Preparation Of Nitrogen-doped Carbon/Silica Composites And Study On Its Lithium Storage Properties

At present,the most used anode material for lithium ion battery on the market is graphite material,but its theoretical specific capacity is only 372 m Ah/g,which no longer meets the requirements of human society for high energy density anode materials.The theoretical specific capacity of silica is 1965 m Ah/g,and natural silica is abundant in the earth's crust,therefore silica is expected to become the next generation lithium ion battery anode material.However,the silica material itself is inferior in electrical conductivity,and silica is highly prone to volume expansion during cyclic charging and discharging,resulting in rapid decay of its battery capacity,hence carbon coating is considered to be an effective way to solve the above problems.In this paper,firstly,the modified St?ber method is used,with tetraethyl orthosilicate and concentrated ammonia as the silicon source and the basic catalyst,respectively.The high-yield,particle-controllable,monodisperse and spherical silica nanoparticles have been successfully prepared by controlling experimental factors such as the scale of the reaction system,the addition mode of tetraethyl orthosilicate,the amount of concentrated ammonia and the reaction time.The microstructure and morphology of the samples have been analyzed by XRD,FTIR and FESEM.The results indicate that in the improved St?ber method both adding the silicon source dropwide and the increase in the reaction system scale can significantly increase the sample yield,and the samples possess good microscopic morphology.In addition,as the amount of concentrated ammonia is gradually increased,the particle size of the prepared silica nanoparticles is gradually increased.Furthermore,as the reaction time was prolonged,the obtained nano-silica shows a smoother surface and a narrower particle size distribution,and the sample yield basically reaches the maximum when the reaction time is extended to 6 hours.Then,using the sacrificial template synthesis method for the nitrogen source,different nitrogen sources and carbon sources have been selected and mixed with the prepared nano-silica,as a result,different nitrogen-doped carbon/silica composites have been synthesized by high-temperature heat treatment.Through a series of characterizations of microscopic morphology,composition,structure and electrochemical performance,it is proved that the nitrogen-doped carbon/silica composites synthesized by using dicyandiamide and anhydrous glucose as nitrogen source and carbon source respectively exhibit good cycle stability.Moreover,its reversible specific capacity remains at approximately 254 m Ah/g after 500 cycles of charge and discharge at a current density of 2 A/g.Finally,the lithium storage mechanism of the nitrogen-doped carbon/silica composites is further explored by adjusting the amount and particle size of silica nanoparticles added.These results show that when the appropriate amount of nanosilica is selected for carbon coating,the carbon layer can not only improve the conductivity of the composites but also provide a good buffering effect on the volume expansion of silica material.Besides,as the particle size of the silica nanoparticles added decreases,the reversible specific capacity of the corresponding composites subjected to the high current impact can promptly recover even higher than the initial value,thus demonstrating its excellent rate performance.