Recent Progress and Challenges in the Optimization of Electrode Materials for Rechargeable Magnesium Batteries

Abstract

Rechargeable magnesium batteries (RMBs) have been regarded as one of the promising electrochemical energy storage systems to complement Li‐ion batteries owing to the low‐cost and high safety characteristics. However, the various challenges including the sluggish solid‐state diffusion of highly polarizing Mg2+ ions in hosts, and the formation of blocking layers on Mg metal surface have seriously impeded the development of high‐performance RMBs. In order to solve these problems toward practical applications of RMBs, a tremendous amount of work on electrodes and electrolytes has been conducted in the last few decades. Creative optimization strategies including the modification of cathodes and anodes such as shielding the charges of divalent Mg2+, expanding the layers of host materials, and optimizing the interface of electrode–electrolyte are raised to promote the technology. In this review, the detailed description of innovative approaches, representative examples, and facing challenges for developing high‐performance electrodes are presented. Based on the review of these strategies, guidelines are provided for future research directions on improving the overall battery performance, especially on the electrodes.

Opportunities and Limitations of Ionic Liquid‐ and Organic Carbonate Solvent‐Based Electrolytes for Mg‐Ion‐Based Dual‐Ion Batteries

“…Overall, and in agreement with previous studies, [12i–k] these results indicate that TFSI − intercalation into graphite is possible from a Mg‐ion‐based electrolyte and has a similar performance compared to a Li‐ion‐based system. The achievable SDCs, however, are still lower compared to other cathode materials such as layered sulfides and oxides (often >100 mAh g −1 ) or sulfur (up to 1000 mAh g −1 ) [5c,6d,7,8b–d] . In addition, in DIBs the electrolyte is part of the active material, which is why typically larger amounts of electrolyte are needed in comparison to cells based on the “ion transfer” mechanism, where the electrolyte is only a charge carrier between the electrodes.…”

“…The dual‐ion system is also highly applicable for Mg‐ion‐based systems, as Mg 2+ ‐transport within the cathode (as known for classical RMB) is not a limiting factor. In addition, the high operating potential for anion intercalation into graphite (≈4.4–5.2 V vs. Li|Li + ; 3.7–4.5 V vs. Mg|Mg 2+ can be expected) [17] could result in a significantly increased cell voltage compared to commonly investigated RMB systems [5c,6d, 7] . However, only few reports show that Mg‐ion‐based DIB cell chemistries with graphite can be enabled, even though most of them do not use high‐capacity Mg metal anodes, yielding mean cell voltages below 3 V [12i–k] .…”

“…An alternative to common insertion‐ (e. g., layered oxides or sulfides)[ 5c , 6d , 7 ] or conversion‐type (e. g., sulfur) [8] cathodes is the usage of an anion intercalating graphite as positive electrode (cathode). The dual‐ion battery (DIB) energy storage mechanism is different from the “ion transfer” mechanism, where the cation is transported from the cathode through the electrolyte to the negative electrode (anode) and back during charge and discharge, respectively.…”

“…In addition, the high operating potential for anion intercalation into graphite (≈4.4–5.2 V vs. Li|Li + ; 3.7–4.5 V vs. Mg|Mg 2+ can be expected) [17] could result in a significantly increased cell voltage compared to commonly investigated RMB systems. [ 5c , 6d , 7 ] However, only few reports show that Mg‐ion‐based DIB cell chemistries with graphite can be enabled, even though most of them do not use high‐capacity Mg metal anodes, yielding mean cell voltages below 3 V.[ 12i , 12j , 12k ] This results from the inability to electrochemically dissolve and deposit Mg with reasonable reversibility in known electrolytes for DIBs, such as ionic liquid‐[ 10 , 17 ] or carbonate‐based[ 9a , 12c ] electrolytes. Electrolytes based on ionic liquids or carbonates tend to passivate the Mg surface and result in high overpotentials for Mg electrodissolution/‐deposition.…”

An Overview on Anodes for Magnesium Batteries: Challenges towards a Promising Storage Solution for Renewables

“…Given all the reasons mentioned above, scientists are looking for new types of batteries (the so-called "new chemistries" [87][88][89][90][91]). One of the most interesting solutions seems to be represented by the rechargeable magnesium-ion batteries (MIBs) [92][93][94][95][96], which utilize magnesium cations as the active charge transporting species in solution and (in many cases) metallic magnesium as the anode. A primary advantage of this technology is given by the solid magnesium anode that leads to high energy density values, well above those of lithium-based cells [97][98][99][100][101].…”