Fuminori Mizuno

An Efficient Halogen‐Free Electrolyte for Use in Rechargeable Magnesium Batteries

Unlocking the full potential of rechargeable magnesium batteries has been partially hindered by the reliance on chloride-based complex systems. Despite the high anodic stability of these electrolytes, they are corrosive toward metallic battery components, which reduce their practical electrochemical window. Following on our new design concept involving boron cluster anions, monocarborane CB11H12(-) produced the first halogen-free, simple-type Mg salt that is compatible with Mg metal and displays an oxidative stability surpassing that of ether solvents. Owing to its inertness and non-corrosive nature, the Mg(CB11H12)2/tetraglyme (MMC/G4) electrolyte system permits standardized methods of high-voltage cathode testing that uses a typical coin cell. This achievement is a turning point in the research and development of Mg electrolytes that has deep implications on realizing practical rechargeable Mg batteries.

Magnesium batteries: Current state of the art, issues and future perspectives

Summary “...each metal has a certain power, which is different from metal to metal, of setting the electric fluid in motion...” Count Alessandro Volta. Inspired by the first rechargeable magnesium battery prototype at the dawn of the 21st century, several research groups have embarked on a quest to realize its full potential. Despite the technical accomplishments made thus far, challenges, on the material level, hamper the realization of a practical rechargeable magnesium battery. These are marked by the absence of practical cathodes, appropriate electrolytes and extremely sluggish reaction kinetics. Over the past few years, an increased interest in this technology has resulted in new promising materials and innovative approaches aiming to overcome the existing hurdles. Nonetheless, the current challenges call for further dedicated research efforts encompassing fundamental understanding of the core components and how they interact with each other to offering new innovative solutions. In this review, we seek to highlight the most recent developments made and offer our perspectives on how to overcome some of the remaining challenges.

Boron Clusters as Highly Stable Magnesium-Battery Electrolytes†

Boron clusters are proposed as a new concept for the design of magnesium-battery electrolytes that are magnesium-battery-compatible, highly stable, and noncorrosive. A novel carborane-based electrolyte incorporating an unprecedented magnesium-centered complex anion is reported and shown to perform well as a magnesium-battery electrolyte. This finding opens a new approach towards the design of electrolytes whose likelihood of meeting the challenging design targets for magnesium-battery electrolytes is very high.

Boron-doped graphene as a promising anode for Na-ion batteries.

The Na-ion battery has recently gained a lot of interest as a low-cost alternative to the current Li-ion battery technology. Its feasibility strongly depends on the development of suitable electrode materials. In the present work we propose a novel anode candidate, boron-doped graphene, for the Na-ion battery. Our first-principles calculations demonstrate that the sodiation of boron-doped graphene well preserves its structural integrity. The 2D-BC3 anode has the average sodiation voltage of 0.44 V in an appropriate range to avoid the safety concerns caused by the formation of dendritic deposits. The capacity of the 2D-BC3 anode reaches ∼2.04 times that of the graphite anode in a Li-ion battery and ∼2.52 times that of hard carbon in a Na-ion battery. The high electronic mobility and Na mobility on boron-doped graphene indicates that it has a high potential to reach good rate performance. These suggest the promising potential of boron-doped graphene to serve as an anode for a rechargeable Na-ion battery.

Understanding the electrochemical mechanism of K-αMnO2 for magnesium battery cathodes.

Batteries based on magnesium are an interesting alternative to current state-of-the-art lithium-ion systems; however, high-energy-density cathodes are needed for further development. Here we utilize TEM, EDS, and EELS in addition to soft-XAS to determine electrochemical magnesiation mechanism of a high-energy density cathode, K-αMnO2. Rather than following the typical insertion mechanism similar to Li(+), we propose the gradual reduction of K-αMnO2 to form Mn2O3 then MnO at the interface of the cathode and electrolyte, finally resulting in the formation of K-αMnO2@(Mg,Mn)O core-shell product after discharge of the battery. Understanding the mechanism is a vital guide for future magnesium battery cathodes.

Phase stability and its impact on the electrochemical performance of VOPO4 and LiVOPO4

Vanadyl phosphate (VOPO4) is an attractive candidate as a Li-ion battery cathode with potential to reach higher energy density than that of olivine LiFePO4. However, the limited knowledge of the complex polymorphism of VOPO4 and LiVOPO4 prevents any systematic optimization. Here we present a comprehensive first-principles study on the phase stability of VOPO4 and LiVOPO4 and its impact on the electrochemical properties. βVOPO4 and αLiVOPO4 are predicted to be the most stable phases of VOPO4 and LiVOPO4, respectively. The crucial factor that determines the phase stability is the connection between cation polyhedra including the tilting of VO6 octahedra and the distance between cations, but not the bonding between cations and anions. The calculated data well explain the experimentally observed transition from the αII- to the αI-phase and from the e- to the α-phase induced by lithiation. The transition from the β- to the α-phase is also possible but is associated with much larger structural deformation. The voltages calculated using possible phase transitions taken into account are in excellent agreement with experimental measurements. The diffusivity in βLiVOPO4 and αLiVOPO4 differs by five orders of magnitude at 300 K. These results elucidate the importance of the phase stability of VOPO4 and LiVOPO4 in the application of Li-ion battery cathodes and pave the way for their future improvements.

