The solid-state Li-ion battery (SSB) is regarded as one of the most promising next-generation batteries due to its intrinsic safety and high energy density. Unfortunately, its development is plagued by the poor interface compatibility between the electrode and solid-state electrolyte. Herein, flame-retardant trimethyl phosphate (TMP) solvent along with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and fuoroethylene carbonate (FEC) additive is utilized as a smart wetting agent to improve the interface compatibility between the Li-metal anode and the Li1.5Al0.5Ge1.5(PO4)(3) (LAGP) solid electrolyte. By taking advantage of such wetting agent, highly stable anodic and cathodic interfaces with improved physical contact and chemical stability are realized. As a result, the solid-state Li/LAGP/Li symmetrical cell with such wetting agent exhibits superior cycling performance over 1000 h with small polarization voltages. When adding such wetting agent to the solid-state Li/LAGP/LiFePO4 battery, we achieved a favorable initial capacity of about 140 mAh g(-1) at 0.5 C and can retain 80% of its initial capacity even after 700 cycles. Additionally, the solid-state Li-air battery with such wetting agent also shows stable cycling performance over 100 h. The strategy demonstrated here will encourage more research works on developing functional and safe wetting agents for solid-state batteries.