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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.

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. H...

Aqueous Na-ion batteries are proposed as cheap, safe, environmentally friendly systems for large-scale energy storage owing to the high abundance of sodium in earth’s crust and the benign nature of...

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)

Rechargeable metal–sulfur batteries (RMSBs) represent one of the most attractive electrochemical systems in terms of energy density and cost. In most of the proposed systems, the anode side is metallic and the cathode side is elemental sulfur impregnated in a porous matrix. Despite the relatively low voltage of these systems, they attract a lot of attention and are considered to be very promising as next-generation batteries for the following reasons: (1) utilization of active metal anodes enables a leap in specific energy due to the high capacity of metal anodes in comparison to intercalation compounds, (2) sulfur as a cathode exhibits high theoretical specific capacity (1675 mAh/g), and (3) system components make RMSBs low-cost, less toxic batteries. Nevertheless, the high reactivity of metallic anodes (e.g., Li, Na, Mg, and Al) and the solubility of sulfur species in the electrolyte render these batteries unstable and hinder their practical realization. In this Perspective, we focus on rechargeable sul...

Summary Secondary magnesium batteries are still in the research stage, after the first prototype of a magnesium-based battery was demonstrated almost two decades ago. Since this breakthrough, despite tremendous efforts by numerous research groups, we are not aware of any system that exhibits better performance in terms of Coulombic efficiency over prolonged cycling. The scientific community is now focusing on the basic phenomena that hinder development of practical magnesium-based rechargeable batteries. Today, we have a better understanding of the structure of electrolyte solutions relevant to rechargeable Mg batteries and its effect on the electrochemical performance. New electrolyte solutions that are not based on organometallic moieties currently surpass the performance of the first generations of complex solutions. There is even an attempt to test alternative anode materials for magnesium-based energy storage systems. In this review, we summarize recent studies conducted in the field, with a focus on the anode/electrolyte solutions side.

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.

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...

One of the holy grails in research and development focused on rechargeable magnesium batteries is development of “conventional” electrolyte solutions that are compatible with both anode and cathode...

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In this paper we present a comprehensive structural, chemical and electrochemical characterization of monoclinic Li 0.33 MnO 2 as a positive electrode material for aqueous high-voltage hybrid supercapacitors. The monoclinic Li 0.33 MnO 2 , which is traditionally used as cathode material for lithium ion batteries, was synthesized through a simple thermal solid-state synthesis. The monoclinic Li 0.33 MnO 2 electrode exhibits a wide operational potential window ranging between −1.25 and 1.25 V vs SCE, which enables it to serve as either a negative or a positive electrode. In addition, this electrode material exhibits a high specific capacity of 140 mAh g −1 at a low current density of 0.1 A g −1 , and 76 mAh g −1 at high current density of 1 A g −1 in this range of potentials. Hybrid supercapacitors composed of Li 0.33 MnO 2 positive electrode and activated carbon (AC) negative electrode were fabricated. They exhibit outstanding electrochemical performance in terms of operational potential window, cycleability, and energy and power density. The Li 0.33 MnO 2 /AC hybrid capacitor has an energy density of 13.5 Wh kg −1 at power density of 100 W kg −1 , which is twice than that of MnO 2 /AC and AC/AC supercapacitors, and an energy density of 7 Wh kg −1 at 1000 W kg −1 , which is seven times higher than that of AC/AC capacitors at this power density. Furthermore, this hybrid capacitor presents an excellent cycle life with 80% specific capacitance retention after 12,000 cycles to 2 V. The electrochemical charge storage mechanism of the monoclinic Li 0.33 MnO 2 was investigated by cyclic voltammetry and X-ray diffraction.

Ionic liquids (IL) are attractive candidates for high voltage electrochemical energy storage systems due to their high electrochemical stability. Recently, a unique eutectic mixture of ionic liquids was reported to demonstrate outstanding performance in supercapacitor systems at low temperatures. Yet, many publications using this or similar ionic-liquid mixtures reported only a limited voltage or cyclability when utilizing them with practical activated carbon electrodes. With super-capacitors consisting of symmetric electrodes in which voltages higher than 3 V are applied, fast capacity fading and activity termination are observed. In order to exceed the limit of 3 V for supercapacitors that use electrolyte solutions possessing wide electrochemical windows, we thoroughly investigated the (unexpected) failure mechanism, using several analytical methods. By this, we discovered a pronounced difference in the electrochemical behavior of the negative and the positive electrodes that has significant implications on the operation of full symmetric cells at high voltages. Finally, we propose a solution that enables stable operation of cells up to 3.4 V. By balancing the mass of the electrodes, we prevent high-voltage failure and control the voltage split to use the full electrochemical window of each electrode and obtain a higher cell voltage of 3.4 V and an energy density higher than 40 Wh/kg (of the electrode materials).

Electrochemical capacitors, so-called supercapacitors, play an important role in energy storage and conversion systems. In the last decade, with the increasing volume of scientific activity and publications in this field, researchers have developed better tools to improve electrode materials. Although carbonaceous materials seem the most suitable for supercapacitor applications, a large diversity of materials has been proposed and studied. Yet, in order to accomplish performance beyond the limitations of each material, mainly in terms of energy density and durability, composite materials have been implemented, most of them being the combinations of carbon-based materials and other components. In this review, we present the recent advances in the field of composite materials that include at least one carbon-based component for supercapacitor electrodes. We focus on cases in which a single material by itself suffers from a drawback that can be overcome by combining it with other components, enabling the fabrication of a composite material with enhanced performance. We present several important compositions as well as the major routes of synthesis, characterization and performance of composite materials in this field.

