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Energies 2020, 13, 6216 4 of 19 with different binders were captured using scanning electron microscopy (a FESEM Cambridge Stereoscan 360 electron microscope). Raman spectra were performed for the CGDHC powder and the electrodes, before and after a 12 h OCV period, using a Horiba IHR 320, wavelength 532 nm. Pore characteristics of the CGDHC materials were investigated by N2 adsorption/desorption isotherms at −195.8 ◦C using a Micromeritics ASAP 2020 instrument. Prior to the adsorption/desorption measurement, the CGDHC sample had been outgassed for 12 h at 300 ◦C. The specific surface area and pore size distribution values were calculated by the linear plot in the relative pressure range and the density function theory (DFT), respectively. 2.3. Electrodes Preparation and Cell Assembling Various electrodes were prepared by mixing 80% of active material with 10% of conductive carbon C65 and 10% of different binders, including CMC, alginate, PAA, and PVDF binders. The slurries were casted onto aluminum foil for NIBs and onto copper for LIBs by using a doctor blade to acquire a homogenous thickness film. The solvents used for CMC, PAA, alginate, and PVDF binders were water, ethanol (due to faster drying and better performances), water, and N- methylpyrrolidone (NMP), respectively. After drying the coatings, the electrodes were cut and dried again under vacuum at 120 ◦C overnight to evaporate solvent residues and to yield the electrode layers. Cells were assembled in a glove box (Jacomex GP-campus, oxygen and moisture content less than 0.8 ppm) using T-shape polypropylene Swagelok-type 3 cells. Lithium metal (Sigma-Aldrich) was used as the counter and reference electrodes for LIBs and sodium metal (Alfa-Aesar) were used as the counter and reference for NIBs. For all cells, a glass fiber disk (Whatman GF/A) was used as separator, while 1 M LiPF6 in ethylene carbonate (EC):dimethyl carbonate (DMC) 1:1 v/v (preformulated by Solvionic, France) and 1 M NaClO4 (Sigma-Aldrich) in EC:PC (Sigma-Aldrich) 1:1 v/v were used as electrolytes (400 μL) for LIBs and NIBS, respectively. After assembly, the cells were removed from the glove box for electrochemical characterization. 2.4. Electrochemical Characterization All electrochemical tests were carried out using a VMP-2Z multichannel electrochemical workstation by Bio-Logic (France). Cyclic voltammetry of the samples for LIBs and NIBs were carried out at a scanning rate of 0.05 mV/s in the voltage between 0.02 to 2.2 V to assess the topotactical reaction of Li for LIBs and of Na for NIBs in the synthetized materials. Galvanostatic charge/discharge tests of the electrodes were collected with the voltage ranging between from 0.02 and 2.2 V. All voltages are given vs. Li+/Li and Na+/Na couples for LIBs and NIBs, respectively. For both LIBs and NIBs, 1C rate was assumed as 300 mA/g with respect to active material mass. In order to evaluate the interfacial behavior of electrodes, electrochemical impedance spectroscopy was carried out at each tenth cycle at E = 0.5 V. 3. Result and Discussion 3.1. Chemical, Structural, and Morphological Characterization of the Material Chemical, structural, and morphological characteristic of pristine coffee grounds was carried out. Scanning electron microscopy (Figure S1a) reveals pores at the micrometer scale. Elemental analysis, carried out on pristine and final powders, reveals (Figure S1b,c) the presence of minor amounts of nitrogen in both samples, which can enhance conductivity of the sample. The presence of N is also evidenced in FTIR analysis (Figure S1d) of the pristine coffee grounds, while after the high-T thermal treatment, a flat signal typical of a conductive carbon material is observed. According to XRD patterns of graphite and soft and hard carbon reported in the literature, the level of structure disorder and the interlayer distances increase from soft to hard carbon [22]. Figure 2a illustrates the X-ray diffraction patterns of the CGDHC synthesized according to the procedure reported in the Experimental and calcinated at T = 970 ◦C for 6 h. The XRD diffraction pattern exhibits veryPDF Image | Coffee Ground Sustainable Anodes Sodium-Ion Batteries
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