Preparation And Electrochemical Performance Of SiO/Carbon Composite Anode Materials For Lithium Ion Batteries

Lithium ion batteries(LIB)have broad prospects of the field in electric vehicles and grid storage.However,the limited reversible capacity and poor rate performance of the 372 mAh g^-1 of commercial graphite anodes currently do not satisfy the high energy density lithium ion batteries.The development needs have stimulated many studies on alternative anode materials.Silicon monoxide anode materials have become one of the candidates for lithium ion battery anode materials due to their advantages such as high theoretical specific capacity,suitable lithium intercalation potential,low cost,and abundant reserves.However,the large volume expansion of silicon monoxide anode materials(approximately 200%),low first-time Coulomb efficiency and low electrical conductivity(6.7×10^-4 S cm^-1)and other defects restrict its industrialization.In this thesis,micron-sized SiO is used as the raw material,and gelatin is used as the carbon source to prepare the silicon oxide/carbon composite anode material,and the electrochemical performance of the material is improved by single atom and diatomic doping.The introduction to butyl ester into titanium dioxide further improves the material's first efficiency.A new composite anode material with improved electrochemical perform ance was obtained by combining hetero atom doping and second phase coating.The effects of nitrogen doping on the properties of silicon carbon composite were studied.The effects of sintering temperature,sintering time and nitrogen addition on the electrochemical properties of the materials were studied.The electrochemical analysis shows that the capacity and cycle retention of the material increase with the incorporation of nitrogen atoms.After comparing the electrochemical properties of samples of different sintering temperature and sintering time,the optimal condition of the experiment is 850°C×4 h.Under these conditions,the effects of different sample ratios of the electrochemical properties of the composite were compared.Under these conditions,the combination of carbon coating and nitrogen atom can promote the composite to show better electrochemical performance than SiO material.At the current density of 0.1 A g^-1,the first reversible capacity and the initial coulomb efficiency of SiO/NC are 1294 mAh g^-1 and 70.6%,respectively.Compared with 998mAh g^-1 and 61.2%of original SiO,the first reversible capacity and the initial coulomb efficiency of SiO/NC are significantly improved.At the current density of 4A g^-1,SiO/NC releases 539 mAh g^-1,while SiO only provides 155 mAh g^-1.In addition,at 1 A g^-1 current density,the capacity retention rate of SiO/NC after 200cycles is 88%,much higher than the 10.2%of SiO.At the same time,on the basis of nitrogen doping,calcium carbonate was introduced as a template to prepare SiO/N-doped porous carbon composite,which provided sufficient space to relieve the pressure of volume change of silicon oxide,and further improved the long-term cycle performance of the material.Compared with the unused calcium carbonate as a template,the capacity retention of SiO/N-doped porous carbon composite increased from 88%to 99%after 200 cycles.By introducing additional phosphorus atoms to control the bonding configuration of doped nitrogen,the effects of phosphoric acid and phatic acid as phosphorus source,the amount of phosphorus source and sintering temperature on the electrochemical properties of the materials were studied.After comparing the electrochemical properties of samples of different sintering temperature and sintering time,the best test conditions are obtained.After choosing different doping ratio to treat the samples to obtain the best performance,the electrochemical analysis shows that the performance of SiO/NC is improved to a certain extent after introducing additional phosphorus doping,especially for the samples treated with phosphoric acid,the first effect is further improved The capacity retention rate is also significantly increase d.Through the analysis of XRD,Raman,SEM,TEM,XPS and electrochemical properties,it is found that after the introduction to phosphorus doping,the larger size of phosphorus atom instead of carbon atom will enlarge the plane spacing of carbon layer,induce more defects and edge positions,and obtain high content of pyridine nitro Gen The introduction to high content pyridine nitrogen is considered as an effective method to improve the electrochemical performance.On the one hand,it can enhance the electronic conductivity,so that lithium ions can react with most of the silicon domain.On the other hand,it can promote the diffusion of lithium ion by inducing more defects,and has high reactivity,The results show the first coulomb efficiency of best sampleis 72%,the capacity after 300 cycles is 96%,and the reversible capacity is as high as 650 mAh g^-1 under high current density 4 A g^-1,while the first charge discharge efficiency of the undoped phosphorus sample is 70.6%.The retention rates after 300 cycles are 50.1%.The discharges specific capacity is only 539mAh g^-1 at 4 A g^-1.Using a simple green"one-pot"sintering modification method,the loss of lithium during the first intercalation process is reduced by the reaction of titanium dioxide and silicon dioxide in the silicon monoxide material to g enerate titanium silicate(Ti SiO₄),which significantly improves the initial efficiency of the material.Titanium dioxide reacts with amorphous silicon dioxide in silicon monoxide to form titanium silicate,consuming silicon dioxide produced by the disproportionation reaction to silicon monoxide at high temperature,effectively reducing the side reaction against lithium ions and silicon dioxide,which helps to improve the initial coulombic efficiency of the material.In addition,titanium silicate is stable during the cycle and can be used as a buffer layer to relieve the volume expansion of silicon monoxide during the cycle,thereby improving the cycle stability.Tetrabutyl titanate was used to improve the performance of silicon monoxide.The effects of different temperatures,different titanium doping levels,and whether the carbon layer was coated into the initial Coulomb efficiency and other electrochemical properties were studied.Electrochemical analysis shows that after modification with titanium dioxide,the performance of the sample has been improved to a certain extent.The treated SiO@Ti SiO₄/C sample has a first efficiency of 77.9%,while the first effect of SiO/C is 66.0%,and the first the effect of SiO is only 61.2%.In addition,the capacity of the SiO@Ti SiO₄/C sample remains the initial capacity after 150 cycles is 87.9%,and the reversible capacity is as high as 4 A g^-1 at high current density 486 mAh g^-1.