Side‐Chain Engineering for High‐Performance Conjugated Polymer Batteries

Conjugated polymers are attractive for energy storage but typically require significant amounts of conductive additives to successfully operate with thin electrodes. Here, side‐chain engineering is used to improve the electrochemical performance of conjugated polymer electrodes. Naphthalene dicarboximide (NDI)‐based conjugated polymers with ion‐conducting ethylene glycol (EG) side chains (PNDI‐T2EG) and non‐ion‐conducting 2‐octyldodecyl side chains (PNDI‐T2) are synthesized, tested, and compared. For thick (20 µm, 1.28 mg cm−2) electrodes with a 60 wt% polymer, the PNDI‐T2EG electrodes exhibit 66% of the theoretical capacity at an ultrafast charge–discharge rate of 100C (72 s per cycle), while the PNDI‐T2 electrodes exhibit only 23% of the theoretical capacity. Electrochemical impedance spectroscopy measurements on thin (5 µm, 0.32 mg cm−2), high‐polymer‐content (80 wt%) electrodes reveal that PNDI‐T2EG exhibits much higher lithium‐ion diffusivity (DLi+ = 7.01 × 10−12 cm2 s−1) than PNDI‐T2 (DLi+ = 3.96 × 10−12 cm2 s−1). PNDI‐T2EG outperforms most previously reported materials in thick, high‐polymer‐content electrodes in terms of rate performance. The results demonstrate that the rate performance and capacity are significantly improved through the incorporation of EG side chains, and this work demonstrates a route for increasing the rate of ion transport in conjugated polymers and improving the performance and capacity of conjugated‐polymer‐based electrodes. Side‐chain engineering of naphthalene dicarboximide‐based conjugated polymer significantly enhances battery rate performance and storage capacity. By incorporating oligo‐ethylene glycol side chains, the nanocomposite electrodes demonstrate exceptional rate performance at high charge–discharge rates, high mass loadings, and high polymer contents. The improved performance is attributed to improved ionic and electronic conductivity in the side‐chain modified materials.