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Core–shell, multilayered and coated materials have great importance to electrochemical energy storage systems, sensors, actuators, photonics, and photoactive applications. A deeper understanding of the effect of combining different materials in complex structures on their physical and electrochemical properties is vital for better engineering of such compounds and wise modulation of their physical characteristics. Herein we proposed a model system of thin film MoO3/V2O5 bilayer systems. The crystallinity, texture, and morphology of each layer in the bilayer structures were determined by X-ray diffraction (XRD), Raman spectroscopy, high resolution scanning electron microscopy (HR-SEM), and atomic force microscopy (AFM). The electrochemical properties were studied via slow scan rate cyclic voltammetry (SSCV) in lithium ions containing solution. The study clearly shows that the physical and electrochemical properties of each layer in a bilayer configuration are not similar to the properties of the single mat...

The specific capacity of activated carbon electrodes for supercapacitors may be enhanced with additional faradaic redox reactions by grafting of electroactive aromatic molecules with heteroatoms that act as redox centers. Such enrichment was demonstrated recently with anthraquinone and catechol using diazonium chemistry. Here, trihydroxybenzene, which has obvious advantages, was successfully grafted, yielding a mass enrichment of 25%. Electrochemical characterization in acidic aqueous solution after in situ methoxy deprotection demonstrated an initial specific capacity of 65 mAh g−1, which faded only slightly to 55 mAh g−1 after about 2000 cycles and remained stable for over 4500 cycles.

Mo6S8 Chevrel phase (CP) is still the most effective magnesium ions insertion material that was examined in cathodes for rechargeable Mg battery model systems. Its synthesis includes CuMo6S8 as an intermediate which undergoes Cu ions leaching. Thereby, CP materials may contain small amounts of copper. CuxMo6S8 undergoes complex magnesium insertion processes that include reversible displacement of copper. We report herein on a study of Cu0.09Mo6S8 electrodes in two electrolyte solutions which are important for secondary Mg batteries: C6H6MgCl/AlCl3/THF (all phenyl complex solutions, APC) and MgTFSI2/MgCl2/DME (Mg imide/chloride/DME solutions, ICD). There systems are ideal playgrounds for studying the nature of complex magnesium insertion processes into inorganic hosts. The analysis reported herein shows how the electroanalytical behavior of CP electrodes is affected by the electrolyte solution identity. For instance, the main Mg ions insertion process from the APC solutions occurs at 160 mV lower than from the ICD ones; the heterogeneous rate constant for charge transfer across the interface are significantly higher in APC solutions; in turn, the conversion processes related to the Cu residue in the CP electrodes are kinetically and thermodynamically favorable in the ICD solutions. As expected, the magnesium solid-state diffusion coefficient was found to be solutions independent. The study demonstrates the sensitivity of electroanalytical tools such as slow scanning rate voltammetry for reflecting fine structural changes during intercalation processes and can be generalized to other systems.

Rechargeable magnesium batteries (RMBs) have attracted a lot of attention in recent decades due to the theoretical properties of these systems in terms of energy density, safety, and price. Nevertheless, to date, fully rechargeable magnesium battery prototypes with sufficient longevity and reversibility were realized only with low voltage and low capacity intercalation cathode materials based on Cheverel phases. The community is therefore actively looking for high-capacity cathodes that can work with metallic magnesium anodes in viable RMB systems. One of the most promising cathode materials, in terms of very high theoretical specific capacity, is, naturally, sulfur. A number of recent works studied the electrochemical performances of rechargeable sulfur cathodes in RMB, with success to some extent on the cathode side. Nevertheless, as known from the lithium-sulfur rechargeable battery systems, the formation of soluble polysulfides during discharge affects strongly the behavior of the anode side. In this article and the work it describes, we focus on soluble polysulfides impact on Mg-S electrochemichal systems. We carefully designed herein conditions that mimic the Mg-S battery prototypes containing balanced Mg and elemental sulfur electrodes. Under these conditions, we extensively studied the Mg anode behavior. The study shows that when elemental sulfur cathodes are discharged in the Mg-S cells containing electrolyte solutions in which Mg anodes behave reversibly, the polysulfide species thus formed migrate to the anode and eventually fully passivate it by the formation of very stable surface layers. The work involved electrochemical, spectroscopic, and microscopic studies. The present study clearly shows that to realize practical rechargeable Mg-S batteries, the transport of any sulfide moieties from the sulfur cathode to the magnesium anode has to be completely avoided. Such a condition is mandatory for the operation of secondary Mg-S batteries.

Rechargeable magnesium batteries (RMBs) have attracted a lot of attention in recent decades due to the theoretical properties of these systems in terms of energy density, safety, and price. Nevertheless, to date, fully rechargeable magnesium battery prototypes with sufficient longevity and reversibility were realized only with low voltage and low capacity intercalation cathodes. The community is therefore actively looking for high-capacity cathodes that can work with metallic magnesium anodes in viable RMB systems. One of the most promising cathode materials, in terms of very high theoretical specific capacity is sulfur. A number of recent works studied the electrochemical performances of rechargeable sulfur cathodes in RMB, with success to some extent on the cathode side. Nevertheless, the formation of soluble polysulfide’s during discharge affects strongly the behavior of the anode side. we are focusing on soluble polysulfide impact on Mg–S electrochemical systems. We carefully designed herein conditions that mimic the Mg–S battery prototypes containing balanced Mg and elemental sulfur electrodes. Under these conditions, we extensively studied the Mg anode behavior. The study shows that when elemental sulfur cathodes are discharged in the Mg–S cells containing electrolyte solutions in which Mg anodes behave reversibly, the polysulfide species thus formed migrate to the anode and eventually fully passivate it by the formation of very stable surface layers.

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.