Dynamic Surface Stress Response during Reversible Mg Electrodeposition and Stripping

Abstract

Through the use of in situ electrochemical surface stress measurements, Mg deposition and stripping processes in electrolytes for Mg batteries are studied. We examine four electrolytes: PhMgCl+AlCl 3 /THF, (DTBP)MgCl-MgCl 2 /THF, MgCl 2 +AlCl 3 /THF, and Mg(BH 4 ) 2 +LiBH 4 /diglyme. Each of these electrolytes exhibits common surface stress response features, indicating that the mechanisms of Mg deposition and stripping are similar among the different electrolytes. Combining the measurements with density functional theory calculations, each part of the stress-potential curve is assigned to steps in the deposition and stripping reactions. The analysis suggests the following mechanism: (1) Mg 2+ /anion/solvent complexes adsorb on the substrate prior to the deposition; (2) Mg deposits as random nuclei and the deposition continues without a recrystallization process; (3) during the initial stage of Mg stripping, less coordinated Mg(0) is converted to soluble Mg(II) species and to partially oxidized species, MgO x ; (4) as the anodic reactions proceed further, Mg continues to dissolve and MgO x is removed via chemical processes; (5) due to the strong interaction between Mg and the noble metal substrate atoms, the Mg layer directly bound to the substrate are the last to be anodically converted (and desorb There is much interest in developing rechargeable Mg batteries due to the high theoretical volumetric capacity, abundance, and benign nature of Mg. Finding a suitable electrolyte for reversible Mg deposition and dissolution, however, is challenging due to the difficulty in producing soluble Mg 2+ and the formation of passivation films on the electrode surface.1-3 The first reversible Mg deposition and stripping was performed in Grignard solutions, 4-6 which suffer from low anodic stability and poor ionic conductivity. The anodic stability and the Coulombic efficiency is greatly enhanced in electrolytes based on Mg organohaloaluminate, prepared via an acid-base reaction between a MgR 2 Lewis base and an AlCl 3-n R n Lewis acid. 7 An inorganic magnesium aluminum chloride complex from MgCl 2 -and AlCl 3 -based electrolyte exhibits even higher anodic stability and a lower overpotential. 8,9 However, the corrosivity of chloride and the reactivity of Lewis acids have prompted the development of newer, less corrosive electrolytes, including Mg(BH 4 ) 2 based inorganic salts with LiBH 4 additive electrolytes 10,11 and all-magnesium phenolatebased electrolytes.12 Thus, previous work illustrates that several Mg systems exhibit promise as a battery electrolyte; however, their interfacial chemistries are complicated and need to be better understood. [8][9][10][11][12][13] Efficient electrodeposition and stripping reactions are essentials in rechargeable batteries. One of the effective in operando techniques for studying such processes is monitoring the electrochemical surface stresses developed during the deposition and dissolution of metals.14 In situ surface stress measurements are experimentally less demanding compared to oth...

Effect of Concentration on the Electrochemistry and Speciation of the Magnesium Aluminum Chloride Complex Electrolyte Solution

“…The reason for a change in peak shape can be understood from previous work studying the oxidative processes during stripping. Our group has shown with in situ electrochemical stress experiments coupled with density functional theory (DFT) calculations that the Mg stripping curve can be broken down into a few sequential oxidation processes . As oxidation begins on the positive CV sweep, the majority of the anodic processes involve oxidation of Mg(101̅0) planes to Mg 2+ or MgO x , followed by oxidation of Mg(0001) planes and removal of MgO x as evidenced by two distinct regions in the stress curve .…”

Exploring the Liquid Structure and Ion Formation in Magnesium Borohydride Electrolyte Using Density Functional Theory

“…There are some interesting research directions that can be taken in future studies. In addition to considering polyethylene oxide as discussed above, the use of LiBH 4 as a coelectrolyte to Mg(BH 4 ) 2 24,25,55 could be an interesting topic. In this work, we have approximated the liquid using a continuum solvation method using the static and dynamic dielectric constants of each solvent in its pure form without any dissolved ions.…”

“…This marked the discovery of the first stable, halide-free electrolyte for magnesium batteries. Since then, there have been a number of experimental 13,[20][21][22][23][24][25] and theoretical 12,23,26 studies aimed at improving battery performance. Presently, there are some problems with this electrolyte, including low ionic conductivity and solubility.…”

Elucidating Zn and Mg Electrodeposition Mechanisms in Nonaqueous Electrolytes for Next-Generation Metal Batteries

“…We previously suggested that Mg deposition from a series of electrolytes containing different salts proceeded according to a similar mechanism. 19 In certain cases, the presence of a Mg dimer [Mg 2 (μ-Cl) 3 •6THF] + was correlated with the advent of efficient Mg electrodeposition. 5,8,17,20,21 The monomeric complexes, (MgCl 2 •4THF) and (MgCl•5THF) + resulting from the original dimer, were suggested to be the intermediate species present at the interface.…”