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Energies 2020, 13, 6216 6 of 19 The Raman spectrum of CGDHC powder, shown in Figure 2b, indicates two sharp and intense peaks of D-band at ~1330.12 cm−1 and of G-band at ~1586.77 cm−1. The D-band “the defect-induced band” is correlated with vacancies or the attendance of functional groups, whereas the G-band “the crystalline graphite” corresponds to the graphitic vibrations [8,31]. The intensity ratio ID/IG value describes the ratio between the degree of defects and the degree of graphitization. For CGDHC it is calculated as 0.965, suggesting a relatively high-graphitization degree of hard carbon [8]. Moreover, two broad peaks are located at around 2600 and 2950 cm−1, which correspond to 2D and D+G bands. The 2D band is associated with the degree of graphitization and the D+G band is associated with defect activated process for an elastic scattering [31]. The graphitic domain size of hard carbon sample was also independently calculated from [20,23]: La = 2.4 × 10−10 λ4(ID/IG)−1 (3) where La is the graphitic domain size (nm) and λ is 532 nm (shown in Table 1). The crystallographic parameters were calculated using the equations and the results can be found in Table 1, indicating a certain degree of graphitization besides a disordered structure of CGDHC hard carbon powder [23,31]. As it can be seen in Figure 2c,d, the SEM images of the CGDHC powder show block morphology and fine fragments. It can be mentioned that the particle size is nonuniformly distributed. Heat treatment under high argon flow plays a critical role in enhancing specific surface area and in creating porosity by flushing away the released gases. Not only pyrolysis, but also acid treatment has a considerable effect on enhancing porosity [21]. The N2 adsorption/desorption isotherms were used to study the porosity of CGDHC powder and the results are presented in Figure S2. The calculated Brunauer–Emmett–Teller (BET) surface area, pore diameter (DFT theory), Barrett-Joyner-Halenda (BJH) adsorption average pore diameter, and cumulative surface area of pores are 787.26 m2/g, 1.02 nm, 4.6 nm, and 541.01 m2/g, respectively. The high BET specific surface area and pore size distribution of CGDHC material can provide good electrolyte ion accessibility to carbon layers, resulting in good electrochemical performance [8]. 3.2. Structural and Morphological Characterization of the Electrodes In order to get information about possible modifications of the hard carbon layers arrangement induced by the different binders used for electrodes formulation, Raman spectra was performed on the CGDHC-based electrodes prepared with CMC, Alg, PAA, and PVDF binders, respectively labeled as CGDHC-CMC, CGDHC-Alg, CGDHC-PAA, and CGDHC-PVDF. This information can be obtained by calculating the ratio between the ID and IG bands. The comparison of Raman spectra of electrodes with different binders for LIBs and NIBs is shown in Figure S3, while the calculated ID/IG and La are presented in Table S1. As expected, all the Raman spectra present the same characteristic peaks already observed for the powder at ~1345 cm−1 (D-band) and ~1590 cm−1 (G-band) [8], together with two broad peaks in the range of 2650–2950 cm−1, correlated to 2D and D+G bands [31]. Commonly, the graphitization degree of hard carbon is bound to conductivity [23], while porosity and defects enable surface storage processes (thus relevant for NIBs) [33]. However, in the present case the data provide evidence, for all electrodes, of similar peak shapes and only small variations of the intensity ratio between D and G bands. This behavior may be attributed to only minor interactions of the active CGDHC and of the conductive additive with the functional groups. Therefore, we may exclude a relevant role of the binder in modifying structural or conduction properties of the active materials. On the contrary, the different binders are expected to play a key role in modifying aggregation and morphology of the electrodes, as demonstrated by SEM images of CGDHC-CMC, CGDHC-Alg, CGDHC-PAA, and CGDHC-PVDF electrodes, both for LIBs and NIBs. Figures 3 and 4 show the surface morphology of the prepared electrodes for LIBs and NIBs, respectively. Two magnification levels (1 and 40 kX) are shown. Among the electrodes prepared for LIBs (Figure 3), CGDHC-CMC shows aPDF Image | Coffee Ground Sustainable Anodes Sodium-Ion Batteries
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