The electrochemistry of Mg salts in room-temperature ionic liquids (ILs) was studied using plating/stripping voltammetry to assess the viability of IL solvents for applications in secondary Mg batteries. Borohydride (BH4(-)), trifluoromethanesulfonate (TfO(-)), and bis(trifluoromethanesulfonyl)imide (Tf2N(-)) salts of Mg were investigated. Three ILs were considered: l-n-butyl-3-methylimidazolium (BMIM)-Tf2N, N-methyl-N-propylpiperidinium (PP13)-Tf2N, and N,N-diethyl-N-methyl(2-methoxyethyl)ammonium (DEME(+)) tetrafluoroborate (BF4(-)). Salts and ILs were combined to produce binary solutions in which the anions were structurally similar or identical, if possible. Contrary to some prior reports, no salt/IL combination appeared to facilitate reversible Mg plating. In solutions containing BMIM(+), oxidative activity near 0.8 V vs Mg/Mg(2+) is likely associated with the BMIM cation, rather than Mg stripping. The absence of voltammetric signatures of Mg plating from ILs with Tf2N(-) and BF4(-) suggests that strong Mg/anion Coulombic attraction inhibits electrodeposition. Cosolvent additions to Mg(Tf2N)2/PP13-Tf2N were explored but did not result in enhanced plating/stripping activity. The results highlight the need for IL solvents or cosolvent systems that promote Mg(2+) dissociation.

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ABSTRACT:A rechargeable battery based on a multi-valent Mg/O2 couple is an attractive chemistry due to its high theoretical energy density and potential for low cost. Nevertheless, metal-air batteries based on alkaline earth anodes have received limited attention and generally exhibit modest performance. In addition, many fundamental aspects of this system remain poorly understood, such as the reaction mechanisms associated with discharge and charging. The present study aims to close this knowledge gap and thereby accelerate the development of Mg/O2 batteries by employing first-principles calculations to characterize electrochemical processes on the surfaces of likely discharge products, MgO and MgO2. Thermodynamic limiting potentials for charge and discharge are calculated for several scenarios, including variations in surface stoichiometry and the presence/absence of intermediate species in the reaction pathway. The calculations indicate that pathways involving oxygen intermediates are preferred, as they generally result in higher discharge and lower charging voltages. In agreement with recent experiments, cells that discharge to MgO exhibit low round-trip efficiencies, which are rationalized by the presence of large thermodynamic overvoltages. In contrast, MgO2-based cells are predicted to be much more efficient: superoxide-terminated facets on MgO2 crystallites enable low overvoltages and round-trip efficiencies approaching 90%. These data suggest that the performance of Mg/O2 batteries can be dramatically improved by biasing discharge towards the formation of MgO2 rather than MgO.

One obstacle to realizing a practical, rechargeable magnesium-ion battery is the development of efficient Mg electrolytes. Electrolytes based on simple Mg(BH) salts suffer from poor salt solubility and/or low conductivity, presumably due to strong ion pairing. Understanding the molecular-scale processes occurring in these electrolytes would aid in overcoming these performance limitations. Toward this goal, the present study examines the solvation, agglomeration, and transport properties of a family of Mg electrolytes based on the Mg(BH) salt using classical molecular dynamics. These properties were examined across five different solvents (tetrahydrofuran and the glymes G1-G4) and at four salt concentrations ranging from the dilute limit up to 0.4 M. Significant and irreversible salt agglomeration was observed in all solvents at all nondilute Mg(BH) concentrations. The degree of clustering observed in these divalent Mg systems is much larger than that reported for electrolytes containing monovalent cations, such as Li. The salt agglomeration rate and diffusivity of Mg were both observed to correlate with solvent self-diffusivity: electrolytes using longer- (shorter-) chain solvents had the lowest (highest) Mg diffusivity and agglomeration rates. Incorporation of Mg into Mg-BH clusters significantly reduces the diffusivity of Mg by restricting displacements to localized motion within largely immobile agglomerates. Consequently, diffusion is increasingly impeded with increasing Mg(BH) concentration. These data are consistent with the solubility limitations observed experimentally for Mg(BH)-based electrolytes and highlight the need for strategies that minimize salt agglomeration in electrolytes containing divalent cations.

Non-aqueous magnesium-oxygen (or 'Mg-air') batteries are attractive next generation energy storage devices due to their high theoretical energy densities, projected low cost, and potential for rechargeability. Prior experiments identified magnesium oxide, MgO, and magnesium peroxide, MgO2, as the primary discharge products in a Mg/O2 cell. Charge transport within these nominally-insulating compounds is expected to limit battery performance; nevertheless, these transport mechanisms are either incompletely understood (in MgO2) or remain a matter of debate (in MgO). The present study characterizes the equilibrium conductivity associated with intrinsic (point) defects within both compounds using first-principles calculations. For MgO, negative Mg vacancies and hole polarons-the latter localized on oxygen anions-were identified as the dominant charge carriers. However, the large formation energies associated with these carriers suggest low equilibrium concentrations. A large asymmetry in the carrier mobility is predicted: hole polarons are highly mobile at room temperature, while Mg vacancies are essentially immobile. Accounting for non-equilibrium effects such as frozen-in defects, the calculated conductivity data for MgO is shown to be in remarkable agreement with the three "Arrhenius branches" observed in experiments, thus clarifying the longdebated transport mechanisms within these regimes. In the case of MgO2, electronic charge carriers alone-electron and hole polarons-are the most prevalent. Similar to MgO, the equilibrium concentration of carriers in MgO2 is low, and moderate-to-poor mobility further limits conductivity. If equilibrium behavior is realized, then we conclude that: (i.) sluggish charge transport in MgO or MgO2 will limit battery performance when these compounds cover the cathode support, and (ii.) what little conductivity exists in these phases is primarily electronic in nature (i.e., polaron hopping). Artificially increasing the carrier concentration via mono-valent substitutions is suggested as a strategy for overcoming transport limitations.