Mg-Ion Intercalation

In order to increase battery capacity, materials are needed in which the electro-chemically active ions undergo redox changes of more than one electron (e.g., Ni2+-Ni +, in a narrow voltage window, maintaining capacity at high 

The thermodynamic properties of magnesium make it a natural choice for use as an anode material in rechargeable batteries, as it may provide a considerably higher energy density than the commonly used lead-acid and nickel-cadmium systems, while in contrast to Pb and Cd, magnesium is inexpensive, environmentally friendly, and safe to handle. However, the development of Mg-ion batteries has so far been limited by the kinetics of Mg diffusion and the lack of suitable electrolytes. Actually, in spite of an expected general similarity between the processes of Li and Mg ion insertion into inorganic host materials, most of the compounds that exhibit fast and reversible Li ion insertion perform very poorly in Mg ions. Hence, there 

---

Beside the Mg " ion intercalation compounds, elemental sulfur also was explored as the conversion-type cathode. It was reported that a Mg-S battery had a 1200 mAh/g of specific capacity versus S at 0.89 V in the first discharge, but diminished to 394 mAh/g in the second discharge mainly due to the dissolution of long-chain magnesium poly sulfide (MgS , n > 4) [35]. 

Sutto TE, Duncan TT (2012) Electrochemical and structural characterization of Mg ion intercalation into RUO2 using an ionic liquid electrolyte. Electrochim Acta 79 170-174. doi 10.1016/j.electacta.2012.06.099... 

However, during efforts to develop rechargeable Mg batteries in recent years, it was realized that the selection of materials suitable for the Mg ion insertion presents a great challenge. In fact, in spite of the expected similarity between Li and Mg ion intercalation, almost all inorganic compounds, which prove themselves as suitable cathode materials for Li batteries, show very poor electrochemical performance regarding Mg ion insertion. 

The discovery of Chevrei phases as interesting hosts for reversible Mg ions intercalation resulted from many unsuccessful experiments of Mg insertion into well-known Li" ion hosts, as well as from the literature concerning the possibility of divalent ion intercalation in inorganic materials. This analysis revealed that Chevrei phases are unique materials that allow a relatively fast insertion of divalent cations [62] such as Zn, Cd " ", Ni, Mn " ", Co and Fe. In the intercalation processes into Chevrei phases, the six metal Mo ions can be regarded as a single ion that can accommodate up... 

In the case of stearate ions intercalated in Mg/Al LDHs, the electron density shows a pronounced minimiun in the center of the interlayer region, which is consistent with the presence of a bilayer of guest molecules [151,152], The electron density distribution for an anionic azobenzene derivative intercalated in an Mg/Al LDH has also been reported [ 153]. 

Other types of insertion electrodes of interest — Graphitic carbons can also insert anions at high potentials (e.g., PFg ion potentials of >4 V vs. Li/Li+, from polar aprotic LiPF6 solutions). There are reports on Mg-insertion electrodes. For instance, Mo6 X8 chevrel phases (X = S, Se) intercalate reversibly with magnesium ions (2 ions per formula). Mg ions can be inserted to VOx compounds and to cubic TiS2. 

Figure 13.16 Model geometry of intercalated Mg ions, optimised at the B3LYP/DZ level, (a) perspective view along the molecular normal, (h) view along a crystal plane. This geometry was used for the calculation of the Huang-Rhys factors visualised in Figure 13.15.

Possible co-intercalation of water and solvent molecules has been inferred, which may help shield the charge on the Mg " ion but can also cause severe structural distortion as well as decompose within the cathode material. Water incompatibility with the anode is another issue. 

Hence, when reviewing important aspects of non-aqueous magnesium electrochemistry, it is important to consider the reversibility of Mg deposition processes, to map possible corrosion process of Mg electrodes, and to determine the anodic stability of electrolyte solutions in which Mg electrodes behave reversibly. In the following sections, we will briefly review conventional non-aqueous electrolyte solutions and the passivation of active metals in non-aqueous solutions, after which we will describe systematically the behavior of Mg electrodes in various types of conventional and non-conventional non-aqueous electrolyte solutions. Finally, we will review in brief another important aspect of non-aqueous magnesium electrochemistry, which is the electrochemical intercalation of Mg ions into inorganic hosts. 

It was highly interesting to explore the effect of increasing the polarizability of the anionic framework of Chevrel phases on the nature of Mg intercalation into these compounds. Consequently, Mg MogSeg has also been studied [64]. The replacement of sulfur by selenium as the anionic element in the Chevrel phase may reduce the intensity of attractive interactions between the intercalated Mg ions and the anionic framework of the host, thus reducing the diffusion bairieis. 

The anionic interchange method involves the dispersion of LDH materials into a monomer solution, most often in aqueous mediiun (path 1 in Fig. 6) [52-54]. The dispersion is then stirred for several horns with mild heating. To be able to replace the interlayer anion, the monomer molecules should have such functionality as can stabilize the layered structure by neutralizing the excess charge on the hydroxide sheets of LDH. For example, acrylate anions are intercalated into Mg - Al LDH through ion exchange with Cl" or NO3" present in LDH. Lee and Chen reported intercalation of acrylate and 2-acryloamido-2-methyl propane sulfonate into hydrotalcite [51]. These intercalated hybrids are then dispersed in an alkah-neutralized solution of the monomer and the polymerization is carried out in the presence of an initiator. Leroux and coworkers prepared vinylbenzene sulfonate monomer intercalated Zn-Al LDH [52] and aminobenzene sulfonate monomer intercalated Cu-Cr LDH [53]. Further, Tanaka et al. reported acrylate ion intercalation within LDH by replacing the nitrate anion from Mg-Al LDH [54]. 

These small upfield shifts are similar to the nonspecific Mg + ion effects on the duplex signals in tRNA (Gueron and Shulman, 1975 Gorenstein et al, 1981). This further supports the hypothesis that large (>2 ppm) downfield shifts are only observed for the intercalation mode of drug binding. 

---