Quantitatively Predict the Potential of MnO2 Polymorphs as Magnesium Battery Cathodes.

Despite tremendous efforts denoted to magnesium battery research, the realization of magnesium battery is still challenged by the lack of cathode candidate with high energy density, rate capability and good recyclability. This situation can be largely attributed to the failure to achieve sustainable magnesium intercalation chemistry. In current work we explored the magnesiation of distinct MnO2 polymorphs using first-principles calculations, focusing on providing quantitative analysis about the feasibility of magnesium intercalation. Consistent with experimental observations, we predicted that ramsdellite-MnO2 and α-MnO2 are conversion-type cathodes while nanosized spinel-MnO2 and MnO2 isostructual to CaFe2O4 are better candidates for Mg intercalation. Key properties that restrict Mg intercalation include not only sluggish Mg migration but also stronger distortion that damages structure integrity and undesirable conversion reaction. We demonstrate that by evaluating the reaction free energy, structural deformation associated with the insertion of magnesium, and the diffusion barriers, a quantitative evaluation about the feasibility of magnesium intercalation can be well established. Although our current work focuses on the study of MnO2 polymorphs, the same evaluation can be applied to other cathode candidates, thus paving the road to identify better cathode candidates in future.

Understanding and overcoming the challenges posed by electrode/electrolyte interfaces in rechargeable magnesium batteries

Guided by the great achievements of lithium (Li)-ion battery technologies, post Li-ion battery technologies have gained a considerable interest in recent years. Their success would allow us to realize a sustainable society, enabling us to mitigate issues like global warming and resource depletion. Of such technologies, Magnesium (Mg) battery technologies have attracted attention as a high energy-density storage system due to the following advantages: (1) potentially high energy-density derived from a divalent nature, (2) low-cost due to the use of an earth abundant metal, and (3) intrinsic safety aspect attributed to non-dendritic growth of Mg. However, these notable advantages are downplayed by undesirable battery reactions and related phenomena. As a result, there are only a few working rechargeable Mg battery systems. One of the root causes for undesirable behavior is the sluggish diffusion of Mg2+ inside a host lattice. Another root cause is the interfacial reaction at the electrode/electrolyte boundary. For the cathode/electrolyte interface, Mg2+ in the electrolyte needs a solvation-desolvation process prior to diffusion inside the cathode. Apart from the solid electrolyte interface (SEI) formed on the cathode, the divalent nature of Mg should cause kinetically slower solvation-desolvation processes than that of Li-ion systems. This would result in a high charge transfer resistance and a larger overpotential. On the contrary, for the anode/electrolyte interface, the Mg deposition and dissolution process depends on the electrolyte nature and its compatibility with Mg metal. Also, the Mg metal/electrolyte interface tends to change over time, and with operating conditions, suggesting the presence of interfacial phenomena on the Mg metal. Hence, the solvation-desolvation process of Mg has to be considered with a possible SEI. Here, we focus on the anode/electrolyte interface in a Mg battery, and discuss the next steps to improve the battery performance.

A highly efficient Li2O2 oxidation system in Li–O2 batteries

A novel indirect charging system that uses a redox mediator was demonstrated for Li-O2 batteries. 4-Methoxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl (MeO-TEMPO) was applied as a mediator to enable the oxidation of Li2O2, even though Li2O2 is electrochemically isolated. This system promotes the oxidation of Li2O2 without parasitic reactions attributed to electrochemical charging and reduces the charging time.

Non-Aqueous Primary Li–Air Flow Battery and Optimization of its Cathode through Experiment and Modeling

A primary Li-air battery has been developed with a flowing Li-ion free ionic liquid as the recyclable electrolyte, boosting power capability by promoting superoxide diffusion and enhancing discharge capacity through separately stored discharge products. Experimental and computational tools are used to analyze the cathode properties, leading to a set of parameters that improve the discharge current density of the non-aqueous Li-air flow battery. The structure and configuration of the cathode gas diffusion layers (GDLs) are systematically modified by using different levels of hot pressing and the presence or absence of a microporous layer (MPL). These experiments reveal that the use of thinner but denser MPLs is key for performance optimization; indeed, this leads to an improvement in discharge current density. Also, computational results indicate that the extent of electrolyte immersion and porosity of the cathode can be optimized to achieve higher current density.