PROCESS FOR MANUFACTURING A PRIMARY ZINC-ALKALINE BATTERY AND ANODE MIX USED THEREIN

Abstract: This invention relates to a process for manufacturing a primary zinc-alkaline battery, which comprises the step of preparing a gelled alkaline anode mix containing an aluminiferous zinc alloy powder as an active material, characterized in that a powered calcium compound is used as an ingredient of the anode mix in a sufficient amount to lower the tendency the battery shows to develop a short-circuit when the calcium compound is absent in its anode mix, or the separator paper contains a calcium compound.

Description: Process for manufacturing a primary zinc-alkaline battery and anode mix used therein The present invention relates to a process for manufacturing a primary zinc-alkaline battery, which compπses the step of prepanng a gelled alkaline anode mix containing an aluminiferous zinc alloy powder as an active material A pnmary zinc-alkaline battery, e.g. of the type LR6, is usually composed of an annular cathode compound, a gel-like anode called anode mix, a separator paper located between the cathode and the anode, a current collector for the anode, a metal case surrounding the cathode, an outer housing surrounding the metal case, a plug for closing the opening of the metal case, a bottom plate forming the anode terminal and possibly a cathode terminal cap (see e.g. EP-A-045735 ) According to WO-A-94-19502 the presence of Al in a zinc alloy powder for alkaline batteries enhances the resistance of the powder to corrosion in the electrolyte of the battery, especially after partial discharging of the latter. However, according to the same document the presence of Al also increases the tendency of the battery to develop short-circuits. That's why WO-A-94-19502 proposes to restrict the Al content of the powder to 1-95 ppm It has been found now by the Applicant that even powders with 1-95 ppm Al still might give nse to short-circuits in some battery applications, especially when one or more of the following conditions prevail : the battery is small, e.g. of the LR6 type; a particular periodic discharge scheme is applied ; a rather porous separator paper is used. The aim of the present invention is to provide a process such a defined hereinbefore, which diminishes the possible detrimental influence of the battery size, the discharge conditions and the separator porosity on the tendency of the battery to develop short-circuits and which moreover allows to use higher Al contents without inducing an unacceptable tendency to develop short-circuits. According to the invention, this aim is achieved by using a calcium compound as an ingredient of the anode mix in a sufficient amount to tower the tendency the battery shows to develop short- circuits when the calcium compound is absent in its anode mix. The invention is based on the discovery that the presence of a calcium compound such as e.g. Ca(OH) z in the anode mix neutralizes the detrimental influence of the Al contained in the powder upon the tendency of the battery to develop short-circuits, as will be shown later It should be noted here that JP-A-63138395 discloses a process for manufacturing a primary zinc-alkaline battery comprising dispersing an aluminiferous zinc alloy powder and calcium plumbate (Ca PbO«) in a gelled alkaline electrolyte, the calcium plumbate being added as a corrosion inhibitor for the zinc alloy powder. Consequently, the Applicant waives protection for the use of calcium plumbate as a calcium compound in the process of the present invention US-A-4312931 discusses the addition of Ca(OH) 2 to zinc electrodes for secondary alkaline cells It is said that while this addition avoids the redeposition of zinc dunng the charge/discharge cycling of the battery, the required amount significantly thickens the electrode and therefore lowers the energy density of the cells Furthermore, no mention is made of any effect on the short-circuit behavior. EP-A-0582293 discloses the addition of an hydroxide of alkaline earth metal, specifically of Ba(OH) 2 , to electrolyte for pnmary alkaline cells. It is said that the function of this addition is not certain, but that it provides in the suppression of hydrogen and in the improvement of unspecified electπc characteristics. No mention is made of any effect on the short-circuit behavior. Moreover, as will