Study On Synthesis Of Iron-based Anodes Materials And Their Electrochemical Performance For Lithium/Sodium-ion Batteries

Iron-based materials attract researchers'attention due to their abundant earth reserves,nontoxic and high theoretical capacity?FeOOH:905 mAh/g;Fe₃O₄:926mAh/g;FeNCN:558 mAh/g?.However,like most transition metal oxides,iron-based materials also have problems such as poor electrical conductivity and obvious volume change during cycling process,which cause active materials to be pulverized and drop from the collector,eventually leading to continuous capacity decrease.At present,the solutions to above problems mainly include:synthesis of nano-sized structures,design of special structures and composite with carbon materials,which can improve the rate of electron transfer and ion diffusion in electrode materials,resulting in the enhancement of the rate performance and cycling stability.In this paper,FeOOH,Fe₃O₄ and FeNCN are investigated according to their problems respectively.The main results including:?1?Graphene-like nanosheets/amorphous carbon framework structure was obtained by sulfuric acid assisted hydrothermal method combined with subsequent high temperature KOH activation method with peanuts as raw material.This kind of carbon structure shows good electrical conductivity,which was used as substrate to grow FeOOH.Through adjusting the proportion of solvent water and glycerol,FeOOH/3D carbon composites were successfully fabricated,showing excellent lithium storage performance.?2?FeOOH nano arrays/rGO composites were fabricated by a simple hydrothermal method.Pulverization was controlled by constructing Fe-O-C chemical bonds,which promotes a predominated pseudocapacitance storage mechanism?Contributes 82%of the total capacity?.Such high extent of pseudocapacitive storage in FeOOH/rGO electrode achieves high energy density with superior cycling performance over 200 cycles at different current densities?1443 mAh/g at 0.2 A/g,1181 mAh/g at 1 A/g and 824 mAh/g at 5 A/g?.?3?Sandwich-like thin carbon layer coated Fe₃O₄ nanoparticles?¹5 nm?uniformly anchored on reduced graphene oxide sheets?Fe₃O₄@C/rGO?are achieved by solvothermal method with following pyrolysis.Fe₃O₄@C/rGO composites with nanosized and carbon coated structure show an excellent cycling performance of 594 mAh/g after 1000 cycles at a high current of 5.0 A/g.?4?Thin carbon layer coated porous Fe₃O₄ particles?¹00 nm?anchored on rGO sheets?Fe₃O₄@C/rGO?are sythisized,which shows a stable cycling performance of 356 mAh/g after 300 cycles at 0.1 A/g with a slight decay of 0.098mAh/g per cycle.Moreover,Fe₃O₄@C/rGO composite exhibits a high initial coulombic efficiency of 74%,superior to the values in most reported literatures for Fe₃O₄/carbon composites?⁵0%?.?5?FeNCN is obtained by a novel solid-state approach using CO?NH₂?₂ and?NH₄?₃Fe?C₂O₄?₃·3H₂O as raw materials.Synthesis mechanism of FeNCN is given,which provides a new way for the fabrication of other metal carbodiimides and will trigger the research of metal carbodiimides.Compared with iron oxides,iron carbodiimides show extremely high chemical reversibility and fast reaction rate.Iron carbodiimides exhibit excellent rate performance due to a predominated pseudocapacitance storage mechanism.The capacity of sodium storage can achive about 300 mAh/g at a high current density of 5.0 A/g.