GB2038078A - Sulfur trioxide soluble cathode primary cell - Google Patents

Abstract

A primary electrochemical cell has an oxidizable active anode material; an inert cathode current collector; and an electrolyte solution, in contact with the anode and the cathode current collector, consisting essentially of at least one soluble electrolyte salt and a solvent mixture consisting essentially of co- solvents sulfur trioxide and sulfur dioxide; wherein the sulfur trioxide is the sole active cathode material.

Description

SPECIFICATION Sulfur trioxide soluble cathode primary cell The present invention relates to electrochemical cells, more particularly to primary electrohemical cells having an oxidizable active anode material, an inert cathode current collector and an electrolyte solution comprising a soluble cathode and an electrolyte salt.

Primary electrochemical cells are a class of voltaic cells. Voltaic cells are those electrochemical cells in which chemical changes produce electrical energy, in distinction to electrolysis cells in which electrical energy from an outside source produces chemical changes within the cell.

Primary cells are those voltaic cells which cannot be conveniently recharged, which usually are discarded after a single exhaustion of their component elements, or which require replacement of their exhausted chemical constituents to bring them back to their original condition. These cells are distinguished from another class of voltaic cells, namely, secondary cells, in which the exhausted cell may be recharged by passing electrical current from an outside source through it in the reverse direction to the discharge current.

In a primary cell, chemical energy is converted to electrical energy with a reduction in the free energy of the system. In the course of the cell reaction, negative charge leaves the anode and enters the cathode through a driven external circuit. Thus, the cathode, where reduction is occurring is the positive electrode and the anode, where oxidation is occurring is the negative electrode. By virtue of the established electromotive series, it is possible to select suitable cathodes and anodes to obtain a theoretically high potential. It would be desirable if the cell could be designed such that the theoretical potential could be obtained under load and the loss in free energy would manifest itself entirely as electrical energy outside the cell.However, this ideal is never attained in practice, because the internal resistance of a cell is not zero and the reactions within the cell are never completely reversible. Moreover, problems of incompatibility of the cathode and anode with each other or with the electrolyte, polarization, and other well known problems prevent performance at theoretical values. There is a present need for batteries which have high initial electromotive force, greatly extend storage and operating life, improved total current output, reduce power to weight ratios, and improved constancy of voltage with time of storage and discharge.

A number of promising electrochemical cells have undergone development in recent years.

Among these is a class of cells, usable in heating aids and other medically-related devices, which employ soluble or liquid cathodes as opposed to the more conventional solid cathode cells. In such soluble cathode cells, the active cathode material is usually a solvent, or one of a number of cosolvents, for the electrolyte solutes. During discharge, the solvent or cosolvents are electrochemically reduced on an inert cathode current collector which typically comprises a screen having pressed thereon a mixture of an inert conductive material such as carbon black, graphite, or the like. The anode for these cells is usually lithium metal although other active metals such as sodium, potassium, rubidium, calcium, magnesium, strontium, barium and cesium may be used either singly or in combination.

In U.S. Patent 3,567,515, Maricle and Mohns describe a cell of this general type. They disclose an electrochemical cell comprising an anode of a metal capable of reducing sulfur dioxide, a cathode current collector of a material substantially inert to sulfur dioxide but on which sulfur dioxide is reducible, and an electrolyte salt substantially inert to sulfur dioxide and to the anode metal, wherein the anode and cathode current collector is immersed in the sulfur dioxide solution. The sulfur dioxide solution is used as the soluble cathode or material which undergoes electrochemical reduction.

In U.S. 3,578,500, Maricle and Hoffman disclose a variation of the above cell which uses certain compounds as soluble cathodes together with sulfur dioxide. The disclosed solvents are in general liquid organic and inorganic compounds which have electron rich centers, i.e., contain one or more atoms having at least one unshared pair of electrons, and which lack acidic hydrogen atoms. A large number of compounds are listed as possible cosolvents with sulfur dioxide, among these is sulfuryl chloride. The disclosed cells typically have open circuit potentials of four volts or less. Example XVII shows a cell employing a lithium anode, a nickel plaque cathode and an electrolyte comprising one molar LiClO4 is propylene carbonate and sulfur dioxide together with sulfuryl chloride. The cell gave an open circuit potential of 3.5V.

