Method for manufacturing nickel metal hydride battery, positive electrode for nickel metal hydride battery, and nickel metal hydride battery

Abstract

A method for producing a nickel metal hydride battery (1) according to the present invention comprises a positive electrode production step in which a positive electrode (2) comprising a positive electrode active material layer (21) that has a graphitization degree of 0.4 or less as determined by Raman spectroscopy spectrum is produced by superposing a starting material composition, which contains a positive electrode active material powder containing nickel hydroxide, a cobalt compound and a flake graphite, on a collector. The method also comprises a negative electrode production step in which a negative electrode (3) is produced by superposing a starting material composition, which contains a negative electrode active material powder, on a collector. The method also comprises: an electrode body production step in which an electrode body is produced by arranging a separator (5) between the positive electrode (2) and the negative electrode (3) and impregnating the separator (5) with an electrolyte solution; and an overdischarge step in which the electrode body is charged, is subsequently overdischarged, and is charged again.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a nickel metal hydride battery, a positive electrode for a nickel metal hydride battery, and a nickel metal hydride battery.

Background Art

[0002] Patent Document 1 describes a paste-like nickel electrode for a storage battery containing an alkaline electrolyte. The paste-like nickel electrode includes an active material based on nickel hydroxide and a conductor based on carbon. Further, in order to improve the conductivity of the paste-like nickel electrode, it is described that a cobalt compound such as metal cobalt, cobalt hydroxide, or cobalt oxide is added.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Since cobalt compounds are expensive materials and limited resources, it is desired to reduce the content of cobalt compounds used in the electrode. As an alternative material for cobalt compounds, for example, a carbon material can be mentioned. However, when a carbon material is used as an alternative material for cobalt compounds, there is a risk of corrosion of the carbon material. When the carbon material corrodes, there is a risk of an increase in cell resistance.

Means for Solving the Problems

[0005] The method for manufacturing a nickel metal hydride battery for achieving the above object includes a positive electrode manufacturing step of laminating a raw material composition containing nickel hydroxide positive electrode active material powder, a cobalt compound, and flaky graphite on a current collector to prepare a positive electrode having a positive electrode active material layer in which the content of the cobalt compound is 3% by mass or less and the graphitization degree obtained by Raman spectroscopy is 0.4 or less, a negative electrode manufacturing step of laminating a raw material composition containing negative electrode active material powder on a current collector to prepare a negative electrode, an electrode body manufacturing step of disposing a separator between the positive electrode and the negative electrode and impregnating the separator with an electrolytic solution to prepare an electrode body, and an over-discharge step of charging the electrode body, then over-discharging it, and further charging it.

[0006] In the method for manufacturing a nickel metal hydride battery for achieving the above object, in the raw material composition, the average particle diameter of the flaky graphite is 0.4 times or more the average particle diameter of the nickel hydroxide positive electrode active material powder.

[0007] The positive electrode for a nickel metal hydride battery for achieving the above object is a positive electrode for a nickel metal hydride battery including a current collector and a positive electrode active material layer, where the positive electrode active material layer has a positive electrode active material, a cobalt compound layer covering the positive electrode active material, and flaky graphite, the content of the cobalt compound in the positive electrode active material layer is 3% by mass or less, the coverage rate of the cobalt compound layer with respect to the positive electrode active material is 50% or more, and the graphitization degree obtained by Raman spectroscopy of the positive electrode active material layer is 0.4 or less.

[0008] In the positive electrode for a nickel metal hydride battery for achieving the above object, the average thickness of the cobalt compound layer is 10 nm or less. In the positive electrode for a nickel metal hydride battery for achieving the above object, the average particle diameter of the flaky graphite is 0.3 times or more the average particle diameter of the positive electrode active material.

[0009] In the positive electrode for a nickel metal hydride battery for achieving the above object, the content of the flaky graphite in the positive electrode active material layer is 3% by mass or more and 10% by mass or less. The positive electrode for a nickel metal hydride battery that achieves the above object is such that the cobalt compound layer is also formed on the surface of the flaky graphite.

[0010] The nickel metal hydride battery that achieves the above object includes the positive electrode for a nickel metal hydride battery described above.

Advantages of the Invention

[0011] According to the present invention, it is possible to suppress an increase in cell resistance while suitably reducing the content of the cobalt compound contained in the positive electrode active material layer.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0013] Hereinafter, an embodiment embodying the present invention will be described with reference to the drawings. First, the electrode body constituting the nickel metal hydride battery will be described. <Electrode Body> As shown in FIG. 1, the nickel metal hydride battery 1 has a positive electrode for nickel metal hydride battery 2 (hereinafter also simply referred to as "positive electrode") including a current collector foil 20 as a current collector and a positive electrode active material layer 21. The positive electrode active material layer 21 is laminated on one surface of the current collector foil 20.

[0014] The nickel metal hydride battery 1 has a negative electrode for nickel metal hydride battery (hereinafter also simply referred to as "negative electrode") 3 including a current collector foil 30 as a current collector and a negative electrode active material layer 31. The negative electrode active material layer 31 is laminated on the other surface of the current collector foil 30. Battery The nickel metal hydride battery 1 has a dual

[0015] type electrode 4 in which a positive electrode active material layer 41 is laminated on one surface of a current collector foil 40 as a current collector, and a negative electrode active material layer 42 is laminated on the other surface of the current collector foil 40. Here, "one side" means the upper side in FIG. 1, and "the other side" means the lower side in FIG. 1. Electrode As shown in FIG. 1, a plurality of dual

[0016] type electrodes 4 are laminated via a separator 5. Specifically, in the dual Electrode type electrode 4, the positive electrode active material layer 41 of one dual Electrode type electrode 4 is laminated in a state facing the negative electrode active material layer 42 of another dual Electrode type electrode 4 via the separator 5, and this lamination state is repeated. Electrode The negative electrode active material layer 42 of the dual

[0017] type electrode 4 is laminated on the surface of the positive electrode active material layer 21 of the positive electrode 2 via the separator 5. Also, the positive electrode active material layer 41 of the dual Electrode type electrode 4 is laminated on the surface of the negative electrode active material layer 31 of the negative electrode 3 via the separator 5. In other words, separators 5 are respectively arranged between the positive electrode 2, the negative electrode 3, and the dual Electrode type electrode 4. The negative electrode 3 is arranged at one end in the lamination direction of the dual Electrode type electrode 4, and the positive electrode 2 is arranged at the other end. Electrode In the lamination direction of the dual

[0018] The separator 5 is impregnated with an electrolytic solution. The positive electrode 2, the negative electrode 3, and the dual Electrode type electrode 4 are laminated via the separator 5 to form an electrode body. The electrode body is also referred to as a battery module.

[0019] <Nickel metal hydride battery> As shown in FIG. 1, the plurality of current collector foils 20, 30, 40 included in the battery module each have the same shape. The plurality of separators 5 included in the battery module also each have the same shape. The current collector foils 20, 30, 40 have a shape larger than that of the positive electrode active material layers 21, 41, the negative electrode active material layers 31, 42, and the separator 5. The separator 5 has a shape larger than that of the positive electrode active material layers 21, 41 and the negative electrode active material layers 31, 42.

