Mg Desolvation and Intercalation Mechanism at the Mo₆S₈ Chevrel Phase Surface

Abstract

In this work, we examine the Mg-ion desolvation and intercalation process at the Chevrel phase Mo 6 S 8 cathode surface from first-principles. It is reported that in electrolytes based on chlorides in tetrahydrofuran (THF), Mg 2+ is strongly coordinated by the counterion Cl and can form singly charged MgCl + and Mg 2 Cl + 3 species in solution. During cell discharge, Mg intercalation into the Chevrel phase requires breaking the strong, ionic Mg-Cl bond. Our simulation results indicate that the stripping of Cl is facilitated by the existence of another cationic species, Mo on the Chevrel phase surface. Once Mg is intercalated, it leaves the counterion, Cl , on the surface, bound to Mo. It is found that the chlorinated surface presents higher activation barriers to further intercalate Mg. Instead, the chlorinated surface continues to interact with incoming MgCl + species and form various MgCl y surface adsorbates. With certain energy costs, the neutral MgCl 2 unit may be released from these surface adsorbates to reopen Mo sites on the surface and permit continuous Mg intercalation. Presuming compatibility of chloride electrolytes with the Mg metal anode, our work implies that finding a compatible cathode material will depend critically on its ability to catalyze Mg-Cl bond-breaking. This may explain the success of the Chevrel phase, with its open Mo sites, permitting intercalation of Mg from the halide solutions, whereas higher voltage transition metal oxides, which typically lack open metal sites, require more weakly coordinating anions in their electrolytes.

Layered TiS2 Positive Electrode for Mg Batteries

“…16−21 For example, it has been shown that the low Mg desolvation barrier on the CP surface from the all-phenyl complex (APC) 22 electrolyte is a key factor for its good performance. 23 In our search for sulfide-based Mg positive electrode materials, here we re-examine the first Li insertion positive electrode material: layered TiS 2 . 24 Chemical Mg insertion into this structure was demonstrated long ago, but the Mg sites were not identified.…”

High power rechargeable magnesium/iodine battery chemistry

“…The hurdle is the clumsy Mg 2+ intercalation because of the sluggish solid-state diffusion of the divalent Mg 2+ and the slow interfacial charge transfer. Compared with monovalent cations (Li + , Na + , K + and so on), the high charge density of Mg 2+ (twice as high as Li + ) inevitably raises the energy barriers for breaking its solvation sheath or ion-ligand pair upon interfacial charge transfer67. It also induces strong Coulombic interactions with the host upon ion insertion and hopping8 and causes difficulty for the host to accommodate electrons89.…”

Tuning the interlayer of transition metal oxides for electrochemical energy storage

“…When transferring from a liquid electrolyte, the ion typically needs to lose its solvation shell 48 and, in the case of multivalent cations, other ions. 49 In theoretical descriptions of charge transfer, an intermediate, transient step is assumed between the solvated stage and the intercalated stage. The difference in energy between the solvated stage and the transient stage is the activation energy for charge-transfer during intercalation (E a ), and may be related to the charge transfer resistance (R CT ) 48 :…”

“…MoO 3 hydrates are isostructural with the tungsten oxide hydrates. , which is due to (i) the Coulombic repulsion between the framework transition metals and the diffusing Mg 21 ions, (ii) the need for a transition metal to accept 2e À , which results in an increase in the ionic radius and thus unit cell volume, 70 and (iii) the strong solvation 71 and anion coordination 49 of Mg 21 , which raises the activation energy for charge-transfer at the electrode/electrolyte interface.…”