GB2083942A - Efficiently Rechargeable Totally Inorganic Non-aqueous Li/SO2 Cell with Halogallate Electrolyte Salt - Google Patents

Abstract

A totally inorganic efficiently rechargeable non-aqueous cell having an anode of active metal, such as lithium or alloys of lithium, a sulphur dioxide electrolyte solvent cathode depolariser, and a gallium-containing electrolyte salt with anode metal cation such as LiGaCl4.

Description

SPECIFICATION Efficiently Rechargeable Totally Inorganic Non-aqueous Li/SO2 Cell with Halogallate Electrolyte Salt This invention relates to room temperature rechargeable nonaqueous cells having active metal anodes such as lithium and more particularly to such cells having sulfur dioxide electrolyte solvent/cathode depolarizers.

In the past a considerable amount of effort has been expended in the development of a viable, practical and commercially acceptable room temperature operable rechargeable lithium cell which would have the advantages over the common rechargeable lead-acid and nickei-cadmium batteries of higher efficiency, lower weight and greater primary lifetimes thereby allowing for more time between charging cycles. Such efforts have met with varying degrees of success. However, such cells have rarely achieved greater than 80% recycling efficiencies over extended charge and discharge cycles.

These cycling efficiencies are to be differentiated from the very high lithium plating efficiencies since lithium plates out of electrolyte solutions, commonly used in lithium cells, with close to 100% efficiency (the exchange current of the reaction Li-tLi+e- is very high-on the order of 1 xl 0#3A/cm2).

The recycling efficiencies are instead related to subsequent anodic oxidation of the plated lithium wherein generally the effectiveness of the plated lithium as an anode material decreases rapidly upon cycling despite the high plating efficiencies.

Several reasons have been postulated for the inefficiency of plated lithium for repeated anodic oxidation and of cells containing such anodes. One reason given is that lithium is dendritically deposited and becomes coated, particularly at the narrow point of contact with the anode substrate with an insulating film which electrically insulates it from the anode substrate despite its physical presence on the anode. The plated lithium dendrites as they become electrically insulated become unavailable for efficient anodic oxidation during discharge. Furthermore, the plated lithium dendrites are fragile and may be easily mechanically dislodged from the anode substrate. The dislodged lithium, because it is insoluble in the electrolyte is thereafter lost from discharge and further replating.

Efficiency is therefore reduced by depletion of available lithium for repeated cycling.

The electrically insulated lithium generally results from the interaction of the lithium with the organic solvent or solvents utilized in the cells. As the cell is recycled, the lithium metal is deposited in the form of dendrites having high surface areas which therefore react to an increasingly greater extent with the electrolyte solvent particularly at the plating site to form the insulating surface films of increasingly greater area whereby such plated lithium becomes increasingly electrically isolated from the anode.

These films, when extensively formed, also tend to reduce the rate at which lithium cations enter solution and therefore may also reduce cell capability. Furthermore, reaction products of lithium with commonly used electrolyte solvents are irreversible (particularly with respect to the solvent) in nature.

Accordingly, during repeated cycling the electrolyte solvent itself becomes depleted with loss of conductivity and cell performance. The reaction products generated from the solvents also tend to act as detrimental impurities further destroying cell capability. Additionally, even lithium contained in such reaction products may also be lost thereby causing increasing reduction of available lithium for recycling.

As an example, propylene carbonate will react with lithium to form an insulating film of lithium carbonate and propylene gas which cannot be reversed to obtain the original solvent. Though recovery of some of the lithium is possible under charging, the lithium carbonate is however not efficiently reversible into its component elements. Some lithium is therefore lost from further cycling. Similarly other solvents such as tetrahydrofuran and acetonitrile form complex reaction products with lithium which are also irreversible. In fact the organic solvents by their very nature must react with the lithium anode.In order to dissolve the electrolyte salts needed for conductivity, the organic electrolytes must be somewhat polar, and it is this very characteristic which causes such solvents to react with the lithium in the formation of the irreversible reaction products.

