In the past, rechargeable electrochemical cells have been shown wherein pure lithium metal anodes together with porous carbon cathodes have been made which make use of a lithium tetrachloroaluminate electrolyte salt dissolved in liquid sulfur dioxide. Such cells have been called a soluble positive electrode system because the solvent also acts as the active cathode depolarizer and is electrochemically reduced during discharge. This reduction takes place at the surface of the porous carbon electrode. Satisfactory performance may be obtained with carbons such as acetylene black, which has a specific surface area of between 60 and 70 m.sup.2 /g (as determined by the Brunauer-Emmett-Teller technique of nitrogen gas adsorption). Superior performance is obtained when the carbon electrode includes a mixture of acetylene black with typically 10% or more of a carbon such as a furnace black with a specific surface area between 800 and 1000 m.sup.2 /g. One example of such a carbon is Ketjenblack.RTM., made by Akzo Nobel of Charlotte, N.C.
Cells of this type have a long shelf life and high volumetric and gravimetric energy density, are relatively inexpensive and use readily available active materials, include a highly conductive electrolyte, and tend to resist damage due to overcharging. Despite these desirable features, however, such cells have not met with any significant commercial success. Even though various attempts have been made to improve the overall performance of such cells, it has been found that during charging, metallic dendrites can grow from the negative lithium electrode. If these make contact with the cathode or the cell hardware, the resulting short circuit can in some cases cause the cell to explode with great violence.
Rechargeable lithium organic electrolyte cells with metal oxide positive electrodes have also been proposed in the past in which the negative electrode is not lithium metal but is lithium sequestered by intercalation into a suitable carbon material. In this structure the electrolyte is usually a mixture of carbonate esters and the positive electrode is a lithiated transition metal oxide such as LiCoO.sub.2 or LiNiO.sub.2 which on electrochemical oxidation during the charging cycle releases lithium ions to the electrolyte. In such cells, these lithium ions migrate to the negative electrode and intercalate between layers in the carbon structure. In such a cell, as long as the potential of the carbon remains anodic to the lithium potential, metallic lithium is never deposited on the anode, and no dendrites can be formed. The lithiation and delithiation of both the carbon and metal oxide in these cells show excellent reversibility, and cells containing such electrodes have a very long cycle life. Very strict control of the charging potential is required to preclude the deposition of dendritic metallic lithium on the surface of the negative electrode during any overcharging period.
Many different types of carbon have been shown to accept lithium ions by intercalation during charging by electrochemical reduction. Examples include carbons with highly ordered structures such as graphites, highly disordered carbons prepared by decomposition of solid organic precursors, and carbons of intermediate structure such as cokes. In every case, more charge is consumed during the first charging of the carbon negative electrode than is returned in the first discharge process. After this first charge and discharge cycle, the capacities of subsequent charge and discharge cycles are more nearly equal; that is, lithium ions intercalate into and deintercalate from the carbon reversibly. The excess charge which is consumed in the first charge process is referred to as the irreversible capacity of the carbon. The excess charge is thought to be consumed in irreversible reduction of components of the electrolyte to form an insoluble film of salts on the surface of the carbon.
While a salt film of this type is thought to be desirable for preventing further reaction between the electrolyte and the negative electrode, active cell material, usually from the positive electrode, is irreversibly consumed in its formation. One way to decrease this loss of active material is to minimize the area over which the film is formed. Although there is no exact correlation between the specific surface area of the carbon and its irreversible capacity, it is generally preferable to use a carbon which has a specific surface area of 50 m.sup.2 /g or less.
The use of inorganic additives dissolved in organic electrolytes for forming protective coatings on carbon negative electrodes has been proposed previously. Studies also suggest that carbon electrodes that have first been treated in an organic electrolyte with such film-forming additives can then be used in inorganic electrolytes. But to date no specific battery system has been designed by which lithium ions could be introduced into such a coated carbon negative electrode. The stability of sulfur dioxide as an electrolyte solvent in systems using a lithiated metal oxide such as LiCoO.sub.2 has been noted, but in a system with such a cathode the sulfur dioxide serves only as an electrolyte solvent, not as an oxidant, cathode depolarizer, or positive electrode material.
It is obvious that sulfur dioxide itself cannot act as a source of lithium ions, therefore if sulfur dioxide is to be used as a soluble positive in a cell with a lithium intercalating negative electrode, it is necessary to provide a separate source of lithium ions from which the negative electrode can be charged. The source of lithium ions conventionally used with carbon negative electrodes in organic electrolyte cells is the high voltage lithiated metal oxides. As noted above, if used in the sulfur dioxide electrolyte system, these metal oxides would themselves act as the positive redox material. If a carbon anode is to be used as taught herein, lithium ions must be introduced into a carbon negative electrode from a sacrificial electrode; the present invention uses such an innovation to exploit both the excellent electrochemical reversibility of the sulfur dioxide electrolyte and the excellent safety characteristics of carbon negative electrodes. The nonmetallic negative electrode provides safety not possible with a metallic lithium negative electrode.