Nonflammable, localized high-concentration electrolyte towards a high-safety lithium metal battery

The use of high-concentration electrolytes (HCEs) is a promising way to stabilize the lithium metal anode, but their high viscosity and poor wettability impede good Li-ion diffusion kinetics. More importantly, most electrolytes are still highly flammable that pose safety risks to lithium metal batteries. Introducing diluent in HCEs provides more possibilities for multifunctional electrolyte design. Here we report an ether-based, nonflammable localized high-concentration electrolyte (LHCE), with fire-retardant fluorinated phosphazene as a multifunctional diluent, that provides much better safety and dendrite suppression ability than normal electrolytes and HCEs. The diluent makes the salt anions to dominate the solvation sheath, which weaken the interaction between Li ions and the solvated molecules, lowering the desolvation energy for Li plating and guiding the formation of an inorganic-rich solid electrolyte interphase. As a result, lithium plating/stripping on a Cu foil with such a LHCE has a high average Coulombic efficiency of 97.8% at a high current density and capacity (5 mA/cm2 and 5 mAh/cm2). The assembled full cell has a long cycling stability, even with an ultralow capacity ratio of the negative/positive electrodes of 1.2. This work presents a new strategy for producing practical lithium metal batteries with high safety and excellent stability.