GB2083683A - Electrolytes for electrochemical cells - Google Patents

Abstract

An electrolyte for eg Ni-Zn, Ag-Zn, Ni-Cd, Ni-Fe or Zn-air cells includes a Potassium- or Sodium-Ferro- or Ferri- cyamide.

Description

SPECIFICATION Electrolyte for electro-chemical cells This invention relates to an electrolyte for use in electro-chemical cells current producing cells or electrical accumulators of various types. The electrolyte contains a complexing agent which reduces electrode solubility improves electrolyte conductance and takes part in the charge discharge electrochemical reaction.

In the continuing search for improved power sources considerable work has been done in investigating means for improving the performance of various electrochemical cells and storage batteries particularly in respect of size weight and capacity.

Important applications for these being in such fields as hearing aids portable equipment space and satelite applications and ultimately electrically driven non-poluting vehicles. One aspect of this work has naturally been to investigate also the electrolyte used in such cells. Factors of interest here are conductance current carrying capacity corrosion effects on electrodes and other cell parts etc. these factors vary with the concentration of the electrolyte temperature solubility of the electrodes and additives. Other important considerations are rate of chemical reaction charge transfer and in the case of many rechargeable cells such problems as dendrite growth deterioration of electrode structure loss of capacity and resultant short serviceable life.

Various additives have been tested in electrochemical cells of different types and inorganic as well as organic additives as sometimes also used in the electroplating industry tried so as to combat dendrite growth improve charge transfer and reduce internal resistance orto generally improve cell performance and life.

Some references: U.S. Patent 4,074,028 Will Feb.

14 1978 Dendrite inhibiting electrolyte solution and rechargeable aqueous zinc-halogen cell containing the solution.

U.S. Patent 3,466,195 Spellman ESB Inc. Sept. 9 1969 Addition of silicate ions to electrolyte.

U.S. Patent 3,433,679 Spellman F & B Inc. March 18 1969 Lithium ions added to electrolyte.

U.S. Patent 3,658,593 Caiola Soc. Acc. Franc. April 25 1972 Tetrabutylammonium chloride in propylene carbonate.

U.S. Patent 3,540,935 Keating Nutt El Du Pont Nov.

17 1970 Sodium cyanide addition to prevent dendrites.

U.S. Patent 3,716,412 Peters EPS Ltd. U.K. Feb. 16 1973. Amorphous silica to acid electrolyte.

U.S. Patent 3,669,743 Swindells Melpar June 13 1972 solid zinc salt complex electrolyte.

Additives have also been added to electrode structures to help reduce loss of capacity electrode poisoning effects or to improve electrolyte penetration further into the electrode structure to better utilise the available material. For the main part the modifications and additives so far used and proposed have not shown adequate long term stability on continued recycling use In electrochemical cells as they tend to be oxidised rejected or react irreversibly with the electrode or other cell parts or are dissipated and lose their efficacity. The present invention solves many of the previously mentioned problems and in degree dependent upon the cell type electrode structure and construction in which the electrolyte is used such a cell is improved in efficiency performance and economic desireability.

The chemical reactions during charge and discharge of various e electrochemical cells are not fully known and there are often a number of competing reaction schemes taking part. To one skilled in the art of electrochemical cell reactions be it as applied to electroplating or to electrical or rechargeable cells it is not essential to fully understand all details of the reactions taking part to make use of the invention.

Furthermore the invention should of course be measured solely by means of the claims herein whereas the reactions taking placein a given electrochemical cell combination when using the electrolyte of this invention should not in any way be construed as defining or limiting the invention.

The electrolyte of this invention is particularly advantageous when used in pre-charged dry stored high capacity cells in metal-air cells and in rechargeable cells such as Silver-Zinc (Ag-Zn) Nickel Cadmium (Ni-Cd) Nickel-Zinc (Ni-Zn) Nickel-Iron (Ni-Fe) Lead Oxide (PbO) and metal-Air cells.

