Timothy S. Arthur

Structure and compatibility of a magnesium electrolyte with a sulphur cathode.

Magnesium metal is an ideal rechargeable battery anode material because of its high volumetric energy density, high negative reduction potential and natural abundance. Coupling Mg with high capacity, low-cost cathode materials such as electrophilic sulphur is only possible with a non-nucleophilic electrolyte. Here we show how the crystallization of the electrochemically active species formed from the reaction between hexamethyldisilazide magnesium chloride and aluminum trichloride enables the synthesis of a non-nucleophilic electrolyte. Furthermore, crystallization was essential in the identification of the electroactive species, [Mg2(μ-Cl)3·6THF]+, and vital to improvements in the voltage stability and coulombic efficiency of the electrolyte. X-ray photoelectron spectroscopy analysis of the sulphur electrode confirmed that the electrochemical conversion between sulphur and magnesium sulfide can be successfully performed using this electrolyte.

Magnesium borohydride: from hydrogen storage to magnesium battery.

Beyond hydrogen storage: The first example of reversible magnesium deposition/stripping onto/from an inorganic salt was seen for a magnesium borohydride electrolyte. High coulombic efficiency of up to 94 % was achieved in dimethoxyethane solvent. This Mg(BH(4))(2) electrolyte was utilized in a rechargeable magnesium battery.

An Efficient Halogen‐Free Electrolyte for Use in Rechargeable Magnesium Batteries

Unlocking the full potential of rechargeable magnesium batteries has been partially hindered by the reliance on chloride-based complex systems. Despite the high anodic stability of these electrolytes, they are corrosive toward metallic battery components, which reduce their practical electrochemical window. Following on our new design concept involving boron cluster anions, monocarborane CB11H12(-) produced the first halogen-free, simple-type Mg salt that is compatible with Mg metal and displays an oxidative stability surpassing that of ether solvents. Owing to its inertness and non-corrosive nature, the Mg(CB11H12)2/tetraglyme (MMC/G4) electrolyte system permits standardized methods of high-voltage cathode testing that uses a typical coin cell. This achievement is a turning point in the research and development of Mg electrolytes that has deep implications on realizing practical rechargeable Mg batteries.

Size-dependent hydrogen storage properties of Mg nanocrystals prepared from solution.

Mg nanocrystals of controllable sizes were prepared in gram quantities by chemical reduction of magnesocene using a reducing solution of potassium with an aromatic hydrocarbon (either biphenyl, phenanthrene, or naphthalene). The hydrogen sorption kinetics were shown to be dramatically faster for nanocrystals with smaller diameters, although the activation energies calculated for hydrogen absorption (115-122 kJ/mol) and desorption (126-160 kJ/mol) were within previously measured values for bulk Mg. This large rate enhancement cannot be explained by the decrease in particle size alone but is likely due to an increase in the defect density present in smaller nanocrystals.

Boron Clusters as Highly Stable Magnesium-Battery Electrolytes†

Boron clusters are proposed as a new concept for the design of magnesium-battery electrolytes that are magnesium-battery-compatible, highly stable, and noncorrosive. A novel carborane-based electrolyte incorporating an unprecedented magnesium-centered complex anion is reported and shown to perform well as a magnesium-battery electrolyte. This finding opens a new approach towards the design of electrolytes whose likelihood of meeting the challenging design targets for magnesium-battery electrolytes is very high.

Understanding the electrochemical mechanism of K-αMnO2 for magnesium battery cathodes.

Batteries based on magnesium are an interesting alternative to current state-of-the-art lithium-ion systems; however, high-energy-density cathodes are needed for further development. Here we utilize TEM, EDS, and EELS in addition to soft-XAS to determine electrochemical magnesiation mechanism of a high-energy density cathode, K-αMnO2. Rather than following the typical insertion mechanism similar to Li(+), we propose the gradual reduction of K-αMnO2 to form Mn2O3 then MnO at the interface of the cathode and electrolyte, finally resulting in the formation of K-αMnO2@(Mg,Mn)O core-shell product after discharge of the battery. Understanding the mechanism is a vital guide for future magnesium battery cathodes.

Understanding and overcoming the challenges posed by electrode/electrolyte interfaces in rechargeable magnesium batteries

Guided by the great achievements of lithium (Li)-ion battery technologies, post Li-ion battery technologies have gained a considerable interest in recent years. Their success would allow us to realize a sustainable society, enabling us to mitigate issues like global warming and resource depletion. Of such technologies, Magnesium (Mg) battery technologies have attracted attention as a high energy-density storage system due to the following advantages: (1) potentially high energy-density derived from a divalent nature, (2) low-cost due to the use of an earth abundant metal, and (3) intrinsic safety aspect attributed to non-dendritic growth of Mg. However, these notable advantages are downplayed by undesirable battery reactions and related phenomena. As a result, there are only a few working rechargeable Mg battery systems. One of the root causes for undesirable behavior is the sluggish diffusion of Mg2+ inside a host lattice. Another root cause is the interfacial reaction at the electrode/electrolyte boundary. For the cathode/electrolyte interface, Mg2+ in the electrolyte needs a solvation-desolvation process prior to diffusion inside the cathode. Apart from the solid electrolyte interface (SEI) formed on the cathode, the divalent nature of Mg should cause kinetically slower solvation-desolvation processes than that of Li-ion systems. This would result in a high charge transfer resistance and a larger overpotential. On the contrary, for the anode/electrolyte interface, the Mg deposition and dissolution process depends on the electrolyte nature and its compatibility with Mg metal. Also, the Mg metal/electrolyte interface tends to change over time, and with operating conditions, suggesting the presence of interfacial phenomena on the Mg metal. Hence, the solvation-desolvation process of Mg has to be considered with a possible SEI. Here, we focus on the anode/electrolyte interface in a Mg battery, and discuss the next steps to improve the battery performance.

Anodes and Anode/Electrolyte Interfaces for Rechargeable Magnesium Batteries

Multivalent battery systems like rechargeable magnesium (Mg) batteries are garnering more interest as candidate post-lithium (Li) battery systems, for eventual applications in electric vehicles (EVs) and plug-in hybrid vehicles (PHVs). Mg, being divalent and denser, is theoretically capable of delivering a higher volumetric energy density (3833 mAh cm−3) than Li (2061 mAh cm−3), making it a viable battery system for addressing current range and space concerns in vehicles. To date, various low-voltage electrolytes have been utilized in Mg battery systems, due to the incompatibility of high-voltage conventional battery electrolytes (TFSI−, ClO4−, PF6−) with Mg metal anodes. It is however possible to use conventional battery electrolytes for Mg battery systems, by changing the type of anode, from a Mg metal anode to a Mg-ion insertion-type anode (e.g., Bi and Sn), as recently reported. This fact has produced two primary avenues of research for anodes in Mg batteries: the first being to utilize insertion-type anodes via their engineering as effective nanomaterials and the second focused on fundamental studies of the anode/electrolyte interface.

In-situ/operando soft x-ray spectroscopy characterization of interfacial phenomena in energy materials and devices

Many important energy systems are based on the complexity of material architecture, chemistry and interactions among constituents within. To understand and thus ultimately control the energy applications calls for in-situ/operando characterization tools. Recently, we have developed the in-situ/operando soft X-ray spectroscopic systems for the studies of catalytic and electrochemical reactions, and reveal how to overcome the challenge that soft X-rays cannot easily peek into the high-pressure catalytic or liquid electrochemical reactions. The unique design of in-situ/operando soft X-ray spectroscopy instrumentation and fabrication principle and one example are presented.