[0020] As shown in FIG. 1, the peripheries of the current collector foils 20, 30, 40 are fixed to an outer frame 7 made of a synthetic resin. Inside the outer frame 7, a seal member 6 made of a fluororesin is disposed. The seal member 6 is bonded to the peripheries of the current collector foils 20, 30, 40 on both one side and the other side of the plurality of current collector foils 20, 30, 40. By bonding the seal member 6 to the peripheries of the current collector foils 20, 30, 40, the spaces between the plurality of current collector foils 20, 30, 40 are in a sealed state.

[0021] As shown in FIG. 1, a pair of cooling members 8 are disposed at the ends on one side and the other side of the battery module. Specifically, the cooling member 8 is disposed on the current collector foil 20 of the positive electrode 2 constituting the battery module. Also, the cooling member 8 is disposed on the current collector foil 30 of the negative electrode 3 constituting the battery module. These pair of cooling members 8 are configured in a plate shape having a plurality of through holes 80.

[0022] Among the pair of cooling members 8, a module positive electrode 22 is disposed on the cooling member 8 disposed on the current collector foil 20 of the positive electrode 2. Among the pair of cooling members 8, a module negative electrode 32 is disposed on the cooling member 8 disposed on the current collector foil 30 of the negative electrode 3. The module positive electrode 22 and the module negative electrode 32 are configured in a rectangular plate shape made of metal.

[0023] As shown in FIG. 1, a pair of restraints 9 are arranged on the module positive electrode 22 and the module negative electrode 32. The pair of restraints 9 are fastened by a plurality of bolts and nuts (not shown). By the restraints 9, the battery module is pressed along the thickness direction of the positive electrode 2, the negative electrode 3, and the bipolar electrode 4. A pair of cooling members 8, the module positive electrode 22, and the module negative electrode 32 are arranged in the battery module, and the nickel metal hydride battery 1 is formed by being restrained by the pair of restraints 9. The nickel metal hydride battery 1 having the bipolar electrode 4 is also referred to as a bipolar nickel metal hydride battery or a bipolar metal hydride battery. Electrode As shown in FIG. 1, a pair of restraints 9 are arranged on the module positive electrode 22 and the module negative electrode 32. The pair of restraints 9 are fastened by a plurality of bolts and nuts (not shown). By the restraints 9, the battery module is pressed along the thickness direction of the positive electrode 2, the negative electrode 3, and the bipolar electrode 4. A pair of cooling members 8, the module positive electrode 22, and the module negative electrode 32 are arranged in the battery module, and the nickel metal hydride battery 1 is formed by being restrained by the pair of restraints 9. The nickel metal hydride battery 1 having the bipolar electrode 4 is also referred to as a bipolar nickel metal hydride battery or a bipolar metal hydride battery. Electrode The nickel metal hydride battery 1 having the bipolar electrode 4 is also referred to as Electrode a bipolar nickel metal hydride battery or a bipolar metal hydride battery.

[0024] Hereinafter, the positive electrode 2, the negative electrode 3, the bipolar Electrode electrode 4, and the separator 5 will be described. <Positive Electrode> The positive electrode 2 includes a current collector foil 20 as a current collector and a positive electrode active material layer 21.

[0025] (Current Collector) The current collector is an inert electrical conductor. The current collector continuously conducts current to the positive electrode active material layer 21 during discharge or charging of the nickel metal hydride battery.

[0026] The material of the current collector is not particularly limited, and examples include silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, etc.

[0027] The current collector may be used alone with one of the above materials, or may be used in combination of two or more. When used in combination of two or more, it may be used as a solid solution or an alloy, and for example, stainless steel may be used.

[0028] The shape of the current collector is not limited to foil. A shape capable of passing current through the positive electrode active material layer 21 can be appropriately selected. Examples of shapes other than foil for the current collector include sheet-like, film-like, linear, rod-like, mesh-like, sponge-like, and the like.

[0029] Among these, foil, sheet-like, and film-like are preferable because it is easy to increase the contact area with the positive electrode active material layer 21 and the module positive electrode 22. The thickness of the current collector is not particularly limited. The thickness of the current collector is preferably, for example, 1 μm or more and 100 μm or less.

[0030] (Positive electrode active material layer) The positive electrode active material layer 21 includes a positive electrode active material powder containing nickel hydroxide or nickel hydroxide with an average valence of nickel higher than divalent, a cobalt compound layer covering the positive electrode active material powder, and flaky graphite. The content of the cobalt compound in the positive electrode active material layer is 3% by mass or less, and the coverage rate of the cobalt compound layer with respect to the positive electrode active material is 50% or more. Also, the degree of graphitization obtained from the Raman spectrum of the positive electrode active material layer 21 is 0.4 or less. Here, the positive electrode active material powder containing nickel hydroxide or higher-order nickel hydroxide is simply referred to as the positive electrode active material.

[0031] Nickel hydroxide may be doped with a metal other than nickel. Examples of metals other than nickel include Group 2 elements such as magnesium and calcium, Group 9 elements such as cobalt, rhodium, and iridium, and Group 12 elements such as zinc and cadmium.

[0032] The particle size of nickel hydroxide is not particularly limited, but the average particle size is preferably 3 μm or more and 40 μm or less, more preferably 5 μm or more and 30 μm or less, and even more preferably 7 μm or more and 20 μm or less.

[0033] The content of nickel hydroxide in the positive electrode active material layer 21 is not particularly limited, but is preferably 75% by mass or more and 98% by mass or less, and more preferably 85% by mass or more and 95% by mass or less.

[0034] The cobalt compound constituting the cobalt compound layer is not particularly limited as long as it has high conductivity. For example, higher cobalt oxides such as cobalt oxyhydroxide (CoOOH) can be mentioned. Cobalt oxyhydroxide (CoOOH) is used as a conductive aid.

[0035] The average thickness of the cobalt compound layer covering the positive electrode active material is preferably 10 nm or less. Further, the average thickness of the cobalt compound layer is more preferably 8 nm or less, and even more preferably 6 nm or less. The lower limit value of the average thickness of the cobalt compound layer is not particularly limited. It can be appropriately set within a range that can preferably maintain the function as a conductive aid. Among them, the average thickness of the cobalt compound layer is preferably 0.5 nm or more, and more preferably 1 nm or more.

[0036] When the average thickness of the cobalt compound layer is within the above numerical range, the content in the positive electrode active material layer 21 can be preferably reduced while maintaining the function as a conductive aid. The average thickness of the cobalt compound layer can be measured by observing with a known TEM-EELS or the like.

