GB2083684A - Electrochemical cell - Google Patents

Abstract

Electrochemical power cells having a lithium or calcium anode, a carbonaceous current collector, and an electrolyte solution including an active cathode depolarizer such as thionyl chloride are enhanced by adding copper thereto.

Description

1

SPECIFICATION

Electrochernical cell GB 2 083 684 A 1 This invention relates to high energy electrochemical power cells. More particularly, it relates to cells having an oxidizable anode material and a liquid active cathode material and a solid current collector.

Recently there has been a rapid growth in portable electronic products requiring electrochemical cells-to supply the energy. Examples are calculators, cameras and digital watches. These products, however, have highlighted the deficiencies of the existing power cells for demanding applications. For example, digital watches were developed using the silver oxide cell, and although these watches have become popular, it is 10 now generally recognized that the least developed component of the digital watch system is the powerreil.

In particular, the energy density of the silver cells is such that thin, stylish watches with reasonable operating life are difficult to make. Additionally, these cells have poor storage characteristics, low cell voltages, and leakage problems.

In an effort to develop a cell that addresses one or more of the foregoing problems, substantial work has 15 been done with cell chemistries using a lithium anode. The cathode and electrolyte material consisting of a solvent and solute vary. Indeed the literature is replete with examples of lithium anode cells with different cathodes and electrolytes. The electrical characteristics of these cells such as energy per unit volume, called energy density; cell voltage; and internal impedance vary greatly.

Among all of the known combinations of lithium anodes with different cathodes and electrolytes, those 20 believed to have among the highest energy density and lowest internal impedance use certain inorganic liquids as the active cathode depolarizer. This type of cell chemistry is commonly referred to as "liquid cathode".

Early liquid cathode cells used liquid sulfur dioxide as the active cathode depolarizer as described in U.S.

Patent No. 3,567,515 issued to Maricle, et al. on March 2,1971. Since sulfur dioxide is not a liquid at room temperature and at atmospheric pressure, it proved to be quite a different chemistry with which to work.

More importantly, sulfur dioxide cells are unsafe for most consumer applications due to their propensity to explode under certain circumstances.

A major step forward in the development of liquid cathode cells was the discovery of a class of inorganic materials, generally called exyhalides, that are liquids at room temperature and also perform the function Of 30 being the active cathode depolarizer. Additionally, these materials may also be used as the electrolyte solvent. Liquid cathode cells using oxyhalides are described in U.S. Patent No. 3,926,669 issued to Auborn on December 16,1975, and in British Patent No. 1,409,307 issued to Blomgren et al. on October 18,1975. At least one of the oxyhalides, thionyl chloride (SOCIA in addition to having the general characteristics described above, also provides substantial additional energy density.

As described in the Auborn and Blorngren patents, the anode is lithium metal or alloys of lithium and the electrolyte solution is an ionically conductive solute dissolved in a solvent that is also an active cathode depolarizer.

The solute may be a simple or double salt which will produce an ionically condutive solution when dissolved in the solvent. Preferred solutes are complexes of inorganic or organic Lewis acids and inorganic 40 ionizable salts. The requirements for utility are that the salt, whether simple or complex, be compatible with the solvent being employed and that it yield a solution which is ionically conductive. According to the Lewis or electronic concept of acids and bases, many substances which contain no active hydrogen can act as acids or acceptors or electron doublets. In the U.S. Patent No. 3,542,602 it is suggested that the complex or double salt formed between a Lewis acid and an ionizable salt yields an entity which is more stable than either of the 45 components alone. Typical Lewis acids suitable for use in the present invention include aluminum chloride, antimony pentchloride, zirconium tetrachloride, phosphorous pentchloride, boron fluoride, boron chloride and boron bromide. 50 Inonizable salts useful in combination with the Lewis acids include lithium fluoride, lithium chloride, lithium 50 bromide, lithium sulfide, sodium fluoride, sodium chloride, sodium bromide, potassium fluoride, potassium chloride and potassium bromide. The double salts formed by a Lewis acid and an inorganic ionizable salt may be used as such orthe individual components may be added to the solvent separatelyto form the salt. One such double salt, for example, is thatformed bythe combination of aluminum chloride and lithium chloride to yield lithium aluminum tetrachloride.

In addition to an anode, active cathode depolarizerand ionically conductive electrolyte, these cells require a current collector.

According to Blorngren, any compatible solid, which is substantially electrically conductive and inert in the cell, will be useful as a cathode collector since the function of the collector is to permit external electrical contact to be made with the active cathode material. It is desirable to have as much surface contact as possible between the liquid cathode and the current collector. Therefore, a porous material is preferred since it will provide a high surface area interface with the liquid cathode material. The current collector may be metallic and may be present in any physical form such as metallic film, screen or a pressed powder.

Examples of some suitable metal current collectors are provided in Table 11 of the Auborn Patent. The current 66 2 GB 2 083 684 A 2 collector may also be made partly or completely of carbon. Examples provided in the Blorngren Patent use graphite.

Electrical separation of current collector and anode is required to insure that cathode or anode reactions do not occur unless electrical current flows through an external circuit. Since the current collector is insoluble in the electrolyte and the anode does not react spontaneously with the electrolyte, a mechanical 5 separator may be used. Materials useful for this function are described in the Auborn Patent.

