High-Performance Magnesium Electrochemical Cycling with Hybrid Mg-Li Electrolytes

Kinetics and coulombic efficiency of the electrochemical magnesium plating and stripping processes are to a significant extent defined by the composition of the electrolyte solution, optimization of which presents a pathway for improved performance. Adopting this strategy, we undertook a systematic investigation of the Mg^0/2+ process in different combinations of the Mg²⁺-Li⁺-borohydride-bis(trifluoromethylsulfonyl)imide (TFSI^-) electrolytes in 1,2-dimethoxyethane (DME) solvent. Results indicate that the presence of BH₄^- is essential for high coulombic efficiency, which coordination to Mg²⁺ was confirmed by Raman and NMR spectroscopic analysis. However, the high rates observed also require the presence of Li⁺ and a supplementary anion such as TFSI^-. The Li⁺ + BH₄^- + TFSI^- combination of ionic species prevents passivation of the magnesium surface and thereby enables efficient Mg^0/2+ electrochemical cycling. The best Mg^0/2+ performance with the stabilized coulombic efficiency of 88 ± 1% and one of the highest deposition/stripping rates at ambient temperature reported to date are demonstrated at an optimal [Mg(BH₄)₂]:[LiTFSI] mole ratio of 1:2.

Probing the Polysulfide Confinement in Two Different Sulfur Hosts for a Mg|S Battery Employing Operando Raman and Ex-Situ UV-Visible Spectroscopy

We study here the Mg-polysulfide confinement inside two structurally different model porous materials, viz., toray carbon paper (TC) and multiwalled carbon nanotubes (CNT), using operando Raman and postcycling ex-situ UV-vis spectroscopy. Sulfur encapsulated inside CNT (CNT-S) and TC (TC-S) serves as S-cathodes in a rechargeable room temperature Mg|S battery. Operando Raman spectroscopy indicates the presence of higher-order Mg-polysulfides at the CNT cathode. This is due to the combination of their entrapment inside CNT and also possibly to their localization in the liquid electrolyte in the vicinity of CNT-S. This finding is directly correlated to the ex-situ UV-vis spectroscopy, which shows a lesser degree of Mg-polysulfide dissolution into the electrolyte solution. In comparison, TC-S, where sulfur is encapsulated within the open matrix formed by the nanofiber network of the carbon paper, displays poorer polysulfide confinement. The distinct differences in their abilities to confine the Mg-polysulfides are corroborated by battery performance. In the current density range (0.05-1) C, the battery with CNT-S displays much higher specific capacities, being nearly two times that of TC-S at 1 C.

Enabling Magnesium Anodes by Tuning the Electrode/Electrolyte Interfacial Structure

A new deposition mechanism is presented in this study to achieve highly reversible plating and stripping of magnesium (Mg) anodes for Mg-ion batteries. It is known that the reduction of electrolyte anions such as bis(trifluoromethanesulfonyl)imide (TFSI-) causes Mg surface passivation, resulting in poor electrochemical performance for Mg-ion batteries. We reveal that the addition of sodium cations (Na+) in Mg-ion electrolytes can fundamentally alter the interfacial chemistry and structure at the Mg anode surface. The molecular dynamics simulation suggests that Na+ cations contribute to a significant population in the interfacial double layer so that TFSI- anions are excluded from the immediate interface adjacent to the Mg anode. As a result, the TFSI- decomposition is largely suppressed so does the formation of passivation layers at the Mg surface. This mechanism is supported by our electrochemical, microscopic, and spectroscopic analyses. The resultant Mg deposition demonstrates smooth surface morphology and lowered overpotential compared to the pure Mg(TFSI)2 electrolyte.

Overview of the Structure-Dynamics-Function Relationships in Borohydrides for Use as Solid-State Electrolytes in Battery Applications

The goal of this article is to highlight crucial breakthroughs in solid-state ionic conduction in borohydrides for battery applications. Borohydrides, Mz+BxHy, form in various molecular structures, for example, nido-M+BH4; closo-M2+B10H10; closo-M2+B12H12; and planar-M6+B6H6 with M = cations such as Li+, K+, Na+, Ca2+, and Mg2+, which can participate in ionic conduction. This overview article will fully explore the phase space of boron-hydrogen chemistry in order to discuss parameters that optimize these materials as solid electrolytes for battery applications. Key properties for effective solid-state electrolytes, including ionic conduction, electrochemical window, high energy density, and resistance to dendrite formation, are also discussed. Because of their open structures (for closo-boranes) leading to rapid ionic conduction, and their ability to undergo phase transition between low conductivity and high conductivity phases, borohydrides deserve a focused discussion and further experimental efforts. One challenge that remains is the low electrochemical stability of borohydrides. This overview article highlights current knowledge and additionally recommends a path towards further computational and experimental research efforts.

