Patent Application
20060141360
The present invention relates to a material for the positive electrode of a battery.
Small silver oxide alkaline batteries (commonly called “button” batteries) are in general use. In a silver oxide battery, silver oxide is used as the positive electrode material, zinc dust as the negative electrode material, and for the electrolyte, an alkaline solution, such as an aqueous solution of KOH or NaOH. Silver oxide has a high discharge capacity, but because silver oxide itself has a resistance that is close to that of an insulator, it is usually blended with a conductive material such as graphite to impart conductivity to the positive electrode material.
Reference Nos. 1 to 3 disclose the use of the silver-nickel compound oxide AgNiO2 as a battery positive electrode material. Reference No. 1 (JP S57-849A), for example, describes obtaining a flat discharge voltage curve from a battery having a positive electrode formed of AgNiO2 synthesized by the equimolar (molar ratio=1/1) reaction of silver nitrate and nickel nitrate, and Reference No. 2 (JP H10-188975A) describes a manufacturing method for optimizing a AgNiO2 synthesizing reaction, and a battery with stable discharge characteristics obtained when the AgNiO2 was used as the positive electrode material. Reference No. 3 (US 2002/0127469 A1) describes a button battery with an electrode container containing AgNiO2 mixed with silver oxide or manganese dioxide.
It may be possible for batteries with positive electrodes constituted by the AgNiO2 described in the above references to exhibit high capacity and stable discharge characteristics. However, in practice such a technology has not been realized. Current silver oxide batteries generally use silver oxide as the positive electrode material, blended with an appropriate amount of graphite for conductivity. Reasons that can be cited for this include that a AgNiO2 compound has a lower molding density than silver oxide, and therefore a lower discharge capacity per unit volume, and that although, based on the valences of the silver and nickel in the compound, it can be expected to have a theoretical discharge capacity of 270 mAh per unit weight (gram), when the actual capacity factor is taken into consideration, the effective discharge capacity is in the order of 10% lower than the discharge capacity of silver oxide.
Accordingly, when AgNiO2 is used as the positive electrode active material in a small battery having a small volume in an attempt to obtain a battery performance equivalent to that of a conventional silver oxide battery of the same size, in some cases there is not enough space to load the positive electrode compact, forming an obstacle that has made it impossible to constitute a battery that can advantageously take the place of the small silver oxide batteries currently in use.
The object of the present invention is to provide a battery positive electrode material comprised of a conductive composite oxide of silver and nickel having a higher molding density and higher discharge capacity than AgNiO2.
Based on the results of many experiments and studies, the present inventors succeeded in synthesizing a silver-nickel composite oxide containing more excess silver than AgNiO2. This excess silver composite oxide has a crystal structure that is substantially the same as that of AgNiO2 while at the same time having a higher discharge capacity than AgNiO2 as well as good conductivity, and forms a powder with a high molding density.
In accordance with the present invention, the above object is attained by providing a battery positive electrode material comprising a conductive chemical compound represented by the general formula AgxNiyO2, wherein X/Y is greater than 1, preferably greater than 1.1, and does not exceed 1.9). In X-ray diffraction, the conductive compound has a main peak that is identical to that of AgNiO2 (wherein X=Y=1), and does not exhibit an Ag2O or AgO peak.
It was also found that when Cu or Bi is substituted for part of the Ni in the compound, the excess silver composite oxide thus formed also has a higher discharge capacity than AgNiO2, good conductivity and a high molding density. Thus, in accordance with this invention, the above object is also attained by a battery positive electrode material comprising a conductive chemical compound represented by the general formula AgxNiyMzO2, wherein M represents at least one of Cu and Bi, X/(Y+Z) is greater than 1 and does not exceed 1.9, and Z does not exceed 0.4.
These AgxNiyO2 system (X/Y>1) and AgxNiyMzO2 system (X/(Y+Z)>1, Z<0.4) excess silver composite oxides have excellent molding density, discharge capacity and conductivity, and when used singly or in combination with silver oxide as a positive electrode material, makes it possible to improve battery performance while retaining the same small size as that of a conventional silver oxide battery.
