Despite their advantages complementary to lithium- and sodium-ion batteries, rechargeable magnesium batteries (RMBs) still suffer from multiple limitations that impede their commercialization. Amine-based electrolytes have recently emerged as promising options to overcome the long-standing challenge of sluggish Mg2+ ion transport at the magnesium metal interface; however, their uncontrolled chemical and electrochemical reactivity impairs cycling and calendar life. After screening a range of bidentate methoxyethylamine derivatives, we herein identify aminoacetaldehyde dimethyl acetal (ADMA)functionalized with a methoxy substituentas a single electrolyte solvent that achieves an optimal balance between interfacial ionic transport and (electro)-chemical stability. These optimal properties are enabled by the concerted modulation of electron donicity and steric hindrance in ADMA. Consequently, the ADMA-based electrolyte delivers stable cycling for Mg||SS half-cellseven under operating protocols incorporating intermittent rest periodsas well as for full-cells paired with Mo6S8 and tellurium cathodes. This study demonstrates that longstanding interfacial issues at the Mg metal anode in RMBs can be resolved through the sophisticated molecular engineering of scalable, bidentate methoxyethylamine electrolyte solvents.

The development of efficient electrolytes is crucial for advancing magnesium (Mg) batteries, which hold promise for next‐generation energy storage systems. Previously, electrolytes such as [Mg 2 (μ‐Cl) 3 ⋅ 6THF] + [Ph 4 Al] − , A, and [Mg 2 (μ‐Cl) 3 ⋅ 6THF] + [Ph 3 AlCl] − , B , have been studied, but their performance has been limited by issues related to ion dissociation and electrochemical stability. In this study, we report the synthesis of novel electrolytes by introducing polydentate ligands to these known systems, leading to the formation of [DME ⋅ MgCl ⋅ 3THF] + [Ph 4 Al] − 1 and [DG ⋅ MgCl ⋅ 2THF] + [Ph 4 Al] − 2 , [Mg ⋅ 3DME] 2+ 2[Ph 3 AlCl − ] 3 and [Mg ⋅ 2DG] 2+ 2[Ph 3 AlCl − ] 4 . These firstly discovered compounds were thoroughly characterized using X‐ray crystallography and NMR spectroscopy. Our findings reveal that the choice of counter anion plays a pivotal role in the products and mechanism of the dissociation of the bridged [Mg 2 (μ‐Cl) 3 ⋅ 6THF] + cation upon the addition of polydentate ligands. Specifically, with the [Ph 4 Al] − counter anion (precursor A ), the dissociation results in a [MgCl] + mono‐cation, while with the [Ph 3 AlCl] − counter anion (precursor B ), a [Mg] 2+ divalent cation is formed. The resultant MgCl 2 byproduct enhances solubility, expands electrochemical windows, and improves cyclic stability, leading to superior electrochemical performance of the new electrolytes ( 1 , 2 , 3 , and 4 ) compared to the original precursors. These insights offer valuable guidelines for the design and synthesis of advanced electrolytes for rechargeable magnesium batteries, potentially paving the way for more efficient and stable energy storage solutions.

Rechargeable magnesium metal batteries (RMBs) represent a promising sustainable energy storage technology, complementary to lithium-ion and sodium-ion batteries due to their superior volumetric energy density, cost-effectiveness, and safety. However, their widespread adoption is hindered by limited electrolyte options due to the formation of Mg ion-insulating surface films that cannot behave as solid-electrolyte-interphases. Here, after considering the binding affinity with Mg²⁺ and steric hindrance, we report a single-solvent system based on commercial aminoacetaldehyde dimethyl acetal (ADMA). Our system effectively forms a Mg ion-conducting interphase and enhances the Mg plating-stripping efficiency, without severe corrosion. The average Coulombic efficiency is 97.3% over 500 hours upon galvanostatic cycling in Mg‖stainless steel cells at cycling conditions of 0.5 mA cm⁻² and 0.5 mAh cm⁻², along with capacity retention of 90.3% and 99.2% for 250 and 300 cycles in Mg‖Mo₆S₈ and Mg‖Tellurium full-cells, respectively. This study indicates that high-performance practical RMBs are achievable through solvation structure engineering with commercially available solvents and salts.

