JP7236413B2 - Nickel-metal hydride storage battery and method for manufacturing nickel-metal hydride storage battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a nickel-metal hydride storage battery and a method for manufacturing a nickel-metal hydride storage battery.

As a nickel-metal hydride storage battery, a battery having a positive electrode using nickel hydroxide as a positive electrode active material is known. Nickel hydroxide expands and contracts during charging and discharging, and this expansion and contraction adversely affects battery characteristics such as battery capacity, resulting in a shortened battery life. On the other hand, it has been proposed to form a zinc compound powder layer on the surface of nickel hydroxide (see, for example, Patent Document 1). According to this method, it is believed that the presence of zinc ions between the nickel layers of the nickel hydroxide suppresses the expansion of the positive electrode active material, thereby improving the life.

JP-A-3-77273

However, even when a powder layer of a zinc compound is formed on the surface of the active material layer, protons and various ions contained in the electrolytic solution enter between the nickel layers during charging, and are desorbed during discharging, resulting in the positive electrode active material becoming weaker. May expand and contract. For this reason, there is still room for improvement regarding the shortened battery life resulting from the expansion and contraction of the positive electrode active material.

The present invention has been made in view of such circumstances, and its object is to extend the life of nickel-metal hydride storage batteries by suppressing the expansion and contraction of the positive electrode active material.

A nickel-metal hydride storage battery that solves the above problems is a nickel-metal hydride secondary battery that includes a positive electrode mixture containing a positive electrode active material containing nickel hydroxide as a main component. It includes inserted cathode active material particles and a tungsten complex layer covering the cathode active material particles.

According to the above configuration, by inserting potassium ions between the crystal layers of the positive electrode active material particles and covering the positive electrode active material particles with the tungsten complex layer, it is possible to maintain a large relative distance between the crystal layers. can. Therefore, it is possible to suppress shrinkage of the positive electrode active material particles so that the relative distance between the crystal layers becomes smaller during discharge. In addition, since the crystal layers are already expanded during charging, the amount of expansion can be minimized. By suppressing the expansion and contraction of the positive electrode active material particles in this way, the conductive paths formed between the positive electrode active material particles are maintained. By maintaining good conductive paths between the nickel hydroxide particles, it is possible to suppress a decrease in battery capacity and extend the life of the nickel-metal hydride storage battery.

In the nickel-hydrogen storage battery, the positive electrode active material particles may have lithium ions, sodium ions, and potassium ions inserted between the crystal layers.
According to the above configuration, as the relative distance between the crystal layers starts to increase during charging, lithium ions having the smallest ionic radius are first inserted between the crystal layers of the positive electrode active material particles. Therefore, by maintaining the distance between the crystal layers at least to the size of lithium ions, sodium ions are then easily inserted between the crystal layers. After the sodium ions are inserted, the distance between the crystal layers is maintained at least the size of the sodium ions, thereby facilitating the insertion of potassium ions between the crystal layers. Therefore, more potassium ions can be inserted between the crystal layers than when lithium ions and sodium ions are not inserted between the crystal layers. Therefore, it is possible to increase the relative distance between the crystal layers and suppress the deviation of the relative distance between the crystal layers.

In the nickel-hydrogen storage battery, the concentration of potassium ions contained in the positive electrode active material particles may be lower than the concentration of sodium ions, and the concentration of sodium ions may be lower than the concentration of lithium ions.

According to the above configuration, by inserting more lithium ions, which have the smallest ionic radius, between the crystal layers of the positive electrode active material particles than sodium ions and potassium ions, the relative distance between the crystal layers is increased and the relative distance is reduced. can be homogenized. In addition, by inserting more sodium ions, which have an ionic radius smaller than that of potassium ions, than potassium ions, the relative distance between the crystal layers can be further increased, the relative distances can be made uniform, and potassium ions can be easily inserted. can.

In the nickel-metal hydride storage battery, the concentration of potassium ions inserted into the positive electrode active material particles may be 0.4 mol % or more.
According to the above configuration, since the concentration of potassium ions inserted into the positive electrode active material particles is 0.4 mol %, the distance between the crystal layers can be maximized.

