Title of Invention

"POSITIVE ELECTRODE FOR LEAD-ACID STORAGE BATTERY AND LEAD-ACID STORAGE BATTERY"

Abstract

The present invention lies in a positive electrode for lead-acid storage battery comprising a positive grid made of a lead alloy containing not lower than 1.2% by weight of tin, filled with a paste obtained by kneading a positive active raw material which contains a lead dust and a red lead as main components with dilute sulfuric acid, wherein the positive active raw material contains a red lead in an amount of from not lower than 5% by weight to not greater than 50% by weight and a positive active material porosity is not lower than 58% after formation to raise the positive active material utilization and prevent the deterioration of cycle life performance of positive electrode, thereby providing a positive electrode for lead-acid storage battery excellent in discharge characteristics and cycle life performance.

Full Text

Description Positive electrode for lead-acid storage battery and lead-acid storage battery Technical Field The present invention relates to improvement of positive active material utilization and improvement of self-discharge characteristics of lead-acid storage battery free from deterioration of cycle life performance of lead-acid storage battery. Background Art The improvement of active material utilization in positive electrode for lead-acid storage battery, particularly during high rate discharge, is indispensable for the reduction of weight and size of batteries, and many attempts have heretofore been made therefor. These attempts were mainly intended to increase the porosity of positive active material. The increase of the porosity of positive active material brings forth an extremely great effect but has an adverse effect on the life performance of lead-acid storage batteries. Thus, the increase of porosity is limited, and the upper limit of porosity has heretofore been about 60% from the practical standpoint of view. This is attributed to the fact that when the porosity rises, the bonding force between active materials is reduced, accompanied by the gradual drop of the electrical conductivity of the active material, causing only the active material in the vicinity of the grid to be discharged and hence resulting in the accumulation of lead sulfate, which has an extremely low electrical conductivity, around the grid. In particular, in recent years, the practice of containing tin in the lead alloy of the grid in an amount of about 1% by weight has been spread to improve the corrosion resistance of the positive electrode grid. However, when the composition ratio of tin rises, the bonding properties of active material with grid are deteriorated. This is because a tin oxide film is formed on the surface of the grid during hydrosetting at the step of producing the positive electrode, impairing the bonding properties of active material with grid. It was thus disadvantageous in that when the porosity of the positive active material is raised, the life performance of the positive electrode is deteriorated more than ever, preventing the improvement of the performance of lead-acid storage batteries. The present invention has been worked out to solve the aforementioned problems and its problem (object) is to provide a positive electrode for lead-acid storage battery which contributes to improvement of discharge characteristics and life characteristics of lead-acid storage battery by improving positive active material utilization and preventing the deterioration of life performance of positive electrode. Disclosure of the Invention In order to accomplish the aforementioned object, the present invention is described in the following clauses (1) to (8) . (1) A positive electrode for lead-acid storage battery comprising: a positive grid made of a lead alloy containing not lower than 1.2% by weight of tin, and, a paste filled therein, which is obtained by kneading a positive active raw material comprising a lead dust and a red lead powder as main components with dilute sulfuric acid, wherein said positive active raw material comprises a red lead component in an amount of from not lower than 5% by weight to not greater than 50% by weight and the positive electrode for lead-acid storage battery prepared from a paste made from this positive active raw material has a positive active material porosity of not lower than 58% after formation. (2) The positive electrode for lead-acid storage battery as described in Clause (1), wherein the positive active raw material comprises a red lead component in an amount of from not lower than 5% by weight to less than 30% by weight and the positive electrode for lead-acid storage battery prepared from a paste made from this positive active raw material has a positive active material porosity of not lower than 58% after formation. The positive electrode for lead-acid storage battery as described in Clause (1), wherein the positive active raw material comprises a red lead component in an amount of from not lower than 12% by weight to not greater than 42% by weight, the metal-lead component in the lead dust is from not lower than 31% by weight to not greater than 40% by weight and the positive electrode for lead-acid storage battery prepared from a paste made from this positive active raw material has a positive active material porosity of not lower than 58% after formation. The positive electrode for lead-acid storage battery as described in Clause (3), wherein the positive active raw material further comprises a metal-lead powder, the metal- lead component in the positive active raw material calculated from the amount of metal-lead component in the lead dust and the metal-lead powder thus added is from not lower than 19% by weight to less than 26% by weight and the positive electrode for lead-acid storage battery prepared from a paste made from this positive active raw material has a positive active material porosity of not lower than 58% after formation. The positive electrode for lead-acid storage battery as described in Clause (1), wherein the positive active raw material comprises a red lead component in an amount of from not lower than 10% by weight to not greater than 50% by weight and the red lead powder has an average particle diameter of not greater than 2.2 times that of the lead dust. The positive electrode for lead-acid storage battery as described in Clause (1) / wherein the lead dust is produced from a lead alloy of its antimony containing in an amount of from not lower than 0.005% by weight to not greater than 0.1% by weight. The positive electrode for lead-acid storage battery as described in any one of