Nickel-hydrogen accumulator

Abstract

This invention provides a nickel-metal hydride (NiMH) battery that suppresses cobalt deposition in a separator. The NiMH battery comprises a positive electrode, a negative electrode, a separator separating the positive and negative electrodes, and an alkaline electrolyte. The positive electrode has a positive electrode active material primarily composed of nickel hydroxide, and the positive electrode active material contains cobalt (Co). The negative electrode has a hydrogen storage alloy containing cobalt (Co) and manganese (Mn). In this NiMH battery, (weight of positive electrode Co × positive electrode porosity + weight of negative electrode Co × negative electrode porosity) / (weight of negative electrode Mn × negative electrode porosity) is 0.85 or more and 1.61 or less, the porosity of the separator is 64% or more, and the weight ratio of cobalt (Co) to manganese (Mn) in the negative electrode is 0.26 or less. Therefore, the deposition of cobalt in the separator can be suppressed by Mn, thereby suppressing the formation of Co-based conductive pathways.

Description

Technical Field

[0001] This invention relates to nickel-metal hydride batteries, and more specifically, to nickel-metal hydride batteries with suppressed self-discharge. Background Technology

[0002] Electric vehicles (including hybrid vehicles) equipped with electric motors use electricity stored in secondary batteries to drive the motors. Among such secondary batteries, nickel-metal hydride batteries, for example, are widely used as vehicle secondary batteries because they can safely undergo high-current charging and discharging.

[0003] In such nickel-metal hydride batteries, cobalt hydroxide (Co(OH)2), which has high conductivity, is typically added to nickel hydroxide (Ni(OH)2), which has low conductivity, as the positive electrode active material. This cobalt hydroxide is oxidized during the initial charging process, becoming cobalt hydroxide oxide (CoOOH), thus improving the conductivity of the positive electrode.

[0004] Here, the invention disclosed in Patent Document 1 indicates a high correlation between the amount of cobalt, zinc, and manganese deposited in the separator and the reduction in output characteristics and self-discharge characteristics. The reason for this is that zinc and cobalt contained in the positive electrode, and manganese in the hydrogen storage alloy serving as the negative electrode active material, are slowly but in minute quantities dissolved into the alkaline electrolyte during charge-discharge cycles. More specifically, on the positive electrode side of the separator, cobalt-manganese compounds are deposited in contact with the positive electrode, while on the negative electrode side, zinc-manganese compounds are deposited. Furthermore, as the amount of these deposits increases, the substantial distance between the positive and negative electrodes decreases.

[0005] Therefore, Patent Document 1 proposes the following solution: by addressing such problems and limiting the amount of zinc, cobalt and manganese deposited in the separator, a nickel-metal hydride battery that can maintain high output characteristics and low self-discharge rate for a long time is provided.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2004-296394 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] However, it is desirable to further suppress the precipitation of cobalt in the separator.

[0011] The problem to be solved by the nickel-metal hydride battery of the present invention is to suppress the precipitation of cobalt in the separator.

[0012] Methods for solving problems

[0013] According to one aspect of this disclosure, a nickel-metal hydride (NiMH) battery comprises a positive electrode, a negative electrode, a separator separating the positive and negative electrodes, and a filled electrolyte. The positive electrode has a positive electrode active material mainly composed of nickel hydroxide, and the positive electrode active material contains cobalt (Co). The negative electrode has a hydrogen storage alloy containing cobalt (Co) and manganese (Mn). The NiMH battery is characterized in that (weight of positive electrode Co × positive electrode porosity + weight of negative electrode Co × negative electrode porosity) / (weight of negative electrode Mn × negative electrode porosity) is 0.85 or more and 1.61 or less, the porosity of the separator is 64% or more, and the weight ratio of cobalt (Co) to manganese (Mn) in the negative electrode is 0.26 or less.

[0014] In the aforementioned nickel-metal hydride battery, the electrolyte may contain tungsten (W) at a weight of 0.48% or more relative to the weight of the aforementioned positive electrode active material.

[0015] In the aforementioned nickel-metal hydride battery, the electrolyte may contain tungsten (W) at a weight of 0.72% or more relative to the weight of the aforementioned positive electrode active material.

[0016] In the aforementioned nickel-metal hydride batteries, the porosity of the separator can be below 70%.

[0017] The effects of the invention

[0018] The nickel-metal hydride battery according to the present invention can suppress the precipitation of cobalt in the separator. Attached Figure Description

[0019] Figure 1 This is a cross-sectional view of the electrode assembly in the nickel-metal hydride battery of this embodiment.

[0020] Figure 2 (a) is a schematic diagram showing the self-discharge when there is a large amount of Co deposited in the separator. Figure 2 (b) is a schematic diagram showing the self-discharge in which a large amount of Mn is deposited in the spacer and the deposition of Co is suppressed.

[0021] Figure 3 This is a graph showing the relationship between the ratio of deposited Co / Mn in the separator and the self-discharge [Ah].

[0022] Figure 4 This is a graph showing the relationship between the ratio of Co and Mn dissolved into the electrolyte (RE) and the self-discharge amount (SD[Ah]).

[0023] Figure 5 This is a graph showing the relationship between the porosity [%] of the spacer and the ratio of deposited Co / Mn in the spacer to the self-discharge [Ah].

