Alkaline battery

Abstract: An alkaline battery using a mercury free zinc alloy powder as an anode active material is disclosed. It has a gelled anode which comprises silicon and zinc alloy powder containing at least one of bismuth, aluminum and calcium.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to an alkaline battery, particularly to a gelled zinc alloy anode for alkaline batteries. The term "battery" used herein includes single cells as well as plural cell arrangements. 2. Description of the Prior Art: Amalgamated (mercury-containing) zinc alloy powders have heretofore been employed for an anode of an alkaline battery in order to suppress possible corrosion of the zinc powder as an anode active material of the alkaline battery and to maintain acceptable storage properties of the battery having such anode. However, from the viewpoints of environmental preservation and protection from pollution, decreasing mercury content in the anode zinc alloy powder and commercialization of a battery including a non-amalgamated, mercury free zinc alloy powder have been demanded in recent years. From various studies on a variety of non-amalgamated (mercury free) zinc alloy powders, it was found that a non-amalgamated zinc alloy powder to which is added any of bismuth, aluminum and calcium, had excellent corrosion-resistance properties and suppressed the generation of gas due to the corrosion of the zinc powder. This zinc alloy powder is thus taken as a promising anode zinc material for a mercury free alkaline battery (See, Japanese Laid-Open Patent Publication No. Hei 5-86430). However, if an alkaline battery including such non-amalgamated zinc alloy powder containing one of bismuth, aluminum or calcium in the anode is subjected to a discharge with a specified load resistance or current, an extreme reduction may sometimes be observed in the discharge capacity of the battery. The cause of such extreme decrease is now clarified as follows: A dendrite-like zinc oxide having electron conductivity is formed during the discharge with a specified load resistance or current. This dendrite-like zinc oxide precipitates on the separator between the anode and cathode, and penetrates the separator, as a result of which internal short-circuiting occurs between the anode and the cathode. In order to prevent this internal short-circuiting, an effective measure is to make the distance between the cathode and the anode large by increasing the thickness of the separator. Another effective measure is to increase the shielding property of the separator by employing a thinner fiber for the separator, thereby to give the separator a more dense texture. The filling amount of the gelled zinc anode must however be decreased if the separator is made thicker, and the discharge capacity decreases with the decrease in the filling amount of the active material. If the separator has a more dense texture, the internal resistance of the battery increases, thereby leading to deterioration of the battery performance. US 4 312 931 discloses a zinc electrode containing diatomaceous calcium silicate in an amount of from 5 to 15% based upon dry weight. Migration of the zincate ions to the separator and thus dendrite-like zinc oxide growth is reduced by traping zincate ions in the diatomite skeletal structure of the calcium silicate. SUMMARY OF THE INVENTION Ways were therefore sought of providing an alkaline battery which reduces or effectively prevents the above-mentioned internal short-circuiting due to the formation of dendrite-like zinc oxide and which has good discharge performance. The alkaline battery in accordance with the present invention comprises a cathode mixture, a gelled zinc anode including a mercury free zinc alloy powder and a gelling agent, an anode current collector, a separator between the cathode and the anode, and an alkaline electrolyte. The zinc alloy powder contains at least one member selected from the group consisting of bismuth, aluminum and calcium. The gelled anode further includes silicon in an amount from 25-1,500 ppm by weight of said zinc alloy powder and the battery also includes a cathode terminal. Incorporation of the element silicon in the anode helps suppression of the possible growth of the dendrite-like zinc oxide during discharging and also suppresses the decrease in the discharge capacity caused under specified conditions. In accordance with the present invention the content of silicon in the above-mentioned anode is 25 - 1,500 ppm by weight of the zinc alloy powder. In one preferable form of the present invention, the above-mentioned anode contains the silicon in an organic compound. In another preferable form of the present invention, the above-mentioned silicon is contained in the anode in the form of silicate ions. In another preferable form of the present invention, the silicon is contained as a component in the