LAUSR

Summary Development of the Li-O 2 battery into a practical technology hinges on the availability of a stable electrolyte. Because of the high reactivity of oxygen species in the system, no known organic electrolytes meet the stability requirements. The search for a suitable electrolyte system remains an outstanding challenge in Li-O 2 battery research. Here, we show that the issue can be solved with the use of a water-in-salt electrolyte system that involves no organic solvents. In essence, the electrolyte consists of super-concentrated LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), in which H 2 O molecules are locked to the ions and exhibit little reactivity toward Li 2 O 2 or other oxygen species. The net result is a highly effective electrolyte that permits stable Li-O 2 battery operations on the cathode with superior cycle lifetimes. A new door to practical Li-O 2 batteries with high performance is opened up.