The chemistry of the electrolyte solutions that enable reversible Mg deposition is not trivial. Such solutions are currently limited to ethereal solvents and most of them contain chlorides complexes. These ionic complexes have important role in the performance. However, the presence of chlorides in these solutions complicates the cathode side because such solutions are not compatible with the commonly used metallic current collectors for cathodes. Consequently, it is questionable whether it is possible to synthesize fully functional Cl-free electrolyte solutions suitable commercial Mg-ion batteries. Noked et al. reported that by adding DME to the precursor electrolyte [Mg2Cl3*6THF]+ [Ph3AlCl]- in THF, it was possible to create a new electroactive complex Mg salt, namely, [Mg-3.DME]2+ 2[AlPh3Cl]-, which solution performs better than the precursor’s solution. This solution introduces a new case of chlorides free electrolyte solution, with stabilized Mg2+ cations and anions containing chloride ligands which cannot lead to corrosion phenomena. In this work a full evaluation of selected electrolyte solution was carried out, considering rigorously the role of the electroactive species and the anion complexes in the electrochemical process. In addition, we study the effect of the anion on the electrochemical performance by comparing this solution with other previously presented solutions.

Battery technologies based in multivalent charge carriers with ideally two or three electrons transferred per ion exchanged between the electrodes have large promises in raw performance numbers, most often expressed as high energy density, and are also ideally based on raw materials that are widely abundant and less expensive. Yet, these are still globally in their infancy, with some concepts (e.g. Mg metal) being more technologically mature. The challenges to address are derived on one side from the highly polarizing nature of multivalent ions when compared to single valent concepts such as Li+ or Na+ present in Li-ion or Na-ion batteries, and on the other, from the difficulties in achieving efficient metal plating/stripping (which remains the holy grail for lithium). Nonetheless, research performed to date has given some fruits and a clearer view of the challenges ahead. These include technological topics (production of thin and ductile metal foil anodes) but also chemical aspects (electrolytes with high conductivity enabling efficient plating/stripping) or high-capacity cathodes with suitable kinetics (better inorganic hosts for intercalation of such highly polarizable multivalent ions). This roadmap provides an extensive review by experts in the different technologies, which exhibit similarities but also striking differences, of the current state of the art in 2023 and the research directions and strategies currently underway to develop multivalent batteries. The aim is to provide an opinion with respect to the current challenges, potential bottlenecks, and also emerging opportunities for their practical deployment.

The development of efficient electrolytes is crucial for advancing magnesium (Mg) batteries, which hold promise for next-generation energy storage systems. Previously, electrolytes such as [Mg2(µ-Cl)3•6THF]+ [Ph4Al]-, A, and [Mg2(µ-Cl)3•6THF]+ [Ph3AlCl]-, B, have been studied, but their performance has been limited by issues related to ion dissociation and electrochemical stability. In this study, we report the synthesis of novel electrolytes by introducing polydentate ligands to these known systems, leading to the formation of [DME•MgCl•3THF]+ [Ph4Al]- 1 and [DG•MgCl•2THF]+ [Ph4Al]- 2, [Mg•3DME]2+ [Ph3AlCl-]2 3 and [Mg•2DG]2+ [Ph3AlCl-]2 4. These firstly discovered compounds were thoroughly characterized using X-ray crystallography and NMR spectroscopy. Our findings reveal that the choice of counter anion plays a pivotal role in the products and mechanism of the dissociation of the bridged [Mg2(µ-Cl)3•6THF]+ cation upon the addition of polydentate ligands. Specifically, with the [Ph4Al]- counter anion (precursor A), the dissociation results in a [MgCl]+ mono-cation, while with the [Ph3AlCl]- counter anion (precursor B), a [Mg]2+ divalent cation is formed. The resultant MgCl2 byproduct enhances solubility, expands electrochemical windows, and improves cyclic stability, leading to superior electrochemical performance of the new electrolytes (1, 2, 3, and 4) compared to the original precursors. These insights offer valuable guidelines for the design and synthesis of advanced electrolytes for rechargeable magnesium batteries, potentially paving the way for more efficient and stable energy storage solutions.

