CA1121448A

CA1121448A - Cell having improved chargeability by oxidation of lower manganese oxides

Info

Publication number: CA1121448A
Application number: CA000317517A
Authority: CA
Inventors: Pentti J. Tamminen
Original assignee: Imatra Paristo Oy
Current assignee: Imatra Paristo Oy
Priority date: 1977-12-08
Filing date: 1978-12-06
Publication date: 1982-04-06
Also published as: FR2411493B1; AT380588B; NL7811958A; ATA877278A; DE2852668A1; US4268589A; FR2411493A1

Abstract

ABSTRACT

A cell of the type employing an alkaline electrolyte, a positive electrode mass containing a depolarizing agent comprising finely divided manganese dioxide and containing a conductive inert material, and a metallic negative electrode mass, characterized in that it contains at least one compound capable of permitting oxidation of lower manganese oxides during passage of recharging current through the cell hereby the recharging capability of the cell from a substantially completely discharged state is improved.

Description

IL4~8 S P ~ C I F I C A T I O N

Title of Invention C~LL HAVING Ir~ROVED RECHARG~ABILITY

EESCRIPTION

This invention relates to c~lls of the type employing an alkaline electrolyte 7 a positive electrode mass containing a depolarizing agent comprising finely divided manganese dioxi~e and containing a conductive inert material, and a metallic negative electrode mass.
A conventional alkaline manganese dioxide/zinc cell can be recharged if about 25 per cent at most of the primary capacity has been used. In this case, about 50 discharge-charge cycles may be obtained. In specially built cells ~here the active layers are very thin, about 150 discharge-charge cycles have been obtained with this discharge depth~
Recently, it has been discovered that by means of adding nickel oxide to the positive electrode mass the recharging capacity of a cell of the above-mentioned type can ~e inrreased and also the rechargeability itself can be improved. This is, however,not yet ~ .

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sufficient to make a practical secondary battery.
It is an object of this invention to provide a cell of the above-mentioned type whose recharging characteristics have been improved to the extent that a battery Eormed of such cells may be considered to be a practical secondary battery~
According to the present invention there is provided a cell of the type employing an alkaline electrolyte, a nega~
tive electrode mass comprising at least one metal, and a positive electrode mass containing a depolarizing agent comprising finely divided manganese dioxide a~nd containing a conductive inert material, ~herein the manganese dioxide of the positive electrode mass is in chemical contact with free negative ions of at least one weak acid that does not in the conditions prevailing in the battery form hydrated crystalline salts with manganese, said negative ions being present in an amount sufficient to permit oxidation of lower manganese oxides during passage of recharging current through the cell, whereby the recharging capability of the cell from a subtantially completely discharged state is improved.
Also according to the invention there is provided a method of improving the rechargeability from a substantially completely discharged state of a cell of the type employing an alkaline electrolyte t a negative electrode mass comprising at least one metal, and a positive electrode mass containing containin~ a clepolarizing agent comprising finely divided manganese dioxide and containing a conductive inert material, wherein the manganese dioxide oE the positive electrode mass is in chemical contact with free negative ions of at least one weak acid that does not in t:he conditions prevailing in the battery form hydrated crystalline salts with manganese, said negative ions being present in an amount sufficient to permit oxidation of lower manganese oxides during passage of recharging current through the cell.
Thus, said at least one weak acid does not form salts with the metal(s) of the negative electrode mass, the crystals of which contain water of crystallization. Preferably the ions are selec-ted such that migration of metallic ions from the negative electrode towards the positive electrode is minimized.
The invention also embraces a battery comprising two or more such cells.
The negative ions may be inorganic in nature such as, for example, borate, carbonate, cyanide, sulphide or silicate, or may be organic in nature, such as for example acetate.
Preferred ions are borate and carbonate. The acids per se may be employed, where they exist, or salts of the acids, such as with alkali or alkaline earth metals, may be employed.
It should be said that it is known to add boric acid to alkaline/silver/zinc cells to reduce the pH of the electro]yte thus to prevent or reduce zinc corrosion. In this case the boric acid was added in an amount of 1.87 g/ 25ml of 40% sodium hydroxide solution. It has previously been thoug~ that the addition of small quanti-ties of borate to an electrolyte would cause passiva-tion of the zinc, The effect of boric acid addi-tion on the rechargeability of an alkaline cell employing manganese dioxide has, as far as is known, never been dis-covered.
Without wishing to be limit:ed in any way by the explanation it is thought at the present time that the additives of this invention work by providing an acidic environment for the manganese dioxide in which the solu-bility of the lower manganese oxides (e.g. Mn304) is increased,whereby oxidation of these oxides can occur during passage of recharging current through the cell.
The weak acid or~salt thereof employed in the present invention may be used either in solid or liquid form depending upon the nature of the acid and its solubility in the electrolyte. The liquid may be the natural state of the weak acid employèd or may be a solution thereof or a solution of a salt thereof.
If the weak acid or the salt thereof which is to be 2Q employed is used as a solid r then the solid is intimately -, 5 ~'Z~4'~

