Future rechargeable Li metal batteries (LMBs) require a rational electrolyte design to stabilize

the interfaces between the electrolyte and both the lithium metal anode and the high voltage

cathode. This remains the greatest challenge in achieving high cycling performance in

LMBs. We report an ether-aided ionic liquid electrolyte which offers superior Li metal

deposition, high voltage (5 V) stability and non-flammability. High performance cycling of

LiNi0.8Mn0.1Co0.1O2 (4.4 V) and LiNi0.6Mn0.2Co0.2O2 (4.3 V) cells is demonstrated with high

coulombic efficiency (>99.5%) at room temperature and elevated temperatures, even at high

practical areal capacity for the latter of 3.8 mAh/cm2 and with a capacity retention of 91% after

100 cycles. The ether-ionic liquid chemistry enables desirable plated Li microstructures with

high packing density, minimal ‘dead’ or inactive lithium formation and dendrite-free long-term

cycling. Along with XPS studies of cycled electrode surfaces, we use molecular dynamics

simulations to demonstrate that changes to the electrolyte interfacial chemistry upon addition

of DME plays a decisive role in the formation of a compact stable SEI.

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