[0037] The coverage rate of the cobalt compound layer with respect to the positive electrode active material is preferably 70% or more, 80% or more, 85% or more, 90% or more, 95% or more. Note that the coverage rate of the cobalt compound layer with respect to the positive electrode active material means the ratio of the cobalt compound layer occupying the surface of the positive electrode active material. When the surface of the positive electrode active material is completely covered with the cobalt compound layer, the coverage rate is 100%. The coverage rate of the cobalt compound layer with respect to the positive electrode active material can be measured by observing with TEM-EELS or the like.

[0038] The content of the cobalt compound in the positive electrode active material layer 21 is preferably 2.5% by mass or less. Further, it is preferably 0.4% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more.

[0039] When the content of the cobalt compound is within the above numerical range, when forming a cobalt compound layer that coats the positive electrode active material by the method described later, it becomes easy to make the average thickness of the cobalt compound layer 10 nm or less. Further, since the capacity utilization rate of the battery can be 90% or more, the battery characteristics can be made suitable.

[0040] Flaky graphite is used as a conductive assistant. The flaky graphite is not particularly limited, and known flaky graphite can be used. Here, for the dimensions of the flaky graphite, the dimension in the stacking direction in the six-membered ring of the graphite is defined as the thickness t, and the maximum length in the direction along the plane of the six-membered ring, in other words, the dimension of the long side is defined as the diameter r. And those that satisfy the following relational expression are referred to as flaky graphite.

[0041] Relational expression: r / t > 1, t < 1 μm In addition, the above diameter r is regarded as the particle diameter of the flaky graphite. The flaky graphite has a larger diameter r than the thickness t, and the overall shape is flat. The flaky graphite is also referred to as nanographene.

[0042] The average particle diameter, which is the average value of the diameter r of the flaky graphite, is preferably 25 μm or less, 20 μm or less, 15 μm or less, 10 μm or less, 8 μm or less, 7 μm or less. The average particle diameter of the flaky graphite is preferably 1 μm or more, 3 μm or more, 5 μm or more.

[0043] Further, the average particle diameter of the flaky graphite is preferably 0.3 times or more, preferably 0.5 times or more, of the average particle diameter of the positive electrode active material. The average particle diameter of the flaky graphite is preferably 3 times or less, preferably 2 times or less, preferably 1.5 times or less, and more preferably 1.25 times or less, of the average particle diameter of the positive electrode active material.

[0044] When the average particle diameter of the flaky graphite is 0.3 times or more the average particle diameter of the positive electrode active material, the particle diameter of the flaky graphite does not become too small with respect to the particle diameter of the positive electrode active material and has a certain size. Therefore, in the positive electrode active material layer 21, the flaky graphite can be in a state of straddling a plurality of positive electrode active materials, in other words, the flaky graphite can be in a state of contacting a plurality of positive electrode active materials. Thereby, a conductive path can be suitably secured and an increase in the internal resistance of the positive electrode active material layer 21 can be suppressed.

[0045] Also, when the average particle diameter of the flaky graphite is 3 times or less the average particle diameter of the positive electrode active material, it becomes possible to contain more flaky graphite when the content of the flaky graphite is the same. More conductive paths can be secured and an increase in the internal resistance of the positive electrode active material layer 21 can be suppressed.

[0046] The dimensions of the flaky graphite can be measured by observing with a known scanning electron microscope. The flaky graphite preferably has a graphitization degree obtained by Raman spectroscopy of 0.3 or less, more preferably 0.25 or less. The method for evaluating the graphitization degree will be described later.

[0047] The specific surface area of the flaky graphite is preferably 20 m 2 / g or less, 15 m 2 / g or less, 10 m 2 / g or less, 5 m 2 / g or less. When the specific surface area of the flaky graphite is small, the ratio of carbon bond defects tends to be small. Therefore, the smaller the specific surface area of the flaky graphite, the lower the reactivity tends to be.

[0048] The specific surface area of the flaky graphite can be measured, for example, by the BET method. The BET specific surface area can be obtained by the one-point method after measuring the adsorption / desorption isotherm with nitrogen gas using a specific surface area and pore size analyzer (QUADRASORB evo manufactured by Anton Paar).

[0049] The content of flaky graphite in the positive electrode active material layer 21 is not particularly limited, but is preferably 1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 8% by mass or less, and even more preferably 3% by mass or more and 8% by mass or less.

[0050] When the content of flaky graphite is within the above numerical range, the conductivity can be improved while maintaining the required content of the positive electrode active material and the cobalt compound in the positive electrode active material layer 21. The degree of graphitization of the positive electrode active material layer 21 obtained by Raman spectroscopy is 0.4 or less. Preferably, the degree of graphitization of the positive electrode active material layer 21 obtained by Raman spectroscopy is 0.3 or less.

[0051] (Evaluation of the degree of graphitization by Raman spectroscopy) In the Raman spectrum, the intensity IG of the peak detected at 1580 - 1620 cm -1 is the intensity of the peak called the G - Band derived from graphite. In the Raman spectrum, the intensity ID of the peak detected at 1300 - 1400 cm -1 is the intensity of the peak called the D - Band derived from the carbon bond defect. The intensity of each peak may be the height of the peak or the area of the peak.

[0052] It is considered that the smaller the ratio R of the intensity ID to the intensity IG is, the better. The reason is as follows. Incidentally, the above R (hereinafter also referred to as the R value) is also referred to as the degree of graphitization.

[0053] When using a positive electrode equipped with a carbon-based conductive additive, the relatively high battery resistance is considered to be caused by the decomposition of the carbon-based conductive additive contained in the positive electrode during charge and discharge, generating substances such as CO and CO2. And the part with bond defects in the carbon-based conductive additive is considered to be a highly reactive site. Therefore, a carbon-based conductive additive with a small R value, which means a low ratio of bond defects, can be said to have lower reactivity than a carbon-based conductive additive with a large R value. Thus, a carbon-based conductive additive with a small R value has excellent resistance to decomposition, suppressing the generation of substances that cause an increase in battery resistance. Therefore, a positive electrode equipped with a carbon-based conductive additive with a small R value can suppress the Rise increase in battery resistance.

[0054] Since the R value of flaky graphite is 0.3 or less, corrosion of the flaky graphite as a carbon-based conductive additive can be suppressed. Similarly, since the R value of the positive electrode active material layer 21 is 0.4 or less, corrosion of the flaky graphite can be suppressed. Thereby, an increase in cell resistance can be suppressed.

[0055] The numerical range of the ratio R of ID / IG, which is the graphitization degree of flaky graphite, is not particularly limited, but 0≦ID / IG<0.4, 0<ID / IG≦0.3, 0<ID / IG≦0.25, 0.002≦ID / IG≦0.2, 0.002≦ID / IG≦0.15, 0.002≦ID / IG≦0.14, 0.05≦ID / IG≦0.2, 0.08≦ID / IG≦0.15, 0.1≦ID / I G G≦0.14 can be exemplified.

[0056] The reactivity of flaky graphite can also be evaluated by the numerical value obtained by multiplying the R value of flaky graphite by the B.E.T specific surface area (m 2 2 / g). The numerical value obtained by multiplying the R value of flaky graphite by the B.E.T specific surface area (m 2 2 / g) is preferably 3.0 or less, 2.5 or less, 2.0 or less, 1.5 or less, 1.0 or less, 0.5 or less.

