GB2200791A - Cell corrosion reduction - Google Patents

Description

- --- 1; t 1:1 1 1 7 9 '1 CELL CORROSION REDUCTION 230P51647A This

invention relates to methods and materials used for reducing gassing in electrochemical cells as well as the amount of mercury required in anode amalgamations for such cells.

Metals such as zinc have been comwuly utilized as anodes in electrochemical cells, particularly in cells with aqueous alkaline electrolytes. In such tells the zinc is amalgamated with mercury in order to prevent or reduce the extent of reaction of the zinc wi.th the aqueous electrolyte vith the detrimental evolution of hydrogen gas. In the past it bat been necessary to utilize about 6-7Z.by weight of mercury amalgamation IV in the anode to reduce the amount of "gassing" to acceptible levels. ROvever, because of environmental considerations it has become desirable to eliminate or, at the very least, reduce the amount of mercury utilized in such cells but vithout concomitant increase in cell gassing. Various expedients have been utilized, to achieve such mercury reduction. such as r special treatment of the zinc, the use of additives and exotic amalgamation methods. Boveyer, such methods have either had economic 21 0 dravbacks or limited success. It is an object of the present invention to provide an economic meant for reduction of gassing in electrocbemical cells. It is a further object of the present invention to provide a relatively economic meant for permitting the reduction of amounts of mercury used in amalgamation of aqueous electrochemical anode metals vithout significant concomitant increase in call gassing or.reduction of cell performance. 2.s These and other objects, features and advantages of the present invention will become more evident from the following discussion as vell as the "dravings" in which: Figure I is a pbotonicrograph of cross sectioned polycrystalline zinc particles; and Figure 2 is a comparative photomicrograpb of cross sectioned polycrystalline zinc as treated in accordance vith the present invention.

i 1 I- Generally the present invention comprises a method for making an electrochemical cell, with reduced gassing. The invention further comprises the cell containing the treated anode material. The method of the present invention generally comprises reducing the number of grains in the polycrystalline anode metal to one third or less of the original number of grains. Thereafter, the reduced grain anode metal is formed into an anode such as by compression of povder particles either on a substrate or within a cavity. Alternatively, the anode metal nay be in the form of a sheet with the anode being convolutely wound in a "jelly roll" 10configuration together with the cell separator and cathode. The sheet metal may also be used, Irithout winding, in a prismatic cell. If desired, the anode metal (particularly zinc) is amalgamated with mercury after the grain reduction and prior to placement of the anode metal in the cell. In all the aforementioned embodiments, with such extent of grain reduction there is a concomitant reduction in the extent of grain boundaries and a reduction of gassing at such sites.

To further reduce the extent of gassing a small amount of a surface active beteropolar substance (surfactant) of a type that will act as a hydrogen evolution inhibitor is added to the cell. Because of the heteropolar nature of the surfactaut it is generally at least slightly soluble in the cell electrolyte and has a polar affinity to the surface of the anode metal particles with a coating being formed thereby. Such affinity is particularly marked with respect to zinc particles commonly utilized in anode& of alkaline elctrolyte cells. The surfactant may be 2S effectively incorporated in the cell in various ways. For example, it say be added to the anode, incorporated in the electrolyte, or in the separator by pre-vetting or impregnating the separator with the additive. The surfactant say even be added to the cathode. In all such instances the surfactant migrates to th surface of the anode metal particles to form 3C) the requisite hydrogen ga: inhibit ing coating. Adding the surfactaut to the anodic material is by direct addition to the povdered metal (amalgamated or unamalgamated) to form a surface coating for the anode X h 91 1r, I 3 metal. Alternative ly, the surfactant is added to the electrolyte which is then admixed with the anode metal.particles with resultant migration of the.surfactant to the surface of the anode metal particles. Migration of the surfactant to the anode metal particles way also be effected by the S' addition of the surfactant to the separator or the cathode.

