Microwave-assisted Preparation And Modification Of Silicon-based Lithium Storage Materials

Silicon is very rich in natural matter?26.0%,second only to oxygen?.With the maturity of its related synthetic processes,silicon-based lithium storage functional materials have gradually entered the field of lithium battery electrode materials.Among them,the representative polyanion cathode material Li₂MnSiO₄ has attracted much attention because of its high theoretical capacity(333 mAh g^-1),good safety performance,environmental protection and low production cost.At the same time,silicon can also be used as a silicon-based anode material for lithium batteries,it is safer than traditional carbon anode materials,has higher lithium storage performance and theoretical capacity up to(4200 mAh g^-1),and is the most negative electrode material,so it is regarded as extremely potential lithium battery anode material with the highest capacity.In addition,microwave radiation is a new type of material synthesis method,which has the advantages of lowering the activation energy and cleaning efficiency compared with the traditional sintering method.This paper mainly focuses on the defects of low conductivity and poor cycle stability of the cathode material Li₂MnSiO₄.Firstly,the synthesis method of the material is screened and optimized by microwave conditions,in order to realize the microwave efficient synthesis of the material;Secondly,microwave assisted in situ the coating method is carbon-modified to obtain a more uniform carbon layer,thereby improving the conductivity of the material;on the basis of this,a microwave-assisted synergistic composite of mossy SiO₂-C and rare earth metal oxide CeO₂-C in order to further stabilize the structure on the basis of improving the conductivity of the material;After that,the ion core doping synergistic modification is carried out on the basis of microwave-assisted in-situ carbon coating,in order to form a pinning effect inside the material,thereby the material conductivity is improved and the material space structure is stabilized.In addition,this paper explores the microwave-assisted microwave heating mechanism and successfully prepares a new fluoropolyanion cathode material Li₃MnSiO₄F.In view of the large volume effect and poor conductivity of silicon-based anode materials during charge and discharge,the method of screening and optimizing porous silicon synthesis was firstly carried out,trying to create a porous structure on the surface of micro-silicon to inhibit its volume effect during the charge and discharge process;on this basis,combined with the unique heating mechanism of microwave,the microwave composite modification of porous silicon material and polypyrrole is realized,thereby further improving the conductivity of the material on the basis of stabilizing its spatial structure.The microwave assisted modification of the cathode material Li₂MnSiO₄ found that:Firstly,when the microwave conditions are 900 W?650°C and 150min.The Li₂MnSiO₄phase synthesized by microwave assisted sol-gel method has the highest purity and best electrochemical performance.Secondly,the microwave assisted in-situ carbon coating method can make the carbon layer more uniformly coated on the surface of the material than the conventional sintering method,and when the ascorbic acid is used as the carbon source and the carbon content is wt=10%,The material has the best crystallinity and the highest phase purity,and the first discharge specific capacity reaches 180 mAh g^-1.On the basis of the best microwave-assisted in-situ carbon coating,in order to further stabilize the spatial structure of the material,the rare earth metal oxide CeO₂ with good absorbing property was introduced.When the coating amount was 4wt%,the purity of the material phase was the highest.The best performance,and the cycle and rate performance is significantly improved compared to the blank material,the first discharge specific capacity reached 225 mAh g^-1,the capacity stabilized at 120 mAh g^-1 after 50 cycles,the capacity at 2.0 C large rate still There is about 100 mAh g^-1.Then,the Li₂MnSiO₄ material was synergistically coated with moss-like SiO₂-C.Although the electrochemical performance was not improved,the uniformly dispersed SiO₂-Li₂MnSiO4/C composite was obtained.Furthermore,on the basis of microwave-assisted in-situ carbon coating,Ni and Na are doped with Ni and Ce in the Li₂MnSiO₄/C material.When Na is introduced,the unit cell parameters of the microwave-assisted preparation become larger,and when the doping amount is 0.09,the crystallinity and phase purity of the material are optimal,and the first discharge specific capacity reaches 211 mAh g^-1,after 50 cycles.After being able to stabilize at 80 mAh g^-1;when introducing a rare earth element Ce with good absorbing properties,the?011?main peak of the material shifts significantly from a high angle to a low angle,and when the doping amount is 0.03,the material The phase purity is the highest and the crystallinity is the best.The first discharge specific capacity reaches 210 mAh g^-1,the coulombic efficiency is 90%,and after 50 cycles,the capacity is stable at 90 mAh g^-1;when the Ni doping amount is 0.05,the material The phase purity is the highest,and the capacity retention rate is 43%after 30 cycles.When the three ions are synergistically doped,the Li_1.91Na_0.09Mn_0.96Ni_0.03Ce_0.01SiO₄/C material is discharged for the first time at 0.1 C rate.The specific capacity is above 200 mAh g^-1.After 50 cycles,the capacity can be stabilized at about120 mAh g^-1,and the electrochemical performance is remarkably improved.Based on Li₂MnSiO₄,a new high-energy density fluoropolyanion cathode material Li₃MnSiO₄F was synthesized by microwave-assisted solid-phase method?liquid phase method?sol-gel method and hydrothermal method.It was found that the microwave-assisted hydrothermal method can successfully synthesize the fluorinated group into Li₂MnSiO₄,which makes the crystal structure of the material change significantly,and successfully synthesizes the Li₃MnSiO₄F material.Characterization by SEM and EDS revealed that the material was nano-sized particles and the F element was evenly distributed in the material.Afterwards,the optimum hydrothermal synthesis conditions were found to be 120°C for 12 h;the optimum microwave synthesis conditions were 900 W,650°C,and 90 min.On the basis of this,Li is doped with Na and Mn is doped with Co.After relevant tests,it is found that when the doping amount is Na-0.10 and Co-0.05,the discharge platform of the material is improved,and then the electrochemical performance is improved..In addition,the non-in-situ carbon coating method further improves the cycle performance of the Li₃MnSiO₄F material.For microwave-assisted modification of silicon-based anode materials,it is found that:Firstly,the de-alloying method is a fast and efficient method for pore-forming of commercial micro-silicon.After correlation test,it is found that the porous silicon prepared by this method has a large pore size?4.9399 nm?and a large Specific surface area?205.3001m²/g?,and after 100 cycles,the capacity is stable at about 400 mAh g^-1;Furthermore,in order to further improve the electrochemical performance of porous silicon,the polymer conductive polymer polypyrrole is selected and coated with microwave heating mechanism to modify the initial discharge specific capacity at a current density of 500mA/g.It reaches 4100 mAh g^-1 and above and the capacity is stable at 500 mAh g^-1 after 100cycles.