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Materials 2022, 15, 6231 5 of 15 achieved a high initial capacity (606 mAh·g−1), but its capacity gradually decreased due to the instability of the electrode structure without buffering C. Moreover, the electrode with only a buffering matrix (TiO2–C) showed very low electrochemical efficiency, close to the theoretical capacity (116 mAh·g−1) (Table S2). The low-capacity contribution of the TiO2–C (~35%) indicated its main role as a buffering matrix. Due to interfacial Na ion storage and electrolyte breakdown, the measured capacities of Ga2Te3–TiO2–C(10%) and Ga2Te3–TiO2 in the SIBs were higher than their theoretical capacities (336 and 333 mAh·g−1, respectively, as computed in Table S3). The change in the reversible capacity of Ga2Te3–TiO2–C for the SIBs was studied using the CE (Table S4) and DCP test of the first 300 cycles (Figure S9). The CE of Ga2Te3–TiO2–C(10%) reached ~99.82% after 150 cycles, slightly decreased, and then stabilized at 98.5% after 300 cycles. The DCP analysis revealed that, for 250 cycles, the main oxidation (at ~0.16, ~1.27, and ~1.42 V) and reduction (at ~0.79 and ~1.58V) peaks remained stable before becoming wider and shifting. However, this polarization had an almost negligible effect on sodiation/desodiation. The reversible capacity of Ga2Te3– TiO2–C(10%) was 436.6 mAh·g−1 (capacity retention (CR) of 97.7%) after 300 cycles at 100 mA·g−1, which was greater than those of Ga2Te3–TiO2–C(20%) (323.8 mAh·g−1) and Ga2Te3–TiO2–C(30%) (264.9 mAh·g−1) (Figure 3b). As shown in Figure S10, although some aggregated particles were observed, the Ga2Te3–TiO2–C(10%) electrode morphology was generally well maintained after 300 cycles. This is because of the presence of TiO2–C, which effieicntly stabilized the electrode structure and mitigated the significant volume variation. In addition, in EDS spectra after 300 cycles, the composition of the Ga2Te3 composite electrode was not significantly changed without impurities (Figure S11). This further proved the stability and good retention of the electrode after the electrochemical reaction. At 500 mA·g−1 (Figure 3c), the reversible capacity of Ga2Te3–TiO2–C(10%) slightly increased until 200 cycles, followed by a gradual decrease. The capacity variation depends on the variation of the redox peaks, in which the oxidation and reduction peaks gradually rise with the cycling, leading to a decrease in polarization and an increase in capacity. In contrast, the oxidation and reduction peaks gradually decrease with the increase in cycling, resulting in a reduced capacity due to the increase in polarization [10,61,62]. This trend was also shown in the DCP analysis (Figures S12 and S13) and CE variation (Table S5). The magnitudes of the reduction (at 0.59 and 1.48 V) and oxidation (at 0.16, 1.27, and 1.69 V) peaks gradually raised over 200 cycles, with a reduction in polarization (Figure S12), and then reduced after 200 cycles, with a rise in polarization (Figure S13). At 100 and 500 mA·g−1, the fluctuation of the DCP profile was examined as a function of the cycle number (Figure S14). The DCP curves of the Ga2Te3–TiO2–C(10%) electrode showed that the overall intensity of the redox peaks was generally stable as the cycle number increased to 300 at 100 mA·g−1. At 500 mA·g−1, the overall magnitudes of the redox peaks increased up to 250 cycles and then decreased with an increase in polarization. Despite the decrease in capacity after 250 cycles, the overall capacity of Ga2Te3–TiO2–C(10%) was still the highest over 500 cycles, reaching 204 mAh·g−1 after 500 cycles with a CR of 76.4%. Figure S15 shows a comparison of the CE variations in Ga2Te3–TiO2–C with varying C contents at 100 and 500 mA·g−1. Table S6 (at 100 mA·g−1) and Table S7 (at 500 mA·g−1) provide summaries of the detailed CE values for the electrodes throughout the first 10 cycles. As shown in Table S6, the ICE of the Ga2Te3–TiO2–C(10%) electrode was slightly higher (69.2%) than that of the Ga2Te3–TiO2–C(20%) (ICE = 64.8%) and Ga2Te3–TiO2–C(30%) electrodes (ICE = 60.5%). Then, after 10 cycles, the CE of the Ga2Te3–TiO2–C(10%) electrode marginally increased and reached the highest among the three various electrodes. This tendency was also discovered at 500 mA·g−1 (Table S7). After the first cycle, the high CE of the Ga2Te3– TiO2–C(10%) electrode showed a high degree of sodiation/desodiation reversibility. The CV curves of the Ga2Te3–TiO2–C(10%) electrode for the first five cycles in the voltage range of 0.005–2.5 V vs. Na/Na+ are shown in Figure 3d. A large reduction peak was observed at 1.37 V during the first discharge process, which denoted the intercalation of Na into Ga2Te3 to form Na2Te and Ga. The reaction between Ga and Na to generate NaGa4 was attributed to being responsible for the peak at 0.52 V. Thus, Na2Te and NaGa4 were thePDF Image | Ga2Te3-Based Anodes for Sodium-Ion Batteries
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