Rechargeable Mg batteries are one of the most investigated polyvalent-metal storage batteries owing to the increased safety associated with the nondendritic nature of Mg electrodeposition, high volumetric capacity, and low cost. To realize the commercial applications of Mg batteries, there are still a number of challenges remaining unsolved, in particular, the lack of halogenfree Mg electrolytes, as the use of the halogens remains a major limiting factor to achieving high voltage cathodes. Work presented here introduces an innovative approach to prepare a halogen-free Mg-based electrolyte in a simple, nonsynthetic method that can plate and strip Mg reversibly. Results suggest that by introducing a secondary amine cosolvent the magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI) 2 ) salt can be easily dissolved into a wide array of polar but aprotic ether solvents. A systematic structural investigation of a representative Mg(TFSI) 2 electrolyte in the cosolvent systems with the secondary amine was performed using pair distribution function (PDF) analysis, single crystal diffraction analysis, and NMR. The experimental atomic scale understanding reveals an ion pair structure of Mg 2+ coordinated with six oxygen donors from the bis-(trifluoromethanesulfonyl)imide (TFSI) anions and the THF solvent located in the first solvation shell. The as-formed neutral ion pair structure acts as the active component for reversible Mg deposition. We believe this new route of preparing Mg electrolytes can extend the current understanding of Mg electrolyte functionality for rechargeable Mg batteries and offers more guidance for the future electrolyte design.

The magnesium–sulfur (MgS) battery is a promising alternative to the post-lithium battery because of its low-cost construction, eco-friendliness, high theoretical energy density, and safety. However, the lack of simple compatible electrolytes, self-discharge, polysulfide shuttle effect, and the slow conversion reaction pathway still limit its practical applications. Here, we propose a simple halogen-free electrolyte (HFE) based on Mg­(NO3)2 dissolved in the cosolvent of acetonitrile (ACN) and tetraethylene glycol dimethyl (G4) that applies to a Mg/S full cell. The as-prepared Mg-ion electrolyte exhibits efficient Mg plating/stripping performance, high anodic stability (vs Mg/Mg2+), and a high ionic conductivity of ∼10–4 S cm–1 at 313 K. Chronoamperometry (CA), scanning electron microscopy, and energy-dispersive spectroscopy examinations report that the HFE supports flat, dendrite-free, and translucent Mg deposits. Polymer layer interface (PLI)-based polyvinylidene fluoride (PVDF) and Mg­(O3SCF3)2 have been designed to isolate the surface of the Mg anode from the liquid electrolyte. A sulfur cathode with the anchoring materials of silicon carbide and barium titanate-based material has been designed and characterized. The Mg/S battery has been constructed with an initial discharge capacity of up to 1200 mAh g–1, and it has retained a reversible capacity at 100 mAh g–1 after 10 cycles. This study offers a pivotal role in designing a promising HFE candidate for a high-performance MgS battery.

Formation and evolution of the microscopic solid electrolyte interphase (SEI) at the Mg electrolyte/electrode interface are less reported and need to be completely understood to overcome the compatibility challenges at the Mg anode–electrolyte. In this paper, SEI evolution at the Mg electrolyte/electrode interface is investigated via an in situ electrochemical quartz crystal microbalance with dissipation mode (EQCM-D), electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectrometry (FTIR). Results reveal remarkably different interfacial evolutions for the two Mg electrolyte systems that are studied, a non-halogen Mg­(TFSI)2 electrolyte in THF with DMA as a cosolvent (nhMg-DMA electrolyte) versus a halogen-containing all-phenyl complex (APC) electrolyte. The nhMg-DMA electrolyte reports a minuscule SEI formation along with a significant Coulomb loss at the initial electrochemical cycles owing to an electrolyte reconstruction process. Interestingly, a more complicated SEI growth is observed at the later electrochemical cycles accompanied by an improved reversible Mg deposition attributed to the newly formed coordination environment with Mg2+ and ultimately leads to a more homogeneous morphology for the electrochemically deposited Mg0, which maintains a MgF2-rich interface. In contrast, the APC electrolyte shows an extensive SEI formation at its initial electrochemical cycles, followed by a SEI dissolution process upon electrochemical cycling accompanied by an improved coulombic efficiency with trace water and chloride species removed. Therefore, it leads to SEI stabilization progression upon further electrochemical cycling, resulting in elevated charge transport kinetics and superior purity of the electrochemically deposited Mg0. These outstanding findings augment the understanding of the SEI formation and evolution on the Mg interface and pave a way for a future Mg-ion battery design.

The integration of electrochemistry and materials has developed rapidly over the past decade and is now a key contributor in many important applications, one emerging area is to design efficient energy storage devices at low cost. Many of these savings are driven by utilizing the higher abundance chemical elements from the earth's crust, for instance Zn, Mg or Ca. Despite some successful research advances in recent decades, there is a lack of an in-depth analysis of the electrolyte/electrode interface for multivalent battery systems. One major difficulty is the lack of capability to provide direct in-situ interfacial information without perturbing its chemical environment. Efforts from the presented work aim to probe and quantify the solid electrolyte interface formation in-situ. We aim to understand at the interface, how parasitic reactions affects or disrupts the mass transport processes and shuts down the cycling of the multivalent electrode material.

Rechargeable magnesium batteries are one of the most promising multivalent energy storage batteries due to high volumetric capacity, high abundance in the earth’s crust and low cost. Electrolyte development plays an essential role for the Mg battery design. Due to the lack of suitable Mg electrolytes that can be compatible with high voltage cathodes limited the commercialization of the rechargeable Mg batteries, the development of the halogen-free Mg-based electrolytes has received considerable attention and achieved significant advancements in recent years. In this study, our goal is to investigate the electrochemical properties and reaction mechanisms for the reversible Mg plating/stripping electrolytes in the non-halogen Mg electrolytes, which are strikingly impacted by solid electrolyte interphase (SEI) at the Mg electrolyte/electrode interface. This work might help to understand and overcome the challenges of the Mg anode-electrolyte compatibility issues.