Bi(benzimidazole)-based Super-Electron-Donors in Redox Polymers as Battery Electrode Materials

Organic electrode-active materials (OAMs) represent an alternative to (transition-) metalbased materials used in conventional battery cells. Reversibly oxidizable p-type OAMs allow realizing full-organic battery cells operating in an anion-rocking chair mechanism. In the search for p-type materials with a low redox potential so-called super-electron-donors (SEDs) are a promising class of molecules. Herein, we implement a bi(benzimidazole) (BBI)-based SED into three polymers, PBBI, a conjugated homopolymer, PSBBI as a styrene-based side chain polymer, and X-PSBBI as its crosslinked counterpart. Their properties as potential OAMs in lithium-organic half cells were investigated, and PBBI was found to be electrochemically inactive. The sidechain polymers showed reversible cycling behavior in binder-free powder electrodes in LiBF4-based electrolytes, even though the accessible capacity quickly faded. As possible degradation mechanism we propose decomposition via a dicarbene species as a plausible, reactive key species. This study showcases bi(benzimidazole)s as redox-active groups in OAMs with low redox potential and provides insight into challenges associated with obtaining a reversibly cycling behavior in battery electrodes.