Anion-Assisted Delivery of Multivalent Cations to Inert Electrodes

To understand and control key electrochemical processes-metal plating, corrosion, intercalation, etc.-requires molecular-scale details of the active species at electrochemical interfaces and their mechanisms for desolvation from the electrolyte. Using free energy sampling techniques we reveal the interfacial speciation of divalent cations in ether-based electrolytes and mechanisms for their delivery to an inert graphene electrode interface. Surprisingly, we find that anion solvophobicity drives a high population of anion-containing species to the interface that facilitate the delivery of divalent cations, even to negatively charged electrodes. Our simulations indicate that cation desolvation is greatly facilitated by cation-anion coupling. We propose anion solvophobicity as a molecular-level descriptor for rational design of electrolytes with increased efficiency for electrochemical processes limited by multivalent cation desolvation.

Boron: Its Role in Energy-Related Processes and Applications

Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy-efficient products has seen boron playing key roles in energy-related research, such as 1) activating and synthesizing energy-rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron-deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability-in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy-related processes and applications.

Beyond Typical Electrolytes for Energy Dense Batteries

The ever-rising demands for energy dense electrochemical storage systems have been driving interests in beyond Li-ion batteries such as those based on lithium and magnesium metals. These high energy density batteries suffer from several challenges, several of which stem from the flammability/volatility of the electrolytes and/or instability of the electrolytes with either the negative, positive electrode or both. Recently, hydride-based electrolytes have been paving the way towards overcoming these issues. Namely, highly performing solid-state electrolytes have been reported and several key challenges in multivalent batteries were overcome. In this review, the classes of hydride-based electrolytes reported for energy dense batteries are discussed. Future perspectives are presented to guide research directions in this field.

Photoelectron spectroscopy and computational investigations of the electronic structures and noncovalent interactions of cyclodextrin-closo-dodecaborate anion complexes χ-CD·B12X122− (χ = α, β, γ; X = H, F)

We report a joint negative ion photoelectron spectroscopy and computational study on the electronic structures and noncovalent interactions of a series of cyclodextrin-closo-dodecaborate dianion complexes, χ-CD·B₁₂X₁₂²⁻ (χ = α, β, γ; X = H, F).

Ion Agglomeration and Transport in MgCl2-Based Electrolytes for Rechargeable Magnesium Batteries

Magnesium halide salts are an exciting prospect as stable and high-performance electrolytes for rechargeable Mg batteries (RMBs). By nature of their complex equilibria, these salts exist in solution as a variety of electroactive species (EAS) in equilibrium with counterions such as AlCl₄^-. Here we investigated ion agglomeration and transport of several such EAS in MgCl₂ salts dissolved in ethereal solvents under both equilibrium and operating conditions using large-scale atomistic simulations. We found that the solute morphology is strongly characterized by the presence of clusters and is governed by the solvation structures of EAS. Specifically, the isotropic solvation of Mg²⁺ results in the slow formation of a bulky cluster, compared with chainlike analogues observed in the Cl-containing EAS such as Mg₂Cl₃⁺, MgCl⁺, and Mg₂Cl₂²⁺. We further illustrate these clusters can reduce the diffusivity of charge-carrying species in the MgCl₂-based electrolyte by at least an order of magnitude. Our findings for cluster formation, morphology, and kinetics can provide useful insight into the electrochemical reactions at the anode-electrolyte interface in RMBs.

The Power of Stoichiometry: Conditioning and Speciation of MgCl2/AlCl3 in Tetraethylene Glycol Dimethyl Ether-Based Electrolytes

