Unlocking higher performance of dual-ion storage via fluorene substitution in bipolar conjugated polymers featuring dual acceptors

Redox-active conjugated polymers (CPs) are promising electrode materials for sustainable, economical, and safer batteries. However, conventional polymer cathodes featuring donor-donor (D-D) or donor-acceptor (D-A) often exhibit limited voltages (~3 V vs. Li⁺/Li). To address this, bipolar dual-acceptor (A1-A2) materials -BT-DPP and its fluorinated variant FBT-DPP incorporating benzo[c][1,2,5]thiadiazole (BT/FBT) and diketopyrrolopyrrole (DPP) were developed. Both CP electrodes yield discharge capacities of 70–80 mAh g⁻¹ at 0.2 A g⁻¹ current density with an output voltage of 3.9 V vs. Li⁺/Li and surpass the PF₆⁻ storage performance of graphite under similar testing conditions, which is the most reliable anion-storage matrix known to date. Density functional theory (DFT) calculations confirm that fluorination lowers the energy gap and enhances interaction with PF₆⁻ ions in FBT-DPP, improving reactivity, binding properties, and stability, making it a strong candidate for dual-ion battery cathode. Hence, the FBT-DPP aids anion storage electrochemistry further, thereby improving specific capacity, coulombic efficiency, and solid-state anionic mobility at the bulk, enabling a stable cycle life of ≥ 2000 cycles at an extremely high current density of 3 A g⁻¹, and controlling the growth of resistance with the progression of cycling compared with the non-fluorinated analog in half-cell and full-cell configurations.