Determining the Degree of Graphitization of Graphite Anode Materials using Powder X-ray Diffraction
The degree of graphitization has an impact on the chemical and physical properties of synthet-ic graphite, for example, when used as an anode material in Li-ion batteries. Two methods for determining the degree of graphitization using X ray powder diffraction are explored. The first method involves using a parallel beam geometry to address and overcome peak shifts arising from sample transparency, while the second method utilizes a more traditional divergent beam geometry on a graphite sample mixed with an internal standard.
Introduction
Graphite is one of three naturally occurring forms of carbon (also known as allotropes) that consists of numerous stacked layers of hexagonal graphene. Under normal conditions, it is considered the most stable form of carbon and is found in great abundance within metamorphic rocks across the planet. Along with the high abundance rates, graphite also exhibits some highly desirable properties and characteristics, such as high stiffness and strengths at extreme temperatures (> 3600 °C) and high chemical resistance. Moreover, graphite is the most commercially successful anode material in Li-ion batteries, making it a key material in the rapidly growing field of energy storage.
A closer look at the internal structure of graphite reveals that the carbon atoms are covalently bonded to three neighboring carbons that ultimately form the layered hexagonal or honeycomb structure. The bonds between the carbon atoms in the layers are known to be extremely strong which gives graphene its strength, while the layers are held together by weaker van der Waals interactions and easily separate or slide past each other giving graphite its smearing behavior.
While natural graphite is formed by the reduction of sedimentary carbon, synthetic graphite is formed during the graphitization process of non-graphitic or amorphous carbon. The process involves heating amorphous carbon for prolonged periods of time to allow the rearrangement of the atoms into the layered structure. By performing X-ray diffraction on the synthetically formed graphite, the degree of graphitization (i.e. how close the structure is to the ideal graphite structure) can be determined. When graphite is used as an anode material in batteries, a higher degree of graphitization can lead to improved performance via better cycling stability and larger charge/discharge capacities.
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