Preparation and Discharge Characteristics of Solid Redox Polymerization Electrodes Employing Disulfide Polymers and Copolymers

Recent work in our laboratory on polymeric organodisulfides has shown these materials to perform well as positive electrodes in solid-state batteries. The polymeric materials have been named solid redox polymerization electrodes (SRPE's) due to the reversible polymerization/depolymerization reaction that occurs on charge/discharge of the electrode. The cell reaction for SRPE-based cells can be described fora simple case as, 2n M + (SRS)_n = n M₂SRS, where M is an alkali metal (LiNaK) and R is an organic group. In the broader sense SRPE's can have more than two S groups per monomer R unit, and are reversible to other monovalent and divalent metals. In the fully charged state SRPE's consist of polydisulfide polymers and are depolymerized on discharge by scission of sulfur-sulfur bonds, leading to the formation of dithiolate salts in the fully discharged cell. SRPE's are easy to synthesize, are air stable, and should be very inexpensive in bulk quantities. Depending on the redox potential of the polydisulfide and reaction conditions, many disulfides can be copolymerized by oxidizing a mixture of dithiols, x HSRSH + y HSR'SH + (x+y) l₂ = (SRS)_x(SR'S)_y + 2(x+y) HI, allowing modification of the physical and/or redox properties of the SRPE's. A series of simple aliphatic dithiols including (HSCH₂CH₂SH), (HSCH₂CH₂OCH₂CH₂ SH), and (HSCH₂CH₂SCH₂CH₂SH) have been oxidized to polydisulfides and mixtures of the dithiols have been copolymerized. All of the resulting polymers and copolymers were evaluated in solid-state lithium cells, with some of the new materials demonstrating high levels of performance. The utilization of available capacity in the positive electrode was observed to be independent of electrode thickness for a number of SRPE's at loading levels up to 45% by weight. At 90°C, relatively thick positive electrodes based on (SCH₂CH₂S)_n have been discharged to surface capacities of over 20 coulombs/cm² at a current density of 0.5 mA/cm². The discharge profiles for most of the aliphatic polydisulfides are exceedingly flat at slightly over 2 volts versus lithium. Although other polydisulfides such as those derived from the dithiazoles exhibit higher cell voltages, the low equivalent weight of materials such as (SCH₂CH₂S)_n [46 g/equiv] and the low cost of such polymers indicates a potential for commercial application.