The present inventors discovered a chemical compound that, even with a higher silver content than AgNiO2, has a crystal structure similar to that of AgNiO2, discharge characteristics and conductivity equivalent or superior to those of AgNiO2, and forms a powder having high molding density. Although the molar ratio of Ag/Ni in this compound is larger than 1, and thus is larger than in the case of AgNiO2, it has a crystal structure equivalent to that of AgNiO2, and can therefore be represented by the general formula AgxNiyO2 (X/Y>1). Preferably, X/Y is 1.1 or above or, depending on the case, 1.2 or above, and does not exceed 1.9.
The compound can be called an excess Ag—Ni delafossite type oxide, which hereinbelow is shortened to “excess Ag—Ni oxide.” As in the case of AgCoO2 and the like, AgNiO2 can be regarded as being a delafossite oxide represented by ABO2, with A as a univalent metal and B as a trivalent metal. A delafossite oxide has a crystal structure comprising alternating packed beds of A-O-B-O-, and although it is an oxide, it is conductive. While the excess Ag in the case of the Ag—Ni oxide of this invention also can be described as a departure from previously-defined delafossite oxides in which the Ag and Ni are equimolar, what this invention has clarified is that it has the same crystal structure and exhibits good conductivity.
With the horizontal axis in the graph of
As described in the Examples below, it was also confirmed that the excess Ag—Ni oxide according to the invention has good conductivity (reduced resistivity). With respect also to moldability, under identical conditions, the excess Ag—Ni oxide of the invention exhibited higher molding density than AgNiO2.
Thus, the excess Ag—Ni oxide of the invention can be used on its own as a positive electrode material. Moreover, by substituting the excess Ag—Ni oxide for part of the silver oxide used as the positive electrode material in a silver oxide battery, it is possible to obtain a high-performance positive electrode material having good conductivity and moldability, even without blending graphite or the like with the material. Improvements in the conductivity, moldability and pellet strength of silver oxide can be achieved by blending in amounts of the excess Ag—Ni oxide starting from a few weight percent. Therefore, the amount of such addition is set at 2 wt % or more, preferably 3 wt % or more, and more preferably 5 wt % or more.
Part of the Ni in the excess Ag—Ni oxide can be replaced by element M (Cu and/or Bi). Specifically, a compound represented by a general formula AgxNiyMzO2 (wherein M is at least one of Cu and Bi, X/(Y+Z) is greater than 1 and does not exceed 1.9, and Z does not exceed 0.4) exhibits the same conductivity, moldability and discharge characteristics as the above excess Ag—Ni oxide. Such a compound in which part of the Ni is replaced by the element M (Cu and/or Bi) is called an excess Ag—Ni-M oxide.
An excess Ag—Ni-M oxide has the advantage that, compared to an excess Ag—Ni oxide, it can be produced at a lower cost achieved by substituting cheap Cu or Bi for expensive Ni. Moreover, by blending 3 wt % or more excess Ag—Ni-M oxide with the silver oxide used as the positive electrode material in a silver oxide battery, it is possible to obtain a high-performance positive electrode material having good conductivity and moldability, without having to blend graphite or the like with the material.
The positive electrode material of the invention can be produced by a method comprising reacting an inorganic acid salt of Ag and an inorganic acid salt of Ni, or an inorganic acid salt of Ni and an inorganic acid salt of M, in an oxidizing alkaline aqueous solution. This is done using the following procedure.
(1) Reacting the inorganic acid salt of Ag and the inorganic acid salt of Ni, or the inorganic acid salt of Ni and the inorganic acid salt of M, in water with an alkali hydroxide to obtain a neutralizing precipitate.
