Determining Factor on the Polarization Behavior of Magnesium Deposition for Magnesium Battery Anode

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.

Chloride-Free Electrolytes for High-Voltage Magnesium Metal Batteries: Challenges, Strategies, and Perspectives

The demand for high-energy-density and safe energy storage devices has spurred increasing interest in high-voltage rechargeable magnesium batteries (RMB). As electrolytes are the bridge connecting the cathode and anode materials, the development of high-voltage electrolytes is the key factor in realizing high-voltage RMBs. This concept presents an overview of three chloride-free electrolyte systems with wide electrochemical windows, together with the degradation mechanisms and modification strategies at the anode/electrolyte interphase. Finally, future directions in stabilizing Mg anodes and realizing high-voltage RMBs are highlighted.

Tailoring a Hybrid Functional Layer for Mg Metal Anodes in Conventional Electrolytes with a Low Overpotential

The development of high-voltage Mg metal batteries is hampered by the incompatibility between a Mg metal anode and conventional electrolyte, leading to a high overpotential for Mg plating/stripping processes. In this work, we tailored a hybrid functional layer consisting of Bi/MgCl₂/polytetrahydrofuran (PTHF) by an in situ THF polyreaction during the reaction of the Mg anode with BiCl₃ solution. The introduction of PTHF inhibits the growth of Bi particles and fills the layer interstice with MgCl₂-containing PTHF, improving the structural integrity of the functional layer and insulation between the electrolyte and Mg anode. As a result, compared to a simply modified Bi/MgCl₂ layer, the Bi/MgCl₂/PTHF functional layer exhibits a lower polarization voltage of 0.25 V and longer cycling life of more than 2000 h at 0.1 mA cm^-2. Mechanism analysis shows that Mg is plated on the surface of Bi particles within the layer. The Mo₆S₈/Mg full battery with the hybrid functional layer achieved a low voltage hysteresis of ∼0.25 V and long cycling life over 500 cycles at 50 mA g^-1. This work provides a facile and effective hybrid functional layer strategy to realize Mg metal batteries in conventional electrolytes.

Evolution of the Dynamic Solid Electrolyte Interphase in Mg Electrolytes for Rechargeable Mg-Ion Batteries

Formation and evolution of the microscopic solid electrolyte interphase (SEI) at the Mg electrolyte/electrode interface are less reported and need to be completely understood to overcome the compatibility challenges at the Mg anode-electrolyte. In this paper, SEI evolution at the Mg electrolyte/electrode interface is investigated via an in situ electrochemical quartz crystal microbalance with dissipation mode (EQCM-D), electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectrometry (FTIR). Results reveal remarkably different interfacial evolutions for the two Mg electrolyte systems that are studied, a non-halogen Mg(TFSI)₂ electrolyte in THF with DMA as a cosolvent (nhMg-DMA electrolyte) versus a halogen-containing all-phenyl complex (APC) electrolyte. The nhMg-DMA electrolyte reports a minuscule SEI formation along with a significant Coulomb loss at the initial electrochemical cycles owing to an electrolyte reconstruction process. Interestingly, a more complicated SEI growth is observed at the later electrochemical cycles accompanied by an improved reversible Mg deposition attributed to the newly formed coordination environment with Mg²⁺ and ultimately leads to a more homogeneous morphology for the electrochemically deposited Mg⁰, which maintains a MgF₂-rich interface. In contrast, the APC electrolyte shows an extensive SEI formation at its initial electrochemical cycles, followed by a SEI dissolution process upon electrochemical cycling accompanied by an improved coulombic efficiency with trace water and chloride species removed. Therefore, it leads to SEI stabilization progression upon further electrochemical cycling, resulting in elevated charge transport kinetics and superior purity of the electrochemically deposited Mg⁰. These outstanding findings augment the understanding of the SEI formation and evolution on the Mg interface and pave a way for a future Mg-ion battery design.

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.