[0024] Figure 6This is a graph showing the relationship between the ratio of dissolved Co / Mn in the negative electrode and the self-discharge [Ah].

[0025] Figure 7 This is a schematic diagram showing the reaction when tungsten (W) is absent in the electrolyte.

[0026] Figure 8 This is a schematic diagram illustrating the reaction in the presence of tungsten (W) in the electrolyte.

[0027] Figure 9 This is a graph showing the relationship between the amount of tungsten in the electrolyte and the ratio of Co / Mn deposited in the separator.

[0028] Figure 10 This is a flowchart illustrating the manufacturing process of the nickel-metal hydride battery according to this embodiment. Detailed Implementation

[0029] The following is for reference Figures 1-10 The present invention will be described through an embodiment of a nickel-metal hydride battery.

[0030] (Summary of this implementation method)

[0031] <Ni-MH battery of this embodiment>

[0032] The nickel-metal hydride battery of this embodiment is a battery module housed in a battery pack for driving electric vehicles, etc. Each battery module has a battery casing with multiple (e.g., eight) electrode cells. Each electrode cell houses an electrode assembly 1, which includes a positive electrode plate 2 and a negative electrode plate 3, which are battery elements. Electrolyte is filled into the multiple electrode cells. Thus, each electrode cell constitutes a battery cell. Multiple battery cells are connected in series, and power is input and output via external terminals.

[0033] <Electrode Group 1>

[0034] Figure 1 This is a cross-sectional view of electrode assembly 1 in the nickel-metal hydride battery of this embodiment. Figure 1 As shown, electrode assembly 1 is constructed by laminating rectangular positive electrode plate 2 and negative electrode plate 3 with a separator 4 in between. In this case, the direction in which the positive electrode plate 2, negative electrode plate 3, and separator 4 are laminated is the thickness direction, and the horizontal direction is the width direction.

[0035] The positive electrode plate 2 and negative electrode plate 3 of the electrode assembly 1 have lead-out portions 21 and 31 that protrude from opposite sides in the direction of the electrode surface. The positive electrode current collector 27 is joined to the side edge of the lead-out portion 21 of the positive electrode plate 2 by spot welding or the like. Similarly, the negative electrode current collector 37 is joined to the side edge of the lead-out portion 31 of the negative electrode plate 3 by spot welding or the like.

[0036] A separator 4 is disposed between the positive electrode plate 2 and the negative electrode plate 3, and the separator 4 is impregnated with electrolyte 5.

[0037] <Positive Plate 2>

[0038] The positive electrode plate 2 has a porous substrate such as foamed nickel, which maintains the shape of the positive electrode plate 2. A positive electrode composite material layer is filled in the porous substrate. In addition, the substrate functions as a current collector for collecting electricity from the positive electrode active material of the filled positive electrode composite material layer. The positive electrode composite material layer includes a positive electrode active material with nickel hydroxide (Ni(OH)2) as the main component, conductive materials, thickening materials, and binder materials. The particles of the positive electrode active material have a coating layer provided on the surface of the nickel hydroxide particles. This coating layer has cobalt hydroxide oxide (CoOOH) as the main component. It should be noted that, regarding the cobalt contained in the positive electrode composite material layer, when the nickel-metal hydride secondary battery is first charged, the cobalt hydroxide (Co(OH)2) contained therein undergoes electrochemical oxidation, thereby precipitating in the form of cobalt hydroxide oxide. A high-density layer is formed by the coating layer formed before charging and the cobalt hydroxide oxide precipitated after charging.

[0039] <Negative Plate 3>

[0040] In the negative electrode plate, a negative electrode composite material layer is coated onto a perforated metal or similar substrate. The negative electrode composite material layer contains a hydrogen storage alloy (MH) as the negative electrode active material. This hydrogen storage alloy contains cobalt (sometimes referred to as "Co") and manganese (sometimes referred to as "Mn"). The negative electrode plate 3 will be described in detail below.

[0041] <Separator 4>

[0042] The separator 4 is made of nonwoven fabric of olefin resins such as polypropylene, or, as needed, the nonwoven fabric is subjected to hydrophilic treatment such as sulfonation. The separator 4 functions to isolate the positive electrode plate 2 from the negative electrode plate 3 to prevent short circuits. In addition, the separator 4 functions to retain the electrolyte 5 so that a reaction can occur at the electrode.

[0043] The separator 4 will cause Co, Mn, and other metals that have dissolved into the electrolyte 5 to precipitate on the separator 4. Such metal precipitation essentially shortens the distance between the electrodes and increases the self-discharge SD[Ah].

[0044] <Structure of this embodiment>

[0045] As described in the prior art, when highly conductive Co is deposited in the separator 4, the inter-electrode distance substantially decreases and the self-discharge SD[Ah] increases. However, Co is essential for improving conductivity in the positive electrode, making it difficult to eliminate Co. In view of this situation, the inventors have discovered a method that allows the presence of Co within the electrode while suppressing Co deposition in the separator 4 on the other hand. The conditions for implementing this method will be explained.

