JP4120768B2 - Non-sintered nickel electrode and alkaline battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to non-sintered nickel electrodes for alkaline batteries, and alkaline batteries such as nickel metal hydride batteries, nickel cadmium batteries, and nickel zinc batteries using the nickel electrodes.
[0002]
[Prior art]
In recent years, alkaline batteries such as nickel metal hydride batteries and nickel cadmium batteries have been widely used as power sources for small information terminal devices such as mobile phones, personal computers, and electric tools. Nickel zinc primary (also called nickel zinc dry batteries) and secondary batteries are also attracting attention as batteries having high energy. In particular, nickel-metal hydride batteries have been conventionally unsuitable for applications that require high output. However, by improving high-rate discharge characteristics, nickel-metal hydride batteries can be used not only for the aforementioned applications but also as a power source for hybrid electric vehicles (HEV). Is also being used and its demand is increasing.
[0003]
In the above application, the battery is generally stored in a limited space. In order to save space, the battery is also required to be downsized. In addition, there is a tendency that the electrical load of the device to be applied tends to increase, and a further increase in capacity is required for the battery.
[0004]
The alkaline battery employs a configuration in which a nickel electrode containing nickel hydroxide as a main active material is used as a positive electrode, and a hydrogen storage alloy electrode, a cadmium electrode or a zinc electrode is used as a negative electrode. As for the positive electrode, non-sintered (also called paste type) nickel electrodes are widely used instead of sintered nickel electrodes because of their high productivity and high capacity. Yes.
[0005]
The non-sintered nickel electrode is an aqueous solution in which a thickener is dissolved in a mixed powder of an active material nickel hydroxide and a conductive agent, or a nickel hydroxide powder whose surface is coated with a conductive agent or a precursor of a conductive agent. Is added and kneaded to make a paste, the porous substrate is filled with the paste, dried, and pressed.
[0006]
As the thickener, an organic polymer compound such as carboxymethylcellulose (hereinafter referred to as CMC) is generally used because it is inexpensive and can obtain an appropriate paste viscosity when made into an aqueous solution. .
[0007]
Incidentally, various improvements have been made to the nickel electrode in order to meet the demand for higher capacity. Previously, a fine powder such as cobalt monoxide powder was added to the nickel hydroxide powder as a conductive agent. However, the addition of the fine powder as the conductive agent has a drawback of greatly reducing the packing density of the active material powder.
[0008]
One of the improvements is that a conductive layer is formed on the surface of the nickel hydroxide powder. Specifically, the surface of nickel hydroxide is coated with a precursor of a conductive substance such as cobalt hydroxide, and this is oxidized to form a higher-order compound of cobalt (also referred to as cobalt oxyhydroxide) which is a conductive substance. It is a method to change. Since this method forms a dense conductive network in the positive electrode, the conductive function is excellent, and good conductivity can be obtained by adding a small amount of a conductive agent. Moreover, there exists an advantage which can make the packing density of a powder high.
[0009]
The second is to oxidize nickel hydroxide powder particles having a cobalt compound deposited on the surface using an oxidizing agent. For example, Japanese Patent Laid-Open Nos. 8-213010 and 12-307130 have proposed methods for performing the oxidation treatment.
[0010]
Conventionally, in the case of a nickel metal hydride battery or a nickel cadmium battery, the battery is charged after being incorporated into the battery, whereby the precursor of the conductive substance such as cobalt hydroxide is converted into a conductive substance made of a higher order compound of cobalt. It was changing. As disclosed in Japanese Patent Application Laid-Open No. 10-199564, etc., a method for efficiently changing to a higher cobalt compound by devising a charging condition has been studied.
[0011]
However, the change from cobalt hydroxide to a higher-order cobalt compound is an irreversible reaction, and when oxidized by charging, the amount of electricity spent in the irreversible oxidation reaction is accumulated in the negative electrode as a discharge reserve. Therefore, it is necessary to fill the negative electrode as much as that, which is an impediment to increasing the capacity of the battery.
[0012]
By the chemical oxidation treatment, a cobalt compound such as cobalt hydroxide deposited on the nickel hydroxide powder can be oxidized, and a higher order compound of cobalt can be generated without being charged. Moreover, not only cobalt hydroxide but also a part of nickel hydroxide can be oxidized simultaneously. When the active material is applied to the positive electrode of an alkaline battery such as a nickel metal hydride battery or a nickel cadmium battery, the discharge reserve of the battery can be reduced, and the active material filling amount of the negative electrode can be reduced and the filling amount of the positive electrode can be increased. it can. The discharge capacity of the battery can be increased by increasing the filling amount of the positive electrode.
