GB2195201A - Batteries having an aqueous alkaline electrolyte - Google Patents

Abstract

A battery electrolyte, more especially for metal-air batteries, comprises an aqueous mixture of sodium hydroxide and potassium hydroxide. The use of such an electrolyte alleviates the problems of sludge formation whilst maintaining good conductivity and other properties.

Description

SPECIFICATION Batteries having an aqueous alkaline electrolyte This invention relates to batteries having an aqueous alkaline electrolyte, more especially to metalair batteries, and above all to aluminium-air batteries.

Aluminium-air battery systems are of two distinct types, namely those in which the aqueous electrolyte comprises a solution of a neutral chloride (such as sodium chloride, potassium chloride or ammonium chloride) and those in which an aqueous alkaline electrolyte is used. The alkaline system provides both high specific energy and power density, whereas the saline system, whilst still having reasonably high specific energy density, has much lower power density. An air electrode requires a highly alkaline (or acidic) electrolyte in order to achieve high current densities, and highly acidic electrolytes are unacceptably corrosive towards the aluminium anode and other cell materials.

During the operation of metal-air batteries with alkaline electrolytes, the active anode material is dissolved by the electrolyte, and a build-up of electrical resistance occurs within the electrolyte owing to the accumulation of the resulting reaction products. For example, in the case of an aluminium-air battery with a sodium hydroxide electrolyte, progressive dissolution of the aluminium anode leads to the formation of a highly insoluble sludge comprising precipitated sodiumaluminium hydroxide, which in turn leads to increased cell resistance, reduced performance and cloaking of the electrodes.

Conventional attempts to solve these problems involve pumping the electrolyte to a precipitator or hydrocyclone to separate out the sludge. Such expedients, however, are cumbersome and impractical except in the case of large batteries where there is a definite requirement to recirculate the electrolyte and thereby remove excess heat. Moreover, there are inherent limitations on the proportion of a finely dispersed sludge that can be removed by such methods.

The present invention provides a metal-air battery, more especially an aluminium-air battery, in which the electrolyte comprises an aqueous mixture of sodium hydroxide and potassium hydroxide.

The use of a mixed electrolyte according to the invention is in principle applicable to other metal-air battery systems such as iron-air, cobalt-air and zinc-air, but an especially important application of the invention is in aluminium-air systems, and the invention will accordingly be described hereinafter with particular reference to such systems.

The use of a mixed electrolyte according to the invention is in principle also applicable quite generally to batteries having aqueous alkaline electrolytes. Examples of such batteries include: metal/alkali/MnO2 (for instance zinc/alkali/Mn02) cadmium, cobalt or iron/alkali/NiO(OH) metal/alkali/silver oxide, where the metal is, for example, magnesium, aluminium, iron, cobalt or zinc.

Surprisingly, it has been found that the use of a mixed electrolyte according to the invention greatly alleviates the problems caused by sludge formation without any unacceptable deterioration in conductivity. The battery can be operated for a significantly longer period before the onset of sludge formation and the precipitate that does form tends to agglomerate to form large granules which are relatively easy to flush or clean away.

Examples of mixed electrolytes according to the invention are equal volume mixtures of the following, all percentages being on a weight/volume basis: (1) 50% NaOH and 50% KOH (2) 50% NaOH and 30% KOH (3) 30% NaOH and 50% KOH (4) 30% NaOH and 40% KOH (5) 40% NaOH and 30% KOH (6) 40% NaOH and 40% KOH Relevant properties of the mixed electrolytes designated (1) to (6) respectively were tested as follows: A. Solubility of aluminates/Al(OH)3 reaction product Test tubes containing the electrolytes were immersed in a heated water bath to maintain a constant working temperature (25"C.). Pieces of an aluminium alloy designated Q4 were then dissolved in each electrolyte until electrolyte saturation. The test tubes were cork-sealed to prevent carbon dioxide absorption.On attaining saturation, the solutions were left to stand, at the constant temperature, for at least 24 hours to allow all undissolved particulate matter to settle down. Aliquot samples of the clear saturated solutions were pipetted into graduated flasks.

The samples were then diluted by sufficient factors to enable absorbance readings to be taken, within the linearity range for aluminium, using an Atomic Absorption spectrophotometer.

Each electrolyte sample was prepared in duplicate. The Atomic Absorption spectrophotometric method was then used to determine the amount of aluminium dissolved in each saturated electrolyte. The average of the concentrations given by the two samples prepared for each electrolyte was then used to obtain the solubility of the aluminium part of the aluminate/Al(OH)3 formed during the reaction.

