Preparation And Electrochemical Properties Of Metal Oxide/carbon Composite As Sodium-Ion Batteries Anode Materials

With the depletion of fossil fuel reserves and the increasingly serious environmental problems,developing green energy system is becoming extremely important.lithium-ion batteries are widely applied to all aspects of society,In many systems,due to their high energy density and long cycle life.However,the shortage and uneven distribution of lithium resources limit the application of lithium-ion batteries in large-scale energy storage.Sodium ion batteries is similar to lithium ion battery.At the same time,sodium resource has abundant reserves and low cost,so they have greater advantages in the field of large-scale energy storage.The same as lithium ion batteries,excellent anode materials have great influence on the performance of sodium ion batteries.In the types of transition metal oxide materials,iron oxide has great potential for development because of its high capacity,low price and environmental friendliness.Unfortunately,the rapid volume change of iron oxide and poor conductivity restrict the development of iron oxide materials and hamper its application in sodium ion batteries.Based on this background,a new composite anode material of iron oxide and graphene is synthesized.(1)In the experiment,the graphene oxide was synthesized by improved Hummers method.Then Fe2O3/RGO composite was prepared by hydrothermal method and its sodium storage property was studied.After characterization and research,it is found that Fe2O3 in Fe2O3/RGO composites presents hollow ellipsoidal structure,which is uniformly loaded on graphene sheet.The sodium storage capacity of Fe2O3/RGO composites increased greatly compared with that of pure Fe2O3 Among them,the capacity remains of FRG-3 is 224.8 mAh·g-1 after 50 cycles at current density of 50 mA·g-1,the efficiency of coulombic efficiency is about 98%,and the rate performance has also been improved.(2)After synthesizing Fe2O3/RGO composites,nitrogen was introduced into graphene,and Fe2O3/N-GNS composites were prepared by hydrothermal method.The results show that the composite structure is three-dimensional network and iron oxide particles are homogeneous.Under nitrogen doping,three kinds of nitrogen atoms in graphene enhance the conductivity of the material and improve the sodium storage capacity of the composite.When the current density is 50 mA·g-1,the capacity of Fe2O3/N-GNS composite material remains 306 mAh·g-1 after 50 cycles.In addition,Fe2O3/N-GNS also has good rate performance.The capacity retention at 500 mA g-1 and 1000 mA g-" is 178 mAh·g-1 and 132 mAh·g-1 respectively,showing excellent storage properties of sodium ions.(3)In the experiment,graphene oxide was reduced to graphene by thermal expansion first,and then Fe2O3/GNS composite was synthesized by solvent thermal method,the sodium properties of the composite were studied.The results show that iron oxide particles in Fe2O3/GNS composites are irregular in shape.Among them,FGN-3 has the highest initial charge and discharge capacity,661.7 mAh·g-1 and 386 mAh· g-1,respectively,but the capacity decay is too fast.The cycle stability of FGN-1 is the best,after 50 cycles,the capacity is 223.4 mAh·g-1.The performance of Fe2O3/GNS composites is not ideal and needs improvement.(4)After synthesizing Fe2O3/GNS composites,Fe2O3@GNS composites were prepared by nanocoating methods.In the experiment,Fe2O3 particles were placed on the graphene lamellae,and the crystallinity of Fe2O3 particles was low,approaching the amorphous form.The Fe2O3 particles with indefinite form have small volume expansion and increase the transport channel of sodium particles.The conductive network composed of graphene can also improve the charge transport and ion migration rate,and buffer volume changes.Therefore,Fe2O3@GNS composites exhibit excellent sodium storage properties.The results show that when the current density is 50 mA· g-1,the specific capacity is 357 mA·g-1 after 100 cycles.In terms of rate performance,the specific capacity of Fe2O3@GNS composite material is 149.4 mAh·g-1 and 120.6 mAh·g-1 respectively at 500 mA·g-1 and 1000 mA·g-1.