The Role of Protective Layers in Preventing Dendrite Growth in Lithium Metal Anodes

Abstract

Lithium metal is considered to be the “Holy Grail” of anode materials used in lithium ion batteries due to its extremely high theoretical capacity (3860 mAh/g), low density (0.59g/cm3), and the most negative electrochemical potential (-3.04V) with respect to standard hydrogen electrode (SHE)[1]. However, extensive research has indicated that dendritic growth and severe capacity fade, due to side reactions, during charge and discharge are the two major problems that prevent wide usage of Li-metal as anode in lithium-based batteries[2]. Usage of excessive amount of lithium in the cathode can potentially compensate for the problem of capacity fade without sacrificing energy density. However, the problem of lithium dendrite growth and subsequent short-circuit over multiple cycle remains as major concern[3]. During cell fabrication or the first cycle, the Li metal in contact with organic solvent and electrolyte forms a passivating film, which is also known as the solid electrolyte interphase (SEI) layer (see Figure 1). Experimental studies indicate that presence of stiff inorganic compounds (LiF, Li2CO3) within the solid electrolyte interface layer can potentially retard the growth of dendrites for a few cycles (approximately tens of cycles)[4]. On the contrary, all the existing theories proposed to understand the growth of dendrite tend to ignore the effect of the SEI. Similarly, stiff artificial SEI’s are also envisioned as a means of protecting the Li metal from dendrite formation[5]. Clearly, thin protective layers, either formed in situ, or fabricated on the surface play a critical role in understanding dendrite formation. The purpose of the present study is to develop a mathematical model, building on the work of Monroe and Newman[6, 7], to understand the effect of the thin protective layer on the propensity to form dendrites in the cell. There exist at least four different parameters that can significantly impact the stress and fracture of the SEI layer: i) Width of the lithium nucleus (W), ii) Height of the dendritic protrusion (H), iii) Thickness of the protection layer (tSEI), and iv) Elastic modulus of the protection layer (shear modulus has been investigated here, GSEI). Figure 2 show the propensity for dendrite formation described as the difference in the electrochemical potential (induced by mechanical deformation) between the peak and the valley (Δμeff = Δμe,peak – Δμe,valley) vs. GSEI for different tSEI values. The figure provides understanding on how thin stiff layers may play a part in dendrite prevention. In this talk, we will explore the impact of thin layers on lithium metal and provide an understanding on how the mechanical and electrochemical properties can impact dendrite formation.