Improving Cycle Life through Fast Formation Using a Superconcentrated Phosphonium Based Ionic Liquid Electrolyte for Anode-Free and Lithium Metal Batteries

Cell formation of lithium-ion cells impacts the evolution of the solid electrolyte interphase (SEI) and the cell cycle stability. Lithium metal anodes are an important step in the development of high energy density batteries owing to the high theoretical specific capacity of lithium metal. However, most lithium metal battery research has used a conventional lithium-ion formation protocol; this is time-consuming and costly and does not account for the different properties of the lithium metal electrode. Here, we have used a recently reported promising phosphonium bis­(fluorosulfonyl)­imide ionic liquid electrolyte coupled with an NMC622 high areal capacity cathode (>3.5 mAh/cm²) to investigate the effect of cell formation rates. A faster formation protocol comprised of a pulsed 1.25C current decreased the formation time by 56% and gave a 38% greater capacity retention after 50 cycles when compared to formation at C/20. Electrochemical impedance spectroscopy measurements showed that the fast formation gave rise to a lower resistance SEI. Column-like lithium deposits with reduced porous lithium domains between the particles were observed using scanning electron microscope imaging. To underline the excellent performance of these high energy density cells, a 56% greater stack specific energy was achieved compared to the analogous graphite-based lithium-ion cell chemistries.