In order to attain a highly efficient rechargeable cell in accordance with the present invention, plated lithium should have reduced dendrite character and should not be coated with a non-reversible insulative film. It is also essential that there must be a complete cycling of substantially all of the active cell components without the introduction or formation of additional reaction by-products which are irreversible in nature. Accordingly, free organic solvents or co-solvents are excluded from the cells of the present invention. Furthermore, though the problem of recharging efficiency has been described as being inherent in organic solvent lithium cells, cells having only inorganic components such as a lithium cell with an inorganic thionyl chloride solvent/cathode depolarizer may have similar problems of inefficient recycling.Reaction of the thionyl chloride with the lithium produces reaction products of lithium chloride and an unstable 'SO' species which cannot be effectively recombined to the original starting materials. Thus the electrolyte solvent and cathode depolarizer, even if inorganic, must only react with the anode metal only to the extent of formation of totally reversible reaction products.

Cathode depolarizing materials have recently been discovered which are in themselves highly rechargeable. Examples of such materials include the layered metal chalcogenide compounds described by Whittingham in the U.S. Patent No. 4,009,052 which intercalate lithium ions within the spacing between the layers without undergoing full reactions. This property makes them effectively reversible and rechargeable. However, such materials are utilized in ambient temperature lithium cells with organic electrolytes whereby the cell as a whole remains inefficiently rechargeable.

Various attempts have been made to improve the efficiency and the rechargeability of the lithium anodes in nonaqueous cells. Such expedients generally attempted to minimize the dendritic plating of the lithium with the use of various means such as additives, alloying of the lithium anode, utilization of specific electrolyte salts and solvents etc. U.S. Patents Nos. 3,953,302 and 4,091,1 52 describe the use of metal salt additives comprised of metals which are reducible by lithium and which coplate with the lithium on charging to form lithium-rich metallics or alloys. The use of polyalkylene glycol ethers in U.S. Patent No. 3,928,067 was described as improving the recycling characteristics of lithium cells by improving the morphology of the plated lithium.Though such expedients improve rechargeability, such cells still contain organic elements which preclude truly efficient rechargeable cells as described.

Dendritic plating of lithium in secondary cells is described in U.S. Patent No. 4,139,680 as being effectively prevented with the use of clovoborate electrolyte salts. However, such electrolyte salts are difficult to synthesize and are accordingly very expensive. U.S. Patent No. 3,580,828 describes specific electrolyte salt concentrations and current density limits which, if observed, improve lithium deposition characteristics. Other methods for improving rechargeability of plated lithium include the initial utilization of lithium alloy anodes particularly with aluminum as described in U.S. Patent No.

4,002,492.

General improvements in rechargeable lithium cells include the use of complexed inorganic lithium salts as charge transfer agents (U.S. Patent No. 3,746,385) and the judicious use of organic cosolvents with SO2 in order to improve solubility of the electrolyte salts (U.S. Patent No. 3,953,234).

The use of solvents which are relatively stable with respect to the lithium anode was in fact recognized in patents such as U.S. Patent No. 3,540,988 as being required in order to provide enhanced rechargeability of the cells.

Various systems requiring external mechanical components include molten lithium cells (not inherently subject to dendritic plating) which require extensive heating and shielding components but which are the most feasible efficient rechargeable lithium cells since there are no dendrites or films on molten lithium. Other systems include cycling electrolytes such as in U.S. Patent No. 4,154,902 which require complex circulating mechanisms.

Electrolytic processes for lithium deposition, however, generally require organic solvent carriers of the electrolyte salt for high conductivity and efficient plating out of the lithium metal. Exemplary of such lithium deposition procedures are U.S. Patents Nos. 3,791,945 and and 3,580,828. Similarly, cells as described above (except for those containing clovoborate electrolyte salts) require organic solvents for high conductivity and efficient lithium plating.

Electrodeposition of lithium in an electrolyte comprised of lithium and sodium tetrachloroaluminate or lithium and sodium tetrabromoaiuminate dissolved in pure sulfur dioxide (without organic cosolvents) is described in U.S. Patent No. 3,493,433. However discharge performance of such cells is severely limited with a discharge capacity substantially less than theoretical capacity. Since such enumerated salts are described therein as being the only salts having sufficient solubility and conductivity in pure liquid SO2 for plating efficiency, cells having other salts in liquid SO2 have as a rule required the further utilization of organic cosolvents as described in U.S.

