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

Co-solvent strategy for rechargeable post-lithium metal batteries

The potential increase in cost of lithium-ion batteries owing to the limited supply of lithium has prompted investigations into alternative and complementary rechargeable batteries that use post-lithium charge carriers with higher elemental abundance. However, achieving highly reversible post-lithium metal anodes with sufficient kinetics remains challenging. The addition of co-solvents to conventional electrolytes is emerging as an important strategy to resolve these issues. In this Perspective, we discuss the progress of the co-solvent strategy for sodium, potassium, magnesium, calcium, zinc and aluminium post-lithium metal batteries. The coordination ability of co-solvents with post-lithium charge carriers is presented as a useful guide for selecting co-solvents for the respective battery electrolytes, owing to its correlation with several influential factors that affect the electrochemical performance of the metal anodes, such as solvation structure, de-solvation process and solid electrolyte interphase formation. Additionally, a discussion is provided on the importance of unravelling the effects beyond the solvation sheath of cationic charge carriers and for the development of sustainable electrolytes.

Critical Ingredients Revitalize Magnesium-Metal Batteries: Rationality and Challenges

Multivalent-metal batteries hold tremendous promise in solving safety and sustainability problems encountered by common lithium-ion batteries, but the lack of ideal electrolyte solutions restricts their large-scale adoption. Tuning electrolyte structures with functional ingredients, especially amines/methoxy-based amines and phosphates, can revitalize multivalent-metal anodes and high-voltage cathodes in conventional electrolytes, unlocking their full potential. However, a rational and clear understanding of the implications of these ingredients, notwithstanding critically important to commercially available electrolyte design, has not been widely accepted. This concise perspective aims to provide timely analysis and discussion on ingredients' functionalities of solvation shell speciation, interphase evolution, and consequently metal plating/stripping kinetics acceleration. In addition to prevailing coordination interactions, fresh understandings of intermolecular ionization/association and unique interphase formation are underscored by the close relationship between electrolyte chemistries and weakly passivated interphase properties. The existing understandings and proposed outlooks are expected to promote the next breakthroughs for rechargeable multivalent-metal batteries.

Anion-Regulated Solvation Structure and Electrode Interface toward Rechargeable Magnesium Batteries

Developing chlorine-free electrolytes enabling fast Mg2+ transport through a solid/cathode-electrolyte interphase (SEI/CEI) remains critical for rechargeable magnesium batteries (RMBs). However, single-anion electrolytes often lack the necessary redox properties for this requirement. Here, we propose a dual-anion electrolyte combining magnesium bis(trifluoromethanesulfonyl)imide and 1-butyl-1-methylpiperidinium trifluoromethylsulfonate (PP14CF3SO3) in diglyme and 2-methoxyethylamine (MOEA) solvent, achieving efficient Mg plating/stripping, cathode compatibility, and high anodic stability. The electrostatic interactions between MOEA and Mg2+/CF3SO3- stabilize the Mg-anode SEI while fostering CxNy-rich CEI formation. This leads to a significantly improved performance in Mg∥Mg and stainless steel (SS)∥Mg cells, with an extended lifespan over 2500 h and average Coulombic efficiency of 98.1%, respectively. Mo6S8∥Mg full cells exhibit excellent rate performance, while poly(6,6',6″-(benzene-1,3,5-triyl)tris(9,10-anthracenedione)) (PBAQ)∥Mg cells operate at 2.8 V (1 A g-1) with ∼70% capacity retention after 200 cycles. The work highlights anion-mediated solvation regulation, providing insights into advanced electrolyte engineering in high-performance RMBs.

Influence of Chloride and Electrolyte Stability on Passivation Layer Evolution at the Negative Electrode of Mg Batteries Revealed by operando EQCM-D

Rechargeable magnesium batteries are promising for future energy storage. However, among other challenges, their practical application is hindered by low coulombic efficiencies of magnesium plating and stripping. Fundamental processes such as the formation, structure, and stability of passivation layers and the influence of different electrolyte components on them are still not fully understood. In this work, we gain unique insights into the initial Mg plating and stripping cycles by comparing magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2)- and magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip)4]2)-based electrolytes, each with and without MgCl2, on gold electrodes by highly sensitive operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) applying hydrodynamic spectroscopy. With the stable Mg[B(hfip)4]2-based electrolytes, highly efficient and interphase-free cycling is possible and passivation layers are attributed to electrolyte contaminants. These are forming and degrading during the so-called initial conditioning process. With the more reactive Mg(TFSI)2-based electrolyte, thick passivation layers with small pores are growing during cycling. We demonstrate that the addition of chloride lowers the amount of passivated Mg deposits in these electrolytes and accelerates the currentless dissolution of the passivation layer. This has a positive effect since we observe the most efficient cycling and uniform deposition when no interphase is present on the electrode.

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.

Defect Engineering: Can it Mitigate Strong Coulomb Effect of Mg2+ in Cathode Materials for Rechargeable Magnesium Batteries?

Rechargeable magnesium batteries (RMBs) have been considered a promising "post lithium-ion battery" system to meet the rapidly increasing demand of the emerging electric vehicle and grid energy storage market. However, the sluggish diffusion kinetics of bivalent Mg2+ in the host material, related to the strong Coulomb effect between Mg2+ and host anion lattices, hinders their further development toward practical applications. Defect engineering, regarded as an effective strategy to break through the slow migration puzzle, has been validated in various cathode materials for RMBs. In this review, we first thoroughly understand the intrinsic mechanism of Mg2+ diffusion in cathode materials, from which the key factors affecting ion diffusion are further presented. Then, the positive effects of purposely introduced defects, including vacancy and doping, and the corresponding strategies for introducing various defects are discussed. The applications of defect engineering in cathode materials for RMBs with advanced electrochemical properties are also summarized. Finally, the existing challenges and future perspectives of defect engineering in cathode materials for the overall high-performance RMBs are described.

Revealing the Structural Evolution of Electrode/Electrolyte Interphase Formation during Magnesium Plating and Stripping with operando EQCM-D

Rechargeable magnesium batteries could provide future energy storage systems with high energy density. One remaining challenge is the development of electrolytes compatible with the negative Mg electrode, enabling uniform plating and stripping with high Coulombic efficiencies. Often improvements are hindered by a lack of fundamental understanding of processes occurring during cycling, as well as the existence and structure of a formed interphase layer at the electrode/electrolyte interface. Here, a magnesium model electrolyte based on magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2 ) and MgCl2 with a borohydride as additive, dissolved in dimethoxyethane (DME), was used to investigate the initial galvanostatic plating and stripping cycles operando using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D). We show that side reactions lead to the formation of an interphase of irreversibly deposited Mg during the initial cycles. EQCM-D based hydrodynamic spectroscopy reveals the growth of a porous layer during Mg stripping. After the first cycles, the interphase layer is in a dynamic equilibrium between the formation of the layer and its dissolution, resulting in a stable thickness upon further cycling. This study provides operando information of the interphase formation, its changes during cycling and the dynamic behavior, helping to rationally develop future electrolytes and electrode/electrolyte interfaces and interphases.