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<sup>–</sup> 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<sup>–</sup> anions with
appropriate concentration can significantly alter the SEI quality,
establishing a meaningful guideline for designing electrolyte formulation
for stable lithium metal batteries
