 WO 2011036759 A1 WO2011036759 A1 WO 2011036759A1 JP 2009066600 W JP2009066600 W JP 2009066600W WO 2011036759 A1 WO2011036759 A1 WO 2011036759A1
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- active material
- electrode active
- material layer
- positive electrode
- negative electrode
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- 229910052744 lithium Inorganic materials 0.000 title claims abstract description 131
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 title claims abstract description 130
- 238000000034 method Methods 0.000 title claims description 21
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Images
Classifications
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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
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
- B60L50/00—Electric propulsion with power supplied within the vehicle
- B60L50/50—Electric propulsion with power supplied within the vehicle using propulsion power supplied by batteries or fuel cells
-
- 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
- H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
-
- 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
- H01M4/1391—Processes of manufacture of electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
-
- 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
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
- H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
- H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/058—Construction or manufacture
-
- 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
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
- H01M2010/4292—Aspects relating to capacity ratio of electrodes/electrolyte or anode/cathode
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M2220/00—Batteries for particular applications
- H01M2220/20—Batteries in motive systems, e.g. vehicle, ship, plane
-
- 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
-
- 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
-
- 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
- Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
- Y02T10/00—Road transport of goods or passengers
- Y02T10/60—Other road transportation technologies with climate change mitigation effect
- Y02T10/70—Energy storage systems for electromobility, e.g. batteries
Definitions
- the present invention relates to a lithium secondary battery. Specifically, the present invention relates to a lithium secondary battery suitably used as a vehicle-mounted power source under high-rate charge / discharge and a method for manufacturing the battery.
- a lithium secondary battery (typically a lithium ion battery) that is lightweight and has a high energy density is expected to be preferably used as a high-output power source for mounting on a vehicle (for example, an automobile, particularly a hybrid automobile or an electric automobile).
- an electrode active material layer (specifically, a positive electrode active material layer and a negative electrode capable of reversibly inserting and extracting lithium ions on the surface of the electrode current collector). Active material layer).
- a paste-like composition in which a positive electrode active material such as a lithium transition metal composite oxide is dispersed in an appropriate solvent (a slurry-like composition is included in the paste-like composition.
- This type of composition is simply referred to as a paste.)
- some secondary batteries are supposed to be used in such a manner that high-rate pulse charge / discharge in which a large current flows instantaneously is repeated in a short time.
- a lithium secondary battery used as a high output power source mounted on a vehicle is a typical example, but a battery used in such a mode is an electrode accompanying movement of a charge carrier as compared with a battery used in a household electric product. Since the load on the active material layer is large, internal resistance may increase due to repeated charge and discharge. As one of the causes of such an increase in internal resistance, the amount of electrolyte retained in the gap formed in the electrode active material layer and the ion concentration distribution balance in the electrolyte are biased toward one electrode. Etc.
- the amount of electrolyte impregnation per predetermined area of the positive electrode active material layer and the negative electrode active material layer is calculated as the electrolyte retention capacity, respectively, and the electrolyte retention capacity (a) of the positive electrode active material layer and the negative electrode active capacity are calculated.
- a lithium secondary battery in which the relationship between the material layer and the electrolyte solution holding capacity (b) satisfies 0.9 ⁇ a / b ⁇ 1.3 is disclosed.
- the suitable liquid amount of electrolyte solution with respect to the sum total of the void volume of each of a positive electrode, a negative electrode, and a separator is examined.
- Patent Document 3 the percentage of void volume of the positive electrode active material layer (V p) and the void volume of the negative electrode active material layer (V n), 0.3 ⁇ ( V p / V n) ⁇ 0.5 A lithium secondary battery that satisfies the requirements is disclosed.
- the present invention was created to solve the above-described conventional problems related to lithium secondary batteries, and the object of the present invention is to determine the relative void volume of each of the positive electrode active material layer and the negative electrode active material layer. It is intended to provide a lithium secondary battery having excellent battery characteristics (cycle characteristics or high rate characteristics) as a vehicle-mounted high-output power source that suppresses an increase in internal resistance and a manufacturing method thereof. Another object is to provide a vehicle including such a lithium secondary battery.
- a positive electrode having a positive electrode active material layer including a positive electrode active material formed on the surface of a positive electrode current collector, and a negative electrode including a negative electrode active material formed on the surface of the negative electrode current collector There is provided a lithium secondary battery including a negative electrode having an active material layer.
- the positive electrode active material of the lithium secondary battery according to the present invention has at least lithium and nickel and / or cobalt as main constituent elements (typically nickel and / or cobalt among constituent metal elements other than lithium).
- the positive electrode active material layer has a porosity of 30% or more and 40% or less, and the negative electrode active material layer has a porosity of 30. % To 45%.
- the void volume ratio (Sa / Sb) between the void volume (Sa) per unit area in the positive electrode active material layer and the void volume (Sb) per unit area in the negative electrode active material layer is 0.9 ⁇ ( Sa / Sb) ⁇ 1.4 is satisfied.
- the “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by movement of lithium ions between the positive and negative electrodes.
- a secondary battery generally referred to as a lithium ion battery is a typical example included in the lithium secondary battery in this specification.
- the “positive electrode active material” refers to a positive electrode capable of reversibly occluding and releasing (typically inserting and desorbing) chemical species (here, lithium ions) that serve as charge carriers in a secondary battery.
- the “negative electrode active material” refers to a negative electrode material capable of reversibly occluding and releasing the chemical species.
- the “porosity” refers to the ratio of the volume of the void (space) portion existing inside the positive electrode active material layer or the negative electrode active material layer to the entire volume.
- the lithium secondary battery for a high-output power source used in a mode where high-rate pulse charge / discharge is repeated in a short time is a reaction in the electrolyte solution on the positive electrode side during discharge (lithium ions occluded on the negative electrode side). Is moved to the positive electrode side).
- the present inventor makes the reaction on the positive electrode side during discharge into a diffusion-controlled state by forming the voids in the positive electrode active material layer to be equal to or larger than the void volume in the negative electrode active material layer, and the internal resistance is reduced. It was found that the increase can be suppressed. Therefore, the lithium secondary battery disclosed herein has a void volume ratio (Sa /) between the void volume (Sa) per unit area in the positive electrode active material layer and the void volume (Sb) per unit area in the negative electrode active material layer.
- the lithium composite oxide constituting the positive electrode active material has the following formula: Li (Ni 1-x Co x ) O 2 (1) (X in the formula (1) is a composite oxide represented by 0 ⁇ x ⁇ 0.5).
- a positive electrode active material for a lithium secondary battery according to a preferred embodiment is composed of a lithium composite oxide containing nickel, which has a large theoretical lithium ion storage capacity and is inexpensive, and cobalt which improves electronic conductivity.
- the lithium complex oxide is configured such that the cobalt molar ratio x satisfies the relationship of 0 ⁇ x ⁇ 0.5, and the nickel molar ratio is larger than the cobalt molar ratio.
