Engineered Polyimide Binders Fortifying Ni- and Co-Free Disordered Rock-Salt Li-Ion Cathodes

Ni- and Co-free Mn-based disordered rock-salt (DRX) cathodes are promising candidates for low-cost, high-energy-density lithium-ion batteries but suffer from rapid performance degradation under high-voltage operation and from large volume changes during cycling. Here, we report a fluorinated polyimide (PI) binder platform that stabilizes DRX cathodes through molecular design. By systematically varying the degree of fluorination in the diamine backbone and incorporating carboxylic acid functionalities, we establish clear structure-property relationships linking binder electronic structure, interfacial adhesion, and electrode processability. Combined theoretical and experimental analyses show that increased fluorination stabilizes the valence band and enhances anodic stability, while carboxylic acid groups promote strong interfacial interactions and homogeneous electrode formation. When applied to partially ordered and fully disordered DRX cathodes, the optimized binder suppresses impedance growth, mitigates mechanical degradation, and reduces transition-metal dissolution, leading to improved cycling performance in both conventional and high-concentration electrolytes (HCEs). For example, the optimized PI binder leads to a 30% improvement in energy density retention compared with a conventional PVDF binder after 100 cycles in HCEs between 1.5 and 4.6 V. These results establish fluorinated PIs as a robust and tunable binder platform and highlight binder design as critical for sustainable, high-capacity Ni- and Co-free lithium-ion batteries.