Organosulfur-Based Cathode Materials for Li-Ion Battery Applications

                               
                       Scheme 1: Redox Reaction Scheme for 2,5-dimercapto-1,3,4-thiadiazole (DMcT)    

In the Abruña group, we have been developing energy-storage materials for lithium-ion rechargeable battery and redox capacitor applications. In particular, we have focused on the design of organosulfur-based multi electron-transfer systems for cathode offering higher energy density than conventional lithium metal oxides and phosphates. The charge/discharge reactions of the systems are based on the redox chemistry of thiolates (RS^-), which can be oxidized to give the corresponding radical (RS) which can, in turn, couple to form disulfides (RSSR). For instance, 2,5-dimercapto-1,3,4-thiadiazole (DMcT), one of the most promising organosulfur compounds due to its high theoretical capacity (362 Ahkg^-1), form a disulfide polymer by coupling reactions of the electrochemically generated radical species (Scheme 1). Therefore, the capability of the thiolate redox chemistry to release and capture lithium ions during charge/discharge allows their easy incorporation into the so-called “rocking-chair” type system employed in the current lithium-ion battery technology. Organic materials also offer the advantage of being relatively low cost and derived from abundant resources, as opposed to the metal oxides currently employed.

                     
 Figure 1: Cyclic Voltammograms for different electrodes          Figure 2: Schematic Depiction of Electrocatalytic Acceleration

However, the charge transfer kinetics of both oxidation and reduction of these materials tend to be too sluggish at room temperature for the use in viable lithium/lithium-ion batteries. In order to accelerate these redox reactions, our group has previously investigated the use of conducting polymers as electrocatalysts and shown that the redox reactions of DMcT can be dramatically accelerated by the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) as shown in Figures 1 and 2. The composite cathode film composed of DMcT polymer (PDMcT) and PEDOT has exhibited a high gravimetric capacity of 200 mAhg^-1 with ca. 2.8 V potential plateau (versus Li/Li⁺) as a result of electrocatalytic effect by PEDOT (Figure 3).

Based on the understanding of the electrocatalytic effect by conducting polymers (e.g., PEDOT) towards the redox reactions of organosulfur compounds (e.g., DMcT), our recent work has focused on designing new hybrid cathode materials in which thiolate-based high-energy storage sites are covalently attached to a conducting polymer backbone as shown in Figure 4. Implicit in this strategy is the search for “modifiable” organosulfur compounds, the redox reactions of which can be electrocatalytically accelerated by conducting polymers such as PEDOT. We have established an efficient process for the material design, i.e., designing materials via computational modeling, synthesizing, and characterizing them in our own laboratory in order to meet properties required as a cathode electroactive mass.