GB2238160A - Lead accumulator - Google Patents

Abstract

A lead accumulator comprises a grid substrate formed of a Pb-Sb alloy containing one of Na, Li and K and Sn, and an electrolyte containing alkaline earth metal ions. As the grid substrate, use may be made of a lead alloy containing Ca and Sn or Ca, Sn and Sb or Ca, Sn, Sb and Al. Grids substrates obtained from a plate piece formed by making a Pb-Ca base alloy plate integral with a Pb-Sn base alloy plate may be used in combination with an electrolyte containing alkaline metal ions, alkaline earth metal ions or phosphoric acid ions. An electrode obtained by plating a Sb-free Pb-Ca alloy substrate with Sn or a Pb-Sn alloy may be used in combination with an electrolyte containing at least one of alkaline metal ions and alkaline earth metal ions. The electrode may further be immersed in dilute sulfuric acid for the achievement of more improved effects. The electrolyte may contain alkaline metal ions in a concentration of at least 500 ppm and, at the same time, ions of an alkaline earth metal having no water and/or crystallized water.

Description

LEAD ACcUMULToRs The present invention is concerned with improvements in or relating to a lead accumniator.

Upon being allowed to stand for an extended period or in an over-discharged state lead accumulators are so frequently unchargeable due to selfdischarging that they are prematurely unserviceable. To solve this problem, various attempts have been made in terms of grid alloys for lead accnmulators, In one effort, the Sb content of grids has been decreased to reduce selfdischarging or, for the same purpose, Pb-Ca base alloys have been used as antimony-free alloys.In another effort, the grids have been coated on their surfaces with a conductor with a view to prevent the formation of PW. or lead sulfate on the interface of positive grids and associated active substances, which is responsible for a lowering of the performance of lead accumulators upon being allowed to stand in an over-charged state.

In a further effort, an electrically conductive resin having a pellobskite base compound or Au, Ag or Pt powders dispersed therein has been added to the active substances in order to permit condnction to be maintained, even though such a highly resistive material is formed. In a still further effort, phosphoric acid or alkaline metal ions have been incorporated into an electrolyte for the same purpose.

The grid alloys now put to practical use include Pb-Sb and Pb Ca base alloys. However, the Sb base alloys are disadvantageous in that a hydrogen-generating potential of Sb is too noble to reduce selfdischarging, and a water electrolysis voltage is so low that the amount of a water reduction is increased. The Pb-Ca alloys, on the other hand, have a limited or reduced service life at the time when used in deep charge/discharge cycles, and need a special load circuit for the purpose of preventing over-discharging. At a reduced content of Sb, say, 3.5 Z by weight or less, such alloys have a demerit similar to that of Sb-free alloys. Thus, the problems as mentioned above cannot substantially be solved by binary grid alloy compositions such as Pb-Ca and Pb-Sb. With this in mind, ternary alloys of Pb-Sn As and Pb-Ca-Sn have been proposed in recent years.These alloys show limited selfdischarging, are comparable to the Pb-Ca alloys in terms of water electrolysis and exhibit good properties even upon being allowed to stand in an over-discharged state. In the case of the Pb Ca-Sn alloys, however, considerable elongation of grid collectors per se is caused by intergranular corrosion at a Ca concentration of 0.09 Z by weight or higher (see Figure 1), thereby making it impossible to hold their associated active substances. When such an alloy is used for a positive grid, it comes into contact with a strap on the negative electrode side, giving rise to a short-circuit or, in extreme cases, a failure of an accumulator cell. As a result, the accumulator becomes prematurely unserviceable.Further, these alloys are inferior to Pb-Ag alloys in terms of corrosion resistance, so that difficulty is involved in holding active substances. Still further, an increase in the content of Sn leads to a rise in the cost.

Referring on the other hand to the concentration of sulfuric acid forming an electrolyte, it is considerably decreased in an overdischarged state with the result that the conductivity of the electrolyte is considerably decreased so that the accumulator becomes unchargeable. Thus, problmes arise in connection with not only the grid alloys but also the conductivity of electrolytes to be maintained. In this respect, it has been proposed to add HOP04 into electrolytes in the prior art. However, although H3PO, improves the chargeability of lead accumulators during over-discharging, it rather gives rise to a lowering of discharge capacity at the initial stage of service life and increased self-discharging.

SUMMARY OF THE INVENTION An object of the present invention is to provide a lead accumulator which satisfactorily solves the problems as mentioned above or improves the performance and properties of conventional lead accumulators upon being permitted to stand in an over-discharged state.

