Cathodically Stable Li-O2 Battery Operations Using Water-in-Salt Electrolyte

Development of the Li-O2 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-O2 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 H2O molecules are locked to the ions and exhibit little reactivity toward Li2O2 or other oxygen species. The net result is a highly effective electrolyte that permits stable Li-O2 battery operations on the cathode with superior cycle lifetimes. A new door to practical Li-O2 batteries with high performance is opened up. As an electrochemical energy storage technology that holds the highest theoretical energy density, the Li-O2 battery has been studied for over two decades. Its reported performance, however, remains much lower than expected. An important reason for the slow progress is the lack of stable electrolytes. Here, we present a solution to such a challenge. We show that the “water-in-salt� electrolyte, which is essentially a super-concentrated aqueous solution, enables stable operation of a Li-O2 battery on the basis of reversible formation and decomposition of Li2O2 for superior long cycle lifetimes. More importantly, it presents a platform for future optimizations to realize the full potential of the Li-O2 battery as a stable, high-capacity electrochemical energy storage technology. Development of the Li-O2 battery as a practical energy storage technology has been underpinned by the lack of a stable electrolyte to enable reversible conversion between O2 and Li2O2, given that previous applied organic electrolytes all show reactivity toward reactive oxygen species. Wang and colleagues show that this issue can be solved with a water-in-salt electrolyte that contains no organic solvent molecules. The net result is a highly effective electrolyte that enables stable Li-O2 battery operations up to 300 cycles.