Clauses (1) to (6) , wherein the positive grid is an expanded grid. (8) A lead-acid storage battery comprising a positive electrode for lead-acid storage battery as described in any one of Clauses (1) to (7). Brief Description of the Drawings Fig. 1 is a graph illustrating the change of initial capacity with the change of red lead component. Fig. 2 is a graph illustrating the change of discharge capacity at 5th cycle with the change of red lead component. Fig. 3 is a graph illustrating the change of capacity ratio (B/A) with the change of red lead component. Fig. 4 is a graph illustrating the relationship between the tin content and attached amount of active material with the change of red lead component. Fig. 5 is a graph illustrating the relationship between the discharge capacity at 5th cycle and metal-lead component with the change of red lead component. Fig. 6 is a graph illustrating the relationship between the initial capacity and metal-lead component with the change of red lead component. Fig. 7 is a graph illustrating the relationship between the discharge capacity at 5th cycle and metal-lead component with the change of red lead component. Fig. 8 is a graph illustrating the relationship between the capacity ratio and metal-lead component with the change of red lead component. Fig. 9 is a graph illustrating the results of cycle life performance test on a valve-regulated lead-acid storage battery. Fig. 10 is a graph illustrating the change of capacity of a valve-regulated lead-acid storage battery during 60°C float test. Fig. 11 is a graph illustrating the distribution of particle diameter of red lead. Fig. 12 is a graph illustrating the change of self-discharge rate with the change of red lead component. Fig. 13 is a graph illustrating the change of cycle ife ratio with the change of weight percentage of antimony in lead alloy over various red lead components. Best Mode for Carrying Out the Invention • First embodiment of implementation In the first embodiment of implementation, the positive active raw material is mixed with red lead in a predetermined amount, improving the life performance of the positive electrode even if the porosity of the positive active material is not lower than 58%. The first embodiment will be further described hereinafter. A grid having a height of 72 mm, a width of 45 mm and a thickness of 2.9 mm was prepared by expanding a lead alloy containing 0.05% by weight of calcium and 1.5% by weight of tin. These grids were then filled each with positive active material pastes prepared according to the formulation set forth in Table 1 to a thickness of 3.0 mm to prepare Sample Nos. 1 to 12, respectively. Several plates were prepared for each of these samples. Subsequently, these samples were allowed to stand in an atmosphere of temperature of 50°C and humidity of 90% for 72 hours so that they were hydrosetted. Three plates were arbitrarily picked up from each of these samples, and then subjected to dropping test of active material involving 20 repetitive spontaneous dropping from a height Df 90 cm. The attached amount of active material on the surface of the grid was then measured. Separately, 9 plates were picked up from the aforementioned samples, and then each subjected to anodization (formation) with a total charge of 20 Ah involving one discharge in dilute sulfuric acid having a concentration of 28% for 24 hours to form positive electrodes. The red lead powder used had a purity of not lower than 95%. Subsequently, 6 plates were picked up from these positive electrodes. Three out of the 6 electrodes were then measured for porosity of positive active material by mercury penetration method while the other three electrodes were measured for weight percentage of PbO2 after formation by iodometry. The attached amount of active material, weight percentage of PbO2 after formation and porosity thus measured were the average values of each three's. The results are set forth in Table 1. The metal-lead component in the lead dust used in the present embodiment of implementation is in an amount of from 25% by weight to 30% by weight for use in the production of an ordinary positive electrode. The amount of dilute sulfuric acid is represented per 10 kg of positive active raw material. (Table Remove)Subsequently, the other three of positive electrodes were incorporated in three cells each containing an excessive amount of dilute sulfuric acid having a concentration of 40% and comprising a lead plate as a counter electrode. These cells were then discharged. These cells were then measured for initial capacity A and discharge capacity B at 5th cycle. Referring to the measuring conditions, the discharge current was a constant current of 0.7 A, and the discharge termination voltage was - 500 mV with respect to a paste type electrode made of lead dioxide and lead alloy dipped in dilute sulfuric acid having a concentration of 40% which had been separately prepared. The initial capacity A was the maximum capacity shown during 3 cycles of discharge after formation. The discharge capacity at 5th cycle was the discharge capacity shown when 5th cycle discharge was effected supposing that the aforementioned discharge showing the initial capacity is at zero cycle. These results are set forth in Table 2 with the results of the ratio of discharge capacity B at 5th cycle to the initial capacity A (capacity ratio B/A). Table 2(Table Remove) Subsequently, Sample Nos. la, 2a and 3a were prepared in the same manner as in Sample Nos. 1, 2 and 3 of Table 2 comprising 100% by weight of lead dust, respectively, except that positive active material pastes prepared from a positive active raw material comprising 12% by weight of a red lead component according to various formulations set forth in Table 3 and these positive active material pastes were then filled in the aforementioned grids similarly. Further, Sample Nos. Ib, 2b and 3b were prepared by preparing positive active material pastes from a positive active raw material comprising 18% by weight of a red lead component according to various formulations set forth in Table 3, respectively, and then filling these positive active material pastes in the aforementioned grids similarly. Further, Sample Nos. Ic, 2c and 3c were prepared by preparing positive active material pastes from a positive active raw material comprising 24% by weight of a red lead component according to various formulations set forth in Table 3, respectively, and then filling these positive active material pastes in the aforementioned grids similarly. Further, Sample Nos. Id, 2d and 3d were prepared by preparing positive active material pastes from a positive active