- the void volume (Sa) per unit area in the positive electrode active material layer and the void volume (Sb) per unit area in the negative electrode active material layer The void volume ratio (Sa / Sb) satisfies 1 ⁇ (Sa / Sb) ⁇ 1.1.
- the void volume of the positive electrode active material layer is too small, it is not preferable because the reaction in the electrolyte solution on the positive electrode side during high-rate discharge stagnates as described above, but on the other hand, if the void volume of the positive electrode active material layer is too large, This time, the amount of electrolyte retained in the positive electrode active material layer becomes too large, and the amount of electrolyte retained in the voids in the negative electrode active material layer decreases, resulting in an increase in internal resistance.
- the positive electrode active material layer has a layer density of 2 g / cm 3 or more and 2.5 g / cm 3 or less.
- layer density refers to the density of solid content forming the positive electrode active material layer. The smaller the layer density of the positive electrode active material layer, the larger the void volume of the positive electrode active material layer. Therefore, in order to control the diffusion reaction on the positive electrode side during discharge, the positive electrode active material layer has a layer density of 2 g / cm 3 or more and 2.5 g / cm 3 or less, whereby a void volume is suitably formed and charge transfer is performed. Is performed with high efficiency. As a result, it is possible to provide a lithium secondary battery in which an increase in internal resistance is suppressed even when high-rate pulse charge / discharge is repeated.
- the positive electrode having a positive electrode active material layer including a positive electrode active material formed on the surface of the positive electrode current collector and the surface of the negative electrode current collector are provided as other aspects for realizing the above object.
- a method for manufacturing a lithium secondary battery comprising a negative electrode having a negative electrode active material layer containing a negative electrode active material.
- the production method disclosed herein has, as the positive electrode active material, at least lithium and nickel and / or cobalt as main constituent elements (typically nickel and / or of constituent metal elements other than lithium).
- the active material layer is formed so that the porosity of the positive electrode active material layer is 30% or more and 40% or less, and the negative electrode The active material layer is formed so that the porosity of the active material layer is 30% to 45%.
- the void volume ratio (Sa / Sb) between the void volume (Sa) per unit area in the positive electrode active material layer and the void volume (Sb) per unit area in the negative electrode active material layer is 0.9 ⁇ ( The positive electrode active material layer and the negative electrode active material layer are formed so as to satisfy Sa / Sb) ⁇ 1.4.
- the lithium secondary battery used in a mode in which high-rate pulse charging / discharging in which a large current flows instantaneously is repeated in a short time is a reaction in the electrolyte solution on the positive electrode side during discharge (lithium ions occluded on the negative electrode side (Which moves to the positive electrode side) is diffusion-controlled. Therefore, the present inventor makes the reaction on the positive electrode side during discharge into a diffusion-controlled state by forming the voids in the positive electrode active material layer to be approximately the same as or larger than the void volume in the negative electrode active material layer, It has been found that an increase in resistance can be suppressed.
- the void volume ratio (Sa / Sb) between the void volume (Sa) per unit area in the positive electrode active material layer and the void volume (Sb) per unit area in the negative electrode active material layer is 0.9.
- ⁇ (Sa / Sb) ⁇ 1.4 is satisfied, and the positive electrode active material layer has a porosity of 30% to 40% and a negative electrode active material layer has a porosity of 30% to 45%.
- a material layer and a negative electrode active material layer are formed. Thereby, the amount of the electrolyte solution retained in the gap is suitably maintained in each electrode active material layer, and the ion concentration distribution balance in the electrolyte solution is not biased to one electrode side even under high rate pulse charge / discharge, An increase in internal resistance can be suppressed. As a result, it is possible to provide a method for producing a lithium secondary battery having excellent battery characteristics (cycle characteristics or high rate characteristics) as a vehicle-mounted high-output power source, particularly good low-temperature cycle characteristics under low-temperature pulse charge / discharge conditions. it can.
- the lithium composite oxide constituting the positive electrode active material is represented by the following formula: Li (Ni 1-x Co x ) O 2 (1) (X in the formula (1) uses a composite oxide represented by 0 ⁇ x ⁇ 0.5).
- a preferred embodiment of the positive electrode active material made of a lithium composite oxide satisfying the above formula (1) includes nickel and cobalt as constituent metal elements other than lithium.
- the composite oxide containing nickel has a large theoretical lithium ion storage capacity, and the raw material cost can be kept low.
- electronic conductivity is improved by containing cobalt with a molar ratio smaller than the molar ratio of nickel. Therefore, a lithium secondary battery having excellent battery characteristics (cycle characteristics or high rate characteristics) can be manufactured by using a composite oxide having such a composition ratio as a positive electrode active material.
- the void volume ratio (Sa / Sb) between the void volume (Sa) per unit area in the positive electrode active material layer and the void volume (Sb) per unit area in the negative electrode active material layer is 1 ⁇
- the positive electrode active material layer and the negative electrode active material layer are formed so as to satisfy (Sa / Sb) ⁇ 1.1.
- each active material layer such that the void volume ratio (Sa / Sb) between the positive electrode active material layer and the negative electrode active material layer satisfies 1 ⁇ (Sa / Sb) ⁇ 1.1, the internal resistance Is further suppressed, and a lithium secondary battery having more excellent battery characteristics (cycle characteristics or high-rate characteristics), in particular, good low-temperature cycle characteristics under low-temperature pulse charge / discharge can be manufactured.
- the active material layer is formed so that the positive electrode active material layer has a layer density of 2 g / cm 3 or more and 2.5 g / cm 3 or less.
- a void volume is suitably formed in the active material layer.
- a vehicle provided with any lithium secondary battery disclosed herein (which may be a lithium secondary battery produced by any production method disclosed herein).
- the lithium secondary battery provided by the present invention has battery characteristics (cycle characteristics or high-rate characteristics) particularly suitable as a power source for batteries mounted on vehicles, particularly good low-temperature cycle characteristics under low-temperature pulse charge / discharge. May be shown. Therefore, the lithium secondary battery disclosed herein can be suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile equipped with an electric motor such as a hybrid vehicle or an electric vehicle.
- FIG. 1 is a perspective view schematically showing the outer shape of a lithium secondary battery according to an embodiment.
- 2 is a cross-sectional view taken along line II-II in FIG.
- FIG. 3 is a perspective view schematically showing the shape of the 18650 type lithium secondary battery produced in the example.
- FIG. 4 is a graph showing the relationship between the void volume ratio and the resistance increase rate.
- FIG. 5 is a side view schematically showing a vehicle (automobile) including the lithium secondary battery according to the embodiment.
- the lithium secondary battery provided by the present invention is suitably used particularly as a high output power source by having the above-described configuration.
- Lithium secondary batteries which are used for a long time in a manner that repeats high-rate pulse charging / discharging in a short time to flow a large current instantaneously, charge / discharge because the load on the electrode active material layer accompanying the movement of charge carriers (lithium ions) is large.
- the amount of the electrolytic solution held in the gap formed in the electrode active material layer and the ion concentration distribution balance in the electrolytic solution are biased toward one electrode side, and the internal resistance tends to increase.
