GB2157480A - Electrodes for lead accumulators - Google Patents

Abstract

Electrodes for lead accumulators, characterized by being prepared by admixing powdered lead for us as an active material in lead accumulators with at least one member selected from lead sulfate salts to obtain a mixture and then having the thus obtained mixture held uniformly on grids without need of previous kneading, aging and drying of the mixture, thereby to obtain electrodes.

Description

SPECIFICATION Electrodes for lead accumulators This invention relates to electrodesforlead accumulators and more particularly it relates to electrodes for high performance lead accumulators prepared by a simplified process which comprises mixing powdered lead useful as the active material in lead accumulators with at least one lead sulfate sa It without subsequent- ly kneading the sulfate and powdered lead togetherto obtain a mixture (hereinafter sometimes referred to as "active material mixture") and then having the thus obtained mixture mounted onto a grid for an electrode.

The term "lead sulfate salts" used herein is intended to mean lead sulfate (PbSO4), monobasic lead sulfate (PbO#PbSO4),tribasic lead sulfate (3PbO#PbSO4#nH2O),tetrnbasic lead sulfate (4PbO-PbSO4), andthe like.

Lead accumulators have heretofore been improved in various ways to provide higher performance ones than conventional, however, further higherperform- ance lead accumulators are now in demand and they are desired to be produced at a low cost.

It has hitherto been generally known that many treating steps are required in the production of a conventional lead accumulator using electrodes holding thereon active material for lead accumulators. The treating steps mainly include a paste preparing step comprising thoroughly kneading powdered lead as the active material for lead accumulators with dilute sulfuric acid to make the powdered lead pasty, an applying step comprising applying the pasty powdered lead to grid, an aging step comprising aging and drying the paste-applied grids, a formation step comprising charging the aged and dried grids to obtain electrodes, a step comprising vçaterwashing and drying the thus obtained electrodes and a fabricating step comprising placing the washed and dried electrodes intra suitable case for an accumu latorto obtain a lead accumulator.

The "paste meth.od"wasdeveloped as early as in 1881, and since then, many people who were engaged inthefield had made many useful improvements until the present state ofthe art was achieved. In the production of such pasted type electrodes, powdered lead is kneaded with dilute sulfuric acid to obtain pasty powdered lead and the thus obtained pasty powdered lead is then applied to lead alloy-madegridsto obtain green electrodes. When the thus obtained green electrodes having the paste thereon are allowed to stand astheyare in a room,the paste will crack or cause fissure as it gets drier, whereby electrodes having a smooth uniform surface are not obtained.

Thus it is considered that the aging and drying step carried out in such a highly moist atmosphere as above is the most important one in the production of conventional electrodes for lead accumulators. In the aging and drying step carried out in a highly moist atmosphere, the use of a lowtemperature range (30-500C) and the use of a considerably high temperature range (70-90 C) have been proposed. The duration is long (30-70 hours) for the lowtemperature range and is short (15-20 hours or more) forthe high temperature range.

In the production of electrodes by the paste method, the use of a suitable amountofsulfuric acid in said kneading step will enable the resulting paste to have such suitable softness as to be applied to grids. When the water contained in the paste and necessary to keep the paste suitably soft as mentioned above, is evaporated, the paste will crack. Thus, said aging and drying step is an inevitably important one in the production of pasted type electrodes. However, the step of aging and drying the active material for accumulators is disadvantageous in that it renders the processforthe production of accumulators complicated and increases the production costthereof.

This invention has been made to solve these conventional problems and an object thereof is to provide inexpensive high-performanceelectrodesfor lead accumulators by using a simplified process.

The present inventors made various studies to attain the object and, as the result of their studies, found that such basic lead sulfates as prepared by kneading powdered lead with dilute sulfuric acid by the conventional method may also be prepared by another method which dispenses with many steps such as the conventional kneading step and the aging and drying step to produce such basic lead sulfates.

This invention is based on this finding.

