The Electrochemical Performances Of Spinel Anode Materials For Lithium Ion Batteries And The Component Of SEI Film On Graphite

As one of the most important energy sources,lithium ion batteries（LIBs）have attracted more and more attentions.LIBs have high energy density,less safety problems,long cycling life and low toxicy.With the rapid development of electric vehicles,the demand for LIBs with high power density and enery density is increasing gradually.Meanwhile,it is very important to implore the suitable electrode materials to match the development.Graphite as a commen curmercial anode material has many advantage,such as:low price,good conductive and low charge/discharge voltage platform,but its theoretical capacity is only 372 mAh g^-1.In this way,transition metal oxides have been studied for their high theoretical capacities as a new kind of anode materials.In addition,electrolyte is another important part of LIBs.The solid electrolyte interphase（SEI）film is formed on the surface of the anode material during the cycling process,which is directly related to the performance of the LIBs.It is meaningful to further study on SEI film.This dissertation is focusing on the study of the storage performances of lithium of spinel transiton mental oxides,the performances of binder free graphite（BF-graphite）with two different electrolytes and the SEI films formed on the surface of BF-graphite.Co₃O₄ electrode is prepared by a hydrothermal method with Ti foil as the base.According to the results of XRD and SEM,the morphology of Co₃O₄ is nanowire with a length of 1²μm,and the diameter of Co₃O₄ nanowire is¹00 nm.The galvanostatic charge and discharge test is carried out to measure the electrochemical performance of Co₃O₄ electrode.The result shows that the initial discharge and charge specific capacities are 1019 and 887 mAh g^-1,respectively,with a coulombic efficiency of81.3%.Afer 50 cylces,the charge specific capacity of Co₃O₄ electrode is 489 mAh g^-1,and the corresponding capacity retention is 55.1%.Fe₃O₄ is synthesized by a one-step method,with the assistant of SDBS.According to the results of XRD and SEM,the morphology of Fe₃O₄ is uniform nanosphere with a diameter of¹00 nm.Fe₃O₄/C electrode materal is further prepared by a carbon-coated process of Fe₃O₄.The diameter of Fe₃O₄/C nanosphere is also¹00 nm,and the thickness of the carbon layer coated on the surface of Fe₃O₄ nanophere is⁵ nm.From the date by galvanostaticcharge and discharge test,the lithium storage perforce of Fe₃O₄/C electrode materal is better than Fe₃O₄.It displays that the initial discharge and charge specific capacities of Fe₃O₄/C electrode are 1359 and 1017 mAh g^-1,respectively,with a coulombic efficiency of 77.7%.Afer 100 cylces,the charge specific capacity of Fe₃O₄/C electrode is797 mAh g^-1,and the corresponding capacity retention is 78.3%.The rate performance of Fe₃O₄/C electrode can be described as:the stable charge specific capacities of Fe₃O₄/C electrode under five current densities（0.1 C,0.5 C,1 C,3 C and 4 C）are 974,795,621,462and 264 mAh g^-1,respectively;when the current density is gradually reduced to 0.1 C,the charge specific capacity of Fe₃O₄/C electrode can rebound to 910 mAh g^-1.MgFe₂O₄ is manufactured through a sol-gel method.According to the results of XRD,SEM and TEM,the MgFe₂O₄ particle is nonuniform with a diameter ranging from 2μm-0.1μm,while the large MgFe₂O₄ particle is composed of small particles（¹0 nm）.The galvanostatic charge and discharge test is applied to value the electrochemical performance of MgFe₂O₄ electrode.The result exhibits that the second discharge specific capacity of MgFe₂O₄ electrode is 674 mAh g^-1 under 1 C.Afer 50 cylces,the discharge specific capacity of MgFe₂O₄ electrode is⁴93 m Ah g^-1,and the corresponding capacity retention