Abstract:
A procedure is described for making niobium triselenide for use in nonaqueous cells. The procedure involves synthesis of NbSe 3  from Nb 2  Se 9  which is rapid, easily adapted to manufacturing conditions and yield product with excellent characteristics for lithium cell applications.

Description:
This application is a continuation of application Ser. No. 07/286,433, filed Dec. 19, 1988 now abandoned. 
    
    
     TECHNICAL FIELD 
     The invention is a process for making a nonaqueous cell and cell made by this process. In particular, the procedure involves an advantageous procedure for synthesizing the active cathode material. 
     BACKGROUND OF THE INVENTION 
     Battery technology has undergone considerable change in recent years in response to demands for higher reliability, greater capacity per unit weight or volume, longer shelf life and greater cycle life. This has been due to market demands for more portable equipment (e.g., telephones, tools, computers, etc.) as well as back-up power sources for a variety of devices including memories and other components associated with computers. 
     A particularly attractive type battery for many applications is a nonaqueous battery, especially those featuring lithium as the active material in the negative electrode. Such battery cells feature very high cell potential and low weight density and result in cells of high energy density (see, for example, High Energy Batteries, by R. Jasinski, Plenum Press, New York, 1967, pp. 140-144). 
     Exceptionally good active materials for the positive electrode of lithium non-aqueous batteries are niobium diselenide, niobium triselenide, and niobium trisulfide (see for example, U.S. Pat. No. 3,864,167 issued on Feb. 4, 1975 to J. Broadhead et al, incorporated herein by reference). These electrode materials not only have high energy density, good charge and discharge characteristics (cycle performance) and good stability, but also are compatible with a large variety of electrolyte systems. Tests carried out on lithium cells made with these positive electrode materials (especially with niobium triselenide) confirm the advantages outlined above. 
     Electrodes comprising NbSe 3  can be fabricated by providing a thin Nb foil and reacting it with Se vapor. The resulting fibrous sheet of NbSe 3  is then rolled onto a metal grid that serves as current collector. The above process has several shortcomings, including relatively high materials cost, and relatively long reaction time. Furthermore, it is generally difficult to produce thin sheets of active material of uniform thickness and morphology by means of the prior art technique. Another procedure involves reacting niobium powder with chalcogenide vapor under conditions where the desired niobium chalcogenide is formed. For niobium triselenide, the conditions involve heating the niobium powder in the presence of selenium to a temperature of about 625°-680° C. Often, a two stage heating process is preferred; first heating to about 580° C. for about 15 hours and then heating to about 680° C. for 15 hours. (See for example, the recently filed application entitled &#34;Non-Aqueous Cell Comprising Niobium Triselenide&#34; with inventors Wei-Chou Fang and Brijesh Vyas filed Sept. 14, 1988 with Ser. No. 244,218U.S. Pat. No. 4,489,145). Niobium-selenium compounds are discussed in a paper by V. E. Fedorov et al., Russian Inorganic Materials, 20, 935 (1984). 
     In order to increase the commercial value of using chalcogenides such as NbSe 3  as the active positive electrode material in non-aqueous cells, it is desirable to reduce the cost of fabricating cells with chalcogenide active material and to make the fabrication procedure more easily adaptable to mass production. In particular, it is desirable to find a synthesis procedure for NbSe 3  active electrode material that is less expensive, that can be used to produce thin sheets of active material of uniform thickness, and that is less cumbersome and/or more easily adapted to mass production under manufacturing conditions than those known to the prior art. 
     SUMMARY OF THE INVENTION 
     The invention is a process for making a nonaqueous cell comprising positive electrode, electrolyte and negative electrode in which the active material in the positive electrode comprises niobium triselenide and the niobium triselenide is made by heating Nb 2  Se 9  to a temperature where it is substantially converted to NbSe 3  typically between 625° C. and 780° C. Preferred is a temperature range from 630° C. to 690° C. with 650° C. to 680° C. most preferred. Generally, the time of reaction may vary over large limits, including from about one minute to about 25 hours. Shorter reaction times are advantageous. Use of higher temperatures shortens reaction times. Removal of selenium (e.g., by venting to the atmosphere or use of a cold region on the closed reaction tube) also shortens reaction times and often permits use of a lower temperature range in the formation of NbSe 3 . Also, use of fine powders of Nb 2  Se 9  shortens reaction times. Reaction times as short as 10 minutes to 4 hours are achieved. 
     At higher temperatures (e.g., 680° C. to 780° C.) in closed containers, caution should be used because of selenium overpressure. 
     A typical procedure typically preferred for making lithium cells involves deposition of a fine powder of Nb 2  Se 9  onto a substrate and then conversion of the Nb 2  Se 9  to niobium triselenide by heating in a sealed quartz tube. Often, a uniform thin layer of niobium triselenide is desired and this is conveniently done by making a slurry of Nb 2  Se 9  powder, depositing the well-mixed slurry onto the substrate (e.g., by dipping, spraying, etc.), removing the liquid, for example by evaporation and then carrying out the reaction to form the niobium triselenide. The niobium triselenide obtained in this way is in the form of fibrous sheets which is highly advantageous for battery applications. Also, the reaction time can be made very short by suitable choice of temperature, bulk of Nb 2  Se 9  being decomposed and pressure of selenium. The niobium triselenide formed in this way can be used as is in a battery after removal from the substrate. 
     In some applications, the niobium triselenide is removed from the inert substrate, rolled onto a metal mesh (e.g., a nickel mesh) and shaped for use in a cylindrical lithium cell. The procedure is rapid, easily adapted for mass production and can be carried with a minimum of operator attention. The resulting positive electrode exhibits excellent electrochemical characteristics including high capacity per unit weight, good recycling characteristics and excellent reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The Figure shows a schematic diagram of the relevant parts of an exemplary rolled nonaqueous cylindrical cell made in accordance with the invention. 
    