raw material comprising 27% by weight of a red lead component according to various formulations set forth in Table 3, respectively, and then filling these positive active material pastes in the aforementioned grids similarly. Further, Sample Nos. le, 2e and 3e were prepared by preparing positive active material pastes from a positive active raw material comprising 30% by weight of a red lead component according to various formulations set forth in Table 3, respectively, and then filling these positive active material pastes in the aforementioned grids similarly. Further, Sample Nos. If, 2f and 3f were prepared by preparing positive active material pastes from a positive active raw material comprising 36% by weight of a red lead component according to various formulations set forth in Table 3, respectively, and then filling these positive active material pastes in the aforementioned grids similarly. Further, Sample Nos. Ig, 2g and 3g were prepared by preparing positive active material pastes from a positive active raw material comprising 42% by weight of a red lead component according to various formulations set forth in Table 3, respectively, and then filling these positive active material pastes in the aforementioned grids similarly. These samples were each then subjected to hydrosetting and anodization (formation) under the aforementioned conditions to form positive electrodes. These positive electrodes were each then measured for weight percentage of Pb02 and porosity after formation. The other three of positive electrodes were then used to prepare cells as mentioned above. These cells were each then similarly measured for initial capacity, discharge capacity at 5th cycle and ratio of discharge capacity B at 5th cycle to initial capacity A (B/A). The results are set forth in Table 3. The change of initial capacity with the change of red lead component, the change of discharge capacity at 5th cycle with the change of red lead component and the change of the ratio (B/A) with the change of red lead component are shown in Figs. 1, 2 and 3, respectively. The aforementioned ratio (B/A) gives an indication of the life performance of positive electrode. Table 3(Table Remove) In accordance with Fig. 1 and Table 3, all of Sample Nos. la to Ig, which have substantially the same porosity (54 to 55%) as that of Sample No. 1 of Table 1, Sample Nos. 2a to 2g, which have substantially the same porosity (56 to 57%) as that of Sample No. 2 of Table 1, and Sample Nos. 3a to 3g, which have substantially the same porosity (58 to 59%) as that of Sample No. 3 of Table 1, show an initial capacity improve with the increase of red lead component in the positive active raw material to the range of from 12% by weight to 42% by weight as compared with the initial capacity of Sample No. 1 set forth in Table 2. In particular, Sample Nos. 3a to 3g (58 to 59%) show a remarkable improvement of initial capacity with the increase of red lead component in the positive active raw material to the range of from 24% by weight to 36% by weight. On the other hand, as can be seen in Fig. 1 and Table 3, so far as the porosity is within the range of from 54 to 57%, when the red lead component in the positive active raw material increases, these samples show an initial capacity improvement but show not only a smaller value of initial capacity itself but also a smaller rate of increase of initial capacity than the samples having a porosity of from 58 to 59%. Similarly, so far as the porosity is within the range of from 62 to 64%, when the red lead component in the positive active raw material increases, these samples show an initial capacity improvement but show a smaller value of initial capacity itself than the samples having a porosity of from 58 to 59%. Accordingly, it can be expected that so far as the porosity is not lower than 58%, when the red lead component in the positive active raw material increases, the initial capacity improves. This is presumably because when the red lead component in the positive active raw material increases, the weight percentage of Pb02 after formation rises, resulting in the improvement of initial capacity. Further, in accordance with Fig. 2 and Table 3, all of Sample Nos. la to Ig, which have substantially the same porosity (54 to 55%) as that of Sample No. 1 of Table 1, Sample Nos. 2a to 2g, which have substantially the same porosity (56 to 57%) as that of Sample No. 2 of Table 1, and Sample Nos. 3a to 3g, which have substantially the same porosity (58 to 59%) as that of Sample No. 3 of Table 1, show an improvement of discharge capacity at 5th cycle with the increase of red lead component in the positive active raw material to 30% by weight as compared with the discharge capacity at 5th cycle of Sample No. 1 set forth in Table 2. In particular, Sample Nos. 3a to 3g (porosity = 58 to 59%) show a remarkable improvement of discharge capacity at 5th cycle with the increase of red lead component in the positive active raw material to 30% by weight. On the other hand, as can be seen in Fig. 2 and Table 3, so far as the porosity is within the range of from 54 to 57%, when the red lead component in the positive active raw material increases, these samples show an improvement of discharge capacity at 5th cycle but show not only a smaller value of discharge capacity at 5th cycle itself but also a smaller rate of increase of discharge capacity at 5th cycle than the samples having a porosity of from 58 to 59%. Further, so far as the porosity is within the range of from 62 to 64%, when the red lead component in the positive active raw material increases to the range of from 5% by weight to 30% by weight, the discharge capacity at 5th cycle improves drastically. This tendency is remarkable particularly when the red lead component is within the range of from not lower than 5% by weight to not greater than 24% by weight. Accordingly, it can be expected that so far as the porosity is not lower than 58%, when the red lead component in the positive active raw material increases, the discharge capacity at 5th cycle improves. Further, in accordance with Fig. 3 and Table 3, all of Sample Nos. la to Ig, which have substantially the same porosity (54 to 55%) as that of Sample No. 1 of Table 1, Sample Nos. 2a to 2g, which have substantially the same porosity (56 to 57%) as that of Sample No. 2 of Table 1, and Sample Nos. 3a to 3g, which have substantially the same porosity (58 to 59%) as that of Sample No. 3 of Table 1, show an improvement of capacity ratio (B/A) with the increase of red lead component in the positive active raw material to the range of from 5% by weight to 27% by weight as compared with the