be shown later, Ba(OH) 2 has no effect on the tendency of the battery to develop short-circuits. The calcium compound is preferably used in such an amount that the anode mix contains at least 0.001 wt% Ca as a compound based on the weight of the alloy powder ; otherwise, the tendency of the battery to develop short-circuits is in many cases not substantially lowered. A calcium content of at least 0.005 wt% is still more preferred and the most preferred minimum content of Ca as a compound is 0.01 wt% It is not recommended to add more that 2 wt% Ca as a compound. It is preferable to add no more that 1 wt%, preferably no more than 0.45 wt%, and still more preferable to add no more than 0.1 wt% Ca. Any calcium compound can be used such as those descπbed in "Nouveau Traite de Chimie Minerale, Paul Pascal, Tome IV, Masson et Cie, Editeurs, Pans 1958", pages 299-545, but the preference is given to Ca(OH) 2 , CaO and calcium compounds that are less stable in the electrolyte than Ca(OH) 2 , i.e. calcium compounds that react with the electrolyte to form Ca(OH) 2 In addition to the alkaline electrolyte, gelling agent, alloy powder and calcium compound, the anode mix may also contain organic and/or inorganic corrosion inhibitors such as e.g. ln(OH) 3 or ln 2 0 3 . The aluminiferous zinc alloy powder contains advantageously, in addition to zinc SHG (Special High Grade) and Al, at least one of the elements Pb, Bi, In, Ga so a to have an increased resistance to corrosion in the electrolyte, the content of Al and of the other element(s) in the powder ranging preferably from 0.001 wt% to 0.5 wt%, more preferably from 0.002 wt% to 0.1 wt%, and still more preferably from 0.005 wt% to 0.1 wt%. The powder may be composed of the said elements, in which case it contains, in addition to Zn, Al and one or more of Pb, Bi, In and Ga, only unavoidable impurities The powder may also contain other elements that improve its corrosion resistance such as e.g Ca in an amount of 0.001-0.1 wt% The powder is advantageously free from added toxic elements such as mercury, cadmium, thallium and lead. Aluminiferous zinc alloy powders that can be used in the process of the present invention are disclosed a.o. in EP-A-0427315, EP-A-0457354, WO-A-94- 19502, JP-A-63138395, EP-B-0500313, JP-A-61077259, US-A-4735876, JP-A-62040162 and JP-A-62123654 The present invention relates also to an anode mix for a pπmary zinc-alkaline battery, compnsing a dispersion of an aluminiferous zinc alloy powder as an active mateπai in a gelled alkaline electrolyte, characterized in that the electrolyte contains also a calcium compound in a sufficient amount to lower the tendency the anode mix without the calcium compound shows to develop a short-circuit in the battery, wherein rt is used. Because of the disclosure in the above discussed JP-A-63138395 the Applicant waives protection for an anode mix obtained by using calcium plumbate as an ingredient of the mix Preferred compositions of the anode mix according to the invention are subject of the enclosed claims 11-15. When carrying out the process of the present invention, it is convenient to prepare firstly an intimate mixture of the aluminiferous zinc alloy powder, the calcium compound and optionally the gelling agent, and to use this mixture as an ingredient of the anode mix. In a variant of the process of the present invention, the calcium compound is incorporated with the separator paper, instead of being incorporated with the anode mix. Therefore, the present invention also relates to a process for manufacturing a primary zinc-alkaline battery, which comprises the steps of (a) prepanng a gelled alkaline anode mix containing an aluminiferous zinc alloy powder as an active matenal, and (b) providing a separator paper between the anode mix and a cathode compound, characterized in that the separator paper contains a calcium compound in a sufficient amount to lower the tendency the battery shows to develop a short- circuit when the calcium compound is absent in the paper The process of the invention is illustrated by the following example. A zinc alloy powder containing 500 ppm Bi, 500 ppm In and 70 ppm Al, is intimately mixed with powdered Ca(OH) 2 , in such ratios as to obtain mixed powders with Ca concentrations of 50, 100 and 250 ppm. Also, the zinc alloy powder without any addition of Ca is used as a reference