    
     DETAILED DESCRIPTION 
     The invention is based on the discovery that niobium triselenide suitable for use in the positive electrode of nonaqueous cells can be produced rapidly and reliably from Nb 2  Se 9  (diniobium nonaselenide). The process for forming the niobium triselenide involves heating the Nb 2  Se 9  to a temperature where Nb 2  Se 9  loses selenium to form NbSe 3 . Generally, this can be done over a wide temperature range (probably as low as 500° C. or even lower if selenium pressure is reduced) but generally not over about 780° C. because NbSe 3  is unstable above that temperature, forming NbSe 2  and selenium. 
     In currently preferred embodiments of the invention, powder Nb 2  Se 9  is deposited as a more or less uniform layer on a substrate before reaction to NbSe 3 . This uniform layer may be obtained by making a slurry of powdered Nb 2  Se 9 , insuring uniformity of the slurry by stirring or other means and depositing the slurry on a substrate. The liquid is then removed from the deposited slurry generally by some evaporation procedure (heating, vacuum drying, etc.). After drying, the reaction is carried out typically by heating to about 680° C. to 720° C. for 6 to 15 hours. Six hours is generally needed to insure complete reaction although shorter times are often sufficient, particularly at high temperatures. The reaction is generally complete at 15 hours but longer times are not detrimental. 
     The slurry may be deposited on the substrate in a variety of ways including dipping, spraying, etc., and the liquid is generally removed by heating. An advantage of this procedure is the ease and reliability of obtaining uniform layers of Nb 2  Se 9  which results in uniform layers of NbSe 3 . Also, this procedure results in good adherence of the powdered Nb 2  Se 9  to the substrate, an advantage during commercial manufacture. 
     Various substrate materials may be used including materials to be incorporated into the finshed cell (such as a metal current collector, as well as inert substrates such as alumina and quartz glass. 
     The slurry composition may vary over large limits. The liquid should be chemically inert to the Nb 2  Se 9  powder and the substrate. It should have reasonable viscosity so the slurry suspension does not separate too fast and should have a reasonable boiling point (e.g., less than about 200° C.) to facilitate removal. Propylene carbonate is quite suitable and is currently preferred but other liquids (glycerol, various oils, etc.) are also useful. Various other liquids may be useful as may occur to those skilled in this art. The concentration of powdered Nb 2  Se 9  may also vary over large limits, typically from 10 to 90 weight percent Nb 2  Se 9 , with 30 to 70 weight percent Nb 2  Se 9  preferred. Particle size of the powder may also vary over large limits but mesh size less than about 325 is currently preferred because of adherence of powder to substrate and more rapid reaction to form the NbSe 3 . 
     The Nb 2  Se 9  may be made in a variety of ways well known in the art. A typical procedure, preferred at the present time is to react stoichiometric amounts of Nb and Se in a closed vessel (such as a quartz tube) in the temperature range from 450° C. to 680° C. for about 12 hours. A preferred temperature range is from 560° C. to 600° C. 
     The cell fabrication may be completed in a variety of ways well known in the battery art. Both large and small cells can be made with a variety of electrolytes and separator materials. Various shapes and cell structure are useful. Particularly attractive are rolled cylindrical cells such as those described in U.S. Pat. No. 4,740,433 issued to W. P. Lu on Apr. 26, 1988 and U.S. Pat. No. 4,753,859 issued to L. E. Brand on June 28, 1988. These patents described both separator material and electrolyte systems useful in the practice of the invention and are incorporated herein by reference. 
     Relevant parts of a useful rolled cylindrical cell 10 are shown schematically in the Figure. The cell comprises four layers rolled into a cylindrical shape. The four layers are a negative lithium electrode 11, a separator 12, a positive electrode 13, and another separator layer 14. The active material in the positive electrode is advantageously manufactured in accordance with the instant invention. The four-layer roll is generally put into a cylindrical container with suitable electrical connections to the positive and negative electrodes, respectively. The container is filled with an appropriate electrolyte to permit electrochemical action. These parts can be conventional and are not shown.