is73.1%.The rate performance of MgFe₂O₄ electrode can be presented as:the stable charge specific capacities of MgFe₂O₄ electrode under four current densities（0.1 C,0.2 C,0.5 C and 1 C）are 837,712,590 and 408 mAh g^-1,respectively;when the current density is decreased to 0.2 C,the charge specific capacity of Fe₃O₄/C electrode can rebound to 716m Ah g^-1.ZnCo₂O₄ is manufactured by a solvethermal method with the assistant of SDBS and a further calcination process.According to the results of XRD,SEM and TEM,the morphology of ZnCo₂O₄ is uniform nanocluster with a diameter of¹00 nm,and the cluster is constituted by small particles（¹0 nm）.To evalue the electrochemical performance of ZnCo₂O₄ electrode,the galvanostatic charge and discharge test is applied.It can be seen that the initial discharge and charge specific capacities of ZnCo₂O₄ electrode are 1100 and 940m Ah g^-1,respectively,with a coulombic efficiency of 84.7%.Afer 100 cylces,the charge specific capacity of ZnCo₂O₄ electrode is 700 mAh g^-1,and the corresponding capacity retention is 74.4%.The rate performance of ZnCo₂O₄ electrode shows that the stable charge specific capacities of ZnCo₂O₄ electrode under five current densities（0.1 C,0.5 C,1 C,2 C and 3 C）are 884,731,505,345 and 188 mAh g^-1,respectively;when the current density is gradually dropped to 0.1 C,the charge specific capacity of ZnCo₂O₄ electrode can rebound to 698 m Ah g^-1.BF-graphite electrode is fabricated by electrophoresis method to evaluta the electrochemical performance of graphite with two different electrolytes（LiTFSI/PCand LiTFSI/G₃）and the SEI film formed on the surface of graphite electrode after charge/discharge cycling.On the study of LiTFSI/PC electrolyte system,the performance of BF-graphite electrode in six different concentrations of LiTFSI/PC electrolytes（2 M,2.2 M,2.4 M,2.6M,2.8 M and 3 M）is examined by the galvanostatic charge and discharge test.It can be concluded from the results that lithium ion can insert into and extract from BF-graphite electrode reversibly with a current density of C/20 in high electrolyte concentration（no less than 2.2 M）;the initial coulombic efficiency of BF-graphite electrode asends with the increase of the concentration of Li TFSI/PC electrolyte;the initial coulombic efficiency of BF-graphite electrode is 77.9%in 3 MLiTFSI/PC electrolyte.The results from the ex-situ characterization,a combination of IR and XPS,display that SEI film can be formed on the surface of BF-graphite electrode after cycling in high electrolyte concentration（no less than2.2 M Li TFSI/PC）;the main components of formed SEI film are Li₂CO₃,ROCO₂Li and LiF;With the increase of the concentration of LiTFSI/PC electrolyte,the contents of Li₂CO₃and ROCO₂Li are decreased while the content of LiF is increasing in SEI film on the surface of BF-graphite electrode.To evalue the performance of BF-graphite electrode in four different concentrations of LiTFSI/G₃ electrolyte（LiTFSI:G₃=1:1,1:2,1:3 and 1:4）,the galvanostatic charge and discharge test is used.It can be summarized from the results that lithium ion can insert into and extract from BF-graphite electrode reversibly in high electrolyte concentration（LiTFSI:G₃=1:1）,and the initial charge specific capacity is 172 mAh g^-1 with a coulombic efficiency of 46.0%;BF-graphite electrode in low concentration of LiTFSI/G₃ electrolyte（LiTFSI:G₃=1:2,1:3 and 1:4）can not show visibly reversible capacity.Interestingly,it can be clearly observed from the TEM images of BF-graphite electrode before and after cycling,a solid film can be formed on the surface of BF-graphite electrode in both LiTFSI:G₃=1:1and LiTFSI:G₃=1:4 electrolytes after cycling.The stable and passivated SEI film can only be formed on the surface of BF graphite electrode in high electrolyte concentration（LiTFSI:G₃=1:1）.