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

Abstract

Cyclic voltammetry and linear sweep voltammetry with an ultramicroelectrode (UME) were employed to study Zn and Mg electrodeposition and the corresponding mechanistic pathways. CVs obtained at a Pt UME for Zn electroreduction from a trifluoromethylsulfonyl imide (TFSI − ) and chloridecontaining electrolyte in acetonitrile exhibit current densities that are scan rate independent, as expected for a simple electron transfer at a UME. However, CVs obtained from three different Mg-containing electrolytes in THF exhibit an inverse dependence between scan rate and current density. COMSOL-based simulation suggests that Zn electrodeposition proceeds via a simple one-step, two-electron transfer (E) mechanism. Alternatively, the Mg results are best described by invoking a chemical step prior to electron transfer: a chemical−electrochemical (CE) mechanism. The chemical step exhibits an activation energy of 51 kJ/mol. This chemical step is likely the disproportionation of the chloro-bridged dimer [Mg 2 (μ−Cl) 3 •6THF] + present in active electrodeposition solutions. Our work shows that Mg deposition kinetics can be improved by way of increased temperature.

The Influence of Interfacial Chemistry on Magnesium Electrodeposition in Non-nucleophilic Electrolytes Using Sulfone-Ether Mixtures

“…The local maximum is likely representative of the initial magnesium nucleation. Because the electron transfer reaction is hypothesized to be preceded by a chemical equilibrium step (Ta et al, 2018), it is likely that the chemical equilibrium step causes the initial decrease in current for each electrolyte. The Cottrell equation could not be fit to either electrolyte; therefore, the Scharifker and Hill models for instantaneous and progressive nucleation could not be directly applied to this system (Scharifker and Hills, 1983).…”

Controlling electrochemical growth of metallic zinc electrodes: Toward affordable rechargeable energy storage systems

“…This finding is consistent with a few separate studies that report on Zn full batteries using an AN-based electrolyte (116)(117)(118). It could also be attributable to the one-step, two-electron transfer mechanism of Zn as predicted by simulation (114), which results in a large reaction constant five orders of magnitudes larger than that of Mg electrodeposition from tetrahydrofuran that includes two steps. Together, the kinetics in the bulk (i.e., diffusivity) and at the interface (i.e., reaction constant) single out AN-based systems as a group of promising electrolytes for Zn batteries.…”

“…As illustrated in Fig. 4F, computation work suggests that Zn electrodeposition can proceed via a one-step, two-electron transfer mechanism in organic electrolyte, e.g., Zn(TFSI) 2 in acetonitrile (AN), which is comparatively faster than the kinetics of Mg, another HCP metal (114). Han et al (115) reported on a comprehensive experimental study of the electrochemical properties of nonaqueous Zn electrolytes including the combinations between one of the salts: Zn(TFSI) 2 , Zn(CF 3 SO 3 ) 2 , Zn(BF 4 ) 2 , Zn(PF 6 ) 2 , and one of the organic solvents: diglyme (G2), AN, propylene carbonate (PC), and N,N′-dimethylformamide (DMF).…”

Direct Nano‐Synthesis Methods Notably Benefit Mg‐Battery Cathode Performance

“…Despite these desirable qualities, there are major challenges hindering the development of Mg batteries due to kinetic limitations associated with the transport and electrodeposition of the divalent cation. [ 13,14 ] While much progress has been made to increase the anodic stability and improve the efficiency of advanced Mg electrolytes, [ 15–17 ] the paucity of known cathode materials capable of reversible Mg 2+ (de)insertion continues to plague this technology. High voltage oxides generally suffer from sluggish Mg 2+ diffusion due to strong ionic interactions between the migrating cation and the highly polarizing lattice.…”