Facile and Economic Synthesis of Robust Non-Nucleophilic Electrolyte for High-Performance Rechargeable Magnesium Batteries

Iodine Boosted Fluoro-Organic Borate Electrolytes Enabling Fluent Ion-Conductive Solid Electrolyte Interphase for High-Performance Magnesium Metal Batteries

Rechargeable magnesium batteries are regarded as a promising multi-valent battery system for low-cost and sustainable energy storage applications. Boron-based organic magnesium salts with terminal substituent fluorinated anions (Mg[B(ORF)4]2, RF=fluorinated alkyl) have exhibited impressive electrochemical stability and oxidative stability. Nevertheless, their deployment is hindered by the complicated synthesis routes and the surface passivation of Mg metal anode. Herein, we report the design of an advanced electrolyte formulation comprised of tri(hexafluoroisopropyl) borate (B(HFIP)3) and iodine (I2) in 1,2-dimethoxyethane (DME) solvent, which eventually convert into a Mg[B(HFIP)4]2/DME-MgI2 electrolyte system upon interacting with Mg anode. The Mg anode reacts with I2 and the electron-accepting B(HFIP)3, leading to the in situ formation of a solid-electrolyte interphase layer composed of MgF2 and MgI2 species that can facilitate fast and stable Mg plating/stripping. Compared with the pristine Mg[B(HFIP)4]2/DME electrolyte, the Mg[B(HFIP)4]2/DME-MgI2 electrolyte exhibited superior electrochemical performance including an ultra-low overpotential (~80 mV), high Coulombic efficiency and a long-cycling period over 1500 h. In result, the rechargeable magnesium batteries with Mg[B(HFIP)4]2/DME-MgI2 electrolyte and Chevrel-phase Mo6S8 cathode show outstanding compatibility, rapid kinetics, and stable cyclability for over 1200 cycles, surpassing all previously reported boron-based electrolytes. This work introduces a promising halogen-enhancement strategy for boron-based Mg-ion electrolytes, with the overarching goal of establishing favorable solid-electrolyte interphases that are pivotal for the advancement and optimization of multi-valent secondary batteries.

Enhanced Durability of Ca Metal Battery with Dual Salt: Synergistic Effect on Solid Electrolyte Interphase and Solvation Structure for Improved Electrodeposition

The use of Ca metal in battery technology is a promising approach owing to its high energy density and sustainability. However, the increased battery resistance during extended cycling significantly narrows its application range. This study aimed to improve the long-term stability of Ca deposition by employing a dual-salt strategy based on calcium monocarborane, Ca(CB11H12)2, which demonstrated favorable Ca deposition characteristics as a single-salt electrolyte. The addition of LiBr to Ca(CB11H12)2 resulted in a low battery resistance even after 200 h of cycling in contrast to the single-salt electrolyte whose resistance gradually increased. The dual-salt effect was examined by investigating solvation structures and electrolyte decomposition products. The introduction of Li cations into Ca(CB11H12)2 significantly altered the solid electrolyte interphase composition, effectively mitigating the increase in resistance during cycling. Furthermore, the inclusion of LiBr salt induced substantial changes in the solvation structures, reducing the number of solvent molecules surrounding Ca2+ ions. This transformation was accompanied by a noticeable decrease in the amount of CaCO3 among the electrolyte decomposition products and simultaneous increase in the polymer-based solid electrolyte interphase. The application of the dual-salt electrolyte comprising Ca(CB11H12)2 and LiBr demonstrated robust cycling stability over extended periods in two-electrode cells utilizing Ca metal anodes and anthraquinone-based organic cathodes. The capacity retention remained at 75% after 200 cycles, indicating the highest performance observed among the previously reported batteries containing Ca metal anodes and organic cathodes in two-electrode cell systems. This study highlights the efficacy of the dual-salt approach based on the stability of Ca(CB11H12)2 and its exceptional ability to enhance the long-term stability of Ca metal deposition, thereby significantly improving the practical application prospects of Ca-based batteries.

Catalyzing Desolvation at Cathode-Electrolyte Interface Enabling High-Performance Magnesium-Ion Batteries

Magnesium ion batteries (MIBs) are expected to be the promising candidates in the post-lithium-ion era with high safety, low cost and almost dendrite-free nature. However, the sluggish diffusion kinetics and strong solvation capability of the strongly polarized Mg2+ are seriously limiting the specific capacity and lifespan of MIBs. In this work, catalytic desolvation is introduced into MIBs for the first time by modifying vanadium pentoxide (V2O5) with molybdenum disulfide quantum dots (MQDs), and it is demonstrated via density function theory (DFT) calculations that MQDs can effectively lower the desolvation energy barrier of Mg2+, and therefore catalyze the dissociation of Mg2+-1,2-Dimethoxyethane (Mg2+-DME) bonds and release free electrolyte cations, finally contributing to a fast diffusion kinetics within the cathode. Meanwhile, the local interlayer expansion can also increase the layer spacing of V2O5 and speed up the magnesiation/demagnesiation kinetics. Benefiting from the structural configuration, MIBs exhibit superb reversible capacity (≈300 mAh g-1 at 50 mA g-1) and unparalleled cycling stability (15 000 cycles at 2 A g-1 with a capacity of ≈70 mAh g-1). This approach based on catalytic reactions to regulate the desolvation behavior of the whole interface provides a new idea and reference for the development of high-performance MIBs.