Patent No. 3,953,234 and as discussed above.

It is an object of the present invention to provide a highly efficient room temperature rechargeable inorganic lithium or other alkali or alkaline earth metal cell having substantially only reversible reaction products which is both efficiently dischargeable and substantially completely rechargeable over extended periods of cycling.

This and other objects, features and advantages of the present invention will become more evident from the following discussion and drawing.

The sole figure of the drawing is a voltage profile of charge and discharge cycles of a cell of the present invention.

Generally the present invention comprises a totally inorganic non-aqueous efficiently rechargeable cell having a lithium or other active metal (generally alkali or alkaline earth metal or alloys thereof) anode, a totally inorganic electrolyte solvent consisting essentially of sulfur dioxide which may also function as cathode depolarizer (with an inert generally carbon cathode) and an inorganic gallium salt such as gallium halides having anode metal cations and LiGaCI4 (with a lithium cation and a GaCI4- anion in a lithium anode cell) in particular, dissolved in said sulfur dioxide electrolyte solvent. Other gallium salts include Li2O (GaCI3)2 and Li2S(GaCl3)2 (with lithium cations and O(GaCl3)2#2 and S(GaCI3)2#2 anions respectively) described in co-pending application (M-3465). A completely reversible solid cathode depolarizer such as an intercalation compound may optionally be utilized with the SO2 electrolyte solvent. Examples of such solid cathode depolarizers include chromium oxide (Seloxette), titanium disulfide, manganese dioxide, etc. The cell is efficiently rechargeable since all reactions therein including internal reactions between cell compounds such as between the lithium anode and the SO2 solvent and the electrochemical cell reactions product substantially only reversible products of, for example, lithium dithionite which is 100% reversible on recharge or intercalated or similarly reacted lithium which is also completely reversible. Additionally, use of the gallium salts appreciably reduces dendritic plating as well.

Generally, all of the organic co-solvents commonly used in non-aqueous lithium cells/SO2 cells such as propylene carbonate, acetonitrile, tetrahydrofuran, dioxolane, gamma-butrolactone and the like are detrimental as co-solvents in the present invention since they tend to form complex non-reversible reaction products with the active metal anodes such as lithium. Thus it is a requirement of the present invention that the electrolyte solvent be entirely inorganic. It is, however, not sufficient that the electrolyte solvent be entirely inorganic since the most common inorganic solvent used in completely inorganic cells, thionyl chloride, as described above, also forms irreversible reaction products with an active metal anode such as lithium.Accordingly, the inorganic solvent of the present invention is specifically SO2 which reacts with lithium in the formation of the completely reversible lithium dithionite. However, sulfur dioxide is a relatively poor solvent for lithium salts since it is an acceptor solvent which interacts primarily with the electrolyte salt anion rather than cation. Accordingly, in order to promote solubility and conductivity, organic solvents (normally donor solvents) have been invariably coupled therewith in order to complete solvation, with the organic solvents solvating the electrolyte salt cations as fully described in U.S. Patent No. 3,953,234. The only salts generally described as being sufficiently soluble in the SO2 alone are the aforementioned lithium and sodium tetrachloroaluminates and borates, and clovoborates.However, while a salt may be soluble in SO2 it must also provide a cationically conductive solution (greater than 1 x 10-3 ohm-1 cm-') for it to be considered as providing an acceptable electrolyte. Thus, materials such as LIAICI, which are soluble and electrically conductive in SO2 are nevertheless generally unsuitable for the cells of the present invention because of the low cationic conductivity of the electrolyte. The clovoborate salts which are soluble and cationically conductive are, however, very expensive. The electrolyte salts of the present invention which are specifically gallium salts such as halides with anode metal cations have been discovered to be soluble and highly cationically conductive in pure SO2.Furthermore, when compared to clovoborate salts such salts are relatively inexpensive.

It is preferred that the anode metal be supported on a metal foil substrate. A preferred substrate for a lithium anode is a copper foil with the lithium being applied to both sides of the copper in the form of sandwich.

With an SO2 cathode depolarizer, the cathode is an inert material such as carbon supported on a usually metallic substrate such as an expanded metal, for example aluminum.