One of the major issues in developing electrolyte solutions for rechargeable magnesium batteries is understanding the positive effect of chloride anions on Mg deposition-dissolution processes on the anode side, as well as intercalation-deintercalation of Mg2+ ions on the cathode side. Our previous results suggested that Cl- ions are adsorbed on the surface of Mg anodes and Chevrel phase MgxMo6S8 cathodes. This creates a surface add-layer that reduces the activation energy for the interfacial Mg ions transportation and related charge transfer, as well as promotes the transport of Mg2+ from the solution phase to the Mg anode surface and into the cathodes' host materials. Here, this work further examines the effect of adding chlorides to the state-of-the-art Mg[B(HFIP)(4)](2)/DME electrolyte solution, specifically focusing on reversible magnesium deposition, as well as the performance of Mg cells with benchmark Chevrel phase cathodes. It was observed that the presence of chlorides in these solutions facilitates both Mg deposition, and Mg2+ ions intercalation, whereby this effect is more pronounced as the purity level of the solution is lowered.

The design of electrolyte solutions that permit reversible and efficient Mg metal electrodeposition is one of the most important tasks in the development of rechargeable Mg batteries. Several types of electrolyte solutions for Mg metal anodes have been developed and explored over the last two decades. These investigations have contributed to a better understanding of the Mg deposition and stripping processes. However, the Coulombic efficiency (CE) for reversible electrodeposition reported for these various systems and their performance in comparison to one another remained unclear. We used rigorous electrochemical methods to accurately quantify the average CE of the major electrolyte solutions considered for secondary Mg metal batteries. We demonstrated how changes in the experiential protocols influence CE measurements, resulting in inconsistent reports. Even though exceptional efficiency has been reported for a variety of systems, we discovered that the only candidate that currently meets the 99% CE benchmark during a prolonged cycling procedure is the dichloro-complex, which is a first-generation Grignard-based electrolyte solution. Second- and third-generation Grignard-free and chloride-free solutions showed reasonable CE only when the deposition currents densities were lowered. This comprehensive and systematic investigation will help to create a more accurate treasure map for potential electrolyte solutions for rechargeable Mg metal anodes.

One of the greatest challenges toward rechargeable magnesium batteries is the development of noncorrosive electrolyte solutions with high anodic stability that can support reversible Mg deposition/dissolution. In the last few years, magnesium electrolyte solutions based on Cl-free fluorinated alkoxyborates were investigated for Mg batteries due to their high anodic stability and ionic conductivity and the possibility of reversible deposition/dissolution in ethereal solvents. Here, the electrochemical performance of Mg[B(hexafluoroisopropanol)4]2/dimethoxyethane (Mg[B(HFIP)4]2/DME) solutions was examined. These electrolyte solutions require a special "conditioning" pretreatment that removes undesirable active moieties. Such a process was developed and explored, and basic scientific issues related to the mechanism by which it affects Mg deposition/dissolution were addressed. The chemical changes that occur during the conditioning process were examined. Mg[B(HFIP)4]2/DME solutions were found to enable reversible Mg deposition, albeit with a relatively low Coulombic efficiency of 95% during the first cycles. Prolonged deposition/dissolution cycling tests demonstrate a stable behavior of magnesium electrodes. Overall, this system presents a reasonable electrolyte solution and can serve as a basis for future efforts to develop chlorine-free alternatives for secondary magnesium batteries. It is clear that such a conditioning process is mandatory, as it removes reactive contaminants that lead to unavoidable passivation and deactivation of Mg electrodes from the solution.

Zinc-based batteries are gaining attention as a promising candidate for large-scale energy storage systems due to their safety, abundance of elemental zinc, low cost, and ease of handling in air. However, only a few zinc storage materials, namely, intercalation cathode materials, were reported, and there is a need to develop host structures with improved performance. Here, we investigate copper vanadate as a cathode material and uncover its proton and zinc storage behavior by combined electrochemical characterization, XRD analysis, and ion migration barrier calculations for the cation diffusion pathways. The material showed a highly reversible capacity of similar to 315 mA h/g at 20 mA/g with a good capacity retention.