In many Mg-based battery systems, the reversibility of Mg deposition and dissolution is lowered by parasitic formation processes of the electrolyte. Therefore, high Coulombic efficiencies of Mg deposition and dissolution are only achieved after several "conditioning" cycles. As this phenomenon is especially reported for AlCl₃-containing solutions, this study focuses on the "conditioning" mechanisms of MgCl₂/AlCl₃ and MgHMDS₂/AlCl₃ (HMDS = hexamethyldisilazide) in tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes. Electrochemical (cyclic voltammetry) and spectroscopic investigations (²⁷Al nuclear magnetic resonance spectroscopy, Raman spectroscopy, inductively coupled plasma optical emission spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy) reveal that cationic AlCl₂⁺ species in TEGDME-based electrolytes with an AlCl₃/MgCl₂ ratio higher than 1:1 corrode the Mg metal. According to a cementation reaction mechanism, the corrosion of Mg is accompanied with Al deposition. In effect, the consumption of Mg results in low Coulombic efficiencies of Mg deposition and dissolution during the electrolyte "conditioning". After understanding the mechanism of this process, we demonstrate that a careful adjustment of the stoichiometry in MgCl₂/AlCl₃ and MgHMDS₂/AlCl₃ in TEGDME formulations prevents Mg corrosion and results in "conditioning"-free, highly efficient Mg deposition and dissolution.

Colloidal Bismuth Nanocrystals as a Model Anode Material for Rechargeable Mg-Ion Batteries: Atomistic and Mesoscale Insights

At present, the technical progress of secondary batteries employing metallic magnesium as the anode material has been severely hindered due to the low oxidation stability of state-of-the-art Mg electrolytes, which cannot be used to explore high-voltage (>3 V versus Mg²⁺/Mg) cathode materials. All known electrolytes based on oxidatively stable solvents and salts, such as Mg(ClO₄)₂ and Mg bis(trifluoromethanesulfonimide), react with the metallic magnesium anode, forming a passivating layer at its surface and preventing the reversible plating and stripping of Mg. Therefore, in a near-term effort to extend the upper voltage limit in the exploration of future candidate Mg-ion battery cathode materials, bismuth anodes have attracted considerable attention due to their efficient magnesiation and demagnesiation alloying reaction in such electrolytes. In this context, we present colloidal Bi nanocrystals (NCs) as a model anode material for the exploration of cathode materials for rechargeable Mg-ion batteries. Bi NCs demonstrate a stable capacity of 325 mAh g^-1 over at least 150 cycles at a current density of 770 mA g^-1, which is among the most-stable performance of Mg-ion battery anode materials. First-principles crystal structure prediction methodologies and ex situ X-ray diffraction measurements reveal that the magnesiation of Bi NCs leads to the simultaneous formation of the low-temperature trigonal structure, α-Mg₃Bi₂, and the high-temperature cubic structure, β-Mg₃Bi₂, which sheds insight into the high stability of this reversible alloying reaction. Furthermore, small-angle X-ray scattering measurements indicate that although the monodispersed, crystalline nature of the Bi NCs is indeed disturbed during the first discharge step, no notable morphological or structural changes occur in the following electrochemical cycles. The cost-effective and facile synthesis of colloidal Bi NCs and their remarkably high electrochemical stability upon magnesiation make them an excellent model anode material with which to accelerate progress in the field of Mg-ion secondary batteries.

Multifunctional Additives Improve the Electrolyte Properties of Magnesium Borohydride Toward Magnesium-Sulfur Batteries

Highly reductive magnesium borohydride [Mg(BH₄)₂] is compatible with metallic Mg, making it a promising Mg-ion electrolyte for rechargeable Mg batteries. However, pure Mg(BH₄)₂ in ether-based solutions displays very limited solubility (0.01 M), low oxidative stability (<1.8 V vs Mg), and nucleophilic characteristic, all of which preclude its practical utilization for any battery applications. Herein, we present a multifunctional additive of tris(2 H-hexafluoroisopropyl)borate (THFPB) for preparing Mg(BH₄)₂-based electrolytes. By virtue of the strong electron-acceptor ability of the THFPB molecule, a transparent and high-concentration Mg(BH₄)₂/THFPB-diglyme (DGM) electrolyte (0.5 M, almost 50 times higher than that of the pristine Mg(BH₄)₂-DGM electrolyte) is first obtained, which shows dramatic performance improvements, including high ionic conductivity (3.72 mS cm^-1 at 25 °C) and high Mg plating/stripping Coulombic efficiency (>99%). The newly-generated active cation and anion species revealed by Raman, NMR and MS spectra, increase the electrochemical potential window from 1.8 V to 2.8 V vs Mg on stainless steel electrode, rendering electrolytes the ability to examine high voltage cathodes. More importantly, on account of the non-nucleophilicity of active electrolyte species, we present the first example of magnesium-sulfur (Mg-S) batteries using Mg(BH₄)₂-based electrolytes, which exhibit a high discharge capacity of 955.9 and 526.5 mA h g^-1 at the initial and 30th charge/discharge cycles, respectively. These achievements not only provide an efficient and specific strategy to eliminate the major roadblocks facing Mg(BH₄)₂-based electrolytes but also highlight the profound effect of functional additives on the electrochemical performances of unsatisfied Mg-ion electrolytes.

