Patent Application
20050221191
1. Field of the Invention
The present invention relates to a highly corrosion resistant lead alloy and a lead storage battery using it for a current collector, particularly relates to the extension of the life and the improvement of the reliability of a lead storage battery, by using the rolled sheet of the highly corrosion resistant lead alloy with retarded intergranular corrosion, for the current collector.
2. Background Art
A lead storage battery has features of a low cost and high reliability, and therefore it is widely used as an uninterruptible power supply in an automobile, a computer backup unit or the like. For the electrode, a current collector made of a lead alloy coated with an active material is used. In these applications, the lead storage battery normally stands by in a charged state by trickle charge, and discharges electricity when power has failed. One of important technical subjects in these applications is to retard the deterioration of a positive current collector (the increase of resistance due to oxidation or a deformation due to a cubical dilatation) due to overcharge.
On the other hand, recently, there has been a need for increasing power and utilization factor, and accordingly in order to increase a contacting area with an active material, a current collector tends to be a thinner flat shape or have holes. Accordingly, a current collector is exposed to an increasingly severe corrosive environment, and an improvement in the corrosion resistance of a lead alloy used for it is a great development challenge.
For a lead alloy in a current collector, conventionally, a Pb—Sn—Sb or Pb—Sn—Ca lead alloy has been used. Particularly, the Pb—Sn—Ca lead alloy has high strength and causes little self-discharge, so that it is frequently used as the grid current collector of an enclosed lead storage battery. In addition, in order to improve the corrosion resistance of the current collector, lead alloys with various compositions have been proposed until now. For instance, JP Patent Publication (Kokai) No. 2000-77076 discloses a lead-base alloy used in a positive grid plate made of a Pb—Ca—Sn—X alloy, where an X element is at least one or more additives selected from the group consisting of Li, Sr and Ba. Specifically, the proposed lead alloy is a Pb-0.05 to 0.20 wt. % Ca-0.50 to 2.0 wt. % Sn alloy containing at least one or more elements among 0.01 to 0.3 wt. % Li, 0.01 to 3 wt. % Sr and 0.01 to 0.3 wt. % Ba However, a Pb—Ca—Sn alloy essentially has coarse crystal grains, so that it easily causes intergranular corrosion when used in a positive current collector, and anodically oxidized in a high-temperature environment, both of which cause the problems of the elongation of a plate, the deformation of a grid, consequent poor contact between a grid and an active material, and lead to the degradation of cell characteristics.
Subjects in the present invention are to solve the problem of intergranular corrosion in a conventional lead alloy, by controlling a structure, specifically, by refining crystal grains, and to provide a positive current collector superior in corrosion resistance; and objects of the present invention are thereby to inhibit the deterioration of a positive current collector due to overcharge and to provide a long-life lead storage battery superior in cycle characteristics.
The present inventors have thought that in order to enhance corrosion resistance and prolong the life of a lead alloy by inhibiting intergranular corrosion, crystal grains should be refined by the control of a structure (the crystal grains). Specifically, in a constant-potential battery environment, so long as grain refining does not give a basic adverse effect on a corrosion reaction mechanism and a corrosion rate, a controlled small grain size extends the total length of crystal grain boundaries per unit weight and unit area, prolongs the rupture life by increasing corrosion length, and consequently increases the corrosion resistance. In the constant-current battery environment as well, so long as the grain refining does not give the basic adverse effect on a corrosion reaction mechanism and a corrosion rate, it should extend the total length of the crystal grain boundaries, reduce a corrosion current per unit length in the grain boundaries, and consequently increase the corrosion resistance.
However, even if the crystal grains of a Pb—Sn alloy and a Pb—Ca—Sn lead alloy are refined by such plastic working as rolling, the alloys have a recrystallization temperature in about room temperature as is conventionally known, so that recrystallization proceeds to coarsen the grains, which means that grain refining is nearly impossible.