above-mentioned zinc alloy. It is particularly preferable that the above-mentioned gelled anode comprises a mercury-free zinc alloy powder containing at least one of 0.01 - 0.5 wt% bismuth, 0 - 0.5 wt% aluminum and 0.005 - 0.5 wt% calcium. It is .preferable that the zinc alloy powder further contains 0.01% - 0.5 wt% indium. BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a front view of a partially sectioned cylindrical alkaline battery of Type LR6 (UM-3) in accordance with a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The battery shown in FIG. 1 is prepared in the following manner. In a nickel-plated metal casing 1 which also serves as a cathode terminal, there are placed a plurality of cathode mixture pieces 2 molded into short annular cylinders, and the combined body of the cathode mixture pieces 2 was pressed again in the metal casing 1. After a separator 3 and an insulating cap 9 are inserted into the center of the annular cathode mixture 2, a gelled zinc anode 4 is injected into the recess formed inside the separator 3. Then, an anode current collector 6 combined with a resin sealing member 5, a bottom disk 7 which also serves as an anode terminal and an insulating washer 8, are inserted into the gelled zinc anode 4, and the open end of the metal casing 1 is roll-caulked for securing tight sealing of the battery. Thereafter, the surface of the metal casing 1 is covered with an exterior label 10 to obtain a single cell. The gelled zinc anode 4 is prepared by mixing zinc oxide with an alkaline electrolyte of an aqueous solution containing 40 wt% potassium hydroxide, a gelling agent and a zinc powder twice the weight of the electrolyte. In the following paragraphs, specific examples of the batteries will be described. EXAMPLE 1 (BIC zinc alloy with silicon in an organic compound) Cylindrical alkaline batteries of Type LR6 as shown in FIG. 1 were prepared by employing a gelled zinc anode comprising an anode active material of zinc alloy powder containing bismuth, indium and calcium, each 500 ppm by weight (hereinafter referred to as "BIC zinc") and as a gelling agent a water-soluble polyvinyl alcohol polymer containing silicon (available from KURARE Co. Ltd., under the tradename "R-polymer R-2130"). The amount of the gelling agent was stepwisely changed so as to adjust the concentration of silicon element in the anode to 25 - 3,000 ppm by weight of the zinc alloy powder as listed in Table 1 below. As Comparative Example 1, another alkaline battery was prepared under the same conditions as in Example 1, except that a conventional gelling agent, viz polyvinyl alcohol free from silicon, was used. These batteries were subjected to discharge tests to evaluate their discharge performances, that is, discharge durations at an initial stage and after storing at 60°C for one month, which is a temperature proven to cause a decrease in the discharge performance of conventional batteries. For testing, the batteries were loaded with a resistance of 3.9 Ω for 5 minutes per day, to make them intermittently discharge. The discharge durations given below represent cumulative discharge hours until the battery voltage is lowered to the level of 1.0 V. Table 1 shows the results of the discharge tests. Zinc alloy Silicon element concentration (ppm by weight) Discharge performance (Discharge duration: minute) Initial After storing at 60°C for one month BIC zinc 0 (Comparative Example 1) 250 200 25 292 278 50 298 288 100 300 290 250 303 293 500 303 293 1000 297 281 1500 288 279 3000 267 258 As is apparent from the results shown in Table 1, both the initial discharge performances of the batteries and the discharge performances after storage for one month are greatly improved by adding the gelling agent containing silicon. In particular, the decreases in the discharge capacities after storage for one month are small, which shows favorable storage properties of the batteries. Increased amounts of silicon in the anode exceeding 1,500 ppm by weight of the zinc alloy powder, however, have no effect on the discharge performance of the battery. Conversely, the discharge capacity decreases, as a result of which the maintenance voltage during discharge also decreases. It is therefore concluded that an excellent discharge performance can be obtained by adding the gelling agent so as to adjust the concentration of the silicon in the anode to a range from 25 to 1,500 ppm by weight of the zinc alloy powder. EXAMPLE 1A (Comparison with other zinc alloys and elementary zinc) Next, other cylindrical alkaline batteries were prepared by employing the gelled zinc anode comprising an anode active material selected from the group consisting of BIC zinc, another zinc alloy powder containing bismuth and calcium, each 500 ppm by weight (hereinafter referred to as "BC zinc"), and still another zinc alloy powder containing bismuth, indium and aluminum, each 500 