Waterborne-Polyurethane as High-Performance Binder for Cathodes of Lithium-Ion Batteries

In lithium-ion batteries, the electrodes are strongly influenced by the polymer binder swelling in liquid electrolytes, which governs electrode integrity, adhesion, flexibility and Li+ transport. Herein, a series of polyacrylate-terminated waterborne polyurethanes (WPUAs) are designed by systematically varying the proportions of nonionic, anionic, and zwitterionic monomers to regulate swelling behavior in liquid electrolytes. The compositional modulation influences the polymer polarity, polymer entanglement, polymer mechanical property and their swelling in electrolytes, and finally the final electrochemical performance of WPUAs. The optimized WPUA exhibits a balanced swelling ratio, strong adhesion, and significantly enhanced Li+ diffusion. Density functional theory calculation reveals that the molecular configuration is rational and conducive to the Li+ transport, which are supported by its superior Li+ conductivity and diffusion coefficient of lithium ions much higher than those of the polyvinylidene fluoride (PVDF) binder. The calculated diffusion coefficient of lithium ions in WPUA is 2.43 × 10-12 cm2 s-1, which is twice times higher than that in PVDF. Employing the synthesized WPUA binder, the assembled LiFePO4@Li half cells show improved electrochemical performance of 110.8 mAh g-1 at 3C for over 800 cycles with 80.4% capacity retention and the high-loading LiFePO4@artificial graphite full cells also show improved performance of 135.0 mAh g-1 after 340 cycles with 87.1% capacity retention, both of which exhibit better electrochemical performance than PVDF. This work highlights controlled binder swelling as a key design parameter for balancing electrolyte accessibility, Li+ transport, and electrode integrity, offering useful guidance for the development of high-performance waterborne binders for lithium-ion batteries.