[0057] (Other components) The positive electrode active material layer 21 may contain, in addition to nickel hydroxide as the positive electrode active material, a cobalt compound layer that coats the positive electrode active material, and flaky graphite, other components.

[0058] Examples of other components include conductive aids other than flaky graphite, binders, additives, antioxidants, and the like. The conductive aid other than flaky graphite is not particularly limited, and examples thereof include acetylene black and carbon black.

[0059] The content of the conductive aid other than flaky graphite is not particularly limited, but it is preferably less than the content of flaky graphite, and more preferably half or less of the content of flaky graphite. The content of the conductive aid other than flaky graphite is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less.

[0060] The binder has the role of fixing the materials contained in the positive electrode active material layer 21 to the surface of the current collector. The binder is not particularly limited, and those used as binders for electrodes of nickel metal hydride batteries can be appropriately employed.

[0061] Specific examples of the binder include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, polyolefin resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, cellulose derivatives such as carboxymethyl cellulose, methyl cellulose, and hydroxypropyl cellulose, copolymers such as styrene-butadiene rubber, and (meth)acrylic resins such as polyacrylic acid, polyacrylate, polymethacrylic acid, and polymethacrylate that contain (meth)acrylic acid derivatives as monomer units.

[0062] The content of the binder is not particularly limited, but it is preferably 0.1% by mass or more and 15% by mass or less, more preferably 0.3% by mass or more and 10% by mass or less, and even more preferably 0.5% by mass or more and 7% by mass or less.

[0063] Specific examples of the additive include, for example, zinc oxide, yttrium oxide and the like. The content of the additive is not particularly limited, but is preferably 0.05% by mass or more and 5% by mass or less, more preferably 0.1% by mass or more and 10% by mass or less, and even more preferably 0.1% by mass or more and 5% by mass or less.

[0064] The antioxidant is not particularly limited, and known antioxidants can be used. Specific examples of the antioxidant include, for example, phosphorus-based antioxidants, amine-based antioxidants, sulfur-based antioxidants, phenol-based antioxidants and the like.

[0065] These may be used alone or in combination of two or more. A phosphorus-based antioxidant and a phenol-based antioxidant may be used in combination. Since the amine-based antioxidant may reduce the self-discharge characteristics due to the shuttle effect by the nitrogen compound, the content is preferably small or not contained.

[0066] The content of the antioxidant is not particularly limited, but is preferably 0.1% by mass or more and 5% by mass or less, more preferably 0.3% by mass or more and 2% by mass or less, and even more preferably 0.5% by mass or more and 1% by mass or less.

[0067] The total content of the other components is not particularly limited, but is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 7% by mass or less.

[0068] Note that the positive electrode active material layer 21 of the present invention does not include those produced using nickel hydroxide particles as a raw material and pre-coated with a cobalt compound. <Negative electrode> The negative electrode 3 includes a current collector foil 30 as a current collector and a negative electrode active material layer 31.

[0069] (Current collector) The material and shape of the current collector are not particularly limited. Materials and shapes similar to those used in the positive electrode can be adopted.

[0070] (Negative electrode active material layer) The negative electrode active material layer 31 contains a hydrogen storage alloy as the negative electrode active material. The hydrogen storage alloy is an alloy of metal A, which easily reacts with hydrogen but has poor hydrogen release ability, and metal B, which hardly reacts with hydrogen but has excellent hydrogen release ability.

[0071] The hydrogen storage alloy is not particularly limited, and those used as the negative electrode active material of nickel-metal hydride batteries can be appropriately adopted. Specific examples of metal A include, for example, Group 2 elements such as Mg, Group 3 elements such as Sc and lanthanoids, Group 4 elements such as Ti and Zr, Group 5 elements such as V and Ta, mischmetal containing a plurality of rare earth elements (hereinafter sometimes abbreviated as Mm), Pd, etc.

[0072] Specific examples of metal B include, for example, Fe, Co, Ni, Cr, Pt, Cu, Ag, Mn, Zn, Al, etc. Specific examples of the hydrogen storage alloy include, for example, AB5 type showing a hexagonal CaCu5 type crystal structure, AB2 type showing a hexagonal MgZn2 type or cubic MgCu2 type crystal structure, AB type showing a cubic CsCl type crystal structure, A2B type showing a hexagonal Mg2Ni type crystal structure, a solid solution type showing a body-centered cubic crystal structure, and AB3 type, A2B7 type, and A5B type in which the crystal structures of AB5 type and AB2 type are combined. 19 Type, etc. can be mentioned.

[0073] The hydrogen storage alloy may have one of the above crystal structures alone, or may have two or more. Also, in each crystal structure, some metals may be substituted with one or two or more other metals or elements.

[0074] The particle size of the hydrogen storage alloy is not particularly limited. It is preferably 1 μm or more and 40 μm or less, more preferably 3 μm or more and 30 μm or less, still more preferably 4 μm or more and 20 μm or less. Further, it is even more preferably 5 μm or more and 15 μm or less, and most preferably 5 μm or more and 12 μm or less.

[0075] Similar to the positive electrode active material layer 21, the negative electrode active material layer 31 may contain other components such as a conductive assistant and a binder. Further, a hydrogen storage alloy with an oxidized surface may be used as the negative electrode active material.

[0076] <Double Electrode -type electrode> Double Electrode The bipolar electrode 4 has a current collector foil 40 as a current collector, a positive electrode active material layer 41 laminated on one surface of the current collector foil 40, and a negative electrode active material layer 42 laminated on the other surface of the current collector foil 40.

[0077] As the current collector foil 40, the positive electrode active material layer 41, and the negative electrode active material layer 42, the same materials and shapes as those used in the positive electrode 2 and the negative electrode 3 can be adopted. Double Electrode The bipolar electrode 4 is also referred to as a bipolar electrode.

[0078] <Separator> The separator 5 separates the positive electrode 2, the negative electrode 3, and the bipolar electrode 4, and prevents short circuits caused by their contact, while providing a storage space and a passage for the electrolytic solution. Electrode The material of the separator 5 is not particularly limited, and known materials can be appropriately adopted.

[0079] Specific examples of the material of the separator 5 include, for example, synthetic resins such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramide, polyester, and polyacrylonitrile, polysaccharides such as cellulose and amylose, natural polymers such as fibroin, keratin, lignin, and suberin, and porous bodies, non-woven fabrics, woven fabrics, etc. made of electrically insulating materials such as ceramics. ​

[0080] As for the material of the separator 5 above, one kind may be used alone, or two or more kinds may be used in combination. The electrolyte impregnated in the separator 5 is an aqueous solution in which a hydroxide of an alkali metal is dissolved. Examples of the hydroxide of an alkali metal include lithium hydroxide, sodium hydroxide, potassium hydroxide and the like.