A method for manufacturing a nickel-metal hydride storage battery that solves the above problems is a method for manufacturing a nickel-metal hydride secondary battery comprising a positive electrode mixture containing a positive electrode active material containing nickel hydroxide as a main component, wherein the positive electrode mixture is applied to a positive electrode base material. The positive electrode provided, the negative electrode provided with the negative electrode mixture on the negative electrode base material, and the nickel-metal hydride storage battery containing the electrolytic solution containing tungsten and potassium are overcharged at a charge rate of 2C or less in an overcharge region above a predetermined state of charge. a discharging step of discharging the nickel-metal hydride secondary battery after the overcharging step; and a step of charging after the discharging step until reaching a state of charge lower than the state of charge reached in the overcharging step. and including.

According to the above configuration, in order to overcharge the nickel-metal hydride storage battery, γ-type nickel oxyhydroxide can be generated, and the gap between the crystal layers can be expanded to insert potassium ions. In addition, since the nickel-metal hydride storage battery is charged at a charge rate of 2C or less in the overcharge region, the gap between the crystal layers of the positive electrode active material particles can be gradually expanded, and many potassium ions can be inserted between the crystal layers. In addition, after the overcharging step, the nickel-metal hydride storage battery is discharged and charged to a state of charge lower than the state of charge reached in the overcharging step, so that a tungsten complex layer is formed around the positive electrode active material particles, It is possible to suppress destruction of the tungsten complex layer due to excessive expansion. Therefore, even if charging and discharging are repeated, the relative distance between the crystal layers can be kept large. By suppressing the expansion and contraction of the positive electrode active material particles in this way, the conductive paths formed between the positive electrode active material particles are maintained. By maintaining a good conductive path between the positive electrode active material particles, it is possible to suppress a decrease in battery capacity and extend the life of the nickel-metal hydride storage battery.

According to the present invention, it is possible to extend the life of the nickel-metal hydride storage battery by suppressing the expansion and contraction of the positive electrode active material.

1 is a perspective view of a nickel-metal hydride storage battery according to one embodiment; FIG. The schematic diagram which shows the crystal structure of the positive electrode active material in the same embodiment. The figure explaining the manufacturing method of the nickel-hydrogen storage battery in the same embodiment. The graph explaining the manufacturing method of the nickel-hydrogen storage battery in the same embodiment. The graph which shows the charge rate in the same embodiment, and the relationship of the content rate of each component. The figure which expanded the principal part of the graph of FIG. The graph which shows the relationship between potassium content rate and c-axis length in the same embodiment. 4 is a graph showing the relationship between the number of cycles and the content of the tungsten complex layer in the same embodiment. 4 is a graph showing the deterioration rate based on the amount of capacity decrease in Examples and Comparative Examples. The graph which shows the expansion rate of an Example and a comparative example.

An embodiment of a nickel-metal hydride storage battery and a method of manufacturing the same will be described below. The nickel-metal hydride storage battery of this embodiment is a battery used as a power source for vehicles such as electric vehicles and hybrid vehicles.

As shown in FIG. 1, the nickel-metal hydride storage battery is a prismatic sealed storage battery comprising a battery module 11 configured by electrically connecting a plurality of single cells 30 in series to obtain a required power capacity. The battery module 11 has an integrated battery container 10 composed of a case 13 that can accommodate a plurality of cells 30 and a lid 14 that seals an opening 15 of the case 13 .

A partition wall 18 that partitions the plurality of cells 30 is formed inside the integrated battery case 10 , and the portion partitioned by the partition wall 18 serves as a case 13 for each cell 30 . In the integrated battery case 10, for example, each of the six cases 13 constitutes a cell 30. As shown in FIG. In the case 13 thus partitioned, the electrode body 20 and the positive electrode current collector plate 24 and the negative electrode current collector plate 25 joined to both sides of the electrode body 20 are housed together with an electrolytic solution. The electrode body 20 is configured by stacking a rectangular positive electrode plate 21 and a negative electrode plate 22 with a separator 23 interposed therebetween. The positive electrode plate 21 and the negative electrode plate 22 of the electrode assembly 20 are formed by protruding to opposite side portions in the plane direction of the electrode plate, and a current collecting plate is provided at the side edge of the lead portion of the positive electrode plate 21 . 24 is joined, and a collector plate 25 is joined to the side edge of the lead portion of the negative electrode plate 22 . Through holes 32 used for connecting the cases 13 are formed in the upper part of the partition wall 18 . The through hole 32 is formed by two connecting projections, one projecting from the upper part of the current collector plate 24 and the other projecting from the upper part of the current collecting plate 25 . The electrode bodies 20 of the cases 13 adjacent to each other are electrically connected in series by being joined through each other. On the other hand, the lid body 14 is provided with an exhaust valve 33 for reducing the internal pressure of the integrated battery case 10 to below the valve opening pressure, and a sensor mounting hole 34 for mounting a sensor for detecting the temperature of the electrode body 20 . there is

The positive electrode plate 21 includes a positive electrode current collector and a positive electrode mixture layer provided on the positive electrode current collector. The positive electrode material mixture layer has a positive electrode active material containing nickel oxide such as nickel hydroxide as a main component, and an additive. Additives include conductive materials, thickeners, binders, and the like. During charging, nickel hydroxide is reduced to nickel oxyhydroxide as shown in the following reaction formula (1).