The most preferred gallium halide salt having the requisite conductivity in SO, without the need for organic cosolvents is one having a gallium tetrachloride anion such as a LiGaCI4 salt for use in a lithium anode cell as described in U.S. Patent No. 4,177,329. In said patent the gallium salts are, however, specifically described as being utilized in an inorganic SOCK, containing cell which is not rechargeable in accordance with the present invention. It has been discovered that such salts may be formed in situ in a pure SO2 solvent by reaction between, for example, LiCI and GaCI3 to form LiGaCI4.

This is in addition to such in situ salt formation in an SOCK, solvent as described in said patent.

The LiGaCI4 salt may also be prepared by direct fusion of LiCI and GaCI3 by melting such materials together in stoichiometric amounts and allowing the melt to crystallize.

As shown in Table I, LiGaCI4 electrolyte salts provides a high cationic conductivity over a wide range of temperature even when dissolved in a relatively poor electrolyte solvent of pure SO2: Table I Conductivity (?M LiGaCI4-S02) Temperature (ohm cm)-1 400C 5.32x 10-2 300C 5.27 x 10-2 200C 5.17x10-2 100C 5.05x10-2 0 C 4.76x10-2 -100C 4.45x10-2 -16.80C 4.19x10-2 It has also been discovered that the aforementioned LiGaCI4 salt reduces dendritic plating of anode materials such as lithium since the electrolyte solutions remain clear of lithium particles and limited insoluble precipitates even under repeated cycling.It is also postulated that because the electrolyte solution remains clear of such particles, the LiGaCI4 also forms scavenging species during charging which scavenge disconnected lithium dendrites, and lithium from lithium reaction products both at the anode and the cathode. As a result, cell reaction-generated products even if isolated from the anode or cathode substrates are returned to solution to reform both anode metal and electrolyte solvent.

In order to illustrate the efficacy of the present invention in providing an efficiently rechargeable cell, the following examples and comparative data (relative to other materials in the prior art) are presented. It is understood that such examples are for comparative purposes and that any enumeration of detail should not be construed as a limitation on the present invention. Unless otherwise indicated all parts are parts by weight.

Example 1 AD' size cell is constructed with convolute wound lithium foil (20"(50.8 cm)x 1 8" (4.13 cm)xO.012"(0.03 cm)) and porous carbon (an aluminum expanded metal substrate) (20"(50.8 cm)x" (4.4 cm)xO.025" (0.063 cm)) electrodes with a polypropylene separator therebetween. The cell is filled with a solution of 1 M LiGaCI4 in pure SO2 (about 40 grams). The theoretical capacity of the lithium anode is about 13 Ahr and the capacity of the SO, is about 14 Ahr. The cell is cycled at 0.5A on 2 hour discharge followed by 2 hour charge. The voltage profiles of the second and sixty ninth cycles (the cell failed abruptly after the 69th cycle) are shown in the figure.There is almost a negligible change in the discharge and charging voltages thereby indicating an almost 100% cycling efficiency.

Rechargeability alone is not, however, a sufficient criterion of cell utility. The cell must also have good primary cell characteristics. The following examples 2 4 illustrate such capability of the cells of the present invention.

Example 2 A 'D' cell is constructed as in Example 1 and is polarized at room temperature (250C) and -300C with the results shown in Table II: Table II i (Amp) Volts at 250 Volts at open circuit 2.91 2.97 .026 2.89 2.85 .050 2.89 2.80 0.10 2.87 2.75 0.25 2.84 - 0.35 - 2.58 0.50 2.79 2.53 1.0 2.63 - 1.5 2.58 2.32 2.0 2.53 2.27 3.0 2.47 2.15 5.0 - 1.85 The relatively high voltages obtained at currents over 1 ampere show the high conductivity of the electrolyte salt and the good rate capability of the cell at both ambient and low temperatures.

Example 3-4 Two 'D' cells are constructed as in Example 1 and are discharged at various rates and temperatures with the conditions and results given in Table Ill: Table Ill Discharge Ex.# Temperature rate or load Capacity to 2v 3 -300C 4.4 ohm 3.5 Ahr 4 250C 0.25 A 9.0 Ahr Since the cell has a theoretical capacity of 10 Ahr (C limited) the room temperature capacity obtained (90%) at a rate of 0.25 A indicates very good primary cell performance. The 3.5 Ahr obtained at -300C is also excellent for low temperature performance.

