| In recent years,lithium-ion(Li-ion)batteries have more widespread application in digital products,hybrid electric vehicle and electric vehicle fields.There are higher requirements about specific capacity,cycling and rate performance of Li-ion batteries.Traditional anode materials could not meet the increasing demand for high energy-density power sources.Therefore,great efforts should be devoted to search for the new alternatives with superior electrochemical performance.Citrate is a typical organic acid salt that composes of citrates and metal cation through complexing.By carbonization treatment under inert atmosphere,the metal cation can form metal oxide while the citrates are able to form porous carbon framework materials(C).As a result,the metal oxide is uniformly dispersed in porous carbon framework,further form metal oxide/ porous carbon in situ composite material.For calcium citrate,the carbonization products of CaO are attributed to in situ template and porous carbon material can be successfully synthesized through a carbonization method combined with a simple acid pickling treatment.Finally,the as-prepared porous carbon material is a new anode material to that of Li-ion batteries.As for zinc citrate and manganese citrate,the carbonization products of ZnO and MnO are common anode materials.After forming the composite structure with porous carbon material,porous carbon framework can be served as three-dimensional conduct network,further ensuring the fast and continuous transport of electrons in the electrode.Meanwhile,ZnO and MnO possess uniformly dispersion status in the porous carbon framework,leading to positive mitigation of the structure damage that caused by stress concentration after volume effect.In consequence,the special composite structure can not only guarantee the structural stability,but also directly solve the capacity decay problem that coming from volume effect and low electric conductivity.On the whole,the research mainly includes following three aspects.(1)At the basis of sheet,fiber and sphere calcium citrate,the influence of carbonization temperature,filtration and drying filtration on morphology and structure are studied.The results indicate that the optimal carbonization temperature is 700°C,the optimal filtration method is common filter funnel and the drying method is freeze drying respectively.Assessing the above conditions,shape-controlled porous carbon materials can be successfully synthesized.From the XRD patterns,it can be found a broad diffraction peak at 2?=24o,which can be identified as the(002)crystallographic plane of amorphous carbonaceous structure.The specific surface area for sheet,fiber and sphere porous carbon is 422,1090,1489m2/g,and the average pore diameter is ~18.11,19.17,18.18 nm,respectively.While using as anode material,the initial discharge/charge capacities of three porous carbon are 1046/600,1025/729,1108/714mAh/g.After 50 cycles,they exhibit a reversibility capacity of 515,589 and 643mAh/g respectively.Even at a current density of 1000mA/g,the discharge capacity of three samples is as high as graphite.By comparison,the sphere porous carbon with hierarchical three-dimensional(3D)assembled structure demonstrates the best cycling capacity and rate performance.(2)In this section,we studied the influence of preparation methods on morphology,structure and sample size.Furthermore,the property tendency of ZnO/C composite material under different carbonization temperature and holding time were investigated.The results indicate that the optimal preparation method is precipitation method with stirring action,the optimal carbonization temperature is 700°C and the optimal holding time is 3h respectively.As a result,the ZnO/C composite material possess typical sphere morphology with a size of 5~15?m.The specific surface area is 10.466m2/g,the average pore diameter is ~19.046 nm and the pore volume is 0.031cm3/g respectively.The atomic percent of Zn,C and O is 20.53%,46.11% and 33.35%.When serving as anode material,the 1st discharge/charge capacity is 985 /711mAh/g.After 50 cycles,the discharge capacity reaches as high as 522mAh/g.Under different current densities,the discharge capacity is 692.1,360.6,272.9,156.5 and 67.4mAh/g.Therefore,the ZnO/C composite material is able to effectively mitigate the volume effect,further improve the electric conductivity,specific capacity and cycling performance.As anode material,ZnO/C composite material exhibits a certain rate performance.(3)As for MnO/C composite material,the influence of carbonization temperature and holding time on morphology,structure and electrochemical performance were studied.The results indicate that the optimal carbonization temperature is 800°C and the optimal holding time is 3h.Meanwhile,the specific surface area is 10.482m2/g,the average pore diameter is ~18.873 nm and the pore volume is 0.032cm3/g respectively.From XPS results,it can be seen that the atomic percent of Mn,C and O is 11.78%,53.44% and 34.78%.Further,the initial discharge and charge capacity of MnO/C electrode is 831.7mAh/g and 599.1mAh/g.After 50 cycles,the discharge capacity is also as high as 522mAh/g.Under the current density of 50mA/g and 100mA/g,the discharge capacity is 550mAh/g and 415mAh/g.However,when the current density reaches above 200mA/g,obvious capacity decay occurs and cycling stability quickly deviates with the increasing values of current density.As a consequence,the MnO/C composite material demonstrates high specific capacity and good cycling performance.But the rate performance needs to further improve. |
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