Structural Design Of Novel Carbide-based Composite Nanomaterials And Energy Storage Applications

With the continuous depletion of fossil fuels and climate change,the exploration of renewable energy conversion and energy carrier has turned out to be an important research topic.For energy storage devices,alkaline metal ion secondary batteries?LIBs/SIBs/PIBs?are considered to be one of the key promising devices.So far,they've partially satisfied our need for small devices with powerful energy storage.However,when it is applied to electric vehicles,smart grid and aerospace fields,it requires much higher performance and lower cost by means of higher multiplier capacity,energy density,recyclability and resource reserve.In order to achieve these goals,the development and modification of new electrode materials play an important role.Several feasible strategies have been widely used,including constructing nanostructures,enhancing electrical conductivity,and increasing intrinsic activity.Based on this,in this paper,we designed and synthesized a new type of metal carbide and carbon matrix composite nanomaterials by combining the above strategies,and optimized their electrochemical properties.In addition,we deeply studied the electrochemical mechanism of high-activity electrode materials in detail through various characterization techniques and density functional calculation?DFT?.The main contents of this paper are as following parts:First,due to the high conductivity,low average working voltage and high theoretical capacity of molybdenum-based materials,it is expected to be an ideal LIBs anode material.In order to further improve its electrochemical performance,researchers have developed several common strategies,such as constructing various nanostructures and composite conductive materials.Apart from increasing the number of additional active sites and enhancing the electronic conductivity,further enhancing the specific activity of the intrinsic active sites is also an effective way to improve the electrochemical performance of the molybdenum-based electrode.Here,we designed and synthesized a series of 3D cross-linked macroporous Mo_xC@N-C nanocrystals with abundant Mo vacancy,high specific activity of intrinsic active sites,and nitrogen-doped carbon?NC?coating via a simple method.Benefitting from its unique structure,Mo_xC@N-C-2.5 can quickly transmit and store extra Li⁺,showing a high initial reversible capacity of 879.3 m Ah g^-1 at 0.05 A g^-1.In addition,Moreover,the Mo_xC@N-C-2.5shows a high discharge capacity of 825.3 m Ah g^-1 at 0.5 A g^-1 with an initial capacity retention of 61.9%after 200 cycles..As expected,this facile strategy can be extended to the synthesis of other high-performance LIBs electrode nanocomposites with abundant defects,numerous porous structures,and heteroatom-doped carbon coating.Second,carbon materials have attracted much attention as the most promising LIBs anode materials due to their advantages of high theoretical capacity and abundant natural resources.Inspired by the first part of the work,we consider to go the other way,the introduce of metal single atom or atomic clusters into the nitrogen-doped carbon framwork,adjusts its electronic structure and enhances its electronic conductivity,and simultaneously activates the intrinsic activity of the hybrid carbon materials in cooperation with single atom catalysis.Here,we have developed a simple and effective method for the synthesis of Co_n@N-C hybrids with Co single atoms and small Co atoms clusters?i.e.,Co_n@N-C-1 and Co_n@N-C-2?.Benefiting from the interconnected porous carbon nanostructures and a large number of highly intrinsic active sites?Co-N-C?,Co_n@N-C-1 electrode materials exhibit excellent lithium-ion storage properties when used as LIBs anode materials.At 0.1 C,it has a high initial reversible capacity of 1587m Ah g^-1,and it still maintains a high reversible capacity of 1000 m Ah g^-1 after 800 cycles at 5C.The experimental and theoretical results show that the Co-N-C with high specific activity can be used as catalyst to promote the formation of stable SEI membrane and the interconnected porous carbon nanostructure can promote the transport and storage of Li⁺.Based on this,we explore a new effective way to promote the transportation and storage of Li⁺.Thirdly,the development and synthesis of lithium/potassium ion battery?LIBs/PIBs?anode materials with high specific capacity and high stability is the top priority in the development of energy storage system.Inspired by the work in the first and second parts,we consider to develop a kind of material which can promote the formation of stable SEI with high conductivity of carbides.Here,we synthesized a highly active electrode material by template sacrifice.The sample has an N-doped 3D interconnect carbon framwork structure,the uniform distribution of Fe/Fe₅C₂ nanoparticles and a large specific surface area.Benefitting from its unique structure,abundant N-C/Fe active sites in the carbon matrix,enhanced electron transport and catalytic activity by Fe/Fe₅C₂.3DM Fe/Fe₅C₂@NC electrode material exhibited excellent lithium/potassium storage performance.For LIBs,3DM Fe/Fe₅C₂@NC exhibits an initial discharge capacity of 1513 m Ah g^-1 at 0.1 A g^-1.At 1 A g^-1,after 1000 cycles,the reversible specific capacity can still be maintained at 839 m Ah g^-1?capacity retention rate is 81.9%?.As for PIBs,at 0.05 A g^-1,it showed a high reversible specific capacity of 460 m Ah g^-1 and high cycling stability of 205 m Ah g^-1 at 0.5 A g^-1 after 1000 cycles with an extremely slow capacity decay of only 0.013%per cycle from the 10^th to 1000^th cycle.Finally,the strategy of the synthesis of 3DM Fe/Fe₅C₂@NC nanocrystals can be extended to fabricate other nanocomposites with metal nanoparticles and abundant active sites as anodes electrodes toward high-performance LIBs/PIBs in future.Finally,based on the lack of high voltage,high stability,and low cost SIBs anode materials,we developed a new Alluaudite-type compounds-Na₂VMn₂?PO₄?₃ for the first time,and investigated the structure information of this compound in detail?including lattice parameters,atomic sites,and coordinates?x,y,z??.It has a monoclinic lattice with the space group of C2/c,and the cell parameters of a=12.0494 A,b=12.6198 A,c=6.5170 A,?=114.7°.In addition,when used as SIBs cathode electrode,it shows a reversible discharge capacity of 80.4 m Ah g^-1 at 5m A g^-1.At 20 m A g^-1,Na₂VMn₂?PO₄?₃/C shows an initial charge capacity of 46.1 m Ah g^-1with the capacity retentions of 63.7%after 100 cycles.The electrochemical reaction process of the electrode material was analyzed via the Rietveld structure data and in-situ XRD technique,and determine the mechanism of insertion/extraction of sodium.