Highly Efficient Br-/NO3- Dual-Anion Electrolyte for Suppressing Charging Instabilities of Li-O2 Batteries.

The main issues with Li-O2 batteries are the high overpotential at the cathode and the dendrite formation at the anode during charging. Various types of redox mediators (RMs) have been proposed to reduce the charging voltage. However, the RMs tend to lose their activity during cycling owing to not only decomposition reactions but also undesirable discharge (shuttle effect) at the Li metal anode. Moreover, the dendrite growth of the Li metal anode is not resolved by merely adding RMs to the electrolytes. Here we report a simple yet highly effective method to reduce the charge overpotential while protecting the Li metal anode by incorporating LiBr and LiNO3 in a tetraglyme solvent as the electrolyte for Li-O2 cells. The Br-/Br3- couple acts as an RM to oxidize the discharge product Li2O2 at the cathode, whereas the NO3- anion oxidizes the Li metal surface to prevent the shuttle reaction. In this work, we found that both anions work synergistically in the mixed Br-/NO3- electrolyte to dramatically suppress both parasitic reactions and dendrite formation by generating a solid Li2O thin film on the Li metal anode. As a result, the charge voltage was reduced to below 3.6 V over 40 cycles. The O2 evolution during charging was more than 80% of the theoretical value, and CO2 emission during charging was negligible. After cycling, the Li metal anode showed smooth surfaces with no indication of dendrite formation. These observations clearly demonstrate that the Br-/NO3- dual-anion electrolyte can solve the problems associated with both the overpotential at the cathode and the dendrite formation at the anode.