capacity ratio (B/A) of Sample No. 1 set forth in Table 2. It was also found that in Sample Nos. 11 and 12 of Table 1 and Sample Nos. le to 3e, If to 3f and Ig to 3g of Table 3, the positive active material had been accumulated in the cells after the measurement of discharge capacity at 5th cycle. This is presumably because the excessive red lead component causes the reduction of the bonding force of the positive active materials, destroying the conductive path between the positive active material particles with the elapse of charge-discharge cycle. Therefore, when the positive active raw material comprises a red lead powder in an amount of from not lower than 5% by weight to less than 30% by weight and the porosity of the positive active material is not lower than 58%, preferably from not lower than 58% to not greater than 62%, a positive electrode for lead-acid storage battery which can be expected to have an improvement of initial capacity and discharge capacity at 5th cycle can be obtained. It is also obvious from Sample Nos. 4 and 6 to 11 of Table 1 that when the weight of the red lead powder comprising in the positive active raw material increases, the attached amount of the active material increases. This is because even when a grid made of a lead alloy containing tin in an amount of 1.5% by weight, which can form a tin oxide film on the surface of grid to impair the bonding properties of the grid with the positive active material, is used, the presence of the red lead component can improve the bonding properties of the grid with the positive active material. In order to substantiate this mechanism, a positive active material paste prepared according to the same formulation as for Sample Nos. 4 and 6 to 11 of Table 1 was filled in grids having a tin content of 1.2% by weight, 1.1% by weight and 1.0% by weight, respectively. The attached amount of active material to these grids were then similarly studied. The results are set forth in Fig. 4. It can be seen in Fig. 4 that the grids having a tin content of 1.0% by weight and 1.1% by weight show a small rate of increase of attached amount of active material even if the weight of the red lead powder comprising in the positive active raw material increases while the grids having a tin content of 1.2% by weight and 1.5% by weight show a remarkable increase of attached amount of active material when the positive active raw material comprises a red lead powder in an amount of not lower than 5% by weight. It can be thought that this results in the contribution to the improvement of life performance. Therefore, when a positive active material paste prepared from a positive active raw material comprising a red lead component in an amount of from not lower than 5% by weight to less than 30% by weight is filled in a grid having a tin content of not lower than 1.2% by weight, a positive electrode for lead-acid storage battery can be obtained which can be expected to have an increase of discharge capacity at 5th cycle and can have an improved life performance. • Second embodiment of implementation The second embodiment of implementation concerns the provision of the positive electrode according to the aforementioned first embodiment of implementation, wherein the arrangement that the red lead component is in an amount of not lower than 30% by weight, which is insufficient in the life performance, is improved so that the resulting positive electrode stands comparison with the first embodiment of implementation even if the porosity thereof is not lower than 58%. The lead dust of the first embodiment of implementation is premised on an assumption that its metal- lead component is in an amount of from 25% by weight t An object of the second embodiment of the implementation is to make the effective use of a lead dust even if its aforementioned metal-lead component is in an amount of greater than 30% by weight because a great deal of equipment investment is required to produce a lead dust having a stabilized quality invariably. In other words, the second embodiment of implementation has an object of obtaining a positive electrode comparable to that of the first embodiment of implementation by predetermining the amount of red lead component in even this lead dust to be not lower than 30% by weight. In the aforementioned first embodiment of implementation, when the positive active raw material is mixed with red lead powder, contribution can be made to the improvement of life performance of positive electjrode. However, when the red lead component is in an amount of not lower than 30% by weight, the bonding force between the active materials is lowered, hence the increase of the red lead component is restricted and it makes impossible to exert sufficiently the effect of prolonging the life performance of the positive electrode by comprising red lead powder. In the second embodiment of implementation, this problem is solved by adjusting the content of metal-lead in the lead dust in the positive active raw material. The second embodiment of implementation will be further described hereinafter. A grid having a height of 72 mm, a width of 45 mm and a thickness of 2.9 mm was prepared by expanding a lead alloy containing 0.05% by weight of calcium and 1.5% by weight of tin. These grids were then filled each with positive active material pastes prepared according to the formulation set forth in Table 4 to a thickness of 3.0 mm to prepare samples set forth in Table 4, respectively. Several plates were prepared for each of these samples. These samples were hydrosetted under the same conditions as used for the first embodiment of implementation, and then each subjected to anodization (formation) with a total charge of 20 Ah involving one discharge in dilute sulfuric acid having a concentration of 28% for 24 hours to form positive electrodes. The red lead powder used had a purity of 95%. (Evaluation test 1) The various positive electrodes prepared according to the formulation set forth in Table 4 were each then measured for weight percentage of Pb02 and porosity after formation under the same condition as used for the first embodiment of implementation. The results are set forth in Table 4. Table 4(Table Remove) (Evaluation test 2) The other three of positive electrodes corresponding to the various Sample Nos. different from the positive electrodes subjected to the evaluation test 1 were each measured for initial capacity A and discharge capacity B at 5th cycle under the same conditions as in the first embodiment of implementation. The results are set forth in Table 5 with the results of the ratio of discharge at 5th cycle to initial capacity A (capacity