Effects of Concentrated Salt and Resting Protocol on Solid Electrolyte Interface Formation for Improved Cycle Stability of Anode-Free Lithium Metal Batteries

The combined effect of concentrated electrolyte and cycling protocol on the cyclic performance of the anode-free battery (AFB) is evaluated systematically. In situ deposition of Li in the AFB configuration in the presence of a concentrated electrolyte containing fluorine-donating salt and resting the deposit enables the formation of stable and uniform SEI. The SEI intercepts the undesirable side reaction between the deposit and solvent in the electrolyte and reduces electrolyte and Li consumption during cycling. The synergy between the laboratory-prepared concentrated 3 M LiFSI in the ester-based electrolyte and our resting protocol significantly enhanced cyclic performances of AFBs in comparison to the commercial carbonate-based dilute electrolyte, 1 M LiPF₆. Benefitting from the combined effect, Cu∥LiFePO₄ cells delivered excellent cyclic performance at 0.5 mA/cm² with an average CE of up to 98.78%, retaining a reasonable discharge capacity after 100 cycles. Furthermore, the AFB can also be cycled at a high rate up to 1.0 mA/cm² with a high average CE and retaining the encouraging discharge capacity after 100 cycles. The fast cycling and stable performance of these cells are attributed to the formation of robust, flexible, and tough F-rich conductive SEI on the surface of the in situ-deposited Li by benefiting from the combined effect of the resting protocol and the concentrated electrolyte. A condescending understanding of the mechanism of SEI formation and material choice could facilitate the development of AFBs as future advanced energy storage devices.