According to one aspect of the present invention, this object is achieved by the provision of a lead accumulator which uses a grid substrate formed of a Pb-Sb alloy containing any one of Na, Li and K and Sn, and in which an electrolyte contains alkaline earth metal ions.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the present invention may be more readily understood, rererence will now be made to the azoomnening drawings, in which: Figure 1 is a view illustrating the relationship between the concentration of Ca and the rate of grid elongation in a trickle life test of a lead accuuulator of 1.2 Ah-6 V using a Pb-Ca alloy, Figure 2 is a graphical view showing the results of a cycle life test carried out with lead accumulators using grids obtained from a Pb-Na-Sb-Sn alloy according to the first aspect of the present invention and a conventional Pb-Ca-Sn alloy, Figure 3 is a comparative view illustrating the recovery performance of capacities of lead accumulators after permitted to stand in an over-discharged state, said accumulators using grids obtained from a Pb-Na-Sb-Sn alloy according to the first aspect of the present invention and a conventional Pb-Ca-Sn alloy, Figure 4 is a comparative view illustrating the recovery of capacities of lead accumulators according to the second aspect of the present invention, Figure 5 is a graphical view illustrating the chargeability of lead accumulators after permitted to stand in an over-discharged state for the purpose of comparison, Figure 6 is a graphical view illustrating the relationship between the days elapsed and the remaining capacities, Figure 7 illustrates the chargeability of lead accumulators after permitted to stand in an over-discharged state,which are affected by plating and acid immersion treatments and the addition of Na' and Mug2', hatched regions showing lead accumulators which are not treated with an acid, and Figure 8 is a view illustrating the relationship between the days elapsed and the remaining capacities.

Description of the Aspects of the Invention According to a first aspect of the present invention, there is provided a lead accumulator which uses a grid substrate formed of a Pb-Sb alloy containing any one of Na, Li and K and Sn, and in which an electrolyte contains alkaline earth metal ions.

According to a second aspect of the present invention, there is provided a lead accumulator which uses a grid substrate comprising a lead alloy containing Ca and Sn or Ca, Sn and Sb or Ca, Sn, Sb and AQ, and in which an electrolyte contains alkaline earth metal ions.

According to a third aspect of the present invention, there is provided a lead accumulator in which a grid substrate is formed of a plate piece obtained by making a Pb-Ca base alloy plate integral with a Pb-Sn base alloy plate, and an electrolyte contains alkaline metal ions, alklaine earth metal ions or phosphoric acid ions.

According to a fourth aspect of the present invention, there is provided a lead accumulator which uses an electrode obtained by plating the surface of a Pb-Ca alloy electrode containing no Sb with Sn or an Pb-Sn alloy, and in which an electrolyte contains at least one of alkaline metal ions and alkaline earth metal ions.

According to a fifth aspect of the present invention, there is provided a lead accumulator in which an electrolyte contains alkaline earth metal ions, and which includes a positive grid obtained by forming an unformed, active substance-filled grid using a grid member obtained from a Ca and Sn-containing lead alloy free from antimony and immersing said grid in dilute sulfuric acid.

According to a sixth aspect of the present invention, there is provided a lead accumulator in which an electrolyte contains 500 ppm or higher of alkaline metal ions and, at the same time, ions of an alkaline earth metal having no water and/or crystallized water.

According to a seventh aspect of the present invention, there is a provided a lead accumulator in which an electrolyte contains alkaline metal ions and/or alkaline earth metal ions, and which includes a positive grid obtained by plating the surface of a collector with a Pb-Sn alloy and forming a grid using said collector, followed by its immersion in dilute sulfuric acid.

DETAILED DESCRIPTION OF THE INVENTION Upon being allowed to stand for an extended period without charging, lead accumulators become unchargeable, or upon being left alone after deep charging, they again become unchargeable. This is caused by a considerable increase in the internal resistance of positive grids probably due to the following factors. Upon overdischarged, the specific gravity of electrolytes decreases, and its decrease is more considerable in the vicinity of the grids in the grid sbustrates than on the surfaces thereof. In this case, the solubility of Pb in the grids increases to yield Pb+.When Pb@ is formed, PbO, that is an active substance in the vicinity of the positive grids becomes unstable at an increased pH level, so that a local cell reaction, as represented by the scheme: PbO2 + Pb + ZH2SO, -+ 2PbSO, + 2H2O, occurs, thus yielding PbSO. At the same time, PESO, undergoes repeated dissolution and precipitation at an increased pH level for crystal growth, so that the grid interface is coated with non-reducing PbSO4 Further, as H2SO, is consumed by the aforesaid local cell reaction, the positive grid potential decreases to a base level of -400 to -200 mV (vs. Hg/HgSO4). At this potential, another local cell reactions, as expressed in terms of the scheme:: PbO2 (active substance) + H20 + 2e b PbO(PbOx) + 20H- and Pb (grid) + SO,2 - PESO, + 2e-, occur synergistically (E. = - 370 mv vs. Hg/Hg2SO4) so that a highly resistive film is formed by the growth of PbSO; and the formation of PbOx, thus making charging impossible. For that reason, there may be available a compound which makes it difficult to form such a resistive film or make conduction between Pb (grid) and PbO2 (active substance), even though it is formed. As already pointed out, Sn is effective for the properties of lead accomiators upon being allowed to stand in an over-discharged state.