Ni(OH) ₂ +OH ⁻ →NiOOH+H ₂ O+e ⁻ (1)
The nickel hydroxide particles have a coating layer on their surface. This coating layer is mainly composed of cobalt oxyhydroxide (CoOOH). Further, when the nickel-metal hydride storage battery is charged for the first time, the cobalt contained in the positive electrode mixture is electrochemically oxidized and deposited as cobalt oxyhydroxide. A high-density coating layer is formed by the coating layer formed before charging and the cobalt oxyhydroxide deposited after charging. This coating layer forms a conductive path between the nickel hydroxide particles.

The negative electrode plate 22 includes a negative electrode current collector and a negative electrode mixture provided on the negative electrode current collector. A hydrogen storage alloy is an alloy that reversibly absorbs and releases hydrogen. The hydrogen storage alloy is any one of AB type, AB ₅ type, AB ₂ type, and A ₂ B ₇ type, where "A" is an element that forms a hydride, and "B" is an element that does not form a hydride. one or a combination thereof can be used. TiCo, ZrCo, or the like can be used as the AB type hydrogen storage alloy. _MmNi5 or the like can be used as the _AB5 type hydrogen storage alloy. "Mm" refers to misch metal, which is an alloy containing a plurality of rare earth elements. In particular, MmNi ₅ includes MmNi _5-x (Co, Mn, Al) _x- based alloys in which part of nickel (Ni) is replaced with Co, Mn, Al, etc., MmNi _5-x (Co, Mn, Al, Fe) _x -based alloys can be preferably used. The misch metal contains at least one of lanthanum (La), cerium (Ce), neodymium (Nd), samarium (Sm), and the like. Also, instead of or in addition to the alloys described above, vanadium (V)-based alloys and magnesium (Mg)-based alloys may be used.

The electrolyte is held in the separator 23 . The electrolytic solution is an alkaline aqueous solution in which a solute such as potassium hydroxide (KOH) is dissolved in an aqueous solvent. The electrolytic solution contains alkali metal ions such as potassium ions, sodium ions, and potassium ions in a state enclosed in the integrated battery case 10 . These alkali metal ions may exist in the electrolytic solution by dissolving the compound contained in the positive electrode mixture or the negative electrode mixture in the electrolytic solution, or may be contained as a solute of the electrolytic solution. There may be. Examples of the solute of the electrolytic solution include potassium salts such as potassium hydroxide, sodium salts such as sodium hydroxide (NaOH), and lithium salts such as lithium hydroxide (LiOH).

Also, the electrolytic solution contains a tungsten element in a state of being sealed in the integrated battery case 10 . A tungsten element is contained in at least one of the positive electrode mixture, the negative electrode mixture, and the electrolytic solution, and is enclosed in the integrated battery case 10 . Moreover, the tungsten element may be present in the electrolytic solution when repeating the charging step S4 and the discharging step S5, which will be described later. When the positive electrode mixture and the negative electrode mixture contain a tungsten compound, the tungsten compound dissolves in the electrolytic solution in the form of, for example, tungstate ions.

The tungsten compound contained in at least one of the positive electrode mixture, the negative electrode mixture, and the electrolytic solution includes, for example, tungsten oxides such as WO ₂ , WO _{3 and} W ₂ O ₅ (WxOy, x and y are real numbers), WO ₃ . Hydrates _of tungsten oxide such as _H2O , _W2O5.H2O can be used _. In addition, _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 , Na2WO _ _ _ 4.2H 2} O, (NH ₄ ) ₃ PO _{4.12WO 3.3H 2} O, Na ₃ ( _PO 4.12WO ₃ ).xH ₂ O, WF ₅ , WF ₆ and the like can be used.

