JP6234190B2 - Nickel metal hydride secondary battery - Google Patents

Description

  The present invention relates to a nickel metal hydride secondary battery.

  Nickel metal hydride secondary batteries are used in various applications such as various portable devices and hybrid electric vehicles because they have higher capacity and better environmental safety than nickel cadmium secondary batteries. It is like that.

  By the way, although a general nickel metal hydride secondary battery has a high capacity but has a large self-discharge, if the leaving period is long, the remaining capacity decreases, and it is often necessary to charge the battery immediately before use.

  Therefore, many studies have been made to improve the self-discharge characteristics of nickel-metal hydride secondary batteries, and various self-discharge suppression type nickel-hydrogen secondary batteries have been developed (see, for example, Patent Document 1).

  Such a self-discharge suppression type nickel metal hydride secondary battery, if the user precharges it, there is little decrease in the remaining capacity even if it is left, so there is a situation where recharging is necessary just before use There is an advantage that the frequency can be reduced. By taking advantage of such merits, the self-discharge suppression type nickel-hydrogen secondary battery is a very excellent battery having both usability as if it was a dry battery and a higher capacity than that of the dry battery.

JP 2007-149646 A

By the way, in the nickel metal hydride secondary battery, further improvement in self-discharge characteristics is desired in order to improve usability.
However, the self-discharge characteristics of the nickel metal hydride secondary battery as disclosed in Patent Document 1 are not yet sufficient to meet such demands.
For this reason, development of a nickel metal hydride secondary battery having better self-discharge characteristics is desired.

  The present invention has been made based on the above circumstances, and an object of the present invention is to provide a nickel-metal hydride secondary battery having improved self-discharge characteristics as compared with the prior art.

  In order to achieve the above object, the present inventor has intensively studied to further improve the self-discharge characteristics of the nickel metal hydride secondary battery. In the course of this study, the present inventor added NaF to the alkaline electrolyte, and this NaF is one of the main factors of self-discharge, namely, hydrogen dissociation from the hydrogen storage alloy of the negative electrode and self-decomposition of the positive electrode Based on this knowledge, the inventors have found that it is effective to improve self-discharge characteristics based on the knowledge that the reaction is suppressed.

  That is, according to the present invention, a container and an electrode group housed in an airtight state together with an alkaline electrolyte in the container, the electrode group is composed of a positive electrode and a negative electrode superimposed via a separator, A negative electrode includes a rare earth-Mg—Ni-based hydrogen storage alloy, and the alkaline electrolyte includes a nickel-hydrogen secondary battery including NaF.

  Preferably, the concentration of NaF in the alkaline electrolyte is 0.25% by mass to 1.17% by mass.

  Moreover, Preferably, it is set as the structure which further contains powdery NaF.

Further, the hydrogen storage alloy has the general _{formula: Ln 1-x Mg x Ni} ya Al a ( In the formula, Ln is Zr, Ti, at least one element selected from Y and rare earth elements, subscripts x, y , A represents a composition represented by 0.05 ≦ x ≦ 0.30, 2.8 ≦ y ≦ 3.8, and 0.05 ≦ a ≦ 0.30, respectively. It is preferable to do.

  The nickel metal hydride secondary battery of the present invention contains NaF in the alkaline electrolyte. This NaF suppresses hydrogen dissociation from the hydrogen storage alloy of the negative electrode and suppresses the self-decomposition reaction of the positive electrode. As a result, a nickel metal hydride secondary battery having improved self-discharge characteristics than the conventional one can be obtained.

It is the perspective view which fractured | ruptured and showed the nickel-hydrogen secondary battery which concerns on one Embodiment of this invention.

  Hereinafter, a nickel-hydrogen secondary battery (hereinafter simply referred to as a battery) 2 according to the present invention will be described with reference to the drawings.