For this reason, it is necessary for refinement of crystal grains to increase recrystallization temperature. The present inventors found that the addition of Sr inhibits crystal growth, that is, gives a pinning effect, and added Sr to a Pb—Sn alloy. Here, the amount of Sr to be added is necessary to balance with the amount of Sn. When a molten alloy solidifies, Sr not only refines a solidification structure through forming a Pb compound (a crystal nucleus) and a Sn compound (an eutectoid), but is also dispersed in the form of fine precipitates in a matrix after plastic-working such as rolling, thereby inhibits the crystal growth and increases recrystallization temperature. In the present invention, for the purpose of securing the hardness, or equivalently, the strength of the above described Pb—Sn—Sr alloy, a trace amount of either of Ba and Te, or Ca is also added. The lead alloy having the crystals thus controlled and the hardness thus adjusted is cold-rolled into a sheet of which at least one part has a recrystallized structure, and the sheet is used for the current collector of a lead battery.
In a lead alloy according to the present invention, Sr added in a Pb—Sn alloy refines a cast structure and the recrystallized structure of a rolled material to inhibit intergranular corrosion, and further added Ca, Ba and Te enable the extensive adjustment of hardness. In addition, the application of the rolled sheet of the lead alloy to the positive current collector of a lead storage battery improves corrosion resistance greatly, and can prolong the life and improve the reliability of the lead battery in the wide range of use.
A lead alloy according to the present invention is basically a Pb—Sn alloy comprising Pb containing 1.3 to 3.0 wt. % Sn. The first alloy includes 0.05 to 0.4 wt. % Sr added in the base alloy to improve corrosion resistance. Sr is added to refine the solidification structure of a cast steel, to raise the recrystallization temperature of the rolled material, to refine recrystallized grains, and to inhibit intergranular corrosion. Added Sr in an amount of less than 0.05 wt. % has an inadequate refining effect for the recrystallized grains, and Sr exceeding 0.4 wt. % has a tendency of increasing an amount of uniform corrosion. Accordingly, the additive amount of Sr is preferably 0.05 to 0.4 wt. %.
The second lead alloy according to the present invention is an alloy comprising a Pb—Sn base alloy comprising Pb containing 1.3 to 3.0 wt. % Sn, 0.05 to 0.4 wt. % Sr, and further 0.05 to 0.20 wt. % one or more elements selected from the group consisting of Ba and Te. Ba and Te are added to improve the hardness of the alloy. An additive amount of less than 0.05 wt. % shows no effect of improving hardness, and an additive amount of more than 0.15 wt. % has a tendency of impairing rollability. Consequently, the additive amount of Ba and Te is preferably 0.05 to 0.20 wt. %. Here, the additive amount of Ba and Te is determined corresponding to the amount of Sr in the specified range.
The third lead alloy according to the present invention is the alloy which comprises a Pb—Sn alloy comprising Pb containing 1.3 to 3.0 wt. % Sn as a base, 0.05 to 0.4 wt. % Sr and further 0.01 to 0.05 wt. % Ca. The element Ca is added to improve the hardness of the alloy. The additive amount of less than 0.01 wt. % shows no effect of improving hardness, and the additive amount of more than 0.05 wt. % lowers a recrystallization temperature to coarsen recrystallized grains, and consequently promotes intergranular corrosion. Accordingly, the additive amount of Ca is preferably 0.01 to 0.05 wt. %. Here, the additive amount of Ca is determined corresponding to the amount of Sr in a specified range.
In an alloy according to the present invention, the concentration ratio Sn/Sr of Sn to Sr is determined in consideration of the concentration of Sn dissolved in an alloy matrix and the amount of Sn and Sr compounds. When the ratio is 7 or lower, the concentration of Sn in the alloy matrix is so low that corrosion resistance decreases, and the amount of the compound is so much that the rollability of the alloy is impaired. In addition, when the ratio is 30 or higher, the amount of the compound having the effect of inhibiting the growth of recrystallized grains and increasing a recrystallization temperature is too little to show the effect of the addition. Accordingly, the ratio is preferably 7 to 30, and further preferably 15 to 25 in terms of cell characteristics.
In addition, a lead alloy according to the present invention is cast, rolled and heat-treated at 160° C. or lower, and then at least one part of the rolled texture acquires a recrystallized structure with an average grain size of 20 μm or smaller, which is suitable for the positive current collector of a lead storage battery. As described above, even when a lead alloy according to the present invention has received a heat load at 160° C. or lower in manufacture and use, at least one part of the rolled texture retains the recrystallized structure with the average grain size of 20 μm or smaller.