This invention resides in electrodes for a lead accumulator, which are characterized in that they are prepared by adding at least one lead sulfate salt to powdered lead for use as the active material in lead accumulators to obtain an active material mixture and then having the thus obtained mixture mounted onto grids without previous kneading of the mixture.

In the conventional paste method, dilute sulfuric acid is added to powdered lead and kneaded together to obtain a paste which is applied to a grid and then aged and dried to obtain conventional electrodes. In this invention, on the other hand, powdered lead is mixed with at least one lead sulfate salt to obtain a mixture which is applied (for example, by pressing) directly to grids to obtain electrodes without being subjected to conventional kneading ofthe mixture and to conventional aging and drying of the applied mixture. In brief, this invention is characterized by adding at least one lead sulfate salt to powered lead thereby to enable satisfactory electrodes to be produced without being treated in the steps of kneading, aging and drying of the active material mixture.

In this invention at least one lead sulfate salt is added to powdered lead for use as the active material for electordes, whereas in the conventional method dilute sulfuric acid is poured into powdered lead and then kneaded to obtain a pasty powdered lead.

In the kneading step of the conventional method, the useofsulfuric acid in an amount by weight of 5-7% will producetribasic lead sulfate (3PbO-PbSO4-nH20), whereas the use thereof in an amountbyweightof more than 7% will tend to produce monobasic lead sulfate (PbO-PbSO4). It is well known that not only tribasic lead sulfate but also tetrabasic lead sulfate (4PbO.PbSO4) are produced in a high-temperature atmosphere in the conventional aging step.

The reason why these basic lead sulfates are useful in a lead accumulator is that they facilitate the electrodes in theaccumulatorto be charged andthe formationthereofto PbO2 by the charging results in rendering the electrodes porous due to the volume of the PbO2so produced being small whereby supply of the electrolyte to and withdrawal thereof from the electrodes are facilitated in the battery reaction ofthe accumulator.

Thus, the battery performance of the accumulator will tend to increase when the basic lead sulfates increase in amount. In the actual kneading step, however, the concentration of su Ifu ric acid in a sulfuric acid solution added must be increased to increase the amount of sulfuric acid added while keeping the softness ofthe mixture (powdered lead and dilute sulfuric acid) at the optimum level, whereby the optimum paste is not obtained due to the tendency oftheto-be-kneaded powdered lead to solidify and heat generation is remarkable thereby to renderthe operation undesirably difficult since the sulfuric acid concentration is increased.On the other hand, it is undesirable to make conventional electrodes solely from monobasic lead sulfate (PbO-PbSO4) or lead sulfate (PbS04) since even if such electrodes are attempted to be charged the charging reaction will not smoothly proceed and the active material will shed from the electrodes in some cases. Since suitable active material for use in lead accumulators will thus be obtained only when dilute sulfuric acid is used in the optimum amount,various measures have now been taken in an attemptto addtheoptimum amount of dilute sulfuric acid to powdered lead.

As previously mentioned, according to this invention, powdered lead is incorporated with at least one member of lead sulfate salt. It is possible to mix the powdered lead with the at least one lead sulfate salt in any mixing ratios.

In the conventional kneading step,the use of sulfuric acid (H2SO4as pure one) in an amount of around 10wt. % or more will produce monobasic lead sulfate (PbO~PbSOd simultaneoulsywith decreasing the amount of tribasic lead sulfate (3PbO-PbSO4-nH2O) produced.

In this invention, on the other hand, it is revealed that monobasic lead sulfate (PbO-PbSO4) will not be produced even if at least one lead sulfate salt is added in an amount byweight of 30-50% or more. Thus, this invention facilitates the content of lead sulfate to increase, this having heretofore been difficult to attain, and it also promotes the active material of the elctrodesto be made more porous whereby the battery performance is expected to be enhanced.