[0046] <1st condition>

[0047] The first condition is that, when RE = (weight of positive electrode Co × porosity of positive electrode + weight of negative electrode Co × porosity of negative electrode) / (weight of negative electrode Mn × porosity of negative electrode), the ratio RE is 0.85 or higher and 1.61 or lower. Here, the weight of positive electrode Co is the weight of Co contained in the positive electrode, the weight of negative electrode Co is the weight of Co contained in the negative electrode, and the weight of negative electrode Mn is the weight of Mn contained in the negative electrode.

[0048] The first condition is the ratio RE of the amount of Co dissolved into electrolyte 5 to the amount of Mn. The inventors have discovered that Mn inhibits the deposition of Co and helps to improve the internal resistance (DC-IR).

[0049] Here, the amount of Co dissolved from the positive electrode into electrolyte 5 depends on the amount of Co contained in the positive electrode, and moreover, the amount of Co dissolved from the positive electrode also depends on the porosity of the positive electrode. Similarly, the amount of Co dissolved from the negative electrode into electrolyte 5 depends on the amount of Co contained in the negative electrode, and moreover, the amount of Co dissolved from the negative electrode also depends on the porosity of the negative electrode. In addition, the amount of Mn dissolved from the negative electrode into electrolyte 5 depends on the amount of Mn contained in the negative electrode, and moreover, the amount of Mn dissolved from the negative electrode also depends on the porosity of the negative electrode.

[0050] Therefore, based on these technical concepts, the amount of Co dissolved from the positive electrode is defined as "positive electrode Co weight × positive electrode porosity", and the amount of Co dissolved from the negative electrode is defined as "negative electrode Co weight × negative electrode porosity". On the other hand, the amount of Mn dissolved from the negative electrode is defined as "negative electrode Mn weight × negative electrode porosity". Here, in this application, the ratio of the amount of Co dissolved into the electrolyte 5 (weight of Co) to the amount of Mn dissolved into the electrolyte 5 (weight of Mn) is sometimes simply referred to as "dissolved Co / Mn". And the ratio RE (dissolved Co / Mn) is calculated using the following formula.

[0051] RE = (Polyoxal Co weight × Positive electrode porosity + Negative Electrode Co weight × Negative electrode porosity) / (Negative Electrode Mn weight × Negative electrode porosity)

[0052] It was found that under these conditions, the ratio of dissolved Co / Mn (RE) became the inhibitory condition for Co precipitation in septum 4.

[0053] Specifically, as a first condition, when the RE ratio is 0.85 or higher and the RE ratio is 1.61 or lower, the precipitation of Co in the spacer 4 can be effectively suppressed.

[0054] <Relationship between the ratio of dissolved Co / Mn (RE) and the self-discharge rate (SD)>

[0055] Figure 2 (a) is a schematic diagram showing the self-discharge when there is a lot of Co deposited in the separator 4. Figure 2 (b) is a schematic diagram showing the self-discharge when a large amount of Mn is deposited in the separator 4 and the deposition of Co is suppressed.

[0056] Here, as Figure 2 As shown in (a), when the Co / Mn dissolution ratio RE is greater than 1.61, multiple Mn-based conductive paths Cp2 (indicated by dashed arrows) and multiple Co-based conductive paths Cp1 (indicated by solid arrows) are generated in the separator 4. Co has high conductivity, therefore the resistance in the Co conductive path Cp1 is low, and current flows easily. Thus, a path for easy current flow is formed close together between the positive and negative electrodes, thereby increasing the self-discharge SD[Ah].

[0057] On the other hand, as shown in Figure 2(b), when the ratio of dissolved Co / Mn (RE) is 0.85 or higher and 1.61 or lower, the proportion of Mn precipitation increases in the separator 4, thus suppressing the formation of the Co-based conduction pathway Cp1, indicated by the solid arrow. Compared to the highly conductive Co-based conduction pathway Cp1, the Mn-based conduction pathway Cp2, indicated by the dashed arrow, has lower conductivity, resulting in increased resistance and difficulty in current flow. Therefore, the self-discharge SD[Ah] is suppressed to a very low level.

[0058] <Graph of Co / Mn deposition ratio RP versus self-discharge SD>

[0059] Sometimes the ratio of the amount of Co precipitated in the separator 4 (by weight of Co) to the amount of Mn precipitated in the separator 4 (by weight of Mn) is simply referred to as "precipitated Co / Mn".

[0060] Figure 3 This is a graph showing the relationship between the ratio of Co deposited in separator 4 to Mn deposited in separator 4 (deposited Co / Mn), i.e., the ratio RP, and the self-discharge amount SD[Ah].

[0061] It can be seen that the ratio RP of Co / Mn deposited in the separator depends on the ratio RE of Co / Mn dissolved in electrolyte 5.

[0062] like Figure 3 As shown, when the ratio RP of precipitated Co / Mn in septum 4 increases, as... Figure 2 As explained in (a), the increased number of conductive pathways Cp1 formed by Co in the separator 4 leads to an increase in the self-discharge quantity SD[Ah]. On the other hand, as... Figure 3 As shown, when the ratio RP of precipitated Co / Mn in septum 4 decreases, as... Figure 2 As illustrated in (b), the number of conductive pathways Cp1 formed by Co in the separator 4 is reduced, and the self-discharge SD[Ah] is suppressed to a very small extent. Figure 3 The plotted points also confirm a strong correlation between the deposited Co / Mn ratio RP in separator 4 and the self-discharge rate SD[Ah]. Therefore, it can be said that the relationship between the deposited Co / Mn ratio RP in separator 4 and the self-discharge rate SD[Ah] is... Figure 3 The relationship is shown in the diagram.