Self-Driven Electrochromic Window System Cu/WOx-Al3+/GR with Dynamic Optical Modulation and Static Graph Display Functions

Electrochromic devices with unique advantages of electrical/optical bistability are highly desired for energy-saving and information storage applications. Here, we put forward a self-driven Al-ion electrochromic system, which utilizes WO_x films, Cu foil, and graphite rod as electrochromic optical modulation and graph display electrodes, coloration potential supplying electrodes, and bleaching potential supplying electrodes, respectively. The inactive Cu electrode can not only realize the effective Al³⁺ cation intercalation into electrochromic WO_x electrodes but also eliminate the problem of metal anode consumption. The electrochromic WO_x electrodes cycled in Al³⁺ aqueous media exhibit a wide potential window (∼1.5 V), high coloration efficiency (36.0 cm²/C), and super-long-term cycle stability (>2000 cycles). The dynamic optical modulation and static graph display function can be achieved independently only by switching the electrode connection mode, thus bringing more features to this electrochromic system. For a large-area electrochromic system (10 × 10 cm²), the absolute transmittance value in its color-neutral state can reach about 41% (27%) at 633 nm (780 nm) by connecting the Cu and WO_x electrodes for 140 s. The original transparent state can be readily recovered by replacing the Cu foil with the graphite rod. This work throws light on next-generation electrochromic applications for optical/thermal modulation, privacy protection, and information display.

Organo-metallic electrolyte additive for regulating hydrogen evolution and self-discharge in Mg–air aqueous battery

Metal–air battery with cutting-edge electrolyte modification technologies.

Modeling of Electron-Transfer Kinetics in Magnesium Electrolytes: Influence of the Solvent on the Battery Performance

The performance of rechargeable magnesium batteries is strongly dependent on the choice of electrolyte. The desolvation of multivalent cations usually goes along with high energy barriers, which can have a crucial impact on the plating reaction. This can lead to significantly higher overpotentials for magnesium deposition compared to magnesium dissolution. In this work we combine experimental measurements with DFT calculations and continuum modelling to analyze Mg deposition in various solvents. Jointly, these methods provide a better understanding of the electrode reactions and especially the magnesium deposition mechanism. Thereby, a kinetic model for electrochemical reactions at metal electrodes is developed, which explicitly couples desolvation to electron transfer and, furthermore, qualitatively takes into account effects of the electrochemical double layer. The influence of different solvents on the battery performance is studied for the state-of-the-art magnesium tetrakis(hexafluoroisopropyloxy)borate electrolyte salt. It becomes apparent that not necessarily a whole solvent molecule must be stripped from the solvated magnesium cation before the first reduction step can take place. For Mg reduction it seems to be sufficient to have one coordination site available, so that the magnesium cation is able to get closer to the electrode surface. Thereby, the initial desolvation of the magnesium cation determines the deposition reaction for mono-, tri- and tetraglyme, whereas the influence of the desolvation on the plating reaction is minor for diglyme and tetrahydrofuran. Overall, we can give a clear recommendation for diglyme to be applied as solvent in magnesium electrolytes.

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

The electrolyte based on magnesium bis(diisopropyl)amide (MBA), a low-cost and non-nucleophilic organic magnesium salt, is proposed to be an admirable alternative for rechargeable magnesium batteries but suffers from limited ionic conductivity and an inferior electrochemical window in the commonly used ether solvents. In this work, the 1-butyl-1-methylpiperidinium bis(trifluoromethyl sulfonyl)imide (PP₁₄TFSI) ionic liquid as the cosolvent of tetrahydrofuran (THF) in chlorine-free MBA-based electrolytes has been first demonstrated to remarkably improve the ionic conductivity and broaden the oxidative stable potential (2.2 V vs Mg/Mg²⁺) on stainless steel. Reversible Mg electrochemical plating/stripping with a low overpotential below 200 mV and ca. 90% Coulombic efficiency are obtained. The current density of Mg plating/stripping is increased 238 times after the addition of PP₁₄TFSI, where the mechanism of competitive coordination of TFSI^- making an easier Mg plating/stripping is proposed theoretically. The MBA-2AlF₃ electrolyte with a ratio-optimized THF/PP₁₄TFSI cosolvent exhibits good compatibility with the Mo₆S₈ cathode. Furthermore, the Se@pPAN|Mg full cell exhibits an initial capacity of 447.8 mAh g^-1 and as low as ∼0.66% capacity decay per cycle for more than 70 cycles at 0.2 C with the synergy of LiTFSI additives. The facile modification strategy of ionic liquid in the MBA-based electrolyte sheds inspiring light on exploring non-nucleophilic and chlorine-free electrolytes for practical rechargeable magnesium batteries.