Alternatively, or in addition, the anode material particles, ouch to zinc, are prealloyed with a small amount of one or wore of indium, cadmium, gallium, thallium, bismuth, tin, and lead and then changed into particles with reduced number of grains or into individual discrete single crystal particles which are thereafter amalgamated with mercury.

In order to effect reduction in the number of grains. polycrystalline anode materials such as zinc are heat treated at a temperature below the melting point thereof for a sufficient time whereby the number of grains in the polycrystalline material is reduced to one third or less of the Isoriginal material.

Though the anode material remains polycrystalline after this heat treatment, the amount of grain boundaries are reduced with the reduction in number of grains. As a result, the amount of gassing in the cell, with the treated particles, is markedly reduced since it is the area of the 2,0 grain boundaries which is most conducive to high chemical activity and &at formation. In addition. mercury infiltrates into grain boundaries readily. With the reduction of grain boundaries there is a reduction in the amount of mercury required for amalgamation with the anode material. With the reduced grain anode materials the amount of mercury required for ejamalgamation can be effectively reduced from about 6-7% to up to about 4%.

Reat treatment of the anode material is dependendent upon the factors of purity of the polycrystalline starting material, the teaperature at which the heat treatment is effected, and the duration of such beat treatment. It is understood that heat treatment of powder particles of different bulk quantity may differ in length of time required since the interior of the aggregate is somewhat insulated by exterior material and does not "tee" the same amownt of beat as external material in direct receipt of the beat. In practice. a continuous tumbling calcined furnace i 4- will provide most effective beating and as a result. with properly designed calciner, less than ten minutes at temperatures above 370C is sufficient to effect sufficient grain reduction. Recrystallization and grain coarsening depends upon many factors such an temperature, time, strain energy within the material, and the purity. As a result, exact beat treatment parameters are determined in accordance with the specific heat treatment equipment being utilized. For clarity, the effective heat and temperature, hereinafter referred to. relate to a direct application of beat to the material. In all events, a reduction of the number of grains in the material to one third or )V less of the original material is the desired result.

The beat treatment of the polycrystalline anode material is effective with both povdered material generally used in the construction of compressed anodes in cells having a bobbin type structure, and such treatment is also effective with respect to the treatment of metal strips or sheets utilized in IS- prismatic or convolutely wound cell structures.

The purity of the initial polycrystalline anode material determines, in part, the length of time required to provide the requisite reduction of grains or conversely the temperature at which the material should be heated for a given period of time; the lover the purity, the higher the temperature -U or the longer the time period required. The most common anode material for electrochemical cells is zinc with the most coon impurity contained therein being lead. Other, less common, anode materials include cadmium, nickeli magnesium, aluminum, manganese, calcium, copper, iron, lead, tin and mixtures thereof including mixtures with zinc.

23 The alkaline electrolyte solution in which the anode material is placed and which generally is a factor in the gas generation (usually the anode reacts vith the electrolyte with resultant gas formation) is usually an aqueous solution of a hydroxide of alkali or alkaline earth metals such as NaOR and KOH. Common cathodes for the alkaline cells include manganese dioxide, cadmium oxide and hydroxide, mercuric oxide, lead oxide, nickel oxide and hydroxide, silver oxide and air. The reduced grain number anodes of the present invention however are also of utility in cells having other electrolytes in which gassing of the anode is problematical such as in acid type electrolyte#.

i AC f S 4 M 1 1 c X 5- Common alkaline type cells contain compressed polycrystalline zinc particles having an average particle size of about 100 microns. Rach of such particles has about 16 or more grains and in accordance with the present invention the number of grains in each of the particles is reduced X by beating the zinc particles at an effective temperature between about 50 to 419.5C (the latter being the melting point of zinc) for a minimum period of time ranging from about two hours at 50C to about five minutes at 419.5% to reduce the amount of grains to an average of about 3 to 5 grains per particle. Zinc particles having lead impurities require a temperature of about 100C for the minimum two hour period to achieve a similar reduction in number of grains.