The constituent parts of the electrolyte and the optional initial additive of Fluoride in suitable form can equally well be used in cells having different electrolytes as also for the formation and activation of various electrodes prior to assembly into a cell as also for certain electroplating and electrochemical surface treatment processes. The electrolyte of this invention has in general improved conductivity particularly at low temperatures reducing cell internal resistance acting as a charge transfer medium it increases cell potential and improves cell stored capacity and efficiency by better utilisation of the electrode electrochemical capacity. In the case of rechargeable cells it will in addition combat dendrite formation by attenuation of electrode solubility and alteration of electrode conditions.In cells of the Silver-Zinc type cell degradation due to Ag migration to the zinc electrode is also greatly attenuated and recycling life considerably extended even without recourse to special separators.

The electrolyte comprises a Potassium or Sodium Ferro or Ferri-Cyanide with or without a cyanide or ferrcyanide of the electrode materials of the cell as electrolyte in the form of a solution or as an addition in solution or salt form to an electrolyte for electrochemical cells current producing cells or accumulators. The metalic (Fe) link retains the (CN) and avoids loss during recycling the (Fe(CN)) complex actually taking part in the charge discharge reaction.

Dependent upon cell type electrodes and electrolyte composition a reduction of the electrode solubility follows which reduces or eliminates effects such as shape change loss of electrode capacity dendrite growth etc.

In certain cases particularly in rechargeable cells with Zinc electrodes an additional small addition of a suitable Fluoride NaF or KF is also advantageous saturating the electrolyte with zincate ZnO can also be useful. The electrolyte of this invention provides the following improvements when used in electrochemical cells as prescribed. These advantages are present in all aspects of the invention hereafter listed to a greater or lesser degree and will not be listed again separately in each example.

1) Lower internal cell resistance.

2) Better low temperature performance.

3) High output potential.

4) Better efficiency.

5) Better utilisation of electrode material.

6) Combatting of dendrite formation.

7) Longer cell life.

The electrolyte of this invention can be prepared as follows: Potassium Hydroxide (KOH) solution is prepared to a density of 1.12 g/mi 20 C. (2.6 molar) 13 wt% of this is then added Pottassium Ferricyanide (K3Fe(CN)6) till a density of 1.14 to 1.2 g/ml is attained. Alternatively 15 grams of (KOH) granules 85% KOH and 4 grams of Pottassium Ferricyanide (K3Fe(CN6) can be dissolved per 100 ml of distilled water. The absolute ratios are not at all critical and in many cases even a very small amount of Pottassium Ferricyanide added to an existing electrolyte of many cells is sufficient to improve performance considerably.

The electrolyte density used in various types of cells differs widely and is chosen to suit the particular operating temperature and conditions as well as the cell components and electrodes concerned the criteria for the choice of electrolyte composition or electrolyte additive used as per this invention by any manufacturer for a given application or set of conditions may likewise vary and could well fall outside the prefered range here indicated as a guide.

This in no way detracts from the value of the invention or the benefits claimed herein.

Example 1 In accordance with one aspect of this invention a normal Silver-Zinc (Ag-Zn) pre-charged dry stored cell is supplied with the electrolyte of this invention consisting of Pottassium Hydroxide (KOH) solution of density 1.12 g/ml 20 C. to which is added Pottassium Ferricyanide (K3Fe(CN)6) till a density of 1.14 g/ml is attained. The cell will provide a higher output have a lower internal resistance give better efficiency and capacity.

Example 2 In accordance with a further aspect of this invention a normal cell of the Nickel-Cadmium (Ni-Cd) type is provided with the electrolyte of this invention prepared as in example 1 above instead of its more conventional electrolyte of KOH or NaOH. The cell will similarly provide improved characteristics as well as a higher output level than usual for conventional cells. The extent of other improvements will depend upon details of the electrode and cell construction.