The data so obtained were used to construct the bar charts shown in Figs. 1 to 5.

The percentage composition (w/w) of other elements in the test aluminium alloy Q4 is as follows: Zn 0.005 Mn 0.002 Cu 0.010 Ti 0.001 Mg < 0.005 Ga 0.027 Si < 0.010 Pb 0.160 Fe 0.020 Bi 0.160 In order to provide a basis for comparison, Figs. 2 to 4 compare the solubility of aluminium in each electrolyte mixture with the solubilities given by the corresponding solutions of the individual hydroxides. Fig. 2 shows an increase in solubility on mixing equal volumes of 30 % NaOH and 50 % KOH. A gain of about 75 % in solubility is achieved as compared with 30 % NaOH and about 62 % as compared with 50 % KOH. In Fig. 3, a remarkable increase in solubility is shown. There is an immense gain of about 140 % in solubility for 30 % KOH on mixing with 50 % NaOH; there is about 58 % gain for 50 % NaOH.

Fig. 4 shows only a slight increase in solubility on mixing equal volumes of 50 % NaOH and 50 % KOH. This could be due to the fact that too many ions already exist in solution, with the result that the solution has almost reached its saturation point even before the introduction of the aluminium ions.

Fig. 5 compares the solubility value given by electrolyte No. 10 (50 % NaOH and 30 % KOH) with the solubilities given by the solution concentrations conventionally used as electrolytes for aluminium-air batteries and also with the concentrations of individual hydroxides at which our experiments have shown that very high solubility values are obtained. A very large gain of 140 % in solubility is found as compared with 30 % KOH. There is also a gain of about 84 % as compared with 30 % NaOH.

The remarkable gains in solubility for the electrolyte mixtures as compared with the conventional alkaline electrolytes mean that the electrolyte mixture, when used in aluminium cells, will result in a considerable improvement in useful discharge times.

As will be appreciated, it is very important that the advantages of reduced sludge formation, and/or delayed onset of sludge formation, are not achieved at the expense of other electrolyte properties, especially conductivity. Accordingly, electrolytes (2) and (3), which gave the best results in the solubility experiments, were investigated for other relevant electrolyte properties.

B. Other properties Tests were carried out on electrolytes (2) and (3) to determine the following: (i) The polarisation characteristics of aluminium in the electrolytes; (ii) The anodic efficiency of aluminium in the electrolytes; (iii) The self-discharge characteristics of aluminium in the electrolytes; (iv) The conductivity of the electrolytes; (v) The pH of the electrolytes; (vi) The viscosity of the electrolytes; and (vii) The nature of the reaction products formed on dissolution of aluminium in the electrolytes was examined using Scanning Electron Microscopy (SEM) and X-ray fluorescence and diffraction techniques.

In order to provide a basis for comparison, the same tests were carried out on conventional single hydroxide electrolytes, 30 % (w/v) NaOH and 30 % (w/v) KOH.

The electrochemical behaviour of the aluminium in the electrolyte mixtures showed improved polarisation as compared with the individual hydroxides at 30 % w/v. In the case of other electrochemical properties, including conductivity, the results with the mixed electrolytes were comparable with those obtained using the individual hydroxides.

The SEM experiments showed that the precipitates formed from the electrolyte mixtures tend to agglomerate in large blocks and can therefore readily be flushed or cleaned away.

The concentration of each hydroxide in a mixed electrolyte according to the invention will in general be at least 2 % (w/v), and usually at least 5 Yo.

The concentration of sodium hydroxide in a mixed electrolyte according to the invention is advantageously in the range of from 10 to 70 % (w/v), preferably in the range of from 20 to 60 %, and is more especially in the range of from 30 to 50 %.

The concentration of potassium hydroxide in a mixed electrolyte according to the invention is advantageously in the range of from 10 to 80 % (w/v), preferably in the range of from 30 to 60 %, and is more especially in the range of from 30 to 50 %.

The ratio of NaOH to KOH by volume may be in the range of from 2:1 to 1:1 preferably in the range of from 1.5:1 to 1:1, and is more especially approximately 1:1.

In general there will be little or no advantage in using a concentration of either hydroxide in excess of about 60 % w/v, and indeed it is to be expected that the solubility of the aluminium hydroxide sludge will tend to decrease above this figure.

