A Chlorine-Free Electrolyte Based on Non-nucleophilic Magnesium Bis(diisopropyl)amide and Ionic Liquid for 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.

Recent Advances in Electrolytes for Magnesium Batteries: Bridging the gap between Chemistry and Electrochemistry

Rechargeable magnesium batteries (RMBs) have the potential to provide a sustainable and long-term solution for large-scale energy storage due to high theoretical capacity of magnesium (Mg) metal as an anode, its competitive redox potential (Mg/Mg2+:-2.37 V vs. SHE) and high natural abundance. To develop viable magnesium batteries with high energy density, the electrolytes must meet a range of requirements: high ionic conductivity, wide electrochemical potential window, chemical compatibility with electrode materials and other battery components, favourable electrode-electrolyte interfacial properties and cost-effective synthesis. In recent years, significant progress in electrolyte development has been made. Herein, a comprehensive overview of these advancements is presented. Beginning with the early developments, we particularly focus on the chemical aspects of the electrolytes and their correlations with electrochemical properties. We also highlight the design of new anions for practical electrolytes, the use of electrolyte additives to optimize anode-electrolyte interfaces and the progress in polymer electrolytes.

Ionic Liquid Boosting the Electrochemical Stability of a Poly(1,3-dioxolane) Gel Electrolyte for High-voltage Solid-State Lithium Batteries

Poor interfacial contact between solid-state electrolytes and electrodes limits high-voltage performance of solid-state lithium batteries. A new gel electrolyte is proposed via in-situ polymerization, incorporating fluoroethylene carbonate (FEC) solvent and ionic liquid1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide (PP14 TFSI). This combination synergistically enhances Li ion transport, achieving a transfer number of 0.58 and improved electrochemical performance. FEC protects the Al current collectors from LiPF6 corrosion and promotes a protective interfacial layer formation. PP14 TFSI improves interfacial contact and provides stable components. An interface layer of fluorine and nitrogen composites forms, preventing side reactions. LiCoO2 ||PPE||Li cell exhibits robust cycling stability at 4.45 V, retaining ~80 % capacity after 200 cycles at room temperature with 0.2 C and 1 C rates, showing increased coulombic efficiency. NCM811||PPE||Li cell also displays exceptional cycling. In-situ polymerization and FEC-ionic liquid coordination enable high-voltage solid-state lithium metal batteries for practical use.

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.

Energy Applications of Ionic Liquids: Recent Developments and Future Prospects

Ionic liquids (ILs) consisting entirely of ions exhibit many fascinating and tunable properties, making them promising functional materials for a large number of energy-related applications. For example, ILs have been employed as electrolytes for electrochemical energy storage and conversion, as heat transfer fluids and phase-change materials for thermal energy transfer and storage, as solvents and/or catalysts for CO2 capture, CO2 conversion, biomass treatment and biofuel extraction, and as high-energy propellants for aerospace applications. This paper provides an extensive overview on the various energy applications of ILs and offers some thinking and viewpoints on the current challenges and emerging opportunities in each area. The basic fundamentals (structures and properties) of ILs are first introduced. Then, motivations and successful applications of ILs in the energy field are concisely outlined. Later, a detailed review of recent representative works in each area is provided. For each application, the role of ILs and their associated benefits are elaborated. Research trends and insights into the selection of ILs to achieve improved performance are analyzed as well. Challenges and future opportunities are pointed out before the paper is concluded.

High-Energy Aqueous Magnesium Ion Batteries with Capacity-Compensation Evolved from Dynamic Copper Ion Redox

The low specific capacity and low voltage plateau are significant challenges in the advancement of practical magnesium ion batteries (MIBs). Here, a superior aqueous electrolyte combining with a copper foam interlayer between anode and separator is proposed to address these drawbacks. Notably, with the dynamic redox of copper ions, the weakened solvation of Mg²⁺ cations in the electrolyte and the enhanced electronic conductivity of anode, which may offer effective capacity-compensation to the 3,4,9,10-perylenetetracarboxylic diimide (PTCDI)-Mg conversion reactions during the long-term cycles. As a result, the unique MIBs using expanded graphite cathode coupled with PTCDI anode demonstrate exceptional performance with an ultra-high capacity (205 mAh g^-1 , 243 Wh kg^-1 at 5 A g^-1 ) as well as excellent cycling stability after 600 cycles and rate capability (138 mAh g^-1 , 81 Wh kg^-1 at 10 A g^-1 ).

