The Controllable Synthesis Of The Structure And Morphology Of Transition Metal Oxalate-based Composites And Study On Their Electrochemical Performances

Lithium-ion batteries(LIBs)have become the main power source for portable electronic devices,electric vehicles and electrochemical energy storage due to their light weight,large reversible capacity,high energy density and long service life.In recent years,the similar working principle of sodium-ion batteries(SIBs)and lithiumion batteries has attracted widespread attention,and sodium resources are abundant,economic costs are low,and large-scale energy storage applications are prospective.However,LIBs and SIBs,as the mainstream energy storage systems at present,cannot meet the demand for efficient new energy due to the influence of anode materials.Therefore,the research on battery anode materials with high energy density,high stability and high safety is a key factor for the development of LIBs and SIBs.Metal oxalates(MC2O4:M=Fe,Co and Ni)have received widespread attention due to their potential high capacity,and are one of the most promising anode materials for LIBs and SIBs.However,the problems of the transition metal oxalate electrode materials such as poor electrical conductivity and large volume change in the cycle process limit their further application in batteries.In this paper,the solvothermal method is used to control the morphology and structure and the introduction of graphene oxide to improve the conductivity and structural stability of the material,and the electrochemical reaction mechanism is preliminary discussed by in-situ XRD,in-situ Raman and other methods.The main research results of this paper are as follows:(1)The CoC2O4·2H2O with rod structure was synthesized by solvothermal method,and then reacted with graphene oxide by hydrothermal method to form reduced graphene oxide(rGO)encapsulated CoC2O4·242O composites(CoC2O4·2H2O/rGO).When it is applied to lithium-ion/sodium-ion batteries,it exhibits excellent cycle stability and rate performance,which can be attributed to the improved conductivity and structural stability of electrode materials after the introduction of graphene oxide.In the lithium-ion half-cell,the CoC2O4·2H2O/rGO electrode delivers a specific capacity of 1011.5 mA h g-1 at 0.2 A g-1 after 200 cycles,when the current density at 10 A g-1,the capacity remains at 434.5 mA h g-1 after 2500 cycles.In the lithium-ion full battery,the electrode provides capacity of 138.1 mA h g-1 after 200 cycles at the current density of 0.2 A g-1.In the sodium-ion half-cell,the electrode material capacity remains at 221.1 mA h g-1 after 100 cycles under a current density of 0.2 A g-1.Subsequently,we explored and analyzed the lithium storage mechanism of CoC2O4·2H2O/rGO electrode materials by in-situ XRD and in-situ Raman testing methods.(2)The rod-shaped Ni0.55Co0.45C2O4-2H2O/rGO composite was synthesized by a simple solvothermal method.When applied to lithium-ion/sodium-ion batteries,Ni0.55Co0.45C2O4·2H2O/rGO electrode material exhibits excellent cycle and rate performance,which is mainly due to:? NiO.55Co0.45C2O4·2H2O rod structure can provide more active sites,shorten the ion diffusion path;? the synergistic effect between the Ni0.55Co0.45C2O4·2H2O rod structure and rGO;?the rGO in the composite material can inhibit the volume expansion of the electrode material during the cycle.As electrode material of lithium-ion half-cell,at a current density of 0.5 A g1,the electrode material capacity remained stable at 1179.9 mA h g-1 after 300 cycles,when the current density at 5 A g-1,the capacity remains at 646.5 mA h g-1 after 1200 cycles.Combined with commercial LiCoO2 to form a lithium-ion full-cell,the capacity can be maintained at 144.6 mA h g-1 after 100 cycles at a current density of 0.2 A g-1.As electrode material of sodium-ion half-cell,the electrode provides capacity of 221.6 mA h g-1 after 100 cycles at the current density of 0.5 A g-1.As electrode material for sodium-ion full battery,the electrode material capacity remains at 269.9 mA h g-1 after 150 cycles under a current density of 0.2 A g-1.Apart from this,the reaction mechanism of Ni0.55Co0.45C2O4·2H2O/rGO electrode material was explored and analyzed by means of in-situ Raman test,and it was revealed that it is a conversion reaction mechanism in the electrochemical reaction process.