Boron Clusters as Highly Stable Magnesium‐Battery Electrolytes

Abstract

Boron clusters are proposed as a new concept for the design of magnesium-battery electrolytes that are magnesiumbattery-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.The possibility of developing a rechargeable magnesium battery has been a topic of great interest for several decades. [1] Magnesium batteries are particularly attractive owing to their high theoretical capacity (3832 mA h cm À3 ) as well as the low cost and high abundance of Mg metal as compared to Li. [1,2] Additionally, Mg possesses an important safety advantage over Li in that Mg metal is not prone to dendrite formation. [3] However, before Mg batteries can see practical application, several challenges must be overcome, including the identification of a suitable solvent/electrolyte combination.[2] It has proven particularly challenging to identify such a system for magnesium-based batteries owing to the tendency of many common electrolytes to form nonconductive ion-blocking layers at the electrode surface.[4] Thus, any solvent/electrolyte combination used in a Mg battery must be stable across the entire potential window at which the battery operates.This requirement is particularly problematic for the design of high-voltage battery systems, for which an electrolyte candidate must be stable to both Mg metal (À2.37 V versus the normal hydrogen electrode, NHE) and the cathode-active material selected, which should ideally operate near + 1 V (vs. NHE). Furthermore, previous research has suggested that Mg metal is reactive toward many common polar solvents and electrolyte anions.[5] In fact, the majority of previously reported electrolyte systems have been limited to highly inert ethereal solvents, such as tetrahydrofuran (THF).[6] This limitation makes the design of electrolyte materials quite difficult, as a successful candidate must be highly soluble in these nonpolar solvents in addition to being electrochemically robust. As a result, only a handful of potential electrolytes are known to be compatible with Mg anodes. [6] Recent studies at the Toyota Research Institute of North America have demonstrated Mg(BH 4 ) 2 to be a highly competent electrolyte for magnesium-battery applications. [7] This electrolyte was the first non-organomagnesium electrolyte compatible with Mg metal and provided excellent electrochemical performance in glyme. In tandem with further research into this system, we sought to pursue potential routes to materials with enhanced oxidative stability as compared to the observed oxidation onset potential of 1.7 V (vs. Mg on Pt) for Mg(BH 4 ) 2 .High oxidative stability is crucial for the development of Mg batteries for operation with future high-voltage cathodes. Although organo...

Electrolytes for Batteries with Earth‐Abundant Metal Anodes

“…[41][42][43] These studies showed that modified structures would lead to high magnesium plating efficiencies with higher potential windows. After the first introduction of ar eversible magnesium battery,m any studies concerning magnesium organoaluminates and organoborates have been reported.…”

“…After the first introduction of ar eversible magnesium battery,m any studies concerning magnesium organoaluminates and organoborates have been reported. [41][42][43] These studies showed that modified structures would lead to high magnesium plating efficiencies with higher potential windows. However,f or battery development, it is necessary to find as impler salt/electrolyte combination with high performance because the organosalts of magnesium are too expensive for real applications.T he first inorganic salt tested for am agnesium battery electrolyte was Mg(BH 4 ) 2 .…”

Magnesium Ethylenediamine Borohydride as Solid-State Electrolyte for Magnesium Batteries

“…In particular electrolytes based on magnesium borohydride, Mg(BH 4 ) 2 , have received significant attention since 2012, when Mohtadi et al . demonstrated the first fully inorganic and halide-free Mg electrolytes enabling reversible Mg plating and stripping78. In an effort to improve the safety, attempts have been made to replace the flammable organic solvents by ionic liquids91011.…”

Computational Model of Magnesium Deposition and Dissolution for Property Determination via Cyclic Voltammetry

“…z E-mail: kthorn@umich.edu their practical application in batteries. 14 To avoid this problem, inorganic salts such as Mg(TFSI) 2 and Mg(BH 4 ) 2 in solutions based on traditional solvents 10,[15][16][17][19][20][21] or ionic liquids [22][23][24]26 have also been explored.Despite the recent growth in efforts to develop efficient magnesium electrolytes, many challenges remain. For example, formation of ionblocking passivation films on magnesium surfaces and compatibility with cathode materials (allowing reversible intercalation of Mg 2+ ) both remain challenges.…”

“…2,7,8,11,12,27 Due to their halide content, Grignards, organohaloaluminates, and MACC are corrosive to non-noble metal substrates, which presents a challenge to their practical application in batteries. 14 To avoid this problem, inorganic salts such as Mg(TFSI) 2 and Mg(BH 4 ) 2 in solutions based on traditional solvents 10,[15][16][17][19][20][21] or ionic liquids [22][23][24]26 have also been explored.…”