Dual-anion ionic liquid electrolyte enables stable Ni-rich cathodes in lithium-metal batteries

High-energy-density lithium-metal batteries face the challenge of developing functional electrolytes enabling both the stabilization of the lithium-metal negative electrode and high-voltage positive electrodes (> 4 V versus Li+/Li). Herein, a low-volatility and non-flammable ionic liquid electrolyte (ILE) incorporating two anions, bis(fluorosulfonyl) imide (FSI) and bis(trifluoromethanesulfonyl)imide (TFSI), is successfully applied to overcome this challenge, employing the high-energy, low-Co, and Ni-rich positive-electrode material, LiNi0.88Co0.09Mn0.03O2 (NCM88), in Li-metal batteries. With this specific electrolyte, the cathode exhibits remarkable electrochemical performance, achieving an initial specific capacity of 214 mAh g−1 and outstanding capacity retention of 88% over 1,000 cycles. More importantly, this electrolyte enables an average Coulombic efficiency of 99.94%. The excellent compatibility of the dual-anion ILE with both the lithium metal (50 μm) and the high-voltage positive-electrode material enables the realization of Li-metal cells achieving specific energies of more than 560 Wh kg−1 based on their combined active material masses.