  The battery 2 to which the present invention is applied is not particularly limited. For example, a case where the present invention is applied to an AA size cylindrical battery 2 shown in FIG. 1 will be described as an example.

  As shown in FIG. 1, the battery 2 includes an outer can 10 having a bottomed cylindrical shape with an upper end opened as a container. The outer can 10 has conductivity, and its bottom wall 35 functions as a negative electrode terminal. A sealing body 11 is fixed to the opening of the outer can 10. The sealing body 11 includes a cover plate 14 and a positive electrode terminal 20, and seals the outer can 10 and provides the positive electrode terminal 20. The cover plate 14 is a disk-shaped member having conductivity. A cover plate 14 and a ring-shaped insulating packing 12 surrounding the cover plate 14 are disposed in the opening of the outer can 10, and the insulating packing 12 is formed by caulking the opening edge 37 of the outer can 10. It is fixed to the opening edge 37. That is, the lid plate 14 and the insulating packing 12 cooperate with each other to airtightly close the opening of the outer can 10.

  Here, the cover plate 14 has a central through hole 16 in the center, and a rubber valve body 18 that closes the central through hole 16 is disposed on the outer surface of the cover plate 14. Further, a metal positive electrode terminal 20 having a flanged cylindrical shape is electrically connected to the outer surface of the cover plate 14 so as to cover the valve element 18. The positive terminal 20 presses the valve body 18 toward the lid plate 14. The positive electrode terminal 20 has a gas vent hole (not shown).

  Normally, the central through hole 16 is hermetically closed by the valve body 18. On the other hand, if gas is generated in the outer can 10 and its internal pressure increases, the valve body 18 is compressed by the internal pressure and opens the central through hole 16. As a result, the central through hole 16 and the positive electrode terminal 20 are opened from the outer can 10. Gas is released to the outside through the vent holes. That is, the central through hole 16, the valve body 18, and the positive electrode terminal 20 form a safety valve for the battery.

  The outer can 10 contains an electrode group 22 together with an alkaline electrolyte (not shown). Each of the electrode groups 22 includes a strip-like positive electrode 24, a negative electrode 26, and a separator 28, which are wound in a spiral shape with the separator 28 sandwiched between the positive electrode 24 and the negative electrode 26. That is, the positive electrode 24 and the negative electrode 26 are overlapped with each other via the separator 28. The outermost periphery of the electrode group 22 is formed by a part of the negative electrode 26 (the outermost periphery) and is in contact with the inner peripheral wall of the outer can 10. That is, the negative electrode 26 and the outer can 10 are electrically connected to each other.

  In the outer can 10, a positive electrode lead 30 is disposed between one end of the electrode group 22 and the lid plate 14. Specifically, the positive electrode lead 30 has one end connected to the positive electrode 24 and the other end connected to the lid plate 14. Therefore, the positive electrode terminal 20 and the positive electrode 24 are electrically connected to each other via the positive electrode lead 30 and the cover plate 14. A circular upper insulating member 32 is disposed between the cover plate 14 and the electrode group 22, and the positive electrode lead 30 extends through a slit 39 provided in the insulating member 32. A circular lower insulating member 34 is also disposed between the electrode group 22 and the bottom of the outer can 10.

  Further, a predetermined amount of alkaline electrolyte (not shown) is injected into the outer can 10. Most of the injected alkaline electrolyte is held in the electrode group 22, and the charge / discharge reaction between the positive electrode 24 and the negative electrode 26 proceeds. As the alkaline electrolyte, an alkaline electrolyte containing NaOH as a main solute is prepared. Specifically, a sodium hydroxide aqueous solution mainly containing NaOH is prepared as the solute of the alkaline electrolyte. In addition, the solute of the alkaline electrolyte may include at least one of KOH and LiOH. Here, when KOH and LiOH are also included as the solute of the alkaline electrolyte, the amount of NaOH is larger than the amount of these KOH and LiOH.