In an electrolytic solution in which a tungsten compound is dissolved, a complex is formed having a tungsten element as a central metal and a hydroxyl group as a ligand. A hydroxyl group as a ligand facilitates transfer of electrons.

Next, nickel hydroxide, which is a positive electrode active material, will be described in detail with reference to FIGS. 2 and 3. FIG.
FIG. 2 is a diagram schematically showing the crystal structure of a typical positive electrode active material particle 40. As shown in FIG. In addition, the "positive electrode active material particles" are made of nickel hydroxide as a raw material, and include nickel oxyhydroxide reduced by a battery reaction. The positive electrode active material particles 40 have a layered structure and have a plurality of layers 50 (crystal layers) arranged along the c-axis of the crystal structure. When the battery module 11 is charged, it expands in a direction substantially parallel to the c-axis of the crystal structure so that the relative distance of the layers 50 increases, and the c-axis length L increases. As a result, in addition to the protons 53, lithium ions and the like contained in the electrolyte are intercalated between the layers. When the battery module 11 is discharged, the relative distance between the layers shrinks substantially parallel to the c-axis, and the positive electrode active material particles 40 shrink. As the c-axis length L is shortened, protons 53, lithium ions, and the like are separated from the interlayer.

When the amount of expansion and contraction of the positive electrode active material particles 40 increases, the conductive paths between the positive electrode active material particles 40 are destroyed. As a result, the capacity of the nickel-metal hydride storage battery is reduced. This tendency becomes more conspicuous as the range of utilization of the state of charge (SOC, State Of Charge) of the positive electrode increases. That is, if the content of the positive electrode active material or the like is reduced without changing the actual capacity of the battery for the purpose of reducing the cost of the material of the positive electrode, the range of utilization of the SOC of the positive electrode is expanded. As the utilization range of SOC expands, the amount of expansion and contraction of the positive electrode active material increases.

FIG. 3 is a diagram schematically showing the crystal structure of nickel hydroxide of the present embodiment. When the c-axis length L (see FIG. 2) is lengthened by charging the nickel-metal hydride storage battery, as shown in the leftmost diagram in FIG. A lithium ion 54 with a small radius is inserted. When the lithium ions 54 are interposed between the layers 50 in this way, the c-axis length between the layers is made uniform to at least about the size of the lithium ions 54, and as shown in the middle diagram of FIG. Sodium ions 55 having a large ionic radius easily enter between the layers. Since the sodium ions 55 are interposed between the layers, the c-axis length of the space between the layers is made uniform to at least about the size of the sodium ions 55, so that the potassium ions 56 having a larger ionic radius easily enter the layers. Become.

Further, as described above, the lithium ions 54, the sodium ions 55, and the potassium ions 56 are sequentially inserted into the positive electrode active material particles 40, so that the concentration of the alkali metal ions in the positive electrode active material particles 40 is reduced to potassium ions 56, The sodium ions 55 and the lithium ions 54 become higher in this order. That is, the ion with a smaller ion radius has a higher concentration in the positive electrode active material particles 40 .

A tungsten complex layer is formed around the positive electrode active material particles 40 . A tungsten complex layer is an aggregate of tungsten complexes in which a large number of hydroxyl groups are added around a tungsten element. The tungsten complex layer also exists at the ion entrance/exit, which is the opening between the layers. The tungsten complex layer allows protons 53 to pass through and prevents lithium ions 54, sodium ions 55, and potassium ions 56 from leaving the layers. As a result, the state in which the lithium ions 54, the sodium ions 55, and the potassium ions 56 are interposed between the layers is maintained while the charge/discharge reaction proceeds by allowing the protons 53 to pass through. That is, the positive electrode active material particles 40 are maintained in a state of being expanded to or near the maximum during discharge, and the amount of expansion is reduced during charge. When the amount of expansion and the amount of contraction decrease in this way, breakage of the conductive paths is suppressed, and a decrease in the capacity of the battery module 11 is suppressed.

Next, with reference to FIG. 4, a method for manufacturing a nickel-metal hydride storage battery will be described.
First, a battery module 11 containing a plurality of electrode bodies 20 and an electrolytic solution is prepared. Then, the battery module 11 is connected to a charging/discharging circuit (not shown), and the battery module 11 is charged until reaching a predetermined SOC (charging step S1). A charging rate in the charging step S1 is not particularly limited. In this embodiment, the predetermined SOC reached in the charging step S1 is "100%", but is not limited to 100%.

