Alkyl Chain Engineering of NDI-Based Conjugated Polymer Cathodes for Improved Capacity and Rate Performance

Organic cathode materials have attracted increasing attention as sustainable and structurally tunable alternatives to conventional inorganic electrodes for rechargeable batteries. In particular, conjugated polymers are promising because their redox properties, solubility, processability, and solid-state packing can be controlled through molecular design. Among various redox-active units, naphthalenediimide (NDI) is a representative electron-deficient building block widely used in organic electronics and is attractive for organic cathodes owing to its reversible n-type redox behavior.

Here, we demonstrate that alkyl side-chain engineering is an effective strategy to improve the electrochemical performance of NDI-based conjugated polymer cathodes. By shortening the alkyl chains attached to the polymer structure, the fraction of electrochemically inactive mass was reduced, increasing the theoretical capacity from 85.5 to 96.7 mAh g⁻¹. Consistent with this design, the experimentally observed discharge capacity increased as the alkyl chain length decreased.

The shortened alkyl chains also enhanced the rate capability. At a high current density of 2000 mA g⁻¹, the capacity retention improved from 46% to 60% with decreasing alkyl chain length. This improvement is attributed to enhanced intermolecular packing of the conjugated backbones, which can facilitate charge transport and redox-site accessibility within the electrode. These results indicate that alkyl side chains determine not only the theoretical capacity by controlling inactive mass, but also the solid-state packing and rate performance of NDI-based polymer cathodes. Overall, this work highlights alkyl chain engineering as a simple and effective molecular design approach for high-performance organic cathode materials