Types of Natural Graphite

Natural graphite is classified into three general types flake (also known asp/umbago), crystalline (vein), and amorphous, varying in physical properties, appearance, chemical composition, and impurities. These differences stem from the type of precursor material (oil, coal, or other carbonaceous deposits) and the natural process by which graphite was formed. Table 10.2 summarizes the characteristics ofthesethreetypes.I lPl 

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Table 10.1. Characteristics and Properties of the Three Types of Natural Graphite...

Synthetic carbonaceous materials are widely used in these applications. Several types of synthetic materials (e.g. graphitized mesophase carbon microbeads (MCMB), graphitized milled carbon fiber, and even, initially, hard carbons) became the materials of choice at the time of commercialization of first successful lithium-ion batteries in late 1980s. New trends, mainly driven by cost reduction and need for improved performance, currently shift focus towards application of natural graphite. 

Fig. 3 (A) Schematic of structures of various types of carbons that can insert lithium reversibly. (B) Charge and discharge characteristics of natural graphite powder (NG-7) at the first cycle in the 1 M LiC104/EC -I-DEC (1 1 vol/vol). (From Refl l)...

Since PAN-based carbon fibers tend to be fibrillar in texture, they are unable to develop any extended graphitic structure. Hence, the modulus of a PAN-based fiber is considerably less than the theoretical value (a limit which is nearly achieved by mesophase fibers), as shown in Fig. 9. On the other hand, most commercial PAN-based fibers exhibit higher tensile strengths than mesophase-based fibers. This can be attributed to the fact that the tensile strength of a brittle material is eontrolled by struetural flaws [58]. Their extended graphitic structure makes mesophase fibers more prone to this type of flaw. The impure nature of the pitch preciusor also contributes to their lower strengths. 

Figure 65 shows commercialized LiCo02-natural graphite cells and Table 18 shows the specification of these batteries. There are two types of batteries cylindrical and prismatic. The cylindrical-type battery in Fig. 65 is called 18650 because its diameter is 18 mm and its length is 65 mm. 

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