After the battery module 11 reaches a predetermined SOC, the overcharging step S2 is performed. In the overcharge step S2, the battery is overcharged until it reaches the state of charge Sth1. In this embodiment, the state of charge Sth1 is higher than the SOC reached in the charging step S1 and 80% or more. When the SOC reaches 80% or more, γ-nickel oxyhydroxide, which is the γ-type structure of nickel oxyhydroxide, is produced. In γ-nickel oxyhydroxide, the c-axis length is longer than in the β-type structure and the like. Also, in the overcharge region, charging is performed at a low rate of 2C or less. When the state of charge Sth1 exceeds 100%, the c-axis length becomes particularly long.

Further, a discharging step S3 is performed to discharge the battery module 11 to a predetermined SOC. At this time, since the tungsten complex layer is not formed entirely around the positive electrode active material particles 40, some of the ions remain between the layers, but other ions are desorbed. After that, the charging step S1, the overcharging step S2 and the discharging step S3 are repeated. As a result, each alkali metal ion interposed between the layers of the positive electrode active material particles 40 increases. Although the charging step S1, the overcharging step S2, and the discharging step S3 are repeated here, they do not have to be repeated when sufficient alkali metal ions are inserted between the layers of the positive electrode active material particles 40. Alternatively, the combination of the charging step S1, the overcharging step S2 and the discharging step S3 may be repeated three times or more.

Next, a cycle consisting of the charging step S4 and the discharging step S5 is repeated a predetermined number of times. The SOC reached in the charging step S4 may be 100% or less, for example 80%. Moreover, the SOC reached in the discharge step S5 is not particularly limited, and may be different from the SOC reached in the overcharge step S2. By repeating charging and discharging within a predetermined SOC range in this way, a tungsten complex layer is formed around the positive electrode active material particles 40 . The reason why the SOC reached in the charging step S4 is less than 100% is that when the SOC exceeds 100%, γ-nickel oxyhydroxide is formed as described above and expands in the c-axis direction, and tungsten in the process of forming This is because the complex layer is destroyed. Here, although the cycle consisting of the charging step S4 and the discharging step S5 is repeated a predetermined number of times, it may not be repeated if the tungsten complex layer is sufficiently formed in one cycle. FIG. 4 illustrates a mode in which three cycles are repeated, but it may be repeated twice, or four or more times.

Next, with reference to FIG. 5, the charge rate and the contents of lithium ions, sodium ions, and potassium ions in the positive electrode will be described. The horizontal axis indicates the charge rate, and the vertical axis indicates the content of each component in the positive electrode. The charging rate is the rate at which the battery modules 11 are charged in the initial overcharging step S2. The content of each alkali metal ion is the ratio (mol %) of the content of the alkali metal ion to the amount of the positive electrode active material. The content of alkali metal ions was measured by ICP (Inductively Coupled Plasma) after removing the positive electrode from one electrode body 20 of the overcharged battery module 11 and washing it with water.

Charge rate/component content ratio lines 100 to 102 indicate the content ratio of lithium ions, the content ratio of sodium ions, and the content ratio of lithium ions with respect to the charge rate, respectively. When compared at the same charging rate, the lithium ions, sodium ions, and potassium ions have higher contents in that order. Also, the higher the charging rate, the smaller the content of each alkali metal ion.

FIG. 6 is an enlarged view of the area indicated by the dashed line in the graph of FIG. When the charging rate exceeds 2C for both the charge rate/component content rate lines 101 and 102, the amount of change in the alkali metal ion content rate becomes small. The content of potassium ions is the same. The reason why the content of potassium ions and the like decreases when the charge rate exceeds 2C is that when the charge rate exceeds 2C, it becomes difficult to overcharge to an SOC of 100% or more, which shortens the c-axis length and increases the amount of potassium between the layers. This is because it becomes difficult for ions and the like to enter.

Next, the relationship between the potassium ion content in the positive electrode and the c-axis length will be described with reference to FIG. The horizontal axis of the graph shown in FIG. 7 indicates the potassium content (mol %) in the positive electrode, and the vertical axis indicates the c-axis length. The c-axis length on the vertical axis represents the ratio of the c-axis length at each potassium ion content to the c-axis length in the crystal structure of the positive electrode of the battery module 11 that is not overcharged, assuming that the c-axis length is "100". ing. After charging the battery module 11 at a predetermined charging rate, the c-axis length was measured by taking out a part of the positive electrode from one electrode body 20 and measuring it by X-ray diffraction (XRD). The potassium ion content was varied by charging the battery modules 11 at different charging rates. Another part of the positive electrode was washed with water, and the amount of potassium relative to the amount of positive electrode active material was detected by ICP (Inductively Coupled Plasma). The ratio of the potassium content to the amount of the positive electrode active material was defined as the potassium ion content.

