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Batteries 2022, 8, 6 14 of 18 the TBA alginate-based Na0.67MnO2 electrode when compared to PVDF binder-based electrodes, while measurements of electrode surface and SEI prior to and after cycling suggest these electrodes also possess good interfacial stability. 4. Materials and Methods Na0.67MnO2 was prepared by solid-state synthesis. Stoichiometric amounts of Mn2O3 (≥99%, Sigma-Aldrich, Darmstadt, Germany) and Na2CO3 (≥99%, Sigma-Aldrich, Ger- many) were ball-milled for 2 h. The mixture was pressed into pellets and heated at 900 ◦C for 24 h in air. The samples were then cooled down to 500 ◦C, taken out of the furnace, and transferred to an Ar-filled glove box for further cooling and processing to avoid unnecessary contact with moisture, oxygen, and CO2. A procedure similar to that used by Babak et al. [34] and Pawar et al. [36] was followed. Thus, 120 mL 0.6 M HCl was mixed with 120 mL 96% ethanol. Na-alginate was added to the mixture and stirred for 12 h at 4 ◦C to obtain alginic acid. The mixture was filtered under vacuum and washed with alcohol and acetone. The solid product was dried in vacuum drying oven at 60 ◦C for 12 h. 1 g of fully dried alginic acid was dispersed in 150 mL water, then 1.5 M tetrabutylam- monium hydroxide (TBA-OH) solution was added dropwise with continuous stirring at room temperature until the alginic acid was dissolved and the pH was adjusted to 8 ± 1. The mixture was dried in vacuum oven at 60 ◦C for 24 h until fully dry solid product was obtained. The crystal structure of Na0.67MnO2 was characterized by X-ray diffraction (XRD) using Rigaku MiniFlex600 X-ray diffractometer with Cu Kα radiation at room temperature. The patterns were recorded in the 2θ range of 10–90◦. Rietveld refinement was conducted with Bruker TOPAS software. An Agilent 8900 ICP-QQQ Inductively Coupled Plasma Mass Spectrometer (ICP-MS) equipped with a MicroMist nebulizer was used to determine Na and Mn ratio in NaMnO2. Before measurements, NaMnO2 was dissolved in nitric acid. Quantification of Na and Mn was done by 5-point calibration graph method in He mode. Scanning electron microscopy (SEM) images of Na0.67MnO2 powder and electrodes were taken by Thermo Scientific (Waltham, MA, USA) Helios 5 UX scanning electron microscope. Powders of the Na alginate, alginic acid and TBA alginate were analyzed with Fourier transformation infrared spectroscopy (FTIR) using Bruker Vertex 80v vacuum FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) was carried out for Na0.67MnO2 powder as well as battery electrodes before and after cycling (post-mortem) using ThermoFisher ESCALAB Xi+ instrument with monochromatic Al Kα X-ray source. The samples were transferred from the argon-filled glovebox to the XPS instrument in an inert gas transfer vessel. The instrument’s binding energy scale was calibrated to give a binding energy at 932.6 eV for Cu 2p3/2 line of freshly etched metallic copper. A charge compensation system was used, with the surface of the sample irradiated with a flood of electrons to produce nearly neutral surface charge. The experimental data has been referenced to C 1s adventitious carbon. The spectra were recorded by using an X-ray beam size 650 × 10 microns, a pass energy of 20 eV and step size 0.1 eV. The electrodes were prepared with wet slurry-coating method. The active material (Na0.67MnO2) was mixed with carbon black and binder (PVDF, Na alginate or TBA alginate) in a weight ratio of 75:15:10. Solvents N-methyl-2-pyrrolidone (NMP), water or N,N- dimethylformamide (DMF) were used respectively, the substances were mixed using mill mixer Retsch MM200. The obtained slurry was coated on an Al foil using a doctor blade and dried in a vacuum drying oven at 80 ◦C for 24 h. The electrochemical performance was tested in Swagelok type cells assembled in Ar-filled glove box. Metallic sodium was used as counter electrode. Electrochemical cells were assembled, using glass fiber filter (Whatman GF/B) as separator and 1 M NaClO4 solution in propylene carbonate (PC) with 5 wt.% fluoroethylene carbonate (FEC) as electrolyte. For selected measurements, described further in the text, the use of FECPDF Image | Na-Ion Batteries Tetrabutylammonium Alginate Binder
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