GB2038536A - 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 either of cosolvents sulfur trioxide and sulfuryl chloride, or of trisolvents sulfur trioxide, sulfur dioxide and sulfuryl chloride; wherein the sulfur trioxide is the sole active cathode material. In a preferred embodiment where the oxidizable active anode material is lithium metal, single cell open circuit potentials of 4.6V and above can be obtained.

Description

SPECIFICATION Sulfur trioxide soluble cathode primary cell The present invention relates to electrochemical cells, more particularly to primary electrochemical 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 withhin 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 extended storage and operating life, improved total current output, reduced 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 hearing aids and other medically-related devices, which employ soluble or liquid cathodes as opposed to the more conventional soid 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 salts. 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 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 material substantially inert to sulfur dioxide but on which sulfur dioxide is reducible, and an electrolyte salt substantially inert to sulfur dioxide and the anode metal, wherein the anode and cathode current collector are 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 empllying a lithium anode, a nickel plaque cathode and an electrolyte comprising one molar Lilo4 in 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 soluble cathode and solvent for the electrolytic solution. Sulfuryl chloride is disclosed as a suitable soluble cathode either alone or in admixture with other materials. The examples show a variety of cells employing lithium anodes and different cathode 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 characteris#tics 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, sulfuryl chloride and sulfur trioxide.

It is broadly disclosed lhat 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 l,i2B10Cl10 as the electrolyte salt and showed an open circuit potential of about 3.62 i 0.05V.

In Patent Application No. (Ref. No. D-21137), 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 consisting essentially of at least one electrolyte salt and-a solvent mixture consisting essentially- of sulfur dioxide and sulfur trioxide, wherein the sulfur trioxide is the sole active cathode material. These cells are capable of producing open circuit potentials(OCP) of 4.6V and above.

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

It is a further object of the prese#nt invention to provide a new primary electrochemical cell which can be safely hermetically sealed.

It is a preferred object of the present invention to produce a new primary electrochemical cell containing a lithium metal anode which exhibits a relatively low self-discharge rate.

It is another specific object of the present invention to provide a new primary electrochemical cell containing a lithium metal anode wherein relatively low corrosion of the anode occurs.

It is yet another specific object-of the present invention to provide a new primary electrochemical cell having a liquid cathode with useful conductivity, useful electrolyte solubility and high lithium anode stability.

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 the cathode current collector, consisting essentially of at least one soluble electrolyte salt and a solvent mixture consisting either of cosolvents sulfur trioxide and sulfu#ryl chloride, or of trisolvents sulfur trioxide, sulfur dioxide and sulfuryl chloride; 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 discharge curve for a cell according to the present invention prepared in Example 3; Figure 2 shows the construction of an AA size cell according to the present invention prepared in Example 4; and Figure 3 shows the potential as a function of time for this cell while being discharged at 40 microamps.

This invention relates to a primary electrochemical cell having: 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 either of cosolvents sulfur trioxide (SO3) and sulfury#l chloride (SO2CI2), or of trisolvents sulfur trioxide (SO3), sulfur dioxide (SO2) and sulfuryl chloride (SO2CI2); wherein the sulfur trioxide is the sole active cathode material. The electrolyte salt provides an effective degree of conductivity to provide an operable cell.

The cells produced according to the present invention, and those produced in our aboveidentified copending application, are the only operabl & nd practical soluble cathode cells known to us that will discharge on inert cathode current collectors at 4.EV 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, functionprimarily 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 known to be effective with sulfur trioxide.

Solvents such as POCI3 and S2O5C12 have been found too corrosive when mixed with sulfur trioxide. Likewise, metal based halides a#nd oxyhalides such as SeOCI2, VOCI3, CrO2CI2, 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; Group IIA and B elements, which are 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 10-5 ohm~' cm-' will typically be employed.Preferably, the conductivities should be above 1 X 10-4 Ohm-' cm - 1 Among the useful electrolyte salts are those which provide at least one anion of the general formula SO3X-, MX4-, M'X6- and M"Cl6- 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; and X is chlorine or fluorine.Preferred salts provide at least one anion selected from the group consisting of: SO3Cl, S03F, BF4-, BCl4-, AIC14-, AIF6-- -, PF6, AsF6-, SbF6-, SbCI6-, NbF6-, TaF6-, SiF6--, SiCI6--, 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 this type disclosed.

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, strontium and barium, or 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 selected from the group consisting of H, F, Cl, Br, I, 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 Li2B,2CI,2, LOCI, LiF, LiBF4, LiAsF6, LiSbF6, and LiPF6. The nature of the solubilization of electrolyte salts with concomitant useful conductivity in this invention does not seem to be the same for the various classes of salts and the precise interaction for all is not presently known. The solubility of Li2BrO612CI,2 in the solvent mixtures of this invention increases with the presence of 503. Generally, our experience with Li2B,2CI,2 suggests that as the weight percentage of SO, and SO2CI2 increases, the solubility of this salt declines.Solubility in pure liquid SO, 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 SO, is apparently operative with the Li2B,2CI,2 electrolyte salt and allows for the observed solubility and conductivity when the weight percentages of SO, and SO2Cl2 are carefully adjusted.

