Suppressing the effect of lithium dendritic growth by the addition of magnesium bis(trifluoromethanesulfonyl)amide

Practical applications of Li-S and Li-air batteries require the morphology of the Li metal negative electrode during charge/discharge (i.e., Li-deposition/dissolution) cycling to be precisely controlled. Herein, we used magnesium bis(trifluoromethanesulfonyl)amide [Mg(TFSA)2] as an electrolyte additive to suppress the growth of Li dendrites, utilizing the occurrence of an alloying reaction between the initially substrate-deposited Mg and the subsequently deposited Li. Notably, no metallic Mg formation and no change in Li deposition morphology were observed at an electrolyte composition of 0.1 M Mg(TFSA)2 + 0.9 M LiTFSA/triglyme, irrespective of the applied potential. In contrast, increasing the Mg salt concentration to 0.5 M resulted in the deposition of interconnected granules, reflecting a dramatic morphology improvement. X-ray diffraction analysis revealed the occurrence of the abovementioned alloying, which finally afforded a deposit composition of Li0.9Mg0.1via the formation of an intermediate Li0.14Mg0.86 phase. Importantly, the deposits obtained under various applied potentials were relatively smooth, with no needle-like morphology observed.