Electrochemically-Matched and Nonflammable Janus Solid Electrolyte for Lithium–Metal Batteries

Solid-state batteries based on ceramic electrolytes are promising alternatives to lithium-ion batteries with better safety and energy density. While solid electrolytes such as the garnet-type Li7La3Zr2O12 (LLZO) are chemically stable with lithium metal, their rigidity leads to poor interfacial contact with the cathodes. Nonflammable organic phosphates, however, are characterized by a liquid nature and can immerse the conventional porous cathodes to form a good contact. However, the phosphates are unstable with lithium metal anodes. We design a quasi-solid Janus electrolyte based on the ceramic LLZO and a trimethyl phosphate (TMP) gel which combines the best of both worlds. The electrochemical window of the Janus electrolyte is significantly extended compared with the TMP to accommodate the lithium metal anode. The contact between the cathode and the electrolyte is maintained by the semifluid nature of the TMP gel. A lithium–metal battery with such a Janus electrolyte can stably cycle at room temperature at 1C while still retaining a capacity of 115 mAh g–1 over 100 times. In contrast, the batteries based on LLZO and TMP individually cannot function properly. More importantly, despite the quasi-solid nature, the battery does not contain flammable functional parts and can alleviate the safety concerns of current batteries containing organic-type electrolytes. This work provides a simple but effective strategy for safe, inexpensive, and energy-dense solid-state batteries.