  And in this invention, NaF is added to this prepared alkaline electrolyte, and it is set as the alkaline electrolyte containing NaF. This NaF adsorbs to Mg etc. on the surface of the hydrogen storage alloy and suppresses hydrogen dissociation from the hydrogen storage alloy of the negative electrode. Moreover, since the amount of Na ions in the alkaline electrolyte increases when the alkaline electrolyte contains NaF, the oxygen overvoltage increases and the self-decomposition reaction of the positive electrode is suppressed. These two actions improve the self-discharge characteristics of the nickel-hydrogen secondary battery.

  Here, the concentration of NaF in the alkaline electrolyte is preferably 0.25% by mass or more. When the concentration of NaF is less than 0.25% by mass, the effect of improving the self-discharge characteristics described above is small. The higher the concentration of NaF, the higher the effect of improving the self-discharge characteristics. Therefore, the upper limit of the NaF concentration is preferably the saturated concentration of NaF with respect to the alkaline electrolyte, specifically 1.17% by mass. .

  In the present invention, as an aspect in which NaF is included in the alkaline electrolyte, NaF powder is added to the prepared alkaline electrolyte and stirred to dissolve NaF to prepare an alkaline electrolyte containing NaF in advance. The mode to do is mentioned. In the case of this embodiment, it is preferable to add a desired amount of NaF to the alkaline electrolyte and stir, because an alkaline electrolyte containing a desired concentration of NaF can be easily obtained. However, in the present invention, the embodiment in which NaF is contained in the alkaline electrolyte is not limited to this embodiment, and an appropriate portion in the outer can 10, for example, NaF in the form of powder in the upper part of the electrode group 22 is used. And a mode in which powdered NaF is added to the negative electrode mixture to be described later. In addition, these aspects may be employed alone or in combination.

  As a material of the separator 28, for example, a polyamide fiber nonwoven fabric or a polyolefin fiber nonwoven fabric such as polyethylene or polypropylene provided with a hydrophilic functional group can be used. Specifically, it is preferable to use a non-woven fabric mainly composed of polyolefin fibers that have been subjected to sulfonation treatment and are provided with sulfone groups. Here, the sulfone group is imparted by treating the nonwoven fabric with an acid containing a sulfuric acid group such as sulfuric acid or fuming sulfuric acid.

  The positive electrode 24 includes a conductive positive electrode base material having a porous structure and having a large number of holes, and a positive electrode mixture held in the above-described holes and on the surface of the positive electrode base material.

  As such a positive electrode base material, for example, a net-like, sponge-like or fibrous metal body plated with nickel, or foamed nickel (nickel foam) can be used.

  The positive electrode mixture includes positive electrode active material particles, a conductive material, a positive electrode additive, and a binder. The binder serves to bind the positive electrode active material particles, the conductive material, and the positive electrode additive, and at the same time, bind the positive electrode mixture to the positive electrode base material. Here, as the binder, for example, carboxymethylcellulose, methylcellulose, PTFE (polytetrafluoroethylene) dispersion, HPC (hydroxypropylcellulose) dispersion, and the like can be used.

  The positive electrode active material particles are nickel hydroxide particles or higher order nickel hydroxide particles. In addition, it is preferable to dissolve at least one of zinc, magnesium, and cobalt in these nickel hydroxide particles.

As the conductive material, for example, one or more selected from cobalt compounds such as cobalt oxide (CoO) and cobalt hydroxide (Co (OH) ₂ ) and cobalt (Co) can be used. This conductive material is added to the positive electrode mixture as necessary, and is added to the positive electrode mixture in the form of a powder and a coating covering the surface of the positive electrode active material. It may be.

  In order to improve the characteristics of the positive electrode, the positive electrode additive is appropriately selected as necessary. Examples of the main positive electrode additive include yttrium oxide and zinc oxide.