(2) Carrying out oxidation treatment (increasing metal ion valences) by adding an oxidizing agent to the solution prior to, or during, the above neutralization reaction, or to the obtained precipitate suspension. It is preferable for the addition of the oxidizing agent to be divided into a plurality of stages before, during and after the neutralization reaction.
(3) Separating the oxidized precipitate from the liquid, washing and drying the precipitate, and crushing the resulting dried cake to powder.
NaOH or KOH can be employed as the alkali hydroxide used in the neutralization reaction. Although nitrate, sulfate, hydrochloride, and phosphate and the like of each metal can be used as the inorganic acid salts of Ag, Ni and M, it is preferable to use the nitrate or sulfate of the metals. Typically, the nitrate of each metal may be used. For AgNO3, for example, Ni(NO3)2 having a desired number of moles can be used, or the required amount of Cu(NO3)2 or Bi(NO3)3.
A higher alkalinity is better for the neutralization treatment. In the case of Ag+Ni+(M), for example, the reaction proceeds more readily in the presence of around five times the alkalinity, in terms of molar ratio. Neutralization and oxidation treatments can both be conducted at a reaction temperature of from room temperature to 100° C., and preferably are conducted at 30 to 50° C. It is necessary to stir at an intensity at which the neutralization and oxidation reactions proceed uniformly. Even after the treatments end, the stirring should be maintained at the required temperature for ripening.
The Ag/Ni ratio in the end compound, and by extension the atomic ratio of the Ag/Ni in the particles, can be adjusted to a value within the range 1 to 1.9 by adjusting the molar ratio of the silver nitrate and nickel furnished for the reaction. Similarly, when element M is included, the atomic ratio of the Ag/(Ni+M) in the end compound, and by extension the atomic ratio of the Ag/(Ni+M) in the particles, can be adjusted to a value within the range 1 to 1.9 by adjusting molar ratio of the Ag/(Ni +M) in the salt concerned.
The oxidation treatment comprises using an oxidizing agent to increase metal ion valences. The oxidizing agent can be added at the start of the neutralization reaction or during the neutralization, or added to the precipitate suspension. Thus, the neutralization and oxidation treatments can be done separately or at the same time. It is preferable for the addition of the oxidizing agent to be divided into a plurality of stages before, during and after the neutralization reaction. The oxidation treatment should be done with stirring, and at not more than 100° C., since too high a temperature will promote decomposition of the oxidizing agent. Substances that can be used as the oxidizing agent include, for example, KMnO4, NaOCl, H2O2, K2S2O8, Na2S2O8, and ozone, but K2S2O8, Na2S2O8 or ozone are preferable, since impurities in the excess Ag—Ni oxide powder can be reduced by the use of an oxidizing agent thus constituted. It is necessary to use an amount of oxidizing agent that is enough to change the valences. This can be achieved by adding an amount of oxidizing agent that is at least equivalent to the valences concerned, and preferably is around twice the amount.
Before moving on to the following Examples of the invention, the methods used to measure the properties of the powder obtained in each case will be explained.
The Ag content, discharge capacity, molding density and resistivity of powder type silver oxide (Ag2O manufactured by Dowa Mining Co., Ltd. having an average particle diameter of 15 μm) used for commercial positive electrodes were measured. The results are shown in Table 1. This silver oxide had a purity of at least 99.9%, as calculated from the Ag content. The discharge capacity was 220 mAh/g, close to the theoretical capacity of silver oxide.
The Ag content, discharge capacity, molding density and resistivity of granular type silver oxide (manufactured by Dowa Mining Co., Ltd. with an average particle diameter of 105 μm) used for commercial positive electrodes were measured. The results are shown in Table 1. The discharge capacity and resistivity were the same as those of Comparative Example 1.
2.0 liters of pure water, 9 mols of NaOH and 1 mol of sodium persulfate were put into a 5-liter beaker and the temperature of the solution adjusted to 30° C. One liter of an aqueous solution of silver nitrate equivalent to 1.0 mol of silver was added to the solution over a thirty-minute period, and the solution was maintained at 30° C. for one hour.
