Modification And Properties Of Separators For Magnesium-sulfur Batteries

Magnesium（Mg）metal is a very potential multivalent battery anode material.Compared with metal lithium,which has been more researched at present,metal Mg as a battery anode electrode has many advantages,such as high volumetric capacity（3833 m Ah/cm³）,which is about twice that of metal lithium（2062 m Ah/cm³）;The reserves are abundant and the price is cheap,only 4%of lithium.More importantly,Mg is not prone to dendrite morphology during the electrodeposition process,and there is no obvious safety hazard.Sulfur（S）is a promising conversion-type cathode material with a specific capacity of 1672 m Ah/g,which is also low in price.The matched Mg/S cell has a unique advantage:the theoretical energy density is greater than 3260 Wh/L.However,Mg/S cell also have some defects,including the shuttle effect of the intermediate product of charge and discharge,magnesium polysulfide,and the non-conductivity of sulfur and its discharge product,magnesium sulfide.These problems make the development of Mg/S cell face many technical difficulties.Modification of the separator by rationally designing the material structure can effectively solve the above-mentioned problems and improve the cycle performance of the Mg/S cell.Therefore,we mainly carried out the following work:（1）The TiO₂/rGO nanocomposite is prepared by hydrothermal reaction,which is covered on the surface of the GF separator by means of vacuum filtration,and the multifunctional separator is obtained after vacuum drying.Because Ti O₂is compounded with r GO,one side of the separator has conductivity,and it is tightly combined with the positive electrode when the battery is assembled,thus acting as an upper current collector.Through testing of separators with different modification layer thicknesses,it is found that a thinner modification layer does not significantly improve the cycle performance of the battery.Only a modification layer with a certain thickness can effectively inhibit the shuttle of magnesium polysulfide and act as the upper current collector.The electrochemical reaction during the charge and discharge process provides additional reaction sites,improves the utilization rate of sulfur,and improves the cycle stability of the Mg/S cell.（2）A nanocomposite SnO₂/C with SnO₂loaded on the surface of carbon spheres was prepared by a simple hydrothermal method.It is used as a coating material to assemble a Mg/S cell to form a multifunctional intercalation layer between the sulfur positive electrode and the separator.It exhibits excellent cycle stability and good rate performance during electrochemical testing.Studies have shown that the polar Sn O₂/C intercalation layer inhibits the diffusion and shuttle of magnesium polysulfide through chemical adsorption and physical barrier.At the same time,the conductive Sn O₂/C intercalation layer can also be used as the current collector on the sulfur electrode to improve the transmission of ions and electrons on the surface of the electrode and improve the electrochemical reaction power rate on the surface of the cathode.