High energy density lithium-ion batteries with preferable cycling stability are critical for the development of all-electric vehicles. Silicon (Si) has demonstrated a remarkable potential for application as anode materials due to its superior capacity performance and worldwide abundance. However, Si intrinsically undergoes substantial volume fluctuation during repeated lithiation/delithiation processes, which pulverizes the Si particles and undermines the integrity of the electrode structures, thus resulting in frustrating cycling stability. We developed a polymer binder with a highly stretchable and elastic network structure that can accommodate volume variation of Si. This was realized by an in situ cross-linking of polyacrylic acid (PAA) with isocyanate-terminated polyurethane oligomers that consist of polyethylene glycol (PEG) chains and 2-ureido-4-pyrimidinone (UPy) moieties through the reaction between isocyanate and carboxyl during the electrode preparation process. In this binder network, PAA could strongly adhere to the Si particles by forming hydrogen bonding with the surface hydroxyl groups. The PEG chains induce the flexibility of the polymer network, while the UPy moieties endow the polymer network with desirable mechanical strength through the formation of reversible and strong quadruple H-bonding cross-linkers. This binder not only can sufficiently accommodate the volume change of Si but can also provide a strong mechanical support to effectively sustain the integrity for the Si anode, consequently enhancing cycle stability and rate performance.
Keywords: cross-linked polymer binder; lithium-ion batteries; polyurethane; silicon; stretchable and elastic ability.