In Situ Scanning Electron Microscopy Observation Of Si/Graphite Composite Anodes

A great challenge in novel lithium-ion battery(LIB)industry is the increasing energy density demand of portable electronic devices and electric vehicles.Si has been considered as the most promising anode materials for LIB because of its high gravimetric and volumetric capacities.But its capacity will severely decline because of the great volume change in application.Carbon materials have a higher electrical conductivity,stable structure,and small volume expansion(less than 10%)during lithiation and delithiation.In application,Si/C composite anode combines the advantages of both Si and carbon materials and thus become one of hot topics in research.In this dissertation,by in-situ electro-chemical-scanning-electron-microscopy(ECSEM)technique,we have systematically investigated the behavior of Si/C composite anodes during charging and discharging processes.(1)The in-situ ECSEM system was designed and built.Combining the electrochemical tester and SEM,the real-time observation of electrode materials during charging and discharging processes could be realized by using this system.We can acquire variety performances of samples(such as cyclic voltammetry(CV)and galvanostatic cycling)by using the electrochemical tester.SEM equipped with an energy dispersive spectrometer(EDS)can characterize morphology,the types and contents of the constituent elements of electrode materials.(2)Two-dimensional(2D)unstructured Si/C composites were prepared using a tape-casting film coater,and line/grid structured Si/C composite materials were generated by laser processing.Through testing the electrochemical performance of the three different structured Si/C composite anodes,we found that the performance of three-dimensional(3D)structured Si/C composite anodes was better than that of 2D unstructured Si/C composites.This is because of the channels generated by laser can release the volume change of Si/C composite.(3)The in situ observation of morphology variation of the three types of Si/C anodes(unstructured and 3D-line/grid-structured)was performed during the lithiation and delithiation processe The effect of different structures on electrode materials' performance and the mechanism of better performance caused by 3D structure were studied.We found that the expansion of the 3D-line-structured anode exhibits a linear response to the charge capacity(lithium intercalation)before the defect formation in the contact area.Based on the in situ experimental results,a coarse optimization of 3D-line-structured anodes was proposed.(4)The lithiation mechanism of Si particles was studied by XRD and TEM techniques.We found that Si Ox layer reacted with Li,forming Li2 O islands on the surface of Si nanoparticles during the early stage.At the later stage of lithiation,pulverization of Si particles was observed,which lead to the loss of capacity and the failure of batteries.