Dendrite-Free Epitaxial Growth of Lithium for Efficient Charging of Li–O2 Batteries with Improved Cycle Life

Abstract

Realization of rechargeable Li-metal batteries with high energy density is hindered by uneven Li deposition leading to dendrite formation during repeated cycles. Uncontrolled dendrite growth induces internal short-circuits which limit the cell lifetime and generate ohmic heat leading to catastrophic failure when a volatile electrolyte is used. Several strategies have been conceived by the researchers to suppress dendritic growth and delay the premature death of the cell. These efforts include using electrolyte additives to form a porous ion-conducting solid electrolyte interphase (SEI) layer on metal preventing further reaction,1 implementing solid electrolytes with optimum mechanical properties,2 designing a Li matrix,3 reducing local space charge on the Li surface,4 and so on. Although considerable improvements have been achieved from these earlier studies, microscopically smooth Li surface covered with thin and uniform SEI layers is yet to be realized. Alternatively, here we show an unprecedented dendrite-free epitaxial electrodeposition of Li in the presence of LiBr–LiNO3/glyme ether electrolyte under O2 atmosphere. Stripping and plating of Li metal was investigated by using a symmetrical Li│Li cell and it has been found that in 1 M LiNO3+0.05 M LiBr/tetraglyme electrolyte the plating of Li metal shows dendrite-free epitaxial growth up to a thickness of >20 μm which is clearly observed by the scanning electron microscopic (SEM) images and electron backscatter diffraction (EBSD) signal in Figure 1. The epitaxial growth of Li metal seems to be benefited from the synergistic effect of the anions Br− and NO3 − forming a thin and homogeneous Li2O–rich solid electrolyte interphase (SEI) layer which is formed during the very first discharge (stripping) process, where the corrosive nature of Br− removes the original thick passivation layer on Li surface which is then again oxidized (passivated) by the NO3 − to prevent further reactions with the electrolyte. Because of this, the SEI layer remains thin and facilitates the electropolishing effect and gets ready for the epitaxial electroplating of Li in the following charge process. Similar epitaxial growths of Li have been observed for both symmetrical Li│Li cell and Li–O2 full cell resulting in a much improved performance with longer cycle life.5,6 However, in contrast, LiNO3 alone or other lithium salts, such as Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), Lithium trifluoromethanesulfonate (LiTf), do not exhibit any epitaxial Li deposition. This demonstrates the bi-functional activity of the dual anion electrolyte LiNO3–LiBr towards dendrite-free epitaxial growth of Li. The role of deposition rate, effect of electrolyte solvent and a detailed mechanism of epitaxial growth will be discussed in the presentation.