Useful surfactants, dhich can be added to the cell, in accordance with the present invention in order to further reduce thedegree of gassing, include ethylene oxide containing polymers such as those having phosphate groups, saturated or unsaturated monocarboxylic acid with at least two ethanolaside groupings; tridecyloxypoly(etbylenoxy) ethanol; and most preferably organic phosphate esters. The preferred organic phosphate esters generally are nonoesters or diesters having the following formula: [RO(ItO) U Ix - P - 0 " (OK) where m - a, ania, amino, or an alkali or alkaline earth metal ' it - phenyl or alkyl or alkylaryl of 6-28 carbon atoms Specific useful organic phosphate ester surfactants include materials which can he identified by their commercial designation as GAFAC RA600 (an 2S anionic organic phosphate eater supplied by GAY Corp. as the free acid, based on a linear primary alcohol, and being an unmentralized partial cater of phosphoric acid); GAYAC RA610 (an anionic complex organic phosphate ester supplied by GAY Corp. as the free acid, having an aroxatic hydrophobe, and being an unneutralized partial cater of phosphoric acid); and ITAKAR AC AA-040 (an anionic zono substituted ortho phosphate eater supplied by BASY Wyandotte Corp).

61 It has been f6and that the incorporation of a surfactant additive of the type referred to herein in a call in an amount of from 0.001% to 5Z, preferably 0.005 to 1Z, and most.preferably 0.01 to 0.3Z by weight of the active anode component of the cell, precludes or at least 5r significantly inhibits the evolution of hydrogen within the cell, and thereby increase its shelf life and its useful work life.

The addition of the surfactant to cell containing reduced number of anode metal grains or single crystals of such anode metals provides a synergistic further reduction of call gassing.

JO Though the use of single crystal anode material and the use of organic phosphate aster surfactants (US Patent Nos. 4,487,651 and 4,195,120 owned by the same assignee as the present invention) have separately been known to effectively reduce cell gassing or to permit some reduction of mercury content in the anode without detrimental increase in gassing, the effect 15, of the combination has unexpectedly been discovered to be considerably sore than additive. Thus, in cells having amalgamated single crystal zinc anodes, the amount of mercury in the amalgam can be effectively reduced from about 6-7Z to about 4% or stated differently the rate of gassing of polycrystalline zinc amalgam containing 1.5% mercury can be reduced by about 2-fold with the use of single crystal zinc. Similarly the utilization'of an organic phosphate ester surfactant such as GAFAC RA600 with polycrystalline zinc amalgam anodes results in about a 4-fold reduction of gassing with for example O.1Z GAFAC RA600. Rowever, in accordance with the present invention, a combination of the two, i.e. a single crystal zinc amalgam with a surfactaut unexpectedly permits the effective reduction of the mercury to about 1.5Z with about a 20-fold gassing rate inhibition or about double what might have been expected. As a matter of course, combination of chemical gas'reduction expedients does not usually even provide an additive effect nor does excessive utilization of additives. The use of the surfactant material with the reduced grain number zinc anode material provides an economical synergistic reduction of cell gassing to above that obtained with single crystal anode material but well belov that obtained with the high grain number polycrystalline zinc.

1 W t 1 I;M e- 17 The single crystals of zinc are preferably prepared as described in #aid US Patent No. 4,487,651, the disclosure of which is incorporated herein by reference thereto. Such procedure involves the formation of a thin skin crucible on each of the zinc particles by oxidation in air at a temperature just below the melting point (41910 of the zinc, heating of the skin enclosed zinc particles in an inert atmosphere above the melting point of the zinc and slow cooling thereafter with removal of the.oxide skins. Zinc particle sizes generally range between 80 and 600 microns for utility in electrochemical cells and such method provides an effective io means for making single crystal particles of such &mall dimensions.

The amount of mercury in the &node amalgam way range from 0 - 4% depending upon the cell utilizatioa and the degree of gassing to he tolerated.

