Concentrated Electrolyte for Stable Lithium Metal Anode

Abstract

Secondary lithium-ion batteries have been widely used as energy storage systems for portable electronics and electric vehicles. Instead of applying graphite as the anode, lithium (Li) metal has an ultrahigh theoretical capacity (3860 mAh/g) that could significantly increase battery’s energy density. Li dendrite formation is considered the main issue when using Li-metal anodes. Therefore, anode-free batteries are suggested as a solution to this obstacle. However, the major problem with anode-free batteries is their inferior cycling performance than reported lithium metal batteries, resulting from the formation of a fragile and fractured Solid-Electrolyte Interphase (SEI). To establish a robust SEI, novel lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) and LiNO 3 -based electrolytes with four different ratios were prepared in the co-solvent system (1,3-dioxolane and 1,2-dimethoxyethane). Compared to the other three electrolytes, concentrated electrolyte (4M LiTFSI + 2 wt.% LiNO 3 or 4+2) in Li||LiFePO 4 cells indicated the highest initial discharge capacity of 154 mAh/g with a capacity retention of 78% after 50 cycles. Scanning electron microscopy was conducted to investigate the SEI morphology after the initial charge/discharge process. The deposited Li on the Li metal was highly dense, and no spiky or needle-like Li clusters were detected. Additionally, electrochemical impedance spectroscopy of the cells was measured after 1, 3, 5, and 10 cycles, respectively. The cells with 4+2 showed the smallest SEI resistance after different cycles, corresponding to a stable and less-resistive SEI formation.