ppm by weight (hereinafter referred to as "BIA zinc") as listed in Table 2 below, and the silicon-containing gelling agent. In this example the content of silicon in the anode is fixed at 500 ppm by weight of the zinc alloy powder. In addition, another battery was prepared in a similar manner by employing elementary zinc in place of the zinc alloys for the anode zinc powder of the gelled zinc anode. These batteries were subjected to discharge tests to evaluate their discharge performances, that is, discharge durations at an initial stage and after storing at 60°C for one month. For testing, the batteries were loaded with a resistance of 3.9 Ω for 5 minutes per day, to make them intermittently discharge. Table 2 shows the discharge durations until the battery voltage decreases to the level of 1.0 V. Zinc alloy Silicon element concentration (ppm by weight) Discharge performance (Discharge duration: minute) Initial After storing at 60°C for one month BIC zinc 500 303 293 BC zinc 500 304 294 BIA zinc 500 300 291 Zinc powder 500 302 278 As is apparent from the results shown in Table 2, the different zinc alloy compositions do not alter the technical advantage obtained by the addition of the silicon-containing gelling agent. An alkaline battery produced by employing a non-amalgamated zinc powder containing neither bismuth, aluminum, indium nor calcium as its anode active material also has a favorable initial discharge performance. However, the discharge performance of such an alkaline battery is greatly decreased after storage for one month because the corrosion- resistance properties of the non-amalgamated zinc powder are inferior to those of the zinc alloys and thus the battery having such zinc powder cannot secure favorable leakage-resistance properties and is in danger of causing leakage of electrolyte. Therefore, the non-amalgamated zinc powder cannot be employed in an alkaline battery free from mercury. EXAMPLE 2 (BIC zinc with ionic silicon) Another series of cylindrical alkaline batteries was prepared by employing a gelled zinc anode comprising BIC zinc as an anode active material, a conventional gelling agent which is polyvinyl alcohol polymer free from silicon, and an aqueous solution of potassium silicate (available from TOKYO OHKA KOGYO Co. Ltd., under a tradename "OHKASEAL"). The concentration of silicon element in the anode was varied from 25 to 3,000 ppm by weight of the zinc alloy powder as listed in Table 3 below. Except for the above conditions, procedures similar to those in Example 1 are generally followed. As Comparative Example 2, another battery was prepared under the same conditions as in Example 2, except that an anode which did not contain an aqueous solution of potassium silicate was used. These batteries were subjected to discharge tests to evaluate their discharge performances, that is, discharge durations at an initial stage and after storing at 60°C for one month. For testing, the batteries were loaded with a resistance of 3.9 Ω for 5 minutes per day, to make them intermittently discharge. Table 3 shows the discharge durations until the battery voltage decreases to the level of 1.0 V. Zinc alloy Silicon element concentration (ppm by weight) Discharge performance (Discharge duration: minute) Initial After storing at 60°C for one month BIC zinc 0 (Comparative Example 2) 250 200 25 295 280 50 300 290 100 303 290 250 305 295 500 305 295 1000 295 283 1500 290 280 3000 270 260 As is apparent from the results shown in Table 3, both the initial discharge performances of the batteries and the discharge performances after storage for one month are greatly improved by employing the gelled zinc anode containing silicate ions. In particular, the decreases in the discharge performances due to storage are small, and the batteries demonstrate a favorable discharge performance. Increased amounts of silicon in the anode exceeding 1,500 ppm by weight of the zinc alloy powder, however, has no effect on the discharge performance of the battery. Conversely, the maintenance voltage during discharge decreases. It is therefore concluded that an alkaline battery having excellent discharge performance can be obtained by adding silicate ions so as to adjust the content of silicon in the anode to a range from 25 to 1,500 ppm by weight of the zinc powder. EXAMPLE 2A (Comparison with the other zinc alloys and elementary zinc) Next, other cylindrical alkaline batteries were prepared by employing a gelled zinc anode comprising an anode active material selected from the group consisting of BIC zinc, BC zinc and BIA zinc, a conventional gelling agent and an aqueous solution of potassium silicate. In this example the content of silicon in each of the anodes was fixed at 500 ppm by weight of the zinc alloy powder. In addition, another alkaline battery was prepared in a similar manner by employing an elementary zinc powder as an