Confined Growth And Electrochemical Performances Of Molybdenum Nitrides For Lithium Ion Batteries

Molybdenum nitrides is a compound in which nitrogen occupies a gap in a tightly packed metal structure.Molybdenum nitrides have higher theoretical specific capacity,but their measured capacity is much lower than their respective theoretical specific capacity.Therefore,how to make full use of the higher molybdenum nitrides'theoretical capacity has become the current research focus.It has been shown that the nanoscale space between the layers formed by the accumulation of graphene sheets and the nanoscale space formed by the cross-linked graphene network can be used as a template for the confined growth of nanomaterials.In this study,a series of molybdenum nitride electrode materials were prepared by different preparation methods and composites with graphene,and their electrochemical properties and lithium storage mechanism were studied.The specific research contents and results are as follows:Mo₂N thin film electrodes were prepared by DC magnetron sputtering.The film electrodes with different morphologies?Mo₂N-3,Mo₂N-4,Mo₂N-5?were obtained by adjusting the ratio of Ar to N₂?3:1,4:1,5:1?.The XRD results show that all the prepared Mo₂N film electrodes have face-centered cubic structure.When the gas ratio is 4:1,the thickness of the prepared Mo₂N thin film electrode is about 1.5?m,and it has nanocolumn array structure with an average column width of about 2-5 nm.This structure endows Mo₂N-4 with better lithium storage performance.The effects of different atmosphere?N₂ and H₂/N₂ mixture?on the molybdenum nitrides phase were investigated by program temperature control method.The XRD test results show that the samples treated in pure N₂ atmosphere contain some MoO₂in addition to Mo₂N,while the samples treated by annealing in H₂/N₂ mixture are Mo₂N.In order to further investigate the lithium storage performance of Mo₂N,the effects of temperatures?600?,700?and 800??on the crystal form andelectrochemical performance of Mo₂N in H₂/N₂ atmosphere were compared.The results show that the samples obtained at 600?and 700?are Mo₂N??-Mo₂N?with face-centered cubic crystal,while the samples obtained at 800?are Mo₂N??-Mo₂N?with tetragonal crystal.Electrochemical test results show that the?-Mo₂N prepared at700?has relatively good rate and long-term cycling performance.Based on the optimized temperature,using graphene oxide as a template and adjusting the contents of graphene oxide?20%,30%,50%?,composite materials of Mo₂N quantum dots and nitrogen-doped graphite were prepared?Mo₂N-QDs@Ngs?.In this process,the controlled growth of Mo₂N was realized,which changed the Mo₂N from the initial bulk morphology to the quantum dot morphology.There Mo₂N quantum dots were evenly distributed on the surface of the graphene sheet.The results show that the Mo₂N-QDs@Ngs electrode material with graphene oxide compound content of 30%has good electrochemical performance:it exhibited highly reversible capacity of 403 mAh g^-11 after 100 cycles under the current density of 0.5 A g^-1.Due to the samples calcined in N₂ atmosphere contain part of MoO₂,which has higher theoretical specific capacity.Therefore,the advantages of Mo₂N and MoO₂can be fully utilized to further improve the lithium storage performance of molybdenum nitride.Under the condition that the optimal content of graphene oxide,the effect of differrent temperature in the N₂ atmosphere on the electrochemical lithium storage performance of molybdenum oxynitride quantum dots and nitrogen-doped graphene composite electrode materials?MON-QDs@Ngs?were explored.The electrochemical performance tests showed that the MON-QDs@Ngs electrode material prepared at 600?has the best electrochemical performance,which exhibited highly reversible capacity of about 200 mAh g^-11 after 260 cycles at the current density of 5 A g^-1.