Morphological Stability of Metal Anodes: Roles of Solid Electrolyte Interphases (SEIs) and Desolvation Kinetics

Abstract

Achieving stable lithium metal anodes requires control over the solid-electrolyte interphase (SEI) and desolvation kinetics. Here, we develop a unified theoretical framework integrating ion transport, desolvation, charge transfer, and SEI breakdown to predict morphological instabilities during electrodeposition. Using linear stability analysis, we identify six dimensionless parameters that govern the onset and evolution of instabilities. We show that SEI transport and desolvation rate effectively modulate apparent reaction kinetics, shifting the system toward a stable, reaction-limited regime. Extending the classical limiting current concept, we demonstrate that a thick, poorly conductive SEI and sluggish desolvation significantly reduce the limiting current. We introduce an apparent Damköhler number to quantify the critical balance: suppressing diffusion-limited instabilities by reaction rate reduction, while maintaining a high limiting current. Our theory enables predictive mapping of electrodeposition morphologies across diverse materials and operating conditions, guiding the rational design of stable lithium metal anodes.

Figures (4)

• Figure 1: Multiscale modeling of lithium metal anodes. (A) Schematic of the macroscale symmetric cell. (B) Microscale depiction of ion transport and kinetics, illustrating sequential steps of bulk transport, desolvation, SEI transport, and charge transfer at the lithium interface. (C) Physical picture of local SEI breakdown: high local current density leads to a reduction in SEI thickness ($L_{\text{SEI}}$), increased effective diffusivity $D_{\text{SEI}}$ and active lithium concentration $c_{\text{SEI}}$, which collectively reduce the SEI parameter $\delta$. (D) SEI breakdown rate and operational constraints. Applied current density ($j_{\text{app}}$) is bound by the limiting current density ($j_{\text{lim}}$). The SEI breakdown rate is quantified by the SEI breakdown parameter $\beta$.
• Figure 2: Effect of the SEI parameter $\delta$ and the desolvation rate (quantified by the dimensionless solvation exchange current density $j_{0,\text{solv}}/j_{\text{lim}}^c$) on (A) limiting current density $j_{\text{lim}}$; (B) apparent exchange current density $j_0^p$; and (C) apparent Damköhler number $\text{Da}^p$.
• Figure 3: Stability results. (A) Dispersion curves for two applied current densities. The critical wavenumber $\tilde{k}_c$ defines the stability boundary ($\sigma=0$) and the maximum growth rate $\sigma_{\text{max}}$ occurs at $k_{\text{max}}$. (B) Impact of the dimensionless applied current density $j_{\text{app}}/j_{\text{lim}}^c$ on the dimensionless critical wavenumber $k_cL$ for three values of the SEI parameter $\delta$. (C) Effect of the dimensionless desolvation rate ($j_{0,\text{solv}}/j_{\text{lim}}^c$) on the stability boundary $\tilde{k}_c$. Dashed lines in (B) and (C) indicate the approaching proximity between the applied current and the limiting current. (D) Effect of the SEI breakdown parameter ($\beta$) on the maximum growth rate for two applied current densities.
• Figure 4: Qualitative morphology instability map for lithium electrodeposition. This map is defined by the critical wavenumber $k_c$ and the maximum growth rate $\sigma_{\text{max}}$. Different regions correspond to distinct morphologies: Mossy dendrites (large $k_c$, large $\sigma_{\text{max}}$), whisker dendrites (small $k_c$, large $\sigma_{\text{max}}$), nodule-like dendrites (small $k_c$, small $\sigma_{\text{max}}$), and random, small-amplitude morphologies (large $k_c$, small $\sigma_{\text{max}}$). The region characterized by minimal $k_c$ and $\sigma_{\text{max}}$ is associated with a compact morphology. Arrows illustrate the trajectory and influence of the six dimensionless governing parameters on the morphology.