A number of batteries of LR6 and LR14 type is produced using these powders. For each battery, a gel-like anode mix is prepared consisting of 68 % mixed powder, 0.5 % gelling agent (Carbopol 941) and 31.5 % electrolyte consisting of 38 % KOH, 2.9 % ZnO and 59.1 % H 2 0. A separator paper of the type FS2182 from Freudenberg (Germany) is used and a Mn0 2 cathode. The capacity and the gassing characteristics of the corresponding batteries were measured and are summarized in table 1 hereafter. Procedures and results are discussed below. Table 1 ppm Ca Number of batteries with relative capacities Mean Batteπes added as (%) relative tested Ca(OH) 2 capacity (%) <60 60 to 80 80 to 90 90 to 95 >95 0 17 9 4 3 3 63.9 36 50 1 3 94.8 4 100 4 98.5 4 250 8 100.0 8 Table 1 (continued) Capacity tests are performed on the R6 type batteries, subjecting them to a periodic discharge scheme of 1 hour every 12 hours, over a 10 ohm resistor. This discharge scheme is continued until the cell voltage drops below 0.9 V. The capacity of the battery is represented by the total discharge time. Some batteries display an unusually low observed capacity due to short-circuits. This is evidenced by a sudden decrease in voltage and is corroborated by visual inspection of the battery contents. The relative capacity as mentioned in Table 1 is the ratio of the actual capacity to the capacity when no short-circuits occur. A relative capacity of less than 90 % is considered to be caused by short-circuits. The example shows that frequent short-circuits occur when using the reference powder, thus when no Ca compound is added. An amount of 50 ppm Ca in the mixed powder proves to be sufficient to inhibit the formation of most short-circuits. An amount of 100 ppm or 250 ppm Ca completely suppresses the occurrence of short-circuits Gassing tests before discharge are performed on a mixture of 25 g mixed powder and 160 ml electrolyte consisting of 38 % KOH, 2 % ZnO and 60 % H 2 0 This mixture is maintained at 45°C The resulting gas production is expressed as μl of gas produced per g mixed powder per day Gassing tests after partial discharge are performed on LR14 type batteries. The batteries are first discharged for 6 h over 2.2 ohm, and then maintained for 7 days at 71 °C. The resulting gas production is expressed as ml of gas per battery It is found that both gassing tests give satisfactory results and are essentially unaffected by the addition of the Ca compound to the zinc alloy powder. A companson is made using the same zinc alloy powder as in the example above, i.e. containing 500 ppm Bi, 500 ppm In and 70 ppm Al. This powder is intimately mixed with powdered Mg(OH) 2 , Ba(OH) 2 or Sr(OH) 2 , in such ratios as to obtain mixed powders with alkaline earth concentrations of 50, 100 and 250 ppm. The results of the same capacity tests as described above are shown in tables 2, 3 and 4 The mean relative capacities all lie well below 90 %, indicating that the tendency to develop short-circuits is not suppressed The clear beneficial effect of Ca is thus not obtained with hydroxides of other common alkaline earth metals. Table 2 ppm Mg Number of battenes with relative capacities Mean Battenes added as (%) relative tested Mg(OH) 2 capacity (%) <60 60 to 80 80 to 90 90 to 95 >95 0 17 9 4 3 3 63.9 36 50 1 1 2 81.3 4 100 2 1 1 62.5 4 250 1 1 2 81.3 8 Table 3 ppm Ba Number of batteπes with relative capacities Mean Battenes added as (%) relative tested Ba(OH) 2 capacity (%) <60 60 to 80 80 to 90 90 to 95 >95 0 17 9 4 3 3 63.9 36 50 1 1 2 83.1 4 100 1 1 2 81.3 4 250 5 1 1 1 57.8 8 Table 4 ppm Sr Number of batteries with relative capacities Mean Batteπes added as (%) relative tested Sr(OH) 2 capacity (%) <60 60 to 80 80 to 90 90 to 95 >95 0 17 9 4 3 3 63.9 36 50 2 1 2 73.0 4 100 1 1 2 77.5 4 250 2 1 1 62.5 8

