Syntheses And Electrochemical Performances Of Anode Materials Based On Carbon Coated Composite Structure For Lithium Ion Batteries

Energy crisis and environmental pollution are two main problems today.It has become an urgent task to develop green and environmental friendly clean energy and energy conversion and storage devices.Lithium-ion batteries have been used in various fields like 3C electronic products and electric vehicles because of their their high energy density,working voltage,no memory effect and environmental friendly.In the development of many lithium-ion battery anode materials,the theoretical specific capacity of graphite anode materials is only 372 mAh·g^-1,which can not meet the market demand,while that of other new anode materials,such as silicon-based,tin-based and transition metal oxides,sulfide,phosphates,etc.,which is found that there are two problems:low conductivity of the materials and large volume change in the process of recycling,to which carbon coating is the most common modification method.Based on the method,the anode materials of silicon oxycarbide?SiO_x/C@C,SiO_x/C@void@C?nanospheres and cobalt phosphide?Co_xP@NC?nanowires with carbon coated composite structure are successfully prepared,which can effectively improve the electrochemical performance of the materials from the experiment result.The specific research contents are as follows:In the first part of the work,the SiO_x/C@C-x?x is carbonization temperature?nanospheres with carbon coating structure are prepared by Stober method using 3-Mercaptopropyltrimethoxysilane?MPTS?as template and sulfur source to form a thiol-modified?-SH?organosilicon template,dopamine as carbon source and nitrogen source for coating,calcined at different temperatures?800,900,1000 and 1100??,and then etched with HF for a certain time to obtain SiO_x/C@void@C-x nanospheres with yolk shell structure.The experimental results show that with the increase of carbonization temperature,the lithium storage performance of SiO_x/C@C-x and SiO_x/C@void@C-x nanospheres are also improved correspondingly,which is due to the increase of graphitization degree of carbon,and the performance at 1100? is not improved much compare to 1000?,which may be the reason of the destruction of spherical shell surface structure caused by carbonization at higher temperature,indicating that the best lithium storage performance at 1000?.In addition,the introduction of nitrogen and sulfur in the synthesis process also greatly improves the conductivity of nanospheres and it is found that the lithium storage performance of SiO_x/C@void@C nanospheres is also better than SiO_x/C@C-1000 nanospheres,which is mainly due to the fact that SiO_x/C@void@C nanospheres have more buffer space after HF etching part of SiO2,which can better reduce the volume change rate during the cycle.The first discharge specific capacity of SiO_x/C@void@C electrode material is 1923 mAh·g^-1,and the specific discharge capacity after the 2 and 100 cycles are 946 mAh·g^-1 and 879 mAh·g^-1,respectively,which repesents the capacity retention rate of 93.6%,while the capacity retention rate of SiO_x/C@C-1000 electrode material is 78.0%at 0.1 A·g^-1.At the same time,the discharge specific capacity of SiO_x/C@void@C electrode material is kept at 584 mAh·g^-1 after 500 cycles at a high current density of 1.0 A·g^-1,while that of SiO_x/C@C-1000 electrode material is only 384 mAh·g^-1,and its stability is also better than that of SiO_x/C@C nanospheres,so it is concluded that SiO_x/C@void@C-1000 has the best electrochemical performance.In the second part,a kind of CoOH?CO3?0.5·11H2O?Co-CH?nanowires are synthesized by hydrothermal method,which are used as Co metal source and template,while 2-methylimidazole?2-MeIM?are used as organic ligand.A layer of MOF?ZIF-67?is grown in situ on the surface of Co-CH nanowire.After high temperature carbonization and phosphating treatment,the Co_xP@NC nanowires with carbon coating structure derived from ZIF-67 are obtained.At the same time,as a contrast material,Co-CH nanowires are directly phosphated to obtain CoP nano wires.Through comparative study,it is found that the lithium storage performance of Co_xP@NC electrode material is much better than that of CoP electrode material.At the low current density of 0.1 A·g^-1,the first discharge specific capacity of Co_xP@NC electrode material is 2146 mAh·g^-1 and the specific discharge capacity after the 2 and 100 cycles are 1143 mAh·g^-1 and 928 mAh·g^-1,respectively,which shows the capacity retention rate is 81.2%,while the discharge specific capacity of CoP electrode material is 191 mAh·g^-1 after 100 cycles,and the capacity retention rate is only 18.1%After 700 cycles at a high current density of 1.0 A·g^-1,the specific discharge capacity of the Co_xP@NC electrode material remains 526 mAh·g^-1,while that of the CoP electrode material is only 140 mAh·g^-1.This is due to the unique structure of the Co_xP@NC electrode material with the ZIF-67 derived carbon coating layer,which not only can improve the electron mobility and lithium ion diffusion rate on the one hand but also slow down the circulation of the Co_xP nanoparticles of volume expansion during the process.In addition,the method of in-situ growth on the nano wires template can avoid the agglomeration of ZIF-67 particles in the process of high-temperature carbonization,leading to Co_xP@NC have better lithium storage performance.