Liquid Electrolytes with Inorganic Salts for Mg-ion Batteries
Gregory et al. [14] first synthesized organic electrolytes and anodes that are capable of Mg electrodeposition. The compounds, such as Mg(BBu,Ph2)2, Mg(C104)2, Mg(BF4)2, and C2H5MgCl, were chosen and they inferred that the bulky groups connected with Mg ions have a tendency to behave in an ionic nature and share in a covalent nature. Among these compounds, reversible intercalation of Mg2+ ions with high specific capacity and energy densities are low. Solute-solvent interaction in electrode materials is also very low. Similarly, the type of solvent also has a major role. Aurbach et al. [15] chose tetrahydrofuran (THF), butyl triphenyl borates, and tetraphenyl borate to investigate the behavior of solvent in the electrochemical performance of the battery. Triphenyl borate and butyl triphenyl borates have low solubility. Similarly, the effect of organohalo-aluminate-based electrolytes were chosen by same group [16]. Organohalo aluminates such as Mg(AlCl,R)2, Mg(AlCl2R’R)2, are some of the electrolytes that exhibit high anodic stability. The use of organohalo aluminates along with THF solvent the electrolyte shows better ionic conductivity and electrochemical properties however the presence halide group in organohalo aluminates, for example Cl group in Mg(AlCl3R)2, Mg(AlCl2R’R)2 leads to corrosion of the electrodes and battery case. The first prototype cell was assembled with Mg/0.25 M Mg(AlCl,BuEt)2 /Chevrel phase and exhibited specific capacity of 122mAh g-' (at current of 0.3 mAcnr2). Replacing alkyl groups with phenyl groups increases anodic stability up to 3.3 V vs. Mg. Alkyl parts and halogen parts play an important role in the complex adsorption phenomena. Similarly, a group of compounds, including organomagnesium and amidomagnesium, dissolved in THF, for which good oxidative stability was observed by Mg(BBu2Ph2)2 compounds [17]. Peter et al. [18] reported different metal oxides and sulfides dissolved in MgC104, whereas V205 have similar morphological characteristics of Ti02. This matrix provides high coulombic efficiency because the water molecules in the electrolytes allow' intercalation of Mg ions. Though many studies using MgC104 with different compounds were investigated, incorporation of ionic liquid is characterized with CoO, as working electrodes, as reported by Sutto et al. [19]. Cations of 1, 2-dimethyl-3-R-imidazolium with R = butyl (MMBI) or octyl (MMOI) ionic liquid w'as utilized wnth anion as bis(trifluoromethanesulfonyl)imide (TFSI). Though MgC104 is a powerful oxidizer, it has better solubility in ionic liquids compared to that of other electrolytes. Of these combinations, MMOITFSI provides the widest electrochemical stability window, although it has lower ionic conductivity. Figure
8.2 show's a schematic diagram of the three- and two-electrode systems. Copper foil/double-sided tape electrode intercalating Mg2+/CoO, was shown below inferring CoO, is not suitable for the choice of anode materials (Figure 8.2).
Later, MgC104 was found to be incompatible with anode materials and borohy- dride-based materials appeared as common, which was first discovered by Mohtadi et al. [21] in which halide-free materials were used to avoid corrosion. Different solvents, such as THF and dimethyl ether (DME), were used, in which DME provides better solubility, low potential and high current density. Shao et al. [22] explained the detailed coordination chemistry in MgBH4 with different ethereal solvents. Since electrochemical performance of Mg closely related to coulombic efficiency, dentic- ity and donating strength of ethereal solvent is vital and these factors influences the thermodynamic as well as kinetic behavior on Mg. A solvated Mg complex was formed by oxygen donor denticity. New electrolytes are able to perform 100% coulombic efficiency with stable cycling performance.
Organic/Inorganic Halo-Salt-Based Electrolytes for Mg-ion Batteries
Organohalo aluminate systems were first introduced by Aurbach et al. [23], introducing different solvents, such as glymes, THF, polyethers, 1,3-dioxine etc. Different Mg(AX4.„Rn)2 complexes were chosen [A = Al, B, Sb, P, As, Fe;
FIGURE 8.2 (a) Schematic illustration on the three electrode system used for the ionic
conductivity measurements and electrochemical characterization ionic liquid characterization, (i) working electrode (glassy carbon), (ii) reference electrode (Vyccor tipped Ag/AgO, and (iii) counter electrode (graphite rod), (b) two-electrode cell collecting charge-discharge cell, (iv) gold foil with Co03, and (v) anode (Mg foil), (c) copper foil electrode with graphitic tape, and (d) removing protecting backing and cathodic material being pressed. Adapted and reproduced with permission from reference [20]. Copyright 2012 Elsevier.
R = butyl, ethyl, phenyl, and benzyl; X = Cl, Br, and F]. The presence of chlorine increases ionic conductivity, and high anodic stability among different combinations of AlCl,Et and Bu2R mixtures delivers better efficiency. Because a matter of corrosion persisted in A1C13, AlOPh, was introduced by Nelson et al. [24] with a 1:4 ratio of AlOPh,:PhMgCl. Later on, Barlie et al. [11] investigated the chemistry behind the Mg-ion stripping, deposition, dissolution, and operating mechanisms of organohalo aluminates with Grignard reagent of EtMgBr. Figure 8.3 shows gold nanoparticles deposited in the Mg metal after the first and 50th cycle. The morphology difference was noted only with EtMgBr, which appears to have cracked in the 50th cycle.
Inorganic halide-based electrolytes were synthesized with a non-nucleophilic source of MgCl2 and Lewis acid, such as A1C1„ AlPh„ and AlEtCl, [25] Both MgCl2-AlEtCl2 and MgCl2-AlCl, are compatible with sulfur atoms and, therefore, can be used in Mg-S batteries. Robert et al. [9] chose the same system using different solvents having anodic stability greater than 3.1 V, including THF and DME. The Mg aluminum chloride complex was formed by an acid-based reaction similar
FIGURE 8.3 FE-SEM images on surface morphology of Au electrodes after charge- discharge cycling: with ethylene Mg bromide electrolyte (EtMgBr), (a) first cycle, and (b) 50th cycle, with organo haloaluminate electrolyte (Mg(AlCl2EtBu)2), (c) first cycle, and (d) 50th cycle. Adapted and reproduced with permission from reference [11]. Copyright 2014 Elsevier.
to that of transmetalation of a Grignard-based complex. Equation 8.3 represents the reaction:
Inorganic salts are capable of forming highly reversible Mg deposition. Homoleptic dialkoxide improves performance of halide ions containing Mg ions. Bulkier phenyl groups reduce deposition overpotential, low cycling performance, and cell impedance. Homoleptic compounds are a class of compounds containing Mg (HMDS)2, Mg dialkoxide, and Mg bisamide compounds, but the reversible plating of halide- free Mg compounds is poor. Mg (OCPh3)2:AlCl3 is reactive and imparts low deposition overpotentials. It is inferred that adding a small amount of alkoxide increases anodic stability and improves deposition morphologies [26].
