Understanding the Effect of Surface Energy on Dendrite Growth in Lithium Metal Batteries

Lithium metal batteries have shown promise in combating the limitations of lithium ion (Li-ion) batteries, including having a theoretical capacity that is an order of magnitude greater than that of the Li-ion battery. However, a main hinderance is the uncontrolled growth of Li dendrites on the anode during cycling. These dendrites lead to a decrease in coulombic efficiencies and can pierce the separator or break off forming “dead lithium” resulting in short circuit and thermal run away. While there has been extensive research into the growth and morphology of Li dendrites, little is known about the nucleation process of Li along the anode surface. The Li growth process is influenced by several factors including surface heterogeneities such as lattice defects and grain boundaries, the overpotential, local lithium ion (Li⁺) concentrations and varying surface energies along the anode. <br/> <br/>Due to the nature of the anode surface, it is difficult to observe the complex physics experimentally. Therefore, computational modeling is a powerful tool for understanding dendrite nucleation. It resolves the electrode-electrolyte interface where dendrite growth occurs and can track the complex chemical and physical phenomena contributing to dendrite nucleation and growth. Here a detailed model is presented that uses an extended, concentration dependent Butler-Volmer equation (eBV) with an additional term for nucleation to describe the rate of reaction at the electrode-electrolyte interface. By utilizing the eBV with the nucleation term, the rate at which Li is locally deposited on the anode surface can be carefully tracked along with other important phenomena, i.e. current density, overpotential, and Li⁺ concentrations. In addition, the nucleation term allows tracking of the growth rate of Li nucleates along the surface when influenced by changes in surface energy. <br/> <br/>The model is used to study how defects in the anode surface effect dendrite growth. These defects include lattice defects such as substitutional impurities, interstitial impurities, and vacancies in the Li-metal crystal lattice. In addition to surface defects, grain boundaries can also lead to a change in surface energy. Results show that when defects that decrease the surface energy are present, we see a more homogenous growth, while defects that increase the surface energy led to more growth. Thus, understanding the effects of nucleation locations and rates of dendrite growth could lead to the design of engineered surfaces to extend Li metal battery lifetime and performance.