According to the first aspect of the present invention, Sn is incorporated as a grid alloy eleient into a Pb-Na-Sb alloy. Although the effect of Sn is still ucclarified, improved chargeability is probably achieved for the following reasons. In the Pb matrix, there say be formed an intermetallic compound such as CaSO4 or PbxcaySnz which, when the grid is anodically oxidized, is dispersed throughout the resulting oxide film to maintain conductivity. Alternatively, Sn may be oxidized to form a compound such as SnO or SnO2 having semiconductive properties (n-type semiconductor).Sb that acts as another grid alloy element dissolves into the oxide film in the form of Sb or Sb203, which then destroy the highly resistive film and prevent the lead accumulator from becoming unchargeable. Further, Sb also serves to reduce an increase in the internal resistance, when the accumulator is allowed to stand in an overcharged state. However, since increased and decreased amounts of Sn bring about a considerable decrease in the consumption of the electrolyte and a lowering of the strength of the grid, respectively, Na is used for keeping the grid strength in an amount which allows Sb to be effective for overdischarging and the amount of the electrolyte to be not decreased.

Referring to the additives in the electrolyte, on the other hand, an oxidation reaction occurs on the positive electrode side during charging, so that electrons flow toward the negative electrode and electron charges are carried from the negative elelctrode by Ht and SO2. in the electrolyte. After over-discharging, however, the pH is in the vicinity of 7, so that H20 increases, while SO2. and Ht decrease. Thus, cations of SO2. salts should be selected as the charge carrier, since most anions such as halogen ions or NO 2- react with Pb.In particular, such cations should be selected from those forming sulfates, in view of the influences upon other properties of the accumulator (inclusive of service life, capacity, etc.). Table 1 shows the ion conductivities at 25w: of various sulfates and a phosphate, of which the sulfates of alkaline earth metals are preferred and advantageously used. The alkaline earth metals are more inexpensive than the alkaline metals.

Table 1 (25 C)

Conductivities (#/cm) Alkaline | Na,S04 @ 0.005 Na,SiO 0.006 Metals K,SO 0.006 Alkaline CaSO4 0.002 (Concentration : 0.02 M) Earth Metals VgSOs I 0.004 l CuSOs 0.003 Other La2(SO*)s 0.001 Elements ZnSO4 0.003 H3PO4 0.006 According to the second aspect of the present inventions Sn is incorporated as a grid substrate alloy element into a Pb-Ca base alloy.More specifically, the grid substrate to be used comprises any one of Pb-Ca-Sn, Pb-Ca-Sn-Sb and Pb-Ca-Sn-Sb-AQ alloys which limits the consumption of an electrolyte, shows limited self-discharging and has improved properties upon being allowed to stand in an overdischarged state, and the electrolyte to be used contains alkaline earth metal ions for the purpose of maintaining its conductivity.

In this aspect, Sn and Sb act in a similar manner as explained in connection with the first aspect of the present invention.

If the amounts (in X by weight) of Ca and Sb are increased in the Pb-Ca-Sb base alloys, then Ca and Sb forms a dross compound in the form of Ca2Sb, incapable of forming a solid solution. In this case, however, the presence of AQ causes Ca to be fixed into the alloy without forming any dross compound. This is why Al is added to that alloy.

The additives for electrolytes mentioned in connection with the first aspect of the present invention are again used in the second aspect of the present invention.

According to the third aspect of the present invention, there is provided a lead accumulator in which an electrode substrate is formed of a plate piece obtained by making a Pb-Ca base alloy plate integral with a Pb-Sn base alloy plate, and an electrolyte contains alkaline metal ions, alklaine earth metal ions or phophoric acid ions.