The positive electrode 24 can be manufactured as follows, for example.
First, a positive electrode mixture slurry including a positive electrode active material powder composed of positive electrode active material particles obtained as described above, a conductive material, a positive electrode additive, water, and a binder is prepared. The obtained positive electrode mixture slurry is filled in, for example, nickel foam and dried. After drying, the nickel foam filled with nickel hydroxide particles and the like is rolled and then cut. Thereby, the positive electrode 24 carrying the positive electrode mixture is produced.

Next, the negative electrode 26 will be described.
The negative electrode 26 has a conductive negative electrode core having a strip shape, and a negative electrode mixture is held in the negative electrode core.
The negative electrode core is made of a sheet-like metal material in which through holes are distributed. For example, a punching metal sheet can be used. The negative electrode mixture is not only filled in the through hole of the negative electrode core, but also held in layers on both surfaces of the negative electrode core.

  The negative electrode mixture includes hydrogen storage alloy particles, a negative electrode additive, a conductive material, and a binder. Here, the hydrogen storage alloy is an alloy capable of storing and releasing hydrogen which is a negative electrode active material. The above-described binder serves to bind the particles of the hydrogen storage alloy, the negative electrode additive, and the conductive material to each other and simultaneously bind the negative electrode mixture to the negative electrode core. Here, a hydrophilic or hydrophobic polymer or the like can be used as the binder, and carbon black or graphite can be used as the conductive material.

  As the hydrogen storage alloy in the hydrogen storage alloy particles, it is preferable to use a rare earth-Mg—Ni-based hydrogen storage alloy containing a rare earth element, Mg, Ni and excluding Co and Mn. Specifically, this rare earth-Mg—Ni-based hydrogen storage alloy has a composition represented by the following general formula (I).

Ln _1-x Mg _x Ni _ya Al _a (I)
However, in the general formula (I), Ln is at least one element selected from Zr, Ti, Y and rare earth elements, and the subscripts x, y and a are 0.05 ≦ x ≦ 0.30 and 2. It is necessary to satisfy the conditions of 8 ≦ y ≦ 3.8 and 0.05 ≦ a ≦ 0.30. In addition, the above-mentioned rare earth elements specifically indicate Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

Here, since the hydrogen storage alloy according to the present invention has a composition excluding Co and Mn, there is little precipitation of the conductive material on the separator, which contributes to the improvement of the self-discharge characteristics. In addition, the hydrogen storage alloy according to the present invention is formed by laminating AB ₂ type subunits and AB ₅ type subunits when Ln and Mg in the general formula (I) are A components and Ni and Al are B components. A so-called superlattice structure having an A ₂ B ₇ type structure or an A ₅ B ₁₉ type structure is formed. The rare earth-Mg—Ni-based hydrogen storage alloy having such a superlattice structure is advantageous in that the storage and release of hydrogen, which is a characteristic of the AB ₅ type alloy, is stable, and the hydrogen, which is a characteristic of the AB ₂ type alloy. It has the advantage of having a large amount of occlusion. For this reason, since the hydrogen storage alloy which concerns on this invention is excellent in hydrogen storage capacity, it contributes to high capacity | capacitance of the battery 2 obtained.

Next, the hydrogen storage alloy particles described above are obtained, for example, as follows.
First, metal raw materials are weighed and mixed so as to have a predetermined composition, and this mixture is melted in an induction melting furnace, for example, and then cooled to an ingot. The obtained ingot is subjected to heat treatment by heating for 5 to 24 hours in an inert gas atmosphere at 900 to 1200 ° C. Thereafter, the ingot cooled to room temperature is pulverized and sieved to obtain hydrogen storage alloy particles having a desired particle size.

  As the negative electrode additive, one appropriately selected as necessary is added to improve the characteristics of the negative electrode. Here, when adding NaF in a negative electrode mixture, it is preferable to add as a kind of this negative electrode additive. In this case, NaF is used in the form of powder. When NaF is added in the form of a powder, NaF in the negative electrode mixture gradually dissolves in the alkaline electrolyte, acts on the negative electrode and the positive electrode, suppresses hydrogen dissociation from the hydrogen storage alloy of the negative electrode, and Suppresses decomposition reaction.