[0013]
Recently, a nickel-zinc primary battery (nickel-zinc dry battery), which is superior in high-rate discharge characteristics as compared with conventional manganese dry batteries, is being commercialized. The main component of the positive electrode active material of the primary battery is a higher-order nickel compound (also referred to as nickel oxyhydroxide) obtained by oxidizing nickel hydroxide. Similar to the above, a higher-order compound of cobalt can be generated on the surface of the active material powder to impart conductivity. The difference from a nickel electrode for a secondary battery such as a nickel metal hydride battery is that at least most of the nickel hydroxide is changed to a higher nickel compound by chemical oxidation treatment.
[0014]
Thus, in order to increase the capacity of an alkaline secondary battery using a nickel electrode as a positive electrode, or to produce a nickel electrode of an alkaline primary battery, the nickel hydroxide, which is the active material of the nickel electrode, is subjected to a chemical oxidation treatment. It has become an important technique to oxidize at least part of the application. However, when trying to prepare an active material paste for a nickel electrode using the nickel hydroxide powder subjected to the oxidation treatment, the paste viscosity decreases rapidly, and the solid powder and the thickener solution are separated. There was a drawback that the porous substrate could not be filled.
[0015]
Further, in order to improve the cycle characteristics of an alkaline secondary battery using a non-sintered nickel electrode as a positive electrode, further improvement has been demanded on the binding property of the active material powder of the nickel electrode.
[0016]
[Problems to be solved by the invention]
The present invention has been made in view of the above-mentioned problems of the prior art, and in producing a non-sintered nickel electrode, nickel hydroxide or higher-order nickel compounds are mainly used by preventing a rapid decrease in paste viscosity. The present invention provides an alkaline battery that achieves uniform filling of a porous substrate with an active material powder as a component and has good characteristics. Moreover, it is for improving the binding property of the active material powder of a non-sintered nickel electrode.
[0017]
[Means for Solving the Problems]
The present invention applies a CMC having a high degree of etherification of 0.9 to 2.5 (hereinafter referred to as a high degree of etherification CMC) as a thickener when producing a non-sintered nickel electrode. The problem is solved.
[0018]
Conventionally, CMC having a low degree of etherification of 0.6 to 0.8 (hereinafter referred to as a low degree of etherification CMC) has been used as a thickener because it is inexpensive and easily available. However, it has been found that the low etherification degree CMC is inferior in oxidation resistance and alkali resistance.
[0019]
The chemical oxidation treatment of the nickel hydroxide powder needs to be performed in a caustic concentrated solution. After the oxidation treatment, the powder is washed with water to remove the alkali, but it is difficult to completely remove the powder.
[0020]
The rapid decrease in viscosity of the nickel hydroxide powder paste is a phenomenon caused by decomposition of the low etherification degree CMC by the caustic contained in the nickel hydroxide powder after the oxidation treatment. This decomposition reaction is promoted by the oxidizing power of the active material powder subjected to oxidation treatment.
[0021]
CMC added to the non-sintered nickel electrode also functions as a binder for the active material powder. However, the low etherification degree CMC is unstable with respect to caustic which is an electrolytic solution even after the nickel electrode to which the CMC is applied is incorporated in the battery, and is oxidized and decomposed by the oxidizing power of the nickel electrode during charging. Therefore, it cannot function sufficiently as a binder because it changes to a low molecular weight substance.
[0022]
In addition, the oxidative decomposition of CMC is an irreversible reaction and contributes to the generation of discharge reserves in nickel metal hydride batteries and nickel cadmium batteries.
[0023]
On the other hand, the high etherification degree CMC applied to the present invention is superior to the low etherification degree CMC in terms of stability against caustic and oxidation resistance. Therefore, the paste to which the high degree of etherification CMC is applied suppresses the decomposition of CMC and suppresses the decrease in viscosity. Applying a high degree of etherification CMC to the nickel electrode is effective in suppressing discharge reserve generation in alkaline batteries.