Taking into consideration all relevant properties, we believe that optimum performance will in general be obtained with electrolyte mixtures of equal volumes of a % NaOH and b % KOH, where each of a and b is at least 30 and the sum of a and b is in the range 70 to 90, more especially 75 to 85, and is in particular 80.

In general, it is considered that useful guidance as to appropriate individual concentrations of the sodium and potassium hydroxides to be used in mixed electrolytes according to the invention is provided by consideration of the dependence of aluminate/Al(OH)3 solubility on concentration for each hydroxide taken alone. This broad general rule, however, is subject to there being no unacceptably detrimental effect on other electrolyte properties, especially conductivity. With these considerations in mind, experiments have been performed using the test aluminium alloy Q4 as hereinbefore specified, and the solubility and conductivity results so obtained (at 25 C.) are given in Fig. 6 for sodium hydroxide and in Fig. 7 for potassium hydroxide.

Referring to Fig. 6, it will be seen that there is a pronounced fall-off in conductivity at sodium hydroxide concentrations above 30 % w/v. At the same time, there is only a small increase in solubility between 30 % and 50 % w/v. On this basis, 30 % w/v sodium hydroxide may be especially recommended for use in formulating a mixed electrolyte according to the invention.

In the case of potassium hydroxide (Fig. 7), it will be seen that the decreases in conductivity between 30 % and 50 % w/v are not as pronounced as the increases in solubility over that range. Accordingly, 50 % w/v potassium hydroxide may be highly recommended for use in formulating a mixed electrolyte according to the invention.

It should be noted, however, that for neither of the hydroxides does the solubility curve follow exactly the conductivity curve and therefore, in general, the choice of hydroxide concentration will represent a compromise between the two. In particular, in neither case does maximum aluminate/Al(OH)3 solubility coincide with maximum conductivity.

It may be advantageous in some cases to operate a battery according to the invention initially with an electrolyte comprising only one of the hydroxides (for example, potassium hydroxide) and then to add the other hydroxide after a period of operation. Such a procedure can offset, at least to some extent, what might otherwise be an increased start-up time in those instances where the over-voltage is higher at room temperature with a mixed electrolyte according to the invention.

The temperature effect on a cell performance is one of the primary limiting factors. Advantageously, an aluminium-air battery according to the invention is operated at an internal temperature in the range of from 40 to 45 C. As a result of the heat generated on discharge in use, the battery can be used in environments at temperatures as low as -20"C and in such environments it will self-heat to an adequate working temperature.

A further problem encountered in the operation of aluminium-air batteries is that, when aluminium alloy anodes are in contact with aqueous electrolytes, parasitic reaction occurs leading to the release of hydrogen which in turn presents explosion hazards.

The release of hydrogen arises as follows: Al + 30H- Al(OH)3 + 3e 3e + 3H2O 30H- + 1.5 H2 In addition to the explosion hazard, diffusion of the hydrogen into voids in the aluminium metal may lead to loss of strength and, ultimately, fracture.

Conventional attempts to alleviate the problems of parasitic hydrogen evolution involve the use of hydrogen/oxygen catalytic recombination devices. In practice, however, the presently available devices are not completely reliable, especially if used intermittently.

Advantageously, a mixed electrolyte according to the invention includes a hydrogen evolution inhibitor. Preferably, the inhibitor is dissolved mercury (as HgO) in an amount up to that which gives a saturated solution. The HgO may be introduced by incorporating HgCI2 with the electro lyte whilst stirring at room temperature. The presence of HgO in solution in the electrolyte acts as an effective hydrogen evolution inhibitor without causing undesirable passivation of aluminium alloy anodes. Typically, the HgO content in solution in the electrolyte will be in the range of from 1 to 2 ppb, more especially about 1.5 ppb. It is remarkable that such a small amount of dissolved mercury can give such useful results.Not only does the presence of dissolved mercury reduce the rate of hydrogen evolution on the aluminium anode surface (presumably by increasing the over-voltage for hydrogen evolution), but the mercury also couples with atoms on the metal surface to produce a negative (anodic) shift in potential of up to as much as 200 mV.

It may be noted in passing that the incorporation of mercury as such in aluminium alloy anodes does not give satisfactory results, because the reactivity of such anodes is very much too high (the mercury having inhibited the formation of the normal protective oxide layer).

The effectiveness of HgO as a hydrogen evolution inhibitor is found to be greater at higher temperatures (for example, 40 C) than at room temperature (25 C). Even at lower temperature, however, the total working surface area of a typical aluminium electrode is such that even a marginal improvement in hydrogen evolution inhibition is worthwhile.