- the present inventor has observed that the reaction in the electrolyte solution on the positive electrode side during discharge (lithium ions occluded on the negative electrode side moves to the positive electrode side) is diffusion-controlled, and the positive electrode active material
- the aspect of the void in the electrode active material layer more specifically by defining it from the relative ratio of the void volume of the layer and the negative electrode active material layer and the respective suitable void ratio, It was found that the increase can be suppressed.
- the positive electrode active material layer includes a positive electrode active material capable of inserting and extracting lithium ions.
- a positive electrode active material capable of inserting and extracting lithium ions.
- the positive electrode active material of the lithium secondary battery disclosed herein at least lithium (Li) and nickel (Ni) and / or cobalt (Co) as main constituent elements (typically other than lithium) Lithium composite oxide is used in which the total molar composition ratio of nickel and / or cobalt among the constituent metal elements is 50% or more.
- a more preferable positive electrode active material is a composite oxide having lithium, nickel, and cobalt as essential constituent elements, and has the following formula: Li (Ni 1-x Co x ) O 2 (1) (X in the formula (1) satisfies the condition 0 ⁇ x ⁇ 0.5).
- Such a complex oxide includes nickel, which has a large theoretical lithium ion storage capacity and is inexpensive, and cobalt which improves electronic conductivity.
- the molar ratio of nickel in the lithium composite oxide is configured so as to be larger than the molar ratio of cobalt.
- the composite oxide may contain at least one or more metal elements other than lithium, nickel, and cobalt, typically in a smaller proportion than cobalt and nickel.
- minute elements include aluminum (Al), manganese (Mn), chromium (Cr), iron (Fe), vanadium (V), magnesium (Mg), titanium (Ti), zirconium (Zr), and niobium.
- Nb molybdenum (Mo), tungsten (W), copper (Cu), zinc (Zn), gallium (Ga), indium (In), tin (Sn), lanthanum (La) and cerium (Ce) It may be one or more metal elements selected from the group.
- the lithium composite oxide for example, a lithium composite oxide powder prepared and provided by a conventionally known method can be used as it is.
- the oxide can be prepared by mixing several raw material compounds appropriately selected according to the atomic composition at a predetermined molar ratio and firing by an appropriate means.
- a granular lithium composite oxide powder substantially composed of secondary particles having a desired average particle size and / or particle size distribution is obtained.
- the particle size of the lithium composite oxide is not particularly limited.
- the positive electrode active material layer may contain optional components such as a conductive material and a binder as necessary in addition to the positive electrode active material.
- a conductive powder material such as carbon powder or carbon fiber is preferably used.
- carbon powder various carbon blacks such as acetylene black, furnace black, ketjen black, and graphite powder are preferable.
- conductive fibers such as carbon fibers and metal fibers, metal powders such as copper and nickel, and organic conductive materials such as polyphenylene derivatives can be contained alone or as a mixture thereof. In addition, only 1 type may be used among these, or 2 or more types may be used together.
- the binder the same binder as that used for the positive electrode of a general lithium secondary battery can be appropriately employed. It is preferable to select a polymer that is soluble or dispersible in the solvent used.
- cellulose polymers such as carboxymethyl cellulose (CMC) and hydroxypropyl methyl cellulose (HPMC); polyvinyl alcohol (PVA); polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene
- PVA polytetrafluoroethylene
- PTFE polytetrafluoroethylene-hexafluoropropylene
- Water-soluble or water-dispersible polymers such as fluorine resins such as polymers (FEP); vinyl acetate copolymers; rubbers such as styrene butadiene rubber (SBR) and acrylic acid-modified SBR resins (SBR latex); Can be adopted.
- a polymer such as polyvinylidene fluoride (PVDF) or polyvinylidene chloride (PVDC) can be preferably used.
- PVDF polyvinylidene fluoride
- PVDC polyvinylidene chloride
- Such a binder may be used individually by 1 type, and may be used in combination of 2 or more type.
- the polymer material exemplified above may be used for the purpose of exhibiting the function as a thickener or other additive of the above composition.
- any of an aqueous solvent and a non-aqueous solvent can be used.
- the aqueous solvent is typically water, but may be any water-based solvent as a whole, that is, water or a mixed solvent mainly composed of water can be preferably used.
- the solvent other than water constituting the mixed solvent one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used.
- a particularly preferred example is a solvent consisting essentially of water.
- preferable examples of the non-aqueous solvent include N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone, toluene and the like.
- the positive electrode active material is mixed with an appropriate solvent (aqueous solvent or non-aqueous solvent) together with a conductive material, a binder and the like to prepare a paste or slurry-like paste for forming a positive electrode active material layer.
- an appropriate solvent aqueous solvent or non-aqueous solvent
- the ratio of the positive electrode active material to the positive electrode active material layer is preferably about 50% by mass or more (typically 50 to 95% by mass), and about 70 to 95% by mass. % (For example, 75 to 90% by mass) is more preferable.
- the proportion of the conductive material in the positive electrode active material layer can be, for example, about 2 to 20% by mass, and is usually preferably about 2 to 15% by mass. Further, in the composition using the binder, the proportion of the binder in the positive electrode active material layer can be, for example, about 1 to 10% by mass, and usually about 2 to 5% by mass.
- the paste prepared by mixing the constituent materials in this way is applied to the positive electrode current collector 32, and after the solvent is evaporated and dried, the paste is compressed (pressed). Thereby, the positive electrode of the lithium secondary battery in which the positive electrode active material layer is formed on the positive electrode current collector is obtained.
- a conductive member made of a highly conductive metal is preferably used as the positive electrode current collector to which the paste is applied.
- a conductive member made of a highly conductive metal is preferably used.
- aluminum or an alloy containing aluminum as a main component can be used.
- the shape of the positive electrode current collector can vary depending on the shape of the lithium secondary battery, and is not particularly limited, and may be various forms such as a rod shape, a plate shape, a sheet shape, a foil shape, and a mesh shape.
- the paste can be suitably applied to the positive electrode current collector by using an appropriate application device such as a slit coater, a die coater, a gravure coater, or a comma coater.
- an appropriate application device such as a slit coater, a die coater, a gravure coater, or a comma coater.
- it can dry favorably by using natural drying, a hot air, low-humidity air, a vacuum, infrared rays, far-infrared rays, and an electron beam individually or in combination.
- a conventionally known compression method such as a roll press method or a flat plate press method can be employed. In adjusting the thickness, the thickness may be measured with a film thickness measuring instrument, and the press pressure may be adjusted to compress a plurality of times until a desired thickness is obtained.
- the negative electrode disclosed here has a negative electrode active material layer containing a negative electrode active material formed on the surface of a negative electrode current collector.
- a conductive member made of a metal having good conductivity is preferably used as the negative electrode current collector.
- copper or an alloy containing copper as a main component can be used as the negative electrode current collector.
- the shape of the negative electrode current collector is not particularly limited because it can be different depending on the shape of the lithium secondary battery as in the case of the positive electrode current collector.
