METHOD OF PRODUCING PLATE FOR LEAD STORAGE BATTERY

Abstract: An obtd. paste is filled into an active material supporter. The weight of minium in the lead oxide (1) is specified to lie between 10% and 50% of the total weight of the lead powder. The lead powder can contain both a lead oxide (1) having a minium content of up to 90% and a lead oxide having a minium content of over 90%, and the amount of minium in the former is more than 80% of the total amount of minium. It is specified that the miniumzation of the lead oxide (1) should start at the surface and that the interior of the lead oxide (1) should comprise particles of which the main component is PbO. In practice, the paste is produced by kneading 1Kg mixt. of 2 powders, one with at least 80% (31.3 pts.wt.) ( and the other comprising lead oxide and lead (the wt. ratio is 3:1) (67.8 pts.wt.) with water (100g), sulphuric acid (density:1.40) (147g) and short synthetic resin fibres (2g). The plate is obtd. by coating this mixt. on the active material supporter and drying. The lead oxide having a min content of 80% used for producing the paste can be replaced by lead oxide having a min content of 70%.

Description: The present invention relates to a method for making an electrode plate for lead storage batteries and more particularly to attainment of efficient formation charging of paste-type elecrode plate and improvements of both the working characteristics and the reliability of battery when the paste-type electrode plate is used in lead storage batteries. In particular, the present invention relates to a method for making an electrode plate for lead storage battery by filling in a support for an active material a paste prepared by kneading water, sulfuric acid and a lead powder containing a lead oxide including red lead (Pb₃O₄). BACKGROUND ART In lead storage batteries, there are commonly used paste-type electrode plates made by coating a paste prepared by kneading a partially oxidized lead powder, water and sulfuric acid as main components, a short fiber of synthetic resin, a fine powder of carbon, organic additives and, if necessary, small amounts of other components on a continuous porous body such as a cast grid, a punching sheet, or an expanded metal and, if necessary, cutting it to shapes of electrode plate, followed by drying. This electrode plate is combined with a separator and an electrode plate assembly is made therefrom. The assembly is placed in a cell, to which dilute sulfuric acid as an electrolyte is added and formation of the electrode plates is carried out in the cell. Alternatively, electrode plates are subjected to formation outside the cell and then are placed in the cell to impart a function of battery. It is generally known to add red lead, namely Pb₃O₄, which is a high grade oxide in order to avoid loss of power and time because efficiency of formation is low especially at a positive electrode in the stage of formation of electrode plate. In US-A-2 422 437 the preparation of pasted electrodes starting with kneaded pastes comprising for instance 90 % PbO, 10 % Pb₃O₄ (red lead) and water containing sulfuric acid is described. When Pb₃O₄ (red lead) is applied to a paste-type electrode plate, efficiency of formation charging increases with increase in the amount of red lead and a lead storage battery made using such electrode plates is improved in initial working characteristics, but tends to deteriorate in life of battery. Therefore, red lead is used commonly for a cladding-type electrode plates comprising a porous tube in which an active material is packed, but practically it is hardly used for paste-type electrode plates which are predominantly used at present and a few examples are seen to form a red lead layer on electrode plate for a special purpose and thus it has not yet been really practically used. Generally, for mass production, red lead is prepared by further firing (oxidizing) a lead powder in which a part or the whole of lead has been oxidized into lead monoxide (PbO). Ratio of Pb converted to Pb₃O₄ is taken as "conversion ratio to red lead" here. The above-mentioned deterioration in life of battery occurs under the condition that in the course of production of red lead through oxidation of a lower oxide such as lead monoxide (PbO), at least 95 % of Pb is converted to red lead (Pb₃O₄) according to conception of grade of red lead. Red lead of such grade has been used as the additive. However, this red lead is lower in reactivity with sulfuric acid than the ordinary lead powder of 60-80 % in oxidation degree and besides, has a hard surface and becomes excessively porous and gives an adverse effect on binding power between pasty active materials per se at coating on a support, aging of the paste and working as a battery. It is considered that the above properties are also influenced by graduation effect or increase of hardness of secondary particles when lead powder is exposed to high temperature for a long time at the final stage of enhancing the conversion ratio to red lead to 95 %. Thus, the object of the present invention is to solve the problem of deterioration in life of battery which occurs in spite of the advantage of improvement in efficiency of formation charging when the above-mentioned red lead (Pb₃O₄) is added to the paste. According to the invention the above problem is solved by a method according to claim 1. Preferred embodiments are claimed in the dependent claims. That is, the characteristic of the present invention is not to use red lead of such grade of 95 % or more in conversion ratio to red lead Pb₃O₄ which has been higher considered to be added to paste, but to use purposely a low grade lead oxide of 90 % or less, preferably 70 % or less, in conversion ratio to red lead Pb₃O₄ and to make efficient use of the