Example 3 In accordance with a further aspect of this invention a cell of the normal Nickel-Iron (Ni-Fe) type is likewise improved by the use of the electrolyte.

Example 4 In accordance with a further aspect of this invehtion a cell of the metal-air type preferably of the Zinc-Air type is similarly improved by the use of the electrolyte of this invention.

Example 5 In accordance with a further aspect of this invention a cell of the Nickel-Zinc (Ni-Zn) type is similarly improved by the use of the electrolyte.

Example 6 In accordance with a further aspect of this invention a cell of the Nickel-Zinc (Ni-Zn) rechargeable type using the electrolyte of this invention will also have Zincate ZnO added to the electrolyte and a very little Fluoride KF or NaF for instance.

In a prefered embodiment of this invention the electrolyte of this invention is used in a Nickel Cadmium (Ni-Cd) rechargeable cell of normal construction the electrode structure of which can be of the pocket or the sintered or other type etc. and with suitable separators. The electrolyte of this invention being made up as previously described of Pottassium Hydroxide KOH to a density of 1.12 g/ml to which is added Pottassium Ferricyanide (K3Fe(CN)6) to increase the density to around 1.14 to 1.18 g/ml.

The absolute ratios are not at all critical as even a very small amount of (K3Fe(CN)6) added to an electrolyte is sufficient to considerably improve performance. In a further prefered embodiment of the invention the electrolyte is used in a Nickel-Zinc (Ni-Zn) cell of normal construction and can be composed as in the previous prefered embodiment of KOH solution to 1.12 g/ml to which is added Pottssium Ferricyanide to bring the density to 1.14 to 1.18 glml. In the case of Zinc based electrodes in rechargeable cells it has been found beneficial to add a small amount of Pottassium Fluoride or Sodium Fluoride KF or NaF to saturate the electrolyte with Zincate ZnO so as to further arrest the solubility of the Zinc electrode.The electrolyte of this invention is in no way limited to use with the above examples and can also be used with other electrode systems and additives. Such additives are often used to modify electrode conductivity or overpotential or to reduce gassing and self discharge etc. some such examples being graphite carbon Ni Pb Mn Mo V Ti Zr Hf Nb Hg etc. When using the electrolyte of this invention some such additives can to advantage be added in the form of a cyanide or ferricyanide such as Pottassiumcyanomercurate (K2Hg (CN)4).

The principles prefered embodiments examples various advantages and improvements obtained from the use of the present invention have been described in the foregoing specification. The invention which is to be protected herein is however not to be construed as limited to the particular examples of its use described or disclosed as these are to be regarded only as illustrative rather than restrictive.

Variations and changes may well be made by those skilled in the art without in any way departing from the spirit of the invention.

Claims (9)

1. An electrolyte for electrochemical cells current producing cells or accumulators using alkaline neutral weak acidic or organic electrolytes containing Pottassium or Sodium-Ferro or Ferri-Cyanide with or without cyanides or Ferricyanides of the electrode materials used in the cell.

2. The use of an electrolyte as in claim 1 together with a Fluoride for use as in claim 1.

3. The use of an electrolyte as in claims 1 and 2 together with an Oxide-Hydroxide of the cell electrode materials as for instance ZnO in a Nickel-Zinc cell for use as in claim 1.

4. The use of an electrolyte as in claims 1 to 3 for Silver-Zinc cells.

5. The use of an electrolyte as in claims 1 to 3 for Nickel-Cadmium cells.

6. The use of an electrolyte as in claims 1 to 3 for Nickel-Iron cells.

7. The use of an electrolyte as in claims 1 to 3 for Metal-Air cells for instance Zinc-Air cells.

8. The use of an electrolyte as in claims 1 to 3 for Nickel-zinc cells.

9. The use of an electrolyte as in claims 1 to 8 where in addition the electrolyte cell or electrode system may contain oxides or compounds used to modify electrode conductivity or overpotential etc.

such as for instance transitional elements mercury oxide or a mercury amalgamated electrode.