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the next discharge [21,22]. The EDX spectra of the KFAM-X after cycling were collected Energies 2022, 15, 5659 (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 cycli5colfif7e at 2 C was performed. After 200 cycles, the capacity retention of KFAM-2 (78%) was obvi- ously higher than that of KFAM-1 (58%) and KFAM-3 (26%) (Figure 4f). At the beginning of cycling, a part of K+ escapes from the interlayer and results in a greater numbe+r of active escapes from the interlayer and results in a greater number of active sites for Na storage, sites for Na+ storage, which leads to an initial capacity increase [22]. which leads to an initial capacity increase [22]. Figure 4. (a) EIS plots of KFAM-X. The 1st and 2nd charge–discharge curves of (b) KFAM-1, Figure 4. (a) EIS plots of KFAM-X. The 1st and 2nd charge–discharge curves of (b) KFAM-1, (c) (c) KFAM-2 and (d) KFAM-3. Cyclic life of KFAM-X at (e) 0.1 and (f) 2 C. KFAM-2 and (d) KFAM-3. Cyclic life of KFAM-X at (e) 0.1 and (f) 2 C. The electrochemical properties of KFAM-2 were surveyed further. The 1st–3rd cyclic The electrochemical properties of KFAM-2 were surveyed further. The 1st–3rd cyclic voltammetry (CV) curves are exhibited in Figure 5a. Two pairs of main redox peaks at + v2o.l1t/am2.7maentdry3(.1C/V3).5cVurcvoeusldarbeeesxeheinb,itredfleicntiFniguarsete5paw.iTsweNoapainrseorftimona/inderiendsoerxtipoena.kTshaet 2c.1h/a2r.7gea–nddisc3h.1a/r3g.5e Vcucrvouesldatbedisffeerne,nrtedflencstintigesasshtoewpwedisea Nsima+ilianrsesrhtaiopne/,dseuingsgersttiionng. tThhee cohuartsgtea–ndiisncghaerlegcetrcoucrhvems aictadlidffyenreanmtidcsenosfiKtieFsAsMho-2w(eFdigausriem5ibla).rAshtacpuerr,esnutgdgenstsiintigesthoefo0u.1t,- −1 st0a.2n,d0i.n5g, 1e,le2catrnodch5eCm,itchaeldiysnchamargicescoafpKaFciAtiMes-w2(eFrieg1u3r5e,51b2)9.,A1t1c6u, 1rr0e7n,t9d6eansdit8ie0smofA0h.1g,0.2,, respectively. The cyclic life of KFAM-2 at a large current density of 5 C was tested to 0.5, 1, 2 and 5 C, the discharge capacities were 135, 129, 116, 107, 96 and 80 mAh g−1, re- prove its superiority (Figure 5c). After 200 cycles, a high capacity retention of 72% was spectively. The cyclic life of KFAM-2 at a large current density of 5 C was tested to prove obtained. To calculate the diffusion coefficient of Na+, the CV curves at scan rates from its superiority (Figure 5c). After 200 cycles, a high capacity retention of 72% was obtained. 0.2 to 2 mV s−1 were collected (Figure 5d).+ The diffusion coefficient was counted by the To calculate the diffusion coefficient of Na , the CV curves at scan rates from 0.2 to 2 mV −f1ollowing Equation (1) [23]: s were collected (Figure 5d). The diffusion coefficient was counted by the following Equation (1) [23]: Ip = 2.69 × 105n3/2ADNa1/2CNaV1/2 (1) Ip = 2.69 × 105n3/2ADNa1/2CNaV1/2 (1) where Ip, n, A, DNa, CNa and V are the peak current, the number of electron transfers in the redox process, the contact area between the electrolyte and the active substance, the diffusion coefficient, the concentration of sodium ions in the lattice and the scan rate. It could be simplified to: Ip = 6487.6DNa1/2V1/2. Figure 5e,f were obtained according to the reduction and oxidation peaks at different scan rates. According the plots of Ip–V1/2, the diffusion coefficients were 3.68 × 10−11 and 4.69 × 10−11 cm2 s−1, respectively, reflecting the fast charge transfer and storage of KFAM-2. The XRD patterns of the KFAM-X after 50 cycles at 1 C were tested to detect their struc- tural stability, as shown in Figure S7. There were no obvious changes for the characteristic peaks of KFAM-1 and KFAM-2, reflecting their good structural stability. A few peaks of NasFexAlyMnzO2 could be found in KFAM-2, revealing its poor structural stability and phase transformation.PDF Image | Material as a High-Performance Cathode Sodium-Ion Batteries
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