Solid‐State Electrolytes for Rechargeable Magnesium‐Ion Batteries: From Structure to Mechanism

Abstract

In recent years, scientists have explored various metals, such as sodium, potassium, zinc, aluminum, and magnesium, for metal-ion batteries to replace lithium from various applications. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Among different metal-ion batteries, rechargeable magnesium-ion batteries (MIBs) are expected to be a potential candidate for large-scale energy storage systems owing to the following excellent intrinsic advantages. 1) Magnesium has a lower electrode potential (-2.37 V versus SHE) than Zn and Al metals. 2) Magnesium is a light metal with a density of 1.74 g cm -3 and is abundantly present in the earth's crust (i.e., approximately 2.1%, which is 10 4 times that of lithium metal). 3) Two electrons can be provided during redox reactions, leading to a higher theoretical volumetric capacity of 3866 mAh cm -3 than that of lithium metal (2046 mAh cm -3 ). 4) A magnesium anode with higher stability than a lithium anode is significantly less susceptible to dendrite formation. Similar to other metal-ion batteries, the core components of MIBs are also anode, cathode and electrolyte. Commonly, the Mg 2+ ions are released from the anode and inserted into the cathode during the discharge process, and the process is reversed during the charging process. However, the electrochemical performance of magnesium is different from that of lithium. The passivation layer formed between magnesium and ordinary electrolyte prevents the diffusion of Mg 2+ ions and limits its reversible deposition from salt-containing aprotic electrolytes such as magnesium perchlorate, magnesium tetrafluoroborate, imide, carbonate, and nitrile. Hence, achieving reversible deposition/dissolution of Mg ions in the electrolyte is very important for MIBs.Because Mg is active in an aqueous solution and easily forms a passivation layer on the surface, researchers have explored different organic systems to realize electrodeposition. [27][28][29] The concept of rechargeable MIBs was first raised in 1990. Gregory et al. constructed Mg//1.0 M N-ethylaniline magnesium chloride, 0.20 M AlCl 3 , THF//Cu batteries, which demonstrated compatibility between Mg organoboranes electrolytes (Mg(BPh 2 Bu 2 ) 2 or Mg(BPhBu 3 ) 2 ) and Mg anodes. [30] Then, in 2000, Aurbach's group reported the first study on electrochemically deposited Mg metal. [31] Subsequently, the same research group prepared a high-voltage (>3 V) all-phenyl complex electrolyte as the product of the reaction between PhMgCl Rechargeable magnesium (Mg)-ion batteries have received growing attention as a next-generation battery system owing to their advantages of sufficient reserves, lower cost, better safety, and higher volumetric energy density than lithium-ion batteries. However, Mg as an anode can be easily passivated during charging/discharging by most common solvents, which are inconducive for magnesium deposition/stripping. Based on this, the development of Mg-ion solid-state electrolytes in the last decades led to the formulization of several concepts beyond prev...

Fast divalent conduction in MB12H12·12H2O (M = Zn, Mg) complex hydrides: effects of rapid crystal water exchange and application for solid-state electrolytes

“…For instance, Mg, Ca, and Zn metals have higher volumetric capacities of 3832, 2072, and 5854 mA h cm −3 than 2066 mA h cm −3 of Li metal. [40][41][42][43] However, designing solidstate divalent ionic conduction materials exhibiting sufficiently high ionic conductivities is a formidable challenge because higher-valence M 2+ cations are strongly trapped by counter anions. Therefore, the ionic conductivities of divalent-cationcontaining conductors continue to improve owing to experimental and theoretical studies on [BH 4 ] − -based complex hydrides.…”

Boron–Based Electrolytes for Rechargeable Magnesium Batteries: Biography and Perspective

“…[3] Among them, magnesium has attracted considerable attention because of its moderate volumetric capacity (3833 mAh cm À 3 ), relatively low redox potentials (À 2.36 V vs. SHE), and ideal abundance (2.1 % crustal reserves, the sixth abundant metal element). [4] Meanwhile, another important advantage of rechargeable magnesium batteries (RMBs) is the extremely rare dendrite phenomenon in the process of repeated anodic electrochemical plating/stripping (Figure 1b). Although sporadic Mg dendrites have recently been reported in Grignard-based electrolytes, their extraordinary Mg plating current density and Grignard-restricted precondition (e. g., 10 mA cm À 2 in 0.4 M all-phenyl complex (APC), [5] and 0.921 mA cm À 2 in 0.5 M MeMgCl/THF [6] ) may not obscure the comparable "dendrite-free" virtues of metallic Mg.…”

“…Among them, magnesium has attracted considerable attention because of its moderate volumetric capacity (3833 mAh cm −3 ), relatively low redox potentials (−2.36 V vs . SHE), and ideal abundance (2.1 % crustal reserves, the sixth abundant metal element) [4] . Meanwhile, another important advantage of rechargeable magnesium batteries (RMBs) is the extremely rare dendrite phenomenon in the process of repeated anodic electrochemical plating/stripping (Figure 1b).…”

Organic Crystalline Solid Electrolytes with High Mg-Ion Conductivity Composed of Nonflammable Ionic Liquid Analogs and Mg(TFSA)2

“…The development of solid electrolytes with Mg-ion conductivity at room temperature is an important issue to achieve all-solid magnesium batteries. − Multivalent Mg ions tend to form strong interactions with the anionic moiety; thus, room temperature Mg-ion conduction has been recognized as a challenging issue in developing solid electrolytes for MGB. − However, high Mg-ion conductivity has been recently reported on the inorganic (ceramics, glass) and organic (polymers, MOF) solid electrolytes based on the studies on Li-ion conductive solid electrolytes. − For example, the ionic conductivity of about 10 –4 S cm –1 at room temperature has been reported for the ceramic electrolyte, MgSc 2 Se 4 . Polymer electrolytes showing high conductivities of 10 –4 –10 –3 S cm –1 have been fabricated using magnesium salts such as Mg­(CF 3 SO 3 ) 2 , Mg­(NO 3 ) 2 , and Mg­(ClO 4 ) 2 . − It has also been reported that a Cu-based MOF with an electrolyte containing MgBr 2 exhibits ionic conductivity of 10 –4 S cm –1 at room temperature .…”

“…1−6 Multivalent Mg ions tend to form strong interactions with the anionic moiety; thus, room temperature Mg-ion conduction has been recognized as a challenging issue in developing solid electrolytes for MGB. 7−20 However, high Mg-ion conductivity has been recently reported on the inorganic (ceramics, glass) and organic (polymers, MOF) solid electrolytes 21 based on the studies on Li-ion conductive solid electrolytes. 22−28 For example, the ionic conductivity of about 10 −4 S cm −1 at room temperature has been reported for the ceramic electrolyte, MgSc 2 Se 4 .…”