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Molecules 2022, 27, 6516 7 of 32 degree of internal disorder, and more microporous structures. As shown in Figure 4, in terms of crystal parameters, La and Lc (the average width and thickness of crystals stacked along axis-a and axis-c) of HC and soft carbon are relatively close. The rise of pyrolysis temperature increased the La and Lc of soft carbon obviously; therefore, increasing its crystallinity and gradually turning it susceptible to graphitization. However, Molecules 2022, 27, x FOR PEER REVIEW 8 of 34 the change of La and Lc in HC is relatively gentle [59], proving that it is more difficult to undergo graphitization. Figure 4. Schematic diagram of La and Lc of soft and HC changing with treatment temperature Figure 4. Schematic diagram of La and Lc of soft and HC changing with treatment temperature (modified by reference [59]). 3.3. Sodium Storage Mechanism of HC (modified by reference [59]). 3.3. Sodium Storage Mechanism of HC The research on the sodium storage mechanism of HC materials provides SIBs with The research on the sodium storage mechanism of HC materials provides SIBs with several advantages in the development of a large-scale energy storage system with several advantages in the development of a large-scale energy storage system with long- long-standing stability. According to previous reports, the way Sodium-ions are stored in standing stability. According to previous reports, the way Sodium-ions are stored in HC HC materials can be through adsorption at defect-sites, edges, and heteroatoms, reversi- materials can be through adsorption at defect-sites, edges, and heteroatoms, reversible ble insertion and extraction between carbon layers, and filling or adsorption in mi- insertion and extraction between carbon layers, and filling or adsorption in microporous croporous regions. So far, there are several assumptions on the mechanism as follows. regions. So far, there are several assumptions on the mechanism as follows. 3.3.1. Intercalation-Adsorption 3.3.1. Intercalation-Adsorption Since Dahn proposed the “House of Card” model and used it to explain the “inter- Since Dahn proposed the “House of Card” model and used it to explain the “intercalation- calation-adsorption” mechanism, a large number of reports began to hypothesize and adsorption” mechanism, a large number of reports began to hypothesize and prove the prove the principle of sodium storage. As shown in Figure 5a, Dahn [38] indicated that principle of sodium storage. As shown in Figure 5a, Dahn [38] indicated that the inclined the inclined potential curve was attributed to the intercalation of sodium between paral- potential curve was attributed to the intercalation of sodium between parallel graphite lel graphite sheets, and the insertion potential changed with the increase in Sodium-ion sheets, and the insertion potential changed with the increase in Sodium-ion content, which content, which restricted the further insertion of Sodium-ions. Moreover, the reason for restricted the further insertion of Sodium-ions. Moreover, the reason for the plateau area the plateau area of the potential curve is the filling behavior of Sodium-ions in the mi- of the potential curve is the filling behavior of Sodium-ions in the microporous region, croporous region, which was formed by the stacking of disorderly layers of nanocrystal- which was formed by the stacking of disorderly layers of nanocrystalline graphite (the line graphite (the original quote is “through a process analogous to adsorption”). In the original quote is “through a process analogous to adsorption”). In the following year, following year, Dahn confirmed, via wide-angle in situ X-ray diffraction, that Sodi- Dahn confirmed, via wide-angle in situ X-ray diffraction, that Sodium-ions can be inserted um-ions can be inserted between the graphite layers of amorphous carbon [60], which led between the graphite layers of amorphous carbon [60], which led to an increase in the to an increase in the spacing between the graphite layers, and thus a low angle shift of the spacing between the graphite layers, and thus a low angle shift of the (002) peak and (002) peak and a decrease in its intensity. It was also proposed for the first time that the a pdoescirteioanseoifntuitrsboinstreantiscitsyt.acIktinwgaissadlissotripbruotpedosiendafochretmheicafilrsetnvtimroenmtheantt,thcaeupsionsgittihoen of tuprbootesntrtaiatlicustravcektiongbeisindcilsintreidbudtuerdinign tahcehcehmarigcealaenndvdirioscnhmaregnet., causing the potential curve tobeinFculrintheedrmduorein,gKtohmeacbhar[g6e1]ancdondfisrmcheadrgteh.e correlation between a graphite layer spaFcuinrgthaenrdmSordei,uKmo-mioanbiant[e6r1c]alcaotinofinr(mcheadngtheeocfo0r0r2elpaetaiokninbwetiwdtehenanadgarnagplhe)i,teaslasyhoerwsnpac- in Figure 5b. Raman spectroscopy was also used to investigate the mechanism; the elec- ing and Sodium-ion intercalation (change of 002 peak in width and angle), as shown in tron orbital of π-bonding was affected when an Sodium-ion was intercalated into the Figure 5b. Raman spectroscopy was also used to investigate the mechanism; the electron graphite layer, which will lead to the change of C–C bond strength. Therefore, the red orbital of π-bonding was affected when an Sodium-ion was intercalated into the graphite shift of G peak at the sloping potential region proved the intercalation of Sodium-ion into layer, which will lead to the change of C–C bond strength. Therefore, the red shift of the graphite layer. In addition, SAXs were also applied, and the sodium storage behavior G peak at the sloping potential region proved the intercalation of Sodium-ion into the at the low-potential platform was accompanied by a decrease in the electron density of graphite layer. In addition, SAXs were also applied, and the sodium storage behavior at the micropores, which was attributed to the filling of Sodium-ions in the microporous the low-potential platform was accompanied by a decrease in the electron density of the region, as shown in Figure 5c. From the various characterization results, larger layer micropores, which was attributed to the filling of Sodium-ions in the microporous region, spacing and lower graphitization increase sodium intercalation (at an inclined potential curve), while porosity greatly affects the adsorption capacity (at a low potential plateau) [62].PDF Image | Hard Carbons as Anodes in Sodium-Ion Batteries
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