Moreover, the negative electrode 26 can be manufactured as follows, for example.
First, a hydrogen storage alloy powder composed of hydrogen storage alloy particles, a negative electrode additive, a conductive material, a binder and water are kneaded to prepare a negative electrode mixture slurry. The obtained negative electrode mixture slurry is applied to the negative electrode core and dried. After drying, the negative electrode core to which the hydrogen storage alloy particles and the like are attached is subjected to roll rolling and cutting, whereby the negative electrode 26 is produced.

  Here, when NaF is employed as the negative electrode additive, since NaF does not hinder the thickening of the thickener, dripping of the negative electrode mixture slurry is suppressed, and the negative electrode can be manufactured satisfactorily.

  The positive electrode 24 and the negative electrode 26 manufactured as described above are spirally wound with the separator 28 interposed therebetween, and are formed in the electrode group 22.

  The electrode group 22 thus obtained is accommodated in the outer can 10. Subsequently, a predetermined amount of alkaline electrolyte is injected into the outer can 10.

  Here, NaF can be directly supplied into the outer can 10 in a powder state. In this case, the place where NaF is introduced is not particularly limited. For example, the NaF is supplied to the bottom of the outer can 10 or the upper part of the accommodated electrode group 22. NaF supplied in the form of powder gradually dissolves in the alkaline electrolyte, acts on the negative electrode and the positive electrode as described above, suppresses hydrogen dissociation from the hydrogen storage alloy of the negative electrode, and suppresses self-decomposition reaction of the positive electrode. .

  It is preferable to supply as much NaF powder as possible. However, too much NaF powder in the space inside the outer can 10 affects the arrangement of the battery components and so on, and it is preferable to supply an amount that does not have such an effect. Specifically, when NaF is dissolved in an alkaline electrolyte, it is preferable to supply NaF powder in an amount equal to or less than an amount that results in an addition concentration of 0.5% by mass.

  Thereafter, the outer can 10 containing the electrode group 22 and the alkaline electrolyte is sealed by the cover plate 14 provided with the positive electrode terminal 20, and the battery 2 according to the present invention is obtained. The obtained battery 2 is subjected to an initial activation process and is in a usable state.

[Example]
1. Production of battery (Example 1)

(1) Production of positive electrode Nickel sulfate, zinc sulfate, magnesium sulfate and cobalt sulfate are weighed so that they are 3% by mass of zinc, 0.4% by mass of magnesium and 1% by mass of cobalt with respect to nickel. In addition to a 1N aqueous sodium hydroxide solution containing ions, a mixed aqueous solution was prepared. While stirring the resulting mixed aqueous solution, a 10N sodium hydroxide aqueous solution was gradually added to the mixed aqueous solution to cause a reaction. During the reaction, the pH was stabilized at 13 to 14, and nickel hydroxide was mainly used. Then, nickel hydroxide particles in which zinc, magnesium and cobalt were dissolved were produced.

  The obtained nickel hydroxide particles were washed three times with 10 times the amount of pure water, then dehydrated and dried.

  Next, 10 parts by mass of cobalt hydroxide powder was mixed with 100 parts by mass of the positive electrode active material powder made of nickel hydroxide particles produced as described above, and further 0.5 parts by mass of yttrium oxide, 0.3 parts by mass. A positive electrode mixture slurry was prepared by mixing 40 parts by mass of zinc oxide and 40 parts by mass of HPC dispersion liquid, and this positive electrode mixture slurry was filled in a sheet-like nickel foam as a positive electrode substrate. The nickel foam holding the positive electrode mixture was dried and then rolled. The nickel foam holding the positive electrode mixture was cut into a predetermined shape after the rolling process, and formed on the positive electrode 24 for AA size. The positive electrode 24 holds the positive electrode mixture so that the positive electrode capacity is 2000 mAh.