[0024]
Moreover, since the high degree of etherification CMC exists stably even after being incorporated in the battery, it functions as a binder for the nickel electrode.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
In the process of producing a paste having nickel hydroxide powder as a main constituent amount, a high etherification degree CMC having an etherification degree of 0.9 to 2.5 is applied as a thickener. The degree of etherification refers to the number of 3 hydroxyl groups (—OH) substituted with carboxymethyl groups (—CH 2 COONa) per anhydroglucose, which is a constituent unit of CMC molecules.
[0026]
It was found that the higher the degree of etherification, the better the alkali resistance and oxidation resistance. However, the synthesis of CMC having a degree of etherification exceeding 2.5 is substantially difficult and is not suitable for practical use.
[0027]
The ratio of the high degree of etherification CMC to the nickel hydroxide powder is preferably 0.05 to 2.0% by weight. When the ratio is less than 0.05% by weight, it is difficult to obtain a solution viscosity necessary for paste formation. If it exceeds 2.0 wt%, except that the paste viscosity filling is difficult for too high into the porous substrate, since lowering the active material powder filling amount is not preferable.
[0028]
Although the polymerization degree of CMC applied to this invention is not specifically limited, The thing of 1500-2500 equivalent to the general purpose goods with easy acquisition can be applied.
[0029]
The procedure for producing the paste is not particularly limited. Nickel hydroxide powder and CMC powder may be mixed in advance, water may be added to the mixed powder and kneaded, or an aqueous solution of CMC may be prepared in advance, and an aqueous solution of CMC may be added to the nickel hydroxide powder. It may be added and kneaded.
[0030]
The ratio of the moisture contained in the paste is not particularly limited. What is necessary is just to set so that the powder in a paste may not isolate | separate, and it should have the softness which is not bulky and is easy to fill a board | substrate. Specifically, it is appropriate to set the ratio of moisture contained in the paste to a range of 10 to 35% by weight.
[0031]
The active material powder for nickel electrodes mainly composed of nickel hydroxide applied to the present invention is not particularly limited. However, the present invention is particularly effective when applied to an active material powder that has been subjected to a chemical oxidation treatment using an oxidizing agent suitable for increasing the capacity.
[0032]
As described above, in the nickel electrode for an alkaline secondary battery, the cobalt compound is deposited on the surface as an active material of the nickel electrode in order to increase its capacity or reduce the generation of discharge reserve, A nickel hydroxide powder in which a part of nickel is oxidized at the same time as the cobalt compound is oxidized by oxidation treatment to form a higher cobalt compound is applied.
[0033]
The degree of oxidation of the nickel hydroxide powder can be expressed by an average oxidation number of nickel and cobalt contained in the nickel hydroxide powder. The degree of oxidation can be controlled by changing the oxidation conditions such as the amount of oxidizing agent added to the reaction bath. In order to increase the capacity of an alkaline secondary battery such as a nickel metal hydride battery that is the subject of the present invention, the average oxidation number is preferably set in the range of 2.04 to 2.4. Further, in the case of a nickel electrode for an alkaline primary battery, it is oxidized almost 100% and almost all of the nickel compound is converted into a higher order nickel compound.
[0034]
The oxidizing agent used for the chemical oxidation treatment is not particularly limited. Specifically, ammonium peroxodisulfate {(NH ₄ ) ₂ S ₂ O ₈ }, potassium peroxodisulfate (K ₂ S ₂ O ₈ ), sodium peroxodisulfate (N ₂ S ₂ O ₈ ), hypochlorous acid An oxidizing agent such as sodium (NaClO) or sodium chlorite (NaClO ₂ ) can be applied.
[0035]
Hereinafter, a nickel metal hydride battery is taken as an example, and the details of the present invention will be described based on one example. However, the present invention mainly uses nickel hydroxide or a higher-order compound of nickel (nickel oxyhydroxide) as a main active material. It can be applied to all alkaline batteries having a non-sintered nickel electrode. Therefore, the battery configuration, electrode constituent materials, and the like of the present invention are not limited to the examples described below.
[0036]
【Example】
(Preparation of active material powder for nickel electrode)
As the nickel electrode active material, a powder (hereinafter simply referred to as nickel hydroxide powder) composed mainly of high-density nickel hydroxide used as a high-capacity nickel electrode active material was applied. The nickel hydroxide powder is a powder having an average particle diameter of about 10 μm, and nickel hydroxide in which zinc (Zn) and cobalt (Co) are dissolved in 4 wt% and 5 wt%, respectively, in a metal ratio. The core layer is formed by coating the surface with β-cobalt hydroxide {Co (OH) ₂ }. In addition, the ratio of the said cobalt hydroxide to the nickel hydroxide powder was 6 weight%.