- a negative electrode active material the 1 type, or 2 or more types of the substance conventionally used for a lithium secondary battery can be used without limitation.
- a carbon particle is mentioned as a suitable negative electrode active material.
- a particulate carbon material (carbon particles) containing a graphite structure (layered structure) at least partially is preferably used. Any carbon material of a so-called graphitic material (graphite), non-graphitizable carbon material (hard carbon), easily graphitized carbon material (soft carbon), or a combination of these materials is preferably used. obtain.
- graphite particles can be preferably used. Since the graphite particles can suitably occlude lithium ions as charge carriers, they are excellent in conductivity.
- the particle size is small and the surface area per unit volume is large, it can be a negative electrode active material suitable for high-rate pulse charge / discharge.
- various polymer materials that can function as the binders listed in the constituent elements of the positive electrode can be suitably used for the negative electrode active material layer.
- the negative electrode active material is mixed with a binder or the like in an appropriate solvent (water, organic solvent and mixed solvent thereof) to prepare a paste or slurry-like paste for forming a negative electrode active material layer.
- the paste thus prepared is applied to the negative electrode current collector, the solvent is volatilized and dried, and then compressed (pressed). Thereby, the negative electrode of the lithium secondary battery which has the negative electrode active material layer formed using this paste on a negative electrode collector is obtained.
- coating, drying, and the compression method can use a conventionally well-known means similarly to the manufacturing method of the above-mentioned positive electrode.
- the lithium secondary battery disclosed herein is defined in a multifaceted manner from the relative ratio of the void volume between the positive electrode active material layer and the negative electrode active material layer and the respective suitable void ratios. First, the relative ratio of the void volume between the positive electrode active material layer and the negative electrode active material layer will be described.
- a void volume ratio (a void volume (Sa) per unit area in the positive electrode active material layer) to a void volume (Sb) per unit area in the negative electrode active material layer ( Sa / Sb) is typically 0.9 ⁇ (Sa / Sb) ⁇ 1.4, preferably 1 ⁇ (Sa / Sb) ⁇ 1.4, more preferably 1 ⁇ (Sa / Sb) ⁇ 1.
- a positive electrode active material layer and a negative electrode active material layer are formed.
- the void volume per unit area of the positive electrode active material layer is formed to be approximately the same as or larger than the void volume per unit area of the negative electrode active material layer.
- electrolysis on the positive electrode side during discharge is performed. Reaction in the liquid (lithium ions occluded on the negative electrode side move to the positive electrode side) is promoted.
- the amount of electrolyte solution retained in the gap is suitably maintained in each electrode active material layer, and the ion concentration distribution balance in the electrolyte solution is not biased to one electrode side even under high-rate pulse charge / discharge, An increase in internal resistance can be suppressed.
- the void volume per unit area (mL / cm 2 ) of the positive electrode active material layer is determined by first punching a predetermined area from the produced positive electrode with a punch or the like, and mass of the positive electrode active material layer per unit area (g / cm 2 ) is measured.
- the porosity of the positive electrode active material layer and the negative electrode active material layer is preferably set as follows.
- the porosity of the positive electrode active material is typically 30% or more and 40% or less, preferably 33% or more and 39% or less, while the porosity of the negative electrode active material layer is typically 30% or more and 45%.
- it is preferably set to 30% or more and 40% or less, respectively.
- the electrode active material layer having a suitably set porosity is a conductive path. Is formed efficiently and improves the conductivity of the lithium secondary battery.
- the shape of the void may take various shapes depending on the material constituting the active material layer and the manufacturing method, but may be any shape, and is generally spherical or its deformation in many cases.
- the layer density of the positive electrode active material layer is typically 2 g / cm 3 or more and 2.5 g / cm 3 or less, for example, 2.2 g / cm 3 or more and 2.5 g / cm 3 .
- the smaller the layer density of the positive electrode active material layer the larger the void volume of the positive electrode active material layer. Therefore, in order to make the reaction on the positive electrode side during diffusion to be diffusion-controlled, by setting the layer density of the positive electrode active material layer in the above range, a void volume is suitably formed and charge transfer is performed with high efficiency.
- a rectangular lithium secondary battery will be described as one specific example of the lithium secondary battery according to the present invention, but the present invention is not intended to be limited to such an example.
- matters other than the matters specifically mentioned in the present specification and matters necessary for carrying out the present invention for example, the configuration and manufacturing method of an electrode body including a positive electrode and a negative electrode, the configuration and manufacturing of a separator and an electrolyte
- General techniques related to the construction of a method, a lithium secondary battery, and other batteries can be understood as design matters of a person skilled in the art based on the prior art in this field.
- FIG. 1 is a perspective view schematically showing a rectangular lithium secondary battery according to an embodiment
- FIG. 2 is a cross-sectional view taken along the line II-II in FIG.
- the lithium secondary battery 100 according to the present embodiment includes a rectangular parallelepiped battery case 10 and a lid body 14 that closes the opening 12 of the case 10.
- a flat electrode body (wound electrode body 20) and an electrolyte can be accommodated in the battery case 10 through the opening 12.
- the lid 14 is provided with a positive terminal 38 and a negative terminal 48 for external connection, and a part of the terminals 38 and 48 protrudes to the surface side of the lid 14. Also, some of the external terminals 38 and 48 are connected to the internal positive terminal 37 or the internal negative terminal 47, respectively, inside the case.
- a wound electrode body 20 is accommodated in the case 10.
- the electrode body 20 includes a positive electrode sheet 30 in which a positive electrode active material layer 34 is formed on the surface of a long sheet-like positive electrode current collector 32, and a negative electrode active material layer 44 on the surface of a long sheet-like negative electrode current collector 42. Is formed from a negative electrode sheet 40 on which is formed, and a long sheet-like separator 50.
- one end portion 35 along the longitudinal direction is a portion where the positive electrode current collector 32 is exposed without forming the positive electrode active material layer 34 (positive electrode active material layer non-formation portion 36).
- one end portion 46 along the longitudinal direction thereof is a portion where the negative electrode current collector 42 is exposed without forming the negative electrode active material layer 44 (negative electrode active material layer non-formed) Part 46).
- the electrode sheets 30 and 40 are slightly shifted and overlapped so that the portion 46 is disposed separately at one end and the other end along the longitudinal direction. In this state, a total of four sheets 30, 50, 40, 50 are wound, and then the obtained wound body is crushed from the lateral direction to be ablated, thereby obtaining a flat wound electrode body 20.
- the internal positive electrode terminal 37 is joined to the positive electrode active material layer non-formation portion 36 of the positive electrode current collector 32 and the internal negative electrode terminal 47 is joined to the exposed end portion of the negative electrode current collector 42 by ultrasonic welding, resistance welding or the like. And it electrically connects with the positive electrode sheet 30 or the negative electrode sheet 40 of the winding electrode body 20 formed in the said flat shape.
- the lithium secondary battery 100 of this embodiment can be constructed by injecting an electrolyte and sealing the injection port.
- a suitable separator sheet 50 used between the positive / negative electrode sheets 30 and 40 what was comprised with porous polyolefin resin is mentioned.
