Nongraphitized Carbon Materials

As mentioned earher, the nongraphitic carbons can be classified into two categories, soft and hard carbons. Different levels of graphitization can be obtained from the nongraphitic carbon materials, depending essentially on the heat treatment... 

The mechanism of lithium intercalation into nongraphitized carbon materials is as yet unclear. But at least three types of interaction between lithium and the carbon material have been suggested interachon with graphene layers, with the surface of polynuclear aromatic planes, and hthium intercalation into microcavihes on the front surface of the carbon material. 

It was established already in the first period of works on sodium ion batteries that contrary to the lithium ion, the sodium ion is not intercalated into the interlayer space of graphite. Sodium ions penetrate nongraphitized carbon materials, but the nature of this penetration is not intercalation. In the case of oil coke, the capacity values of 90 - 95 mAh/g were obtained, which approximately corresponds to the composition of NaC24. In the case of carbon black electrodes, capacity of about 200 mAh/g was obtained. Quite suitable materials for the negative electrode could be different varieties of nanoporous hard carbon (obtained, e.g., by pyrolysis of glucose). In this case, intercalation of sodium ions is provided not only by their intercalation into the interlayer space, but also by their adsorption on the inner nanopore surface. The capacity of electrodes of nanoporous hard carbon reaches 300 mAh/g. Most recently, negative electrodes of carbon nanotubes with nearly similar sodium intercalation capacity were described. 

Furthermore, the preparation of graphitic carbon materials requires high-temperature treatment, which is disadvantageous in production cost. The nongraphitic carbon obtained at around... 

FIG U RE 12.2 Typical voltage profiles from carbon materials with different heat-treatment temperatures and different structural variations. The profiles were obtained at the second cycle, and (a) and (b) show discharge and charge cycles by using graphitizable carbon heat-treated at 3000°C, graphitizable carbon heat-treated at 2000°C, and nongraphitic carbon obtained at 700°C. (Reprinted from Endo, M., et al., Carbon, 38, 183, 2000. With permission.)... 

Figure 10.20 Structure model of different carbon materials, (a) nongraphitic carbon exhibiting inherent slit-shaped micropores, (b) multiwall carbon nanotube (MW-CNT),...

The obvious questiou now arises What is the impact of the precursor heteroatoms on the end-product at HTT > 2800 C The auswer should be obvious from the preceding discussion production of intermediates (i.e., partiaUy graphitized carbons, occurring between the nongraphitized carbons, such as saccharose-based ones, inunature kerogens, and low-rank coals) with P, = 0, or < 0.1 and the highly graphitized materials with Pj > 0.75. 

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