[0063] <Relationship between the Co / Mn leaching ratio (RE) of the electrolyte and the self-discharge rate (SD[Ah])>

[0064] Figure 4 This is a graph showing the relationship between the ratio of Co dissolved in the electrolyte to the amount of Mn dissolved in the electrolyte (dissolved Co / Mn), i.e., the ratio RE, and the self-discharge amount SD [Ah] in this embodiment. As described above, the dissolved Co / Mn ratio of electrolyte 5 is derived from "RE = (weight of positive electrode Co × positive electrode porosity + weight of negative electrode Co × negative electrode porosity) / (weight of negative electrode Mn × negative electrode porosity)". It should be noted that, as described above, the dissolved Co / Mn ratio RE is strongly correlated with the deposited Co / Mn ratio RP in the separator.

[0065] In this embodiment, the self-discharge amount SD[Ah] is set to be approximately less than 1.03[Ah].

[0066] Here, if the ratio of dissolved Co / Mn (RE) is greater than 1.61, then in Figure 2 As shown in (a), a large number of conductive pathways Cp2 are generated by Co on the separator 4, and the self-discharge SD[Ah] is approximately 1.03[Ah] higher. On the other hand, if the ratio of dissolved Co / Mn RE is less than 0.85, then in Figure 2 The number of conductive paths Cp2 generated by Co on the septum 4 shown in (a) is reduced, while a large number of conductive paths Cp1 are generated by Mn, and the self-discharge SD[Ah] is increased by approximately 1.03[Ah].

[0067] Therefore, in this embodiment, the preferred range is a Co / Mn leaching ratio RE of 0.85 or more and 1.61 or less.

[0068] <Condition 2>

[0069] The second condition is that the porosity P[%] of the spacer 4 is 64% or higher. Here, "porosity P[%]" is defined as the percentage of pore volume in the total volume of the material. Measurement methods include optical methods, liquid immersion methods, mercury porosimetry, etc. In this embodiment, mercury porosimetry is used for measurement, and it is taken as the median diameter (d50) in the pore size distribution frequency.

[0070] <Relationship between porosity P [%) of spacer 4, the ratio of deposited Co / Mn RP, and self-discharge SD [Ah]>

[0071] Figure 5 This is a graph showing the relationship between the porosity P [%] of the spacer 4, the ratio RP of deposited Co / Mn in the spacer, and the self-discharge SD [Ah]. Here, Figure L1 represents the ratio RP of deposited Co / Mn, and Figure L2 represents the self-discharge SD [Ah].

[0072] <Lower limit of porosity P [%)>

[0073] If the porosity P[%] of the separator 4 is large, the electrolyte retention capacity of the separator 4 itself will increase. As a result, the amount of electrolyte 5 in contact with the electrode interface increases, which is considered to suppress uneven reaction.

[0074] The hydrogen storage alloy at the negative electrode undergoes micronization due to the lattice volume change caused by the absorption and release of hydrogen. This micronization increases the surface area, enhances the reactivity with electrolyte 5, and facilitates the dissolution of elements such as Co and Mn.

[0075] If the porosity P[%] of the separator 4 is high, the reaction unevenness is suppressed, thereby allowing for uniform use of the negative electrode plate 3. Consequently, the negative electrode as a whole uniformly absorbs and releases hydrogen, leading to micronization of the hydrogen storage alloy within the negative electrode and an increase in the amount of Mn dissolved from the alloy. That is, the overall Co / Mn precipitation ratio RP of the battery increases. This is particularly significant in alloys with a low Co / Mn precipitation ratio RP in the separator.

[0076] The result, such as Figure 5As shown, when the porosity P[%) is less than 64%, the ratio RP of precipitated Co / Mn increases. That is, the precipitation of Mn in separator 4 decreases. As explained above, this is because the ratio RA of dissolved Co / Mn at the negative electrode decreases, while the amount of Co dissolved at the positive electrode does not change as much as at the negative electrode. Therefore, when the porosity P[%) is less than 64%, Figure 3 The relationship between the dissolved Co / Mn ratio RE shown has changed, which lacks the premise of this invention.

[0077] Therefore, by satisfying the second condition, it is possible to make the proportion RE, which is the first condition, 0.85 or more and the proportion RE 1.61 or less.

[0078] <Upper limit of porosity P [%)>

[0079] It should be noted that in this embodiment, the upper limit of the porosity P[%] of the spacer is set to 70%. The reason for this is that if the porosity P[%] of the spacer 4 is greater than 70%, the mechanical strength of the spacer 4 itself will decrease and it will be easily flattened, or its ability to prevent small short circuits will become worse. Therefore, it is not directly related to the first to fourth conditions that are features of this embodiment.

[0080] <3rd condition>

[0081] The third condition is that the ratio of Co / Mn dissolved from the negative electrode, RA, is 0.26 or less. Here, the structure of the negative electrode plate 3 will be described in detail first.

[0082] <Negative Plate 3>

[0083] The negative electrode plate 3 has a substrate and a negative electrode compound layer disposed on the substrate.

