Polymers of Intrinsic Microporosity as Organic Cathodes for Fast-Charging Lithium-Ion Batteries

Developing fast-charging electrodes requires understanding how polymer architecture governs ion-transport, redox-site accessibility, and charge-storage mechanism. While porosity improves electrochemical performance, its role at fast-charging conditions remains unclear. We investigate the effects of porosity by comparing polymers with a naphthalene diimide (NDI) redox-active unit and two contorted monomers (triptycene-like [tryp] and spirobisindane [spiro]) to a nonporous model system (NDI-model). In galvanostatic cycling, NDI-model exhibits low accessible capacity even at slow rates (25.8 mAh g^-1 at C/10), indicating limited ion transport, related to its low surface area (35 m² g^-1). NDI-spiro exhibits a superior performance, retaining 74% of its C/10 capacity at 20C. Cyclic voltammetry reveals a surface-controlled charge-storage mechanism for NDI-spiro, attributed to its higher surface area (273 m² g^-1), which allows greater accessibility of redox sites. In contrast, NDI-tryp achieves high capacity at slow rates (50.4 mAh g^-1 at C/10), but suffers from diffusion limitations and poor rate capability, related with its diffusion-controlled charge-storage mechanism. Extending this study to spiro-based polymers with other redox active units, perylene diimide (PDI) and benzenediimide (BDI), shows that redox-unit identity affects kinetics and rate capability, shifting the charge-storage mechanism from surface-controlled to a diffusion-controlled. These findings establish structure-property relationships for designing high-performance fast-charging organic battery electrodes.