Toward Realistic Full Cells with Protected Lithium‐Metal‐Anodes: the Effect of an Adaptive Self‐healing Artificial SEI

Artificial SEI meets LIM: The successful formation of a LiBFEP-based artificial solid electrolyte interphase (SEI) on thin Li-metal anodes is confirmed using various characterization methods. Extended electrochemical cycling tests in full cells show the positive effect of the artificial SEI resulting in a tripling of the cell's lifetime. Scanning electron microscopy measurements reveal the structural stabilization of the artificial SEI during and after the cycling tests.

Abstract

The application of an artificial LiBFEP-SEI on thin 48 µm Lithium-Metal-Anodes (LiBFEP = Li[O₂P(OCH₂CF₃)₂]; SEI = solid electrolyte interphase) induces improved plating/stripping of lithium at the interphase with an almost doubling of the lifetime for Li–Li symmetrical cells. Four lithium-fluoroalkylphosphates other than LiBFEP are tested, but all perform inferior to the control. Lithium-metal full cells with various NMC-cathodes and LP57 standard electrolyte are employed to confirm the superior performance of the LiBFEP artificial SEI (LP57 = 1 m Li[PF₆] in ethylene carbonate/ethyl methyl carbonate 3:7 wt%). A tripling of the cell lifetime is observed using realistic cell-cycle parameters. The stabilization of the modified anodes is evidenced by SEM-EDX, ToF-SIMS, PEIS measurements and led to an improved anode structure. The performance of the artificial SEI is further improved with the introduction of a novel low-concentration and low-polarity electrolyte (0.2 m lithium perfluoroalkoxyaluminate (Li[Al(OR^F)₄]; R^F = C(CF₃)₃) in 1,2-difluorobenzene). Cycling is sustained over 350 cycles with realistic conditions at a current density of 1 mA cm⁻² and 48 µm thin Lithium-Metal-Anodes. This work provides a comprehensive understanding of how the artificial LiBFEP-SEI influences the performance of Lithium-Metal-Battery full cells, confirming the simplicity/effectiveness of the immersion process for the LiBFEP-coating.