Baris Key

The unexpected discovery of the Mg(HMDS)2/MgCl2 complex as a magnesium electrolyte for rechargeable magnesium batteries

We developed a unique class of non-Grignard, aluminum-free magnesium electrolytes based on a simple mixture of magnesium compounds: magnesium hexamethyldisilazide (Mg(HMDS)2) and magnesium chloride (MgCl2). Through a reverse Schlenk equilibrium, a concentrated THF solution of Mg(HMDS)2–4MgCl2 was prepared to achieve reversible Mg deposition/dissolution, a wide electrochemical window, and a coulombic efficiency of 99%. High reversible capacities and good rate capabilities were obtained in Mg–Mo6S8 cells using these new electrolytes in tests with different rates. The unexpected high solubility of MgCl2 in the solvent of THF with the help from Mg(HMDS)2 provides a new way to develop magnesium electrolytes.

Low temperature stabilization of cubic (Li7−xAlx/3)La3Zr2O12: role of aluminum during formation

The lithium lanthanum zirconium oxide garnet, Li7La3Zr2O12 (LLZ), has received significant attention in recent years due to its high room temperature lithium ion conductivity and its stability against lithium metal. Together these features make it a promising electrolyte candidate for a high energy all solid-state battery. Previous studies have shown that incorporation of aluminum cations during the synthesis stabilizes the higher conductivity cubic phase of LLZ; however the incorporation process and its effect on the phase transition are still unclear. In the present study, we have combined powder X-ray diffraction (XRD), 27Al and 7Li MAS NMR and high-resolution X-ray diffraction (HRXRD) to determine the disposition of Al cations during the formation of low temperature cubic LLZ. At temperatures as low as 700 °C, the aluminum is incorporated into amorphous or nanocrystalline grain boundary phases. Above 700 °C, the Al cations are associated with a poorly crystalline anti-fluorite phase Li5AlO4, composed of molecular [AlO4]5− anions. This phase then reacts with tetragonal LLZ to form cubic LLZ over a 25 hour period at 850 °C. Although the reaction appears complete by powder X-ray diffraction, 27Al NMR spectra showed overlapping resonances suggesting multiple Al environments due to uneven substitution of the 24d Li(1) site. This was confirmed by high-resolution XRD and was consistent with a series of closely related cubic LLZ phases with slightly different Al concentrations, indicating the slower Al(III) diffusion within the lattice has not reached equilibrium in the time allotted. The disorder over the two crystallographic tetrahedral sites by lithium and aluminum cations at this temperature contributes to the observed lattice enlargement associated with the low temperature cubic phase.

High magnesium mobility in ternary spinel chalcogenides

Magnesium batteries appear a viable alternative to overcome the safety and energy density limitations faced by current lithium-ion technology. The development of a competitive magnesium battery is plagued by the existing notion of poor magnesium mobility in solids. Here we demonstrate by using ab initio calculations, nuclear magnetic resonance, and impedance spectroscopy measurements that substantial magnesium ion mobility can indeed be achieved in close-packed frameworks (~ 0.01–0.1 mS cm–1 at 298 K), specifically in the magnesium scandium selenide spinel. Our theoretical predictions also indicate that high magnesium ion mobility is possible in other chalcogenide spinels, opening the door for the realization of other magnesium solid ionic conductors and the eventual development of an all-solid-state magnesium battery.Low magnesium mobility in solids represents a significant obstacle to the development of Mg intercalation batteries. Here the authors show that substantial magnesium ion mobility can be achieved in close-packed ternary selenide spinel materials.

Pristine-state structure of lithium-ion-battery cathode material Li1.2Mn0.4Co0.4O2 derived from NMR bond pathway analysis

Layered lithium ion battery cathode materials have been extensively investigated, of which layered–layered composites xLi2MnO3·(1 − x)LiMO2 (M = Mn, Co, Ni) are of particular interest, owing to their high energy density. Before the structural transformations that occur in these materials with cycling can be understood, the structure of the pristine material must be established. In this work, NMR spectra are measured for the model layered–layered system xLi2MnO3·(1 − x)LiCoO2 and Bond-Pathway-model analysis is applied to elucidate the atomic arrangement and domain structure of this material in its pristine state, before electrochemical cycling. The simplest structural element of an Li2MnO3 domain consists of a stripe of composition LiMn2 parallel to a crystallographic axis in a metal layer of the composite. A simple model of the composite structure may be constructed by a superposition of such stripes in an LiCoO2 background. We show that such a model can account for most of the features of the observed NMR spectra.

Concentration dependent electrochemical properties and structural analysis of a simple magnesium electrolyte: magnesium bis(trifluoromethane sulfonyl)imide in diglyme

Development of Mg electrolytes that can plate/strip Mg is not trivial and remains one of the major roadblocks to advance Mg battery research. Halogen-free electrolyte has attracted great attention due to its high stability, less corrosive nature and compatibility with Mg metal anodes. However, the electrochemical properties of such electrolytes have not been analytically evaluated in the literature. Herein, we report a systematic study of the concentration-dependent electrochemical and mass transport properties of a non-aqueous, halogen-free Mg electrolyte composed of magnesium bis(trifluoromethane sulfonyl)imide in diglyme (Mg(TFSI)2/G2). Specifically, cyclic voltammograms confirm that plating and stripping of Mg in Mg(TFSI)2/G2 electrolyte occur over a wide concentration range. Results suggest a comparably difficult magnesium dissolution in Mg(TFSI)2/G2 electrolyte in contrast to in Grignard based electrolytes. Dissolution overpotential shows a non-monotonic dependence on electrolyte concentration, it requires an ∼2 V overpotential to deposit Mg. Findings also reveal concentration-dependent mass transport properties, including concentration-dependent electrolyte diffusivity and transference number. The atomic environment of the Mg(TFSI)2/G2, as being further explored by Nuclear Magnetic Resonance (NMR) measurement and Molecular Dynamics (MD) simulations, is coupled with the electrochemical measurements to explain the observed concentration-dependent mass transport properties.

Synthesis and Characterization of MgCr2S4 Thiospinel as a Potential Magnesium Cathode

Magnesium-ion batteries are a promising energy storage technology because of their higher theoretical energy density and lower cost of raw materials. Among the major challenges has been the identification of cathode materials that demonstrate capacities and voltages similar to lithium-ion systems. Thiospinels represent an attractive choice for new Mg-ion cathode materials owing to their interconnected diffusion pathways and demonstrated high cation mobility in numerous systems. Reported magnesium thiospinels, however, contain redox inactive metals such as scandium or indium, or have low voltages, such as MgTi2S4. This article describes the direct synthesis and structural and electrochemical characterization of MgCr2S4, a new thiospinel containing the redox active metal chromium and discusses its physical properties and potential as a magnesium battery cathode. However, as chromium(III) is quite stable against oxidation in sulfides, removing magnesium from the material remains a significant challenge. Early attempts at both chemical and electrochemical demagnesiation are discussed.