incorporated into the positive electrode mass and this may be done by mixing the solid with manganese dioxide and conductive inert material and thereafter moistening the mixture with alkaline electrolyte (eg.sodium hydroxide or potassium hydroxide solution)~ Alternatively but not exclusively the solid may be first mixed with the alkaline electrolyte and then rapidly mixed with manganese dioxide and conductive inert material.
An example of a we~k acid, including salts thereof, ~hich is pre~erably incorporated as a solid is boric acid and its salts, although its salts may be incoI~orated as solutions if their solubility in the electrolyte is sufficient.
If the weak acid or salt thereof which is to be employed is used as a liquid then the liquid, ~hich should ge~erally be miscible with the alkaline electrolyte, may be simply mixed with the alkaline electrolyte in the desired proportions. An example of such a li~uid is potassium carbonate solution.
In the case of silicic acid the weak acid may be incorporated as a solid and will subsequently dissolve in the electrolyte. Conversely it may be possible, by making use of temperature-dependent solubility changes 9 to precipitate out of the electrolyte the weak acid or salt thereof to ensure intimate incorporation 4 ~ ~
, of the ~eak acid or its salt in the positive electrode mass.
The weak acid or sa:Lt thereof is generally employed in the cell in a relatively large quantity. For instance in the ease of boric acid this amount may be for example from 5 to 25 grams per 100 grams of manganese dioxide. In the case of potassium carbonate solution used ~ith an alkali hydroxide-containing electrolyte the weight ratio of carbonate:hydroxide may be, for example, from 2:1 ~o 3:1.
Sufficient alkaline electrolyte should generally be presen~ to overcome the neutralizing effects of the weak acid or its salt.
It should be emph2sized however, that for particular purposes the ranges referred to above may be extended. For example, if ~otassium carbonate is used as the salt of the weak acid, its solution may be used alone to provide both the negative ions and the alkaline electrolyte, if a cell be required for small drain applications, such as for use in an electric watch. Conversely for short time high drain applications such as for a car battery9 it may be desirable to use considerably less carbonate than previously specified~
for example, from 35 to 50,' by weight of the total solids in the electrolyte.

.. , Two or more of the weak acids or salts thereof defined for use in the present invention may be employed together.
Examples of cells according to this invention will now be described, by way of example only, wlth reference tv the accompanying drawings, in which:
Fig. 1 is a graph illustrating the effect of a boric acid addition on the first discharge of a battery cell after 18 months' storage time;
Fig. 2 is a comparative and illustrates the rechargeability of a conventional alkaline/manganese dioxide/zinc ba~tery cell when discharged to a depth of about 50 per cent of its effective Amp/hours (Ah) capacity;
Fig. 3 illustrates the rechargeability of a battery cell with a boric acid addition when cor,lpletely discharged after 24 months t storage time;
Fig~ 4 illustrates the effect of potassium carbonate additions to battery cells on fifth and twentieth discharges and on a fifth discharge, respectively;
Fig. S shows a cell of the t~pe described and claimed in my U.S. Patent No. 4060670; and ~Q
Fiq, 6 shows a battery formed from~ of the cells of Fig. 5~
~ eferring firstly to Fig. 5, a steel sheet 1 is a positive current cDllector. Its under-surface i~ covered wikh a paint made conductive by means of graphite and carbon black ~nd having a binding material resistant to the electrolyte used, in the present case ~OH-solution.