(2) Production of Hydrogen Storage Alloy and Negative Electrode First, a rare earth component containing 20% by mass of La, 39.5% by mass of Pr, 39.5% by mass of Nd and 0.01% by mass of Zr was prepared. The obtained rare earth components, Mg, Ni and Al were weighed to prepare a mixture in which these were in a molar ratio of 0.83: 0.17: 3.10: 0.20. The obtained mixture was melted in an induction melting furnace, and the molten metal was poured into a mold, and then cooled to room temperature to form a hydrogen storage alloy ingot. The sample collected from the ingot was subjected to composition analysis by high frequency plasma spectroscopy (ICP). As a result, the composition of the hydrogen storage alloy was (La _0.20 Pr _0.395 Nd _0.395 Zr _0.01 ) _0.83 Mg _0.17 Ni _3.10 Al _0.20 .

  Next, heat treatment was performed on the ingot by heating in an argon atmosphere at a temperature of 1000 ° C. for 10 hours. Then, after the heat treatment, the hydrogen storage alloy ingot cooled to room temperature was mechanically pulverized in an argon gas atmosphere and sieved to select a powder composed of hydrogen storage alloy particles remaining between 400 mesh and 200 mesh. . Specifically, first, the pulverized hydrogen storage alloy was passed through a 200-mesh sieve, and the particles that passed through the 200-mesh mesh were collected. Thereafter, the collected particles were passed through a 400 mesh screen to collect the particles remaining on the 400 mesh screen. The obtained hydrogen storage alloy particles have a particle size of about 35 μm to about 74 μm.

  Dispersion of sodium acrylate 0.4 parts by mass, carboxymethylcellulose 0.1 parts by mass, styrene butadiene rubber (SBR) (solid content 50% by weight) with respect to 100 parts by mass of the powder comprising the obtained hydrogen storage alloy particles 1.0 part by mass (in terms of solid content), 1.0 part by mass of carbon black, and 30 parts by mass of water were added and kneaded to prepare a negative electrode mixture slurry.

  This negative electrode mixture slurry was applied to both surfaces of an iron perforated plate as a negative electrode core so that the thickness was uniform and constant. The perforated plate has a thickness of 60 μm, and the surface thereof is plated with nickel.

  After drying the slurry, the perforated plate to which the hydrogen storage alloy powder is adhered is further rolled to increase the amount of alloy per volume, and then cut and cut for an AA size containing a rare earth-Mg—Ni-based hydrogen storage alloy. A negative electrode 26 was prepared.

(3) Assembly of Nickel Metal Hydride Battery The obtained positive electrode 24 and negative electrode 26 were spirally wound with a separator 28 sandwiched between them, and an electrode group 22 was produced. The separator 28 used for the production of the electrode group 22 here was made of a nonwoven fabric made of polypropylene fiber subjected to sulfonation treatment, and its thickness was 0.1 mm (weight per unit area 53 g / m ² ).

  On the other hand, an alkaline electrolyte composed of an aqueous solution containing NaOH, LiOH and KOH was prepared. Here, NaOH is 7.0 N, LiOH is 0.8 N, and KOH is 0.2 N. Further, NaF was added to the alkaline electrolyte. Here, NaF was added to the alkaline electrolyte in an amount of 0.25% by mass and dissolved by stirring.

  Next, the above-described electrode group 22 is housed in the bottomed cylindrical outer can 10, and 2.362 g of the alkaline electrolyte to which the above-described NaF is added (breakdown: 2.356 g of alkaline electrolyte, 0 of NaF) 0.006 g). Thereafter, the opening of the outer can 10 was closed with the sealing body 11, and the AA size nickel-hydrogen secondary battery 2 having a nominal capacity of 2000 mAh was assembled. This nickel metal hydride secondary battery is referred to as battery a. Here, the nominal capacity was the discharge capacity of the battery when it was discharged at 0.1 It for 16 hours and then discharged at 0.1 It until the battery voltage reached 1.0 V.