properties of such lead oxide. By employing the above means, both the improvements in life and in efficiency of formation can be attained, being different from the case where the conventional oxide of high conversion ratio to red lead is added to raw material lead powder for paste. This effect is a phenomenon utterly different from the construction where a lead oxide of high conversion ratio to red lead is mixed with the conventional raw material lead powder for paste containing predominantly lead monoxide and the like and is diluted therewith. This is also concerned with the structure of particles with progress of conversion to red lead. In general, conversion to red lead progresses from the surface of particles of a low grade oxide which is a raw material and the red lead component surrounds the low grade oxide of inside of the particles and directly bonds to the low grade oxide. This is the difference from the conventional mixture of ordinary lead powder and an oxide of high conversion ratio to red lead of 95 % or more where substantially no low grade oxide is present in the particles. According to a preferred embodiment of the present invention the low grade lead oxide of 90 % or less in conversion ratio to red lead is 80 % by weight or more of the total amount of red lead. According to another preferred embodiment of the present invention the conversion to red lead in the lead oxide of 90 % or less in conversion ratio to red lead proceeds from the surface and the inside comprises particles mainly composed of lead monoxide. According to a particularly preferred embodiment the method of the present invention is characterized by adding 100 g of water, 147 g of sulfuric acid having a specific gravity of 1.40, and 2 g of short fibers of a synthetic resin to 1 kg of a powder prepared by mixing 31.3 parts by weight of a lead oxide of 80 % in conversion ratio to red lead and 68,7 parts by weight of a lead powder comprising a lead oxide and a metallic lead at a weight ratio of about 3:1, kneading the mixture to obtain a paste and coating the paste onto a support for an active material and drying and aging the paste. According to another particularly preferred embodiment of the present invention the method is characterized by adding 100 g of water, 147 g of sulfuric acid having a specific gravity of 1.40, and 2 g of short fibers of a synthetic resin to 1 kg of a powder prepared by mixing 35.7 parts by weight of a lead oxide of 70 % in conversion ratio to red lead and 64,3 parts by weight of a lead powder comprising a lead oxide and a metallic lead at a weight ratio of about 3:1, kneading the mixture to obtain a paste and coating the paste onto a support for an active material and drying and aging the paste. The course of oxidation for obtaining red lead can be divided into an area of 70 % in conversion ratio to red lead (Pb₃O₄) where conversion rapidly proceeds in the vicinity of the surface of particles, an area of 70-90 % in conversion ratio to red lead (Pb₃O₄) where oxidation rate somewhat decreases and an area of high grade conversion of 95 % or more in conversion ratio to red lead (Pb₃O₄), where the conversion slowly proceeds over a long time. In this case, when the particles once converted to red lead are exposed to high temperature for a long time, granulation proceeds, resulting in increase of hardness and such particles are poor in chemical reactivity with sulfuric acid and water at the time of kneading of paste and become fragile. Such undesired change occurs most greatly at the final stage where a high grade lead oxide of at least 95 % in conversion ratio to red lead is produced. The present invention is characterized by using an oxide which is prepared by purposely discontinuing the conversion to red lead (Pb₃O₄) at the stage of 90 % or less, preferably 70 % or less in conversion ratio to red lead (Pb₃O₄), and thus which retains inside a low grade oxide such as PbO which is high in reactivity with sulfuric acid and water and produces binding power at the time of kneading of paste and which directly bonds to red lead. Thereby not only improvement of efficiency at the step of formation, but also prolongation of life of battery are attained. For attaining both the effects of improvement in efficiency of formation and prolongation of life of battery by the red lead contained in the lead oxide of 90 % or less in conversion ratio to red lead, remarkable effects can be obtained when the amount of the red lead (Pb₃O₄) in lead oxide is 10 % by weight or more per total weight of lead powder considering reactivity with sulfuric acid and increase of binding power of paste. However, especially in order to increase binding power of paste and prolong life of battery, it is preferred to set the amount of the red lead (Pb₃O₄) at 50 % by weight or less of total weight of lead powder. If necessary, thereto may be added a high grade red lead powder of 90 % or more in conversion ratio to red lead, but in this case, it is also preferred to add at least 80 % by weight, based on the total weight of red lead, of red lead of low conversion ratio to red lead of 90 % or less. Furthermore, from the point of the above-mentioned binding mechanism, effect to improve the life becomes conspicuous with lowering conversion ratio to red lead to 80 %, 70 % where low grade oxide is present in a large amount in the particles. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 shows relation between PbO₂ production rate in the step of formation charging and red lead content in total lead powder where A indicates characteristic when conversion ratio to red lead is changed and B indicates characteristic when a high grade red lead of 98% in conversion ratio to red lead is mixed with a general lead powder. Fig. 2 is a graph which shows relation between life of battery and red lead content. Fig. 3 is a graph which shows life and mixing ratio of powder A adjusted in conversion ratio to red lead and powder B adjusted with powder of high conversion ratio to red lead. BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be explained in detail by the following examples. First, various lead oxides different in conversion ratio to red lead were prepared with changing firing (oxidizing) time of a raw material lead powder. Then, if necessary, the resulting lead oxide was mixed with a lead powder of 50-80% in oxidation degree or lead oxide different in conversion ratio to red lead to prepare various lead powders different in content of red lead. Then, given amounts of sulfuric acid and water were added to the above lead powder and these were kneaded to prepare a paste in accordance with a conventional method. This paste was coated on a grid, aged and dried to obtain a positive electrode plate. A lead storage battery of 12 V in voltage and 50 Ah in nominal capacity was made using the above positive electrode plate and was subjected to determination of efficiency of formation charging and life test. Total Pb amount in active material filled was nearly the same in all cases. First, with reference to influence on charging efficiency, results are shown in Fig. 1 on case A where the content of red lead in total lead powder was changed from 5% to 98% by adjusting a conversion ratio to red lead and case B for comparison where content of red lead was changed by mixing a conventional lead powder with a high grade red lead of 98% in conversion ratio to red lead. In order to make efficiency of formation charging further clearer, charging quantity in this case was electrical quantity of 100% of stoichiometric number (this was 100% of theoretical amount) required for oxidation of an amount of Pb excluding tetravalent Pb contained in red lead in the total amount of lead (Pb) contained in a positive electrode plate to tetravalence and production rate of PbO₂ was determined. As a result, it was recognized that efficiency of formation charging was also markedly improved in the case A where the content of red lead was changed by adjusting a conversion ratio to red lead like the case B where only the conventional high grade red lead was added. Furthermore, characteristics are somewhat different in cases A and B. In case A, effect of efficiency of formation was higher in the region of somewhat low content of red lead and there was a difference from the case B at a content of red lead of at least 10% by weight. With reference to initial capacity, holding time was examined at -15°C and 300 A as 5 Hr rate discharging property or low temperature rapid discharging property and in all cases showed correlation with production rate of PbO₂. With reference to rapid discharging voltage, better tendency was shown in the region of high red lead content. Life of battery was determined in the following manner. Batteries with electrode plate containing no red lead was charged with an electrical quantity of 100% of stoichiometric value and other batteries were charged with an electrical quantity of 50% to place them at completely charged state and charging of 3 hours at 14.8 V and discharging of 1 hour at 20 A for each battery were repeated and a voltage at 30 seconds after discharging at 300 A was measured every 50 cycles and number of cycles required for reduction to a voltage of 7.1 V was determined. Fig. 2 shows a relation between life and content of red lead in the cases A and B, namely, a case where the content of red lead was changed by changing conversion ratio to red lead and a case where content of red lead was changed by mixing a high grade lead oxide of 98% in conversion ratio to red lead and ordinary lead powder. As is clear from the graph, life was conspicuously deteriorated due to increase of red lead in case B while in case A, life was longer than that of battery with no red lead until 70% in conversion ratio to red lead and showed superiority to case B until 90% in conversion ratio to red lead and was abruptly deteriorated when the conversion ratio exceeded 90%. Remarkable improvement in life was recognized when the content of red lead was up to 10-50%. Results in case C where the content of red lead was changed with a lead oxide of 90% in conversion ratio to red lead and ordinary lead powder and those in case D where the content of red lead was changed by mixing a lead oxide of 90% in conversion ratio to red lead and a lead oxide of 10% in conversion ratio to red lead are also shown in Fig. 2. As a result, it was found that as in case A, life was improved than case B by mixing a lead oxide of 90% or less in conversion ratio to red lead. With reference to a mixture of lead oxides of the same content of red lead which were obtained in cases A and B, Fig. 3 shows a relation between mixing ratio and life in case E where a conversion ratio to red lead in A was 10%, in case F of the conversion ratio of 25%, in case G of the conversion ratio of 50% and in case H of the conversion ratio of 70%. As a result, it was found that when oxide of 90% or less in conversion ratio to red lead was mixed in an amount of 80% or more, the effect of the present invention was not basically damaged and, if necessary, oxide of high conversion ratio to red lead may be partially added for the purpose of improving rapid discharging characteristics. Specific examples of preparation of the paste according to the resent invention are shown below. Example 1 100 g of water, 147 g of sulfuric acid having a specific gravity of 1.40 and 2 g of short fibers of a synthetic resin were added to 1 kg of powder comprising a mixture of 68.7 parts by weight of lead powder composed of a lead oxide and metallic lead at a weight ratio of about 3:1 and 