- a porous separator sheet made of synthetic resin for example, made of polyolefin such as polyethylene
- a separator is unnecessary (that is, in this case, the electrolyte itself can function as a separator).
- the same electrolyte as the non-aqueous electrolyte conventionally used for lithium secondary batteries can be used without any particular limitation.
- a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent.
- the non-aqueous solvent include one or two selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like. More than seeds can be used.
- the supporting salt examples include LiPF 6 , LiBF 4 , LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , LiC 4 F 9 SO 3 , LiN (CF 3 SO 2 ) 2 , LiC (CF 3 SO 2 ). 3.
- Lithium compounds (lithium salts) such as LiI can be used.
- concentration of the support salt in a nonaqueous electrolyte solution may be the same as that of the nonaqueous electrolyte solution used with the conventional lithium secondary battery, and there is no restriction
- An electrolyte containing a suitable lithium compound (supporting salt) at a concentration of about 0.1 to 5 mol / L can be used.
- the lithium secondary battery constructed in this way has excellent battery characteristics (cycle characteristics or high-rate characteristics) as a vehicle-mounted high-output power supply in which the increase in internal resistance is suppressed, particularly low-temperature pulse charge / discharge. It may exhibit good low temperature cycle characteristics under conditions.
- the lithium secondary battery (sample battery) disclosed herein was constructed and its performance was evaluated. However, it is not intended to limit the present invention to those shown in the specific examples.
- ⁇ Test Example 1> A lithium secondary battery in which the porosity of the positive electrode active material was changed while the porosity of the negative electrode active material was constant was constructed.
- a negative electrode (negative electrode sheet) for a lithium secondary battery was produced. That is, ion-exchanged water such that graphite as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) are in a mass ratio of 98: 1: 1.
- SBR styrene butadiene rubber
- CMC carboxymethyl cellulose
- the negative electrode for a lithium secondary battery thus obtained had a layer density of the negative electrode active material layer of 1.34 g / cm 3 , a porosity of 39%, and a void volume per unit area of 3.0 mL / cm 2. It was.
- a positive electrode (positive electrode sheet) for a lithium secondary battery was produced. That is, lithium composite oxide (LiNi 0.8 Co 0.2 O 2 ) powder as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder, A paste for forming a positive electrode active material layer was prepared by mixing with N-methylpyrrolidone (NMP) so that the mass% ratio of the materials became various ratios. The paste is applied to both surfaces of a sheet-like aluminum foil having a thickness of about 10 ⁇ m as a positive electrode current collector, and after moisture in the paste is dried, the paste is stretched into a sheet shape by a roller press machine.
- NMP N-methylpyrrolidone
- the sample was molded to a thickness of about 75 ⁇ m (both sides). 1 to 8 positive electrode sheets were obtained. Sample No. obtained in this way. The layer density (g / cm 3 ), porosity (%), and void volume per unit area (mL / cm 2 ) of the positive electrode active material layer of the positive electrode for lithium secondary batteries 1 to 8 were calculated. In Table 1, Sample No. Each data of 1 to 8 is shown.
- a cylindrical lithium secondary battery having a diameter of 18 mm and a height of 65 mm (18650 type) as shown in FIG. 3 was constructed using the positive electrodes 1 to 8 (positive electrode sheet) in the following procedure. That is, a negative electrode sheet and a positive electrode sheet were laminated together with two separators having a thickness of 25 ⁇ m, and this laminated sheet was wound to produce a wound electrode body. This electrode body is housed in a container together with the electrolytic solution, and the opening of the container is sealed. A total of eight types of lithium secondary batteries (sample batteries) using 1 to 8 different positive electrode sheets were constructed.
- a volume ratio of 3 Using a solution obtained by dissolving the support salt LiPF 6 at a concentration of 1 mol / L in a mixed solvent of 7 ethylene carbonate (EC) and diethyl carbonate (DEC).
- EC ethylene carbonate
- DEC diethyl carbonate
- the void volume ratio (Sa / Sb) between the void volume (Sa) per unit area in the positive electrode active material layer and the void volume (Sb) per unit area in the negative electrode active material layer is 0.
- the lithium secondary batteries showing .93 (sample No. 4), 1.00 (sample No. 5), 1.10 (sample No. 2), and 1.03 (sample No. 1) have a resistance increase rate. Was smaller than 1.25, and it was confirmed that an increase in internal resistance could be suppressed even after a cycle due to high-rate pulse charge / discharge under low temperature conditions.
- the porosity of the positive active material layer had a large resistance increase rate. Focusing on the porosity of the positive active material layer, the porosity of the positive electrode active material layer of the lithium secondary battery having a small resistance increase rate is 35 to 39%, and the layer density is 2.30 to 2.45 g / cm 3 . (The porosity of the negative electrode active material layer is all 39%.)
- a lithium secondary battery in which the porosity of the negative electrode active material was changed while the porosity of the positive electrode active material was constant was constructed.
- a positive electrode (positive electrode sheet) for a lithium secondary battery was produced. That is, lithium composite oxide (LiNi 0.8 Co 0.2 O 2 ) powder as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder, A paste for forming a positive electrode active material layer was prepared by mixing with N-methylpyrrolidone (NMP) so that the mass% ratio of the material was 87: 10: 3.
- NMP N-methylpyrrolidone
- the paste is applied to both surfaces of a sheet-like aluminum foil having a thickness of about 10 ⁇ m as a positive electrode current collector, and after moisture in the paste is dried, the paste is stretched into a sheet shape by a roller press machine. The thickness was molded to about 75 ⁇ m (both sides) to obtain a positive electrode sheet.
- the positive electrode for the lithium secondary battery thus obtained had a layer density of the positive electrode active material layer of 2.45 g / cm 3 , a porosity of 10%, and a void volume per unit area of 2.6 mL / cm 2. It was.
- the negative electrode (negative electrode sheet) for lithium secondary batteries was produced. That is, ion-exchanged water such that graphite as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) are in a mass ratio of 98: 1: 1. And a paste for forming a negative electrode active material layer was prepared. And paste was apply
- SBR styrene-butadiene rubber
- CMC carboxymethyl cellulose
- the negative electrode active material layer was formed to a thickness of about 80 ⁇ m (both sides) by stretching it into a sheet with a roller press. 9 to 13 negative electrode sheets were obtained. Sample No. obtained in this way. The layer density (g / cm 3 ), porosity (%), and void volume per unit area (mL / cm 2 ) of the negative electrode active material layer of the negative electrode for lithium secondary batteries 9 to 13 were calculated. Table 2 shows the sample numbers. Each data of 9-13 is shown.
- the lithium secondary battery has a resistance increase rate smaller than 1.25 and has an internal resistance even after cycling by pulse charge / discharge under low temperature conditions. It was confirmed that the increase in the amount could be suppressed.
- Sample No. whose void volume ratio was smaller than the above sample. 9 and sample no. 10 and Sample No. in which the void volume ratio was larger than the above sample. In 13, the resistance increase rate was large.
