| Rechargeable magnesium(Mg)batteries are one of such promising "beyond lithium-ion batteries"owing to several distinctive advantages,namely,the bivalent nature of Mg2+ leading to a high volumetric capacity(3833 vs 2062 mAh cm-3 for Li),the abundant magnesium resources in the earth's crust,and the operation safety of Mg metal anode due to its dendrite-free property.However,the incompatibility of many cathode materials with electrolyte hinders the development of Mg batteries.Two-dimensional transition metal carbon/nitrogen compounds(MXene)have attracted more and more attention because of their high electrical conductivity,abundant surface functional groups and adjustable layer structure.This thesis takes MXene as the research object,and constructs MXene-based composite materials by introducing different types of materials(C nanospheres,metal ions,nanoparticles)between the layers,the purpose is to increase the layer spacing and make full use of the terminal groups.Furthermore,the Mg storage performance is significantly improved by adjusting the composition,structure and morphology of the MXene-based composite material,and the Mg storage mechanism and structure-activity relationship are systematically studied.First,the Ti3C2@C nanocomposite with sandwich structure was prepared by electrostatic self-assembly method.The successful introduction of C nanospheres has significantly enlarged the layer spacing and increased the specific surface area of Ti3C2 MXene,and this method can effectively alleviate the stacking of Ti3C2 nanosheets.When used as the cathode of Mg batteries,Ti3C2@C nanocomposite possesses a high reversible specific capacity of 198.7 mAh g-1 at 10 mA g-1 and 85%capacity retention after 400 cycles.The de-intercalation mechanism and migration kinetics of Mg2+ions in Ti3C2@C nanocomposite were explored via the theoretical calculations.Compared with Ti3C2,V2C has a thinner structure and larger specific surface area;however,compared to the magnesium storage performance of Ti3C2@C nanocomposite,V2C@C nanocomposite have a lower the Mg storage capacity(67.4 mAh g-1)and first Coulombic Eficiency(CE)(27.5%).In view of this,pre-lithiation of V2C is carried out through the principle of self-discharge.The pre-intercalation of Li+ ions can combine with F on the surface of V2C to form LiF,which can make up for the Li+ ions consumed during the formation of the SEI film,thereby improving the the reversible specific capacity and first CE.Furthermore,in order to promote the rapid transmission of Mg2+ ions,a Mg-Li hybrid battery was constructed by introducing LiCl to 0.4 M(PhMgCl)2-AlCl3/THF.The pre-lithiated V2C realizes the Mg2+/Li+co-intercalation in this battery system.It possesses a considerable reversible specific capacity of 230.3 mAh g-1 at 20 mA g-1 and 82%capacity retention after 480 cycles.Since the C nanospheres in Ti3C2@C nanocomposite materials only play a role in increasing the interlayer spacing,they hardly provide magnesium storage capacity.In order to obtain a MXene-based composite with a large interlayer spacing and multiple types of Mg storage mechanism,the Ti3C2/CoSe2 composite material was obtained by selenizing Ti3C2/ZIF-67.The CoSe2 nanoparticles increase the interlayer spacing of Ti3C2,and CoSe2 can generate a strong Co-O-Ti covalent bond with the oxygen functional groups on the surface of Ti3C2.The formation of this covalent bond significantly enhances the interaction between the two materials,accelerates the transmission speed of electrons and ions,thereby enhancing the structural stability of the Ti3C2/CoSe2 electrode.Thanks to the synergy of the Mg storage mechanism of the intercalated MXene Ti3C2 and the converted CoSe2,Ti3C2/CoSe2 composite cathode has excellent rate performance(20 mA g-1 135 mAh g-1;1000 mA g-1,75.7 mA h g-1),and the capacity retention can upto 76%after 500 long cycles.Furthermore,the Ti3C2/CoSe2 also exhibits good rate performance(20 mA g-1,116 mAh g-1)and cycle stability(the capacity is maintained at 67.3 mAh g-1 after 70 cycles)in a quasi-solid Mg battery. |
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