In order to remarkably inhibit intergranular corrosion, the average size of recrystallized grains is preferably 20 μm or less. A heat treatment temperature is determined corresponding to the recrystallization temperature of the alloy, but the heat treatment temperature of 160° C. or higher proceeds grain growth into the grains with 20 μm or larger, so that the temperature is preferably 160° C. or lower.
By using a lead alloy according to the present invention, a current-collecting plate for a lead storage battery can be produced. A lead storage battery can be produced by using the current-collecting plate for the lead storage battery with the use of the lead alloy according to the present invention as a component. The lead storage battery is suitable not only for a winding type but also applicable to a multilayered type.
A lead storage battery provided with a current collector, particularly a positive current collector with the use of a lead alloy according to the present invention, can be used as an industrial battery required to have high input characteristics and output characteristics, such as in an electric vehicle, a parallel hybrid electric vehicle, a simple hybrid car, a power storage system, an elevator, an electric power tool, an uninterruptible power supply and a dispersion type power source.
(Experiment)
A rolled sheet having the thickness of 1 mm was prepared by smelting an alloy comprising a Pb—Sn alloy containing Sr, an alloy comprising a Pb—Sn—Sr alloy containing one element selected from the group consisting of Ba and Te, and an alloy comprising a Pb—Sn—Sr alloy containing Ca and cold-rolling them. The rolled sheet was subjected to microstructure observation, micro-Vickers hardness measurement and a corrosion test.
[Corrosion Resistance Evaluation]
Subsequently, a corrosion test was conducted to evaluate corrosion resistance. The corrosion test was conducted by taking test pieces with the size of 10×50×1 mmt from a rolled material, and continuously applying the current of 10 mA/cm2 for 36 hours onto the test pieces in a sulfuric acid electrolytic solution of 30° C. having the specific gravity of 1.280 (20° C.). After the test, a corrosion product formed on the surface of the test piece was removed with a nitric acid solution, and the depth of intergranular corrosion was measured with a laser microscope.
A cycle corrosion test was conducted in order to evaluate corrosion resistance under severer conditions, by repeating the cycle of charging and leaving the test pieces described below for each 6 hours with the current density of 1.25 mA/cm2 in a sulfuric acid electrolytic solution of 75° C. having the specific gravity of 1.280 (20° C.), for continuous six weeks. The test pieces with the size of 10×50×1 mmt were taken from the rolled material. After the test, the cross sections of the test pieces including a corroded layer were observed, and the corroded quantity (the total of a uniformly corroded thickness and an intergranular-corroded depth) was determined.
Referring to specific examples, the present invention will be now described in further detail below, but the present invention is not limited to the examples unless being beyond the purpose of the present invention. In addition, the examples to which the present invention is applied will be described in detail in comparison with a lead storage battery (a comparative example) prepared for confirming the effect of the examples.
At first, a method for preparing a lead storage battery in each example and a comparative example will be described. In examples 2 or higher numbers and comparative examples 1 or higher numbers, the description of the same producing methods as in the example 1 will be omitted, and different methods will be described.
[Preparation of Positive Current Collector]
A Pb—Sn—Sr alloy having a composition according to the present invention was smelted, cold-rolled into a rolled sheet with the thickness of 0.8 mm and was formed into an expanded shape, and the product was used for a positive current collector. The alloy composition of the example 1 is shown in Table 1.
[Preparation of Negative Plate]
A negative plate was prepared by the steps of: at first adding 0.3 wt. % lignin, 0.2 wt. % barium sulfate or strontium sulfate, and 0.1 wt. % carbon powder with respect to lead powder, and kneading them with a kneading machine for about 10 minutes to arrange the mixture; subsequently, adding 12 wt. % water with respect to the lead powder to the lead powder, mixing them, and further adding 13 wt. % dilute sulfinuric acid (with the specific gravity of 1.26 at 20° C.) with respect to the lead powder to prepare the paste of an active material for a negative electrode; and charging 50 g of the paste of the active material for a negative electrode to a current collector made of an expanded lead alloy with the thickness of 0.8 mm, leaving the product in the atmosphere with the humidity of 95% at 50° C. for 18 hours to age it, and then leaving it at 110° C. for two hours to dry it and prepare an unformed negative electrode.