[0084] Hydrogen storage alloys are alloys that reversibly absorb and release hydrogen. When "A" represents the element that forms hydrides and "B" represents the element that does not form hydrides, any one or a combination of AB, AB5, AB2, and A2B7 types can be used. AB-type hydrogen storage alloys can use TiCo, ZrCo, etc. AB5-type hydrogen storage alloys can use MmNi5, etc. It should be noted that "Mm" refers to an alloy containing multiple rare earth elements, i.e., mixed rare earth metals. Specifically, as MmNi5, MmNi5-x(Co,Mn,Al)x series alloys and MmNi5-x(Co,Mn,Al,Fe)x series alloys, in which a portion of nickel (Ni) is replaced by Co, Mn, Al, etc., can be appropriately used. Mixed rare earth metals include at least one of lanthanum (La), cerium (Ce), neodymium (Nd), and samarium (Sm). Alternatively, vanadium (V) series and magnesium (Mg) series alloys can be used to replace the above alloys, or vanadium (V) series and magnesium (Mg) series alloys can be used in addition to the above alloys.

[0085] The hydrogen storage alloy of this embodiment contains cobalt (Co) and manganese (Mn).

[0086] The negative electrode plate 3 is manufactured as follows: carbon black and other thickening materials and styrene-butadiene copolymer and other binding materials are added to the hydrogen storage alloy and processed into a paste. The resulting negative electrode paste is then attached to a substrate made of metal material, and the substrate is dried, rolled and cut to manufacture the negative electrode plate.

[0087] <Ratio of dissolved Co / Mn in the negative electrode RA>

[0088] Figure 6 This is a graph showing the relationship between the ratio RA of dissolved Co / Mn in the negative electrode and the self-discharge amount SD[Ah].

[0089] like Figure 6 As shown, if the Co / Mn dissolution ratio RA at the negative electrode is greater than 0.26, the self-discharge SD[Ah] increases sharply. This is due to the increased proportion of Co dissolution from the negative electrode. Since the positive electrode contains virtually no Mn, if the proportion of Co dissolution from the negative electrode increases, the Co / Mn dissolution ratio RE in the electrolyte also increases. Therefore, since the amount of Co dissolution from the positive electrode is not easily changed, it is difficult to suppress the self-discharge SD[Ah] to 1.03 or less, and the Co / Mn dissolution ratio RE in the electrolyte to 1.61 or less. Thus, it is difficult to achieve the objective of this embodiment.

[0090] Therefore, by satisfying the third condition, the proportion RE, which is the first condition, can be 0.85 or more and the proportion RE can be 1.61 or less.

[0091] <Condition 4>

[0092] In condition 4, the amount of tungsten (hereinafter sometimes simply referred to as "W") contained in the electrolyte 5 is 0.48% by weight or more relative to the weight of the positive electrode active material of the positive electrode plate 2. Here, the electrolyte 5 will be described first. It should be noted that conditions 1 to 3 are essential components of this embodiment, while condition 4 is a preferred optional condition.

[0093] <Electrolyte 5>

[0094] Electrolyte 5 is held in spacer 4. The material of spacer 4 is not particularly limited, and may be, for example, nonwoven fabric, a resin membrane with a large number of micropores, other sheets capable of holding liquid, or combinations thereof. As described above, the porosity P of spacer 4 is configured to be 64% or more and 70% or less.

[0095] The electrolyte is an alkaline aqueous solution with potassium hydroxide (KOH) as the main solute and contains at least tungsten. Tungsten is contained in tungsten compounds, which are used as solutes.

[0096] The tungsten compounds used as solutes can be tungsten oxides such as WO2, WO3, and W2O5 (WxOy, where x and y are real numbers), hydrates of tungsten oxides such as WO3·H2O and W2O5·H2O. Among these, tungsten compounds that can be used include ZrW2O8, Al2(WO4)3, WC, CaWO4, FeWO4, MnWO4, WCl6, WBr6, WCl2F4, W(CO)6, WO2Cl2, Li2WO2, H2WO4, K2WO4, Na2WO4, Li2WO4·2H2O, H2WO4·2H2O, K2WO4·2H2O, Na2WO4·2H2O, (NH4)3PO4·12WO3·3H2O, Na3(PO4·12WO3)·xH2O, WF5, and WF6.

[0097] <Tungsten W in electrolyte 5>

[0098] Figure 7 This is a schematic diagram illustrating the reaction when tungsten (W) is absent in electrolyte 5. For example... Figure 7 As shown, in electrolyte 5 where W is absent, there are few available hydroxyl groups, and only the reactions at sites where electron donation and acceptance are easily carried out are promoted.

[0099] As mentioned above, if the Co content is high, it will precipitate on the separator 4, resulting in an increased self-discharge rate SD[Ah]. However, the reduced Co content in the hydrogen storage alloy reduces the number of usable hydroxyl groups in the electrolyte 5 (where W is absent), promoting reactions only at sites where electron donation and acceptance are easily achieved. In this case, the surface reaction of the hydrogen storage alloy becomes uneven, leading to the precipitation and phase separation of hydroxides derived from metals other than nickel and nickel metal derived from nickel. When a hydroxide and metal coating is formed on the surface of the hydrogen storage alloy, its reactivity is locally reduced. Furthermore, this coating is only partially formed on the surface. On the other hand, in areas where no coating is formed, electrons concentrate due to high reactivity, resulting in localized overcharging and over-discharging. As a result, uneven reaction occurs within the negative electrode plate 3, increasing the internal resistance of the negative electrode as a whole. In addition, in areas where localized overcharging and over-discharging occur, micronization occurs due to expansion and contraction associated with hydrogen absorption and release. This tendency is also observed in hydrogen storage alloys that contain Co in the form of alloys, regardless of the composition of the alloy.