Chloride-Free Electrolyte Based on Tetrabutylammonium Triflate Additive for Extended Anodic Stability in Magnesium Batteries

Rechargeable magnesium batteries (RMBs) have been proposed as a promising alternative to currently commercialized lithium-ion batteries. However, Mg anode passivation in conventional electrolytes necessitates the use of highly corrosive Cl- ions in the electrolyte. Herein for the first time, we design a chloride-free electrolyte for RMBs with magnesium bis(hexamethyldisilazide) (Mg(HMDS)2) and magnesium triflate (Mg(OTf)2) as the main salts and tetrabutylammonium triflate (TBAOTf) as an additive. The TBAOTf additive improved the dissolution of Mg salts, consequently enhancing the charge-carrying species in the electrolyte. COMSOL studies further revealed desirable Mg growth in our modulated electrolyte, substantiated by homogeneous electric flux distribution across the electrolyte-electrode interface. Post-mortem chemical composition analysis uncovered a MgF2-rich solid electrolyte interphase (SEI) that facilitated exceptional Mg deposition/dissolution reversibility. Our study illustrates a highly promising strategy for synthesizing a corrosion-free and reversible Mg battery electrolyte with a widened anodic stability window of up to 4.43 V.

Research development on electrolytes for magnesium-ion batteries

Magnesium-ion batteries (MIBs) are considered strong candidates for next-generation energy-storage systems owing to their high theoretical capacity, divalent nature and the natural abundancy of magnesium (Mg) resources on Earth. However, the development of MIBs has been mainly limited by the incompatibility of Mg anodes with several Mg salts and conventional organic-liquid electrolytes. Therefore, one major challenge faced by MIBs technology lies on developing safe electrolytes, which demonstrate appropriate electrochemical voltage window and compatibility with Mg anode. This review discusses the development of MIBs from the point-of-view of the electrolyte syntheses. A systematic assessment of promising electrolyte design strategies is proposed including liquid and solid-state electrolytes. Liquid-based electrolytes have been largely explored and can be categorized by solvent-type: organic solvent, aqueous solvent, and ionic-liquids. Organic-liquid electrolytes usually present high electrochemical and chemical stability but are rather dangerous, while aqueous electrolytes present high ionic conductivity and eco-friendliness but narrow electrochemical stability window. Some ionic-liquid electrolytes have proved outstanding performance but are fairly expensive. As alternative to liquid electrolytes, solid-state electrolytes are increasingly attractive to increase energy density and safety. However, improving the ionic conductivity of Mg ions in these types of electrolytes is extremely challenging. We believe that this comprehensive review will enable researchers to rapidly grasp the problems faced by electrolytes for MIBs and the electrolyte design strategies proposed to this date.

De-Ammonium Ba0.18V2O4.95/NH4V4O10 Film Electrodes as High-Performance Cathode Materials for Magnesium-Ion Batteries

Magnesium-ion batteries (MIBs) have been pushed into the research boom in the post-lithium-ion batteries era due to their low cost, no dendrite hazard, and high capacity. However, finding suitable cathode materials to improve the slow kinetics of Mg2+ is an ongoing challenge. In this work, Ba0.18V2O4.95/NH4V4O10 film electrodes were grown in one step on indium tin oxide (ITO) conductive glass using a low-temperature liquid-phase deposition method. Temperature was used as the probe condition, and it was concluded that the films annealed at 400 °C had suitable crystallinity and de-ammonium lattice space. At lower current density, with 0.5 M Mg(ClO4)2/PC as the electrolyte, it exhibited an initial discharge capacity of 130.99 mA h m-2 at 210 mA m-2 and 106.52% capacity retention after 100 cycles. In addition, it exhibited excellent electrochemical performance in long-term cycling (92.98% capacity retention after 300 cycles at 600 mA m-2). According to the results of ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM), the removal of NH4+ created more lattice space, assisting Ba0.18V2O4.95 to increase the transfer channels of Mg2+, providing more active sites to promote diffusion kinetics (the average DMg2+ was 2.07 × 10-12 cm2 s-1) and specific capacity. Therefore, these film electrodes for scalable Mg2+ storage are promising MIB cathode candidates that exhibit good performance advantages in storage applications.

Energy Storage Mechanism of C12-3-3 with High-Capacity and High-Rate Performance for Li/Mg Batteries

The low specific capacity and Mg non-affinity of graphite limit the energy density of ion rechargeable batteries. Here, we first identify that the monolayer C12-3-3 in sp2-sp3 carbon hybridization with high Li/Mg affinity is an appropriate anode material for Li-ion batteries and Mg-ion batteries via the first-principles simulations. The monolayer C12-3-3 can achieve high specific capacities of 1181 mAh/g for Li and 739 mAh/g for Mg, higher than those of most previous anodes. The Li storage reaction is an "adsorption-conversion-intercalation mechanism", while the Mg storage reaction is an "adsorption mechanism". The 2D carbon material of C12-3-3 displays fast diffusion kinetics with low diffusion barriers of 0.41 eV for Li and 0.21 eV for Mg. As a new carbon-based anode material, the monolayer C12-3-3 will promote the practical application of batteries with high-capacity and high-rate performance.