ELECTROCHEMICAL CELL AND METHOD

Abstract: An electrochemical cell and a method of forming such a cell, wherein the cell comprises a lithium anode and a cathode spaced therefrom prior to activation by an electrically insulating film of one or more lithium oxides or lithium carbonate formed on the anode.

Description: ELECTROCHEMICAL CELL AND METHOD Technical Field The United States Government has rights in this invention pursuant to Purchase Order No. 5513309 with The Continental Group, Inc. under Prime Contract No.- W-7405-ENG-48 between ^ The University of California and the U. S. Department of Energy. The present invention relates generally to an improved electrochemical cell and a method of forming such a cell and, more specifically, the invention relates to an improved electrochemical cell having a lithium anode which is activated by contact with an aqueous electrolyte. Background Art Electrochemical cells utilizing aqueous electrolytes and anodes of highly chemically and electrochemically reactive metals, such as sodium and lithium, are well known. Such cells are described in detail in Rowley U.S. Patent No. 3,791,871, Momyer U.S. Patent No. 4,001,043 and Momyer et al U.S. Patent No. 4,269,907, the disclosures of which are all hereby incorporated by reference. Generally in such a cell a reactive metal anode is spaced from a cathode, initially by an electrically insulating film formed on the anode. Activation of the cell is effected " by contact of the anode and cathode with an aqueous electrolyte. The insulating film on the anode dissolves in the electrolyte, and the water reacts with the anode-forming metal to form an alkaline electrolyte solution, generally an alkali metal hydroxide solution. As the reaction proceeds, the insulating film is further eroded, and the cathode is placed in electrochemical contact with the anode through the electrolyte. In some cases, as in Momyer et al 4,269,907, an additional element such as a nonconductive screen or other porous element is disposed between the anode and cathode to maintain proper spacing. Alkali metal electrochemical cells utilizing lithium anodes are very popular due to the relatively high energy density obtainable therewith. Lithium anodes are especially desirable when the cathode comprises a so-called "air cathode" in which depolarization at the --strraπ OMPI cathode is accomplished by reduction of oxygen in the air. Giner U.S. Patent No. 3,438,815, the disclosure of which is hereby incorporated by reference, provides a good disclosure of a fuel cell utilizing an air cathode. In most applications wherein activation of a lithium anode cell is carried out by addition of an aqueous electrolyte, it is desirable that activation be deferred until it is desired to utilize the cell. However, it may be equally desirable that activation be effected with a minimum of delay. Thus, it is required both that the lithium anode be protected and spaced from the cathode by an insulating film and that the film be subject to rapid decomposition upon addition of the aqueous electrolyte to the cell for activation. In prior lithium anode cells, the electrolyte-soluble insulating film was typically formed on the anode by prior contact with an aqueous lithium hydroxide solution. However, films formed in this manner tend to contain an excess of free moisture, which is known to catalyze the room temperature reaction between lithium and nitrogen in the atmosphere to form lithium nitride (Li,N). Thus, the formation on a lithium anode of a hydrated lithium hydroxide film tends to promote the formation of lithium nitride occlusions in the film if the anode is subject to contact with the atmosphere. The formation of lithium nitride on an anode surface is undesirble as it decreases the rate of battery activation and reduces the shelf life of the cell. Further, the ultimate power output of the cell is substantially reduced by the presence of lithium nitride on an anode. Disclosure of Invention The present invention is directed to overcoming one or more of the problems described above. According to the present invention, the lithium anode of an electrochemical cell is provided with an electrolyte-soluble, electrically insulating film by reaction of the lithium metal of the anode with oxygen or carbon dioxide, in the substantial absence of moisture. The resulting protective film of lithium oxides or lithium carbonate prevents lithium nitride formation, resulting in an _ O PI electrochemical cell which has long shelf life, is rapidly activated an produces high power output. Further objects and advantages of the invention will b apparent from the * following detailed description, taken in conjunctio with the appended claims. Best Mode of Carrying Out the Invention The formation of lithium nitride (Li_N) on lithium metal is the only known reaction which occurs between a metal and atmospheri nitrogen at room temperature. As set forth above, the formation of lithium nitride on a lithium anode is undesirable for a variety of reasons. Thus, it is highly desirable to prevent or at least greatly retard the formation of lithium nitride on lithium anodes of electrochemical cells utilizing lithium anodes * and an air cathode in which the anodes are subject to prolonged exposure to air. For reasons set forth above, the formation on an anode of an insulating film which contains free moisture (defined as water which is capable of catalyzing the lithium-nitrogen reaction) must be avoided. Thus, according to the invention, it has been found that an electrolyte-soluble insulating protective film can be formed on a lithium anode of an elecrochemical cell by reaction of the lithium in the anode with substantially moisture free oxygen, preferably from the atmosphere, or with carbon dioxide. The resulting films comprise lithium oxides and lithium carbonate, respectively. The films may be formed in a straightforward manner. A thin film of one or more lithium oxides is readily formed on a lithium anode by exposure of the anode to air at room temperature in a low humidity environment. It has been found that air with a relative humidity of 3% or less is suitable, and that the resulting film is substantially devoid of "free" moisture. in the case of lithium carbonate, the anode can readily be reacted with gaseous carbon dioxide to form a lithium .carbonate (Li 2 C0 3 ) film. The resulting thin, opaque lithium carbonate film on the lithium anode surface retards or substantially eliminates the normal oxidation processes which occur in the atmosphere. Further, the lithium carbonate film on the anode does not contain any waters of hydration ("free" moisture), which explains the effectiveness of the film in eliminating lithium nitride formation. However, lithium carbonate is relatively insoluble in aqueous lithium hydroxide electrolyte solutions, thus resulting in a relatively slow activation rate for the cell. Thus, air oxidation of the anode is the preferred procedure in carrying out the invention. The resulting lithium oxide film is also substantially devoid of "free" moisture. It will be appreciated that the prior method of "filming" a lithium anode with an aqueous lithium hydroxide electrolyte is unsatisfactory because excess moisture inevitably present in the film tends to catalyze lithium nitride formation unless air is rigorously excluded from the cell structure after assembly. Without such air exclusion, the possibility exists for lithium nitride formation during assembly and activation of the battery. This problem is particularly acute with an air cathode because the porous structure allows nitrogen from the air supply to contact the lithium anode surface prior to and during the activation sequence. This problem is not encountered with the method of the present invention. Since the resulting lithium oxide and lithium carbonate films of the invention are relatively thin, an electrically insulating electrode separator disposed between the anode and cathode is required in order to eliminate possible short circuiting of the electrical cell before or at activation. Various non-conductive structures are suitable, such as metal screens coated with Teflon (polytetrafluoroethylene) or various plastic resins such as Vexar brand low and high density polyethylene or polypropylene netting. The use of such non-conductive screens is disclosed in Momyer et al 4,269,907. Once an electrochemical cell utilizing the improved protected anode of the invention is activated, no difficulties associated with lithium nitride formation are experienced, because the anodically formed lithium anode film is quite protective. While the protected anode and method of the invention are applicable to any type of electrochemical cell having a lithium anode and activated by addition of an aqueous electrolyte, the invention is especially applicable to primary batteries utilizing an aqueous electrolyte and an air cathode because the lithium anode is generally subjected to prolonged exposure to air prior to activation. The foregoing detailed description is given for clearness of understanding only., and no unnecessary limitations are to be inferred therefrom, as obvious modifications within the scope of the invention will be apparent to those skilled in the art. C'--t?I

