Alkaline solid polymer electrolyte, electrode and electrochemica

Details Alkaline solid polymer electrolyte, electrode and electrochemica Alkaline solid polymer electrolyte, electrode and electrochemica

1995-03-01

1996-10-29

Willis, Jr., Prince

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

429189, 429191, 429198, 429206, 252 622, H01M 1026, H01M 1028

Patent

active

055695590

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to an alkaline polymer solid electrolyte, to the electrodes containing such an electrolyte and to their applications, especially electrochemical generators.
The term alkaline polymer solid electrolyte denotes a material which possesses characteristics inherent in materials based on organic polymers and characteristics of conduction of an electric current by displacement of ions, in particular hydroxyl ions OH.
Such a material may be used for the production of electrochemical devices: electrodes, sensors, electrochromic devices, generators, etc.
The present invention relates more particularly to so-called rechargeable or secondary generators and is of particular interest in the case of generators containing a so-called soluble negative electrode, especially one of zinc.
It is known that commercial accumulators operate with a liquid electrolyte: either an aqueous acid solution, of which the most widespread are lead accumulators, or an aqueous alkaline solution, of which the most widespread are those with a cadmium or zinc negative electrode, or a solution of an organic or inorganic salt in an organic solvent, of which the most widespread are those with a lithium negative electrode.
The electrolyte is maintained in a quantity which is sufficient for ion exchange between the electrodes, by capillarity in porous separators. These separators are in different forms, such as fibers in woven or feltlike form consisting of various materials, such as glass, cellulosic polymers, vinyl polymers, polyolefins or polyamides, for example.
Nevertheless, not all of these separators are always satisfactory, owing to their permeable character:
either to zincate ions which are formed in discharge, leading to the formation of well-known dendrites on recharge,
or to the oxygen which comes from the positive electrode in recharge, which prevents the complete charge of the negative electrode, especially when it is made of cadmium.
In order to remedy these drawbacks it has been proposed to replace these porous separators by sheets of polymers such as carboxymethylcellulose, carboxyl cellulose, regenerated cellulose and polyvinyl alcohol, although this measure does not give complete satisfaction.
These drawbacks are particularly notable in the case of nickel-zinc accumulators. In effect, this type of accumulator exhibits a serious defect in cyclability in aqueous potassium hydroxide because of the fact that potassium zincate, which is soluble, stratifies during discharge, and the zinc is deposited in the course of recharge at locations other than that where it dissolves. The result is a modification of the form and behavior of the zinc electrode. Moreover, during recharge, zinc dendrites form, which lead rapidly to short-circuits and render the accumulator unusable. Although specific membranes and various devices have been proposed for rendering the zinc electrode cyclable, it is found that these solutions do not give complete satisfaction, with the result that few nickel-zinc accumulators are on the market.
These parasitic phenomena are evidently not possible with solid polymer electrolytes (SPEs). These solid polymer electrolytes, moreover, are known to allow the production of thin generators which are easy to prepare by techniques involving the use of a polymer film.
By means of solid polymer electrolytes in salt or acid form it is hoped to obtain lithium accumulators of high performance in terms of capacity and cycling, or efficient fuel cells. These generators generally function at above ambient temperature (from 60.degree. to 200.degree. C.) since the conductivity of these SPEs is often poor at 25.degree. C. and below. Moreover, lithium accumulators exhibit additional transfer resistances, principally at the positive electrode, which limits still further the power at ambient temperature and below.
Furthermore, in general, it is known that in aqueous media alkaline electrolytes exhibit a number of advantages with respect to acidic electrolytes (H.sub.2 SO.sub.4) or saline electrolytes; in particular,

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Mark and Kroshwitz, eds. Encyclopedia of Polymer Science and Engineering, vol. 6, John Wiley and Sons, 1986 (no month) p. 246.
Aldrich Chemicals Catalogue, 1994, (no month) p. 1188.
Database WPIL AN=81-52429, 30 May 1981.

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