With the advantages of organic and inorganic solid electrolytes, composite electrolytes are a promising option for use in all-solid-state Li-metal batteries. However, the considerable disparity in interfacial energy between ceramic and polymer electrolytes results in poor solid-solid contacts and the internal creation of a space charge layer in the composite electrolyte. Here, we report a melamine (MA) transition layer for the sake of strengthening the bond between Li1.5Al0.5Ge1.5(PO4)3 (LAGP) and poly(ethylene oxide) (PEO) to enhance physical and electrochemical properties. The MA is absorbed on LAGP by electron transfer from LAGP to MA's triazine ring, resulting in intimate contact and good mechanical stability. Simultaneously, the MA stabilizes the Li-salt anion, reduces its decomposition reactions at the interface between PEO and LAGP in the electrolyte, and promotes free Li+ dissociation, resulting in superior ionic conductivity and interfacial stability. Thus, the solid electrolyte film enables symmetric Li/Li batteries to achieve steady Li plating/stripping for more than 1300 h at a current density of 0.25 mA cm-2. The all-solid-state Li|PEO-MA@LAGP|LFP cell exhibits improved cycling stability. The Li/PEO-MA@LAGP/NCM523 cell shows a cycling life that is a factor of 5 times greater than that of a cell based on PEO-LAGP.
Keywords: Li1.5Al0.5Ge1.5(PO4)3; composite electrolyte interface; lithium-metal battery; melamine; poly(ethylene oxide).