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Energies 2022, 15, 5659 4 of 7 Figure 2. SEM images of (a,d) KFAM-1, (b,e) KFAM-2 and (c,f) KFAM-3. Fiigurre3..((aa))ADFFiimaaggee,,((b))EDXsspeeccttrrumand((cc))ellementtallmappiingoffKFAM-2. The electrochemical properties of the KFAM-X were tested in a 2020 coin-type cell. The electrochemical properties of the KFAM-X were tested in a 2020 coin-type cell. The electrochemical impedance spectra (EIS) were then compared (Figure 4a). The charge The electrochemical impedance spectra (EIS) were then compared (Figure 4a). The charge transfer and interfacial resistances could not be separated; thus, they were denoted as transfer and interfacial resistances could not be separated; thus, they were denoted as R . The ohmic resistance (R ) and R of KFAM-2 were 16 and 440 Ω, respectively, Rcct+ti+nitn.tThe ohmic resistance (Ro)oand Rct+citn+tinotf KFAM-2 were 16 and 440 Ω, respectively, smaller than those of KFAM-1 (21 and 710 Ω, respectively) and KFAM-3 (35 and 570 Ω, smaller than those of KFAM-1 (21 and 710 Ω, respectively) and KFAM-3 (35 and 570 Ω, respectively). The slope of the line at the low-frequency area of KFAM-2 was also larger respectively). The slope of the line at the low-frequency area of KFAM-2 was also larger than that of KFAM-1 and KFAM-3, indicating the fastest+Na+ transfer rate of KFAM-2 thanthatofKFAM-1andKFAM-3,indicatingthefastestNa transferrateofKFAM-2from from the electrolyte to the electrode surface. The excellent conductivity of KFAM-2 was attributed to its high crystallinity and purity. Figure 4b–d exhibits the charge–discharge curves of the KFAM-X and their curves showed a similar shape. The 1st and 2nd discharge capacities of KFAM-2 were 61 and 141 mAh g−1, respectively, larger than those of KFAM-1 (59 and 113 mAh g−1, respectively) and KFAM-3 (60 and 113 mAh g−1, respectively). Previous studies have found that sodium ions enter the structure of KFAM-X by both an ion exchange and electrochemical intercalation during the 1st discharge process and Na+ can be extracted after the 1st charge process, which leaves a greater number of vacancies for the next discharge [21,22]. The EDX spectra of the KFAM-X after cycling were collected (Figures S4–S6). Both Na and K elements could be found, proving the above-mentioned mechanism. The cyclic life of the KFAM-X was further compared. At a current density of 0.1 C, KFAM-2 displayed a good coulombic efficiency and the highest discharge capacity (Figure 4e). After 50 cycles, the discharge capacity of KFAM-2 was 124 mAh g−1, higher than that of KFAM-1 (114 mAh g−1) and KFAM-3 (83 mAh g−1). Furthermore, a cyclic life at 2 C was performed. After 200 cycles, the capacity retention of KFAM-2 (78%) was obviously higher than that of KFAM-1 (58%) and KFAM-3 (26%) (Figure 4f). At the beginning of cycling, a part of K+PDF Image | Material as a High-Performance Cathode Sodium-Ion Batteries
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