Revisiting the Criterion for Mechanical Supression of Dendrites at the Li/Electrolyte Interface

Abstract

Large-scale adoption of electric vehicles has been challenging due to the high energy density required to achieve a reasonable driving range. One route to increasing the energy density has been to develop “beyond Li-ion” chemistries1 like Li-air and Li-S. Another route is to use Li metal anode coupled with a traditional Li-ion cathode since lithium metal electrode possesses a very high theoretical specific capacity (3860 mAh g-1) and low reduction potential.2 Both routes require a Li metal anode which can reversibly cycle without forming dendrites. Mechanical suppression of dendrite growth through solid or polymer electrolytes has shown potential for alleviating the problem. It has been suggested by treating the electrode and electrolyte as isotropic materials that a solid polymer electrolyte that has roughly a shear modulus twice that of lithium can suppress dendrite growth.3 One potential issue relates to the assumption of isotropic materials as Li metal is highly anisotropic with a drastically different shear and elastic moduli depending on the crystal orientation. This further dictates that the nature of dendrite growth will be affected by the crystal orientation of Li metal in contact with the electrolyte. In this work, we aim to fill the gaps in our current understanding of the mechanical suppression of dendrite growth at electrode-electrolyte interfaces by explicitly accounting for the anisotropic effects. We impose a periodic deformation at the interface and solve two-dimensional linear elasticity equations to get the resulting periodic deformation at the anisotropic electrode-electrolyte interface. We use the full elastic tensor of Li metal anode and solid electrolyte calculated from first-principles density functional theory calculations for the constitutive stress-strain relationship of each material. The elastic tensor was calculated based on the method of homogeneous deformations of the unit cell followed by fitting the energies of the strained cell to energy-strain relationship. Using the calculated deformation, we obtain the stresses at the interface and study their role in the amplification of surface roughness using a kinetic model.4 Our anisotropic model enables us to study the effect of different crystal orientations of Li metal anode and solid electrolyte on the dendrite growth.