Example 5 A cell made in accordance with Example 1 but with an anode of two 0.010"(O.025 cm) foils of lithium sandwiching a copper foil substrate, is cycled at 0.10 A for 10 hours discharge and charge. The cell provides about 104 Ahrs about 5 times original lithium capacity.

Example 6 A glass cell is made using a 2 cox0.5 cm Li anode and a 2.0 cmxO.5 cm catalytic carbon cathode with about 20 cc of 1 M LiGaCI4 in pure SO2 electrolyte. The cell is discharged at 1 ma/cm2 for 5 hours then recharged at 1 ma/cm2 for 5 hours. After 15 cycles the lithium surface is clean and dendrite free and the electrolyte remains clear.

Example 7 Two cells are made in accordance with Example 1 but with one having an electrolyte of 1 M LIAICI, in pure SO, as in U.S. Patent No. 3,493,433. The cells are discharged with the results given in Table IV: Table IV Cap (O.25A Electrolyte OCV discharge rate) 1 M LiGaCI4 2.94V 9.0 A.hr 1 MLiAICI4 (Prior Art) 3.26 0.38 It is evident from the above comparison that the LiAICI4 electrolyte salt described in the prior art provides only a minimal primary cell capability in pure SO, and is certainly unsuitable for the secondary or rechargeable cells of the present invention.It may also be noted that LIAICI, is the electrolyte salt of choice in totally inorganic cells having lithium anodes and thionyl chloride electrolyte solvent/cathode depolarizers.

The following examples are presented as further illustrating the rechargeable efficacy of the cells of the present invention under varying cycling conditions.

Examples 8-10 Three cells are made in accordance with Example 5 and are cycled under conditions and with results shown in Table V: Table V Capacity Utilization Ex. # Test cond. (disch. a ch.) (Ahr) Li So2 8 0.5 Ax2 hrs D/C 73 360% 570% 9 0.25Ax4 hrs 66 330% 500% 10 0.5A (discharge to 2.5v 80 400% 600% charge at 3.5v) The above examples are for illustrative purposes only with changes in cell structure and components being within the scope of the present invention as defined by the following claims.

Claims (14)

Claims:

1. A rechargeable, non-aqueous electrochemical cell comprising an active metal anode and an electrolyte salt having an anode metal cation and an anion characterized in that said anion includes gallium and halogen atoms and that said salt is dissolved in a totally inorganic solvent which consists essentially of sulfur dioxide.

2. The cell of Claim 1 wherein said anode is comprised of lithium or an alloy thereof.

3. The cell of Claim 2 wherein said lithium or alloy thereof is supported on a metal substrate.

4. The cell of Claim 2 wherein said metal substrate comprises a metal foil sandwiched by said lithium or alloy thereof.

5. The cell of Claim 2, 3 or 4 wherein said metal substrate is comprised of copper.

6. The cell of any of Claims 1-5 wherein said sulfur dioxide also constitutes the cathode depolarizer of said cell.

7. The cell of Claim 6 wherein said cell further includes an inert carbon cathode.

8. The cell of Claim 7 wherein said carbon is supported on an expanded aluminium substrate.

9. The cell of any of Claims 1-5 wherein said cell induces a solid cathode depolarizer which when reacted with cations of said active metal anode during cell discharge forms a completely reversible product on cell charging.

10. The cell of Claim 9 wherein said solid cathode depolarizer is an intercalation compound.

1 The cell of Claim 10 wherein said cathode depolarizer is selected from TiS2, MnO2 and chromium oxide.

12. The cell of Claims 1~1 1 wherein said salt has an anion selected from GaCI-4, O(GaCI3)#22, and S(GaCI3)#22.

13. A rechargeable cell as set forth in any of the foregoing examples 1 to 6.

14. A method of improving the rechargeability of a non-aqueous cell having an active metal anode, said method comprising dissolving a salt containing the anode metal cation and an anion which includes gallium and halogen atoms in a solvent characterised in that said solvent is totally inorganic and consists essentially of sulfur dioxide.