We have also found that the loss of active anode material, and hence the loss in charge, on self-discharge of aluminium alloy anodes in the presence of HgO is very much less than that observed without the HgO additive.

Other hydrogen evolution inhibitors which may be used are, for example, K2Cr207, Na2S, Na2O SiO2 solution, or dissolved gallium.

In general, the requirements for a satisfactory inhibitor are a high resulting over-voltage for hydrogen evolution without concomitant passivation at high current densities.

If desired, a hydrogen/oxygen recombination device can be used to eliminate, or at least reduce, any remaining evolved hydrogen.

Advantageously, the or each aluminium anode in an aluminium-air battery according to the invention comprises an alloy of aluminium with one or more of Zn, Ga, In, Pb, Bi and Sn. Such alloying with elements of high hydrogen over-potential has the effect of destabilising the passivating oxide film on the surface of pure aluminium without at the same time increasing the rate of corrosion. An Al-Zn alloy has been found to have relatively little self-discharge, but tends to passivate at high current densities. The addition of Ga or In to the Al-Zn alloy has been found to improve its anodic performance. The performance is subdued, however, if the concentration of Zn in the alloy is high (say, > 4 %). Thus, even though Zn is extremely valuable in improving the electrode properties of aluminium, the amount of Zn added must be closely controlled.

The alloy designated Q4 performs very well at high current densities with a high utilisation, but is also found to have a rather high self-discharge with the evolution of a relatively large amount of hydrogen. This may be attributed to the comparatively high concentration of Fe present in the Q4 alloy. Thus, to reduce the rate of self-discharge, the concentration of Fe must be kept to a minimum. Similar considerations apply to Cu.

Overall, the alloy designated Q4 has proved to be the best of all those we have tested. The problem of self-discharge, which results in loss of capacity under standby conditions, can be avoided to a certain extent by withdrawing the electrolyte from the cell, automatically, whenever power is not required and returning it to the cell when needed. Also, as hereinbefore described, measures can be taken to restrict the extent of parasitic hydrogen evolution and/or to cause recombination with oxygen.

It is not necessary to use pure (distilled) water to make up electrolyte solutions for use according to the invention and, in particular, seawater can be used, offering obvious advantages in submarine and offshore applications. The start-up time will in general be a few seconds longer in such applications, and the useful operational time will not be as great as is obtainable when distilled water is used, but there are in general no unacceptable disadvantages in the use of seawater. A foamy white gel (comprising aluminium chloride) forms in operation, but this can readily be removed as desired by electrolyte circulation or agitation.

The air (or oxygen) electrode may be of any conventional form. Thus, such electrodes typically comprise a porous, conducting solid, for example, graphite, into which both the electrolyte and the gas can penetrate. Several methods are available, dependent upon capillary action, for stabilising the gas-liquid interface within the pores. By wet-proofing some of the pores, the contact angle is raised, and electrolyte cannot penetrate. Such wet-proofing may be effected, for example, by polytetrafluoroethylene (PTFE) or by paraffin wax. Alternatively, if two distinctly different pore sizes are employed, it is possible to arrange the conditions so that the electrolyte floods the fine pores, but is kept out of the coarse pores by a positive gas pressure. In order to prevent the electrolyte from leaking out through the electrode pores, a wet-proof technique may be used. Thus, for example, the oxygen side of the electrode may be cladded with a thin layer of a porous water-repellent plastics material, such as a mixture of PTFE and acetylene black.

Advantageously, each or at least one electrode incorporates an electrocatalyst such as, for example, nickel cobalt oxide (NiCo2O4) or, preferably, lithiated cobalto-cobaltic oxide (Li/Co304).

Such electrocatalysts are known per se and may be used in a customary manner. Thus, for example they may be applied as powders, optionally in admixture with one or more other suitable electrocatalysts, in polymeric dispersion (for example, polytetrafluoroethylene dispersion) to supports (for example, nickel screens) which are dried and then cured to form polymerbonded electrodes.

Similarly, the anode may be of any customary form, but it is an important advantage of the use of mixed electrolytes according to the invention that it is possible to construct the anode in the form of a plurality of small discrete bodies accommodated in a conducting basket formed, for example, of stainless steel mesh (e.g. 18 Cr/2 Mo ferritic steel). With such an arrangement, it is possible to have automatic feeding of aluminium to the cell over long periods. Such an anode construction would not be practicable with a conventional single-hydroxide electrolyte, because the substantially more rapid sludge formation observed with such electrolytes would tend to isolate adjacent spheres electrically from one another. The discrete bodies may be of any form that is capable of being fed freely from a hopper, but are preferably small spheres.