ratio B/A) . Fig. 5 shows the change of discharge capacity at 5th cycle with the change of red lead component for each of various metal-lead components. Table 5 (Table Remove) The discharge capacity at 5th cycle measured in the aforementioned evaluation test 2 is shown with the change of red lead component every component of metal-lead in the lead dust in Fig. 5. As can be seen in Fig. 5 and Tables 4 and 5, Sample A-l, in which a lead dust of its metal-lead component of 26% by weight was used as a main component for a positive active raw material and no red lead powder was mixed therewith, and Samples A-6 to A-8, in which a lead dust of its metal-lead component of 26% by weight was used as a main component for a positive active raw material and not lower than 30% by weight of a red lead component was mixed therewith, explicitly have a lower discharge capacity at 5th cycle and capacity ratio (B/A) than Samples A-2 to A-5, in which a lead dust of its metal-lead component of 26% by weight was used as a main component for a positive active raw material and a red lead component in an amount of from not lower than 12% by weight to not greater than 27% by weight was mixed therewith. However, the respective comparison of Samples A-2' to A-8', in which a lead dust of its metal-lead component of 35% by weight was used as a main component for a positive active raw material and a red lead powder in an amount of from not lower than 12% by weight to not greater than 42% by weight was mixed therewith, with Samples A-2 to A-8 shows that Samples A-6' to A-81 in particular show an improvement of discharge capacity at 5th cycle and capacity ratio (B/A). This tendency is seen also in Samples A-2" to A-8", which comprise a positive active raw material mainly comprising a lead dust of its metal-lead component of 40% by weight similarly mixed with red lead powder. This is presumably attributed to the fact that while Samples A-6 to A-8 have an excessive red lead component in the positive active raw material and hence a low bonding force between the active material particles after formation/ Samples A-6' to A-8' and A-6" to A-8" have an increase of the metal-lead component in the lead dust as a main component of the positive active raw material to 35% by weight or 40% by weight, allowing the chemical reaction of the red lead component with the metal-lead component during hydrosetting that enhances the bonding force between the particles. Further, the study of optimum range of combination of red lead component and metal-lead component in the lead dust from Fig. 5 shows that Samples A-6"' to A-8"' and A-6"" to A-8"", which comprise a lead dust of its metal-lead component in an amount of 28% by weight and 31% by weight, respectively, exhibit an improvement of discharge capacity at 5th cycle and hence an improvement of capacity ratio (B/A) as compared with Samples A-6 to A-8 comprising a lead dust of its metal-lead component in an amount of 26% by weight. It can be thought from this fact that there is an inflection point at which the capacity ratio (B/A) or life performance shows a drastic change between 28% by weight of metal-lead component and 26% by weight of metal-lead component. Since the metal-lead component in the lead dust to be used in the production of an ordinary positive electrode is in an amount of from 25% by weight to 30% by weight, the second embodiment of implementation does not exert a sufficient effect of improving the life performance of a positive electrode prepared from such a lead dust as a positive active raw material but exerts an effect of overcoming the difficulty of a positive electrode prepared from such a lead dust mixed with a red lead powder in comprising not lower than 30% by weight of a red lead component as a positive active raw material. In other words, in the second embodiment of implementation, even when a lead dust of its metal-lead component being in an amount of not lower than 31% by weight is used, it makes possible to obtain a positive electrode having as good a life performance as not lower than 0.90 as calculated in terms of capacity ratio (B/A) by means of predetermining the content of red lead component to be not lower than 30% by weight and hence to contribute to the improvement of yield of lead dust. In particular, when a lead dust of its metal-lead component being in an amount of from not lower than 35% by weight to not greater than 40% by weight is used, it makes possible to obtain a positive electrode having a better life performance, i.e., capacity ratio (B/A) of not lower than 0.91. On the other hand, even when a lead dust of its metal-lead component being in an amount of from not lower than 25% by weight to not greater than 30% by weight is used, it makes possible to obtain a positive electrode having a high capacity ratio (B/A) as in Sample A-6'" so far as the lead dust of its metal-lead component is within the range of from not lower than 28% by weight to 30% by weight. The aforementioned evaluation tests 1 and 2 were made on positive electrodes having a porosity of about 62%. This is because it is intended to prevent a positive electrode having such a porosity from deteriorating in its discharge capacity at 5th cycle and capacity ratio (B/A) when the red lead component exceeds 30% by weight as can be seen in Figs. 2 and 3. It goes without saying that it can provide a positive electrode of its porosity of less than 62% to have a good life performance without using a lead dust of its metal-lead component being so high. Further, when the metal-lead component in the lead dust is in an amount of from not lower than 31% by weight to not greater than 40% by weight, the same effect can be exerted even if the red lead component is in an amount of not greater than 30% by weight, preferably not lower than 12% by weight to not greater than 30% by weight. It is not desirable that the metal-lead component in the lead dust being in an amount of not lower than 40% by weight because the use of a ball mill type lead dust machine, which is a main modern lead dust producing facility, causes the rise of the particle diameter of the lead dust thus produced, not only giving an effect on the performance of the positive electrode but also raising the oxidation rate of the metal-lead and hence undesirably bringing forth a problem in storage of lead dust. Both the aforementioned evaluation tests I and 2 are based on the metal-lead component in the lead dust. However, since the increase of the metal-lead component in the lead powder makes it difficult to control conditions in the production step, it is desirable that the metal-lead component in the lead dust is