active material for the gelled zinc anode containing the silicate ions. These batteries were subjected to discharge tests to evaluate their discharge performances, that is, discharge durations at an initial stage and after storing at 60°C for one month. For testing, the batteries were loaded with a resistance of 3.9 Ω for 5 minutes per day, to make them intermittently discharge. Table 4 shows the discharge durations until the battery voltage decreases to the level of 1.0 V. Zinc alloy Silicon element concentration (ppm by weight) Discharge performance (Discharge duration: minute) Initial After storing at 60°C for one month BIC zinc 500 305 295 BC zinc 500 304 294 BIA zinc 500 303 293 Zinc powder 500 305 280 As is apparent from the results shown in Table 4, the different zinc alloy compositions do not alter the technical advantage obtained by the addition of the potassium silicate aqueous solution. An alkaline battery produced by employing an anode zinc powder containing neither bismuth, indium, aluminum nor calcium as its anode active material also has the favorable initial discharge performance. However, the corrosion- resistance properties of the elementary zinc powder are inferior to those of the zinc alloys because the battery cannot secure favorable leakage- resistance properties of the zinc powder, and thus, the non-amalgamated zinc powder cannot be employed in an alkaline battery free from mercury. EXAMPLE 2B (Comparison with other silicon sources) Other cylindrical alkaline batteries were prepared by employing a gelled zinc anode comprising BIC zinc powder as an anode active material, and a silicon-containing material selected from the group consisting of potassium silicate powder, sodium silicate powder, silicon dioxide powder and silicic acid, respectively. In this example, the content of silicon in the anode was fixed at 500 ppm by weight of the zinc alloy powder. These batteries were subjected to discharge tests to evaluate their discharge performances, that is, discharge durations at an initial stage and after storing at 60°C for one month. For testing, the batteries were loaded with a resistance of 3.9 Ω for 5 minutes per day, to make them intermittently discharge. Table 5 shows the discharge durations until the battery voltage decreases to the level of 1.0 V. Silicon Composition Silicon element concentration (ppm by weight) Discharge performance (Discharge duration: minute) Initial After storing at 60°C for one month Aqueous soln. of potassium silicate 500 305 295 Potassium silicate 500 304 295 Sodium silicate 500 306 294 Silicic acid 500 304 296 Silicon dioxide 500 305 297 Any of the potassium silicate powder, sodium silicate power, silicon dioxide powder and silicic acid can give a similar technical advantage to that of the aqueous solution of potassium silicate. Any of the silicon ion-containing substances viz a silicate salt such as calcium silicate or magnesium silicate, and another silicon compound such as a silicon oxide other than silicon dioxide or silicon nitride can give a similar technical advantage to that of the aqueous solution of potassium silicate. EXAMPLE 3 (Alloy composition containing silicon in various amounts) Another series of cylindrical alkaline batteries was produced by first preparing alloy powders (particle diameter: 35 - 200 mesh) of BIC zinc, to which is added 25 to 3,000 ppm by weight silicon stepwise as listed in Table 6 below, and then employing these alloy powders with a conventional gelling agent for the gelled zinc anode. Except for the above conditions, procedures similar to those in Example 1 are generally followed. As Comparative Example 3, another battery was prepared under the same conditions as in Example 3, except that a BIC zinc containing no silicon was used as the zinc alloy powder. These batteries were subjected to discharge tests to evaluate their discharge performances, that is, discharge durations at an initial stage and after storing at 60°C for one month. For testing, the batteries were loaded with a resistance of 3.9 Ω for 5 minutes per day, to make them intermittently discharge. Table 6 shows the discharge durations until the battery voltage decreases to the level of 1.0 V. Zinc alloy composition (ppm by weight) Discharge performance (Discharge duration: minute) Bi In Ca Si Initial After storing at 60°C for one month 500 500 500 0 250 200 (Comparative Example 3) 500 500 500 25 293 282 500 500 500 50 299 288 500 500 500 100 303 293 500 500 500 250 310 298 500 500 500 500 312 299 500 500 500 1000 303 289 500 500 500 1500 298 286 500 500 500 3000 273 262 As is apparent from the results shown in Table 6, both the initial discharge performance of the batteries and the discharge performance after storage for one month are greatly improved by employing the non-amalgamated zinc alloy powder containing silicon as the anode active material for the gelled zinc anode. In