Understanding Performance Limitations to Enable High Performance Magnesium-Ion Batteries

Abstract

A mathematical model was developed to investigate the performance limiting factors of Mg-ion battery with a Chevrel phase (Mg x Mo 6 S 8 ) cathode and a Mg metal anode. The model was validated using experimental data from the literature [Cheng et al., Chem. Mater., 26, 4904 (2014)]. Two electrochemical reactions of the Chevrel phase with significantly different kinetics and solid diffusion were included in the porous electrode model, which captured the physics sufficiently well to generate charge curves of five rates (0.1C-2C) for two different particle sizes. Limitation analysis indicated that the solid diffusion and kinetics in the highervoltage plateau limit the capacity and increase the overpotential in the Cheng et al. 's thin (20-μm) electrodes. The model reveals that the performance of the cells with reasonable thickness would also be subject to electrolyte-phase limitations. The simulation also suggested that the polarization losses on discharge will be lower than that on charge, because of the differences in the kinetics and solid diffusion between the two reactions of the Chevrel phase. During the past 30 years, Li-ion batteries have come to dominate the market for use in electric vehicles (EVs) and portable electronic devices. However, to extend the driving range of EVs and to make the portable electronic devices more compact and lighter, the energy density of the batteries must be increased. Significant effort has been devoted to finding new battery chemistries with the potential to exceed the energy density of Li-ion batteries. 1-4Among the novel approaches being studied, Mg-ion batteries are promising candidates, 5-24 partly because of the advantages of Mg metal anode. Although the reduction potential of Mg metal is similar to that of Li (-2.37 V vs. H + /H 2 for Mg compared with -3.0 V vs. H + /H 2 for Li), the volumetric capacity is significantly higher (3,833 mAh cm -3 for Mg compared with 2,046 mAh cm -3 for Li). In addition, Mg-ion batteries may be safer than Li-ion batteries, not only because Mg does not form hazardous dendrites when charged and discharged repeatedly 25 but also because a strong and stable oxide develops on the surface, protecting the battery if the metal is exposed to ambient conditions. Finally, the divalent charge on the Mg ion enables exploration of higher-capacity cathode materials that promise increases in the overall cell-energy density.Although multivalent systems are promising from an energydensity perspective, they are limited by the slow transport of the divalent cation. This issue manifests itself in the electrolyte phase, where the Mg ion is thought to form ion pairs, 26,27 and in typical cathode materials, where the diffusion of ions can be fairly slow. 28,29Furthermore, although not clearly explored in the literature, the sluggish movement of Mg ions can also manifest itself at the interface during charge transfer. To date, it is unclear which of these limitations (bulk cathode vs. bulk electrolyte vs. interfaces) limits the rate capability of Mg-ion systems ...

Thermodynamic Origin of Irreversible Magnesium Trapping in Chevrel Phase Mo6S8: Importance of Magnesium and Vacancy Ordering

“…Assuming the same cost for breaking the ordering as the energy difference between ordered and disordered structure, which is 0.15−0.30 eV in our calculations, we estimated that the ordering of Mg and vacancies in the highvoltage regime reduces the chemical diffusivity by 3−5 orders of magnitude to the range of 10 −14 to 10 −16 cm 2 /s. This value agrees well with a recent work 52 in which the diffusivity of Mg in the high-voltage stage was estimated to be 1.7 × 10 −15 cm 2 /s.…”

Composite Polymer Electrolyte for Highly Cyclable Room-Temperature Solid-State Magnesium Batteries

“…Second, there is a lack of compatible electrolytes that support high cyclability with Mg-metal and simultaneously possess a high oxidative stability (voltage window). , This limitation has also affected the research progress in magnesium intercalation (cathode) materials . With the limited development of electrolytes with high voltage windows, it has been challenging to experimentally identify and evaluate the reversibility of prospective cathode materials for Mg-ion batteries. , The Grignard reagents that are compatible with the Mg-metal anode possess a low oxidative stability of <1.5 V, which is not high enough to investigate possible cathode candidates for Mg-ion batteries. , To improve the oxidative stability and the kinetics of Mg 2+ ions within the electrolyte, several liquid electrolytes were designed with organometallic reagents dissolved in solvents such as tetrahydrofuran (THF), diglyme, and tetraglyme. , Although these electrolytes exhibit a high degree of reversibility and oxidative stability, the use of flammable, volatile ethereal solvents such as THF poses a safety risk . This conflicts with the notion of projecting Mg-metal batteries as a safer alternative to Li-metal batteries.…”

“…16 With the limited development of electrolytes with high voltage windows, it has been challenging to experimentally identify and evaluate the reversibility of prospective cathode materials for Mg-ion batteries. 17,18 The Grignard reagents that are compatible with the Mg-metal anode possess a low oxidative stability of <1.5 V, which is not high enough to investigate possible cathode candidates for Mg-ion batteries. 15,19 To improve the oxidative stability and the kinetics of Mg 2+ ions within the electrolyte, several liquid electrolytes were designed with organometallic reagents dissolved in solvents such as tetrahydrofuran (THF), diglyme, and tetraglyme.…”

Two-Dimensional Anode Materials for Non-lithium Metal-Ion Batteries

“…Low reversibility and overall poor kinetics limit their applicability. Especially for MIB systems, the reaction kinetics is of significant concern and the choice of electrode and electrolyte is of utmost concern …”

“…Especially for MIB systems, the reaction kinetics is of significant concern and the choice of electrode and electrolyte is of utmost concern. 258 It is expected that 2D materials with their superior characteristics, high stability, enhanced surface area, improved electrochemical kinetics, and reversibility, etc., will greatly improve the performance of these emerging metal-ion systems. Also, with greater research in 2D materials, scalable and economic fabrication techniques could be developed which may further enhance suitability of 2D materials in CIB and MIB for energy storage.…”