- a~ 4~8 The pcsitiYe electrode is a mix cake 2 compressed from carbon and manganese dioxide powder and moistened with eleztrolyte.
The negative electrode 3 contains amalgamated zinc powder and an electrolyte gel containing carboxymethylcellulose.
The ~egative current collector 4 is a steel sheet which has been treated in a KOH-solution together with excessively amalgamated zinc powder so that no hydrogen evolution takes place on its surface in the ci'rcumstances prevailing within the cell unit. In the middle of thle outer surface of each current collector sheet 1 and 4 there is a sticky i~sulating layer 5 and 10 of bitumen, softenecl with oil. Each electrode with its current collector is separately packed in a separator paper 6 in which there is, at the place of the insulating layer t5,10) a hole of about the same size as said layer. The electrode elements, disposed against each other, are packed in a plastics envelope 7,7' by heat sealing preferably in vacuum whereby-the plastics envelope is tightly compressed around ~h~ ~alva~ic cell formed and against the insulating layers 5 and 10.
Fig. 6 schematically illustrates a battery comprising a cell 12, contact ~lements 11, end plates 13-which may ~e E.G.
of stiff plastics or cardboard, bonds 14 which may be E.G.
rubber rings, and metallic contact strips 15 and 16 which form the positive and negatiYe poles of the battery.
Referring now to Figs. 1 to 3, the tests were carried out with a flat cell structure according to U.S. Patent Specification No. 4060~70 and as shown in Fig. 5. In Fig. 1, broken lines represent the discharge curve of a conven-tional fresh al~aline/man~anese dioxide/zinc battery cell without any additive. In Fig. 1, a distinct decline will be noted at the state ~after about 45 hours) whera the positive electrode beglns to undergo an irreversible reaction. For example, hauE;nannite ~r~04) does not revert to manganese dioxide (~'~2~ ~rhen charged.
The next decline (after about 65 hours) is due to the formation of oxides which are still lotrer in oxygen content.
The ~our other curves show ho~J boric acid added b the positive mass of a battery cell which is similar in other respects affects the shape of the discharge curve and at the same time also the primary capacity o~ the cell, while reducing it in relation to the auantity of boric acid.
It should be noted that there is no stepped shape similar to the previous curve to be seen.

_10 -~ 44 ~

1~hen the conventional cell having no boric aci~ rhose performance inter alia is shown in Fig. 1, tras completely discharged it ~as not rechargeable.
1ihen the conventional cell ~Jas discharged for only about 20 hours (or to a depth of about 50 per cent of its effective primary capacity) and thereafter charged to a constant voltage of about 1~ volts, charge-discharge curves according to Fi~ure 2 were obtained.
As can be seen, the rechargeability of the cell was quickly reduced along ~rith the charging times and after the 5th charging time the effective capacity of the battery ;~as negligible (maybe 1 Ah).
~ n`alkaline/manganese dioxide battery cell was then prepared using mixture ratios as follo~rs: 100 parts by ~Teight of manganese dioxlde (r~02) po~der, 45 parts by eigh~ of graphite (C), 77 parts by weight of a 40 per cent potassium hydroxide solution and 1~ parts by ~eight of finely divided boric acid (H3B03).
The mixing was carried out so that the manganese dioxide and graphite as well as the boric acid were firstly mixed together ~Ihereafter ~-ater and, after a further mixing period, potassium hydroxide flakes were added to the mixture. It has been found that from 5 to - 25 grams of boric acid per 100 grams of manganese dioxide is most suitable for medium current density applications.

, . . .
~ ~ .

.

- 11 - 11;~3~9L48 ~ 'hen a test corresponding to the above ~as made ith a cell provided with such a boric acid addition of 16 g/100 g manganese dioxide, where the boric acid has been added in the dry state, the effective ca~acity of the first discharging time was relatively small (Figure ~a). ~Theni the cell , however, was recharged, the discharg~ curve improved ~hereby, for eYample, the energy content of the fourth discharge corresponds to that of the first discharge according 10 to Figure 2. In the seventeenth discharge of the cell , the discharge curve whereof ~as repeated as the test continued, the capacity obtained frorn the cell noticeably exceeded that of the first di~charge of the above mentioned primary cell. It can thus be 15 said that the shape of the discharge curve markedly improves to a certain limit, although the first discharge as such does not indicate any advantageous~
situation. ~!!hat is especially surprising is the complete rechargeability of a cell with a boric acid -20 addition despite the fact that it has been repeatedly substantially fully discharged.
It should be emphasized again that boric acid can be added either in po~Tder~form to a dry positive mass or as a suspension to the electroly~e.
It is also noted that the negative electrode of ~19t4~3 the cells in the fore~oing tests consisted of a conventional amalgamated zinc paste containing alkaline electrolyte and gellin~ agen~s. In an assembled `cell , the borate ions are? of course, diffused also into the separator and the negative electrode mass. The rechargeability of a zinc electrode in alkaline battery cells is a ~1ell-kno~m difficult problem because zinc dendrites produced ~uring charging tend to gro~l through the separator, thereby producing an internal short circuit.
Surprisingly it has been discovered that when the above mentioned cells t1ith a boric acid addition are opened after several tens of discharge/charge cycles, no growth of zinc den~lrites through the ; 15 sep~rator is noted. A suitable addition of boric acid obviously improves the rechargeability of both electrode masses. ~l~en the cell, whose performance is shotm in Figure 2 was opened, the zinc dendrites h~d thoroughly grotrn into the separator as a gray layer.
Referring now to Fig. 4 of the drawings, two~
batteries were employed having a construction as described above with reference to U.S. Patent Specification No.4060~70 an~ h~ch will be referred to respectively as cell 1 and cell 2.
In cell 1 the relation between potassium ;:

carbonate and potassium hydroxide in the electrolyte ~^ras

2.4:1, and the amount of solid matter in the aqueous - solution ~las about 32%. The anode was mercury-amalgamated zinc powder containing 4% of carboxymethylcellulose as gelling agent. The cathode mix comprised electrolytic manganese oxide and graphite in a weight ratio of 100:45.
All discharges ~ere complete, as seen in the plot. It is surprising to note that discharge No. 20 was better than discharge No. 5.
Cell 2 was in other respects similar to cell 1, out the ~eight ratio of potassium carbonate to that of potassium hydroxide was 2.6:1 in the electrolyte.
Ojther experiments have sllown that the optimumum ratio for potassium carbonate in me~ium current density applications (e.g. 5m A/cm~) is between 2 and 3:1.
The cells 1 and 2 were charged at constant voltage 1.~5 V; no gassing was observed.
Although the foregoing tests have been described in conjunction ~Tith cells using zinc in the negative electrode mass, the invention is also a~plicable to cells containing other negative electrode materials such as, for example, copper, lead~cadmium, and mixtures ! and alloys of two or more of these metals with each other or ~ith æinc. In accordance with procedures known in the art, small quantities of mercury may also be present _ 14 -to prevent hydrogen evolution at the negative electrode.
Additionally, although the above tests have been described in conjunction with flat cells and indeed although such a structure is very suitable for the cells of this invention, the invention is not limited to such cells. It is envisaged that cells according to the present invention can be made in any appropriate shape or si~e and that the characteristics of the cells may be tailored to meet particular reauirements. The cells of the present invention are particularly suitable for use in hermetically sealed units since the problems of gassing and of s~elling are substantially completely elminated thereby, but it is envisaged that the cells of this invention could employ a venting system. A plurality of the cells of this invention can be made up into a battery having a particular qhape~
structure or properties as required.
Moreover, although the above tests have been described in conjunction with potassium hydroxide solution as the alkaline electrolyte other alkalis or mixtures thereof may be employed. îhus, ~or example a solution containing e~ual quantities of potassium hydroxide and sodium hydroxide has been ~ound to be effective. ~Ioreover in certain systems i~ may not be necessary to use an alkali hydroxide and there may be employed another alkalinity-inducing agent.
Finally in each o~ the tests described above the conductive inert material employed was graphite. However ~.l,Z14~B
_ 1 5_ any other suitable conductive material may be substituted, in whole or in part, for the graphite.
It goes ~rithout saying that other additives and procedures which are adopted or employed in the art may be freely employed in the~ present invention where suitable. ~n example of one commonly employed additive is nickel oxide ~hich is often added to the positive electrode mass to improve its characteristics.
This invention has made it possible to recharge a cell after a complete discharge substantially t~ithout reducing the capacity of the cell. one result of this is a deep-dischargeable alkaline/manganese dioxide accumulator. In the above embodiments the watt hours (~;~h) capacity of the accumulator per each unit of weight is approximately as large as the capacity of a lead acid accumulator of the same weight (about 35 '~.~/kg). An additional advantage accrues if the accumulator is hermetically sealed, by which means its charge may be retained for years ~ithout any significant self-discharge.
An accumulator of this kind can be mounted in any position because the electrolyte is bsorbed in the separators and active masses. The accumulator functions the better the thinner its active layers are. Under these circumstances, also a more advantageous capacity per weight ratio is obtained.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A cell of the type employing an alkaline electro-lyte, a negative electrode mass comprising at least one metal, and a positive electrode mass containing a depolarizing agent comprising finely divided manganese dioxide and containing a conductive inert material, wherein the manganese dioxide of the positive electrode mass is in chemical contact with free negative ions of at least one weak acid that does not in the conditions prevailing in the battery form hydrated crystalline salts with manganese, said negative ions being present in an amount sufficient to permit oxidation of lower manganese oxides during passage of recharging current through the cell, whereby the recharging capability of the cell from a substantially completely discharged state is improved.