When the potassium ion content is 0.4 mol % or more with respect to the weight of nickel hydroxide, the c-axis length becomes longer than that of the positive electrode of the conventional battery module 11, and potassium ions are easily inserted. As a result, sufficient potassium ions are inserted between the crystal layers of the positive electrode active material particles, and by covering the positive electrode active material particles with the tungsten complex layer, the relative distance between the crystal layers can be kept large. Therefore, it is possible to suppress shrinkage of the positive electrode active material particles so that the relative distance between the crystal layers becomes smaller during discharge.

With reference to FIG. 8, the relationship between the number of cycles of charging and discharging and the content of the tungsten complex layer in the positive electrode will be described. The horizontal axis of the graph in FIG. 8 indicates the number of cycles, and the vertical axis indicates the content (mol %) of the tungsten complex layer. The content of the tungsten complex layer is the ratio of the mass of the tungsten complex layer to the mass of the entire positive electrode active material. After repeating the charging step S4 and the discharging step S5 for a predetermined number of cycles, the content of the tungsten complex layer is determined by taking out the positive electrode and washing it with water, and measuring the amount of the tungsten element with respect to the amount of the positive electrode active material in ICP (Inductively Coupled Plasma). detected. As shown by the cycle number/content rate curve 110, the content rate of the tungsten complex layer increases exponentially as the cycle number increases. Specifically, the amount of change in the content of the tungsten complex layer decreases as the number of cycles increases, and becomes substantially constant after the number of cycles reaches “10”. Therefore, when the number of cycles is 1 or more and 10 or less, the formation time can be shortened while forming the tungsten complex layer.

Effects of the above embodiment will be described.
(1) By inserting the potassium ions 56 between the layers of the positive electrode active material particles 40, the relative distance between the layers can be kept large. Therefore, it is possible to suppress the contraction of the positive electrode active material so as to reduce the relative distance between the layers during discharge. Moreover, since the c-axis length between the layers has already been expanded during charging, the amount of expansion can be minimized. By suppressing the expansion and contraction of the positive electrode active material particles in this manner, the conductive paths formed between the positive electrode active material particles 40 are maintained. By maintaining good conductive paths of the positive electrode active material particles 40 , it is possible to suppress a decrease in battery capacity and extend the life of the battery module 11 .

(2) Lithium ions having the smallest ionic radius are first inserted between the crystal layers as the layers of the positive electrode active material particles 40 begin to expand during charging. Therefore, the c-axis length, which is the relative distance between the layers, is maintained at least at the size of lithium ions, thereby facilitating subsequent insertion of sodium ions between the layers. After the sodium ions are inserted, the relative distance between the layers is maintained to be at least the size of the sodium ions, thereby facilitating the insertion of potassium ions between the layers of the positive electrode active material particles 40 . Therefore, more potassium ions can be inserted between the layers of the positive electrode active material particles 40 than when lithium ions and sodium ions are not inserted between the layers of the positive electrode active material particles 40 . Therefore, it is possible to increase the relative distance between the layers of the positive electrode active material particles 40 and suppress the deviation of the relative distance between the layers.

(3) By inserting more lithium ions, which have the smallest ionic radius, between the layers of the positive electrode active material particles 40 than sodium ions and potassium ions, the relative distance between the layers is increased and the relative distance is made uniform. can be done. In addition, by inserting more sodium ions, which have an ionic radius smaller than that of potassium ions, than potassium ions, the relative distance between the layers can be further increased, the relative distances can be made uniform, and potassium ions can be easily inserted. .

(4) Since the concentration of potassium ions inserted between the layers of the positive electrode active material particles 40 is 0.4 mol %, the interlayer spacing can be maximized.
(5) Since the battery module 11 is overcharged in the overcharging step S2, γ-type nickel oxyhydroxide is generated, and the gap between crystal layers can be expanded to insert potassium ions. In addition, since the battery module 11 is charged at a charge rate of 2C or less in the overcharge region, the gap between the layers of the positive electrode active material particles 40 can be gradually expanded, and many potassium ions can be inserted between the layers. Further, after the overcharging step, the battery module 11 is discharged and charged until reaching a state of charge lower than the state of charge Sth1 reached in the overcharging step. , it is possible to suppress destruction of the tungsten complex layer due to excessive expansion. Therefore, even if charging and discharging are repeated, the relative distance between the crystal layers can be kept large.