[Preparation of Positive Plate]
A positive plate was prepared by the steps of: at first mixing lead powder with 12 wt. % water with respect to the lead powder and 13 wt. % dilute sulfuric acid (with the specific gravity of 1.26 at 20° C.) with respect to the lead powder, and kneading the mixture to prepare the paste of an active material for a positive electrode; and subsequently charging 60 g of the paste of the active material for a positive electrode to a current collector made of an expanded Pb—Sn—Sr alloy, leaving the product in the atmosphere with the humidity of 95% at 50° C. for 18 hours to age it, and then leaving it at 110° C. for two hours to dry it and prepare an unformed positive plate.
[Preparation and Electrolytic Formation of Multilayered Battery]
A lead battery has a configuration of serially connecting several electric cells to acquire a predetermined electric voltage. Here, the prepared battery has the discharge voltage of 12 V and the charging voltage of 14 V, but the battery having the discharge voltage of 36 V and the charging voltage of 42 V can be produced, and the present invention is not limited to the electric voltage range. Accordingly, in the examples according to the present invention, the battery having the discharge voltage of 12 V was prepared, but various characteristics of the present invention do not change depending on the electric voltage range.
[Deep Cycle Test]
As for a deep cycle test, the obtained lead storage battery was subjected to five repetitive discharge and charge cycles of charging the battery at constant current and constant voltage with 5.6 amperes of charging current within a maximum electric voltage of 14.5 V for six hours and discharging it with 5.6 amperes of discharging current till the voltage reaches 10.5 V. The maintenance factor of service capacity in the fifth cycle with respect to the service capacity in the first cycle was determined. The results are shown in Table 1.
[Preparation of Positive Current Collector]
A Pb—Sn—Sr alloy having a composition according to the present invention was smelted and cold-rolled into a rolled sheet with the thickness of 0.2 mm, which was used for a positive current collector.
[Preparation of Negative Plate]
A negative plate was prepared by the steps of: at first, adding 0.3 wt. % lignin, 0.2 wt. % barium sulfate or strontium sulfate, and 0.1 wt. % carbon powder with respect to lead powder, and kneading them with a kneading machine for about 10 minutes to arrange the mixture; subsequently adding 12 wt. % water with respect to the lead powder to the lead powder, mixing them, and further adding 13 wt. % dilute sulfuric acid (with the specific gravity of 1.26 at 20° C.) with respect to the lead powder to prepare the paste of a negative-electrode active material; and applying the paste of the negative-electrode active material in an amount of 50 g to a current collector consisting of a lead alloy foil with the thickness of 0.2 mm.
[Preparation of Positive Plate]
A positive plate was prepared by the steps of: at first mixing lead powder with 12 wt. % water with respect to the lead powder and 13 wt. % dilute sulfuric acid (with the specific gravity of 1.26 at 20° C.) with respect to the lead powder; kneading the mixture to prepare the paste of an active material for a positive electrode; and subsequently applying 60 g of the paste of the active material for a positive electrode to a current collector with the thickness of 0.2 mm made of a Pb—Sn—Sr alloy foil.
[Preparation and Electrolytic Formation of Winding Battery]
[Five-Hour-Rate Capacity Confirmatory Test]
A five-hour-rate capacity was determined by discharging the obtained lead storage battery at 3 amperes of discharging current till the voltage reaches 10.5 V.
As in the Example 1, a rolled sheet having the thickness of 1 mm was prepared by smelting an alloy comprising a Pb—Sn alloy containing Sr, an alloy comprising a Pb—Sn—Sr alloy containing one element selected from the group consisting of Ba and Te, and an alloy comprising a Pb—Sn—Sr alloy containing Ca and cold-rolling them. A microstructure was observed with the use of this rolled sheet.
Each sample was heat-treated at 80° C. for 20 hours. It is clear that in
A lead storage battery which uses a lead alloy according to the present invention for a current collector, particularly a positive current collector, can be used as an industrial battery required to have high input characteristics and output characteristics, such as in an electric vehicle, a parallel hybrid electric vehicle, a simple hybrid car, a power storage system, an elevator, an electric power tool, an uninterruptible power supply and a dispersion type power source.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-103679 | Mar 2004 | JP | national |