- the porosity of the negative active material layer of the lithium secondary battery which had a small resistance increase rate, was 30 to 35%. (The porosity of the positive electrode active material layer is all 33%.)
- FIG. 4 is a graph showing the relationship between the void volume ratio and the resistance increase rate in Tables 1 and 2.
- the horizontal axis represents the void volume ratio (Sa / Sb) between the void volume per unit area (Sa) in the positive electrode active material layer and the void volume (Sb) per unit area in the negative electrode active material layer.
- the axis indicates the resistance increase rate.
- the increase rate of the internal resistance is small in the lithium secondary battery having the void volume ratio of about 0.9 to 1.4.
- the present invention has been described in detail above, the above embodiments and examples are merely examples, and the invention disclosed herein includes various modifications and changes of the above-described specific examples.
- the battery of the various content from which an electrode body structural material and electrolyte differ may be sufficient.
- the size and other configurations of the battery can be appropriately changed depending on the application (typically for in-vehicle use).
- the lithium secondary battery according to the present invention has excellent battery characteristics (cycle characteristics or high rate characteristics) as described above, it can be suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile. . Therefore, as schematically shown in FIG. 5, the present invention provides a vehicle 1 (typically an automobile, particularly a hybrid) provided with such a lithium secondary battery (typically a battery pack formed by connecting a plurality of series batteries) 100 as a power source. Automobiles, electric vehicles, automobiles equipped with electric motors such as fuel cell vehicles).
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Abstract
Description
この種のリチウム二次電池の一つの典型的な構成では、電極集電体の表面にリチウムイオンを可逆的に吸蔵および放出し得る電極活物質層(具体的には、正極活物質層および負極活物質層)を有する。例えば、正極の場合、リチウム遷移金属複合酸化物等の正極活物質が、適当な溶媒に分散された状態のペースト状組成物(ペースト状組成物にはスラリー状組成物が包含される。以下、この種の組成物を単にペーストと呼称する。)が正極集電体表面に塗布されることにより形成された正極活物質層を有している。
また、本明細書において「正極活物質」とは、二次電池において電荷担体となる化学種(ここではリチウムイオン)を可逆的に吸蔵および放出(典型的には挿入および脱離)可能な正極側の活物質をいい、本明細書において「負極活物質」とは、上記化学種を可逆的に吸蔵および放出可能な負極側の物質をいう。
さらに、本明細書において「空隙率」とは、正極活物質層または負極活物質層の全体積に対する、その内部に存在する空隙(空間)部分の体積の割合をいう。
ここで、ハイレートのパルス充放電を短時間で繰り返す態様で使用される高出力電源用のリチウム二次電池は、放電時の正極側における電解液中の反応(負極側に吸蔵されていたリチウムイオンが正極側に移動する)が拡散律速となっている。本発明者は、正極活物質層の空隙を負極活物質層の空隙体積と同程度か、あるいはそれよりも大きく形成することにより、放電時の正極側の反応が拡散律速状態となり、内部抵抗の上昇を抑制することができることを見出した。従って、ここに開示されるリチウム二次電池は、正極活物質層における単位面積当たりの空隙体積(Sa)と負極活物質層における単位面積当たりの空隙体積(Sb)との空隙体積比(Sa/Sb)が、0.9≦(Sa/Sb)≦1.4を満たし、且つ正極活物質層の空隙率が30%以上40%以下であり、負極活物質層の空隙率が30%以上45%以下に設定される。これにより、空隙に保持されている電解液量がそれぞれの電極活物質層において好適に維持されるため、ハイレートのパルス充放電下においても電解液中のイオン濃度分布バランスが一方の電極側に偏ることなく、内部抵抗の上昇が抑制される。従って、本発明より車両搭載用高出力電源として優れた電池特性(サイクル特性またはハイレート特性)、特に低温パルス充放電条件下において良好な低温サイクル特性を有するリチウム二次電池を提供することができる。
Li(Ni1-xCox)O2 (1)
(式(1)中のxは、0<x<0.5を満たす)で示される複合酸化物である。
好ましい一態様のリチウム二次電池の正極活物質は、理論上のリチウムイオン吸蔵容量が大きく安価でもあるニッケルと、電子導電性を向上させるコバルトとを含むリチウム複合酸化物から成る。また、かかるリチウム複合酸化物のコバルトのモル比xが、0<x<0.5の関係を満たし、ニッケルのモル比がコバルトのモル比よりも大きくなるように構成される。その結果、上記リチウム複合酸化物を用いることにより、優れた電池特性(サイクル特性またはハイレート特性)を有するリチウム二次電池を提供することができる。