The amalgamated reduced grain number or single crystal metal particles with surfactant additives such to GAPAC ILA600 are formed into &nodes for electrochemical cells particularly alkaline electrocheaical cells. Alternatively, the &nodes are formed from the reduced grain number or single crystal metal particles and the surfactant migrates thereto from other cell components such as the electrolyte, separator.or cathode to P.0 which the surfactants have been initially added. Other &node metals capable of being formed into reduced grain number or single crystal powder@ and which are useful in electrochcaical cells include Al, Cd, U, Cu, Ph, Kg, Mi, and Sa.

A further additional or alternative expedicat for reduction of gassing Z57 is the alloying 6f.polyerystallilat &node metal particles with indium or other additives prior to &node metal grain reduction or the formation of the single crystal metal particle# generally in amounts-ranging between 25-5000 ppa and preferably between 100-1000 ppa. The amount of mercury in the &node amalgam way range from 0 - 4% depending upon the cell utilization and the degree of gassing to be tolerated.

1 9 The amalgamated single crystal metal particle with prealloyed inclusions of materials such asindium are then formed into anodes for electrochemical cells particularly alkaline electrochemical cells. Other anode metals capable of being formed into single crystal powder& and which are useful in electrochemical cells include Al, Cd, Ca, Cu, Pb, Ng, Ni, and Sn. It is understood that with anodes of these metals tke prealloy material is not the same as the anode active material but is less electrochemically active.

In order to more clearly illustrate the effectiveness of the present IV invention in reducing cell gassing, the following comparative examples are presented. It is underst?od that such examples are for illustrative purposes only and that details contained therein are not to be construed as limitations on the present invention. Unless otherwise indicated herein and throughout the present specification all parts are parts by weight.

EXAMPLE I

Three batches of polycrystalline zinc of average particle size of about 100 microns are beat treated for varying periods of time and temperatures and are then amalgamated with about 4Z mercury by weight. A fourth batch of 4% mercury amalgamated polycrystalline zinc is not beat treated and is used as a control. Two grams of each batch are placed in 37Z KOH solutions (similar to the electrolyte of alkaline cells) at 90'C with beating parameters and gassing rates given in Table 1:

TABLE I

Zinc Treatment ml gas (24 hours) al gas (93 hours) Z5, Control, not heated 0.62 3.77 114 hours at 400C 0.25 2.07 235 hours at 400% 0.28 2.78 hours at 419% 0.28 2.18 It is evident from the above that the heat treatment of the present IS' invention serves to wore thaw halve the gassing rate of amalgamated zinc. It is further evident that continued long term heating does not significantly affect gassing rates and is generally economically undesirable.

Q RIA LE 2 Polyerystalline zinc powder (average particle size of 100 microns) from the New Jersey Zinc Co. (NJZ) is beat treated at 370C by tumbling for one hour in a rotating calcine furnace. The powder, as received from New Jersey Zinc, has the crystalline structure shown in Figure 1. After the heat treatment the powder has the crystalline structure shown in Figure 2 wherein grain size is markedly increased, the number of grains is reduced and the amount of grain boundaries is concomitantly reduced. The polycrystalline zinc, as received and after beat treatment is amalgamated with 42 mercury and two gram samples of each are tested for gassing as in Example 1. An to additional two gram sample of 7Z mercury amalgamated zinc from Royce Zinc Co., with similar polycrystalline grain structure and average particle size, is also tested for gassing as an additional control (representing prior art amalgamated zinc) with gassing results given in Table 2: TABLE 2 19' Zinc Type

As received from NJZ 4Z Rg Rested at 370C for I hour 4Z Bg 7% Rg Royce Gassing (1al) after 24 hours at 90C 0.85 0.4 0.25 Beat treatment, as described, provides an anode material having markedly 2,v superior gassing properties when compared to untreated polycrystalline zinc and slightly worse than prior art amalgamated zinc having considerably more mercury in the amalgam,.

