Waterborne polyurethane as binder for graphite anodes in lithium-ion batteries

Graphite (Gr) and silicon (Si) are two typical anode materials due to their good conductivity, low redox potential and/or structural stability. However, the commercial carboxymethyl cellulose/styrene-butadiene rubber (CMC/SBR) binder has insufficient adhesion and lithium ion (Li + ) transport and the poly(acrylic acid) (PAA) binder is brittle. Herein, we design a waterborne polyurethane (WPU) binder featuring a polyurethane backbone with the anionic -COOLi moieties and a zwitterionic poly(SBMA- co -AA- co -EHA- co -BMA) terminal. By increasing the content of anionic and zwitterionic moieties in poly(SBMA- co -AA- co -EHA- co -BMA) and introducing crosslinking in the polyurethane backbone to decrease swelling, the typical WPU-2 achieves a high adhesion strength (3.35 N cm −1 ) and a high Li + diffusion coefficient ( D Li + , 1.05 × 10 −13 cm 2 s −1 ). Electrochemical tests show that the Gr/Li half-cell with the WPU-2/Gr anode exhibits a 79.9% capacity retention after 300 cycles at 0.5C, which is much better than the cells employing the SBR binder. Furthermore, the NCM811||Gr full cell retains 81.7% capacity after 600 cycles. Additionally, when applied to silicon anodes, the WPU binder demonstrates superior performance compared to the PAA binder. These results confirm that the WPU binder, characterized by low swelling in liquid electrolytes and enhanced Li + diffusion kinetics, serves as a potential candidate for both graphite and silicon anodes.