The charging and discharging reactions will proceed atthe initial stage even in a case where lead sulfate is not added at all to powdered lead, however, the discharge performance varying with the lapse of time will be worsened. It is accordingly preferably that the amount of at least one lead sulfate salt added be in the range offrom about 5 to about40 wt. % since the use ofthis amount makes the battery performance remarkable. In a case where the amount ofthe lead sulfate salt added increases, the amount of the unreacted lead sulfate salt in the charging step will tend to increase; thus,theamountofthe lead sulfate salt added should not exceed 40-50 wt. %.In this invention, it is possible to use lead sulfate (FbSO4), monobasic lead sulfate (PbO-PbSO4) orthe like as the lead sulfate salt to be added to powdered lead.

In a case where water is added to a mixture of lead sulfate and powdered lead, the following reaction will proceed to produce tribasic lead sulfate as indicated below:

This is also the case for monobasic lead sulfate (PbO-PbS04) instead of lead sulfate.

The battery reaction of the tribasic lead sulfate as shown in the formula (2) exhibited the same performance as that ofthe tribasic lead sulfate as indicated in the formula (1). In addition, it is also possible to constitute the electrodes of this invention by adding tribasic lead sulfate (3PbO-PbSO4-nH2O) to lead oxide powder.

In this invention, powdered lead is incorporated and mixed with at least one lead sulfate salt and the resulting mixture is then applied uniformly to the whole surface of lead alloy-made grids thereby to start the formation sulfate of tribasic lead, or, if desired, a water-impregnated cloth is applied to the mixtureapplied surface of the grids pressed against the grids by the use of an ordinary press to allowthewaterto permeate into the mixture wherebythe formation of tribasic lead sulfate soon begins and ends in 2-3 hours with the resultthatthe lead sulfate is completely extinguished and tribasic lead sulfate is instead produced.

The methods of pressing the water-impregnated cloth are not particularly limited, but include those using a press or rollers. It is preferable at the time of pressing to have powdered lead impregnated with moisture such as water or a dilute aqueous solution of sulfuric acid. The electrodes can be prepared without the use of water, however, those prepared by said water-impregnation method have a solider structure.

As an example of a method of impregnating water, a cloth preliminarily impregnated with water is placed on powdered lead uniformly distributed along the grid and is then pressed.

Another example of a method of water impregnation is as follows: When the pressing is to be carried out by the use of rollers or other rotating bodies, these rotating bodies are preliminarily bored so that they are provided on the surface with many little holes, are wound on the perforated surface with a cloth-like material, permit waterto pass through the inside thereof and then pressed against the powdered lead of agrid for an electrode whereby such secure attachment and water impregnation are carried out at the same time. The pressure for pressing is not particularly limited.

In a case where water is used as desired in this invention, it is preferable that the amount of water impregnated be in the range of for example 5-25%, preferably about 10-20%, by weight of powdered lead used. The relationship between water contents and cracking was investigated with the result that electrodes prepared by the conventional paste method cracked on the surface even when they had a water content of as low as 10-15 wt. %, whereas the electrodes ofthis invention did not crack on the surface even when they had a water content of over 20 wt. %. The reason forthis would be that owing to the pressing, the powdered lead particles reach regions where they can fully interact, and the water permeating in the gaps between the particles easily evaporates without changing the disposition of the particles.In addition, the water impregnated into the electrodes promotes in the presence of oxygen in the air oxidation ofthe metallic lead ingredient in the powdered lead whereby the solidification ofthe electrodes is advantageously further promoted. It is not necessary to add waterto the active material mixture of the electrodes to an extent that the water content exceeds 25 wt. % since the electrodes having such a water content are in the wetted state, whereas it is preferable thatthewater content in the electrodes should be at least about 5 wt. % to maintain promotion ofthe reaction. However, these upper and lower limits maybe changed depending on the status of the electrodes.It is advantageous to add water to the electrodes since not only the water itself promotes the oxidation but also the oxidation heat generated by the promotion ofthe oxidation raises the temperature of the electrodes whereby the oxidation reaction is more vigorous and the water easily evaporates from the electrodesthus accelerating reduction ofthe excess water. Such being the case, the amount of water impregnated in the electrodes is reduced to less than 7-8wt. %,which is less than a half ofthe original water content, in only 1-2 hours when the electrodes are left in a room and it is further reduced to 3-5 wt. %, which is an equilibrium moisture in the room (almost dry state) in 4-5 hours when the electrodes are still left in the room.