Claims: CLAIMS 1 A process for manufactuπng a primary zinc-alkaline battery, which compnses the step of preparing a gelled alkaline anode mix containing an aluminiferous zinc alloy powder as an active material, characterized in that a powdered calcium compound is used as an ingredient of the anode mix in a sufficient amount to lower the tendency the battery shows to develop a short-circuit when the calcium compound is absent in its anode mix, being excluded the use of calcium plumbate as a calcium compound. 2. A process according to claim 1 , charactenzed in that the calcium compound is used in such an amount that the anode mix contains at least 0.001 wt% Ca as a compound based on the weight of the alloy powder. 3 A process according to claim 2, characteπzed in that the anode mix contains at least 0.005 wt% Ca as a compound, preferably at least 0.01 wt%. 4. A process according to claim 1 , 2 or 3, characterized in that the calcium compound is used in such an amount that the anode mix contains no more than 2 wt% Ca as a compound, preferably no more than 1 wt%. 5. A process according to any of the claims 1-4, charactenzed in that the calcium compound is used in such an amount that the anode mix contains no more than 0.45 wt% Ca as a compound, preferably no more than 0.1 wt%. 6. A process according to any of the claims 1-5, characterized in that the calcium compound is Ca(OH) 2 , CaO or a calcium compound that is less stable than Ca(OH) 2 in the electrolyte. 7. A process according to any of the claims 1-6, characterized in that the aluminiferous zinc alloy powder contains, in addition to zinc SHG and Al, at least one of the elements Pb, Bi, In, Ga, its content of Al and the other element(s) ranging from 0.001 to 0.5 wt%, more preferably from 0.002 wt% to 0.1 wt%, and still more preferably from 0.005 wt% to 0.1 wt% 8. A process according to claim 7, characteπzed in that the other constituents of the powder are unavoidable impurities. 9 A process according to claim 7 or 8, characterized in that the powder is free from added mercury, cadmium, thallium and lead. 10. An anode mix for a pπmary zinc-alkaline battery, comprising a dispersion of an aluminiferous zinc alloy powder as an active material in a gelled alkaline electrolyte, characterized in that the electrolyte contains also a calcium compound in a sufficient amount to lower the tendency the anode mix without the calcium compound shows to develop a short-circuit in the battery wherein it is used, being excluded the anode mix obtained by using calcium plumbate as an ingredient of the mix. 11.An anode mix according to claim 10, characteπzed in that it contains 0.001 -2 wt% Ca as a compound based on the weight of the alloy powder. 12. A anode mix according to claim 10 or 1 , characterized in that the calcium compound is Ca(OH) 2 or CaO. 13. An anode mix according to claim 10, 11 or 12, characterized in that the aluminiferous zinc alloy powder contains, in addition to zinc SHG and Al, at least one of the elements Pb, Bi, In, Ga, its content of Al and the other element(s) ranging from 0.001 to 0.5 wt%, more preferably from 0.002 wt% to 0.1 wt%, and still more preferably from 0.005 wt% to 0.1 wt%. 14. An anode mix according to claim 13, characterized in that the other constituents of the powder are unavoidable impurities. 15. An anode mix according to claim 13 or 14, characteπzed in that the powder is free from added mercury, cadmium, thallium and lead. 16. A product for use in the manufactunng of a primary zinc-alkaline battery, said product compπsing an aluminiferous zinc alloy powder, characteπzed in that it is composed of an intimate mixture of said alloy powder and of a powdered calcium compound, the mixture containing 0.001-2 wt% Ca as a compound based on the weight of the alloy powder, being excluded a mixture containing calcium plumbate as a calcium compound. 17. A product according to claim 16, characteπzed in that the calcium compound is Ca(OH) 2 , CaO or a calcium compound that is less stable than Ca(OH) 2 in the electrolyte of the battery. 18. A process for manufacturing a primary zinc-alkaline battery, which comprises the steps of a) prepanng a gelled alkaline anode mix containing an aluminiferous zinc alloy powder as an active material, and b) providing a separator paper between the anode mix and a cathode compound, characterized in that the separator paper contains a calcium compound in a sufficient amount to lower the tendency the battery shows to develop a short-circuit when the calcium compound is absent in the paper 19. A process according to claim 18, charactenzed in that the calcium compound is Ca(OH) 2 , CaO or a calcium compound that is less stable than Ca(OH) 2 in the electrolyte.

Inventors: STRAUVEN YVAN, STRAUVEN, YVAN
Applicants: STRAUVEN YVAN, UNION MINIERE SA, N.V. UNION MINIERE S.A., STRAUVEN, YVAN
Application year: 1997
Main classification: H01M6
IPC codes: H01M4, H01M6