Zwitterion-Modulated Quasi-Solid-State Polymer Electrolyte with Janus Interface toward Low-Temperature Lithium Metal Batteries

Although polymer-based lithium batteries have high theoretical energy densities, they usually suffer from poor low-temperature performance, owing to interfacial instability and sluggish ion transport in solid-state electrolytes. Herein, a unique Janus interface is constructed between the lithium anode and PVDF-based polymer electrolyte via oriented assembly of a zwitterion under an electric field, in which the imidazolium cation is attracted to the anode and the sulfonate anions extend outward into the electrolyte. Such an asymmetric feature effectively lowers the Li⁺ desolvation barrier and suppresses solvent-related side reactions on the anode. Moreover, in the bulk electrolyte, the sulfonate groups of the zwitterion provide additional Lewis basic sites for continuous ion hopping, which act as lithium-ion migration bridges, greatly enhancing ion transport along polymer chains. Thus, the quasi-solid-state polymer electrolyte with a Janus interface exhibits a high ionic conductivity of 0.66 mS cm^–1 (at room temperature), an elevated Li⁺ transference number (0.61), and strong mechanical strength (strength of 8.2 MPa, elongation of 520%). The full cells coupled LiFePO₄ cathode and lithium metal anode exhibit outstanding cycling stability, with 83.6% capacity retention over 1000 cycles at 5 C. Remarkably, even at −10 °C, the full cells exhibit high cycling stability of 86.3% over 700 cycles, superior to most reported polymer-based solid lithium batteries.