Since, in this invention, the electrodes wetted with water bythe pressing are almost dried without cracking when left in a room as mentioned above,the thus dried electrodes are easy to handle. Therefore, this invention does not need a conventional step of aging and drying in a highly moist atmosphere, the step having heretofore been necessary, and it dispenseswith a large-scaled equipmentforthe step and the energy necessaryfor operating the equipment.

It is very advantageous in the electrodes of this invention thattribasic lead sulfate is promptly produced by impregnating water under pressure into powdered lead plus at least one lead sulfate salt. This is also very advantageous from the view-point of production techniques in the production of lead accumulators since the residence time of the electrodes in the process for producing lead accumulators is much shortened, this resulting in the reduction of production cost ofthe accumulators.

As is seen from the foregoing, this invention enables the formation of lead accumulators to start soon afterthe end of the aforesaid simple step which has never been thought of. Thus, the electrodes of this invention are very suitable for use in lead accumulators.

This invention will be better understood by the following Examples and Comparative Examples with reference to the accompanying drawings.

Intheaccompanying drawings, Fig. 1 indicates graphs showing changes, with the lapse of time, in amount of each compound in the active material mixture,the amount being expressed in terms of the height of peaks obtained by X-ray diffraction (target: copper pole); Fig.2indicatesascanning-typeelectro- microscopic photograph (x 8,500) of tri basic lead sulfate in the flower-like crystal form produced in the electrode of Example 1; and Fig. 3 is a schematic view showing the sequence of steps for forming an electrode of this invention.

Example 1 Eighty (80) grams of powdered lead obtained by an ordinary ball-mill method were incorporated with 20 g of powdered lead sulfate and then thoroughly mixed together. With reference to Fig.3,16 of the resulting mixture were weighed out and spread uniformly over a lead alloy-made grid (content of Sb, 2.5 wt. %; 4.5 cm long x 5.8 cm wide x 0.2 cm thick).

Afiltercloth (madeofVinylon; 6cm long x 7cm wide x 0.13 cm thick) was placed on a steel-made plate (12 cm long x 15 cm wide x 1.0 cm thick) and a vinyl resin sheet (0.01 cm thick), which was somewhat largerthan said filter cloth, was then placed on the filter cloth. A half amount, 8 g, of the thus obtained lead/lead sulfate mixturewas spread on a portion of the vinyl resin sheet surface, the portion being of the same area as the surface of said grid. The grid was placed on the mixture spread on the vinyl resin sheet.

The remaining half (8 g) ofthe mixture was spread uniformly on the grid andthesameVinylon-made filter cloth (6 cm long x 7 cm wide x 0.13 cm thick) impregnated with water was then placed on the mixture spread on the grid, after which a vinyl resin sheet (0.01 cm thick) was placed on this filter cloth. A steel-madeplate(12cm long x 15 cm wide x 1.0cm thick) was then placed on this vinyl resin sheet. All the materials so piled up were pressed together at a pressure of 15kg/cm2 by a press to form an electrode.

The pressing pressure may be suitably adjusted depending on the condition ofthe grid and it is preferably in the range of about 5-20 kg/cm2. A pressure of as high as 50-100 kg/cm2 may still be used, but the use ofthe necessary minimum pressure is recommended from the view-point of battery performance. The resulting electrode was withdrawn after release of the pressing and then left in a room for a short time (1-2 hours), during which the formation of tribasic lead sulfate (3PbO-PbSO4),theoxidation of the lead ingredient in the powdered lead and the evaporation of water from the active material mixture did proceed. The water content in the mixture just after the end of the pressing was 16 wt. % based on the weight of powdered lead used, decreased to 8 wt. % in 2 hours and further decreased to 5wt. % in 4 hours.

