Modifications Of Molybdenum Oxides As The Anode Materials For Lithium Ion Batteries

Lithium ion batteries(LIBs)are considered as the most promising chemical power sources due to their high energy density,long life,good cycling performances and environmental friendly features.Therefore,they are widely used in mobile devices such as laptop computers,digital cameras and cellphones.With the development of high power equipment including electric vehicles(EVs)and plug-in hybrid vehicles(PHEVs),higher energy density and charge rate capability are required for LIBs.As one of the three major components of LIBs(cathode,electrolyte and anode),the anode material is important to the performance of LIBs and has attracted a lot of research attention.The ideal anode material should have a high lithium storage capacity,high charge rate capability and fast charge/discharge features.However,the current commercialized anode material,carbon,cannot meet these expectations.Molybdenum trioxide and molybdenum dioxide,which possess a relatively large specific capacity,have been investigated for a long time.However,mechanical degradation and pulverization resulted from severe volume change during charge/discharge,as well as low electrical and ionic dif7usion rate,restrict the applications of these materials.This dissertation focuses on modifications of molybdenum oxides to improve the performances of these materials in LIBs.Nitridation,nanomaterials preparation approaches and carbon coating have been attempted to increase the conductivity and lithium diffusion rate,as well as minimize the pulverization due to volume changes.1,Temperature programmed nitridations of bulk MoO3 and MoO3 nanobelts were achieved by using NH3.During the topotactic reaction between MoO3 and NH3,a fraction of oxygen ions are expected to be substituted by nitrogen ions,generating a new pathway for lithium intercalation and deintercalation,as well as improving the electrical conductivity,thus leading to better performances.The experimental data shows that two molybdenum oxynitride intermediates(phase X and Y)were obtained with improved performances.Phase X exhibits a high capacity of 980 mAh/g at a current density of 50 mA/g with no significant loss in capacity after 35 cycles.Phase Y also shows a high capacity of 878 mAh/g at a current density of 50 mA/g and a capacity of 805 mAh/g remains after 70 cycles.Both performances are much better than that of bulk MoO3.In addition,we used XPS and elemental analysis to obtain the nominal composition of phase X(MoO2.31N0.24).7Li solid-state NMR was used to confirm the conversion mechanism during lithiation and delithiation.2,MoO2/carbon hybrid composites were prepared by high temperature treatment of the MoO3/DDA precursor.The hybrid composites deliver a good capacity of 800 mAh/g and show a much better performance in cycling and rate capability than that of bare MoO2.It can be partially attributed to the layered structure of the composites,which is beneficial for the diffusion of Li+ by decreasing the diffusion path lengths.In addition,combining carbon with MoO2 improves the electronic conductivity of the composites and maintains the mechanical integrity of electrode,thus decreases the mechanical degradation and pulverization,leading to improved performances.3,By using salicylic acid as the template and the reducing agent,MoO3 nanobelts/RGO composites were obtained from in-situ growth of MoO3 nanobelts on RGO with hydrothermal method.The ultrahigh electrical conductivity of RGO can increase the conductivity of the composites while the nanostructure of the material decreases the diffusion length of Li+ in MoO3 and mechanical degradation during charge/discharge.The composites exhibit a high reversible capacity of 1000 mAh/g at a current density of 50 mA/g.There is only 20%capacity loss after 70 cycles,showing much significantly improved electrical performance.