EXAMPLE 3

Two gram of each of the amalgamated zinc materials of Example 2 are similarly tested for gassing at 71C after period of 7.and 14 days with the results given in Tables 3 and 4:

TABLE 3

Zinc Type 7 Days (al gas) 14 days (al gas) As received from NJZ 4Z Rg 0.98 1.95 3 0 Heated at 3708C for 1 hour 4Z Rg 0.50 1.19 7Z Eg Royce 0.46 0.95 Zinc Type As received from NJZ 4% Rg 70 R@ated at 370C for 1 hour 41 HE 36 TABLE 4

Gassing Rate (ullga-day) 0-7 Days 7-14 days 0-14 days 69 49 43 X Both the total amount of evolved &a and the &&&sing rate of beat treated zinc powder&, after extended periods of time, are comparable to those of zinc powder& amalgamated with significantly more mercury.

It is evident from the photomicrographs of Figure 1 and 2 that the numerous polyerystalline grain boundaries have been reduced in number with a concomitant reduction in the number of polyerystalline grain& per particle without general change in the shape of the individual particles. The number of grains in the heat treated particles is a third or less of that of the original particles.

EXAMPLE 4

Zinc powder amalgams containing 1.5% mercury are made with standard grain polyerystalline zinc alone, standard grain polyerystalline zinc with 0.1Z RA600 as an additive element, single crystal zinc, and single crystal zinc with 0.1Z RA600 an an additive element. Equal amounts of the amalgam 1,5r powder are then placed in equal amounts of 37% KOR alkaline solution (typical electrolyte solution of alkaline cells) and tested for &acting at a temperature of 71% The 0.1% GAPAC 1A600 is added to the alkaline solution and stirring of the zinc in such solution resulta in the deposition of the surfactant on the zinc. The amount of gassing, measured in microliteralgraw per day (uL/S-day) and the rate reduction factors (with the polyerystalline zinc control being 1) are act forth in Table 5:

TABLE 5

ANODE MATERIAL GASSING RATE RATE REDUCTION FACTOR Polyerystalline zinc, 1.5% E& 295 1 2,5 Polycrystalline zinc, 1.5% Rg 80 3.7 0.1% RA600 Single crystal zinc, 1.5% Mg 140 2.1 Single crystal zinc, 1.5% Eg is 19.7 0.1% RA600 A rate reduction factor (if any) would at most have been expected to - be about 7.8 (3.7 x 2.1) for a combined utilization of single crystal zinc and RA600 with a gassing rate reduction to about 38 uLlg-day. The combination however synergistically reduces the gassing to ab out double the expected reduction.

t, t z L - 1 0 X X 1 J EXAMPLE 5

Zinc powder alaalgazz of polyerystalline and single crystal zinc with and without the 0.1% GAFAC RA600 additive are tested as in Example 3 but with 0.5% mercury amalgam. The mount of gassing, measured in aicroliteralgram per day (uL/g-day) and the rate reduction factors (with the polyerystalline zinc control being 1) are set forth in Table 6:

TABLE 6

ANODE MATERIAL GASSING RATE Polyerystalline zinc, 0.5% Rg 720 Polycrystalline zinc, 0.5% Elg 130 0.1% RA600 RATE REDUCTION FACTOR Single crystal zinc, 0.5% Eg 265 2.7 Single crystal zinc, 0.5% Eg 26 28 0.1% RA600 1,5 - A rate reduction factor (if any) would at soot have been expected to be about 14.9 (5.5 x 2.7 for a combined utilization of single crystal zinc and RA600 with a gassing rate reduction to about 48 uL/g-day. The combination however synergistically reduces the gassing to nearly double the expected reduction.

It is evident from the above examples and tables that the single crystal zinc with one or more additives of the present invention is markedly effective in permitting large mercury reductions vitkout increase in cell gassing.

EKAMPLE 6 Polycrystalline zinc is prealloyed with 550 pps of gallium and 100 ppm of 2.5r, indium. A first sample thereof is then amalgamated with 1.5% mercury. A second sample is made into individual single crystal alloy particles, as described above, prior to the amalgamation with mercury. Two grams of each of the samples are placed in a 37% KOH electrolyte solution with gassing at the end of 24 and 48 hours being measured at 90% as being representative of -30 corrosion. As control an amalgam is made with polycrystalline zinc with 7% mercury similar to that commonly used in alkaline type cells. Results of such tests are given in Table 7.