In U.S. 3,926,669, to Auborn, there is disclosed another electrochemical cell of this general class which employs a covalent inorganic oxyhalide or thiohalide as the solvent for the electrolytic solution. Sulfuryl chloride is disclosed as a suitable solvent either alone or in admixture with other materials. The examples show a variety of cells employing lithium anodes and different cathodes and solvent materials, which exhibit open circuit potentials of from 2.05 to 3.74V. It is stated at column 5, lines 40 to 59 that the disclosed electrochemical cells specifically exclude sulfur dioxide and other oxidants as cathode materials or as solvent or cosolvent materials, because there is no need for sulfur dioxide where the thiohalide or oxyhalide is employed.

In U.S. 4,020,240, to Schlaikjer, there is disclosed another electrochemical cell of this general type, employing an electrolyte salt containing a clovoborate anion. The disclosed cells are said to have characteristics of high potential and current capabilities at low temperatures, and to be resistant to anode passivation during long-time storage at elevated temperatures. The disclosed electrolyte salts are said to be useful in electrochemical cells utilizing a wide variety of soluble cathode materials. Among these are sulfur dioxide and sulfur trioxide. It is broadly disclosed that these as well as the other compatible solvents can be used alone or in combination.The cell in Example 3 employed a lithium anode, a thionyl chloride solvent with Li2B10C110 as the electrolyte salt and showed an open circuit potential of about 3.62 t 0.05V.

In our Patent Specification No. 7943792 (Ref. D-21839), filed concurrently herewith, there is disclosed a primary voltaic electrochemical cell having: an oxidizable anode material; an inert cathode current collector; and an electrolyte solution, in contact with the anode and cathode current collector, consisting essentially of at least one soluble electrolyte salt and a solvent mixture selected from the group consisting of (a) cosolvents sulfur trioxide and sulfuryl chloride, and (b) trisolvents sulfur trioxide, sulfur dioxide and sulfuryl chloride, wherein the sulfur trioxide is the sole active cathode material.

It is an object of the present invention to produce a primary electrochemical cell having a high open circuit potential which does not require pre-electrolysis to generate the cathode oxidant.

It is another object of the present invention to produce a primary electrochemical cell having a relatively constant voltage over an extended period of discharge.

It is yet another object of the present invention to produce a primary electrochemical cell with an extended shelf life.

These and other objects are accomplished according to the present invention which provides a primary electrochemical cell comprising an oxidizable active anode material, an inert cathode current collector, and an electrolyte solution in contact with the anode and cathode current collector consisting essentially of at least one electrolyte salt and a solvent mixture consisting essentially of sulfur trioxide and sulfur dioxide, wherein the sulfur trioxide is the sole active cathode material.

The present invention will be described in detail below and is illustrated in important detail in the attached drawings wherein: Figure 1 shows the variation of conductivity with concentration of electrolyte salt for a solvent mixture consisting of 49 wt.% sulfur trioxide and 51 wt.% sulfur dioxide, and Figure 2 shows discharge curves for two cells according to the present invention prepared in Example 3.

This invention relates to a primary voltaic electrochemical cell having an oxidizable active anode material, an inert cathode current collector, and an electrolyte solution consisting essentially of at least one electrolyte salt and a solvent mixture consisting essentially of sulfur trioxide (SO3) with sulfur dioxide (SO2) as a cosolvent, wherein the sulfur trioxide is the sole active, i.e., reducible, cathode material.

The cells produced accordingly to the present invention, and those produced in our aboveidentified copending application, are the only known operable and pratical soluble cathode cells that will discharge on carbon cathode current collectors at 4.5 volts or higher versus lithium. For the purposes of the present invention, the cell is defined as having sulfur trioxide as the sole reducible cathode material. The other solvents, sulfur dioxide or sulfuryl chloride, function primarily to promote solubility of the electrolyte salt and to prevent the sulfur trioxide from polymerizing (freezing). They do not function as active, reducible cathode materials. No solvents other than sulfur dioxide and sulfuryl chloride are know to be effective with sulfur trioxide.

Solvents such as POCK, and S205CL2 have been found too corrosive when mixed with sulfur trioxide. Likewise, metal based halides and oxyhalides such as SeOCI2, LOCI3, CrO2Cl2, and the like have been rejected because of toxicity, reactivity, expense, lack of ability to dissolve electrolyte salts, or usually a combination of these reasons. Thus, the present invention is limited to the named solvent mixtures.