正極活物質層の空隙体積が小さ過ぎると、上述のようにハイレート放電時における正極側の電解液中の反応が停滞するため好ましくないが、他方、正極活物質層の空隙体積が大き過ぎると、今度は正極活物質層の電解液保持量が多くなり過ぎて、負極活物質層の空隙に保持される電解液量が少なくなり、結果として内部抵抗を増大させてしまうこととなる。そのため、空隙体積比(Sa/Sb)が、1≦(Sa/Sb)≦1.1を満たすことにより、内部抵抗の上昇がさらに抑制され、より優れた電池特性(サイクル特性またはハイレート特性)、特に低温パルス充放電下において良好な低温サイクル特性を有するリチウム二次電池を提供することができる。
正極活物質層の層密度が小さいほど、正極活物質層の空隙体積は大きくなる。そのため、放電時の正極側の反応を拡散律速させるため、正極活物質層の層密度を2g/cm3以上2.5g/cm3以下に設定することにより、空隙体積が好適に形成され電荷移動が高効率で行われる。その結果、ハイレートのパルス充放電を繰り返しても、内部抵抗の上昇が抑制されたリチウム二次電池を提供することができる。
瞬間的に大電流を流すハイレートのパルス充放電を短時間で繰り返す態様で使用されるリチウム二次電池は、放電時の正極側における電解液中の反応(負極側に吸蔵されていたリチウムイオンが正極側に移動する)が拡散律速となる。そのため、本発明者は、正極活物質層の空隙を負極活物質層の空隙体積と同程度か、あるいはそれよりも大きく形成することにより、放電時の正極側の反応が拡散律速状態となり、内部抵抗の上昇を抑制することができることを見出した。また、正極活物質層における空隙体積が大き過ぎてもまた正極活物質層の空隙に保持される電解液量が過多となり、負極活物質層の電解液保持力が低下するため好ましくない。そこで、本発明では、正極活物質層における単位面積当たりの空隙体積(Sa)と負極活物質層における単位面積当たりの空隙体積(Sb)との空隙体積比(Sa/Sb)が、0.9≦(Sa/Sb)≦1.4を満たし、さらに、正極活物質層の空隙率が30%以上40%以下、負極活物質層の空隙率が30%以上45%以下になるように正極活物質層および負極活物質層を形成する。これにより、空隙に保持される電解液量がそれぞれの電極活物質層において好適に維持され、ハイレートのパルス充放電下においても電解液中のイオン濃度分布バランスが一方の電極側に偏ることなく、内部抵抗の上昇が抑制することができる。その結果、車両搭載用高出力電源として優れた電池特性(サイクル特性またはハイレート特性)、特に低温パルス充放電条件下において良好な低温サイクル特性を有するリチウム二次電池を製造する方法を提供することができる。
Li(Ni1-xCox)O2 (1)
(式(1)中のxは、0<x<0.5を満たす)で示される複合酸化物を使用する。
上記式(1)を満たすリチウム複合酸化物からなる正極活物質の好ましい一態様は、リチウム以外の構成金属元素としてニッケルおよびコバルトを含む。ニッケルを含む複合酸化物は、理論上のリチウムイオン吸蔵容量が大きく、また原料コストを安価に抑えられる。また、ニッケルのモル比よりも少ないモル比でコバルトが含まれることにより、電子導電性が向上されている。そのため、かかる組成比の複合酸化物を正極活物質として用いることにより、優れた電池特性(サイクル特性またはハイレート特性)を有するリチウム二次電池を製造することができる。
正極活物質層と負極活物質層との空隙体積比(Sa/Sb)が、1≦(Sa/Sb)≦1.1を満たされるようにそれぞれの活物質層を形成することにより、内部抵抗の上昇がさらに抑制され、より優れた電池特性(サイクル特性またはハイレート特性)、特に低温パルス充放電下において良好な低温サイクル特性を有するリチウム二次電池を製造することができる。
正極活物質層の層密度(固形分密度)が小さいほど、正極活物質層の空隙体積は大きくなる。そのため、放電時の正極側の反応を拡散律速させるため、正極活物質層の層密度が2g/cm3以上2.5g/cm3以下になるように正極活物質層を形成することにより、該活物質層には空隙体積が好適に形成される。これにより、電極間の電荷移動が高効率で行われるため、ハイレートのパルス充放電を繰り返しても、内部抵抗の上昇が抑制されたリチウム二次電池を製造することができる。
ここに開示されるリチウム二次電池の正極活物質としては、少なくともリチウム(Li)と、ニッケル(Ni)及び/又はコバルト(Co)とを構成元素の主体とする(典型的にはリチウム以外の構成金属元素のうち、ニッケル及び/又はコバルトの合計モル組成比が50%以上である)リチウム複合酸化物が用いられる。
また、より好ましい正極活物質としては、リチウムと、ニッケルと、コバルトとを必須構成元素とする複合酸化物であって、以下の式:
Li(Ni1-xCox)O2 (1)
(式(1)中のxは、0<x<0.5を満たす)で示される複合酸化物が用いられる。かかる複合酸化物は、理論上のリチウムイオン吸蔵容量が大きく安価でもあるニッケルと、電子導電性を向上させるコバルトとを含む。さらに、かかるリチウム複合酸化物のニッケルのモル比はコバルトのモル比よりも大きくなるような組成比で構成されるのが好ましい。
上記正極活物質を導電材及び結着材等と共に上記適当な溶媒(水系溶媒または非水系溶媒)で混合して、ペーストまたはスラリー状の正極活物質層形成用ペーストを調製する。各構成材料の配合比率は、例えば、正極活物質層に占める正極活物質の割合は、凡そ50質量%以上(典型的には50~95質量%)であることが好ましく、凡そ70~95質量%(例えば75~90質量%)であることがより好ましい。また、正極活物質層に占める導電材の割合は、例えば凡そ2~20質量%とすることができ、通常は凡そ2~15質量%とすることが好ましい。さらに、結着材を使用する組成では、正極活物質層に占める結着材の割合を例えば凡そ1~10質量%とすることができ、通常は凡そ2~5質量%とすることが好ましい。こうして各構成材料を混合して調製したペーストを正極集電体32に塗布し、溶媒を揮発させて乾燥させた後、圧縮(プレス)する。これにより正極活物質層が正極集電体上に形成されたリチウム二次電池の正極が得られる。
まず、負極集電体としては、導電性の良好な金属からなる導電性部材が好ましく用いられる。例えば、銅、または銅を主成分とする合金を用いることができる。負極集電体の形状は、正極集電体と同様にリチウム二次電池の形状等に応じて異なり得るため特に制限はない。
なお、負極活物質層には、上記負極活物質の他に、上述の正極の構成要素で列挙した結着材として機能し得る各種のポリマー材料を好適に使用し得る。
上記負極活物質を結着材等と共に適当な溶媒(水、有機溶媒およびこれらの混合溶媒)で混合して、ペーストまたはスラリー状の負極活物質層形成用ペーストを調製する。こうして調製したペーストを負極集電体に塗布し、溶媒を揮発させて乾燥させた後、圧縮(プレス)する。これにより該ペーストを用いて形成された負極活物質層を負極集電体上に有するリチウム二次電池の負極が得られる。なお、塗布、乾燥および圧縮方法は、上述の正極の製造方法と同様に従来公知の手段を用いることができる。
まず、正極活物質層と負極活物質層の空隙体積の相対比率について説明する。 ここに開示されるリチウム二次電池の好ましい一態様では、正極活物質層における単位面積当たりの空隙体積(Sa)と負極活物質層における単位面積当たりの空隙体積(Sb)との空隙体積比(Sa/Sb)が、典型的には0.9≦(Sa/Sb)≦1.4、好ましくは1≦(Sa/Sb)≦1.4、より好ましくは1≦(Sa/Sb)≦1.1、を満たすように正極活物質層および負極活物質層を形成する。このように、正極活物質層の単位面積当たりの空隙体積を負極活物質層の単位面積当たりの空隙体積と同程度か、あるいはそれよりも大きく形成されることにより、放電時の正極側における電解液中の反応(負極側に吸蔵されていたリチウムイオンが正極側に移動する)が促進される。その結果、空隙に保持される電解液量がそれぞれの電極活物質層において好適に維持され、ハイレートのパルス充放電下においても電解液中のイオン濃度分布バランスが一方の電極側に偏ることなく、内部抵抗の上昇が抑制することができる。