decreased. Therefore, the positive active raw material comprising a lead dust and a red lead powder as main components may further comprise a metal-lead powder instead of increasing the metal-lead component in the lead dust. In order to substantiate this, the following evaluation test was made. (Evaluation test 3) A positive active material paste was prepared under the same conditions as in the second embodiment of implementation from a positive active raw material prepared by mixing a lead dust of its metal-lead component of 26% by weight with a red lead powder and a metal-lead powder according to the formulation set forth in Table 6. The positive active material paste thus prepared was then filled in the same grid as used in the second embodiment of implementation to a thickness of 3.0 mm to prepare Sample Nos. set forth in Table 6. Several plates were prepared for each of these samples. These samples were hydrosetted under the same conditions as used for the second embodiment of implementation, and then each subjected to anodization (formation) with a total charge of 20 Ah involving one discharge in dilute sulfuric acid having a concentration of 28% for 24 hours to form positive electrodes. The red lead powder used had a purity of 95%. These positive electrodes were each then measured for PbO2 (% by weight) after formation and porosity under the same conditions as used in the first embodiment of implementation. The results are set forth in Table 6. (Table Remove) (Evaluation test 4) The other three of positive electrodes corresponding to the various Sample Nos. different from the positive electrodes subjected to the evaluation test 3 were each measured for initial capacity A and discharge capacity B at 5th cycle under the same conditions as in the second embodiment of implementation. The results are set forth in Table 7 with the results of the ratio of discharge at 5th cycle to initial capacity A (capacity ratio B/A) . Fig. 6 shows the change of initial capacity with the change of metal-lead component by containing metal-lead powder and with the change of red lead component, Fig. 7 shows the change of discharge capacity at 5th cycle under the same conditions as mentioned above, and Fig. 8 shows the change of capacity ratio (B/A) under the same conditions as mentioned above. The reason why the samples of its metal-lead component 26% by weight were selected in the aforementioned evaluation tests 3 and 4 is that this lead dust showed a remarkable drop of discharge capacity at 5th cycle with the increase of red lead component in the earlier evaluation test 2. (Table Remove)Table 7As can be seen in Figs. 6 to 8 and Tables 6 and 7, when a lead dust of its metal-lead component 26% by weight as a main component is mixed with a metal-lead powder and further with a red lead powder, the weight percentage of metal-lead component in the positive active raw material decreases with the increase of red lead component but can be raised when the metal-lead powder comprises to make up for the loss. It can be seen that the discharge capacity at 5th cycle improves as a result. For example, while Sample No. A-6 (same as Sample No. A-6 in Table 5) comprising 30.0% by weight of a red lead component comprises 18.2% by weight of a metal-lead component, Sample No. a-6' comprises 19.7% by weight of a metal-lead component when mixed with a metal-lead powder in an amount such that the content of metal-lead powder reaches 2.0% by weight. Further, Sample No. a-6" comprises 25.2% by weight of a metal-lead component when mixed with a metal-lead powder in an amount such that the content of metal-lead powder reaches 9.5% by weight. In any case, the weight percentage of metal-lead component in the positive active raw material can be raised. As a. result, the discharge capacity at 5th cycle improves as compared with the case where no metal-lead powder is contained. However, while Sample No. A-7 (same as Sample No. A-7 in Table 5) comprising 36.0% by weight of a red lead component and Sample No. A-8 (same as Sample No. A-8 in Table 5) comprising 42.0% by weight of a red lead component comprise a metal-lead component in an amount of 16.6% by weight and 15.1% by weight, respectively, Sample No. a-7' and Sample No. a-8' comprise a metal-lead component in an amount of 19.6% by weight and 19.5% by weight when mixed with a metal-lead powder in an amount such that the content of metal-lead powder reaches 4.0% by weight and 6.0% by weight, respectively. Further, Sample No. a-7" and Sample No. a-8" comprise a metal-lead component in an amount of 25.2% by weight and 25.1% by weight when mixed with a metal-lead powder in an amount such that the content of metal-lead powder reaches 11.5% by weight and 13.5% by weight, respectively. It was found that as a result, the weight percentage of metal-lead component in the positive active raw material can be improved and the discharge capacity at 5th cycle, too, can be improved as compared with the case where no metal-lead powder is contained, but the initial capacity decreases with Sample Nos. a-7" and a-8" and Sample Nos. a-2"' to a-4"'. This is presumably attributed to the fact that Sample No. a-7" and No. a-8" show a drop of weight percentage of PbO2 after formation as compared with Sample No. A-7 and Sample No. A-8. This is presumably attributed to the fact that when a metal-lead powder is contained in such an amount that the content of metal-lead powder exceeds 10% by weight, the metal-lead powder reacts with the red lead powder, exerting an effect of lessening the efficiency of formation of positive electrode in addition to the effect of enhancing the bonding force between the positive active material particles. This is obvious also from the fact that Sample No. a-2"' to a-4"', which contain a metal-lead powder in an amount such that the metal-lead component exceeds 30.0% by weight, have a drop of initial capacity. Further, it is obvious from Figs. 6 to 8 and Tables 6 and 7 that Sample Nos. a-9' and a-10', which comprise a lead dust of its metal-lead component 26% by weight as a main component mixed with a metal-lead powder and further with a red lead powder to have a red lead component in an amount of 45.0% by weight and 48% by weight, respectively, show a remarkable drop of discharge capacity at 5th cycle. From the aforementioned description, it is obvious that in the case where a lead dust of its metal-lead component 26% by weight is used as a positive active raw material, when the red lead component is predetermined to be from not lower than 30% by weight to not greater than 42% by weight and a metal-lead powder is contained in such an amount that the metal-lead component is