The anode is preferably lithium metal, although other oxidizable anode materials contemplated for use in a cell of this invention include other alkali metals such as sodium, potassium, cesium and rubidium; the alkaline earth metals such as beryllium, magnesium, calcium strontium, barium, zinc and cadmium; the Group IIIA and IIIB metals such as the rare earths, scandium, yttrium, aluminum, gallium, indium and thallium; the Group IVA metals such as tin and lead; and transition metals such as titanium vanadium, manganese, iron, cobalt and copper.

The inert cathode current collector is any material which is inert to the other components of the system and sufficiently electrically conductive to draw off the current that is being produced by the cell. Typically, the current collector is a nickel, nickel alloy or stainless steel grid or screen having applied to it an inert and electrically conductive material such as carbon black, graphite or other electrically conductive material of high surface area. These materials preferably contain binding agents which hold them together and maintain them in position on the screen.

The salts utilized as electrolytes according to the present invention must provide M ions, such as Li+, and anions which are stable to oxidation and Lewis acid addition by SO3. The salts will be present in amounts effective to provide sufficient conductivity to the cell to operate as a primary voltaic electrochemical cell. Typically, the salts will be employed in amounts effective to make the solutions from 0.01 to 2.0M. Specific conductivities above 1 > < X 10-510-# ohm-'cm-' will typically be employed.Preferably, the conductivities should be above 1 X 10-4 ohm-'cm-l. In Fig. 1 there is shown the variation of specific conductivity with concentration for a cell employing 49 wt.% S03 in a SO3-SO2 cosolvent mixture, and employing varying amounts of LiAsF6 as the electrolyte salt.

Among the useful electrolyte salts are those which provide at least one anion of the general formula SO3X-, MX4-, M'X6- and M"C16--, where M is an element selected from the group consisting of aluminum and boron; M' is a Group VA or VB metal selected from the group consisting of phosphorous, arsenic and antimony, niobium and tantalum; M" is a Group IVA or IVB element selected from the group consisting of silicon, tin, zirconium, hafnium and titanium; an X is chlorine or fluorine.Preferred salts provide at least one anion selected from the group consisting of: SO3CI-, SO3F-, BF4-, BC14-, AIC14-, Al F6 - -, PF6-, AsF6-, SbF6-, SbCL6-, NbF6-, TaF6-, SiF6--, SiCl6-#, SnF6--, ZrF6--, HfF6--, TiCI6--, TiF6-#, WF6, MoF6-, and PbCl6--. The disclosure of U.S. Patent 3,926,669 is hereby incorporated by reference with regard to the electrolyte salts of the type disclosed therein.

Also suitable as electrolyte salts are the clovoborates disclosed in U.S. Patent 4,020,240.

Among these are metal clovoborates having a metal cation selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, stontium and barium, or combinations thereof, and a clovoborate anion which has a formula (BmXn)-k wherein m, n and k are integers with m ranging from 6-20, n ranging from 6-18 and k ranging from 1-4, B is boron, and X is selected from the group consisting of H, F, Cl, Br, l, OH and combinations thereof. The disclosure of this patent is also incorporated by reference with regard to the particular electrolyte salts disclosed therein.

Examples of suitable soluble salts yielding Li+ and anions are Li2Br2C112, LiCI, LiF, Lib4, LiASF6, LiSbF6, and LiPF6. The nature of the solubilization of electrolyte salts with concomitant useful conductivity in the SO3-SO2 cosolvent system of this invention does not seem to be the same for the various classes of salts and precise interaction for all is not presently known. The solubility of Li2B,2CI,2 in the cosolvent appears to result from the presence of SO2. Generally, our experience with Li2B,2C1,2 suggests that as the weight percentage of SO, increases, the solubility of this salt declines.Solubility in pure liquid SO2 results from the ability of the solvent to polarize or interact with large multi-electron anions and in such fashion contribute significantly to solvation energy. This propensity of polarizability by liquid SO2 is apparently operative with the Li2S12C112 electrolyte salt and allows for the observed solubility and conductivity when the weight percentage of SO, is carefully adjusted.

The use of simple metallic halides, e.g. MX or MX2 where X = F- or Cl- to form conductive solutions in SO3-SO2 mixtures results from the tendency of the strong Lewis acid, SO2, to add to fluoride and chloride ions, SO3 X + SO3 3SO3X o SnO3nX The reaction in the instant cosolvent system does not stop at simple monomer or halogensulfate but proceeds to polymeric halogenpolysulfate ions which dissociate sufficiently to render the solution conductive.

