Preparation And Electrochemical Properties Of Molybdenum Oxides Anode Nanomaterials For Lithium Ion Battery

Lithium ion batteries（LIBs）,as an energy storage device,have developed rapidly due to its advantages of their high energy density,good cycle lifespan,wide working range,safety and reliability,etc.And they covering all corners of our lives have been widely used in wearable electronic devices,convenient electronic products and other fields.With an increasing requirement for higher energy density,higher power density and more beneficial safety performance in various fields for LIBs,people have been looking for a new material with higher cycle stability and higher specific capacity to meet the ever-increasing energy-consuming demand.As the most traditional anode material,graphite satisfies the requirement for the most ideal electrode namely low potential,stability to electrolyte,good electronic or ionic conductivity,eco-friendliness and rich resources.However,its low theoretical specific capacity(372 mA g h^-1)and lithium ion diffusion rate(10^-10-10^-9cm² s^-1)can’t meet increasing needs of LIBs with superior performance.Molybdenum trioxide（MoO₃）and molybdenum dioxide（MoO₂）,as binary molybdenum oxide materials,both possess a layered structure analogous to graphite and have the high theoretical specific capacities of 1117 m Ah g^-1 and 838 m Ah g^-1,respectively.Compared with other metal oxides,molybdenum oxides have been widely used in photochromic devices,electrochromic,field emission devices,gas sensors and energy storage systems thanks to their appropriate band gaps（3.1 e V for MoO₃,2.8 e V for MoO₂）,their unique anisotropic/isotropic structures and thermostable performances.Molybdenum oxides are also suitable for anode materials of LIBs.However,the general micron-size molybdenum oxides have a poor ionic and electronic conductivity(?_MoO3=9.6×10^-7 s?cm^-1,?_MoO2=8.8×10^-5 s?cm^-1),and a sluggish kinetics for diffusion of Li⁺ions.Also,they suffer a large volume expansion of about 100～250%during the insertion/extraction leading to phenomena of internal stresses,electrode pulverization and loss of interparticle contact.This seriously affects their practical application as anode materials for LIBs.In order to solve the problem of poor electrochemical performance of micron-size molybdenum oxides,nanocrystallization and carbon coating methods have been adopted to improve their electrochemical performance.In this paper,MoO₃ nanosheets were prepared using DC arc discharge plasma as an evaporative heat source,micron-size Mo powder as raw material,argon gas and hydrogen gas as reaction gas.And then the prepared powders were calcined in a tubular furnace.The structure was characterized by XRD、TEM and Raman.The results show that with the increase of temperature,Mo is gradually oxidized to MoO₃,which proves that MoO₃ is a more stable oxidation product of Mo.At a current density of 0.1A g^-1,the initial charge-discharge specific capacities are 1074 m Ah g^-1 and 653.6 m Ah g^-1,respectively.At various current densities of0.1,0.4,1.6,3.2 and 6.4 A g^-1,the discharge specific capacities are 600,350,140,63 and 20m Ah g^-1,respectively.To a certain extent,MoO₃ nanosheets increase the contact area between the material surface and the electrolyte.And the special nano-structure also shortens the path of Li⁺in the internal transmission of electrode,thus improving the performance of LIBs partly.In order to further improve electrochemical performance of molybdenum oxide,（MoO₃NR_S/MoO₂NP_S）@C nanocomposites were prepared by the DC arc plasma method.XRD analysis shows that there are two kinds of molybdenum oxide phases in the composites.TEM analysis shows that MoO₃ is clubbed and MoO₂ is spherical in the composites.The first charge-discharge capacities of（MoO₃NR_S/MoO₂NP_S）@C nanocomposite electrode are 1038 m Ah g^-1and 930 m Ah g^-1 at a current density of 0.1A g^-1,respectively,and the first coulomb efficiency is up to 89.6%.At current densities of 0.1,0.4,1.6,3.2 and 6.4 A g^-1,the specific capacities are910 m Ah g^-1,675 m Ah g^-1,435 m Ah g^-1,320 m Ah g^-1,and 210 m Ah g^-1,respectively.The electrochemical performance of（MoO₃NR_S/MoO₂NP_S）@C is significantly improved compared with that of MoO₃ nanopowders.