The above embodiment can be implemented with the following modifications. Each of the above-described embodiments and the following modifications can be implemented in combination with each other within a technically consistent range.
- In the above embodiment, the charging step S1 was performed before the overcharging step S2, and in the charging step S1, charging was performed at a higher charging rate than in the overcharging step S2. Alternatively, the charging step S1 and the overcharging step S2 may be charged at a constant charging rate of 2C or less.

- Although six battery cells were accommodated in the case 13 in the above-described embodiment, the number of battery cells may be one or a plurality other than six.
- In the above-described embodiment, the nickel-metal hydride storage battery is a stacked battery in which a plurality of positive electrode plates 21 and a plurality of negative electrode plates 22 are alternately stacked with separators 23 interposed therebetween. Alternatively, a wound type battery in which one long positive electrode sheet and one long negative electrode sheet are laminated and wound with a separator interposed therebetween, or a battery having another structure may be used.

- In the above-described embodiment, the nickel-metal hydride storage battery is used as the prismatic sealed battery module 11, but it may be a battery with other configurations such as a cylindrical battery. Also, although the nickel-metal hydride storage battery is the battery module 11 composed of a plurality of single cells 30, the single cell may be used as long as it has a configuration that can be electrically connected to an external load.

Hereinafter, Example 1 and Comparative Example 1, which are examples of the above embodiments, will be specifically described. It should be noted that these examples do not limit the present invention.
<Production of battery>
(Example 1)
As for the positive electrode plate, a foamed nickel substrate was filled with an active material paste containing nickel hydroxide as a main component, dried, rolled and cut to produce a positive electrode plate. Nickel hydroxide coated with cobalt oxyhydroxide was used as the positive electrode active material, and this positive electrode active material was mixed with water, carboxymethyl cellulose (CMC), a thickener, and the like to prepare a paste. Then, a foamed nickel substrate was filled with the paste, dried, and then pressure-molded to produce a positive electrode plate.

MmNi _5-x (Co, Mn, Al) _x -based alloy powder, a thickener, and a binder were added and kneaded to form a negative electrode mixture paste. Also, the negative electrode mixture paste was applied to both sides of a long metal substrate (punching metal), dried and rolled, and cut into a predetermined size to prepare a negative electrode plate.

Furthermore, an electrolytic solution was prepared by dissolving potassium hydroxide and WO ₃ (tungsten compound) in water. The weight percent of the tungsten element was adjusted to 0.1 weight percent with respect to the weight of nickel hydroxide, which is the positive electrode active material. The content of potassium ions inserted into the positive electrode was adjusted to 0.8 mol % with respect to the weight of nickel hydroxide, which is the positive electrode active material. Then, a plurality of the above-described positive electrode plate and negative electrode plate were laminated with a separator made of a non-woven fabric of an alkali-resistant resin interposed therebetween, and pressed. The inter-electrode distance was determined using the inner dimension of the case 13, the number and thickness of the positive electrode plates 21, and the number and thickness of the negative electrode plates 22, as in the above embodiment. Furthermore, the battery module 11 was produced by accommodating the electrode group which the collector part was joined to the laminated body in the battery container with electrolyte solution.

Next, the battery module 11 was connected to a charging/discharging circuit, and an ion insertion step of inserting alkali metal ions between nickel hydroxide layers and a complex layer forming step of forming a tungsten complex layer were performed. Specifically, after repeating the cycle consisting of the charging step S1, the overcharging step S2 and the discharging step S3 twice, the cycle consisting of the charging step S4 and the discharging step S5 was repeated three times. The charge rate in the overcharge process was set to 2C.

(Example 2)
A nickel-metal hydride storage battery was fabricated in the same steps as in Example 1, except that the potassium ion content (mol %) was adjusted to 0.4 mol %.

(Comparative example 1)
A nickel-metal hydride storage battery was fabricated in the same steps as in Example 1, except that the step of inserting alkali metal ions between layers and the step of forming a tungsten complex layer were omitted. That is, Comparative Example 1 is a nickel-metal hydride storage battery having a conventional configuration in which alkali metal ions are not inserted between the layers of positive electrode active material particles.

