Revisiting naphthalenediimide (NDI)-based polymer cathodes for Li-ion batteries: The role of molecular weight on the electrochemical and mechanical properties

Organic cathodes are emerging as promising electrodes for next-generation batteries due to their remarkable structural adaptability, environmental friendliness, cost-effectiveness, and tunable electrochemical properties. However, most small-molecule cathodes encounter significant challenges related to solubility issues and sluggish kinetics. In this study, we investigated n-type conjugated polymer-based organic cathodes, specifically focusing on how variations in the molecular weight of naphthalenediimide (NDI)-based n-type conjugated polymers, PNDI-T2, affect the performance of Li metal batteries. Our results demonstrated that the cathode with the highest molecular weight, P46 (M_n: 46 kg mol⁻¹), exhibited superior performance, characterized by enhanced mechanical properties and Li-ion diffusivity, which contributed to significantly better cycling stability (over 2000 cycles) and rate capability. Additionally, P46 maintains excellent cycling stability even at −10 °C, with 100 % capacity retention over 200 cycles. In contrast, the lowest molecular weight, P7 (M_n: 7 kg mol⁻¹), exhibited poor performance due to increased solubility in liquid electrolyte, poor mechanical properties, and low Li-ion diffusivity, resulting in poor specific capacity and rapid capacity decay under high C-rates. This investigation provides crucial insights into how molecular weight influences the properties of n-type conjugated polymer-based cathodes and underscores the potential of high-molecular-weight conjugated polymers in developing more efficient and durable Li metal batteries.