[0037]
100 g of the nickel hydroxide powder was put into 200 ml of an aqueous sodium hydroxide solution having a temperature of 90 ° C. and a concentration of 30% by weight, and the powder was dispersed by stirring. While maintaining the temperature of the dispersion at 90 ° C., 50 ml of a sodium hypochlorite solution having a concentration of 5% as an oxidizing agent was gradually added dropwise. The reaction bath was slowly stirred for 2 hours while maintaining the temperature of the reaction bath at the above temperature. The nickel hydroxide powder was separated from the reaction bath solution by filtration, washed with water, and then dried. The average oxidation number of the nickel hydroxide powder was 2.17, and the pH of the dispersion obtained when 10 g of the powder was dispersed in 100 ml of water was 11.8.
[0038]
(Preparation of nickel electrode active material paste)
A CMC aqueous solution having a concentration of 0.5% by weight was prepared using CMC having a degree of etherification of 0.7, 0.9, 1.5, 1.8, and 2.5. Let this CMC aqueous solution be CMC aqueous solution 1, 2, 3, 4, and 5, respectively.
[0039]
A paste was prepared by adding and mixing 20 parts by weight of a CMC aqueous solution to 80 parts by weight of the nickel hydroxide powder subjected to the oxidation treatment. The paste is made into pastes 1, 2, 3, 4 and 5 according to the number of the CMC solution.
[0040]
(Evaluation of viscosity stability of paste) The paste was allowed to stand at a temperature of 20 ° C., and the change with time in viscosity after the paste was prepared was examined. A B-type clay meter was used for viscosity measurement. The result is shown in FIG. In order to fill the porous substrate with the paste, a viscosity of at least 3000 millipascal · second (mPa · s) or more is required. As shown in FIG. 1, the paste 1 to which CMC having a degree of etherification of 0.7 is applied In this case, the viscosity rapidly decreases with the passage of time, and decreases to 2000 mPa · s after 1 hour. A rapid reduction in viscosity of the paste 1 is a phenomenon that occurred for CMC is decomposed by the oxidizing power of the caustic with the nickel hydroxide powder contained in the nickel hydroxide powder subjected to oxidation treatment.
[0041]
In paste 1, separation of the powder from the CMC solution occurred after 1 hour. On the other hand, in the case of paste 2 to paste 5 to which CMC having an etherification degree of 0.9 or more was applied, the viscosity of 5000 mPa · s or more was maintained even after 5 hours, and the powder and CMC aqueous solution were hardly separated. I was not able to admit.
[0042]
(Production of nickel electrode)
After 1 hour had elapsed after the paste was prepared, the paste was stirred again and filled into a foamable nickel substrate having a thickness of 1.6 mm, a basis weight of 500 g / m ² , a width of 50 mm, and a length of 50 m. The ratio of the volume of the filled paste to the pore volume of the foamable nickel substrate at this time was evaluated as the filling efficiency (paste filling volume / substrate pore volume × 100%). Thirty samples were sampled at equal intervals from the paste-filled substrate to determine the filling efficiency of each sample. The result is shown in FIG.
[0043]
As shown in FIG. 2, in the case of the paste 1, the average value of the filling efficiency is as low as 90%, the minimum value is about 85%, and the maximum value is as large as 95%. On the other hand, when Paste 2 to Paste 5 are applied, the average filling efficiency is as high as 96 to 97%, the minimum value is 93%, the maximum value is 99%, and the variation is smaller than that of Paste 1. . The paste was agitated prior to filling the substrate, but in the case of paste 1, the viscosity of the paste is too low so that the filled paste tends to fall off the substrate, and the powder and solution are mixed during the filling process. This seems to be the result of the separation.
[0044]
Since the active material filling amount of the nickel electrode is directly reflected in the discharge characteristics of the alkaline battery using the nickel electrode, it is disadvantageous that the filling efficiency is low and the variation thereof is large.
[0045]
(Production of nickel metal hydride battery)
Immediately after producing the paste 1, paste 2 and paste 5, the substrate was filled (while no noticeable viscosity drop occurred). The filling efficiency of each substrate was about 96%, and no difference due to paste was observed. After the paste was filled, it was dried and pressed to adjust the thickness to 0.7 mm to obtain a nickel electrode original plate. The original plate was cut into a predetermined size to obtain a nickel electrode. The obtained nickel electrode was used as electrode 1, electrode 2 and electrode 5 in accordance with the symbol of the paste used for electrode preparation.
