Structural and electrochemical characterization of graphite materials for Li ion batteries

Abstract

The purpose of this study is to find a correlation between the crystallite sizes in thebasal plane direction of graphite materials, used in carbon anodes in Lithium-ionbatteries. Carbon anodes are the most common type of anode used in rechargeableLithium-ion batteries today. The two major causes of capacity fading in graphite anodesis structural disordering during cycling and SEI (solid electrolyte interface) formation,which causes a loss of active graphite material and loss of active lithium-ionrespectively.On this basis, the crystallite size in the basal plane direction La of graphite aremeasured by structural characterization methods for later comparison with the performanceof the graphite powder when used as a carbon anode. In this work, three characterizationmethods have been utilized to characterize four different types of graphitepowders. These three characterization methods are Raman spectroscopy, X-ray powderdiffraction and SEM analysis, with special emphasis on Raman spectroscopy dueto its flexibility when it comes to sample preparation methods. Three different samplepreparation methods have been carried out on the graphite powders before doingRaman measurements.A comparison of the results from the Raman measurements shows a clear trend inwhich the calculated R-ratio is dependent on the sample preparation method, wherethe R-ratio is largest for samples prepared as carbon anodes. It is very likely that thistrend is attributed to the increase in R-ratio due to mechanical milling of the graphitepowders as a part of the processing steps of the carbon anode sample.Determining the La value with X-ray powder diffraction is usually a difficult taskdue to poor resolution of the diffraction peaks necessary to derive the La value. Thisis due to preferred orientation of non-spherical shaped graphite powders when thepowder is applied directly on a Si-sample holder for X-ray diffraction measurements.However, by air spraying the graphite powders mixed with ethanol, the particle aremore random oriented, giving visible diffraction peaks needed to derive the La value.It has been shown that Raman spectroscopy is an unreliable method for determiningthe La value by comparing the results from X-ray with the results from Raman measurements.However, if Raman spectroscopic measurements are used together withX-ray measurements, a new linear relationship between the X-ray measured La valueand the Raman R-ratio can be derived.3