Fabrication And Electrochemical Properties Of Molybdenum-based Nanocomposites

With the actual demand for higher and higher electric vehicle range,how to improve the energy density and cycling stability of lithium batteries has become an international research frontier and hot spot.The current preparation processes of electrode materials in lithium-ion batteries are complicated,in particular the capacity,rate performance and the cycle stability are poor.Moreover,the shuttle effect of lithium polysulfides and the slow electrochemical reaction kinetics restrict the practical application of lithium sulfur batteries.To address these issues,this thesis focuses on platinum-like and metallic phase molybdenum-based compounds with special electronic energy band structure as the research objects,taking advantages of the inherent high conductivity and high electrochemical activity of the two types of compounds,and further improving the electrochemical properties of electrode materials through structure control and interface design:(1)constructing a porous structure with large specific surface area,can promote the penetration of electrolyte and increase the mobility of lithium ions;(2)designing electrode materials with unique nanostructure can enhance the cycle stability;(3)introducing the highly conductive graphene or carbon nanotubes can increase the electrical conductivity of electrode materials and improve the rate performance of batteries;(4)fabricating the synergistic catalytic effect nano-level active materials can reduce the electrode polarization and promote electrochemical reaction kinetics.In addition,the first-principle calculations combined with in situ experiments were used to reveal the mechanism of electrochemical performance enhancement,which provides guidance for the design of electrode materials.The main research contents and results are as follows:1.Pt-like Mo₂C quantum dots composites were designed,constructed and their electrochemical properties were investigated.(1)Metal-like conductivity Mo₂C quantum dots-anchored carbon nanotube conducting network(Mo₂C@CNT)was prepared on a large scale using an industrial spray drying method,and their electrochemical properties were investigated as anode for Li-ion batteries and sulfur host for Li-S batteries,respectively.It is shown that the reversible specific capacity of Mo₂C@CNT is as high as 1065 m Ah g^-1 at a current density of 0.1 A g^-1,and the specific capacity nearly no decay after 1000 cycles at a current density of 1.6 A g^-1,and can be stably cycled even at a very high current of 16 A g^-1.The in situ Raman spectroscopy reveals the electrochemical reaction process and mechanism of Mo₂C@CNT.The Mo₂C@CNT/S(MCN/S)anode discharges at 0.2 C with a specific capacity of 1303.3 m Ah g^-1,and still shows a super-stable cycling performance at 1 C high current with the specific capacity decay rate of only 0.019%after 1200 cycles.The nucleation experiments of Li₂S combined with first principles calculations and in situ Raman characterizations reveal the electrochemical mechanism of the excellent performance of MCN/S.(2)The interfacial layer was designed and constructed on the surface of commercial PP separator with in situ growth of Mo₂C quantum dots on the surface of N-doped graphene nanosheets,and its mechanism study for Li|Li symmetric cells and Li-S cells were carried out.It is shown that the Li|Li symmetric batteries based on MQD@NG/PP maintains stable voltage fluctuation even after cycling for 1600 h at 10m A cm^-2 without short circuit.The interaction between C atoms of Mo₂C and graphene to make it easier to wet electrolyte and effectively promoting the uniform deposition of lithium.The excellent performance mainly comes from the porous nanostructure,high conductivity and strong polarity of MQD@NG.The porous structure facilitates ion diffusion,the high conductivity facilitates electron transfer,and the strong polarity guarantees its strong chemical absorption of lithium polysulfide and thus efficiently suppresses the shuttle effect.MQD@NG/PP separators have enabled a high capacity of1230 m Ah g^-1 and a stable cyclability of over 400 cycles.2.3D porous graphene modified with Pt-like MoP quantum dots(MQD@NG)was designed and prepared as sulfur host,and the electrochemical properties and mechanism of MPQ@G/S cathode were investigated.The MPQ@G/S cathode has enabled excellent electrochemical performance with a specific capacity of 1184.5 m Ah g^-1 after100 cycles at 0.2 C and nearly no decay after 600 cycles at 1 C.The main mechanism of the excellent electrochemical performance of MPQ@G/S is that the three-dimensional porous and highly conductive graphene matrix not only boosts electron transfer,but also facilitates electrolyte permeation and ion diffusion.In addition,MoP has strong polarity and high metal-like conductivity,and its platinum-like electronic structure gives it excellent catalytic properties,making it efficient and fast catalytic conversion of Li PSs while chemisorption of Li PSs,which effectively improves the electrochemical reaction kinetics.3.The porous composites of metallic phase 1T-MoS₂ nanotubes and porous graphene(GMNT)were designed and synthesized.The electrochemical performance and mechanism of GMNT/S cathode were investigated.It is shown that the spray drying method can be used for the efficient preparation of graphene-encapsulated nanocomposite.The reversible specific capacity of GMNT/S is still as high as 1219m Ah g^-1 after 200 cycles at 0.2 C,and the decay rate of specific capacity is as low as0.039%per cycle at 1 C.The main mechanism of the excellent electrochemical performance is that the metallic phase 1T-MoS₂ nanotubes and the highly conductive graphene matrix facilitate electron transfer,and the adsorption experiments confirm the strong chemisorption of the strongly polar 1T-MoS₂ to Li PSs,thus inhibiting the shuttle effect of Li PSs.The high specific area and abundant active sites of 1T-MoS₂ facilitate the efficient catalytic conversion of long-chain Li PSs and promote the kinetics of redox reactions and enhance their electrochemical properties.4.Metallic phase 1T?-MoTe₂ quantum dot graphene modified aerogel(MTQ@3DG)were designed and synthesized.It is shown that MTQ@3DG/S exhibits excellent performance with a high specific capacity of 1310.1 m Ah g^-1 at 0.2 C and a super stable cycling performance,the capacity decay rate is only 0.026%per cycle after 600 cycles at a high current density of 1 C.The mechanism of the excellent electrochemical performance of MTQ@3DG/S is that the metallic phase 1T?-MoTe₂ quantum dots and the high electrical conductivity of the graphene aerogel matrix facilitate electron transfer,the three-dimensional porous structure facilitates electrolyte penetration and lithium ion diffusion,and 1T?-MoTe₂ has abundant active sites thus realize efficient catalytic conversion of long-chain Li PSs and promoting Li₂S nucleation.Theoretical calculations indicate that 1T?-MoTe₂ has strong chemisorption ability and low free energy barrier of the potential limiting step for Li PSs.Electrochemical in situ Raman spectroscopy experimentally confirms the efficient suppression of the Li PSs shuttle effect by MTQ@3DG.The synergistic interactions of the above factors effectively enhance the electrochemical performance of Li-S batteries.