| The metallic Li anode has a high theoretical specific capacity and a low electrochemical redox potential,and therefore metallic Li is the potential anode in next-generation high-energy-density battery systems.Nevertheless,the practical implementation of metallic Li anodes is plagued by nonuniform Li plating/stripping,huge volume change,and unstable Li-electrolyte interface during Li plating/stripping process.Two-dimensional molybdenum nitride（2D MoN）has the characteristics of high electronic conductivity,strong lithiophilicity,high mechanical strength and perfect thermal stability.Meanwhile,the（002）crystal plane of 2D MoN exhibits small lattice mismatch with（001）crystal plane of Li,so that 2D MoN is a new type material for framework construction and modification of current collector.In this dissertation,the application of 2D MoN in the aspects of copper current collector surface modification,Li composite anode construction,Li anode interface optimization and separator modification are discussed,and the influences and mechanism of 2D MoN regulating Li deposition are studied.The main research contents and conclusions are as follows:（1）Aiming at the problems of high nucleation overpotential and lithium dendrite growth on Cu current collector,due to the poor lithiophilicity of Cu,the 2D MoN modified Cu current collector（MoN-Cu）is designed and prepared,so that to decrease the nucleation overpotential,induce flat Li plating and inhibit the growth of Li dendrite.2D MoN nanosheets with a size of 1μm and a thickness of 6.5 nm are prepared by heat treatment in NH3atmosphere with Na2CO3and MoS2as the precursors.The calculation results show that the binding energy between Li and 2D MoN（-5.05 e V）is much stronger than that between Li and Cu（-2.74 e V）.Meanwhile,the lattice mismatch rate between the2D MoN（002）crystal plane and Li（001）crystal plane is relatively low（1.6%）,which helps to induce flat growth of metal Li on the MoN（002）crystal plane.The nucleation overpotential of Li metal on MoN-Cu is 7.1 mV,which is much lower than that of Li on Cu（40.3 mV）,at current density of 1 mA cm-2.The Coulombic efficiency（CE）of Li plating/stripping on MoN-Cu is 97.6%at current density of 0.5 mA cm-2and capacity of 1m Ah cm-2for 400 cycles,and there is no Li dendrite showing on MoN-Cu.（2）Aiming at the problems of large volume change and poor cycle performance of lithium metal anode during the cycle process,the stacked Li-MoN composite anode is prepared to promote the cycling performance of the batteries.The effects of Li-MoN composite anode on Li plating/stripping behavior,Li dendrite growth and the volume change during Li plating/stripping process are investigated.In Li-MoN anode,2D MoN can induce the uniform deposition of Li between the MoN layers.By stripping Li metal with a capacity of 6 mAh cm-2,the thickness of Li-MoN anode decreases by 2.5%,which is much lower than the thickness change of Li foil（6.9%）.After re-plating Li metal with a capacity of 6 mAh cm-2,the thickness of Li-MoN electrode increased by 2.1%,which is still lower than the thickness change of Li foil（10.8%）.The polarization potential of the Li-MoN symmetrical cell is only 24.4 mV at current density of 1 mA cm-2and capacity of1 mAh cm-2for 2500 h.（3）Aiming at the problems of unstable interface of lithium metal anode and poor cycling stability under high current density,the Li3N-Mo layer modified composite anode（Li3N-Mo/Li）is prepared,through the redox reaction between 2D MoN and molten Li.The Young’s modulus（7.6 GPa）of the Li3N/Mo layer is higher than the Young’s modulus（about 3.0 GPa）of Li dendrites,which can inhibit the vertical growth of Li dendrite.The Li3N-Mo mixed protective layer can effectively inhibit the side reactions between Li and electrolyte,promote the diffusion of Li ions,and construct the Li F-rich SEI layer.The symmetric cell with Li3N-Mo/Li electrode can cycle stably for 3000 h at 5 mA cm-2and 1m Ah cm-2.And at high current density of 30 mA cm-2and high capacity of 5 mAh cm-2,the symmetric cell still shows a cycling lifespan of 900 h,which is 2 times as high as that of the symmetrical cell assembled with Li foil,showing excellent cycle stability at high current.（4）In order to solve the problems of poor mechanical properties and thermal stability of polypropylene（PP）separator,2D MoN modified PP composite separator（Mon-PP）is prepared by coating 2D MoN on one side of PP separator by mechanical blading method.The MoN modified layer improves the Young’s modulus（8.1 GPa）and thermal stability of the composite separator.As a consequence,the MoN-PP composite can effectively prevent the penetration of Li dendrite and ensure the structural integrity of the separator under high temperature conditions.The symmetric cell assembled with MoN-PP as separator and Li foil as the electrode can cycle stably for 2000 h at the current density of 60 mA cm-2and the capacity of 20 mAh cm-2.However,the symmetric cell assembled with PP as the separator gets failed after cycling with 200 h under the same conditions,indicating that the MoN modified layer on PP separator can greatly improve the cycle life of the battery. |