Even at this poi nt, the resulting electrode did notcause cracks and fissures on any one of the sides. It was revealed that no cracks and fissures were produced on any one of the sides of the electrode even when the electrode was left for a long time (one month).

Table 1 indicates the water contents ofthe electrode obtained from the weight losses while left under the ambient condition. The results show that no cracks were incurred in the electrode during the moderate drying operation.

Table 1

Time lapse Water content Cracks in after pressingg (!.) electrode 5 min. 16. O None 1 hr 10.0 None 2 hr 8.0 None 4 he 5.0 None 24 hr 4.8 None It is seen from Table 1 thatthewatercontent reached an equilibrium in about4 hours and no cracks were produced in at least 24 hours.

Fig. 1 illustrates the curves showing changes in amount of each of the compounds, appearing and disappearing in the active material mixture, ofthe above electrode. The changes here are expressed in terms of intensities of X-ray diffraction peaks observed with copper target.

In Fig. 1, the axis of ordinates indicates the height (cm) of the maximum peak in X-ray analysis charts and the axis of abscissas indicates time lapse (minute) after water impregnation and pressing.

It is clear from Fig. 1 that in the active material mixture of the electrode ofthis invention, the formation of tribasic lead sulfate is almost completed in 30-40 minutes afterthe water impregnation by the pressing, whereas not onlythe lead sulfate but also the red litharge and metallic lead (which are the original components of the active material mixture) decrease.

Fig. 2 in the accompanying drawings indicates a scanning-type electromicroscopic photograph (x 8,500) of said tribasic lead sulfate. In the electromic roscopic photograph,thetribasic lead sulfate shows its flower-like crystals. The formation of such flowerlike crystals have not been found in the conventional pasty active material. The formation of such crystals according to this invention is considered to be conducive to the improvement of porosity of the active material mixture.

According to this invention, the active material mixture-applied electrodes may be subjected directly to the next formation treatment without being previously subjected to aging and drying steps.

Exam pies 2-5 and Comparative Example 1 Based on the procedure of Example, in Examples 2-5 powdered lead obtained by an ordinary ball-mill method was incorporated respectively with 10,20,30 and 40% byweight of lead sulfate powder to obtain active material mixtures. The lead sulfate powder used was of JIS first grade.

Sixteen (16) grams of each ofthethus obtained mixtures were applied uniformly on both the sides of a grid (Sb 2.5 wt. %; cm long x 5.8 cm), after which a water-impregnated filtering cloth was placed onto the grid and then pressed at a pressure of 15 kg/cm2to obtain a green electrode. The water content in the active material mixture of such an electrode varies depending on thethickness, area andwatercontent ofthefiltering cloth used.In these Examples, there were used Vinyon-madefiltering cloths (No.5026, produced by Shikishima Canvas Co.), cm long x 7 cm wide x 1.3 mm thick, fully impregnated with water, thereby to get the active material mixtures to have a water content of about 16 wt. %. The electrodes so obtained were leftfor3-4 hours in a room and then, without being subjected to the conventional aging and drying steps, directly sub jected to formation step.

In Comparative Example 1, to 16 g of the same lead oxide powder as used in Examples 2-5 were incorporated with an aliquot of 2.3 ml of dilute sulfuric acid Of 1.2 sp. gr., and then a quantity of 0.5 ml of water. The mixture was kneaded at 250C for 0.5 hours to obtain a pasty active material which was applied to the same grid as used in the Examples, aged at 500C and a R.K.

of 90% for24 hours, allowed to stand for 24 hours and then dried at 500C for 24 hou rs to obtain an electrode.

The thus obtained electrode was then treated in the same manner as in the Examples.

One positive electrode was inserted in between two negative electrodeswith separators in a plastic casetoform atest-batteryunitandsubsequently underwent a forming procedure with a forming acid of 1.1 sp. gr. H2SO4, using a forming constant current of0.2Afor48 hours. Dilute sulfuric acid having a specific gravity of 1.28 was poured into the plastics case to obtain a lead accumulator, afterwhich charge/discharge cycle tests were conducted under the conditionsthatthe charge in one cycle was carried out at a currentof0.2Afor 14 hours and the discharge was carried out at a current of 0.4A until a cut-off voltage of 1.7 Vwas indicated, to measure the duration of discharge.There was a pause of one hour between the end of charge and the start of discharge.

