magnesium sulfide (MgS) is formed when Magnesium (Mg) reacts with Sulfur (S), a non-metal anion, to form an ionic bond between the two species. Mg and S have equal charges which cancel out to make the compound neutral.
Chemical Formula : MgS – magnesium sulfide
magnesium sulfide is a wide band-gap direct semiconductor which emits blue/green light at the short wavelength of ultraviolet light. The electrons in the MgS band are resonantly excited by ions in the metal anode, which is why it can be used as a photodetector.
It has a relatively low specific energy on the cell stack level and is not considered to be particularly expensive. However, it does require a very clean environment and the production of cells may be costly.
Compared to Li based electrolytes, the overall oxidation potential of Mg polysulfides is lowered in high dielectric permittivity solvents like DMSO and DMF. This is attributed to an effective solvent complexation of Mg2+, which weakens the cation-polysulfide interaction.
A further way to lower the oxidation potentials of Mg polysulfides in electrolytes is to limit the discharge reaction to the formation of MgS2. This approach results in a lower overpotential and a more reversible charge-discharge cycle at the sulfur cathode90,93,96,97.
The oxidation of Li and Mg polysulfides in ethers is also controlled by the cation-polysulfide interactions, which are strongly dependent on the relative dielectric permittivities. In ethers with comparably low cation-polysulfide repulsion, a strong disproportionation of polysulfides to sulfur and high charge density species like S42- and S22- is observed84.