The rapid proliferation of portable electronic devices in the international marketplace has led to a corresponding increase in the demand for advanced secondary batteries. The miniaturization of such devices as, for example, cellular phones, laptop computers, etc., has naturally fueled the desire for rechargeable batteries having high specific energies (light weight). Mounting concerns regarding the environmental impact of throwaway technologies, has caused a discernible shift away from primary batteries and towards rechargeable systems.
In addition, heightened awareness concerning toxic waste has motivated efforts to replace toxic cadmium electrodes in nickel/cadmium batteries with the more benign hydrogen storage electrodes in nickel/metal hydride cells. For the above reasons, there is tremendous market potential for environmentally benign secondary battery technologies.
Several approaches have been pursued in an effort to develop improved secondary battery technologies, including the recent introduction of the nickel/metal hydride cell and the commercialization of lithium-ion technologies. Among the factors leading to the successful development of high energy density batteries, is the fundamental need for high voltage and/or low equivalent weight electrode materials. Electrode materials must also fulfill the basic electrochemical requirements of sufficient electronic and ionic conductivity, high reversibility of the oxidation/reduction reaction, as well as excellent thermal and chemical stability within the temperature range for a particular application. Importantly, the electrode materials must be reasonably inexpensive, widely available, non-toxic, and easy to process.
It has been found previously that organodisulfide compounds can be used as high energy density electrodes for rechargeable battery systems. In the work by Liu et al. [Liu et al., J. Electrochem. Soc., 138: 1891 (1991); Liu et al., J. Electrochem. Soc., 138: 1896 (1991); Visco et al., Mol. Cryst. Lig. Cryst. 190: 185 (1990); and Visco et al., U.S. Pat. No. 5,162,175 (issued Nov. 10, 1992)], a novel class of polymeric organodisulfides were described having exceptionally low equivalent weights and consequently very high gravimetric and good volumetric capacities. The electrochemical reaction involves the oxidation of a thiolate anion to a sulfur radical which rapidly dimerizes to form a disulfide linkage as shown below: EQU 2[.sup.- SRS.sup.- -e.sup.-.fwdarw..sup.- SRS ] EQU 2.sup.- SRS .fwdarw..sup.- SRS-SRS.sup.-.
Those reactions lead ultimately to the formation of a polymeric organodisulfide.
In the preferred embodiment, the polyorgano-disulfides (PDS) were used as positive electrodes in thin-film polymer electrolyte cells having alkali metal negative electrodes. [See, Visco et al., U.S. Pat. No. 5,162,175.] Since organodisulfides are invariably electronic insulators, carbon black was included in the electrode formulation. The PDS were formulated as an intimate mixture of the disulfide, solid polymer electrolyte (SPE), and carbon black. Although the intermediate temperature performance (60.degree. C. to 120.degree. C.) of the alkali metal solid polymer batteries having PDS cathodes is excellent, utilization of positive electrode capacity can be critically dependent on the intimate connection between the carbon black particles, PDS particles, and current collectors. That interconnectivity appears to be more important as the temperature of operation is reduced to ambient and sub-ambient temperatures.
The instant invention provides a means to overcome the temperature range limitations of the Liu et al. PDS electrodes. The need for a high degree of interconnectivity between and among the carbon black, PDS matrix, and current collectors of the Liu et al. PDS electrodes is eliminated or minimized according to the instant invention by the introduction of metallic conductivity into the polyorganodisulfide polymer chain.
Metallic conductivity and low temperature superconductivity has been reported for inorganic polymers such as (SN).sub.x, and synmetals such as tetracyano-p-quinodimethane-tetrathiafulvalene (TCNQ-TTF) since the early 1970s. In the mid-1970s considerable attention was directed towards the study of organic polymers following the discovery that polyacetylene could be prepared as a film having metallic luster.
