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 (∼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 °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.