One liter of an aqueous solution of nickel nitrate equivalent to 1.0 mol of nickel was added to the solution over a thirty-minute period, and the solution was maintained at 30° C. for four hours and the reaction ended. The reaction slurry was filtered to obtain a black cake. The cake was thoroughly washed with pure water and then dried in a vacuum for 12 hours at 100° C. A pestle was then used to crush the dried cake. When X-ray diffraction was used to identify the black powder thus obtained, it was confirmed to be AgNiO2. The X-ray diffraction chart of the powder is shown in
2.0 liters of pure water, 9 mols of NaOH and 1 mol of sodium persulfate were put into a 5-liter beaker and the temperature of the solution adjusted to 30° C. One liter of an aqueous solution of silver nitrate equivalent to 1.05 mols of silver was added to the solution over a thirty-minute period, and the solution was maintained at 30° C. for two hours.
One liter of an aqueous solution of nickel nitrate equivalent to 0.95 mol of nickel was added to the solution over a thirty-minute period and was followed by the addition of 1 mol of sodium persulfate, and the solution was maintained at 30° C for 12 hours to end the reaction. The reaction slurry was filtered to obtain a black cake. The cake was thoroughly washed with pure water and then dried in a vacuum for 12 hours at 100° C. A pestle was then used to crush the dried cake. When X-ray diffraction was used to identify the black powder thus obtained, a peak similar to that of AgNiO2 was observed. The X-ray diffraction chart of the powder is shown in
The same procedure as that of Example 1 was repeated, except that an aqueous solution of silver nitrate equivalent to 1.10 mols of silver and an aqueous solution of nickel nitrate equivalent to 0.90 mol of nickel were used. When X-ray diffraction was used to identify the black powder thus obtained, a peak similar to that of AgNiO2 was observed. No silver oxide peak was observed. The results of the evaluation test of the powder are shown in Table 1.
The same procedure as that of Example 1 was repeated, except that an aqueous solution of silver nitrate equivalent to 1.20 mols of silver and an aqueous solution of nickel nitrate equivalent to 0.80 mol of nickel were used. When X-ray diffraction was used to identify the black powder thus obtained, a peak similar to that of AgNiO2 was observed. No silver oxide peak was observed. The X-ray diffraction chart of the powder is shown in
25 wt % of the Ag2O of Comparative Example 1 and 75 wt. % of the AgNiO2 synthesized in Comparative Example 3 were thoroughly mixed. The mixture had a silver content that was substantially equivalent to the silver content of the excess Ag—Ni oxide synthesized in Example 3. The results of the evaluation test of this mixed powder are shown in Table 1.
The same procedure as that of Example 1 was repeated, except that an aqueous solution of silver nitrate equivalent to 1.30 mols of silver and an aqueous solution of nickel nitrate equivalent to 0.70 mol of nickel were used. When X-ray diffraction was used to identify the black powder thus obtained, a peak similar to that of AgNiO2 was observed. No silver oxide peak was observed. The results of the evaluation test of the powder are shown in Table 1.
36 wt % of the Ag2O of Comparative Example 1 and 64 wt. % of the AgNiO2 synthesized in Comparative Example 3 were thoroughly mixed. The mixture had a silver content that was substantially equivalent to the silver content of the excess Ag—Ni oxide synthesized in Example 4. The results of the evaluation test of this mixed powder are shown in Table 1.
The same procedure as that of Example 1 was repeated, except that an aqueous solution of silver nitrate equivalent to 1.40 mols of silver and an aqueous solution of nickel nitrate equivalent to 0.60 mol of nickel were used. When X-ray diffraction was used to identify the black powder thus obtained, a peak similar to that of AgNiO2 was observed, and an Ag2O (111) plane peak was also observed. The results of the evaluation test of the powder are shown in Table 1.
