Paper separators for electrochemical cells

Details Paper separators for electrochemical cells Paper separators for electrochemical cells

1999-03-23

2001-04-24

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S247000, C429S129000, C429S145000, C162S123000

Reexamination Certificate

active

06221532

ABSTRACT:

The present invention relates to types of paper useful in the manufacture of separators for electrochemical cells, especially zinc chloride cells.
The history of electrochemical cells goes back to 1866 when Leclanché first discovered the principle on which they are based. The manufacture and design of electrochemical cells has come a long way since that time, but problems still remain. Cells (also known as batteries, although the term technically relates to a series of cells) essentially consist of an anode, a cathode and an electrolyte. In the present day version of the Leclanché cell, the anode is zinc, the cathode is manganese dioxide and the electrolyte is an aqueous solution of varying proportions of zinc chloride and ammonium chloride. In other primary cells, the electrolyte is frequently an aqueous solution of potassium or sodium hydroxide. In any event, it is necessary to seal the various components into a can in order to prevent the possibly dangerous escape of the constituents, as well as to prevent the atmosphere from affecting the constituents.
The problem of leakage of the electrolyte and corrosion of the can (zinc in Leclanché cells) was very largely overcome by the addition of cadmium and mercury, but especially mercury, to the cell ingredients.
Thus, mercury was responsible for reducing perforation of the can during abuse conditions, reducing corrosion and preventing perforation during storage, and it also had the advantage that it assisted in discharge. However, now that mercury is viewed as a major environmental pollutant, there has been a very major push to develop cells with no added mercury and, to a lesser extent, cells with no added cadmium.
In seeking to overcome the problems associated with cells containing no added mercury, it is the electrolyte mix which has been targeted, and a number of different additives, such as arylsulphur compounds, fluoroalkylpolyoxyethylene ether compounds, alkyl polyoxyethylene ethers, alkyl polyoxyethylene phosphate ethers and tetraalkyl and alkyl ammonium compounds, have been tried, all with varying degrees of success. In addition, cell design has also played a large role, with there being a large number of different designs for the seal.
However, the design of the separator has largely been ignored. This is essentially because the purpose of the separator is to prevent contact between the materials used for the cathode and anode, whilst permitting ionic contact between the materials via the electrolyte which permeates the separator. As such, the criteria for the manufacture of a separator have always been rather loose, and conventional separators vary very widely, the only matter which they share in common being that they are made of porous paper and have a coating of starch and gellant to assist in absorption of the electrolyte.
We have now discovered that the type of paper used in the manufacture of the separator can have a very great impact on the properties of the cell in which it is used.
Our tests on a number of different papers which are currently used in the manufacture of separators have established that the type of paper used can make a very great difference to cell performance. Of the papers known in the prior art, that which is currently known as PBDE100 (as disclosed in U.S. Pat. No. 4,001,044) performs well in a variety of tests. However, this paper suffers a number of disadvantages, not least of which is expense.
PBDE100 is a duplex paper, insofar as it is manufactured by the combination of pulp from two sources. This makes PBDE100 expensive to manufacture, but it has a number of advantages, including: the ability to prevent mix penetration; low leakage in low drain continuous testing; high short circuit amperage; high continuous performance; and high performance retention. However, apart from the high cost of manufacture, this paper also suffers from high leakage in high drain continuous testing.
Accordingly, it is desired to find a paper with the advantages of PBDE100 and which is cheaper to manufacture or which has low leakage in the high drain continuous test, or both.
In a first aspect, the present invention provides paper for use as a separator in an electrochemical cell which, at room temperature (20° C.), completely absorbs a 50 &mgr;l drop of water placed thereon in a time of not less than four minutes, and not greater than fifteen minutes.
We have established that the rate of absorption of a drop of water under ambient conditions at room temperature is indicative of the performance of the paper as a separator. Currently, all of the papers used as separators, including PBDE100, absorb the droplet of water in less than four minutes. In fact, most currently used separator papers absorb water in a matter of seconds, while PBDE100 generally takes a little over three minutes.
In addition to the length of time that it takes to absorb the droplet of water, we also prefer that the paper wets virtually instantly (i.e. in less than 5 seconds), preferably in less than 1 second, from one side to the other when the droplet is placed upon it. Whilst this is not crucial, it is desirable, and provides an indication of the thickness of the paper, amongst other things. The paper thickness is not crucial to the present invention, but is merely guided in the same manner as for conventional cells. Paper which is too thin may not provide a good barrier to penetration of the mix, whilst paper which is too thick may obstruct ionic communication between the cathode and anode material, as well as taking up valuable space in the cell. For example, a commonly used paper is Enso 80 (Trademark), and this has a thickness of 160 &mgr;m, and is the thickest paper currently used on a regular basis.
The method of manufacture of the paper has some considerable effect on the properties of the paper. As stated above, PBDE100 is manufactured by bringing together pulp from two different vats, so as to provide a duplex paper.
While this method tends to produce a quality paper, it is costly and complex
Most papers used in conventional separators are actually from a single source of pulp. These papers are considerably cheaper to manufacture, but perform poorly in a number of tests. However, we have established that it is possible to produce papers from a single source of pulp which perform well in all tests, and such papers are characterised by their ability, at a temperature of about 20° C., to absorb a 50 &mgr;l droplet of water in a period of between four and fifteen minutes. More preferably, this period is between five and fifteen minutes and is particularly preferably between five and ten minutes.
If the paper absorbs the droplet of water in less than four minutes, then the density of the paper tends to be too low, and poor results are obtained in the tests (which are outlined below). If the paper absorbs the droplet in greater than fifteen minutes, then this causes practical problems during manufacture, as the individual cells need to be voltage tested soon after assembly, and the delay in absorbing the electrolyte from the mix would mean that there would be an unacceptable storage time before the cells could be tested.
The characteristics of the papers which have the necessary absorption tend to be those of high beat and high density. Beating is performed on the pulp prior to formation of the paper, and the degree of beating can be measured by the use of the “Canadian standard freeness tester”. The test is T 227m-58 of the Technical Association of the Paper and Pulp Industry and is described, for example, in “A Laboratory Handbook of Pulp and Paper Manufacture (Auth. J. Grant, Pub. Edward Arnold, 2nd Ed. 1961, pp.154 et seq.).
Conventional papers, such as Enso 80 (supra) have a density typically in the region of 0.5 g/cm
3
, and even PBDE100 only has a density of 0.62 g/cm
3
.
The single pulp source papers which are preferred for the present invention have densities typically of 0.64 g/cm
3
and above, with preferred densities being between about 0.65 and about 1 g/cm
3
, and more preferred densities being between about 0.65 and ab

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