式:
[単位面積当たりの正極活物質の体積]=[単位面積当たりの正極活物質層の質量]×[正極活物質の配合比率]/[正極活物質の真比重] (2)
式:
[正極活物質層の単位面積当たりの空隙体積]=[単位面積当たりの正極活物質層の体積]-{[単位面積当たりの正極活物質の体積]+[単位面積当たりの導電材の体積]+[単位面積当たりの結着材の体積]} (3)
負極活物質の空隙率を一定にして、正極活物質の空隙率を変化させたリチウム二次電池を構築した。
まず、リチウム二次電池用の負極(負極シート)を作製した。すなわち、負極活物質としてのグラファイトと、結着材としてのスチレンブタジエンゴム(SBR)、とカルボキシメチルセルロース(CMC)とを、これら材料の質量%比が98:1:1となるようにイオン交換水と混合して、負極活物質層形成用ペーストを調製した。そして、負極集電体としての厚み約10μmの銅箔の両面に、調製したペーストを塗布した。次いで、ペースト中の水分を乾燥させた後、ローラプレス機にてシート状に引き伸ばして負極活物質層の厚さを約80μm(両面)に成形し、負極シートを得た。こうして得られたリチウム二次電池用の負極は、負極活物質層の層密度が1.34g/cm3、空隙率が39%、および単位面積当たりの空隙体積が3.0mL/cm2であった。
次に、上記構築した各リチウム二次電池の出力特性を評価する指標として、温度条件-15℃でハイレートのパルス充放電によるサイクル試験をし、サイクル後の内部抵抗増加率を調べた。すなわち、-15℃の温度条件下にて、定電流定電圧(CC-CV)充電によって各電池をSOC60%の充電状態に調整した。その後、20Cで放電し、放電開始から10秒後の電圧を測定し、I-V特性グラフを作成した。このI-V特性グラフの傾きから、-15℃における初期内部抵抗値(mΩ)を算出した。
そして、同様の条件で各電池をSOC60%に調整した後、-15℃の温度条件下にて、20Cで10秒間放電し、2Cで100秒間充電する短形波パルス充放電サイクルを1000サイクル繰り返した。そして、1000サイクル後の電池について、上記初期内部抵抗値の測定と同様にして内部抵抗値を測定し、次式:{(サイクル後IV抵抗値)/(初期IV抵抗値)}×100;により、上記パルス充放電サイクルの前後における内部抵抗値増加率(%)を求めた。結果を表1に示す。
他方、空隙体積比が上記サンプルよりも小さい、0.87(サンプルNo.3)、0.83(サンプルNo.8)、0.77(サンプルNo.7)、および0.70(サンプルNo.6)を示すリチウム二次電池は、抵抗増加率が大きかった。
また、正活物質層の空隙率に着目すると、抵抗増加率が小さかったリチウム二次電池の正極活物質層の空隙率は35~39%であり、層密度は2.30~2.45g/cm3であった。(なお、負極活物質層の空隙率は全て39%である。)
次に、正極活物質の空隙率を一定にして、負極活物質の空隙率を変化させたリチウム二次電池を構築した。
まず、リチウム二次電池用の正極(正極シート)を作製した。すなわち、正極活物質としてのリチウム複合酸化物(LiNi0.8Co0.2O2)粉末と、導電材としてのアセチレンブラックと、結着材としてのポリビニリデンフロライド(PVDF)とを、これら材料の質量%比が87:10:3となるようN-メチルピロリドン(NMP)と混合して、正極活物質層形成用ペーストを調製した。かかるペーストを、正極集電体としての厚み約10μmのシート状のアルミニウム箔の両面に塗布し、ペースト中の水分を乾燥させた後、ローラプレス機にてシート状に引き伸ばして正極活物質層の厚さを約75μm(両面)に成形し、正極シートを得た。こうして得られたリチウム二次電池用の正極は、正極活物質層の層密度が2.45g/cm3、空隙率が10%、および単位面積当たりの空隙体積が2.6mL/cm2であった。
次に、上記構築した各リチウム二次電池の出力特性を評価する指標として、温度条件-15℃でパルス充放電によるサイクル試験をし、試験例1と同様の手順でサイクル後の内部抵抗増加率を調べた。結果を表2に示す。
他方、空隙体積比が上記サンプルよりも小さかったサンプルNo.9およびサンプルNo.10、および空隙体積比が上記サンプルよりも大きかったサンプルNo.13では、抵抗増加率が大きかった。
また、負活物質層の空隙率に着目すると、抵抗増加率が小さかったリチウム二次電池の負極活物質層の空隙率は30~35%であった。(なお、正極活物質層の空隙率は全て33%である。)
図4から明らかなように、空隙体積比が凡そ0.9~1.4を示すリチウム二次電池では、内部抵抗増加率が小さいことが確認される。
Claims (9)
- 正極集電体の表面に形成された正極活物質を含む正極活物質層を有する正極と、負極集電体の表面に形成された負極活物質を含む負極活物質層を有する負極とを備える、リチウム二次電池であって、
前記正極活物質は、少なくともリチウムと、ニッケル及び/又はコバルトとを構成元素の主体とするリチウム複合酸化物により構成されており、
前記正極活物質層の空隙率は30%以上40%以下であり、且つ前記負極活物質層の空隙率は30%以上45%以下であり、
ここで、前記正極活物質層における単位面積当たりの空隙体積(Sa)と前記負極活物質層における単位面積当たりの空隙体積(Sb)との空隙体積比(Sa/Sb)が、0.9≦(Sa/Sb)≦1.4を満たす、リチウム二次電池。 - 前記正極活物質を構成するリチウム複合酸化物は、以下の式:
Li(Ni1-xCox)O2 (1)
(式(1)中のxは、0<x<0.5を満たす)
で示される複合酸化物である、請求項1に記載のリチウム二次電池。 - 前記正極活物質層における単位面積当たりの空隙体積(Sa)と前記負極活物質層における単位面積当たりの空隙体積(Sb)との空隙体積比(Sa/Sb)が、1≦(Sa/Sb)≦1.1を満たす、請求項1または2に記載のリチウム二次電池。
- 前記正極活物質層の層密度は、2g/cm3以上2.5g/cm3以下である、請求項1~3のいずれかに記載のリチウム二次電池。
- 正極集電体の表面に形成された正極活物質を含む正極活物質層を有する正極と、負極集電体の表面に形成された負極活物質を含む負極活物質層を有する負極とを備えるリチウム二次電池を製造する方法であって、
前記正極活物質として、少なくともリチウムと、ニッケル及び/又はコバルトとを構成元素の主体とするリチウム複合酸化物を使用して、前記正極活物質層の空隙率が30%以上40%以下になるように該活物質層を形成し、
前記負極活物質層の空隙率が30%以上45%以下になるように該活物質層を形成し、
ここで、前記正極活物質層における単位面積当たりの空隙体積(Sa)と前記負極活物質層における単位面積当たりの空隙体積(Sb)との空隙体積比(Sa/Sb)が、0.9≦(Sa/Sb)≦1.4を満たされるように正極活物質層および負極活物質層を形成する、製造方法。 - 前記正極活物質を構成するリチウム複合酸化物として、以下の式:
Li(Ni1-xCox)O2 (1)
(式(1)中のxは、0<x<0.5を満たす)
で示される複合酸化物を使用する、請求項5に記載の製造方法。 - 前記正極活物質層における単位面積当たりの空隙体積(Sa)と前記負極活物質層における単位面積当たりの空隙体積(Sb)との空隙体積比(Sa/Sb)が、1≦(Sa/Sb)≦1.1を満たされるように正極活物質層および負極活物質層を形成する、請求項5または6に記載の製造方法。
- 前記正極活物質層の層密度が2g/cm3以上2.5g/cm3以下になるように正極活物質層を形成する、請求項5~7のいずれかに記載の製造方法。
- 請求項1~4のいずれかに記載のリチウム二次電池または請求項5~8のいずれかに記載の製造方法により製造されたリチウム二次電池、を備える車両。
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