Ion Pairing and Diffusion in Magnesium Electrolytes Based on Magnesium Borohydride

One obstacle to realizing a practical, rechargeable magnesium-ion battery is the development of efficient Mg electrolytes. Electrolytes based on simple Mg(BH₄)₂ salts suffer from poor salt solubility and/or low conductivity, presumably due to strong ion pairing. Understanding the molecular-scale processes occurring in these electrolytes would aid in overcoming these performance limitations. Toward this goal, the present study examines the solvation, agglomeration, and transport properties of a family of Mg electrolytes based on the Mg(BH₄)₂ salt using classical molecular dynamics. These properties were examined across five different solvents (tetrahydrofuran and the glymes G1-G4) and at four salt concentrations ranging from the dilute limit up to 0.4 M. Significant and irreversible salt agglomeration was observed in all solvents at all nondilute Mg(BH₄)₂ concentrations. The degree of clustering observed in these divalent Mg systems is much larger than that reported for electrolytes containing monovalent cations, such as Li. The salt agglomeration rate and diffusivity of Mg²⁺ were both observed to correlate with solvent self-diffusivity: electrolytes using longer- (shorter-) chain solvents had the lowest (highest) Mg²⁺ diffusivity and agglomeration rates. Incorporation of Mg²⁺ into Mg²⁺-BH₄^- clusters significantly reduces the diffusivity of Mg²⁺ by restricting displacements to localized motion within largely immobile agglomerates. Consequently, diffusion is increasingly impeded with increasing Mg(BH₄)₂ concentration. These data are consistent with the solubility limitations observed experimentally for Mg(BH₄)₂-based electrolytes and highlight the need for strategies that minimize salt agglomeration in electrolytes containing divalent cations.

H2V3O8 Nanowires as High-Capacity Cathode Materials for Magnesium-Based Battery

Magnesium-based batteries have received much attention as promising candidates to next-generation batteries because of high volumetric capacity, low price, and dendrite-free property of Mg metal. Herein, we reported H₂V₃O₈ nanowire cathode with excellent electrochemical property in magnesium-based batteries. First, it shows a satisfactory magnesium storage ability with 304.2 mA h g^-1 capacity at 50 mA g^-1. Second, it possesses a high-voltage platform of ∼2.0 V vs Mg/Mg²⁺. Furthermore, when evaluated as a cathode material for magnesium-based hybrid Mg²⁺/Li⁺ battery, it exhibits a high specific capacity of 305.4 mA h g^-1 at 25 mA g^-1 and can be performed in a wide working temperature range (-20 to 55 °C). Notably, the insertion-type ion storage mechanism of H₂V₃O₈ nanowires in hybrid Mg²⁺/Li⁺ batteries are investigated by ex situ X-ray diffraction and Fourier transform infrared. This research demonstrates that the H₂V₃O₈ nanowire cathode is a potential candidate for high-performance magnesium-based batteries.

Unraveling the Magnesium-Ion Intercalation Mechanism in Vanadium Pentoxide in a Wet Organic Electrolyte by Structural Determination

Magnesium batteries have received attention as a type of post-lithium-ion battery because of their potential advantages in cost and capacity. Among the host candidates for magnesium batteries, orthorhombic α-V₂O₅ is one of the most studied materials, and it shows a reversible magnesium intercalation with a high capacity especially in a wet organic electrolyte. Studies by several groups during the last two decades have demonstrated that water plays some important roles in getting higher capacity. Very recently, proton intercalation was evidenced mainly using nuclear resonance spectroscopy. Nonetheless, the chemical species inserted into the host structure during the reduction reaction are still unclear (i.e., Mg(H₂O)_n²⁺, Mg(solvent, H₂O)_n²⁺, H⁺, H₃O⁺, H₂O, or any combination of these). To characterize the intercalated phase, the crystal structure of the magnesium-inserted phase of α-V₂O₅, electrochemically reduced in 0.5 M Mg(ClO₄)₂ + 2.0 M H₂O in acetonitrile, was solved for the first time by the ab initio method using powder synchrotron X-ray diffraction data. The structure was tripled along the b-axis from that of the pristine V₂O₅ structure. No appreciable densities of elements were observed other than vanadium and oxygen atoms in the electron density maps, suggesting that the inserted species have very low occupancies in the three large cavity sites of the structure. Examination of the interatomic distances around the cavity sites suggested that H₂O, H₃O⁺, or solvated magnesium ions are too big for the cavities, leading us to confirm that the intercalated species are single Mg²⁺ ions or protons. The general formula of magnesium-inserted V₂O₅ is Mg_0.17H_xV₂O₅, (0.66 ≤ x ≤ 1.16). Finally, density functional theory calculations were carried out to locate the most plausible atomic sites of the magnesium and protons, enabling us to complete the structure modeling. This work provides an explicit answer to the question about Mg intercalation into α-V₂O₅.

