Crystallographic orientation effects on growth dynamics of Li dendrites

Abstract

Lithium metal anodes are the premier technology for increasing lithium ion battery capacity and power output. However, dendrite formation after repeated charge/discharge cycling reduces longevity. Growth dynamics of lithium dendrites in electrochemical systems and their dependence on crystallographic orientation of the electrode are not well understood. Understanding the effect of texture is difficult to do in the presence of grain boundaries, as they are known to enhance diffusivity, promoting dendrite growth during charging and anodic collapse during discharging. On the other hand, single crystals of lithium are difficult to produce and use as an electrode material due to their high reactivity and homologous temperature. While an experimental approach suffers from the inhibitors described above, computational methods, such as molecular dynamics, allow investigation on the effect of texture. As such, the effects of crystallographic orientation of single crystals on the initial growth dynamics of lithium dendrites are studied using molecular dynamics (MD). Substrates of different orientation are simulated and lithium atoms are deposited based on current density and surface area. As an example, an applied current density of 2 mA/cm2 is modelled as 3396 lithium atoms deposited over 1 ns for a surface area of 27.2 nm2, and as 7842 lithium atoms deposited over 1 ns for a surface area of 62.82 nm2. Results are shown to not be artefacts of finite size effects or thermal fluctuations, but are attributed to dendrite nucleation kinetics. Crystallographic orientation is tied to surface energy to explain the results. Orientations with high surface energy, such as planes in the {111} family for lithium, tend to be stable against changes in current density, showing little change in surface roughness after a 0.5 ns annealing period as applied current density increased from 0.5 to 5.0 mA/cm2. Orientations with low surface energy, the {100} family of planes, tend to have low propensity for dendrite formation, typically having the lowest absolute surface roughness when compared to other orientations. As current density increases to 5 mA/cm2, substrate surface orientation is shown to have little effect on the nucleation of dendrites. Overall, the trends found through simulation match nucleation theory and could be used as a basis for understanding the initial growth dynamics of dendrites. Based on the results, lithium metal anodes with the {100} family of planes exposed on the surface would be most suitable for applications without a wide range in applied current density. If the applied current density is being swept across a wide range, then orientations with a high surface energy, such as the {111} family are preferred. Films with high surface energy could also be applied to the surface of the lithium metal anode to suppress dendrite growth. By doing so, the longevity of a lithium metal battery could be extended to the point of commercialization, enabling electrified aviation beyond two-seater planes.