Although the varied cells described in the Blorngren and Auborn Patents may be feasible, much of the recent interest is in cells using thionyl chloride as the active cathode depolarizer and electrolyte solvent. This results from thionyl chloride's apparent ability to provide greater energy density and current delivery capability than other oxyhalide systems. Yet even though thionyl chloride cells have proven to be the best performer among the oxyhalides, they have not lived up to expectations on energy density or internal impedance. Furthermore, the thionyl chloride cell is equally if not more dangerous than the sulfur dioxide cell. As a result, all known efforts to commercialize cells using this chemistry have failed.

The present invention is based on the discovery that a substantially safer lithium or calcium anode, thionyl chloride active cathode depolarizer cell with the added benefits of higher energy density and lower internal is impedance can be made by adding copper to the cell.

According to the invention there is provided an electrochemical cell comprising an anode of lithium or calcium; a carbonaceous current collector spaced from the anode; an ionically conductive electrolyte solution in intimate contact with the current collector and the anode, the electrolyte comprising an ionically conductive solute dissolved in an active cathode depolarizer which is phosphory] chloride, thionyl chloride, 20 sulfuryl chloride or a mixture of two or more thereof; the cell including copper in intimate contact with the - electrolyte.

As mentioned previously, the basis of this invention is the discovery that the addition of elemental copper to the cell provides substantial improvements in both performance and safety. While the theoretical explantion of this phenomenon is not clear, and applicant does not intend to be limited to any theory of 25 invention, at least one explanation of the results involves a reaction with the copper and elemental sulfur.

Some of the several different reactions that could occur in the prior art cells (there is no conclusive evidence as to which reaction or reactions actually occur) are as follows:

3 SOC12 + 8 Li U2S03 + 6 LiCI + 2S 4 SOC12 + 8 Li U2S204 + 6 LiCI + S2C12 4 SOC12 + 8 Li 4 LiC] + S02 + S (1) (2) (3) 1 These reactions aR release the same amount of electrical energy. However, equation (1) requires less thionyl chloride. Thus the energy density of this reaction is higher. That is, more electrical energy can be derived from a given volume of chemicals with the reaction of equation (1) than with the reaction of equations (2) or (3).

40- One possible explanation of the difference in energy of the cell of this invention is that the copper first acts 40 as a catalyst to force the reaction to be a higher energy density reaction such as the reaction of equation (1), and second, combines with the elemental sulfur to form copper sulfide.

The significance of the copper sulfur reaction relates to safety. It is widely believed that the elemental sulfur in contact with lithium will explode above a certain temperature and that this temperature may be easily reached by short circuiting, reverse charging, or other electrical conditions, as well as exposure to 45 high ambient temperatures. However, copper sulfide does not react explosively with lithium at any temperature likely to be experienced by batteries. Experimental evidence tends to support this in that batteries made pursuant to this invention do not explode under circumstances that would cause prior art batteries to explode.

A preferred method of incorporating the copper into the cell is by dispersing it in the current collector 50 although this method is not embraced by the claims of the present application, since it is protected by our copending-application No. 34613178.

The following examples of preferred methods of putting this application into effect are all in accordance with our copending application no. 34613178.

Test results showing the effect of varying percentages of copper on energy density and internal impedance are provided in Table 1. Each of the entries in Table 1 represents a test cell having a nominal internal volume of.035 cu inches constructed in a typical button configuration with single disc shaped current collector, anode and separator. The electrolyte is a 1.5 molar solution of lithium aluminum tetrachloride in thionyl chloride. The anode is.680 inch diameter disc of. 020 inch thick lithium. The separator is a commercially available ceramic paper. The current collector is acetylene black. Particulate copper is 60 dispersed in the acetylene black which is then compacted into a pellet. The only variable in the cell is the ratio by weight of carbon to copper.

3 GB 2 083 684 A 3 1 TABLE 1

Ratio % Life Load Current Internal C:Cu Cu mah voltage Impedance 1:1 50 110 3.29 54.8 5.16 2:1 33 170 3.30 55.0 5.56 3:1 25 180 3.29 54.8 5.65 10 4:1 20 200 3.28 54.6 5.86 6:1 14 210 3.28 54.6 5.86 8:1 11 205 3.24 54.0 6.6 10:1 9.1 195 3.24 54.0 6.6 12:1 7.7 195 3.22 53.6 7.0 20 16:1 5.9 195 3.22 53.6 7.22 -- 0 190 3.15 52.5 8.57 25 The foregoing results were taken from samples that were tested shortly after their construction. The trends generally indicated by the data in Table 1 are accentuated as the batteries are stored at elevated temperatures.

It should be understood that the preferred embodiments described above and claimed in copending 30 application no. 34613/78 do not represent the only way in which the cell may be constructed. Certainly copper in either a more or less finely divided state would be useful, and other battery geometries such as spiral wound may also be employed with the present invention.

Claims (5)

1. An electrochemical cell comprising an anode of lithium or calcium; a carbonaceous current collector spaced from the anode; an ionicafly conductive electrolyte solution in intimate contact with the current collector and the anode, the electrolyte comprising an ionically conductive solute dissolved in an active cathode depolarizer which is phosphoryl chloride, thionyl chloride, sulfuryl chloride or a mixture of two or more thereof; the cell including copper in intimate contact with the electrolyte.

2. An electrochemical cell as claimed in claim 1, wherein the current coi lector is a porous body.

3. An electrochemical cell as claimed in claim 2, wherein the porous body is acetylene black.

4. An electrochemical cell as claimed in anyone of claims 1 to 3, wherein the anode is primarily lithium.

5. An electrochemical cell as claimed in anyone of claims 1 to 4, wherein the active cathode depolarizer 45 is thionyl chloride.

Printed for Her majesty's stationery Office, by Croydon Printing company limited, Croydon, surrey, 1982.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.