In Situ-Cross-linked Supramolecular Eco-Binders for Improved Capacity and Stability of Lithium–Sulfur Batteries

Rechargeable lithium–sulfur batteries (LSBs) have caused widespread concern because of their high theoretical energy density and environmental benefits. However, LSBs accompany a series of phase transitions of which occur significant volume changes during charging and discharging, which severely reduce the lifetimes of LSBs. In this study, a supramolecular eco-binder with in situ-cross-linking is investigated to extend the charge–discharge cycle of LSBs. Specifically, a vegetable-oil-based cationic waterborne polyurethane polymer with phytic acid (WPUP) is developed as an eco-binder for sulfur cathodes. A low-viscosity WPUP is used in the electrode fabrication process to achieve homogeneous, dense coating of the active materials. The WPUP participates in an in situ covalent cross-linking reaction to produce a three-dimensional network with excellent mechanical properties. This ensures robust adhesion and much shorter ion- and electron-transfer paths. The resultant sulfur cathodes compose of high initial discharge capacity of 1051 mAh g^–1 at 0.5 C, promising long-term battery cycling stability (632 mAh g^–1 after 600 charge/discharge cycles at 0.5 C), and high rate cycling performance (634 mAh g^–1 at 4 C). As proof-of-concept, a Li–S full cell is fabricated with a 350% oversized lithium anode. It provides a good capacity of 1.3 mA h cm^–2 and high capacity retention of 99.83% per cycle over 300 cycles. This binder-design strategy will be important in developing high-capacity and long cycle stable LSBs.