The ability of the disclosed halogenometallate complexes, MX4-, M'X6- and M"X6- - to act as electrolytes in the SO3-SO2 cosolvent system of this invention is not entirely understood at this time. For example, LiAsF6 is insoluble in liquid SO2 and in liquid SO3 separately, yet a novel mixture of the two solvents effects dissolution of the salt. In fact, a 1 M LiAsF6 solution has been prepared in 50 wt.% SO, and SO2 which has a specific conductivity of 1.4 X 10-2 S2-' cm-'.

The nature of the solvent interaction may rest with the ability of the electron deficient SO3 molecules to form fluoride bridges with the AsF6- anion and thereby solubilize the salt in this solvent system, i.e.,

In a preferred embodiment of the invention, a primary lithium/sulfur trioxide-sulfur dioxide electrochemical cell is provided which exhibits a single cell open circuit potential (OCP) of 4.6V.

This is the highest OCP obtained to date on a single cell which does not require pre-electrolysis to generate the cathode oxidant. Previous reports on lithium cells, J. Electrochem. Soc., 118, 461 (1971); Abstract No. 6, Extended Abstracts of Battery Division, Fall Meeting 1977, The Electrochemical Society; J. Electrochem. Soc., 125, 186C (1978) have reported high OCP but relied on the electrolysis to produce unstable products. The preferred Li/SO3-SO2 cell described herein develops the OCP immediately upon assembly and shows extended shelf life.

The electrolyte solution consists essentially of a cosolvent mixture of sulfur trioxide and sulfur dioxide, and at least one electrolyte salt dissolved therein. The cosolvent mixture will always be present in more than a major amount of the cathode solvents, and cannot contain other materials which adversely affect the operation of the cells as improved by the sulfur trioxide cosolvent cathode system of this invention. The cosolvents may be utilized in all weight percentages depending on the volumetric amount of sulfur trioxide needed and the desired conductivity of the solution once the electrolyte salt is added. However, they are preferably employed in weight ratios sufficient to fully dissolve the salt employed. In most cases weight percentages of the S02 will be from 10 to 90%.The liquid S02 and S03 cosolvents typically showed conductivities varying between 0.5-5 X 10-6 ohm-' cm-' before addition of the electrolyte salt.

It is preferred that the sulfur dioxide be dried by condensing it onto P4010 at ~78 C and by distilling it from this mixture to yield liquid SO, essentially free from water. Typical specific conductivities for preparations of liquid SO, collected using the aforementioned treatment are less than 10-6 ohm-' cm-'. The sulfur trioxide can be of commercial purity; satisfactory results have been obtained with material from MCB Manufacturing Chemists, a distributor for Allied Chemical Corporation's SULFAN stabilized sulfur trioxide. It is preferred that the S03 be fractionally distilled, to provide material essentially free from stabilizers and sulfuric acid and to promote increased lithium stability.

The following examples are for the purpose of further illustrating and explaining the present invention, and are not to be taken as limiting in any regard. Unless otherwise indicated, all parts and percentages are by weight.

Example 1 This example shows the preparation of a cell according to the invention employing a lithium anode and a lithium clovoborate as an electrolyte salt. The Li2B,2CL,2 electrolyte salt was prepared by known chemical literature procedures (U.S Patent 3,551,120, Substituted Dodecaborates; and Inorg. Chem., 3, 159 #1 964 ] ). The Li2B,2C1,2 prepared in this manner proved to be insoluble in liquid S03 but showed high solubility in liquid SO, ( > 1 M). An electrolyte solution was then prepared using a 36 wt.% S03 and 64 wt.% SO, as the cosolvent mixture, to which the Li2B,2CIr2 electrolyte salt was added in an amount sufficient to make the solution 0.1 M.The electrolyte solution had a specific conductivity of 1 X 10-3 ohm-' cm-'. This liquid cathode, containing dissolved electrolyte salt, was transferred to a glass pressure cell which contained a lithium anode supported on nickel screen, a lithium reference and a cathode comprised of carbon black and binder supported on nickel screen. The initial OCP was 4.6V. The cell discharged for 4 days above 4.2V at a rate of 0.2 mA/cm2.

Example 2 This example describes the preparation and discharge characteristics of a cell according to the invention employing a lithium anode and LiCI to produce the dissolved electrolyte salt.