Table 2 indicates the cumulative duration of discharge ofthe electrodes of this invention in comparison with that of the electrode prepared bythe conventional paste method, the measurement of these cumulative discharge durations having been continued until the 30th cycle/-discharge cycle was completed.

Table 2

Amount of Cumulative Electrode lead sulfate duration of added discharge of electrode Example 2 Electrode (A) 10 at. 90.4 hr Example 3 Electrode (B) 20 wt. 117.2 hr Example 4 Electrode (C) 30 lit 2 128.0 he Example 5 Electrode (D1 40 Vt 129.0 he Eomp.Ex.l Electrode (E) 15 at. 107.0 he It is seen from Table 2 thatthe electrodes (B), (C) and (D) respectively of Examples (3), (4) and (5) were all improved in battery performance as compared with the electrode (E) of Comparative Example 1. The reason why the electrode (A) of Example 2 exhibited a shorter duration of discharge than the electrode (E) of Comparative Example 1, is considered to be that the porosity of the active material mixture of the former was insufficient since the amount of lead sulfate incorporated in the electrode (A) was smallerthan that in the electrode (E). The electrode (Dl of Example 4 exhibited a slightly increased duration of discharge as compared with the electrode (C) of Example Sin spite of the factthatthe amount of lead sulfate contained in (D) is largerthan the one in (C). Thus, the effect of lead sulfate contained in the active material mixture will not appreciably increase when the lead sulfate content exceeds about 30 wt. %.

Example 6 Powdered lead obtained by an ordinary ball-mill method was incorporated with a monobasic lead sulfate salt (PbO-PbSO4) in an amount by weight of 45%, based on the weight of the powdered lead, to obtain a mixture containing lead sulfate (PbSO4) in an amount by weight of 25% of the mixture. The thus obtained mixture was treated in the same manner as in Example 1 to obtain an electrode (F) which was then subjected to the same charge/discharge cycle test as in Examples 2-5 with the result being as shown in Table 3.

Table 3

Lead sulfate Cumulative Electrode (monobasic doration of lead solfate discharge of salt) electrode Example 6 Electrode (F) 25 at. as 120.0 hr Pub504 (45 at. as pbO~PbSOq) Comp Ex l Electrode (E) Lead sulfate 107.0 hr 15 at.

It isseen from the above Table that the Electrode (F) exhibited approximately the same battery performance and also exhibited excellent battery performance as compared with the comparative electrode (E).

Claims (7)

1. Electrodes for lead accumulators, prepared by mixing powdered lead for use as an active material in lead accumulators with at least one lead sulfate salt to obtain an active material mixture and then having the thus obtained mixture held on grids without need of previous kneading, aging and drying of the mixture, thereby to obtain electrodes.

2. Electrodes for lead accumulators according to claim 1, further characterized in that the active material mixture is incorporated with water when it is held on the grids.

3. Electrodes for lead accumulators according to claim 1 or2, wherein the at least one lead sulfate salt is selected from lead sulfate, monobasic lead sulfate, tribasic lead sulfate and tetrabasic lead sulfate.

4. Electrodes for lead accumulators according to claim 1,2 or 3, whereinthe lead oxide powder is incorporated with the at least one lead sulfate salt in an amount by weight of 5-50% of the former.

5. Electrodes for lead accumulators according to claim 1,2 or 3, wherein the lead oxide powder is incorporated with the at least one lead sulfate salt in an amount by weight of 5-40% ofthe former.

6. An electrode for a lead accumulator, substan tially as described in any one ofthe foregoing Examples 1 to 6.

7. Aleadaccumulatorwhich incorporates an electrode as claimed in any one of the preceding claims.