_r )2_ TA 31- 7 VOLMIE OF GAS (UL), 900C 24 Hours 48 Hours LIC 1 polycrystalline alloy 0.7 1.9 single crystal alloy 0.3 1.0 control (7Z Eg) 0.2 0.5 =LE 7 A first portion of polyerystalline zinc powder containing 0.04% lead is amalgamated with 2Z Ng and a second portion is converted to individual single crystal particles prior to the amalgamation. The analgaze are then tested for corrosion rate in ION XOR containing 2% MO. The gassing rates at 71C are 225 uL/ga per day and 80 LIga per day respectively.

It is evident that the corrosion reduction of &node metals such as zinc by the prealloying with corrosion reducing additive materials is greatly enhanced by the formation of single crystals from the &node metaladditive alloy.

It is understood that the above example& are illustrative in nature and that changes in material treatment, material proportions, the specific materials, cell construction and the like are within the scope of the present invention as defined in the following claim.

11 c IQefts,.- v i 13 1. An electrochemical cell comprising an anode, a cathode and an aqueous electrolyte characterized in that said anode is comprised of single crystal anode metal particles and said cell includes a surface active heteropolar material additive having a polar affinity to said anode.

2. The cell of claim 1 wherein said surface active heteropolar material additive is selected from the group consisting of ethylene oxidecontaining polymers, monocarboxylIc acid with at least two ethanolamide groupings, tridecyloxypoly(ethylenoxy) ethanol, and organic phosphate esters.

3. The cell of claim 2 wherein said surface active hetero polar material additive is an organic phosphate ester having the formula:

[RO(EtO) n]x p 0 - (OM) y where x + y = 3 M = H. ammonia. amino, or an alkali or alkaline earth metal and R = phenyl or alkyl or alkylaryl of 6-28 carbon atoms.

4. The cell of claim 3 wherein said organic phosphate ester is comprised of a member of the group consisting 4 t 14 of the free acid of an anionic organic phosphate ester based on a linear primary alcohol, and being an unneutralized partial ester of phosphoric acid; the free acid of an anionic complex organic phosphate ester having an aromatic hydrophobe, and being an unneutralized partial ester of phosphoric acid; and an anionic mono substituted ortho phosphate ester.

5. The cell of claim 4 wherein said organic phosphate ester is comprised of the free acid of an anionic organic phosphate ester based on a linear primary alcohol, and being an unneutralized partial ester of phosphoric acid.

6. An electrochemical cell subj,ect to reduced gassing comprising an aqueous alkaline electrolyte, a cathode and an anode comprised of mercury amalgamated single crystal zinc particles and an organic phosphate ester with said mercury comprising up to 4% by weight of said anode and said organic phosphate ester being comprised of the free acid of an anionic organic phosphate ester based on a linear primary alcohol, and being an unneutralized partial ester of phosphoric acid with said organic phosphate ester comprising from 0.01 to 0.3% by weight of said anode.

1 ZI -2 1 f 11 7. The cell of claim 6 wherein said mercury comprises up to 1.5% by weight of said anode, said cathode is comprised of manganese dioxide and said aqueous electrolyte is comprised of a potassium hydroxide solution.

8. An electrochemical cell comprising an anode, a cathode and an aqueous electrolyte characterized in that said anode is comprised of particles of discrete single crystals of anode metal and one or more members of the group consisting of indium, cadmium, gallium, thall-ium, bismuth, tin and lead, wherein said one or more members are present in said anode in a range of 25-5000 ppm and wherein said one or more members are alloyed with said anode metal, prior to formation of said discrete single crystal particles, whereby said one or more members form part of said single crystal.

9. An electrochemical cell, substantially as herein particularly described.

Published 1988 at The Patent Office, State House, 6671 High Holborn, London WC1R 4TP. Further copies may be obtained from The Patent Mce.

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