Optimizing interfacial wetting by ionic liquid for high performance solid-state lithium metal batteries operated at ambient temperature

Employing solid electrolytes in lithium metal batteries has been proposed to address the safety issues from the organic liquid electrolyte and Li dendrite growth. However, their practical applications are still plagued by the large interfacial resistance between the electrode and solid-state electrolytes (SSEs). Herein, a small amount of nonflammable and nonvolatile ionic liquids (ILs) are introduced into the poly(ethylene oxide) (PEO)/Li₇La₃Zr₂O₁₂ (LLZO) composite SSEs to simultaneously promote the ionic transport within SSEs bulks and through SSEs-electrode interface. When the 17.5 wt% ILs are added, the ionic conductivity of PEO/LLZO SSEs increases by an order of magnitude, and the interfacial resistance decrease by >5 times at 25 °C. Meanwhile, no leakage of ILs from PEO/LLZO SSEs can be observed under high pressure of 10 MPa. Moreover, it is found that the IL of [Py₁₄]TFSI endows the PEO/LLZO SSEs with better interfacial compatibility with both high voltage cathode and Li metal anode than [BMIM]TFSI and [EMIM]TFSI. Benefiting from these merits of PEO/LLZO@[Py₁₄]TFSI (PLP) SSEs, the pouch-type solid-state LiFePO₄||Li cell shows a high average capacity of 124 mAh g⁻¹, average Coulombic efficiency of over 99.5 %, and retains 120 mAh g⁻¹ after 100 cycles at 25 °C.