[0081] As for the hydroxide of the alkali metal above, one kind may be used alone, or two or more kinds may be used in combination, but it is preferable to contain all three kinds. <Method for manufacturing nickel metal hydride battery> The method for manufacturing the nickel metal hydride battery 1 includes a positive electrode manufacturing step, a negative electrode manufacturing step, a dual Electrode type electrode manufacturing step, an electrode body manufacturing step, and an over-discharge step.

[0082] Hereinafter, each step will be described. (Positive electrode manufacturing step) The positive electrode manufacturing step is a step of manufacturing a positive electrode 2 including a positive electrode active material layer 21 having an R value of 0.4 or less by laminating a raw material composition including a positive electrode active material powder containing nickel hydroxide, a cobalt compound such as metal cobalt, and flaky graphite on a current collector foil 20.

[0083] The method for laminating the raw material composition on the current collector foil 20 is not particularly limited. For example, the raw material composition may be kneaded and laminated by applying the kneaded raw material composition on the current collector foil 20. Further, the kneaded raw material composition may be applied on a known transfer sheet, pasted on the current collector foil 20 together with the transfer sheet, and then the transfer sheet may be removed for lamination. Further, after the kneaded raw material composition is formed into a sheet shape, the sheet-shaped raw material composition may be laminated by bonding it to the current collector foil 20. In the positive electrode manufacturing step, known solvents, binders, additives, etc. may be added as other components to the raw material composition. Further, the step of drying the raw material composition may be included.

[0084] The kneading of the raw material composition can be carried out using a known kneader. By changing the kneading conditions, the average particle diameter of the flaky graphite in the kneaded raw material composition can be adjusted. Examples of the kneading conditions include shear force.

[0085] In the raw material composition, it is preferable that the average particle diameter of the flaky graphite is 0.4 times or more the average particle diameter of the positive electrode active material powder. When the average particle diameter of the flaky graphite is 0.4 times or more the average particle diameter of the positive electrode active material, it becomes easier to suppress the particle diameter of the flaky graphite from becoming too small compared to the particle diameter of the positive electrode active material when kneading the raw material composition.

[0086] (Negative electrode manufacturing process) The negative electrode manufacturing process is a process of manufacturing the negative electrode 3 by laminating a raw material composition containing negative electrode active material powder on the current collector foil 30. The negative electrode manufacturing process can be carried out by the same method as the positive electrode manufacturing process.

[0087] (Double Electrode type electrode manufacturing process) Double Electrode The double type electrode manufacturing process is a process of manufacturing a positive electrode by laminating a raw material composition containing positive electrode active material on one surface of the current collector foil 40, and manufacturing a negative electrode by laminating a raw material composition containing negative electrode active material powder on the other surface of the current collector foil 40. The positive electrode and the negative electrode can be manufactured by the same method as the positive electrode manufacturing process and the negative electrode manufacturing process.

[0088] (Electrode body manufacturing process) The electrode body manufacturing process is a process of manufacturing an electrode body by sandwiching the separator 5 with the positive electrode 2, the negative electrode 3, and the double type electrode 4, and impregnating the separator 5 with an electrolytic solution. In the electrode body, the separator 5 is disposed between the positive electrode 2 and the negative electrode 3. Since the double type electrode 4 also has the functions of the positive electrode and the negative electrode, it can also be said that the electrode body is in a state where the separator 5 is disposed between the positive electrode and the negative electrode. Electrode Electrode Electrode Electrode Electrode

[0089] (Over-discharge process) The over-discharge process is a process of charging the electrode body, then over-discharging it, and then charging it again. In the over-discharge process, first, the electrode body is charged. The charging of the electrode body is performed when the cell voltage is 1 V or more.

[0090] Next, the electrode body is over-discharged. The cell voltage in the over-discharge process is preferably 0.2 V or more and less than 1.0 V, and more preferably 0.7 V or more and less than 1.0 V. The above cell voltage may be converted to the positive electrode potential (V vs. Hg / HgO). That is, the preferable range of the over-discharge process may be expressed by the positive electrode potential. The positive electrode potential in the over-discharge process is preferably -0.6 V or more and less than 0.2 V, and more preferably -0.1 V or more and less than 0.2 V.

[0091] After over-discharging, when charging is performed again until the cell voltage becomes 1 V or more, cobalt hydroxide selectively deposits on the positive electrode active material. Then, a coating layer of a cobalt compound such as cobalt oxyhydroxide with an average thickness of 10 nm or less (a higher-order cobalt oxide in which the average valence of cobalt is greater than divalent) is formed on the positive electrode active material.

[0092] Also, a coating layer of a cobalt compound is formed on the surface of the flaky graphite. The thickness of the coating layer on the flaky graphite is thinner than the thickness of the coating layer on the positive electrode active material. Furthermore, a nickel metal hydride battery 1 is manufactured by arranging a pair of cooling members 8, a module positive electrode 22, and a module negative electrode 32 on the electrode body that has undergone the over-discharge process and restraining them with a restraint 9.

[0093] As described above, instead of manufacturing the nickel metal hydride battery 1 after performing the over-discharge process on the electrode body, the over-discharge process may be performed after manufacturing the nickel metal hydride battery 1.

[0094] The operation of this embodiment will be described. First, a case where the over-discharge process is not performed in the manufacturing method of the nickel metal hydride battery 1 will be described.

[0095] As shown in FIG. 2, in the positive electrode active material layer, nickel hydroxide particles as the positive electrode active material, cobalt compounds such as metal cobalt, and flaky graphite are mixed. When an electrolytic solution is injected into the storage space during the assembly of the nickel metal hydride battery, the metal cobalt dissolves. Then, when the first charge is performed, cobalt hydroxide is selectively deposited on the surface of the flaky graphite, and further, the cobalt hydroxide is oxidized to generate cobalt oxyhydroxide having excellent conductivity.

[0096] The mechanism by which cobalt hydroxide is selectively deposited on the surface of the flaky graphite is considered as follows. As shown in FIG. 3, at the time of the first charge, when comparing the nickel hydroxide particles and the flaky graphite in the positive electrode active material layer, the flaky graphite has higher conductivity than the nickel hydroxide particles. Since cobalt hydroxide is more likely to be deposited at a location where the flow of electrons is easier, cobalt hydroxide is selectively deposited on the flaky graphite. Around the flaky graphite, since the concentration of cobalt ions (Co 2+ ) decreases, it becomes easier for cobalt hydroxide to be deposited on the flaky graphite.

[0097] Next, the case of performing the over-discharge process of the present embodiment will be described. As shown in FIGS. 4 and 5, when the over-discharge process is performed and the cell voltage is discharged to be less than 1.0 V, the cobalt oxyhydroxide is reduced and dissolved. After the over-discharge is performed, when the cell voltage is charged again until it exceeds 1 V, cobalt hydroxide is selectively deposited on the positive electrode active material.