Dimethyl sulfoxide as a function additive on halogen-free electrolyte for magnesium battery application

Practical Mg batteries still face significant challenges in their development, like the lack of simple compatible electrolytes, self-discharge, the rapid passivation of the Mg anode, and the slow conversion reaction pathway. Here, we propose a simple halogen-free electrolyte (HFE) based on magnesium nitrate (Mg(NO3)2), magnesium triflate Mg(CF3SO3)2, and succinonitrile (SN) dissolved in acetonitrile (ACN)/tetraethylene glycol dimethyl ether (G4) cosolvents, with dimethyl sulfoxide as a functional additive. The addition of DMSO to the HFE changes the interfacial structure at the magnesium anode surface and facilitates the transport of magnesium ions. The as-prepared electrolyte shows high conductivity (σ b = 4.48 × 10-5, 6.52 × 10-5 and 9.41 × 10-5 S cm-1 at 303, 323, and 343 K, respectively) and a high ionic transference number (tmg +2 = 0.91/0.94 at room temperature/55 °C) for the matrix containing 0.75 ml of DMSO. Also, the cell with 0.75 ml of DMSO shows high oxidation stability, a very low overpotential, and steady Mg stripping/plating up to 100 h. Postmortem analysis of pristine Mg and Mg anodes extracted from disassembled Mg/HFE/Mg and Mg/HFE_0.75 ml DMSO/Mg cells after stripping/plating reveals the role of DMSO in improving Mg-ion passage through HFE by evolving the anode/electrolyte interface at the Mg surface. Further optimization of this electrolyte is expected to achieve excellent performance and good cycle stability when applied to the magnesium battery in future work.

Rational Design Strategy of Novel Energy Storage Systems: Toward High-Performance Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) are promising candidates to replace currently commercialized lithium-ion batteries (LIBs) in large-scale energy storage applications owing to their merits of abundant resources, low cost, high theoretical volumetric capacity, etc. However, the development of RMBs is still facing great challenges including the incompatibility of the electrolyte and the lack of suitable cathode materials with high reversible capacity and fast kinetics of Mg²⁺ . While tremendous efforts have been made to explore compatible electrolytes and appropriate electrode materials, the rational design of unconventional Mg-based battery systems is another effective strategy for achieving high electrochemical performance. This review specifically discusses the recent research progress of various Mg-based battery systems. First, the optimization of electrolyte and electrode materials for conventional RMBs is briefly discussed. Furthermore, various Mg-based battery systems, including Mg-chalcogen (S, Se, Te) batteries, Mg-halogen (Br₂ , I₂ ) batteries, hybrid-ion batteries, and Mg-based dual-ion batteries are systematically summarized. This review aims to provide a comprehensive understanding of different Mg-based battery systems, which can inspire latecomers to explore new strategies for the development of high-performance and practically available RMBs.

Ether-Water Hybrid Electrolyte Contributing to Excellent Mg Ion Storage in Layered Sodium Vanadate

Magnesium ion batteries have potential for large-scale energy storage. However, the high charge density of Mg²⁺ ions establishes a strong intercalation energy barrier in host materials, causing sluggish diffusion kinetics and structural degradation. Here, we report that the kinetic and dissolution issues connected to cathode materials can be resolved simultaneously using a tetraethylene glycol dimethyl ether (TEGDME)-water hybrid electrolyte. The lubricating and shielding effect of water solvent could boost the swift transport of Mg²⁺, contributing to a high diffusion coefficient within the sodium vanadate (NaV₈O₂₀·nH₂O) cathode. Meanwhile, the organic TEGDME component can coordinate with water to diminish its activity, thus providing the hybrid electrolyte with a broad electrochemical window of 3.9 V. More importantly, the TEGDME preferentially amassed at the interface, leading to a robust cathode electrolyte interface layer that suppresses the dissolution of vanadium species. Consequently, the NaV₈O₂₀·nH₂O cathode achieved a specific capacity of 351 mAh g^-1 at 0.3 A g^-1 and a long cycle life of 1000 cycles in this hybrid electrolyte. A mechanism study revealed the reversible interaction of Mg²⁺ during cycles. This organic water hybrid electrolyte is effective for overcoming the difficulty of multivalent ion storage.