Insight into Chemical Reduction and Charge Storage Mechanism of 2,2'-Dipyridyl Disulfide toward Stable Lithium-Organic Battery.

In lithium-organic batteries, organic cathode materials could dissolve in a liquid electrolyte and diffuse through the porous separator to the active lithium-metal anode, resulting in cycling instability. However, 2,2'-dipyridyl disulfide (PyDS) can be cycled 5 times better than diphenyl disulfide (PDS) although both are soluble. We believe this is related to their reactivity with lithium (Li0). Herein, we investigate the chemical reduction of PyDS by lithiated carbon paper (Li-CP) in ether electrolyte. It is found that only 6.3% of PyDS was reduced by Li-CP after 10 days, unlike PDS. Experimental and computational results show that PyDS molecules are ionized by lithium ions of lithium salts delocalizing the charge on pyridine rings of PyDS, which can momentarily store Li0, thus keeping the S-S bond inert in chemical reaction with Li0. This finding is successfully utilized in a membrane-free redox flow battery with PyDS catholyte, showing long cycle life with high energy density and energy efficiency. This work reveals the interesting charge storage mechanism and the different activity of organodisulfides toward electrochemical reduction and chemical reduction due to the organic groups, which can provide guidance for the design of stable lithium-organic batteries.