[0098] The mechanism by which cobalt hydroxide is selectively deposited on the positive electrode active material is considered as follows. As shown in FIG. 6, when performing the over-discharge process, since the flaky graphite has higher conductivity than the positive electrode active material, the positive electrode active material is more likely to be in a state where its potential is higher than that of the flaky graphite. Cobalt hydroxide is likely to precipitate when the cell voltage is 1 V or higher, so it is likely to precipitate on the positive electrode active material with a higher potential. Also, since the content of the positive electrode active material in the positive electrode active material layer is larger than the content of the flaky graphite, in the positive electrode active material layer, the positive electrode active material has a relatively large surface area. Therefore, cobalt hydroxide precipitates thinner in a state where aggregation is suppressed on the positive electrode active material.

[0099] As shown in FIG. 7, by performing the over-discharge process, the cobalt hydroxide precipitating on the flaky graphite can be reduced. Also, cobalt hydroxide can be selectively precipitated on substantially all surfaces of the nickel hydroxide particles in contact with the electrolytic solution to form a cobalt oxyhydroxide layer with an average thickness of 10 nm or less.

[0100] The effects of this embodiment will be described. (1) A raw material composition having a positive electrode active material powder containing nickel hydroxide, a cobalt compound, and flaky graphite is laminated on a current collector. And it has a positive electrode manufacturing process of manufacturing a positive electrode provided with positive electrode active material layers 21 and 41 in which the content of the cobalt compound is 3% by mass or less and the R value is 0.4 or less. Also, it has a negative electrode manufacturing process of laminating a raw material composition having a negative electrode active material powder on the current collector to manufacture a negative electrode, and an electrode body manufacturing process of disposing a separator 5 between the positive electrode and the negative electrode and impregnating the separator 5 with an electrolytic solution to manufacture an electrode body. Also, after charging the electrode body, it has an over-discharge process of over-discharging and then charging again.

[0101] By performing the over-discharge process, cobalt hydroxide can be selectively precipitated on the positive electrode active material. Furthermore, when cobalt hydroxide is oxidized, the positive electrode active material is coated with cobalt oxyhydroxide having excellent conductivity. Therefore, while suitably reducing the content of the cobalt compound contained in the positive electrode active material layer, an increase in cell resistance can be suppressed.

[0102] (2) In the raw material composition, the average particle diameter of the flaky graphite is 0.4 times or more the average particle diameter of the positive electrode active material powder. Therefore, when kneading the raw material composition, it becomes easier to suppress the particle diameter of the flaky graphite from becoming too small with respect to the particle diameter of the positive electrode active material powder.

[0103] (3) A positive electrode for a nickel-metal hydride battery including a current collector and a positive electrode active material layer, wherein the positive electrode active material layer has a positive electrode active material, a cobalt compound layer covering the positive electrode active material, and flaky graphite, and the content of the cobalt compound in the positive electrode active material layer is 3% by mass or less. Also, the coverage rate of the cobalt compound layer with respect to the positive electrode active material is 50% or more, and the degree of graphitization obtained by Raman spectroscopy of the positive electrode active material layer is 0.4 or less.

[0104] Therefore, while maintaining the function of the cobalt compound layer as a conductive auxiliary agent, the content in the positive electrode active material layers 21 and 41 can be suitably reduced. Also, an increase in cell resistance due to corrosion of the flaky graphite can be suppressed.

[0105] (4) The average particle diameter of the flaky graphite is 0.3 times or more the average particle diameter of the positive electrode active material. The particle diameter of the flaky graphite does not become too small with respect to the particle diameter of the positive electrode active material and is in a state of having a certain size. Therefore, in the positive electrode active material layers 21 and 41, the flaky graphite can be in a state of straddling a plurality of positive electrode active materials, in other words, in a state of the flaky graphite being in contact with a plurality of positive electrode active materials. Therefore, a conductive path can be suitably secured and an increase in the internal resistance of the positive electrode active material layers 21 and 41 can be suppressed.

[0106] (5) The content of the flaky graphite in the positive electrode active material layers 21 and 41 is 3% by mass or more and 10% by mass or less. Therefore, conductivity can be improved while maintaining the necessary content of the positive electrode active material and the cobalt compound in the positive electrode active material layers 21 and 41.

[0107] <Modified Example> Note that this embodiment can be implemented with the following modifications. This embodiment and the following modification examples can be implemented in combination with each other within a technically consistent range.

[0108] ○ In this embodiment, the manufacturing method of the nickel-metal hydride battery 1 includes a positive electrode manufacturing step, a negative electrode manufacturing step, a dual Electrode type electrode manufacturing step, an electrode body manufacturing step, and an over-discharge step, but is not limited to this aspect. The dual Electrode type electrode manufacturing step may be omitted. That is, the nickel-metal hydride battery 1 may not have a dual Electrode type electrode 4, and the positive electrode 2 and the negative electrode 3 may be arranged via a separator 5 to form an electrode body.

[0109] ○ In this embodiment, the over-discharge step is performed as one step in the manufacturing method of the nickel-metal hydride battery 1, but is not limited to this aspect. The over-discharge step may be performed as one step in the manufacturing method of the electrode body. That is, the over-discharge step may be performed as a manufacturing method of the electrode body. Similarly, the over-discharge step may be performed as one step in the manufacturing method of the positive electrode. When performing the over-discharge step in the manufacturing method of the positive electrode, an electrode body for performing the over-discharge step may be used separately. A new electrode body or nickel-metal hydride battery may be manufactured using the positive electrode produced through the over-discharge step.

Example

[0110] Hereinafter, examples further embodying the above embodiment will be described. (Example 1) Nickel hydroxide with an average particle diameter of 8 μm was used as the positive electrode active material.

[0111] Metallic cobalt with an average particle diameter of 5 μm was used as the cobalt compound. The flaky graphite had an average particle diameter of 5 μm, an R value of 0.15, and a BET specific surface area of 11.7 m 2 / g and was used.

[0112] The solid content ratio of nickel hydroxide, metal cobalt, and flaky graphite was blended to be 90.2% by mass, 1% by mass, and 6% by mass. Further, 1% by mass of polyolefin and 1% by mass of carboxymethyl cellulose were blended as a binder. Also, 0.3% by mass of zinc oxide and 0.5% by mass of yttrium oxide were blended as additives to prepare a raw material composition. Note that the blending amount of the above raw material composition is substantially constant even after the production of the positive electrode 2. That is, the blending amount in the raw material composition is substantially equal to the content in the positive electrode 2.

[0113] Next, the raw material composition was kneaded using a known kneader. The shearing force during kneading was 524 Pa. The kneaded raw material composition was applied onto the current collector foil 20 and dried to produce the positive electrode 2. Also, the above negative electrode production process and the Electrode dual Electrode type electrode production process were performed to produce the negative electrode 3 and the Electrode dual type electrode 4. Using the produced positive electrode 2, negative electrode 3, and Electrode dual type electrode 4, the above electrode body production process was performed to produce an electrode body. The produced electrode body was charged up to 1.2 V and then overdischarged down to 0.9 V. Thereafter, it was charged up to 1.2 V again. Further, using a pair of cooling members 8, a module positive electrode 22, a module negative electrode 32, and a pair of restraints 9, a nickel metal hydride battery 1 was produced.