from not lower than 19% by weight to less than 26% by weight, the initial capacity and the discharge capacity at 5th cycle can be improved. In particular, when the metal-lead powder to be contained is not greater than 10% by weight, the aforementioned effect can be remarkably exerted as in Sample Nos. a-4' to a-8' and a-2" to a-6". Even in the case where the metal-lead component in the lead dust is predetermined to be from not lower than 19% by weight to less than 26% by weight by containing a metal-lead powder, the red lead component can be predetermined to be not greater than 30% by weight, preferably from not smaller than 12% by weight to not greater than 30% by weight to exert the same effect as mentioned above as can be seen in Figs. 6 to 8. Referring to technically effective scope of the present invention the essence of which lies in the arrangement that a lead dust and a red lead powder are main components of a positive active raw material on the basis of the aforementioned first and second embodiments of implementation and evaluation tests 1 to 4, it is preferred: 1. In a positive electrode for lead-acid storage battery comprising a positive grid filled with a paste obtained by kneading a positive active raw material mainly comprising lead dust and red lead powder with dilute sulfuric acid, the red lead component in the aforementioned positive active raw material be from not lower than 5% by weight to less than 30% by weight, preferably from not lower than 5% by weight to not greater than 24% by weight; The content of red lead component in the aforementioned positive active raw material be from not lower than 12% by weight to not greater than 42% by weight and the metal-lead component in the lead dust be from not lower than 31% by weight to not greater than 40% by weight; The aforementioned positive active raw material further comprises a metal-lead powder to be contained, the red lead component in the raw material be from not lower than 12% by weight to not greater than 42% by weight, and the metal- lead component in the raw material be from not lower than 19% by weight to less than 26% by weight; and 4. Further, the content of metal-lead powder in the raw material be less than 10% by weight. • Third embodiment of implementation The third embodiment of implementation concerns an arrangement that the self-discharge characteristics of positive electrode for lead-acid storage battery according to the first and second embodiments of implementation is improved without impairing the cycle life performance of the lead-acid storage battery by restricting the average particle diameter of red lead powder even if the lead dust comprises a red lead powder to be contained in a large amount. By containing a red lead powder in the lead dust produced by the use of a Shimadzu type ball mill or Burton type lead dust machinb self-discharge characteristics of lead-acid storage e improved. However, it was disadvantageous K when a red lead powder contains in a large amount, powder reacts with sulfuric acid in the electrolsll produce lead sulfate that then grows greater than tlM pores in the positive electrode, resulting in the destruction of the combined structure of active material and hence the deterioration of life performance of positive electrode. In the third embodiment of implementation, the distribution of particle diameter of red lead powder is predetermined to have a proper range with respect to the distribution of particle diameter of lead dust such that even when the red lead powder to be contained in the lead dust which is a main component of the positive active material reacts with sulfuric acid to produce lead sulfate, the particle diameter of lead sulfate does not increase too much with respect to the average particle diameter of lead dust. In this arrangement, the combined structure of active material can be prevented from being destroyed, making it possible to exert an effect of improving the self-discharge characteristics developed by containing red lead powder without deteriorating the life characteristics of lead-acid storage battery. The third embodiment of implementation will be further described hereinafter. The lead dust used in this embodiment of implementation had been produced by the use of a Shimadzu type ball mill, and its average particle diameter and content of metal-lead component were about 2.3 |4m and from 25 to 30% by weight, respectively. The red lead powder had an average particle diameter of about 2.2 pm. The mixture of the lead dust and the red lead powder was kneaded with dilute sulfuric acid to prepare a positive active material paste. The positive active material paste thus prepared was filled in an expanded grid and a casted grid, and then hydrosetted under the same conditions as in the first embodiment of implementation. In the present embodiment of implementation, the average particle diameter was calculated by measuring the sedimentation velocity of the lead dust and the red lead powder while being dispersed in a 3 : 1 mixed solvent of cyclohexanol and methanol. Table 8 indicates the paste-density prepared from a positive active raw material comprising a red lead component in an amount of from 0 to 50% by weight during kneading and after hydrosetting. The reason why the content of red lead component is limited to 50% by weight is that even when the red lead component is contained in an amount exceeding 50% by weight, no effect can be exerted on the cycle life performance described later (see Table 10). The positive active material paste-density prepared by mixing a mixture of lead dust and red lead powder comprising a content of red lead component varying up to 50% by weight with dilute sulfuric acid during kneading and after hydrosetting and drying were almost the same as that of the positive active material pastes prepared free of red lead. It can be thought from this fact that the reactivity of lead dust and red lead powder with dilute sulfuric acid are almost the same at the step of kneading positive active material paste, dilute sulfuric acid, red lead powder, etc. The dilute sulfuric acid used herein had a specific gravity of 1.16 and was mixed in a proportion of 2.0 (litter) per 10 kg of positive active raw material mainly comprising lead dust and red lead powder. Table 8(Table Remove) Under a tension of about 40 kg/dm2 developed by stack, a formed positive electrode prepared by subjecting the hydrosetted and dried positive electrode (comprising a casted grid) set forth in Table 8 to so-called tank formation with dilute sulfuric acid having a specific gravity of 1.10, a negative electrode prepared by formation and drying through almost