The ability of the disclosed halogenmetallate complexes, MX4-, M'X6- and M"X6- - to act as electrolytes in the SO3-SO2CI2 is not entirely understood at this time. For example, LiAsF6 is insoluble in liquid S02C12 and in liquid SO, separately, yet a novel mixture of the two solvents effects dissolution of the salt.In fact, a 0.5M LiAsF6 solution has beeri prepared in 50 wt.% S03 and SO2CI2 which has a specific conductivity of 1.0 x 10-3 2-1 cam~'. The nature of the solvent interaction may rest with the ability of the electron deficient SO, molecules to form fluoride bridges with the AsF6- anion and thereby render the salt soluble in this solvent system,

The electrolyte solution consists essentially of at least one soluble electrolyte salt as described above and a solvent mixture selected from the group consisting of (a) cosolvents sulfur trioxide and sulfuryl chloride, and (b) trisolvents sulfur trioxide, sulfuryl chloride, and sulfur dioxide.The cosolvent or tri#solvent mixture will always be present in more than a major amount, and cannot contain other materials which adversely affect the operation of the cells as improved by the sulfur trioxide liquid cathode system of this invention.

The solvents sulfur trioxide, sulfur dioxide and sulfuryl chloride may be utilized in all weight percentages depending on the volumetric amount of sulfur trioxide needed and the desired cpnductivity -of the solution once the electrolyte salt is added. However, they are preferably employed in weight ratios sufficient to fully dissolve the electrolyte salt employed. In the case of the cosolvent system, the typical weight percentages of the S03 will be from 10 to 90% and the sulfuryl chloride will be from 90 to 10%. In case of the trisolvent system, the typical weight percentages of the SO, will be from 1 to 25%, the S03 will be from 10 to 90%, and the sulfuryl chloride will be from 9 to 89%.The cosolvent system of sulfur trioxide and sulfuryl chloride, and the trisolvent system of sulfur trioxide, sulfuryl chloride and sulfur dioxide, exhibit specific conductivities of less than 1 X 10-7 ohm-' cm-1 before addition of the electrolyte salt.

It is preferred that the sulfur dioxide be dried try condensing it onto P40,4 at ~78 C and distilling it from this mixture to yield liquid SO, free of H20. Typical specific conductivities for liquid SO2 collected using the aforementioned treatment are less than 1 x 10-6 ohm-1 cm-1.

The sulfur trioxide can be obtained from MCB Manufacturing Chemists, a distributor of Allied Chemical Corporation's SULFAN stabilized sulfur trioxide and is preferably fractionally distilled to provide S03 essentially free from H2SO4 and commercial stabilizers. The sulfuryl chloride is also preferably fractionally distilled from lithium metal to remove hydrolysis impurities.

The new electrochemical cells of the present invention provide a number of advantages over prior cells and those described in our above-identified patent application. In general, the cells of this invention are high performance cells with improved safety and reliability over our earlier sulfur trioxide-sulfur dioxide liquid cathode cells. The cells of the present invention are safer by virtue of the vapor pressure reducing effect of the sulfuryl chloride which allows for more secure hermetic seals.

An advantage of the trisolvent system SO3-SO2CI2-SO2 arises from the higher conductivity and higher solubility of electrolyte salts in this mixture as well as a relatively low vapor pressure.

The trisolvent system shows a higher conductivity for LiAsF6 and Li2B12CI12 electrolyte salts than SO3-SO2CI2 mixtures. This trisolvent combines the conductivity and solubility advantages of the 503-502 system with the improved lithium stability of the SO3-SO2CI2 system. Further advantages of this invention are that both the self-discharge of the cell and the rate of corrosion of the anode due to hydrolysis impurities, are relatively low for lithium and like active metal anode cells employing the cathode systems of this invention 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 illustrates the preparation and discharge of a cell according to the invention employing-a lithium metal anode, an electrolyte solution consisting essentially of a cosolvent mixture of sulfur trioxide and sulfuryl chloride and LiAsF6 as the electrolyte salt, wherein sulfur trioxide is the sole cathode material.

The LiAsF6 was used as received from U.S.S. Fluorine Chemicals, Decatur, Georgia. The electrolyte solution contained 50 wt.% S03 and 50 -wt.% SO2CI2. Sufficient LiAsF6 was added to the cosolvents to make the solution 0.5M. The solution was mildly heated for a few minutes to dissolve the electrolyte salt. The specific conductivity of the resulting solution was 1 x 10-3 2-1 cm-1. The liquid cathode containing the electrolyte was transferred to a glass pressure cell which contained a lithium anode supported on nickel screen, a Li reference and a cathode current collector comprised of carbon black and binder-supported on nickel screen. The initial open circuit potential was 4.75V. The cell discharged for two days above 4V at a rate of 0.2 mA/cm2.

