Effect of LiFSI Concentrations To Form Thickness- and Modulus-Controlled SEI Layers on Lithium Metal Anodes

Improving the cyclic stability of lithium metal anodes is of particular importance for developing high-energy-density batteries. In this work, a remarkable finding shows that the control of lithium bis­(fluorosulfonyl)­imide (LiFSI) concentrations in electrolytes significantly alters the thickness and modulus of the related SEI layers, leading to varied cycling performances of Li metal anodes. In an electrolyte containing 2 M LiFSI, an SEI layer of ∼70 nm that is obviously thicker than those obtained in other concentrations is observed through <i>in situ</i> atomic force microscopy (AFM). In addition to the decomposition of FSI^– anions that generates rigid lithium fluoride (LiF) as an SEI component, the modulus of this thick SEI layer with a high LiF content could be significantly strengthened to 10.7 GPa. Such a huge variation in SEI modulus, much higher than the threshold value of Li dendrite penetration, provides excellent performances of Li metal anodes with Coulombic efficiency higher than 99%. Our approach demonstrates that the FSI^– anions with appropriate concentration can significantly alter the SEI quality, establishing a meaningful guideline for designing electrolyte formulation for stable lithium metal batteries