In this aspect, Sn acts in a similar manner as explained in connection with the first aspect of the present invention, and the additives used for electrolytes are identical with those used in the first aspect of the present invention.

According to the fourth aspect of the present invention, an electrode comprises a Pb-Ca alloy which contains no Sb and is plated on its surface with Sn or a Pb-Sn alloy, and an electrolyte contains at least one of alkaline metal ions and alkaline earth metal ions.

A lowering of the chargeability of a lead accumulator occurring upon being allowed to stand in an over-discharged state seems to be due to an insulating PESO, film being formed on the interface of a positive grid and an active substance. However, since chargeability is improved by an increase in the amount of Sn contained in the grid alloy, it is likely that Sn serves to maintain conductivity between the grid and the active substance. If the concentration of Sn on the grid interface is increased by plating with Sn or a Pb-Sn alloy, then the properties of the accumulator upon allowed to stand in an overdischarged state can be improved.When the lead accumulator is permitted to stand in an over-discharged state, on the other hand, the concentration of sulfuric acid in the electrolyte is so decreased that the conductivity of the electrolyte is decreased; this being one factor for a lowering of chargeability. However, the addition of alkaline metal ions or alkaline earth metal ions to the electrolyte causes an increase in the conductivity of the electrolyte, which assures improved flowing of charging currents.

Any appreciable effect is not obtained by sole application of a Sn plating or a Pb-Sn alloy plating or alkaline (earth) metal ions, since they act separately upon individual causes for a lowering of the chargeability of lead accumulators upon allowed to stand in an overdischarged state. However, if the Sn (or a Pb-Sn alloy) plating is used in cooperation with alkaline (earth) metal ions, then extremely improved effects are obtained through synergism.

According to the fifth aspect of the present invention, there is provided a lead accumulator in which an electrolyte contains alkaline earth metal ions, and which includes a positive grid obtained by preparing a grid substrate from a Ca and Sn-containing lead alloy free from antimony, filling an active substance in the grid substrate and forming the grid substrate, followed by its immersion in dilute sulfuric acid.

In this aspect, Sn acts in a similar manner as explained in connection with the first aspect of the present invention.

After a lead accuuulator is permitted to stand in an overdischarged stage, H2504 in its electrolyte is converted to H,O and PbSO*. As a result, charging current-carrying ions are extremely reduced, giving rise to a considerable lowering of the conductivity of the electrolyte. Required in this case are ions giving no damage to the service life, capacity and performance of the accumulator.

Alkaline metal ions are said to be effective for this purpose.

However, alkaline earth metal ions. more inexpensive than alkaline metal ions are comparable thereto in terms of electrical conductivity and effect, as set forth in Table 1. In place of the alkaline earth metal ions, the corresponding hydrates may also be used, as is the case with xagnesina sulfate. In this connection, it is noted that the alkaline petal ion and alkaline earth metal ion, if used simultaneously, cooperate synergistically with each other to improve conductivity, except for in a high concentration region in which the ions do not interact.

In order to prevent PESO, from being formed on the interface of a grid and an active substance during over-discharging, the positive grid after forming may be immersed in dilute sulfuric acid regulated to the given concentration and temperature, whereby the positive grid is maintained at a certain potential and an alpha-PbO2 film is formed on the interface of the grid and the active substance to keep conductivity therebetween. This is due to the presence of alpha-PbO2 which is more inert with respect to a discharging reaction and more intimate than beta-PbO2. In other words, alpha-PbO2 is unlikely to be converted to PbO, through a local reaction, and suppresses inward diffusion of SO42. Thus, the formation of PESO, in the interface film is prevented.Alpha-PbO2 is stable and electrically conductive during charging, and thus functions as a conductor.

Any noticeable effect is not obtained by sole application of such three means. However, if they are applied in combination, then much more improved effects are obtained, since they act upon improvements in the properties of lead accumulators upon permitted to stant in an over-discharged state through difference mechanisms.

According to the sixth aspect of the present invention, there is provided a lead accumulator in which an electrolyte contains alkaline metal ions in a concentration of 500 ppm or more and, at the same time, ions of an anyhydrous alkaline earth metal and/or ions of an alkaline earth metal having crystallized water.