Influence of cations in lithium and magnesium polysulphide solutions: dependence of the solvent chemistry

The disproportionation and dissociation equilibria of chemically prepared “Li₂S₈” and “MgS₈” solutions are studied in a variety of solvents.

Sustainability and in situ monitoring in battery development

The development of improved rechargeable batteries represents a major technological challenge for this new century, as batteries constitute the limiting components in the shift from petrol (gasoline) powered to electric vehicles, while also enabling the use of more renewable energy on the grid. To minimize the ecological implications associated with their wider use, we must integrate sustainability of battery materials into our research endeavours, choosing chemistries that have a minimum footprint in nature and that are more readily recycled or integrated into a full circular economy. Sustainability and cost concerns require that we greatly increase the battery lifetime and consider second lives for batteries. As part of this, we must monitor the state of health of batteries continuously during operation to minimize their degradation. It is thus important to push the frontiers of operando techniques to monitor increasingly complex processes. In this Review, we will describe key advances in both more sustainable chemistries and operando techniques, along with some of the remaining challenges and possible solutions, as we personally perceive them.

Complex metal borohydrides: multifunctional materials for energy storage and conversion

With the limited supply of fossil fuels and their adverse effect on the climate and the environment, it has become a global priority to seek alternate sources of energy that are clean, abundant, and sustainable. While sources such as solar, wind, and hydrogen can meet the world's energy demand, considerable challenges remain to find materials that can store and/or convert energy efficiently. This topical review focuses on one such class of materials, namely, multi-functional complex metal borohydrides that not only have the ability to store sufficient amount of hydrogen to meet the needs of the transportation industry, but also can be used for a new generation of metal ion batteries and solar cells. We discuss the material challenges in all these areas and review the progress that has been made to address them, the issues that still need to be resolved and the outlook for the future.

Mapping the Challenges of Magnesium Battery

Rechargeable Mg battery has been considered a major candidate as a beyond lithium ion battery technology, which is apparent through the tremendous works done in the field over the past decades. The challenges for realization of Mg battery are complicated, multidisciplinary, and the tremendous work done to overcome these challenges is very hard to organize in a regular review paper. Additionally, we claim that organization of the huge amount of information accumulated by the great scientific progress achieved by various groups in the field will shed the light on the unexplored research domains and give clear perspectives and guidelines for next breakthrough to take place. In this Perspective, we provide a convenient map of Mg battery research in a form of radar chart of Mg electrolytes, which evaluates the electrolyte under the important components of Mg batteries. The presented radar charts visualize the accumulated knowledge on Mg battery and allow for navigation of not only the current research state but also future perspective of Mg battery at a glance.

Performance study of magnesium-sulfur battery using a graphene based sulfur composite cathode electrode and a non-nucleophilic Mg electrolyte

Here we report for the first time the development of a Mg rechargeable battery using a graphene-sulfur nanocomposite as the cathode, a Mg-carbon composite as the anode and a non-nucleophilic Mg based complex in tetraglyme solvent as the electrolyte. The graphene-sulfur nanocomposites are prepared through a new pathway by the combination of thermal and chemical precipitation methods. The Mg/S cell delivers a higher reversible capacity (448 mA h g(-1)), a longer cyclability (236 mA h g(-1) at the end of the 50(th) cycle) and a better rate capability than previously described cells. The dissolution of Mg polysulfides to the anode side was studied by X-ray photoelectron spectroscopy. The use of a graphene-sulfur composite cathode electrode, with the properties of a high surface area, a porous morphology, a very good electronic conductivity and the presence of oxygen functional groups, along with a non-nucleophilic Mg electrolyte gives an improved battery performance.