[0100] When a coating is partially formed at the negative electrode, although a certain degree of reaction can occur in the coated portion, electrons concentrate in the uncoated portion. Thus, in the absence of W in the electrolyte, uneven reaction occurs at the negative electrode.

[0101] <The role of tungsten (W) in electrolyte 5>

[0102] Figure 8 This is a schematic diagram illustrating the reaction in the presence of tungsten (W) in the electrolyte. In contrast, when electrolyte 5 contains tungsten (W), a complex is formed with tungsten (W) as the central metal and hydroxyl groups as ligands. The hydroxyl groups, acting as ligands, facilitate electron donation and acceptance. If a large amount of complex is uniformly present between the positive electrode 2 and the negative electrode 3, electrons move from the complex to adjacent complexes and uniformly reach the negative electrode 3. Therefore, compared to nickel-metal hydride secondary batteries, which differ only in that electrolyte 5 does not contain tungsten, uneven reaction at the negative electrode can be suppressed.

[0103] Furthermore, the hydrogen storage alloy at the negative electrode undergoes micronization due to lattice volume changes caused by hydrogen absorption and release. This micronization increases the surface area, enhances reactivity with the electrolyte, and facilitates element dissolution. Here, by utilizing tungsten in the electrolyte to suppress reaction inhomogeneity, the negative electrode plate can be used uniformly, thus allowing micronization caused by hydrogen absorption and release to occur uniformly in the negative electrode. Therefore, in the hydrogen storage alloy at the negative electrode, according to condition 3, the Co / Mn dissolution ratio RA is set to 0.26 or less. Thus, if the micronization of the negative electrode proceeds uniformly, the proportion of Mn dissolved into the electrolyte increases.

[0104] <Relationship between the amount of tungsten (W) in the electrolyte and the ratio of Co / Mn deposited in the separator>

[0105] Figure 9 This is a graph showing the relationship between the amount of tungsten in electrolyte 5 and the ratio of Co / Mn deposited in separator 4. (See graph for example.) Figure 9 As shown, if the W content is low, the Co / Mn precipitation ratio increases; if the W content is high, the Co / Mn precipitation ratio decreases.

[0106] The rationale is that when the W content in electrolyte 5 is high, the proportion of Mn relative to Co increases regarding the dissolution of Co and Mn at the negative electrode. Conversely, the dissolution of Co at the positive electrode does not change as much as at the negative electrode. Therefore, the Co / Mn precipitation ratio RP decreases.

[0107] Conversely, when the W content in electrolyte 5 is low, regarding the dissolution of Co and Mn at the negative electrode, the ratio of Mn to Co decreases, while the dissolution of Co at the positive electrode does not change as it does at the negative electrode. Therefore, the ratio RP of deposited Co / Mn increases.

[0108] <Lower limit of tungsten (W) content>

[0109] Specifically, when the W content in electrolyte 5 is 0.48% by weight or more relative to the weight of the positive electrode active material of positive electrode plate 2, the Co / Mn deposition ratio RP is approximately less than 0.039; when the W content in electrolyte 5 is less than 0.48% by weight relative to the weight of the positive electrode active material of positive electrode plate 2, the Co / Mn deposition ratio RP is approximately greater than 0.039.

[0110] Therefore, based on this reasoning, by ensuring that the amount of W contained in the electrolyte 5 as the fourth condition is 0.48% by weight or more relative to the weight of the positive electrode active material of the positive electrode plate 2, it is easier to achieve the ratio RE of 0.85 or more and 1.61 or less as the first condition. It should be noted that, as described above, the fourth condition is a more preferred condition, but not an essential element of this embodiment.

[0111] <Upper limit of tungsten (W) content>

[0112] It should be noted that the arbitrarily chosen condition is that the amount of tungsten contained in the electrolyte 5 is 0.72% by weight or less relative to the weight of the positive electrode active material of the positive electrode plate 2. The reason for this is that if the amount of tungsten contained in the electrolyte 5 is greater than 0.72% by weight relative to the weight of the positive electrode active material of the positive electrode plate 2, then W becomes a resistor, and the conductivity decreases. Therefore, although this is a preferred method in this embodiment, it does not contribute to making the ratio RE 0.85 or more and the ratio RE 1.61 or less, which is the first condition, easier to achieve. Therefore, this condition is not directly related to the first to fourth conditions, which are features of this embodiment.

[0113] <Manufacturing Method of Nickel-Metal Hydrate Batteries>

[0114] Figure 10 This is a flowchart illustrating the manufacturing process of the nickel-metal hydride battery according to this embodiment. The following is based on... Figure 10 The flowchart below describes the manufacturing process of the nickel-metal hydride battery according to this embodiment. In this embodiment, the nickel-metal hydride battery is manufactured in a manner suitable for conditions 1 to 4 described above. Therefore, the following adjustments are made in each step.