As already mentioned, lead accumulators frequently become unchargeable upon allowed to stand after deep discharging. This is because a non-reactive PESO, film is formed on the interface of a grid and an active substance, giving rise to an increase in the internal resistance of the accumulators. In particular, the internal resistance of the positive grid is increased. This is due to the fact that PESO, is formed on the interface of the grid and the active substance as a result of local cell reactions between Pb, PbO2 and H2S04 occurring on that interface. When a lead accumulator is allowed to stand in an over-discharged state, the specific gravity of the electrolyte is so decreased that the resistance of the electrolyte is increased.This is responsible for a lowering of the properties of the accumulator after allowed to stand in an over-discharged state.

After allowed to stand in an over-discharged state, the specific gravity of the electrolyte drops and approaches nearly to that of water. For that reason, the resistance of the electrolyte is so increased that difficulty is involved in flowing of a charging current. In order to increase the conductivity of the electrolyte and a charging current flowing there through, alkaline metal ions may be added to the electrolyte.

When magnesium sulfate is added to the electrolyte, on the other hand, there is an increase in the solubility of lead sulfate.

For that reason, the passivated lead sulfate formed on the grid interface is dissolved to recover conduction between the grid and the active substance, thus facilitating flowing of a charging current.

Any noticeable effect on the recovery of chargeability is not obtained by sole application Qf such two means. However, if the two means are used in combination, then the chargeability of the lead accumulator after being allowed to stand in an over-discharged state can be largely improved. Alkaline metal ions are substantially ineffective in a concentration of below 500 ppm, and should thus be used in a concentration of at least 500 ppm to achieve the desired effect.

The seventh asepct of the present invention is a combination of the fourth aspect with the fifth asepct of the present invention.

The present invention will now be explained with reference to the following non-restrictive exaaples.

EXAMPLES Examples of the First Aspect Lead accumulators of 10 Ah-2 V were prepared from Pb-Na-Sb-Sn and Pb-Ca-Sn alloys, and were then subjected at 300C to a cycle test of 2A discharge (with a depth of 100 Z) at a constant voltage of 2.55 V (2A restriction). The results are shown in Figure 2, from which it is found that the invented accumulator A based on Pb-Na-Sb-Sn is superior to the conventional accumulator B based on Pb-Ca-Sn in terms of cycle properties. Prepared from the aforesaid two alloys and an electrolyte containing a predetermined amount of MgSOs were accumulator cells of 1.2 Ah-6V, which were then discharged at 8 ohms for 24 hours, allowed to stand at 250C for one month, and subjected to constant-voltage charge at 2.45 V/cell for 24 hours.At that time, the 5H- R capacities were compared with each other before and after over-discharging. The ratios of capacities recovered with respect to the initial capacities are illustrated in Figure 3, from which it is noted that the Pb-Na-Sb-Sn base cell is improved over the Pb-Ca-Sn base cell in terms of recovery properties. It is also found that by the addition of MgSO*, the recovery properties of the Pb-Ca-Sn base cell shows a 15 Z increase, while the recovery properties of the Pb Na-Sb-Sn base cell exhibits a 20 Z increase. This means that Mg50, and the alloy composition interact synergistically with each other.

It is to be understood that the collector to be used may be not only in the grid form but also in the plate, expanded, blanked or rolledplate form.

Examples of the Second Aspect Grids were prepared from a Pb-Ca alloy containing Sn and Sb or AQ, and were used with an electrolyte containing a predetermined concentration (0.1 mole/Q) of MgSOs as an alkaline earth metal to make accumulator cells of 1.2 Ah-6V, which were then discharged at a constant resistance of 8 ohms, open-circuited after the lapse of 24 hours, allowed to stand at 25x: for one month and recovered by constant-voltage charge at 2.45 V to measure the ratios of the capacities recovered with respect to the initial five-hour capacities.

For the purpose of comparison, a Pb-Ca alloy and an electrolyte containing no MgSOs were used to make comparative cells. The test results are illustrated in Figure 4, from which it is found that the Pb-Ca, Pb-Ca-Sn, Pb-Ca-Sn-Sb and Pb-Ca-Sn-Sb-AQ base cells are all poor in the recovery of their capacities, when the electrolytes used therewith contain no MgSO*. This is considered to be due to the synergism of the effects of Sn, Sb and MgSO*, which restrains a high resistor from being formed at the time when the cells are allowed to stand in an over-discharged state.