Preferably, the diameter of the spheres is in the range of from 3 to 5 mm.

Alternatively, it is possible to use a self-perpetuating semi-wedge anode which sinks progressively when it is consumed enabling a constant distance between the cell electrodes to be maintained, thereby maintaining a constant internal resistance within the cell.

A battery according to the invention, especially a metal-air battery, can be used to provide power for applications both onshore and offshore. It can be used as the main power source in submersibles, life-boats, military field equipment and reconnaissance vehicles. It can also be used as an emergency power source for lighting, and for burglar and fire alarms, computer memory banks, as starters for car engines and as back-ups for generators.

An aqueous electrolyte for use according to the invention may be in any suitable physical form, for example, in a liquid or gel form or supported in a porous medium.

In accordance with generally accepted terminology, the term "battery" is used herein to include arrangements comprising only a single cell as well as arrangements comprising more than one cell.

A battery according to the invention is shown by way of example in the accompanying drawing (Fig. 8), which is a schematic perspective view, partly in section, in which the reference numerals have the following meanings: 1 positive terminal 2 aluminium anode 3 battery casing (with electrolyte mixture housed within) 4 air electrode (there is another one on the opposite side of the anode) 5 negative terminal.

Examples of suitable dimensions are as follows: a 120 mm b 80 mm c 80 mm d 100 mm e 10 mm (with internal Cathode to Anode separation of 2.3 mm in each case) The following Example illustrates the invention: EXAMPLE A cell as shown in the accompanying drawing and having an aluminium alloy anode in plate form (total area of each side, approximately 60 sq. cm), and a PTFE-bonded graphite cathode with a nickel screen support, was discharged at 8A, 1.2-1.3V and 40 C. for three hours continuously without clogging. The anode/cathode separation was 2.3 mm and the cathode incorporated an Li/Co304 electrocatalyst. The electrolyte was an equal volume mixture of 30 % w/v KOH and 50 % w/v NaoH saturated with HgO (1.5 ppb). The cell can be recharged, by putting in more aluminium and fresh electrolyte, to operate over any length of time. At a lower current drain, 3A (25 mA/cm2) the same cell was able to operate continuously (without being recharged) for over 48 hours without any serious problems; e.g. the problem of cell clogging.

The precipitate formed, from the dissolution of the active anode material, was granular in nature and could therefore be easily cleaned away as desired.

The energy density of the cell, calculated for a 24 hour operation time, was nearly 400 Wh/Kg; 3,714 Wh/Kg for the Al anode. The cell can be operated from as low as -20"C without the use of an external heater to warm it up.

Claims (14)

1. A battery having an aqueous alkaline electrolyte, characterised in that the electrolyte comprises an aqueous mixture of sodium hydroxide and potassium hydroxide.

2. A battery as claimed in claim 1, wherein the electrolyte comprises a mixture of sodium hydroxide of a concentration in the range of from 30 to 50 % w/v and potassium hydroxide of a concentration of from 50 to 30 % w/v.

3. A battery as claimed in claim 2, wherein the hydroxides are mixed in a ratio by volume in the range of from 2:1 to 1:1 (NaOH:KOH).

4. A battery as claimed in any one of claims 1 to 3, wherein the electrolyte includes dissolved mercury (as HgO).

5. A battery as claimed in any one of claims 1 to 4, wherein the anode is in the form of small spheres accommodated in a conducting basket.

6. A battery as claimed in any one of claims 1 to 5, which is a metal-air battery.

7. A battery as claimed in claim 6, which is an aluminium-air battery.

8. A battery as claimed claim 7, wherein the anode comprises an alloy of aluminium with one or more of Zn, Ga, In, Pb, Bi and Sn.

9. A method of using a metal-air battery as defined in any one of claims 6 to 8 in which the battery is maintained at an internal temperature in the range of from 40 to 45 C.

10. A battery as claimed in any one of claims 1 to 5, which is a metal/alkali/MnO2 battery.

11. A battery as claimed in claim 10, wherein the metal is zinc.

12. A battery as claimed in any one of claims 1 to 5, which is a cadmium, cobalt or iron/alkali/NiO(OH) battery.

13. A battery as claimed in any one of claims 1 to 5, which is a metal/alkali/silver oxide battery.

14. A battery as claimed in claim 13, wherein the metal is magnesium, aluminium, iron, cobalt or zinc.