[0115] In a nickel-metal hydride battery, the first initial step involves fabricating a positive electrode plate 2, a negative electrode plate 3, a separator 4, and an electrolyte 5. Then, the positive electrode plate 2, the negative electrode plate 3, and the separator 4 are laminated to form the electrode assembly 1.

[0116] First, regarding the positive electrode plate 2, a paste for the positive electrode composite layer is prepared by a mixing process (S1) using water, comprising the positive electrode active material (which serves as the raw material for the positive electrode composite layer), a conductive material containing Co, and a binder material that binds them together. At this time, by adjusting the amount of Co added, the paste for the positive electrode composite layer achieves a predetermined mixing ratio.

[0117] Next, the paste of the positive electrode composite material layer produced in the mixing process (S1) is filled into the substrate of the positive electrode made of porous foamed nickel using a coating machine. The positive electrode substrate is then dried. Afterwards, a filling / calendering process (S2) is performed, in which the thickness of the substrate is adjusted by calendering the dried positive electrode substrate using a press. At this time, the Co content of the positive electrode is adjusted by the amount of paste filled in the positive electrode composite material layer. Furthermore, the porosity [%] is adjusted by the filling amount and the pressing pressure.

[0118] Subsequently, the positive electrode plate 2 is completed by adjusting the size of the substrate of the positive electrode after the filling / calendering process (S2) through the single-plate processing process (S3).

[0119] Subsequently, the separator 4 is laminated onto the positive electrode plate 2, which is manufactured in the single-plate processing step (S3), during the separator sealing process (S4).

[0120] On the other hand, regarding the negative electrode plate 3, in the crushing process (S7), the negative electrode active material composed of a hydrogen storage alloy containing Co and Mn is crushed to a set particle size.

[0121] In the alkali treatment step (S8), the negative electrode active material that has been pulverized to a set particle size in the pulverization step (S7) is subjected to alkali treatment.

[0122] For the negative electrode active material that has undergone alkali treatment in step S8, conductive materials, binders, etc., are mixed and kneaded with solvent to prepare a paste for the negative electrode composite material layer. The prepared negative electrode composite material layer paste is coated onto the negative electrode substrate at a specified unit area weight. The Co and Mn content of the negative electrode is adjusted using this unit area weight.

[0123] Next, the paste of the negative electrode composite material layer is coated onto the negative electrode substrate in a specified amount, and the resulting negative electrode plate 3 is calendered using a press at a set pressing pressure to form a set thickness and a set porosity [%). Subsequently, the negative electrode plate 3 is dimensionally adjusted using a single-plate processing step (S10) to complete the negative electrode plate 3.

[0124] Furthermore, regarding the electrolyte 5, a liquid conditioning process (S11) for the electrolyte 5 is performed in parallel with the manufacture of the positive electrode plate 2, the negative electrode plate 3, and the separator 4. In the liquid conditioning process (S11) of the electrolyte 5, a predetermined amount of W is added to the potassium hydroxide solution.

[0125] In the battery cell assembly process (S5), the positive electrode plate 2, negative electrode plate 3, and separator 4, which were fabricated according to the above process, are laminated to form electrode assembly 1. This electrode assembly 1 is housed in the battery case of the nickel-metal hydride battery module, and the battery cell is assembled. The electrode assemblies 1 of the battery cell assembled in this way are connected in series to make them conductive to external terminals.

[0126] In the electrolyte injection process (S6), a predetermined amount of electrolyte is injected into each battery cell assembled in the battery cell assembly process (S5).

[0127] The battery module of the nickel-metal hydride battery of this embodiment is completed through the above processes. As described above, in this embodiment, the nickel-metal hydride battery is manufactured in a manner suitable for conditions 1 to 4 described above. In addition, the upper limit of the porosity [%] of the separator and the upper limit of W contained in the electrolyte are also adjusted.

[0128] This allows for the manufacture of a nickel-metal hydride battery that meets all the requirements of this embodiment.

[0129] (The function of this implementation method)

[0130] By satisfying conditions 1 to 3, the nickel-metal hydride battery of this embodiment can increase the amount of Mn deposited in the separator 4, thereby suppressing the amount of Co deposited in the separator 4.

[0131] When Co precipitates in large quantities in septum 4, Figure 2 As shown in (a), a large number of high-conductivity Co conductive paths Cp1 are formed between the positive electrode 2 and the negative electrode 3. When such a large number of high-conductivity Co conductive paths Cp1 are formed, the self-discharge [Ah] increases through these conductive paths Cp1.

[0132] However, by suppressing the precipitation of Co in septum 4, such as Figure 2 As shown in (b), the formation of the high-conductivity Co conduction pathway Cp1 between the positive electrode 2 and the negative electrode 3 is suppressed. This effectively suppresses the self-discharge [Ah].

[0133] (Effects of this implementation method)

[0134] (1) In the nickel-metal hydride battery of this embodiment, the deposition of Co in the separator 4 can be suppressed. Therefore, the self-discharge [Ah] can be suppressed. Furthermore, the internal resistance (DC-IR) is not increased.

[0135] (2) Since the ratio RE = (weight of positive electrode Co × porosity of positive electrode + weight of negative electrode Co × porosity of negative electrode) / (weight of negative electrode Mn × porosity of negative electrode) is 0.85 or more and the ratio RE is 1.61 or less, the self-discharge [Ah] can be 10.3 [Ah] or less.

