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 m ²  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 m ² 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.