Mg [B (HFIP)4]2 electrolyte salt: A key breakthrough in the interface chemistry of magnesium batteries

Magnesium batteries, as a powerful contender for the next-generation high-energy-density energy storage technology, have long been limited in their development due to the compatibility issues between the electrolyte and the electrode interface. Traditional electrolytes either cause the magnesium anode to become passive or face the problem of insufficient oxidation stability, severely restricting the cycle life and energy density improvement of magnesium batteries. In recent years, magnesium Mg[B(HFIP)4]2, a new type of fluorinated alkoxo-borate electrolyte salt, has emerged, providing a new solution to this problem. This paper systematically reviews the molecular design principles, synthesis methods, electrochemical properties, and interface regulation mechanisms of this electrolyte salt, revealing its unique advantages in achieving high stability and high reversibility magnesium batteries.

Challenges and Innovation Demands of Magnesium Battery Electrolytes

In the current era where lithium-ion batteries dominate, magnesium batteries demonstrate great potential due to their unique advantages. The volumetric capacity of magnesium is as high as 3833 mAh/cm3, nearly 10 times that of lithium, and magnesium is abundantly present in the earth's crust (about 2.3%), significantly reducing the risk of resource constraints. More importantly, the magnesium metal anode does not easily form dendrites during deposition, fundamentally enhancing the safety of the battery. This characteristic makes it an indispensable application prospect in the fields of electric vehicles and large-scale energy storage.

However, the development of magnesium batteries faces severe challenges in the field of electrolytes. The early research on Grignard reagent-based electrolytes was able to achieve reversible deposition of magnesium, but their oxidation stability was extremely low (typically below 2 V vs Mg2+/Mg), which could not match the high-voltage cathode materials. Subsequent boron-based electrolytes often form an insulating passivation layer on the magnesium negative electrode surface, resulting in a significant increase in battery internal resistance and deterioration of cycling performance. The core of these problems lies in the inherent contradiction at the interface between the electrolyte and the electrode: it is necessary to ensure the rapid transmission of magnesium ions while suppressing the occurrence of side reactions.

The molecular design of Mg[B(HFIP)4]2 provides an innovative solution to this contradiction. The core advantage of this electrolyte salt stems from its unique chemical structure: the central boron atom forms a stable tetrahedral configuration with four hexafluoroisopropoxy groups (-OCH (CF3)2). This large-volume weakly coordinating anion can effectively reduce the interaction strength with Mg2+, ensuring the high solubility of the electrolyte and promoting the rapid migration of magnesium ions. At the same time, the strong electronegativity of fluorine atoms endows the electrolyte with excellent oxidation stability, enabling it to match higher-voltage cathode materials.

Synthesis methods and interface control strategies

The synthesis method of Mg[B(HFIP)4]2 has undergone a development process from complex to simplified, reflecting the researchers' continuous deepening understanding of its chemical properties.Synthesis methods and interface control strategies

 The traditional exogenous synthesis method requires strict anhydrous and airtight conditions.0 mS cm-1 at room temperature, laying the foundation for high-rate performance.

Synthesis methods and interface control strategies

The synthesis method of Mg[B(HFIP)4]2 has undergone a development process from complex to simplified, reflecting the researchers' continuous deepening understanding of its chemical properties. The traditional ex situ synthesis method requires strict anhydrous and airtight conditions. Usually, tri-(hexafluoroisopropyl) borate (B (HFIP)3) is used as the precursor, reacting with excess magnesium powder in a 1,2-dimethoxyethane (DME) solvent for more than 24 hours in an argon glove box. Subsequently, the generated MgI₂ impurities are filtered out to obtain a clear electrolyte solution. Although this method can produce high-purity electrolytes, the process is cumbersome and the raw material cost is high, limiting its large-scale application.

The in-situ synthesis strategies developed in recent years have opened up new pathways for the preparation of Mg[B(HFIP)4]2. The research team from Nanjing University proposed an "iodine-enhanced" in-situ transformation method, where B (HFIP)3 and iodine (I2) were directly dissolved in DME, and reacted with the magnesium negative electrode during battery assembly, spontaneously forming a Mg[B(HFIP)4]2/DME-MgI2 electrolyte system. This method not only simplifies the synthesis steps, but more importantly, the generated MgI2 during the reaction can participate in the construction of the solid-state electrolyte interface (SEI) layer, significantly improving the interface compatibility between the electrolyte and the electrode.