[0136] (3) Since the porosity P[%] of the septum 4, which is the second condition, is 64% or more, the unevenness of the reaction on the electrode surface can be suppressed and the dissolution of Mn can be promoted, thus ensuring the realization of the first condition.

[0137] (4) Since the ratio of Co / Mn dissolved from the negative electrode, which is the third condition, is RA less than 0.26, the first condition can be achieved by ensuring the supply of Mn from the negative electrode.

[0138] (5) Since the amount of tungsten contained in the electrolyte 5, which is the fourth condition, is more than 0.48% by weight relative to the weight of the positive electrode active material of the positive electrode plate 2, a complex with tungsten as the central metal and hydroxyl groups as ligands is formed using W in the electrolyte. The hydroxyl groups as ligands facilitate electron donation and acceptance. When a large amount of complex is uniformly present between the positive electrode plate 2 and the negative electrode plate 3, electrons move from the complex towards the adjacent complex and uniformly reach the negative electrode plate 3. Therefore, the unevenness of the reaction on the electrode surface can be suppressed, and the dissolution of Mn can be promoted, making it easy to achieve the first condition.

[0139] (6) It should be noted that in this embodiment, the upper limit of the porosity P[%] of the spacer is set to 70%. Therefore, it is possible to suppress the reduction of the mechanical strength of the spacer 4 itself so that it is easy to be flattened, or to suppress the deterioration of its ability to prevent small short circuits.

[0140] (7) In addition, the amount of tungsten contained in the electrolyte 5 is less than 0.72% by weight relative to the weight of the positive electrode active material of the positive electrode plate 2. Therefore, it is possible to suppress the decrease of W in resistance and conductivity.

[0141] (8) In this embodiment, as described above, the self-discharge of the nickel-metal hydride battery [Ah] can be suppressed by adjusting only the amounts of Co, Mn and W in the electrolyte 5, the porosity of the positive electrode plate 2 and the negative electrode plate 3, and the porosity [%) of the separator.

[0142] (9) Therefore, it can be applied to existing nickel-metal hydride batteries without the need for special devices, special processing, or special materials.

[0143] (Other examples)

[0144] The present invention is not limited to the above-described embodiments, and can be implemented as follows.

[0145] Regarding the nickel-metal hydride battery of this embodiment, a battery module for driving a vehicle has been used as an example for explanation, but the application of the battery is not limited, and it can also be used in aircraft, ships, and stationary battery applications.

[0146] ○The accompanying drawings are schematic diagrams for illustrating the nickel-metal hydride battery of this embodiment. The number and size balance of the constituent elements are sometimes exaggerated and may not be accurate.

[0147] ○ Figure 10 The flowchart shown is an example; you can add, delete, replace, or change the process.

[0148] The values ​​and ranges described in this embodiment are illustrative and are not intended to limit the invention to these values ​​or ranges. Those skilled in the art can optimize the invention based on the configuration of the nickel-metal hydride battery.

[0149] ○As long as it does not depart from the description in the claims, those skilled in the art can certainly add, delete or modify the structure of the present invention to implement it.

[0150] Explanation of reference numerals in the attached figures

[0151] 1…Electrode group

[0152] 2…Positive plate

[0153] 3…Negative electrode plate

[0154] 4…septum

[0155] 5… Electrolyte

[0156] SD[Ah]...Self-discharge

[0157] RE…(electrolyte) Co / Mn dissolution ratio (positive electrode Co weight × positive electrode porosity + negative electrode Co weight × negative electrode porosity) / (negative electrode Mn weight × negative electrode porosity)

[0158] The ratio of Co / Mn precipitated by RP (septum)

[0159] RA… (the ratio of Co / Mn dissolved at the negative electrode)

[0160] Cp2… (Mn-based) conduction pathway

[0161] Cp1… (Co-based) conduction pathway Cp1

[0162] P[%]...(the porosity of the spacer)

Claims

1. A nickel-metal hydride (NiMH) battery comprising a positive electrode, a negative electrode, a separator separating the positive and negative electrodes, and a filled electrolyte, wherein the positive electrode has a positive electrode active material mainly composed of nickel hydroxide, and the positive electrode active material contains cobalt (Co); the negative electrode has a hydrogen storage alloy containing cobalt (Co) and manganese (Mn), characterized in that... The ratio (weight of positive electrode Co × porosity of positive electrode + weight of negative electrode Co × porosity of negative electrode) / (weight of negative electrode Mn × porosity of negative electrode) is greater than or equal to 0.85 and less than or equal to 1.61, and is the ratio (Co / Mn) of the amount of Co deposited in the electrolyte. The porosity of the spacer is 64% or higher. The weight ratio of cobalt (Co) to manganese (Mn) in the negative electrode is less than 0.26, and the electrolyte contains tungsten (W) of more than 0.48% by weight relative to the active material of the positive electrode.

2. The nickel-metal hydride battery as described in claim 1, wherein, The electrolyte contains tungsten (W) at a weight of 0.72% or more relative to the positive electrode active material.

3. The nickel-metal hydride battery as described in claim 1 or 2, characterized in that, The porosity of the spacer is less than 70%.

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