Examples of the Third Aspect A 0.5 mm-thick alloy sheet of Pb containing 2.5 weight Z of Sn was laminated on and rolled with an alloy sheet of Pb containing 0.06 weight Z of Ca and 0.3 weight Z of Sn. Prepared from the thus obtained laminate was an electrode substrate of 3 mm in thickness by punching, which was then filled with a paste, aged and dried to obtain a positive grid. A lead accuinlator was assembled by forming to make an accumulator cell of 4 V-4 Ah.The thus obtained cells were then filled with electrolytes containing a regulated concentration (0.1 mole/Q) of Na,SO,, MgSOr and H3PO4. The thus prepared lead accumclator cells were discharged at 1.7 ohms for 24 hours, allowed to stand at 25'C for 6 months, and were thereafter charged at a constant voltage of 2.45 V/cell (1.2A cut) for 24 hours to measure the rocovery of their capacities with resepct to their initial capacities. The results are set forth in Table 2.

Table 2

Cell i Compositions and Recovery of Capacities I Nos. Configurations of Additives Grides (%) 1 Pb-0.006Ca-1.5Sn No Additive Unchargeable (Cast) 2;; fl Na2SO4 j 87.4 3 " MgSO4 86.2 4 " H3PO4 85.7 Prior 5 " Na2SO4 91.8 MgSO4 Art 6 " Na2SO4 89.6 I I I 7 MgSO4 i 88.5 H3PO4 t 8 I Pb-0.06Ca-0.35n Pb-35n(Punched No Additives 17.6 Rolled Sheet) 9 " Na2SO4 93.8 10 " MgSO4 93.3 11 " H3PO4 71.7 12 " Na2SO4 100.4 Invention MgSO4 13 " Na2SO4 98.8 H3PO4 14 MgSOs 99.3 H3PO4 From a comparison of Cell No. 1 to No. 7 with Cell No. 8 to No.

14, it is found that the grids obtained from the rolled laminates are improved over those from castings in terms of recovery propoerties.

This seems to be due to the fact that the content of Sn on the surfaces of the electrode substrates made by rolling is 1.5 Z or more than that made by castings. Of Cell Nos. 8 to 14, Cell Nos. 9, 10 and 11 are more improved in terms of charge recovery because of the presence of alkaline metal ions, alkaline earth metal ions and phosphoric acid ions. From this, it is found that Sn in the electrode substrates becomes more effective in the presence of the aforesaid additives. Much more increased effects are obtained with Cell Nos.

12, 13 and 14 containing all the possible combinations of these additives than with Cell Nos. 9, 10 and 11. From the foregoing, it is preferred in view of improvements in the properties of the accumulator cells at the time when they are permitted to stand in an overdischarged state that the rolled laminates of Pb-Ca-Sn and Pb-Sn having an increased Sn content on their surfaces be used as electrode substrates in combination with electrolytes containing alkaline metal ions, alkaline earth metal ions and phosphoric acid ions, which interact synergistically with Sn.

Examples of the Fourth Aspect Grids of an Sb-free Pb-Ca alloy plated on its surface with Sn was used as positive grids to prepare accumulators of 1.2 Ah-2 V, which were then filled with an electrolyte containing Nae and Nag24 and an electrolyte to which no Nat and Mg2t were added. The thus obtained accuitilator cells were discharged at a constant resistance for 24 hours, open-circuited, and were thereafter permitted to stand for one month. For the purpose of comparison, accumulator cells were prepared with grids which were not plated with Sn, and were tested under similar conditions as mentioned above. These accumulator cells were then charged at a constant voltage of 2.45 V to measure the charging currents after 10 seconds, 30 seconds and 60 seconds elapsed. The results are illustrated in Figure 5. The charging currents measured are expressed by the ratio with respect to a charging current flowing in the reference accumulator cell comprising a grid which was not plated on its surface with Sn and containing an electrolyte to which Nat was added.

In Figure 5, reference numerals 1, 2 and 3 stand for accumulator cells comprising grids which were not plated on their surfaces with Sn. The cell No. 1 contained an electrolyte to which nothing was added, No. 2 an electrolyte to which Nat was added, and No. 3 an electrolyte to which Mug2+ was added. Reference numerals 4, 5 and 6 stand for accumulator cells having grids plated with Sn on their surfaces. The cell No. 4 contained an electrolyte to which nothing was added, No. 5 an electrolyte to which Nat was added, and No. 6 an electrolyte to which Mg2t was added.

When comparing with the accumulator cells containing an electrolyte to which Nar or Mg2t alone was added or having a grid plated with Sn, the accumulator cells having grids plated with Sn and containing an electrolyte to which both Nat and Mug2' were added are found to carry an about two-fold charging current. This indicates that Sn plating interacts synergistically with Na and Mg2t added to an electrolyte.

In the instant examples, cast grids were used. However, similar effects will be obtained with grids obtained from rolled sheets by punching or expanded grids.