(4) Initial activation treatment After charging the battery a for 16 hours at a temperature of 25 ° C. and at a temperature of 25 ° C., the battery a is initially discharged until the battery voltage reaches 0.5 V at 0.2 It. The activation process was repeated twice. In this way, the battery a was made usable.

(Example 2)
The amount of NaF added to the alkaline electrolyte and dissolved in an amount of 0.50% by mass with respect to the alkaline electrolyte, and the injection amount of the alkaline electrolyte added with NaF into the outer can 10 is 2 A nickel-metal hydride secondary battery (battery b) was produced in the same manner as the battery a of Example 1, except that the amount was .368 g (breakdown: alkaline electrolyte was 2.356 g, NaF was 0.012 g).

(Example 3)
The amount of NaF added to the alkaline electrolyte and dissolved in an amount of 1.00% by mass with respect to the alkaline electrolyte, and the injection amount of the alkaline electrolyte added with NaF into the outer can 10 is 2 A nickel metal hydride secondary battery (battery c) was produced in the same manner as the battery a of Example 1, except that the amount was .380 g (breakdown: alkaline electrolyte was 2.356 g, NaF was 0.024 g).

Example 4
The amount of NaF added to the alkaline electrolyte and dissolved in an amount of 1.00% by mass with respect to the alkaline electrolyte, and the amount of injection of the alkaline electrolyte added with NaF into the outer can 10 is 2 .380 g (breakdown: alkaline electrolyte is 2.356 g, NaF is 0.024 g), and after housing the electrode group 22 in the outer can 10, before closing the opening of the outer can 10 with the sealing body 11 In addition, the battery of Example 1 except that when NaF was dissolved in an alkaline electrolyte, an amount of NaF powder corresponding to an amount of 0.5% by mass was supplied to the upper portion of the electrode group 22. A nickel hydride secondary battery (battery d) was produced in the same manner as a.

(Example 5)
A nickel metal hydride secondary battery (battery) was obtained in the same manner as the battery a of Example 1, except that NaF was not added to the alkaline electrolyte, and instead 0.5 parts by mass of NaF powder was added to the negative electrode mixture slurry. e) was prepared.

(Comparative Example 1)
A nickel metal hydride secondary battery (battery f) was produced in the same manner as the battery a of Example 1 except that NaF was not added to the alkaline electrolyte.

  In addition, regarding the battery a to the battery f, the aspect of the alkaline electrolyte and the concentration of NaF added in the alkaline electrolyte are summarized in Table 1.

2. Evaluation of NiMH secondary battery (Evaluation of self-discharge characteristics)
The batteries a to f after the initial activation treatment are charged at 1.0 It in an environment of 25 ° C., and charged when the battery voltage reaches the maximum value and then decreases by 10 mV from the maximum value. The charging under the so-called -ΔV control (hereinafter simply referred to as -ΔV charging) was performed. Thereafter, discharging was performed at 0.2 It. And the discharge capacity of the battery at this time was measured. This discharge capacity is taken as the initial capacity.

  Then, after carrying out -ΔV charging at 1.0 It in an environment of 25 ° C., this battery was left in a constant temperature bath at 60 ° C. for 2 weeks to allow the battery to self-discharge. The battery after left at 60 ° C. for 2 weeks was discharged at 0.2 It in an environment of 25 ° C. The discharge capacity at this time was measured. This discharge capacity is defined as the remaining capacity.