A. The same procedure as that of Example 2 was repeated, except that the aqueous solution of nickel nitrate equivalent to 0.90 mol of nickel was changed to an aqueous solution of nickel nitrate equivalent to 0.72 mol of nickel and an aqueous solution of copper nitrate equivalent to 0.18 mol of copper. The results of the evaluation test of the powder thus obtained are shown in Table 1.
B. The same procedure as that of Example 2 was repeated, except that the aqueous solution of nickel nitrate equivalent to 0.90 mol of nickel was changed to an aqueous solution of nickel nitrate equivalent to 0.54 mol of nickel and an aqueous solution of copper nitrate equivalent to 0.36 mol of copper. The results of the evaluation test of the powder thus obtained are shown in Table 1.
C. The same procedure as that of Example 3 was repeated, except that the aqueous solution of nickel nitrate equivalent to 0.80 mol of nickel was changed to an aqueous solution of nickel nitrate equivalent to 0.60 mol of nickel and an aqueous solution of copper nitrate equivalent to 0.20 mol of copper. The results of the evaluation test of the powder thus obtained are shown in Table 1.
D. The same procedure as that of Example 3 was repeated, except that the aqueous solution of nickel nitrate equivalent to 0.80 mol of nickel was changed to an aqueous solution of nickel nitrate equivalent to 0.40 mol of nickel and an aqueous solution of copper nitrate equivalent to 0.40 mol of copper. The results of the evaluation test of the powder thus obtained are shown in Table 1.
A. The same procedure as that of Example 3 was repeated, except that the aqueous solution of nickel nitrate equivalent to 0.80 mol of nickel was changed to an aqueous solution of nickel nitrate equivalent to 0.70 mol of nickel and an aqueous solution of bismuth nitrate equivalent to 0.10 mol of bismuth. The results of the evaluation test of the powder thus obtained are shown in Table 1.
B. The same procedure as that of Example 3 was repeated, except that the aqueous solution of nickel nitrate equivalent to 0.80 mol of nickel was changed to an aqueous solution of nickel nitrate equivalent to 0.60 mol of nickel and an aqueous solution of bismuth nitrate equivalent to 0.20 mol of bismuth. The results of the evaluation test of the powder thus obtained are shown in Table 1.
The results shown in Table 1 reveal the following.
(1) The excess Ag—Ni oxide of the present invention contains more excess Ag than AgNiO2, and discharge capacity rises with the increase in the degree of excess Ag (see
(2) As the degree of excess Ag in the excess Ag—Ni oxide of the present invention increases compared to AgNiO2, the molding density thereof also increases, and in terms of discharge capacity per unit volume (mAh/cc), a higher capacity can be obtained than in the case of AgNiO2.
(3) A comparison between the AgNiO2 and Ag2O mixtures of Reference Examples 1 and 2, and the excess Ag—Ni oxides of Examples 3 and 4 having an Ag content equivalent to that of those mixtures, shows that the excess Ag—Ni oxides have a higher molding density and lower resistivity than the mixtures.
(4) The excess Ag—Ni-M oxides of Examples 5 and 6 in which Cu or Bi is substituted for part of the Ni, form positive electrode materials having the same high discharge capacity and low resistivity as an excess Ag—Ni oxide.
The powder of Example 3 (Ag1.2Ni0.8O2) and the granular silver oxide used in Comparative Example 2 were mixed together in various proportions, and the pellet strength, resistivity and discharge capacity of the mixtures measured. The results are shown in Table 2.
As shown in Table 2, when the excess Ag—Ni oxide powder of this invention is mixed with silver oxide, the discharge capacity is maintained, the resistivity decreases in accordance with the mixture amount and pellet strength increases. Those effects are manifested starting with an excess Ag—Ni oxide powder blend amount in the order of 5 wt %. Therefore, initial discharge continuity can be ensured even with a blend amount of 5 wt %. Once discharge begins, metallic silver produced by the discharge exhibits conductivity, thus functioning as a battery. `
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2004-375474 | Dec 2004 | JP | national |
