Importance of Homogeneity in the Solid Electrolyte Interphase for Dendrite-Free Epitaxial Plating of Lithium

Abstract

Realization of practical rechargeable Li-metal batteries is severely hindered by uncontrolled growth of Li that leads to formation of either dendritic or mossy Li during repeated cycles. These three-dimensional (3D) growth modes of Li induce internal short-circuits, cause loss of Coulombic efficiency and limit the lifetime of the cell. Therefore, a two-dimensional (2D) layer-by-layer deposition of Li is highly desirable for safe and efficient long-term cycling of Li-metal batteries. Existing knowledge on Li dendrite formation suggests that uncontrolled reaction of lithium metal with nonaqueous liquid electrolytes forms an inhomogeneous and mechanically weak solid electrolyte interphase (SEI) layer that results in localized Li-deposition and leads to dendrite-growth on top of the electrode surface. Therefore, in-depth knowledge about structural as well as compositional features of the SEI are vital to suppress dendritic growth and also to realize 2D deposition of Li. However, usually the structure of an SEI is quite complex and heterogeneous. Moreover, the stability of Li-deposition varies with properties of the electrolyte. Since multiple destabilizing factors work together, it is a significant experimental challenge to understand the effects of physical and chemical features of the SEI on the nucleation-and-growth of dendrites. Although considerable improvements have been achieved1, 2 to suppress dendritic growth, microscopically smooth dendrite-free two-dimensional deposition of Li is rare3 and there is no proper know-how to its realization. Herein, we comprehensively examine the role of electrolytes as well as physical and chemical properties of the SEI for layer by layer epitaxial plating of Li.Stripping and plating of Li metal have been investigated in a Li│Separator│Li symmetric-cell configuration using different electrolytes, such as LiTFSI, LiTFSI + LiBr, LiNO3, LiNO3 + LiBr etc. in glyme ether solvents. Morphological analysis has shown that in all the investigated electrolytes except LiNO3 + LiBr/tetraglyme, 3D/dendritic/mossy deposition of Li has been observed. In contrast, specifically in 1 M LiNO3 + 0.05 M LiBr/tetraglyme electrolyte the plating of Li metal has exhibited dendrite-free epitaxial growth up to a thickness of ~ 30 μm which has been confirmed by the scanning electron microscopic (SEM) images and electron backscatter diffraction (EBSD) signal as shown in the Figure. Further mechanistic analyses have revealed that the epitaxial growth of Li metal is benefited from the synergistic effect of the anions Br⁻ and NO3⁻ forming a thin and homogeneous Li2O–rich solid electrolyte interphase (SEI) layer which has been formed during the very first discharge (stripping) process, where the corrosive nature of Br⁻ has removed the original thick passivation layer on pristine Li surface which is then again oxidized (passivated) by the NO3⁻ to prevent further reactions with the electrolyte. As a result, the SEI layer has remained thin due to the electropolishing effect and got ready for the epitaxial electroplating of Li in the following recharge process. Our analyses clearly show that dendrite-free stable Li-deposition is a result of favorable combinations of several competitive factors, such as morphology and chemical composition of the SEI layer as well as conductivity, transference number and the type of counter anion in the electrolyte. Besides, more importantly, we highlight that, in addition to other favorable conditions, spatial homogeneity in the SEI layer in terms of surface roughness, conductivity and chemical composition is an essential requirement for 2D growth of Li.