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Energies 2020, 13, 5729 3 of 12 2.2. Electrochemical Measurements The electrochemical measurements were performed with CR2032 coin-type cells. The cathode electrodes were prepared by mixing NaMMO powder with acetylene black and polyvinylidene fluoride (PVDF) (8:1:1) dispersed in N-methy1-2-pyrrolidone (NMP). Then the homogeneous slurry was cast on carbon coated Al foil with a doctor blade and dried at 120 ◦C for 12 h under vacuum. The battery assembly was carried out in a glove box under argon atmosphere. Then sodium metal foil was used as the negative electrode and glassy fiber filter was used as the separator, and 1.0 M NaPF6 in polycarbonate (PC) with fluoroethylene carbonate (FEC, 5% in volume) as the electrolyte. 2.3. Morphology and Crystal Structure Characterizations The phase of NaMMO was investigated with lab X-ray diffraction (XRD; Empyrean, Malvern Panalytical, Almelo, Netherlands) using Cu Kα as the X-ray source. Rietveld refinement was conducted by the program of GSAS. The morphology of the samples was examined by a scanning electron microscope (SEM, G500, Zeiss, Oberkochen, Germany). High resolution lattice stripes and EDS maps of the samples were obtained by transmission electron microscopy (TEM, 200X, Talos, Thermo Fisher Scientific, Waltham, MA, USA). 2.4. Soft X-ray Experiments The electrodes charged/discharged to the selected states of charge were disassembled carefully and cleaned repeatedly with dimethyl carbonate (DMC) to remove the residual carbonate and other impurities on the surface. Then the cleaned electrodes are put into the transition chamber and vacuum dried and stored in an argon atmosphere. The sample to be tested is transferred into the load lock chamber through a specially designed glove box and evacuated. After that, it was transferred to the analysis chamber for testing under ultra-high vacuum. The soft X-ray absorption spectra (sXAS) in TFY and TEY mode were normalized by a gold mesh in the end station. 3. Results The as-prepared NaMMO cathode was synthesized by a traditional solid-state method and its crystal structure was studied by the powder X-ray diffraction (XRD). The XRD patterns are highly consistent with the corresponding Rietveld refinement results as shown in Figure 1a, indicating that NaMMO is well-indexed to the pure P2 phase with a space group P63/mmc. It should be noted that the diffraction peak (enlarged view in Figure 1a) located at 2θ around 21◦ is not related to the space group √ √ 2+ 4+ P63/mmc but to the 3a × 3a-type supercell, implying the existence of long-range Mg /Mn cationic honeycomb ordering within the TM layers [26]. The corresponding crystal structure of NaMMO is depicted in Figure 1b,c, which is composed of alternate TMO6 layers and Na+ layers in the z-direction. Specifically, the oxygen layers stack with the ABBA sequence, in which the cations orderly distribute in the form of honeycomb-type as aforementioned to minimize the electrostatic energy [27]. In addition, there are two types of Na+ sites situated between the TMO6 slabs: one sharing two faces with the TMO6 slabs, called Naf; the other sharing six edges with the TMO6 slabs, called Nae.PDF Image | Cathode Materials for Advanced Sodium-Ion Batteries
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