[0114] (Example 2) A nickel metal hydride battery 1 was produced in the same manner as in Example 1, except that the shearing force during kneading the raw material composition was changed to 131 Pa.

[0115] (Example 3) A nickel metal hydride battery 1 was produced in the same manner as in Example 1, except that the shearing force during kneading the raw material composition was changed to 1833 Pa.

[0116] (Example 4) The nickel metal hydride battery 1 was fabricated in the same manner as in Example 1, except that the blending amount of metallic cobalt was 2% by mass, the blending amount of nickel hydroxide was 89.2% by mass, the R value of the flaky graphite was 0.18, and the shearing force during kneading was changed to 100 Pa.

[0117] (Example 5) The nickel metal hydride battery 1 was fabricated in the same manner as in Example 4, except that flaky graphite having an average particle diameter of 7 μm, an R value of 0.15, and a BET specific surface area of 9.5 m 2 / g was used.

[0118] (Example 6) The nickel metal hydride battery 1 was fabricated in the same manner as in Example 4, except that flaky graphite having an average particle diameter of 3 μm, an R value of 0.20, and a BET specific surface area of 16.4 m 2 / g was used.

[0119] (Example 7) The nickel metal hydride battery 1 was fabricated in the same manner as in Example 1, except that the blending amounts of nickel hydroxide, metallic cobalt, and flaky graphite were as shown in Table 1, and the shearing force during kneading was 100 Pa.

[0120] (Example 8) A nickel metal hydride battery was fabricated in the same manner as in Example 7, except that nickel hydroxide having an average particle diameter of 5 μm was used as the positive electrode active material.

[0121] (Example 9) The nickel metal hydride battery 1 was fabricated in the same manner as in Example 7, except that nickel hydroxide having an average particle diameter of 14 μm was used as the positive electrode active material.

[0122] (Example 10) The nickel metal hydride battery 1 was fabricated in the same manner as in Example 1, except that the shearing force during kneading was 100 Pa.

[0123] (Example 11) A nickel metal hydride battery 1 was produced in the same manner as in Example 10, except that the blending amounts of nickel hydroxide, metal cobalt, and flaky graphite were as shown in Table 1, and the blending amount of carboxymethyl cellulose was 1.5% by mass.

[0124] (Example 12) A nickel metal hydride battery 1 was produced in the same manner as in Example 10, except that the blending amounts of nickel hydroxide, metal cobalt, and flaky graphite were as shown in Table 1.

[0125] (Example 13) A nickel metal hydride battery 1 was produced in the same manner as in Example 12, except that flaky graphite having an average particle diameter of 19 μm, an R value of 0.08, and a BET specific surface area of 4.0 m 2 / g was used.

[0126] (Example 14) A nickel metal hydride battery 1 was produced in the same manner as in Example 12, except that an antioxidant was used. As the antioxidant, IRGAFOS 168 (manufactured by BASF Japan Ltd.), a commercially available phosphorus-based antioxidant, was used. The blending amount of the antioxidant was 5% by mass based on the blending amount of flaky graphite. The antioxidant was coated on the flaky graphite by dry mixing while heating at about 200°C in an air atmosphere.

[0127] (Example 15) A nickel metal hydride battery 1 was produced in the same manner as in Example 12, except that flaky graphite having an R value of 0.02 was used. The flaky graphite having an R value of 0.02 was prepared by heat-treating the flaky graphite of Example 1 at about 2800°C in a nitrogen atmosphere.

[0128] (Comparative Example 1) A nickel metal hydride battery was produced in the same manner as in Example 1, except that the shearing force during kneading of the raw material composition was changed to 2618 Pa.

[0129] (Comparative Example 2) A nickel-metal hydride battery was fabricated in the same manner as in Example 1, except that the shearing force during kneading of the raw material composition was changed to 2094 Pa.

[0130] (Comparative Example 3) A nickel-metal hydride battery was fabricated in the same manner as in Example 1, except that the over-discharge process was not performed on the fabricated electrode body.

[0131] (Comparative Example 4) A nickel-metal hydride battery was fabricated in the same manner as in Comparative Example 3, except that the blending amounts of nickel hydroxide, metal cobalt, and flaky graphite were as shown in Table 1.

[0132] Table 1 shows the average particle diameters of nickel hydroxide, metal cobalt, and flaky graphite, the particle size ratio of metal cobalt to flaky graphite, the blending amounts of raw materials, the graphitization degree of flaky graphite, the BET specific surface area of flaky graphite, and the value obtained by multiplying the R value by the BET specific surface area in the raw material compositions of Examples 1 to 15 and Comparative Examples 1 to 4. They are as described in the columns of "Average particle diameter (μm)", "Particle size ratio", "Blending amount (wt%)", "R value of flaky graphite", "BET specific surface area (m 2 / g)", and "R value × BET specific surface area" in Table 1, respectively.

[0133] Also, the presence or absence of an antioxidant, the shearing force during kneading in the manufacturing process, and the presence or absence of an over-discharge process are as described in the columns of "Antioxidant ○: present ×: absent", "Shearing force during kneading (Pa)", and "Over-discharge process ○: present ×: absent" in Table 1, respectively.

[0134] The BET specific surface area was determined by the one-point method after measuring the adsorption / desorption isotherm with nitrogen gas using a specific surface area and pore size analyzer (QUADRASORB evo, manufactured by Anton Paar).

[0135]

Table 1

[0136]

Table 2

[0137] In nickel metal hydride batteries of Examples 1 to 15 and Comparative Examples 1 to 4, the following evaluations were carried out. (Average particle diameter and particle size ratio) The average particle diameters of the positive electrode active material and the flaky graphite contained in the positive electrode active material layers 21 and 41 were measured using a known scanning electron microscope. Fifty particles were randomly measured, and the average particle diameter D50 was calculated with the long side of the flaky graphite as the particle diameter. The results are shown in the columns of "Average particle diameter (μm)" and "Particle size ratio" in Table 2.

[0138] (Degree of graphitization of the positive electrode active material layer) The positive electrode active material layers 21 and 41 were measured using a known Raman spectrometer. From the obtained Raman spectrum, the degree of graphitization was calculated by the above method. The results are shown in the column of "R value of the positive electrode active material layer" in Table 2.

[0139] The measurement conditions for Raman spectroscopic analysis using a Raman spectrometer are as follows. Apparatus: Raman-11 (manufactured by Nanophoton Co., Ltd.) Measurement mode: XY-average Objective lens: 50 times Laser wavelength: 532 nm Laser output: 0.2 mW Measurement time: 3 seconds Diffraction grating: 300 gr / mm Measurement temperature: Room temperature Measurement atmosphere: Air (Average thickness and coating rate of the cobalt compound layer coating the positive electrode active material) The average thickness and coating rate of the cobalt compound layer coating the positive electrode active material were observed and measured by TEM-EELS or the like.

