Linker-Free Design of Naphthalenediimide Polymer Cathodes for Large Capacity and High Rate Capability

As the demand for sustainable, high-performance rechargeable batteries grows, organic electrode materials have attracted considerable attention due to their elemental abundance and chemical tunability. Among them, naphthalene diimide (NDI)-based main-chain polymers, in which NDI units are linked by aromatic comonomers at the 2,6-positions, are promising because their high electronic conductivity enables excellent rate performance. However, their specific capacities are typically limited to below 55 mAh/g because of the large fraction of redox-inactive molecular components. Previous molecular engineering strategies have mainly focused on shortening bulky N-alkyl side chains to increase theoretical capacity, but removing long alkyl chains reduces polymer solubility and complicates synthesis and electrode fabrication. Meanwhile, the influence of aromatic comonomers on the electrochemical properties of NDI polymer electrodes remains largely unexplored.

In this study, we systematically investigate how comonomer type affects molecular packing, charge distribution, electrical conductivity, interfacial charge transfer, and Li⁺ transport using experimental and theoretical analyses. We prepared two NDI polymers linked by either phenyl or thiophene rings, together with a linker-free NDI polymer. Comonomer linkers play a critical role in molecular coplanarity and interchain stacking, thereby determining intra- and interchain charge transport. In contrast, the absence of a linker causes steric repulsion between adjacent NDI units, inducing backbone twisting and suppressing crystallinity. Unexpectedly, this reduced crystallinity facilitates Li-ion diffusion and interfacial charge-transfer kinetics within the polymer electrodes. Consequently, the linker-free NDI polymer electrode delivers both increased specific capacity and excellent rate performance in Li-organic cells.