From the obtained initial capacity value and remaining capacity value, the self-discharge amount was determined from the following formula (II). The results are shown in Table 2 as the amount of self-discharge. It represents that self-discharge is suppressed, so that this self-discharge amount is small.
Self-discharge amount (mAh) = initial capacity-remaining capacity (II)

Next, the indexed self-discharge amount was obtained from the following formula (III) from the self-discharge amount of the battery of each example. Taking the indexed self-discharge amount of Comparative Example 1 as 100, the ratio of each battery to the indexed self-discharge amount was determined, and the results are shown in Table 2 as the indexed self-discharge amount ratio. A smaller value of the indexed self-discharge amount ratio indicates that the self-discharge characteristics are improved as compared with the conventional battery (Comparative Example 1).
Indexed self-discharge amount = Self-discharge amount of each example / Self-discharge amount of Comparative Example 1 (III)

3. Discussion (i) In the case of Example 5 in which NaF was used as the negative electrode additive, the negative electrode mixture slurry could be applied to the core without any problem, and a battery could be produced.

  Moreover, the self-discharge amount of the battery e is 316.0 mAh, which indicates that self-discharge is suppressed more than the battery f of Comparative Example 1. From this, it can be seen that NaF contributes to the improvement of the self-discharge characteristics even when added to the negative electrode.

(Ii) The batteries a to d of Examples 1 to 4 in which NaF is added to the alkaline electrolyte have a lower self-discharge amount than the battery f of Comparative Example 1 to which NaF is not added. From this, it can be seen that the self-discharge characteristics of the nickel-hydrogen secondary battery are improved by adding NaF to the alkaline electrolyte. This is because NaF contained in the alkaline electrolyte is adsorbed on Mg or the like on the surface of the hydrogen storage alloy, suppressing hydrogen dissociation from the hydrogen storage alloy of the negative electrode, and increasing the amount of Na ions in the electrolyte. This is probably because the oxygen overvoltage increases and suppresses the self-decomposition reaction of the positive electrode.

(Iii) Further, the battery d of Example 4 had a lower self-discharge amount than the batteries a to c of Examples 1 to 3, and was 314.9 mAh. In this battery d, NaF was added to the alkaline electrolyte, and a large amount of NaF powder was supplied onto the electrode group 22. In this case, it is considered that the powder NaF is dissolved in the alkaline electrolyte present above the electrode group 22 and the concentration of the alkaline electrolyte in the battery is maintained at a value close to the saturation concentration as a whole. That is, in the case of this aspect, the alkaline electrolyte contains as much NaF as possible. From this, it was confirmed that the effect of improving the self-discharge of the battery increases as the amount of NaF increases. Moreover, it was confirmed that it is effective for improving the self-discharge characteristics to include NaF in the alkaline electrolyte and to supply powdered NaF into the outer can 10.

  The present invention is not limited to the above-described embodiments and examples, and various modifications are possible. For example, the nickel metal hydride secondary battery may be a prismatic battery and has a mechanical structure. Is not exceptionally limited.

2 Nickel metal hydride secondary battery 22 Electrode group 24 Positive electrode 26 Negative electrode 28 Separator

Claims (3)

A container, and an electrode group housed in an airtight state together with an alkaline electrolyte in the container,
The electrode group is composed of a positive electrode and a negative electrode superimposed via a separator,
The negative electrode includes a rare earth-Mg-Ni-based hydrogen storage alloy,
The alkaline electrolyte, only contains the NaF,
The hydrogen storage alloy is
General _{formula: Ln 1-x Mg x Ni} y-a Al a ( In the formula, Ln is Zr, Ti, at least one element selected from Y and rare earth elements, the subscript x, y, a, respectively, 0.05 ≦ x ≦ 0.30, 2.8 ≦ y ≦ 3.8, 0.05 ≦ a ≦ 0.30) .   The nickel metal hydride secondary battery according to claim 1, wherein the concentration of NaF in the alkaline electrolyte is 0.25 mass% to 1.17 mass%.   The nickel metal hydride secondary battery according to claim 1, further comprising powdered NaF.

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