The cosolvent used in this example contained 47 wt.% S03 and 53 wt.% 503. This solvent was transferred to a glass pressure cell which contained sufficient LiCI (vacuum dried) to make the solution of 0.5M with respect to LiCI. The cell also contained the electrodes described in Example 1. Dissolution of the LiCI occurred followed by subsequent precipitation of a white solid at the bottom of the cell. We believe this to have resulted from the sequential formation of 503C1-, and SO3Cl- with added 503. The OCP was 4.61V and sustained an initial discharge rate of 0.5 mA/cm2 of lithium above 4V.

Example 3 This example describes the preparation and discharge characteristics of two cells according to the invention employing lithium anodes and LiAsF6 as the electrolyte salt.

The LiAsF6 was used as received from U.S.S. Fluorine Chemicals, Decatur, Georgia. The electrolyte solutions contained 50 wt.% SO, and 50 wt.% SO2, with sufficient LiASF6 to make the solution 0. 1M in one cell and 0.16M in the other. The electrolyte solutions were introduced into glass pressure cells containing electrodes as described in Example 1. The OCP for the cells constructed in this fashion averaged between 4.6-4.7V. The cells were discharged across a constant 8200 ohm resistor at an average current density of 0.25 mA/cm2. Fig. 2 shows the discharge curves.

Example 4 This example illustrates a cell according to this invention employing a sodium anode and LiAsF6 as the electrolyte salt. An electrolyte solution containing 50 wt.% SO, and 50 wt.% SO, was made 0.4M with respect to LiAsF6 electrolyte salt and was transferred to a pressure cell containing a sodium anode, sodium reference electrode and a nickel supported carbon cathode.

This cell developed an OCP of 4.48V and was discharged for 2 days above 4.OV.

The above disclosure is for the purpose of explaining the present invention to those skilled in the art. It is not intended to describe all those obvious modifications and variations of the invention which will become apparent upon reading. Applicants do intend, however, to include all such obvious modifications and variations within the scope of the invention which is defined by the following claims.

Claims (12)

1. A primary electrochemical cell comprising: an oxidizable active anode material; an inert cathode current collector; and an electrolyte solution, in contact with the anode and the cathode current collector, consisting essentially of at least one soluble electrolyte salt and a solvent mixture consisting essentially of cosolvents sulfur trioxide and sulfur dioxide; wherein the sulfur trioxide is the sole active cathode material.

2. A primary electrochemical cell according to Claim 1, wherein the oxidizable active anode material is lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium or a combination of these.

3. A primary electrochemical cell according to Claim 1, wherein the oxidizable active anode material comprises lithium.

4. A primary electrochemical cell according to Claim 1, wherein the electrolyte salt comprises a metal clovoborate having a metal cation consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, or barium, or a combination thereof, and a clovoborate anion which has a formula (BmXn)-k wherein m, n and k are integers with m ranging from 6-20, n ranging from 6-18 and k ranging from 1-4, B is boron, and X is H, F, Cl, Br, l, OH or a combination thereof.

5. A primary electrochemical cell according to Claim 4, wherein the clovoborate ion is B12Cl12--.

6. A primary electrochemical cell according to Claim 1, wherein the electrolyte salt provides at least one anion consisting of: S03CI-, 503F -, BF, , BCí4-, AIC14-, AIF6---, PF6, AsF6-, SbF6-, SbCl6-, NbF6-, TaF6-, SiF6- -, SiCI6- -, SnF6- -, ZrF6 - -, HfF6--, TiCL6--, TiF6 -, WF6-, MoF6 - or PbCl6- -.

7. A primary electrochemical cell according to Claim 6, wherein the electrolyte salt comprises LiAsF6.

8. A primary electrochemical cell according to Claim 1, wherein the inert cathode current collector comprises a binding agent and carbon black or graphite.

9. A primary electrochemical cell according to Claim 1, wherein the sulfur trioxide is employed in an amount of from 90 to 10 wt. % based on the combined weight of the sulfur trioxide and sulfur dioxide.

10. A primary electrochemical cell according to Claim 1, wherein the electrolyte salt is present in an amount effective to provide a specific conductivity of the solution of above 1 X 10-5 ohm-1 cm-1.

11. A primary electrochemical cell substantially as described in any one of Examples 1-4 herein.

12. The features as herein described, or their equivalents, in any novel selection.