Surface modification enabling 4.7 V Nickel-rich layered cathode with superior long-term cyclability via novel functional polyimide binders

High voltage operation of LiNi0.8Co0.1Mn0.1O2 (NCM) for high energy density lithium-ion batteries has poor cycling performance, due to the unstable cathode-electrolyte interface. Herein, two functional polyimides (PIs), one polyimide with hydroxyl group is abbreviated as PI-OH and the other polyimide with carboxyl group is abbreviated as PI-COOH, are designed to contain hydroxyl, carboxyl, trifluoromethyl and ether groups and used as cathode binder, which allows a strong hydrogen bond interaction, abundant chemical bonding sites, superior electrochemical oxidation resistance, and excellent flexibility. Experimental results reveal that both PI-OH and PI-COOH binders show the impressive efficiency on improving the cathode-electrolyte interface stability and long-term cyclability. Especially, the NCM cathode using PI-COOH binder exhibits the highest cyclability (205 mAh g−1 at 40 mA g−1 with 73% capacity retention after 100 cycles at 4.7 V), contrasting to NCM cathode with commonly-used polyvinylidene fluoride (PVDF) binder (208 mAh g−1 with 34% capacity retention). Furthermore, the fully-charged NCM cathode with PI-COOH binder releases the less heat than the NCM/PVDF cathode during high-temp treatment to 300 °C, demonstrating the superiority of polyimide binders in improving the safety of the cathode materials.