Mo-based Nanocomposites For Lithium And Sodium Storage

Growing global energy consumption and associated environmental issues have stimulated great efforts to explore clean and renewable energy sources.Lithium ion batteries?LIBs?as a type of traditional energy storage device,are considered as main power sources for small portable electronics,electric vehicles,and hybrid electric vehicles because of its many advantages such as high energy density,high operating voltage,low self-discharge rate,long cycle life,and no memory effect.At present,commercial LIBs mainly use low-capacity graphite as an anode material,which cannot meet the demand for high energy density and power density.It is urgent to develop other anode materials to replace traditional graphite.In addition,sodium ion batteries have been widely studied due to their abundant natural sodium reserves and low cost.Molybdenum-based compounds are considered as promising candidates for efficient electrochemical energy storage systems owing to their unique physicochemistry properties,such as high electrical conductivity,and good mechanical and thermal stability.This dissertation focuses on molybdenum-based bimetal oxides and carbides,dealing with their fabrication,nanostructure regulation,lithium/sodium storage performance and lithium storage mechanism.The main contents are as follows:1.Using CoMoO₆·0.9H₂O nanorods as precursors,a series ofa/b-CoMoO₄heterogeneous nanorods with different r_?/?ratios were prepared by changing the calcination temperature.Among them,thea/b-CoMoO₄ heterogeneous nanorods with a high r_?/?of 6.0 afforded excellent lithium ion storage performance and fast reaction kinetics.This could be ascribed to the synergy between the two phases and the mutual buffering effect,which provided a rich phase interface to shorten the diffusion path of Li⁺,promoted electron transfer,and released the volume expansion during repeated cycles.The Li⁺storage mechanism was further analyzed via in-situ X-ray diffraction and ex-situ transition electronic microscopy.It's revealed that the?-CoMoO₄ follows a one-step conversion reaction;while,?-CoMoO₄ preceded an intercalation pathway followed by a conversion reaction,which could effectively alleviate the volume effect during repeated cycling.2.On the basis of CoMoO₆·0.9H₂O nanorod precursors,the conformal coated CoMoO₄@NC core-shell structures were prepared by in-situ polymerization and followed carbonization processes.The effects of nitrogen-doped carbon coating on the electrochemical performance and sodium ion storage kinetics of half-cells were investigated.Compared with pristine CoMoO₄ electrode,NC coating significantly improved the sodium storage performance and reaction kinetics of the CoMoO₄electrode.On the one hand,the N-doped carbon shells served as a buffer to accommodate severe volume changes during the sodiation/desodiation.On the other hand,nitrogen doping could improve electronic conductivity and activate surface sites,promoting reaction kinetics.Furthermore,CoMoO₄@NC as anode was matched with Na₃V₂?PO₄?₃ cathode for full-cell measurement.The full-cell presented reversible capacity of 151.7 m Ah g^-1,with capacity retention of 75%at 1 A g^-1 after 100 cycles.3.Using ammonium molybdate as molybdenum source and dopamine as carbon precursor,nitrogen-doped carbon-supported Mo₂C microspheres?Mo₂C/NC?were synthesized by a two-step method.The effect of calcination temperature on the performance of electrochemical lithium storage was explored.Moreover,the Li⁺storage mechanism was further analyzed via in-situ X-ray diffraction.It's revealed that the Mo₂C follows an intercalation pathway.The lithium storage mechanism still needed further investigation in combination with other in-situ and ex-situ characterizations.