Claims: 1 ' Claims 2 3 4 1. An electrochemical cell comprising a lithium anode spaced from a 5 cathode by an electrically insulating film formed on said anode, 6 said anode and cathode being adapted to be activated by contact with 7 an. aqueous electrolyte in which said film is soluble, said film 8 being substantially free of moisture and selected from the group 9 consisting of lithium oxides and lithium carbonate. 0 11 2. The cell of claim 1 wherein said cathode is an air cathode. 12 13 3. The cell of claim 1 further including insulating separating means 14 disposed between said anode and said cathode in addition to said 15 film. 16 " 174. The cell of claim 3 wherein said separating means comprises a screen 18 of a synthetic plastic resin. 19 205. The cell of claim 4 wherein said resin is selected from the group 21 consisting of polytetrafluoroethylene, polyethylene and 22 polypropylene. 23 246. in a method of forming an electrochemical cell in which a lithium 25 anode is disposed in spaced relation to a cathode such that said 26 cell is adapted to be activated by contact of said anode and said 27 cathode with an aqueous electrolyte, the improvement which comprises 28 forming an electrically insulating, electrolyte-soluble film on said 29 anode by exposure of said anode to oxygen or gaseous carbon dioxide 30 under substantially moisture-free conditions. 31 327. The method of claim 6 wherein said film is formed by oxidation of 33 S ai anode by exposure to air at room temperature, the relative 34 humidity of said air being less than about 3%. 35 368. The method of claim 6 wherein said film is formed by exposure of OMPI ' said anode to gaseous carbon dioxide. The method of claim 6,wherein said cathode is an air cathode. O PI

Inventors: LITTAUER ERNEST LUCIUS, MOMYER WILLIAM RAYMOND, LITTAUER, ERNEST, LUCIUS, MOMYER, WILLIAM, RAYMOND
Applicants: LOCKHEED MISSILES SPACE, LOCKHEED MISSILES & SPACE COMPANY, INC.
Application year: 1983
Main classification: H01M4
IPC codes: H01M2, H01M4, H01M6, H01M12