Example 2 This example describes the preparation and discharge of a cell according to this invention which comprises a lithium metal anode, an electrolyte solution consisting essentially of a cosolvent mixture of sulfur trioxide and sulfuryl chloride and Li2B12CI,2 as the electrolyte salt, wherein sulfur trioxide is the sole cathode material.

The Li2B12Cl12 was prepared by known chemical literature procedures (U.S. Patent 3,551,120, Substituted Dodecarborates; and Inorg. Chem., 3, 159 [ 1964#). The Li2B12C112 is essentially insoluble in S03 and in SO2CI2 separately but shows sufficient solubility and conductivity in a cosolvent mixture of 50 wt.% S03 and 50 wt.% SO2CI2 to be useful as an electrolyte. The conductivity of a 0.01 M solution Li2B,2CI12 in the-SO3-SO2CI2 solvent mixture is 3 X 10-5 #-1 cm-1 A flooded cell constructed as explained in Example 1, showed an open circuit potential of 4.56V with this electrolyte.This cell was discharged for 1500 hours to a depth of 120 mAhr (milliamp hours) and average cell voltage of 4.35V.

Example 3 This example illustrates the preparation and discharge of a cell according to the present invention having a lithium metal anode, an eletrolyte solution which consists essentially of atrisolvent mixture of sulfur trioxide, sulfuryl chloride and sulfur dioxide, and Li2B12C112 as the electrolyte salt, wherein sulfur trioxide is the sole cathode material.

A 0.01 M Li2B,2CI,2 solution in 45 wt.% S03, 46 wt.% SO2CI2 and 9 wt.% SO2, displayed a conductivity of 1 X 10-4 #-1 cm-1. An electrochemical cell as prepared in Example 1 and composed of a Li anode, Li reference and carbon cathode current collector with the described solution trisolvents and electrolyte salt showed an open circuit potential of 4.6V. Fig. 1 shows the discharge curve for this cell prepared with flooded flag electrodes. The cell was discharged at a rate of 15 juA/cm2 of lithium to a total depth of 63 mAhr. Inspection and disassembly revealed that the termination of discharge resulted from cathode failure caused by the accumulation of discharge products.

Example 4 This example illustrates the preparation and discharge of a limited electrolyte cell having a lithium anode, an electrolyte solution consisting essentially of a trisolvent mixture of SO3, S02Ci2 and SO, and Li2B,2CI,2 as the electrolyte salt, wherein the sole reducible cathode material is the sulfur trioxide.

A 0.01 M Li2B,2C1,2 solution in 50 wt.% S03, 35 wt.% SO2CI2 and 15 wt.% SO2 was utilized as the electrolyte. The electrochemical cell design is shown in Fig. 2. A conventional size "AA" battery can with the cathode current collector and anode as shown was protected within a glass pressure vessel and filled to the top of the can with the electrolyte solution. A load simulating a total drain of 40 juA was imposed. The ensuing potential as a function of time is shown in Fig.

3.

The above disclosure is for the purpose of explaining the present invention to those skilled in the art, and 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 (16)

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 either of cosolvents sulfur trioxide and sulfuryl chloride, or of trisolvents sulfur trioxide, sulfur dioxide and sulfuryl chloride; wherein the sulfur trioxide is the sole active cathode material.

2. A primary electrochemical cell according to Claim 1, wherein the cathode consists essentially of sulfur trioxide dissolved in sulfuryl chloride.

3. A primary electrochemical cell according to Claim 1, wherein the solvent mixture consists essentially of sulfur trioxide, sulfuryl chloride and sulfur dioxide.

4. 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 two or more thereof.

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

6. 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, I, 0H or a combination thereof.

7. A primary electrochemical cell according to Claim 6, 'wherein the clovoborate anion is B12C12- -.

8. A primary electrochemical cell according to Claim 1, wherein the electrolyte salt provides at least one anion consisting of SO3CI-, SO3F-, BF4-, BCl4-, AICI4-, AIF6---, PF6-, AsF#6-, SbF6-, SbCI6-, NbF6-,TaF6--, SF6-, SiCI6-- -, SnF6--, ZrF6--, HfF6 --, TiCl#--, TiF6--, W.F6-, MoF6- or PbCl6--.

9. A primary electrochemical cell according to Claim 8, wherein the salt comprises LiAsF6.

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

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

12. A primary electrochemical cell according to Claim 1, wherein sulfuryl chloride is employed in an amount of from 10 to 90 wt. % based on the combined weight of the solvent mixture.

13. A primary electrochemical cell according to Claim 1, wherein the solvent mixture consists essentially of sulfur trioxide, sulfur dioxide and from 9 to 89 wt. % sulfuryl chloride.

14. 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

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

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