31:3 parts by weight of red lead of 80% in conversion ratio to red lead and the resulting mixture was kneaded for about 20 minutes by a stirrer to prepare a paste. Then, this paste was coated at a thickness of about 1.8 mm on an expanded metal made of a lead alloy by a paste filling machine, dried and aged to make a positive electrode plate. A lead storage battery of 12 V in voltage and 50 Ah in nominal capacity was made using the positive electrode plate. Life characteristic of this lead storage battery is similar to that of C in Fig. 2, and is somewhat lower than that of C. Example 2 In the same manner as in Example 1, 100 g of water, 147 g of sulfuric acid having a specific gravity of 1.40 and 2 g of short fibers of a synthetic resin were added to 1 kg of a powder comprising 64.3 parts by weight of lead powder composed of lead oxide and metallic lead at a weight ratio of about 3:1 and 35.7 parts by weight of red lead of 70% in conversion ratio to red lead and the resulting mixture was kneaded for about 20 minutes by a stirrer to prepare a paste. Then, this paste was coated at a thickness of about 1.8 mm on a lead alloy cast grid which was an active material support by a paste filling machine, dried and aged to make a positive electrode plate. A lead storage battery of the same specification as of the battery made in Example 1 was made using said positive electrode plate. Life characteristic of this lead storage battery was similar to that of C in Fig. 2 and showed medium characteristic between A and C. Collective consideration revealed that life as a battery was improved by incorporating a large amount of red lead powder in case A and life cycle did not reach the maximum value, but showed good characteristic close to the maximum value. Particle size of red lead powder used can be optionally adjusted by grinding and sifting, but preferably the structure is such that as much unoxidized lead monoxide as possible is contained inside and the lead monoxide directly bonds to the red lead component formed on the surface. It is a matter of course that the present invention includes use of such paste for negative electrode plate. INDUSTRIAL APPLICABILITY As explained above, according to the present invention, the contradiction in addition of red lead in conventional manner that efficiency of formation charging can be enhanced while life of battery is deteriorated has been solved by use of lead oxide of low conversion ratio (90% or less) to red lead. Especially, the present invention has attained compatibility of charging efficiency and life performance of paste-type electrode plate which cannot warrant life of battery due to its structure as clad-type electrode plate can. This effect is considered to be essentially based on the fact that amount of red lead component is great which is formed by the surface portion of lead monoxide particle existing as core being partially or mostly converted to red lead and which directly bonds to the lead monoxide of core and the remaining lead monoxide serves enhancement of binding power between active material per se. Therefore, the effect cannot be obtained by mixing separately prepared red lead and ordinary lead powder. The present invention especially attains compatibility of improvements of efficiency of formation charging and life of battery and the industrial value is very high.

Claims: Method for making an electrode plate for a lead storage battery comprising the steps of filling in a support for an active material a paste prepared by kneading water, sulfuric acid and a particulate material containing a low lead oxide and red lead Pb₃O₄ coagulated in single particles, the ratio of Pb converted to red lead defined as conversion ratio of the red lead being 90 % or less, and the content of red lead being 10 % or more of the total weight of said particulate material. Method according to claim 1, wherein the content of red lead being 50 % or less of the total weight of said particulate material. The method according to claim 1 or 2, wherein the low grade lead oxide of 90 % or less in conversion ratio to red lead is 80 % by weight or more of the total amount of red lead. The method according to claim 1, 2 or 3 wherein the conversion to red lead in the lead oxide of 90 % or less in conversion ratio to red lead proceeds from the surface of the particles, while the inside thereof mainly comprises lead monoxide. The method according to any of claims 1 to 4 wherein a low grade lead oxide of 70 % or less in conversion ratio to red lead Pb₃O₄ is used. The method according to any of claims 1 to 5, characterized by adding 100 g of water, 147 g of sulfuric acid having a specific gravity of 1.40, and 2 g of short fibers of a resin to 1 kg of a powder prepared by mixing 31.3 part by weight of a lead oxide of 80 % in conversion ratio to red lead and 68.7 parts by weight of a lead powder comprising a lead oxide and a metallic lead at a weight ratio of about 3:1, kneading the mixture to obtain a paste and coating the paste onto a support for an active material and drying and aging the paste. The method according to any of claims 1 to 5, characterized by adding 100 g of water, 147 g of sulfuric acid having a specific gravity of 1.40, and 2 g of short fibers of a synthetic resin to 1 kg of a powder prepared by mixing 35.7 parts by weight of a lead oxide of 70 % in conversion ratio to red lead and 64.3 parts by weight of a lead powder comprising a lead oxide and a metallic lead at a weight ratio of about 3:1, kneading the mixture to obtain a paste and coating the paste onto a support for an active material and drying and aging the paste.

Inventors: KIMURA, TOSHIAKI, SANO, ICHIRO, TAKAHASHI, KATSUHIRO, YASUDA, HIROSHI, YONEZU, KAZUYOSHI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H01M4
IPC codes: H01M4