Ionic solid-like conductor-assisted polymer electrolytes for solid-state lithium metal batteries

Solid polymer electrolytes (SPEs) have attracted extensive attention by virtue of lightweight and flexible processability for solid-state lithium metal batteries (LMBs) with high energy density and intrinsic safety. However, the SPEs suffer from the trade-off effect between ionic conductivity and mechanical strength. Herein, we report an ionic solid-like conductor with high Li⁺ conductivity and good thermal stability as the conductive phase of polymer electrolytes for advanced LMBs. Using poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as the polymer matrix, the ionic solid-like conductor can be regarded as a solid plasticizer due to its advantages of non-fluidity and non-leakage. It increases the amorphous region and the dissociation degree of lithium salts in SPEs, while minimizing the loss of mechanical properties. As a result, the Li⁺ conductivity of SPEs incorporating the ionic solid-like conductor is enhanced by four orders of magnitude compared to the blank PVDF-HFP-based electrolyte. The optimized SPE membranes can be processed as thin as 50 µm with a high Young’s modulus of 16.8 MPa, therefore ensuring stable long-term cycling of solid-state LMBs. The Li/Li symmetric cells stably cycled for more than 750 h without short circuits, and the LiFePO₄/Li solid-state batteries demonstrate excellent electrochemical performance over 350 cycles with a capacity retention of 82.5%. This work provides a new strategy for designing ionic solid-like conductors as solid plasticizers for high-performance polymer electrolytes.