the same procedure and a fine glass mat retainer were assembled into a valve-regulated lead-acid storage battery having a capacity corresponding to 7AH (20 hour rate). The electrolyte which had been injected into the valve-regulated lead-acid storage battery was dilute sulfuric acid having a specific gravity of 1.300. The valve-regulated lead-acid storage batteries thus prepared were Comparative Example and Examples 1 to 5. These valve-regulated lead-acid storage batteries were each auxiliarily charged with 10 hour rate current by about 150%, allowed to stand at 40°C for 30 days, and then examined for self-discharge characteristics. Similarly, a hydrosetted and dried positive electrode (comprising an expanded grid) set forth in Table 8, a hydrosetted and dried negative electrode prepared through almost the same procedure and a fine glass mat retainer were assembled under a tension of about 40 kg/dm2 developed by stack, and then subjected to so-called case formation to prepare a valve-regulated lead-acid storage battery having a capacity corresponding to 7AH (20 hour rate). As the electrolyte there was used dilute sulfuric acid having a specific gravity of from 1.225 to 1.245 so that the specific gravity thereof reaches 1.300 at the termination of case formation. The valve-regulated lead-acid storage batteries thus prepared, too, were referred to as Comparative Example and Examples 1 to 5 depending on the red lead components, respectively, allowed to stand at 40°C for 30 days, and then examined for self-discharge characteristics similarly. For the determination of self-discharge characteristics, the difference between the discharge at first cycle after 30 days of standing and the averaged value of three cycles of discharge after charge with 10 hour rate current of 8.4 AH and 30 days of standing is divided by 30. The quotient is then multiplied by 100 to give self-discharge rate. Table 9 indicates the results. Table 9(Table Remove) As can be seen in Table 9, both the sample prepared by tank formation and the sample prepared by case formation show a drop of self-discharge rate with the increase of red lead component. The drop of self-discharge rate becomes slightly slow as the content of red lead component is not smaller than 30% by weight. In addition, it is also made obvious that the absolute value of self-discharge rate of valve-regulated lead-acid storage battery comprising a positive electrode prepared by case formation is smaller than that of valve-regulated lead-acid storage battery comprising a positive electrode prepared by tank formation. Subsequently, the aforementioned value-regulated lead-acid storage batteries which had been subjected to tank formation were subjected to cycle life performance We Claim: 1. A positive electrode for lead-acid storage battery comprising: a positive grid made of a lead alloy containing not lower than 1.2% by weight of tin, and a paste filled therein, which is obtained by kneading a positive active raw material comprising a lead dust and a red lead powder as main components with dilute sulfuric acid, wherein said positive active raw material comprises a red lead component in an amount of from not lower than 5% by weight to not greater than 50% by weight and the positive electrode for lead-acid storage battery prepared from a paste made from this positive active raw material has a positive active material porosity of not lower than 58% after formation. The positive electrode for lead-acid storage battery as claimed in claim 1, wherein the positive active raw material comprises a red lead component in an amount of from not lower than 5% by weight to less than 30% by weight and the positive electrode for lead-acid storage battery prepared from a paste made from this positive active raw material has a positive active material porosity of not lower than 58% after formation. The positive electrode for lead-acid storage battery as claimed in claim 1, wherein the positive active raw material comprises a red lead component in an amount of from not lower than 12% by weight to not greater than 42% by weight, the metal-lead component in the lead dust is from not lower than 31% by weight to not greater than 40% by weight and the positive electrode for lead-acid storage battery prepared from a paste made from this positive active raw material has a positive active material porosity of not lower than 58% after formation. The positive electrode for lead-acid storage battery as claimed in claim 3, wherein the positive active raw material further comprises a metal-lead powder contained in, the metal-lead component in the positive active raw material calculated from the amount of metal-lead component in the lead dust and the metal-lead powder thus added is from not lower than 19% by weight to less than 26% by weight and the positive electrode for lead-acid storage battery prepared from a paste made from this positive active raw material has a positive active material porosity of not lower than 58% after formation. The positive electrode for lead-acid storage battery as claimed in claim 1, wherein the positive active raw material comprises a red lead component in an amount of from not lower than 10% by weight to not greater than 50% by weight and the red lead powder has an average particle diameter of not greater than 2.2 times that of the lead dust. The positive electrode for lead-acid storage battery as claimed in claim 1, wherein the lead dust is produced from a lead alloy of its antimony incorporating in an amount of from not lower than 0.005% by weight to not greater than 0.1% by weight. The positive electrode for lead-acid storage battery as claimed in any one of claims 1 to 6, wherein the positive grid is an expanded grid. A lead-acid storage battery comprising a positive electrode for lead-acid storage battery as claimed in any one of claims 1 to 7. A Positive electrode for lead-acid storage battery substantially as herein described with reference to the accompanying drawings.

Documents:

2027-delnp-2005-abstract.pdf

2027-delnp-2005-Assignment-(20-04-2011).pdf

2027-DELNP-2005-Claims-11-04-2008.pdf

2027-delnp-2005-claims.pdf

2027-delnp-2005-Correspondence-Others-(20-04-2011).pdf

2027-DELNP-2005-Correspondence-Others-11-04-2008.pdf

2027-delnp-2005-correspondence-others.pdf

2027-delnp-2005-description (complete).pdf

2027-delnp-2005-drawings.pdf

2027-delnp-2005-form-1.pdf

2027-delnp-2005-form-13.pdf

2027-delnp-2005-form-18.pdf

2027-delnp-2005-form-2.pdf

2027-delnp-2005-form-26.pdf

2027-delnp-2005-form-3.pdf

2027-delnp-2005-form-5.pdf

2027-delnp-2005-GPA-(20-04-2011).pdf

2027-delnp-2005-pct-306.pdf

2027-delnp-2005-pct-search report.pdf