The interfacial electrochemical characteristics of unpassivated Mg metal electrodes have been studied using Staircase Galvano Electrochemical Impedance Spectroscopy (SGEIS) in THF/C6H5MgCl/AlCl3 solutions over a wide range of applied dc currents. The results shed light on the mechanism of electrochemical deposition and dissolution of Mg. We found that at the stationary condition (at the OCV, when no current flows) there is a very high interfacial charge-transfer resistance of around 40,000 Ωcm2. The impedance decreases to several hundred Ωcm2 when dynamic, though steady, processes of Mg deposition or dissolution take place. The alternating process through which impedance spectroscopy is measured is superimposed on this system. We show that the high impedance measured at the OCV with a low direct current is due to interfacial adsorption phenomena. However, the adsorption phenomena affect the interfacial charge transfer in different ways depending on the direction of the reaction. During steady-state deposition, the adsorbed layer leads to a large distance for electron tunneling, which translates to a high charge-transfer impedance across the interface. Application of relatively large steady-state negative currents leads to changes in the composition of the adsorbed layer and a significant reduction in the charge-transfer resistance. During the opposite process, dissolution, the high interfacial impedance measured is probably predominantly due to migration of Mg ions away from the Mg metal surface, across the adsorbed layer, to the bulk solution. At relatively high direct currents (and overpotentials >100 mV) the interfacial resistivity obeys Butler–Volmer kinetics, which are not complicated by adsorption phenomena.

Tunnel-type sodium manganese oxide is a promising cathode material for aqueous/nonaqueous sodium-ion batteries, however its storage mechanism is not fully understood, in part due to the complicated sodium intercalation process. In addition, low cyclability due to manganese dissolution has limited its practical application in rechargeable batteries. Here, the intricate sodium intercalation mechanism of Na0.44MnO2 is revealed by combination of electrochemical characterization, structure determination from powder X-ray diffraction data, 3D bond valence difference maps, and barrier-energy calculations of the sodium diffusion. NaI is proposed as an important electrolyte solution additive. It is shown to form a thin, beneficial, and durable cathode surface film that prevents manganese dissolution. The addition of 0.01 m NaI to electrolyte solutions based on alkyl carbonate solvents and NaClO4 greatly improves the cycling efficiency, raising the capacity retention from 86% to 96% after 600 cycles. This study determines the core aspects of the sodium intercalation mechanism in tunnel-type sodium manganese oxide and shows how it can serve as a durable cathode material for rechargeable Na batteries.

Most electrolyte solutions supporting highly reversible Mg deposition/dissolution contain chlorides. These come as charged or neutral complex species, frequently as Mg-based complex ions, and Lewis-base anions. The electroactive species are usually comprised also of solvent molecules and chlorides as ligands. Numerous studies had shown the critical role of Cl-based species on the electrochemical reaction kinetics, deposition and dissolution overpotentials, electrocrystallization morphology, and many other imperative aspects of the electrochemical responses. Only very few, quite exotic and not fully studied, Cl-free electrolyte solutions have been reported to maintain reversible Mg deposition. Over the years, several compelling theories have been proposed to explain the decisive role of the Cl-containing species on the electrochemical reaction mechanisms during magnesium deposition and dissolution. The role of chlorine-based species in intercalation processes of Mg ions into crystalline hosts was seldom pursued, though. Interestingly, it appears that the Chevrel phase (CP) MgxMo6S8, the main benchmarking Mg intercalation compound, does not intercalate Mg in Cl-free solutions at room temperature (RT). This was established over and again during past decades and recently had been corroborated over a large set of experiments in our lab. Herein, we reveal, via experimental work, that Cl-based species also have a critical role in the electrochemical processes of intercalation. In this work, we studied the function of these during the course of the intercalation process of Mg ions into CP electrodes. We demonstrate that the intercalation process is extremely sluggish, to the point of nonexistent, from Cl-free electrolyte solutions. Quite remarkably, the addition of chlorine-comprising species opens the door to facile magnesium intercalation. On the basis of the numerous experimental results, we suggest a detailed Mg intercalation mechanism into CP. The proposed mechanism emphasizes the importance of the Cl-based species on the charge transfer process across the solid/solution interface. We propose that surface absorbed Cl-containing complexes reduce the activation energy for the interfacial charge transfer stage involving the transport of Mg ions from the solution to the crystalline phase and vice versa. We also propose a relatively simple manner to alleviate the intercalation impediment of Cl-free electrolyte solutions by the cathode's chemical or electrochemical pretreatment. This project has both scientific and applicative significance and may open a hatch for the future development of practical rechargeable Mg batteries (RMBs).