Polynorbornene Brush Polymers With Naphthalene Diimide‐Bithiophene Motifs for Organic Batteries

ABSTRACT Polymer brushes comprising a polynorbornene backbone and redox‐active naphthalene diimide (NDI)‐bithiophene (T2) side chains are prepared and used as cathode materials in organic batteries. High molar masses and high yields are obtained for three brush polymers termed P(Nor‐T2NDIT2), P(Nor‐T2NDIT2‐10X) and P(Nor‐T2NDIT2‐NDIBu). In the case of P(Nor‐T2NDIT2), fast capacity fading during early cycles is related to partial dissolution of the polymer in the electrolyte. Insolubility of the brush polymers in the battery electrolyte is successfully achieved by either cross‐linking (P(Nor‐T2NDIT2‐10X)) or variation of the side chain length (P(Nor‐T2NDIT2‐NDIBu)). When utilized as cathode material in a Li half cell, initial capacities between 55 and 75 mA•h/g are obtained for the different materials, corresponding to 100% of their theoretical capacity (CT). Dissolution is not found for neither P(Nor‐T2NDIT2‐10X) nor P(Nor‐T2NDIT2‐NDIBu), which retain 91% and 87% of their initial capacities, respectively, after 1500 cycles at 1C. P(Nor‐T2NDIT2‐NDIBu) is not cross‐linked but still insoluble in the electrolyte due to an additional NDI unit bearing a short n‐butyl side chain (NDIBu). This allows for an increase in the theoretical capacity to 75 mA•h/g. At 10C, P(Nor‐T2NDIT2‐NDIBu) additionally features an enhanced rate capability, delivering 67% of CT, while P(Nor‐T2NDIT2) and P(Nor‐T2NDIT2‐10X) only yield 43% and 57% of CT, respectively.