A non-aqueous electrochemical cell using lithium as an active anode material has high energy density, good storage characteristics and wide operation temperature range. A non-aqueous electrochemical cell is therefore often used as a power source for a calculator, a watch or a memory back up system. Such a cell comprises an anode, an electrolyte and a cathode. In general, such a cell uses as an anode an alkali metal such as lithium or sodium; as an electrolyte or electrolytic solution, a solution of a solute such as lithium perchlorate or lithium tetrafluoroborate in a non-aqueous solvent such as propylene carbonate, .gamma.-butyrolactone or diglyme; and as a cathode, manganese dioxide or polycarbonmonofluoride.
The combination of relatively high theoretical energy density, potentially long life, and low cost materials such as reported in the sodium-sulfur system high temperature batteries is suitable primarily for low rate performance work such as electric road vehicle propulsion or load leveling of electric power supplies. The sodium-sulfur systems, first proposed in 1966, have had a great deal of effort expended in trying to develop a practical system. The basic operating principle involves the separation of two active molten materials, sodium and sulfur, by either a ceramic membrane of beta alumina or sodium glass, which at about 300.degree. C. or higher allows the passage of sodium ions that form with the sulfur any of the several polysulfides. The open circuit voltage of the system is at just over 2 volts, about the same as the lead-acid cell. Two formidable problems exist at the present time, viz., cracking of the separator and corrosion of the casing and seal.
Another somewhat similar system is the lithium-iron sulfide system, operating at about 450.degree. C. However, insufficient development has been done to date to demonstrate the widespread practicality of this system.
Another of the developments being pursued involves a lithium-based cell, in which the negative electrode is a lithium alloy (typically either lithium-aluminum or lithium-silicon), the positive electrode is an iron sulfide, and the electrolyte is a molten salt, such as the eutectic composition in the lithium chloride potassium chloride system. Because of the high melting point of such salts, such cells must be operated in the temperature range of 400-500 degrees centigrade.
This requirement to operate at such high temperatures has several important disadvantages. One of these is that various degradation processes, such as corrosion of the cell container, seals, and other components are accelerated by such high temperatures. Another is that a substantial amount of energy is lost through heat transfer to the surroundings. Still another is that the voltage obtained from such cells is lower at elevated temperatures, due to the fundamental property of the negative temperature dependence of the free energy of the cell reaction. Furthermore, the higher the temperature of operation, the greater the potential problems related to damage to the cell during cooling to ambient temperature and reheating, whether deliberate or inadvertent. Differences in thermal expansion, as well as dimensional changes accompanying phase changes, such as the freezing of the molten salt, can cause severe mechanical distortions, and therefore damage the cell components.
Cells involving a lower temperature molten salt electrolyte have been investigated where the molten salt is based upon a solution of aluminum chloride and an alkali metal chloride. However, the soluble positive electrode materials are not stable in the presence of the respective alkali metals. As a result, an auxiliary solid electrolyte must be used to separate the alkali metal and the counter electrode. One example of such a cell involves a molten sodium negative electrode, a solid electrolyte of sodium beta alumina, a molten aluminum chloride-sodium chloride salt, and either antimony chloride or an oxychloride dissolved in the chloride salt as the positive electrode reactant. Such a cell can operate in the temperature range of 150-250 degrees centigrade. It has the disadvantage of having to employ an electrolyte, which increases the cell impedance, as well as adding to the cost and complexity.
U.S. Pat. No. 3,844,837 to Bennion et al discloses a non-aqueous battery in which the anode may be lithium and/or graphite on which lithium metal is deposited and as a positive electrode a platinum cup filled with powdered K.sub.2 SO.sub.4 and graphite is utilized. The electrolytes disclosed are LiClO.sub.4, LiCF.sub.3 SO.sub.3 or LiB.sub.4 dissolved in dimethyl sulfite.
U.S. Pat. No. 4,877,695 discloses a non-aqueous electrochemical cell for use in primary rechargeable storage devices in which the cathode comprises an electrically conductive carbonaceous material, the anode is a molten mixture of two elements selected from the group consisting of sodium, potassium, cesium and rubidium, and the electrolyte comprises a solvent and an electrolyte salt selected from the group consisting of an alkali metal tetrafluoroborate and a tetraalkylammonium tetrafluoroborate. The present invention provides a specific improvement in the electrochemical cell of the patent by increasing its energy and power density.
U.S. Pat. No. 4,886,715 discloses a primary rechargeable energy storage device comprising an anode which is an alkaline earth or alkali metal and a carbonaceous fiber cathode. The electrolyte is a membrane comprising lithium laurate.