Examples of the Fifth Aspect Prepared from an Pb-Ca alloy containing Sn were grids, which were filled with an active substance and formed to make positive grids having or having not an alpha-PbO, film thereon. Prepared with these grids were accumulator cells of 1.2 Ah-6 V, which were then filled with electrolytes containing Na,SO,, KgSO, MgSO, 7H20, Na2SOs + MgSO, and Na2SO, + KgSO . 7H,O. In order to prepare a reference accumulator cell comnon to all the cells as mentioned above, a Pb-Ca alloy having no alpha-PbO2 thereon was used with a sulfuric acid electrolyte to which nothing was added.These cells were discharged at a constant resistance of 8 ohms for 24 hours, and were thereafter opencircuited and permitted to stand at 250C for 1 month. The cells were then charged at a constant voltage of 7.35 V (0.3 C restriction) for 24 hours with a stabilized power source.

Table 3 shows the results in terms of the ratios of the 5 H.R.

capacities recovered with respect to the initial 5 H.R. capacities.

Table 3

Cell Grid Additives @-PbO2 Recovery Nos. Film (%) 1 Pb-Ca No Not Unchargeable Additive Form 2 Pb-Ca Na2SO4 Not 15.7 I Form 3 Pb-Ca MgSO4 Not 16.4 Form 4 Pb-Ca MgSO4 7H2O Not 16.9 Form 5 Pb-Ca Na2SO4 Not 21.3 @ MgSO4. 7H2O 1 Form 6 Pb-Ca Na2SO4 Form 49.4 Prior 7 Pb-Ca No Form 25.8 Additive 8 Pb-Ca Na2SO4 Form 42.1 9 Pb-Ca Mg-So4. 7H2O Form 41.9 Art 10 Pb-Ca Na2SO4 Form 48.6 MgSO4 11 Pb-Ca MgSO4 Form 43.5 12 Pb-Ca-Sn No Not 51.3 Additive Form 13 Pb-Ca-Sn Na2SO4 Not 67.2 I Form 14 Pb-Ca-Sn MgSO4. 7H2O Not 68.4 15 Pb-Ca-Sn Na2SO4 Not 73.4 MgSO4 7H2O Form 16 Pb-Ca-Sn Na2SO4 Form 93.0 17 Pb-Ca-Sn No For 76.5 Additive 18 Pb-Ca-Sn MgSO4. 7H2O Form 93.4 Inven tion 19 Pb-Ca-Sn Na2SO4 Form 99.5 MgSO4. 7H2O 20 Pb-Ca-Sn MgSO4 Form 93.1 From the table, it is found that the addition of Sn gives rise to a reovery of about 50 Z (No. 12), the incorporation of alkaline earth metal ions a recovery of about 16 Z (Nos. 3, 4), the formation of an alpha-PbO2 film a recovery of 25 Z (No. 7) and the addition of both alkaline metal and earth metal ions a recovery of 5 Z (No. 5).

Some synergism is obtained through a combination of two of these means. However, more improved effects are obtained by a combination of three means, i.e., Sn, alkaline earth metal ions and an alpha-PbO2 film. Cell Nos. 18 and 20 according to the present invention proved to be more effective than Cell Nos. 13 and 16 according to the prior art. Much more improved effects are obtained in the presence of both alkaline metal and earth metal ions (No. 19), which are found to be very effective for the properties of the cell at the time when it is permitted to stand. This seems to be due to the fact that, as expected, Sn serves to suppress the formation of PbOx, alpha-PbO2 the formation of PbSO*, and Nay , Mug?' and Mg(H0).2 a lowering of conductivity.

An accumulator cell immersed in an acid to form an alpha-PbO2 film and an untreated cell, each of 1.9 Ah-12 V, were completely charged, and were then discharged at 650C for 15 days to examine the relationship between the remaining capacities at 1.25 A discharge and the days elapsed. The results are illustrated in Figure 6. Just after being permitted to stand, both cells shows a capacity of 70 minutes. After the lapse of 15 days, however, the untreated cell exhibits a capacity short of 25 minutes, while the acid-immersed cell naistains a capacity of as long as 40 minutes, which means that such acid immersion is also considerably effective for self-discharge.

Thus, the acid immersion gives rise to a passivated oxide film at the initial stage, which is to be forked when the untreated cell is allowed to stand for an extended period.

