Continuous Long-ch a in PEG Design in Polyimide Binders Enables Durable High-Voltage Cycling of NCM811 Cathodes

High-nickel LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes suffer from severe structural degradation, interfacial side reactions, and mechanical failure under high-voltage operation. To address these challenges, a novel polyimide-polyethylene glycol binder, denoted PI-PEG3(2000), was designed by integrating flexible long-chain PEG segments into a rigid polyimide backbone via a one-step synthesis. The rigid framework enables high-voltage tolerance, while the continuous PEG segments simultaneously establish efficient Li+ transport channels and provide outstanding elasticity to accommodate cycling-induced stress. When applied to NCM811 cathodes, PI-PEG3(2000) forms a uniform and dense coating that effectively suppresses interfacial side reactions, transition metal dissolution and phase transformation. As a result, cells with this binder exhibit significantly improved high-voltage cycling stability, retaining 86.2% and 76.7% capacity after 100 cycles at 4.5 and 4.7 V versus Li/Li+, respectively, outperforming conventional PVDF-based cells (66.1% and 54.2%). The binder also enables excellent rate capability, delivering 146 mAh g–1 at 5C in the 2.5–4.7 V versus Li/Li+ window compared with 113 mAh g–1 for PVDF. In long-term cycling tests, the PI-PEG3(2000) binder retains 71.4% capacity after 250 cycles (2.5–4.5 V vs Li/Li+) and 65.2% after 200 cycles (2.5–4.7 V vs Li/Li+). Additionally, the thermal stability of fully charged cathodes is significantly improved by reducing heat release, enhancing overall battery safety. This study presents a chain-length-based design strategy for advanced binders that simultaneously achieve high-voltage stability, high ionic conductivity, and mechanical resilience, providing a promising approach for developing high-energy-density lithium-ion batteries.