Decoupling Stiffness and Toughness in Solid Polymer Electrolytes via Reversible Crystallization

Solid polymer electrolytes (SPEs) are polymer-based, flexible, and nonflammable electrolytes, making them promising candidates for developing highly stretchable electrochemical devices. However, in conventional designs, toughness improvement is typically coupled with an increase in stiffness. This coupling often arises from the introduction of thermally reversible crystals (TRCs) of polymer, resulting in high stiffness, brittleness, and poor conformability to electrodes. In this study, we develop a material design strategy to decouple toughness from stiffness in SPEs using strain-induced crystallization (SIC) in a homogeneous four-branched poly(ethylene glycol) (Tetra-PEG) network. SIC significantly enhances the toughness without increasing the stiffness, enabling the formation of soft, tough, and stretchable SPEs. Building on this decoupled platform, stiffness was reintroduced through TRCs of PEG, yielding SPEs that were both stiff and highly fracture-resistant. Importantly, upon heating, these materials exhibited thermoplastic behavior, which improved their conformability to metal electrodes. Consequently, Li|Tetra-PEG SPE|Li symmetric cells exhibited reversible lithium plating and stripping with stable long-term cycling. Overall, the proposed design strategy effectively decouples toughness from stiffness, thereby overcoming the conventional trade-offs in SPEs.