(Comparative example 2)
A nickel-metal hydride storage battery was produced in the same steps as in Example 1, except that the step of forming the tungsten complex layer was omitted.

<Capacity measurement>
The nickel-metal hydride storage batteries of Examples 1 and 2 and Comparative Example 1 were charged to full charge (SOC 100%), and the initial capacities were measured.

After that, a durability test was conducted. The endurance test was carried out by repeating 500 cycles of charge/discharge between 20% and 60% SOC at 5C, then charging the nickel-metal hydride storage battery to full charge, and measuring the capacity after the endurance test.

<Measurement of c-axis length>
For Examples 1 and 2 and Comparative Examples 1 and 2, a part of the positive electrode was taken out from the electrode body 20 of the fully charged battery module 11, and the c-axis length was measured by the X-ray diffraction method (XRD).

<Evaluation>
The graph in FIG. 9 is based on Comparative Example 1, which has a conventional configuration in which no alkali metal ions are inserted between the layers of the positive electrode active material particles, and the amount of capacity decrease is “1”. The ratio of the amount of capacity decrease in Examples 1 and 2 is shown as the "deterioration rate." A smaller ratio value indicates a smaller decrease in capacity. The deterioration rate of Example 1 was "0.9", and the deterioration rate of Example 2 was "0.96".

The graph of FIG. 10 indicates the expansion coefficient, which is the ratio of the c-axis length of Example 1 and Comparative Example 2 to the c-axis length of Comparative Example 1, where the c-axis length of Comparative Example 1 is "1". The c-axis length of Comparative Example 2 was 1.4 times the c-axis length of Comparative Example 1. The c-axis length of Example 1 was "0.4", and the c-axis length of Example 2 was "0.7". From this, it can be seen that the effect of suppressing the expansion rate cannot be obtained only by inserting alkali metal ions between the layers, and that the effect of suppressing the expansion rate can be obtained only by forming the tungsten complex layer. In other words, it is possible to suppress destruction of the tungsten complex layer due to excessive expansion only after the insertion of alkali metal ions between the layers and the formation of the tungsten complex layer.

DESCRIPTION OF SYMBOLS 11... Battery module 21... Positive electrode plate 22... Negative electrode plate 23... Separator 40... Positive electrode active material particle 50... Layer

Claims (5)

A nickel-metal hydride secondary battery comprising a positive electrode mixture containing a positive electrode active material containing nickel hydroxide as a main component,
Containing positive electrode active material particles in which potassium ions are inserted between crystal layers that expand and contract with charging and discharging, and a tungsten complex layer covering the positive electrode active material particles,
Lithium ions, sodium ions and potassium ions are inserted between the crystal layers in the positive electrode active material particles,
The concentration of potassium ions contained in the positive electrode active material particles is lower than the concentration of sodium ions, and the concentration of sodium ions is lower than the concentration of lithium ions.
Nickel metal hydride battery. The concentration of potassium ions inserted into the positive electrode active material particles is 0.4 mol % or more
The nickel-metal hydride storage battery according to claim 1. A nickel-metal hydride secondary battery comprising a positive electrode mixture containing a positive electrode active material containing nickel hydroxide as a main component,
Containing positive electrode active material particles in which potassium ions are inserted between crystal layers that expand and contract with charging and discharging, and a tungsten complex layer covering the positive electrode active material particles,
The concentration of potassium ions inserted into the positive electrode active material particles is 0.4 mol % or more
Nickel metal hydride battery. Lithium ions, sodium ions, and potassium ions are inserted between the crystal layers in the positive electrode active material particles.
The nickel-metal hydride storage battery according to claim 3 . In a method for manufacturing a nickel-metal hydride secondary battery comprising a positive electrode mixture containing a positive electrode active material containing nickel hydroxide as a main component,
A nickel-metal hydride storage battery containing a positive electrode having a positive electrode base material provided with the positive electrode mixture, a negative electrode having a negative electrode base material provided with the negative electrode mixture, and an electrolytic solution containing tungsten and potassium in an overcharge region above a predetermined state of charge. an overcharging step of overcharging to a first state of charge of 100% or more at a charge rate of 2C or less;
a discharging step of discharging the nickel-hydrogen secondary battery after the overcharging step;
after the discharging step, charging until a second state of charge of less than 100% is reached ;
After repeating the cycle including the overcharging step and the discharging step multiple times, repeating the cycle including the charging step and the discharging step and not including the overcharging step multiple times.
A method for manufacturing a nickel-metal hydride storage battery.

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