[0140] (Cell resistance) The cell resistance was measured when discharging for 0.2 seconds under the conditions of 25°C and a charging rate (SOC) of 60%. The results are shown in the column of "Cell resistance at 0.2 seconds (mΩ)" in Table 2.

[0141] (Charge-discharge efficiency) For the nickel-metal hydride batteries of Examples 1 to 15 and Comparative Examples 1 to 4, after charging to SOC 100 at a rate of 1 / 3C under the condition of a temperature of 25°C, discharging was performed to 1.0V at a rate of 1 / 3C. Then, the charge-discharge efficiency of each nickel-metal hydride battery was calculated using the following formula.

[0142] Charge-discharge efficiency (%) = 100 × (discharge capacity) / (charge capacity) The results are shown in the column of "Charge-discharge efficiency (%)" in Table 2. The nickel-metal hydride batteries of Examples 4 and 12 to 15 were further evaluated as follows.

[0143] (Increase in cell resistance) The increase in cell resistance before and after a durability test of 14,000 km was measured. The measurement conditions were to measure the cell resistance when discharging for 5 seconds under the conditions of 0°C and a charging rate (SOC) of 60%. The results are shown in the column of "Increase in cell resistance at 0°C for 5 seconds (mΩ) (after 14,000 km of durability - before durability)" in Table 2.

[0144] (Evaluation results) In Comparative Examples 1 and 2, the average particle diameter of the flaky graphite in the positive electrode active material layer was as small as 0.5 μm. Due to the high shearing force during kneading, it is considered that the flaky graphite was broken and fragmented. Since it was difficult for the flaky graphite to be in contact with a plurality of positive electrode active materials, the cell resistance is considered to have increased. In addition, due to the fragmentation of the flaky graphite, the graphitization degree of the positive electrode active material layer exceeded 0.4.

[0145] In Comparative Examples 3 and 4, since the over-discharge process was not performed, the cobalt compound layer was mainly deposited on the flaky graphite in a state where the average thickness exceeded 10 nm. The coverage rate of the cobalt compound layer with respect to the positive electrode active material was about 30%.

[0146] In Examples 1 to 15 and Comparative Examples 1 and 2, the average thickness of the cobalt compound layer was 10 nm or less, and the coverage rate was 70% or more. In Examples 1 to 15, the average particle diameter of the flaky graphite in the positive electrode active material layers 21 and 41 was 3 μm or more, and it was not fragmented. Since the flaky graphite was likely to be in contact with a plurality of positive electrode active materials, it was found that the cell resistance was kept low. Also, the graphitization degree of the positive electrode active material layers 21 and 41 was 0.4 or less. Since the ratio of bond defects was small, the reactivity of the positive electrode active material layers 21 and 41 could be lowered.

[0147] Moreover, since the particle size ratio of nickel hydroxide to flaky graphite was 0.3 or more, it was found that the cell resistance was even lower. Also, in Examples 1 to 15, the charge-discharge efficiency was excellent at 94% or more.

[0148] In Example 12, compared with Example 4, flaky graphite with a small R value was used. That is, flaky graphite with advanced graphitization was used. Since the reactivity of the flaky graphite decreased due to the advanced graphitization, the increase amount of the cell resistance was kept low.

[0149] In Example 13, compared with Example 12, flaky graphite having a large average particle diameter, a small BET specific surface area, and a small R value was used. The value obtained by multiplying the R value and the BET specific surface area was 0.32. Generally, the larger the average particle diameter of the flaky graphite or the smaller the BET specific surface area, the more likely the graphitization is to progress. The more the graphitization progresses, the lower the reactivity of the flaky graphite, so the increase amount of the cell resistance was kept low.

[0150] In Example 14, compared with Example 12, the difference was that an antioxidant was used. By using the antioxidant, the reactivity of the flaky graphite decreased, so the increase amount of the cell resistance was kept low.

[0151] In Example 15, heat-treated flaky graphite was used. Compared with Example 12, the R value was smaller, indicating that graphitization had progressed. The product of the R value and the BET specific surface area was 0.23. Since the reactivity of the flaky graphite was lower, the increase in cell resistance was kept low.

Explanation of Signs

[0152] 1…Nickel metal hydride battery, 2…Positive electrode, 3…Negative electrode, 4…Double Electrode type electrode, 5…Separator, 6…Sealing member, 7…Outer frame, 8…Cooling member, 9…Restraint, 20…Current collector foil, 21…Positive electrode active material layer, 22…Module positive electrode, 30…Current collector foil, 31…Negative electrode active material layer, 32…Module negative electrode, 40…Current collector foil, 41…Positive electrode active material layer, 42…Negative electrode active material layer, 80…Through hole.

Claims

1. A positive electrode manufacturing step of manufacturing a positive electrode including a positive electrode active material layer having a positive electrode active material powder containing nickel hydroxide, a cobalt compound, and flaky graphite laminated on a current collector, wherein the content of the cobalt compound is 3% by mass or less, and the graphitization degree obtained by Raman spectroscopy is 0.4 or less, A negative electrode manufacturing step of manufacturing a negative electrode by laminating a raw material composition having a negative electrode active material powder on a current collector, An electrode body manufacturing step of disposing a separator between the positive electrode and the negative electrode and impregnating the separator with an electrolytic solution to manufacture an electrode body, A method for manufacturing a nickel-metal hydride battery, comprising an over-discharge step of over-discharging and then recharging the electrode body after charging the electrode body.

2. The method for manufacturing a nickel-metal hydride battery according to claim 1, wherein in the raw material composition, the average particle diameter of the flaky graphite is 0.4 times or more the average particle diameter of the positive electrode active material powder.

3. A positive electrode for a nickel-metal hydride battery including a current collector and a positive electrode active material layer, wherein the positive electrode active material layer has a positive electrode active material, a cobalt compound layer covering the positive electrode active material, and flaky graphite, the content of the cobalt compound in the positive electrode active material layer is 3% by mass or less, the coverage rate of the cobalt compound layer with respect to the positive electrode active material is 50% or more, and the positive electrode active material layer has a graphitization degree obtained by Raman spectroscopy of 0.4 or less.

4. The positive electrode for a nickel-metal hydride battery according to claim 3, wherein the average thickness of the cobalt compound layer is 10 nm or less.

5. The positive electrode for a nickel-metal hydride battery according to claim 3, wherein the average particle diameter of the flaky graphite is 0.3 times or more the average particle diameter of the positive electrode active material.

6. The positive electrode for a nickel-metal hydride battery according to claim 3, wherein the content of the flaky graphite in the positive electrode active material layer is 3% by mass or more and 10% by mass or less.

7. The positive electrode for a nickel-metal hydride battery according to claim 3, wherein the cobalt compound layer is also formed on the surface of the flaky graphite.

8. A nickel-metal hydride battery including the positive electrode for a nickel-metal hydride battery according to any one of claims 3 to 7.

Images