2. A cell according to claim 1 in which said at least one weak acid does not form hydrated crystalline salts with said at least one metal of the negative electrode mass.

3. A cell according to claim 1, in which the alkaline electrolyte comprises a solution of an alkali metal hydroxide.

4. A cell according to claim 1, in which the negative ions are those of weak inorganic acid.

5. A cell according to claim 4, in which the nega-tive ions are selected from one or more of borate, carbonate, silicate, cyanide, and sulphide.

6. A cell according to claim 5, in which the negative ions are derived from boric acid which is present in the posi-tive electrode mass as finely divided particles.

7. A cell according to claim 6, in which the boric acid is present in an amount of from 5 to 25 grams per 100 grams of manganese dioxide.

8. A cell according to claim 5, in which the nega-tive ions are carbonate ions derived from carbonate dis-solved in the alkaline electrolyte.

9. A cell according to claim 8, in which the carbonate is present in an amount of from 200 to 300 per cent by weight of the weight of alkali in the alkaline electrolyte.

10. A cell according to claim 8, in which the carbonate is that of alkali metal.

11. A cell according to claim 2, in which the negative ions are derived from weak organic acid.

12. A cell according to claim 11, in which the nega-tive ions are those of acetic acid.

13, A cell according to claim 3, in which the alkaline electrolyte comprises aqueous potassium hydroxide solution.

14. A cell according to claim 1, in which the negative electrode mass comprises zinc.

15. A cell according to claim 1, which is of a laminar flat cell construction.

16. A cell according to claim 1, in which the conductive inert material comprises finely divided graphite.

17. A battery comprising a plurality of cells as claimed in claim 1.

18. A method of improving the rechargeability from a substantially completely discharged state of a cell of the type employing an alkaline electrolyte, a negative electrode mass comprising at least one metal, and a positive electrode mass containing a depolarizing agent comprising finely divided manganese dioxide and containing a conductive inert material, wherein the manganese dioxide of the positive electrode mass is in chemical contact with free negative ions of at least one weak acid that does not in the conditions prevailing in the battery form hydrated crystalline salts with manganese, said negative ions being present in an amount sufficient to permit oxidation of lower manganese oxides during passage of recharging current through the cell.

19. A method according to claim 18 in which said at least one weak acid does not form hydrated crystalline salts with said at least one metal of the negative electrode mass.

20. A method according to claim 18, in which the alka-line electrolyte comprises a solution of an alkali metal hydroxide.

21. A method according to claim 18, in which the negative ions are derived from weak acid present as finely divided particals intimately mixed with the manganese oxide.

22. A method as claimed in claim 21, in which the weak acid or salt thereof is mixed as a dry powder with the manganese dioxide and conductive inert material, and thereafter the mixture is moistened with the alkaline electrolyte.

23. A method as claimed in claim 21, in which the weak acid or salt thereof is first mixed with the alkaline electo-lyte solution, and thereafter the manganese dioxide, the con-ductive inert material, and other optional additives for the positive mass, are added and rapidly mixed therewith.

24. A method according to claim 18, in which the negative ions are derived from boric acid.

25. A method according to claim 24, in which the boric acid is incorporated in or mixed with the manganese dioxide in an amount of from 5 to 25 grams per 100 grams of manganese dioxide.

26. A method according to claim 18, in which the nega-tive ions of the weak acid are derived from weak acid or salt thereof dissolved in the alkaline electrolyte.

27. A method according to claim 26, in which the salt of the weak acid is at least one of carbonate, cyanide, sulphide, silicate and acetate.

28. A method according to claim 27, in which the salt of weak acid comprises carbonate dissolved in the electro-lyte.

29. A method according to claim 28, in which the electro-lyte comprises a solution of alkali metal hydroxide and the weight ratio of the carbonate to the hydroxide is from 2 to 3:1.

30. A method according to claim 29, in which the salt of weak acid comprises potassium carbonate which is employed as a solution to provide both the weak acid and the alkaline electrolyte.