Molecular Engineering of the Prototypical n-Type Polymer P(NDI2OD-T2) Enables Capacity Enhancement and High-Temperature Stability in Lithium-Ion Battery Cathodes

Abstract

Conjugated n-type polymers have long been explored as organic cathodes for lithium-ion batteries (LIBs), yet the widely studied P(NDI2OD-T2) has received limited attention as a practical cathode because of its modest capacity (similar to 55 mAh g-1). Here, we report the first systematic effort to re-engineer this prototypical polymer through side-chain shortening and donor simplification. Replacing bulky 2-octyldodecyl (2OD) chains with 2-butyloctyl (2BO) and simplifying the bithiophene (T2) donor to a vinylene (V) produced P(NDI2BO-V), which delivers a 1.51-fold higher capacity (56.9 -> 86.0 mAh g-1) while retaining excellent cycling stability. Crucially, we show that n-type polymers can achieve remarkable cycling stability at elevated temperatures: both polymers remained stable at 60 degrees C, where small molecules fail, with P(NDI2OD-T2) keeping 97% after 1000 cycles and P(NDI2BO-V) 80% after 600 cycles. Mechanistic studies combining electrochemical analysis with density functional theory and molecular dynamics simulations reveal how donor linker units dictate structure and transport. Crystalline P(NDI2OD-T2) exhibits higher electronic conductivity and undergoes a one-step two-electron redox process, whereas amorphous P(NDI2BO-V) offers enhanced Li+ diffusivity but follows a stepwise pathway. This work establishes a molecular design framework for conjugated polymer cathodes that combine high capacity, efficient charge transport, and long-term thermal stability, advancing their potential for practical LIB applications.