In the late 1970s it was found that the conductivity of polyacetylene could be increased 13 orders of magnitude by doping it with various donor or acceptor species, reaching conductivities of 10.sup.3 S.multidot.cm.sup.- 1. It was later proposed by Heeger and MacDiarmid [MacDiarmid et al., J. Phys. Collog. 44: C3-543 (1983)] to use such materials as electrodes in secondary batteries. Unfortunately, with few exceptions, conducting polymers such as doped polyacetylene tend to be air sensitive, difficult to process, and have very low volumetric capacities. For those reasons, most groups have abandoned conducting polymers for battery applications.
Recently, highly conducting metallo-organic and/or coordination polymers have been reported which exhibit exceptionally good electrical properties and environmental stability, as well as multiple oxidation states. For example, tetrathiolate ligands can be reacted with transition metal salts (see reaction set forth below) to yield black amorphous coordination polymers with exceptionally high electrical conductivity and environmental stability [Dahm et al., Synthetic Metals, 55-77: 884-889 (1993)]. Such materials have achieved electrical conductivities of 10.sup.2 S.multidot.cm.sup.-1. ##STR1##
The instant invention concerns the use of metallo-organic charge-transfer materials as positive electrodes in secondary batteries. Some metal oxides and chalcogenides presently used as battery electrodes have appreciable electrical conductivities; however, many such oxide/chalcogenide materials need to be formulated with carbon black to increase the conductivity of the composite electrode to acceptable levels. In contrast, the inherently high electrical conductivity of charge-transfer materials allows them to be used without significant dilution.
The metallo-organic charge-transfer materials of this invention are essentially redox electrodes and in principle are reversible to an enormous variety of counter electrodes. Oxide/chalcogenide cathode materials presently under investigation by other groups are not reversible to alkali metal ions other than lithium, and thus, lack the flexibility of the electrodes of the instant invention.
The reversibility of the electrodes of the instant invention can furnish large economic advantages in the commercialization of secondary batteries based on charge-transfer materials, in that expensive lithium anodes can be replaced by inexpensive sodium electrodes. Also, the ability to alter the thermodynamic redox potential of the charge-transfer electrode by a suitable choice of the metal ion and/or of the organosulfur chelating ion provides tremendous flexibility in tailoring-the secondary battery characteristics to the specific application at hand.
Teo [U.S. Pat. No. 4,181,779 (issued on Jan. 1, 1980)] describes the use of halogen reactive materials such as organo-metallic polymers limited to the nominal stoichiometry [M(TTL)].sub.x in which M is a transition metal containing complex having at least one transition metal ion selected from Group VIII of the periodic table; wherein TTL has the nominal atom composition C.sub.10 H.sub.4 X.sub.4 and substituted compositions thereof in which X is selected from the group consisting of sulfur (S), selenium (Se), tellurium (Te), and mixtures thereof; and x is equal to or greater than 1. The TTL compounds described by Teo have high formula weights, for example, as shown below, leading to unattractive energy densities (watthours/kilogram) for electrodes formulated with such materials. ##STR2##
In contrast to the Teo electrodes, the electrodes of the instant invention have low equivalent weights and consequently high energy densities.
A key feature of the electrodes of the instant invention is the use of thiolate ligands to chelate metal ions of the electrodes and thereby form coordination polymers having low equivalent weights and enhanced electrical conductivity. The chelation also solves a prior art dissolution problem of the PDS electrodes. For example, thiolate anions from the PDS electrodes most notably in a liquid or gel format, diffuse and migrate to the negative electrode, which may result in deterioration of battery performance. In accordance with the instant invention, the metal-organosulfur polymers are anchored by chelation and locked into position.
The new metal-organosulfur positive electrodes of this invention, thus, overcome as indicated above many of the problems found in prior art battery systems. The invention provides for novel secondary batteries that have high electrical conductivity, high energy densities and polymeric positive electrodes that are stably situated. Further, the secondary cells of this invention represent improvements over prior art batteries in having more cycle life and in being able to perform at a lower temperature without deterioration of performance.