Li dendrite growth is a critical safety issue for batteries with lithium metal anodes. Mg metal electrodes offer a promising way to overcome this safety limitation. Yao et.al. has observed that Li and Mg plating show different morphology, where Li plating is more dendritic but Mg plating is more faceted [1]. The difference of the morphology evolution of the plated Li and Mg is related to the interfacial reactions kinetics at the electrode and electrolyte interface. One major difference is that an electrical insulating but ionic conducting solid electrolyte interphase (SEI) always covers Li anode, but Mg electrode surface is believed to be SEI free[2]. Here, atomistic simulations are combined with phase-field model to evaluate the effect of interfacial reaction kinetics on the plated Li/Mg structural morphology.

Reactivity of Li with liquid electrolytes, typically ethylene carbonate (EC), forms SEI layer on the Li metal. Magnesium aluminum chloride complex in tetrahydrofuran (THF) is one of the few electrolytes showing good reversible Mg deposition/stripping with a relatively large anodic stability (~3.1 V)[3]. Thus we considered Li/SEI/EC-electrolyte and Mg/THF-electrolyte interface models. Using both static first-principles calculations and ab initio molecular dynamics, the interfacial structures, de-solvation processes and Li⁺/Mg²⁺ ion energy landscapes at Li/SEI/EC-electrolyte and Mg/THF-electrolyte, and the effects of electric field on them were investigated. The preliminary results showed that the Li and Mg metal are all negatively charged at zero voltage, but the negative charge density of Li metal is more than that of Mg metal, which is due to SEI can block the electrons into electrolyte from anode. While the Mg plating free energy change for the electrode reaction is higher than Li due to its larger solvation energy (2.5 eV for Mg vs 1.2 eV for Li from thermodynamic cycle).

Coupling with atomistic calculations, phase-field simulations were performed to investigate the Li/Mg plating morphological evolution from mesoscale. With thermodynamic and kinetic parametric inputs from first-principles calculation (i.e. interfacial energy and anisotropy of Li/SEI (Li₂CO₃), surface energy and anisotropy of Mg, diffusion coefficient of Li⁺ in SEI (Li₂CO₃), and relative difference in charge-transfer energy barrier of Li/Mg plating), our non-linear phase-field model captures the notable morphological evolution difference between dendritic Li plating and faceted Mg plating. Furthermore, the simulation results indicate that, among all possible parameters for Li/Mg electrodeposition processes, the plating charge-transfer energy barrier difference is the major contributing factor that accounts for the morphological discrepancy. Specifically, in our systematical tests on the impact of charge-transfer energy barrier on plating morphology (a range of 0.5-10 times of the Li plating reaction energy barrier height were tested), dendrites were prominently suppressed from around 2 times and faceted morphology (hexagonal for Mg) were produced at about 4 times. This study result fundamentally explains the origin of the Li/Mg plating morphological difference, and reveals that the proper elevation of the charge-transfer barrier height (e.g. by anodic surface treatment, protective coating, upgraded electrolyte, etc.) is a promising approach to dendrite suppression in Li metal batteries.

References

[1] H.D. Yoo, Y. Liang, Y. Li, Y. Yao, ACS Appl. Mater. Interfaces 7 (2015) 7001.

[2] K. Xu, Chem. Rev. 114 (2014) 11503.

[3] R.E. Doe, R. Han, J. Hwang, A.J. Gmitter, I. Shterenberg, H.D. Yoo, N. Pour, D. Aurbach, Chem. Commun. 50 (2014) 243.