Interface regulation is the key to the excellent performance of the Mg[B(HFIP)4]2 electrolyte. The team from Chongqing University discovered through characterization methods such as X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) that in the DME - tetrahydrofuran (THF) mixed solvent, this electrolyte can form a uniform organic-inorganic composite SEI layer on the magnesium negative electrode surface. This SEI layer is rich in MgF2 and MgBxOy inorganic components, and also contains C-C/C-O organic components. It not only ensures the high conductivity of magnesium ions but also effectively prevents the continuous decomposition of the electrolyte.

Electrochemical performance and application potential

The electrochemical performance of the Mg[B(HFIP)4]2 electrolyte salt has been outstanding in various tests, demonstrating the ability to address the long-term issues of cycle stability and rate performance faced by magnesium batteries. In the magnesium symmetric battery tests, the electrolyte system using a DME-THF (50 vol%) mixed solvent was able to achieve a stable cycle of up to 2000 hours, with an average Coulombic efficiency of 99.4% and the overpotential remaining at a relatively low level (approximately 80 mV). This stability far exceeds that of traditional electrolytes, indicating that the problem of magnesium anode passivation has been effectively solved.

In the all-cell application, the Mg[B(HFIP)4]2 electrolyte exhibits excellent compatibility with various cathode materials. After 1200 cycles at a 1C rate, the capacity decay rate of the all-cell assembled with the classic Chevrel phase Mo6S8 cathode is only 0.04% per cycle, demonstrating outstanding cycling stability. What is even more remarkable is that the use of this electrolyte in combination with the high-voltage Prussian blue-like Fe-PBA cathode enables the all-cell to maintain a capacity retention rate of 83% after 500 cycles at a discharge platform of 2.1 V, opening up a new path for the development of high-energy-density magnesium batteries.

The rate performance is another prominent advantage of the Mg[B(HFIP)4]2 electrolyte. Thanks to the optimized solvation structure and efficient interface conduction, the Mg||Mo6S8 all-cell still maintains a discharge specific capacity of 61.8 mAh g-1 at a high rate of 500 mA g-1. The magnesium symmetric cell, with an current density of 10 mA cm-2, has an overpotential of approximately 120 mV and no significant increase in polarization. This performance indicator indicates that this electrolyte can meet the requirements of high-power application scenarios.

Challenges and Future Prospects

Although the Mg[B(HFIP)4]2 electrolyte salt exhibits excellent comprehensive performance, it still faces several challenges before it can be applied in practice. The primary issue is the contradiction between synthesis cost and large-scale production. The traditional synthesis method relies on expensive hexafluoroisopropanol derivatives as raw materials and requires strict anhydrous and anoxic conditions, resulting in high electrolyte costs. Although the in-situ synthesis strategy simplifies the preparation process, the introduction of iodine additives may bring new interface problems, and their long-term effects still need to be evaluated.

Environmental stability is another urgent issue that needs to be addressed. Research has shown that Mg[B(HFIP)4]2 and its precursors are prone to hydrolysis in humid environments, releasing corrosive gases, which not only increases the difficulty of battery assembly but also imposes strict requirements for long-term storage. Developing more stable derivatives or effective encapsulation technologies will be an important direction for future research.

From the perspective of material design, current research mainly focuses on ether-based solvent systems such as DME and THF, and exploration of other types of solvents is relatively limited. Expanding the range of solvent selection, especially developing solid or quasi-solid electrolyte systems with higher safety, may further enhance the practical application value of magnesium batteries. The recent research progress of Qingdao Energy Institute in high-temperature polymer electrolytes provides useful references for the application of Mg[B(HFIP)4]2 in solid-state batteries.

The compatibility of the cathode materials still needs to be expanded. Although good matching has been achieved with Mo6S8 and Fe-PBA, the compatibility study of Mg[B(HFIP)4]2 electrolyte with high-capacity oxide cathodes is still insufficient. Developing targeted interface modification strategies, such as pre-modification of the cathode or electrolyte additives, may be an effective way to solve this problem.