Role of NaF in Attenuating Interfacial Instability of Lithium Metal Anode: A Strategy to Modulate SEI for Enabling...

The interfacial instability of naturally fragile solid electrolyte interphase (SEI) on lithium leads to dendritic deposition, inferior cycling stability and substantially low coulombic efficiency of metal batteries. The performance of metal anodes can be improved by creating a robust passivation layer/artificial SEI. Along the same line, the modification of Li anode using sodium fluoride (NaF) is proposed to create a protective layer that mitigates dendrite growth and electrolyte decomposition. The proposed modification substantially improves the cycle life with >500 cycles in carbonate solvents at 1 mA cm^-2 and areal capacity 0.5 mAh cm^-2 in Li||Li symmetrical cells. The post-mortem analyses of cycled lithium reveal a smooth surface with no visible dendrites. In contrast, the unmodified lithium is covered with mossy metal deposits. The efficacy of NaF modification in improving the striping/plating behavior of lithium is also demonstrated using Aurbach's method. The improved performance is attributed to the formation of a stable SEI facilitated by the NaF layer, promoting preferred Li⁺ mass-transfer and mitigated electrolyte degradation as suggested by the post-mortem characterization and computational studies. The substantial improvement in the long-term performance of lithium cobalt oxide in the presence of NaF-modified lithium further underscores the practical utility of this relatively simple SEI modulation strategy.