[0046]
MnNi _3.55 Co _0.75 Mn _0.4 Al _0.3 (Mm represents a misch metal which is a mixture of rare earth elements such as La, Ce, Pr, Nd, etc.) Was made. The ratio of the capacity of the hydrogen storage alloy electrode to the capacity of the nickel electrode was set to 1.6.
[0047]
A wound-type electrode plate group was prepared by combining the nickel electrode and the hydrogen storage alloy electrode, and five AA-sized cylindrical nickel-metal hydride batteries were assembled by a conventional method by applying the electrode plate group. The battery 1, battery 2, and battery 5 were made according to the symbol of the nickel electrode.
[0048]
(Initial activation)
The battery is charged and discharged (called chemical conversion) for activating the battery by a regular method for 5 cycles and discharged at a temperature of 20 ° C. and 1/5 It (A) (current 320 mA) at a final voltage of 1.0 V. Was confirmed to be stable.
[0049]
(Charge / discharge cycle test)
The nickel metal hydride battery is charged at a rate of 1/10 It (A) (current 160 mA) for 15 hours at a temperature of 20 ° C., and discharged at 1/5 It (A) (current 320 mA) final voltage 1 V as one cycle. Went. There was almost no difference in performance among the five batteries belonging to the same symbol. FIG. 3 shows an average characteristic regarding the discharge capacity maintenance rate of the battery of each symbol when the battery 1, the battery 2 and the battery 5 are subjected to the charge / discharge cycle test.
[0050]
As shown in FIG. 3, the example battery 2 and the battery 5 according to the present invention have a higher capacity retention rate than the comparative example battery 1. The first reason is that in the case of the battery 2 and the battery 5, the CMC is excellent in alkali resistance and oxidation resistance, so that it does not deteriorate even when the battery is repeatedly charged and discharged, and the function of the binder is not lost. Because of that. The second reason is considered that in the case of the battery 2 and the battery 5, CMC is excellent in oxidation resistance, so that the amount of discharge reserve generated when charging and discharging are repeated is small.
[0051]
As described above in detail, the cycle performance of the alkaline battery can be improved by applying a high etherified CMC having an etherification degree of 0.9 to 2.5 to the non-sintered nickel electrode of the alkaline battery.
[0052]
In particular, when a non-sintered nickel electrode is prepared by applying nickel hydroxide powder that has been chemically oxidized in advance, it is possible to suppress a decrease in the viscosity of the nickel hydroxide powder paste over time. . As a result, the filling efficiency of the paste into the porous substrate can be increased, and variations in the filling efficiency can be reduced.
[0053]
According to the onset bright, it is possible to change over time of the paste viscosity to provide a very small, high nickel electrodes in the charging efficiency to the substrate. Further, it is possible to provide a non-sintered nickel electrode for an alkaline battery that has a small discharge reserve generation amount and excellent cycle characteristics.
[0054]
According to the onset bright, it is possible to provide a non-sintered nickel electrode for large alkaline battery capacity.
[0055]
According to the onset bright, it is possible to reduce in particular the discharge reserve generation, in fabricating a large non-sintered nickel electrode of the discharge capacity, the charging efficiency is high, and provides a small variation electrode charging efficiency Can do.
[0057]
Alkaline battery according to the present onset Ming, a large discharge capacity. In addition, it is an alkaline battery with excellent charge / discharge cycle characteristics.
FIG. 1 is a graph showing changes with time in viscosity of an active material paste for nickel electrodes according to examples and comparative examples of the present invention.
FIG. 2 is a graph showing filling efficiency when a nickel electrode active material paste according to an example and a comparative example of the present invention is filled in a foamed nickel substrate.
FIG. 3 is a graph showing cycle characteristics of nickel-metal hydride batteries according to examples of the present invention and comparative examples.

Claims (3)

A non-sintered nickel electrode for an alkaline battery using nickel hydroxide as a main active material, wherein the nickel hydroxide is at least partially oxidized by a chemical oxidation treatment; and A non-sintered nickel electrode comprising carboxymethyl cellulose (CMC) having a degree of etherification in a range of 0.9 to 2.5. The non-sintered nickel electrode according to claim 1 , wherein the degree of etherification of CMC is 1.5 to 2.5 . An alkaline battery comprising the non-sintered nickel electrode according to claim 1.

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