Formation of an Artificial Mg2+-Permeable Interphase on Mg Anodes Compatible with Ether and Carbonate Electrolytes

Rechargeable Mg-ion batteries typically suffer from either rapid passivation of the Mg anode or severe corrosion of the current collectors by halogens within the electrolyte, limiting their practical implementation. Here, we demonstrate the broadly applicable strategy of forming an artificial solid electrolyte interphase (a-SEI) layer on Mg to address these challenges. The a-SEI layer is formed by simply soaking Mg foil in a tetraethylene glycol dimethyl ether solution containing LiTFSI and AlCl₃, with Fourier transform infrared and ultraviolet-visible spectroscopy measurements revealing spontaneous reaction with the Mg foil. The a-SEI is found to mitigate Mg passivation in Mg(TFSI)₂/DME electrolytes with symmetric cells exhibiting overpotentials that are 2 V lower compared to when the a-SEI is not present. This approach is extended to Mg(ClO₄)₂/DME and Mg(TFSI)₂/PC electrolytes to achieve reversible Mg plating and stripping, which is not achieved with bare electrodes. The interfacial resistance of the cells with a-SEI protected Mg is found to be two orders of magnitude lower than that with bare Mg in all three of the electrolytes, indicating the formation of an effective Mg-ion transporting interfacial structure. X-ray absorption and photoemission spectroscopy measurements show that the a-SEI contains minimal MgCO₃, MgO, Mg(OH)₂, and TFSI^-, while being rich in MgCl₂, MgF₂, and MgS, when compared to the passivation layer formed on bare Mg in Mg(TFSI)₂/DME.

Progress and Perspective on Rechargeable Magnesium-Sulfur Batteries

Rechargeable magnesium-sulfur (Mg-S) batteries are emerging as a promising candidate for next-generation energy storage technologies owing to their prominent advantages in terms of high volumetric energy density, low cost, and enhanced safety. However, their practical implementation is facing great challenges in finding electrolytes that can fulfill a multitude of rigorous requirements along with efficient sulfur cathodes and magnesium anodes. This review highlights electrolyte design for reliable Mg-S batteries in terms of efficient Mg-based salt construction (cation/anion design of organomagnesium salt-based electrolytes, optimization of all inorganic salt-based electrolytes and choosing of simple salt-based electrolytes), suitable solvent selection, and strategies for confronting corrosivity of Mg electrolytes. Before the comprehensive overview of the research status of Mg-based electrolytes, the understanding of Mg-S electrochemistry and views on the recent progress and potential strategies for high-performance S-based cathode and Mg anode are also provided for a holistic insight into Mg-S systems. At the end, the perspectives on the possible research directions for constructing high performance practical Mg-S batteries are also shared.

Critical Issues of Fluorinated Alkoxyborate-Based Electrolytes in Magnesium Battery Applications

The development of noncorrosive but highly efficient electrolytes has been a long-standing challenge in magnesium rechargeable battery (MRB) research fields. As fluorinated alkoxyborate-based electrolytes have overcome serious problems associated with conventional electrolytes, they are regarded as promising for practical MRB applications. An electrolyte containing representative magnesium fluorinated alkoxyborate Mg[B(HFIP)₄]₂ ([B(HFIP)₄]: tetrakis(hexafluoroisopropoxy) borate) was prepared through general synthetic routes using Mg(BH₄)₂; however, it shows poor electrochemical magnesium deposition/dissolution behavior. Herein, we report an alternative synthetic route of highly reactive Mg[B(HFIP)₄]₂ and several critical issues associated with the use of Mg[B(HFIP)₄]₂/glyme electrolytes in MRBs. The cycling performance of the electrolytes as well as the synthetic reproducibility of the salt was significantly improved upon adopting a transmetalation reaction between certain magnesium and boron compounds for the salt preparation. Despite the outstanding electrochemical activity of Mg[B(HFIP)₄]₂/glyme, the electrolytes were unstable with the magnesium metal. The remarkably high dissociativity of Mg[B(HFIP)₄]₂ in glyme solutions and the resulting enhanced induction interaction of Mg²⁺ with coordinated glymes make the solutions reductively unstable. Surface passivation by [TFSA]-based electrolytes (TFSA: bis(trifluoromethanesulfonyl)amide) effectively suppressed the decomposition of Mg[B(HFIP)₄]₂/glyme electrolytes. This passivation simultaneously caused a large overpotential for electrochemical cycling. The short-circuiting of the cells upon repeated deposition/dissolution cycling is rather problematic. Here, the findings disclose the issues of fluorinated alkoxyborate-based electrolyte solutions that should be resolved for practical MRB materialization. We also emphasize the importance of systematic strategies in manipulating the electrolytes and interfaces as well as base magnesium metal based on each appropriate approach.