| Magnesium hydroxide（MH）and magnesium oxide are important components of magnesium compounds.MH is widely used in environmental protection and flame retardant fields while magnesium oxide is often used as a sintering agentand an auxiliary material for a variety of electronic materials because of its high chemical activity and strong adsorption properties due to the ratio of surface atoms to bulk atoms.Magnesium hydroxide as a flame retardant has attracted much attention due to its high decomposition temperature,green and non-toxic advantages.However,due to its hydrophilic and hydrophobic surface and large surface energy,it is easy to aggregate and has poor compatibility with organic polymer matrix.Therefore,it is necessary for MH to be modified,which can change its surface properties to improve its flame retardant performance.Because of its special properties,magnesium oxide has been used to improve the cycling stability of lithium-sulfur batteries having attracted much attention in recent years.Three silane coupling agents are employed to modify MH by wet modification.By studying the influence of reaction time,reaction temperature,amount of modifier and stirring speed on MH modification,the optimum process conditions for the modification of magnesium hydroxide are obtained and the modification effect of MH with Vinyltriethoxysilane（VTES）is the best among the three modifier.The contact angle of MH modified by VTES（VTES-MH）to water 108.4°is the biggest among the three modified MH while contact angle to dimethyl silicone oil is 66.3°.The surface of VTES-MH changes from hydrophilic and hydrophobic to hydrophilic and hydrophobic.The surface energy and particle size both decrease and the dispersion was improved.Characterized by X-ray photoelectron spectroscopy（XPS）,it was proved that Si-OH in VTES react with hydroxyl on the surface of MH to form Si-O-Mg bond during the modification process so that VTES is coated on the surface of magnesium hydroxideDifferent MH/silicon rubber composites are prepared by adding different amount of MH.The combustion grade of MH/silicone composite with the addition amount of original MH 40%and VTES-MH 50%can reach 94V-1 and 94V-0.The LOI are 35.7%,40.2%,respectively.That is because a more stable network structure forms between VTES-MH and methyl vinyl silicone rubber by the free radical reaction between the vinyl in VTES and the vinyl in methyl vinyl silicone rubber under the existence of vulcanizing agent.MMgO is prepared with cetyltrimethyl ammonium bromide（CTAB）added by hydrothermal method and the influence of Mg2+concentration and the addition amount of CTAB on MMgO morphology is studied.When Mg2+concentration is 0.04mol L-1,the cube MMgO particles with a specific surface area of 123.58 m2g-1,a pore volume of 0.21 cm3g-1and a pore diameter of 6.35 nm are obtained.On the basis of this condition,hexagonal sheet of MMgO particles,of which specific surface area,pore volume and pore diameter are 181.56 m2g-1,0.26 cm3g-1and 9.11 nm,respectively are produced when the addition amount of CTAB is 5%.The composite made by this kind of MMgO mixing with sulfur is used as the positive electrode of Li-S battery.The electrochemical performance of Li-S battery prepared with MMgO:S weight ratio of 90:10 is the best.At 0.2 C,the retention rate of specific discharge capacity after 100 cycles can reach 84.70%,higher than 79.22%when commercial MgO are added at the same ratio.According to the XPS characterization,in addition to the adsorption effect of MMgO on lithium polysulfide,they also interact with each other to form thiosulfate and polysulfate complexes,These substances can promote the transformation of long-chain polysulfide to short-chain polysulfide,which can promote the electrochemical reaction in lithium sulfur batteries. |
