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

Organic electrode-active materials (OAMs) represent an alternative to (transition-) metal-based materials used in conventional battery cells. Reversibly oxidizable p-type OAMs allow realization of 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 incorporate 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 cross-linked counterpart. Their properties as potential OAMs in lithium–organic half-cells were investigated, and PBBI was found to be electrochemically inactive. The side-chain polymers showed reversible cycling behavior in binder-free powder electrodes in LiBF$_4$-based electrolytes with a low charge/discharge potential of 2.1 V vs Li$^{+/0}$, even though the accessible capacity quickly faded. As a 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 a low redox potential and provides insight into challenges associated with obtaining reversible cycling behavior in battery electrodes.