The "sweet spot" in length for contorted conjugated ladders in ultrafast-charging Li and Mg batteries.

Here, we explore a conjugated, contorted polymer framework tailored for ultrafast-rate charging/discharging, leveraging a tunable synthetic strategy to control its molecular length. We systematically explore the helical perylene diimide (hPDI) ladder polymers across three length regimes, short, medium, and long, to determine the optimal electrochemical stability and performance. The intermediate-length polymer strikes a critical balance between electrode integrity, solubility, and rate capability. Its reversible redox activity and structural robustness make it well-suited for both Li and Mg ions. The hPDI-medium cathode delivers a remarkable specific power of 22.4 kW kg after 10 000 cycles in Li batteries and 1.7 kW kg after 3000 cycles in Mg batteries, and we extend this to practically relevant mass loadings. This study highlights the critical role of molecular engineering in the rational design of high-performance organic cathode materials for sustainable energy storage.