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C: Energy Conversion and Storage; Energy and Charge Transport 4
Comparative Investigation of MgMnSiO and OlivineType MgMnSiS as Cathode Materials for Mg Batteries 4
Artuiro Torres, and M. Elena Arroyo-de Dompablo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02369 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018
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The Journal of Physical Chemistry
Comparative Investigation of MgMnSiO4 and Olivine-type MgMnSiS4 as Cathode Materials for Mg Batteries A. Torres and M.E. Arroyo-de Dompablo* MALTA Consolider Team, Departamento de Química Inorgánica, Universidad Complutense de Madrid, 28040 Madrid, (Spain) *Corresponding author:
[email protected] Abstract The identification of potential cathode materials is necessary for the development of a new magnesium-based battery technology. Most attempts focus on oxide and sulfide materials, which in general suffer, respectively, of poor Mg mobility and low intercalation voltage. New chemistries should be explored. In this work, we investigate the basic electrode characteristics of olivine-type thiosilicates MgMSiS4 (M = Fe, Mn) with the double challenge of (1) raise the low intercalation voltage of transition metal sulfides and (2) improving the poor Mg diffusion of the oxosilicate counterparts MgMSiO4 (M = Fe, Mn).
Density Functional Theory (DFT)
calculations corroborate both expectations. The calculated average Mg deintercalation voltage for MgMnSiS4 (2.31 V) is above that of virtual MgMnS2 compounds (around 1.8 V), accounting for the inductive effect of the Si4+ ion on the transition metal. The calculated energy barriers for Mg diffusion in MgxMnSiO4 are 0.75 eV at x ~1, and higher than 1.1 eV at x ~ 0 and x = 0.5. The energy barriers decrease to 0.68 and 0.76 eV in MgxMnSiS4 (x ~ 0, 1), thanks to the more covalent Mn-S bond (compared to the Mn-O bond) that renders a less oxidized Mn ions therefore favoring Mg2+ mobility. Although these results are promising, more work is needed to ensure the potential application of thiosilicates as cathode materials for Mg batteries.
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Introduction While the current state-of-the-art in rechargeable batteries is the Li-ion technology, research efforts are intensified towards the development of alternative electrochemical energy storage technologies to satisfy the ever-increasing demands for enhanced energy density
1-4
. Batteries
based on the intercalation of divalent ions (Ca2+, Mg2+) in inorganic hostages are a promising alternative. Compared to lithium insertion, for a given material and degree of insertion, the divalent cations (Ca2+, Mg2+), would double the delivered specific capacity. However, realization of the maximum degree of insertion (and hence specific capacity) requires an acceptable diffusion coefficient for the divalent ion in the host at any degree of insertion. Several reviews reunite recent attempts to identify potential cathode materials for Mg batteries
2-5
. Sulfur-based
compounds of transition metal ions (MxSy) offer a better diffusivity for Mg2+ than oxides, as (I) the more covalent M-S bond (compared to M-O) screens the electrostatic repulsions between the inserted Mg2+ ions and the transition metal ions and (ii) the softer sulfur anion suffer weaker electrostatic interactions with the Mg2+ ions 6-7. Accordingly, reversible Mg2+ insertion has been proved in several sulfides: Mo6S8-ySey (y=1,2) Chevrel phases, at an average potential of 1.1 V vs. Mg2+/Mg 8, in layered TiS2 at 1.3 V
9
and in the thiospinel-MgTi2S48 at 1.6 V 10. However,
the low average potential of these sulfur-based electrodes (i.e.