particular, the decreases in the discharge performances due to storage are small, and the batteries demonstrate favorable storage properties. Increased amounts of silicon in the anode zinc alloy powder exceeding 1,500 ppm by weight, however, have no effect on the discharge performance of the battery. Conversely, the maintenance voltage during discharge decreases. It is therefore concluded that an alkaline battery having an excellent discharge performance can be obtained by adjusting the silicon content in the zinc alloy to a range from 25 to 1,500 ppm. EXAMPLE 3A (Comparison with other zinc alloys) Next, other cylindrical alkaline batteries were prepared by employing a gelled zinc anode comprising an anode active material selected from the group consisting of BIC zinc, BC zinc and BIA zinc, each containing silicon at 500 ppm by weight. Except for the above conditions, procedures similar to those in Example 3 are generally followed. In addition, another alkaline battery was prepared in a similar manner by employing a zinc alloy containing only silicon in place of the other zinc alloys, for the anode zinc powder of the gelled zinc anode. These batteries were subjected to discharge tests to evaluate their discharge performances, that is, discharge durations at an initial stage and after storing at 60°C for one month. For testing, the batteries were loaded with a resistance of 3.9 Ω for 5 minutes per day, to make them intermittently discharge. Table 7 shows the discharge durations until the battery voltage decreases to the level of 1.0 V. Zinc alloy composition (ppm by weight) Discharge performance (Discharge duration: minute) Bi In Ca Al Si Initial After storing at 60°C for one month 500 500 500 0 500 312 299 500 0 500 0 500 310 296 500 0 0 500 500 308 293 0 0 0 0 500 311 278 As is apparent from the results shown in Table 7, the different zinc alloy compositions containing components other than silicon do not alter the technical advantage obtained by the addition of silicon. An alkaline battery produced by employing the non-amalgamated zinc alloy containing only silicon as its anode zinc powder for the gelled zinc anode also has a favorable initial discharge performance. However, the corrosion- resistance properties of the zinc powder are inferior to those of the other zinc alloys and the battery cannot secure favorable leakage-resistance properties, and thus, the non-amalgamated zinc alloy power containing only silicon cannot be employed in an alkaline battery free from mercury. As shown above, a non-amalgamated zinc alloy powder containing at least one member selected from bismuth, aluminum, indium and calcium as an anode active material, formed into a gelled zinc anode with a gelling agent, which contains the silicon element, regardless of its form, at a concentration of 25 to 1,500 ppm by weight of the zinc alloy powder is effective to prevent short-circuiting between the anode and cathode due to formation of zinc oxide, as a result of which the alkaline battery can have improved discharge performance. Thus, it is possible to provide an alkaline battery having a large discharge capacity and an excellent storing property.

Claims: An alkaline battery comprising: a cathode mixture; a gelled zinc anode which comprises a mercury free zinc alloy powder and a gelling agent; an anode current collector; a separator between said cathode and anode; and an alkaline electrolyte, said zinc alloy powder containing at least one of bismuth, aluminum and calcium, wherein said gelled anode further comprises silicon, in an amount from 25 - 1,500 ppm by weight of said zinc alloy powder. The alkaline battery in accordance with claim 1, wherein said zinc alloy powder further contains indium. The alkaline battery in accordance with claim 1 or 2, wherein said anode contains said silicon as an organic compound containing silicon. The alkaline battery in accordance with claim 1 or 2, wherein said anode contains said silicon as silicate ions. The alkaline battery in accordance with claim I or 2, wherein said silicon is contained as a component in said zinc alloy. The alkaline battery in accordance with any preceding claim, wherein said zinc alloy powder contains at least one of 0.01 - 0.5 wt% bismuth, 0 - 0.5 wt% aluminum and 0.005 - 0.5 wt% calcium. The alkaline battery in accordance with claim 6, wherein said zinc alloy powder further contains 0.01 - 0.5 wt% indium. The alkaline battery in accordance with any preceding claim, wherein said zinc alloy powder contains at least bismuth and calcium. The alkaline battery in accordance with any preceding claim, wherein said zinc alloy powder contains at least bismuth and aluminum.

Inventors: ASAOKA JUNICHI, KOJIMA YURI, MOTOTANI YUJI, NOYA SHIGETO, ASAOKA, JUNICHI, KOJIMA, YURI, MOTOTANI, YUJI, NOYA, SHIGETO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: H01M4
IPC codes: H01M6, H01M4, C22C18