Examples of the Sixth Aspect Accumulator cells of 1.2 Ah-2 V were prepared and filled with a predetermined amount of electrolytes (H2SO, having a specific gravity of 1.320) containing a total of 0.2 moles of Nat and Mg+ in various proportions. After intially charged and subjected to an initial capacity test, these cells were discharged at a constant resistance for 24 hours. Afterwards, they were open-circuited and permitted to stand at 25x for one month. The cells were then charged at a constant voltage of 2.45 V to measure the charging currents after the lapse of 10 seconds, 30 seconds and 60 seconds.The results are set out in Table 4, wherein the current values are given on the basis of 100 which stands for the value for a current flowing in No. 3 after the lapse of 10 seconds, and the amounts of Nat and Mg+ are given in terms of mol Z, provided that the total amount of the additives is fixed at 0.2 moles.

Table 4

No. Amount Amount Current Current Current After Na+ After Mg+ After After After (%) (%) 10 sec. 30 sec. 60 sec.

1 100 0 43.5 102 174 2 80 20 65.2 152 261 3 60 40 100 172 261 4 40 60 95.7 195 261 5 20 80 78.3 143 257 6 10 90 28.3 65.2 137 7 0 100 8.7 19.6 34.8 The foregoing results were obtained with anyhydrous magnesium.

However, similar results were also obtained with magnesium having crystallized water.

The cells (Nos. 2 to 6) containing a mixture of Nat with Mg2t carry more charging currents than do the cells (Nos. 1 and 7) containing Na+ or Mg2'alone. This indicates that Na correlates with Msft. In particular, it is found that the effect of Nat is not appreciably observed in a region of 0.04 moles (about 400 ppm) or less. Especially when Na is not used, the effect of the present invention is not attained at all. As mentioned above, Nat and Mg21 cooperate synergistically with each other to have an improved effect on the properties of lead accuswlators at the time when they are permitted to stand in an over-discharged state.

Examples of the Seventh Aspect Prepared from grids plated on their surfaces with a Pb-Sn alloy were accrmaiator cells, which were then examined on their chargeability and synergistic effects upon being permitted to stand in an over-discharged state with or without acid immersion treatments and in the presence or absence of alkaline metal ions and alkaline earth metal ions.The cells, each of 1.2 Ah-6 V, were discharged at a constant resistance of 8 ohms for 24 hours, and were then permitted to stand for 7 days to measure their chargeability at 7.35 V by constantvoltage charge (25x ). As illustrated in Figure 7, the results are that the addition of Na' and Mg+ and plating have a limited or reduced effect upon the cells which are not immersed in an acid, but have a striking effect upon the cells treated by acid immersion, leading to a considerably increased charging current flowing therein.

More improved effects are obtained with the addition of both Na and Mg" than with the addition of Nat or g:? alone. From the instant examples, it is evident that the three means, i.e., acid immersion, plating and the addition of Nat and Kg'+ cooperate synergistically with one another. Similar effects are also obtained with ions of an alkaline earth metal having crystallized water.

An accu alator cell treated by acid immersion and an untreated cell, each of 1.9 Ah-12 V, were completely charged, and were then discharged at 651 for 15 days to anine a relationship between their remaining capacities at 1.25 A discharge and the days elapsed. The results are illustrated in Figure 8. Just after permitted to stand, both cells shows a capacity of 70 minutes. After the lapse of 15 days, however, the untreated cell exhibits a capacity short of 25 minutes, while the acid-treated cell maintains a capacity of as long as 40 minutes, which means that such acid immersion is also considerably effective for self-discharge. Thus, the acid immersion treatment gives rise to a passivated oxide film at the initial stage, which is to be formed when the untreated cell is allowed to stand for an extended period.

Claims (6)

1. A lead accumulator in which an electrolyte contains alkaline metal ions and/or alkaline earth metal ions, and which includes a positive grid obtained by plating the surface of a collector with a Pb-Sn alloy and forming a plate using said collector, followed by immersion thereof in dilute sulfuric acid.

2. A lead accumulator which uses a grid substrate formed of a Pb-Sb alloy containing any one of Na, Li and K and Sn, and in which an electrolyte contains alkaline earth metal ions.

3. A lead accumulator in which an electrolyte contains 500 ppm or higher of alkali metal ions and, at the same time, ions of an alkaline earth metal having no water and/or crystallized water.

4. A lead accumulator as claimed in any one of Claims 1 to 3, wherein said alkaline earth metal ions are ions of an alkaline earth metal having crystallized water.

5. A lead accumulator as claimed in any one of Claims 1 to 4 wherein said electrolyte further contains alkali metal ions.

6. A lead accumulator substantially as hereinbefore described with reference to any one of the examples.