Dual Cross-Linked Polymer Networks Derived from the Hyperbranched Poly(ethyleneimine) and Poly(acrylic acid) as Efficient Binders for Silicon Anodes in Lithium-Ion Batteries

The condensation of poly(acrylic acid) (PAA) with small amounts of hyperbranched poly(ethyleneimine) (PEI) resulted in the dual cross-linked network cxPAA-PEI(x) (x: wt % of PEI, x = 1, 3, and 5), consisting of physical hydrogen bonds (H bonds) and chemical covalent bonds as the framework-maintaining forces, as efficient binders for silicon anodes in lithium-ion batteries. In addition to strong mechanical strength, the inherent functional COOH and NH2 groups of cxPAA-PEI(x)s also contribute to strong interfacial adhesive forces toward anode components (e.g., copper current collector). The mechanical properties and adhesive forces of the PAA-PEI(x) binders can be appropriately adjusted by varying the PEI content in the binders; as a result, the cxPAA-PEI (3) binder exhibited the best mechanical and binding strengths, superior to cxPAA-PEI (1) and (5). The subtle balance between the mechanical strength and adhesive force is therefore discussed in terms of the cross-linking density and H-bond interaction forces. With superior mechanical and adhesive properties, cxPAA-PEI (3) can be integrated into a silicon anode with high initial Coulombic efficiency and excellent specific capacity, with a value of 2602 mAh g–1 at 0.2 C after 100 cycles and 1230 mAh g–1 at 1 C after 300 cycles.