Ga2Te3-Based Composite Anodes for High-Performance Sodium-Ion Batteries

Recently, metal chalcogenides have received considerable attention as prospective anode materials for sodium-ion batteries (SIBs) because of their high theoretical capacities based on their alloying or conversion reactions. Herein, we demonstrate a gallium(III) telluride (Ga2Te3)-based ternary composite (Ga2Te3–TiO2–C) synthesized via a simple high-energy ball mill as a great candidate SIB anode material for the first time. The electrochemical performance, as well as the phase transition mechanism of Ga2Te3 during sodiation/desodiation, is investigated. Furthermore, the effect of C content on the performance of Ga2Te3–TiO2–C is studied using various electrochemical analyses. As a result, Ga2Te3–TiO2–C with an optimum carbon content of 10% (Ga2Te3–TiO2–C(10%)) exhibited a specific capacity of 437 mAh·g−1 after 300 cycles at 100 mA·g−1 and a high-rate capability (capacity retention of 96% at 10 A·g−1 relative to 0.1 A·g−1). The good electrochemical properties of Ga2Te3–TiO2–C(10%) benefited from the presence of the TiO2–C hybrid buffering matrix, which improved the mechanical integrity and electrical conductivity of the electrode. This research opens a new direction for the improvement of high-performance advanced SIB anodes with a simple synthesis process.


Introduction
In the last few decades, lithium-ion batteries (LIBs) have been utilized as an effective alternative to unsustainable fossil fuels in energy storage systems such as portable electronic devices and electric vehicles [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Nevertheless, by virtue of the limited reserves and the high cost of Li, much attention has been drawn to developing alternative secondary batteries to overcome these issues [15]. Sodium-ion batteries (SIBs) are reputed as one of the most viable secondary batteries among many next-generation batteries because of their many similarities to LIBs and the abundance of Na on earth [16][17][18][19]. Because of the chemical similarity between Na and Li, the Na storage mechanism of anode materials is similar to that of LIB systems, including intercalation/deintercalation, conversion, and alloying/dealloying reactions. Therefore, considerable attempts have been dedicated to finding suitable anode materials for SIBs. However, the slow reaction kinetics owing to the large ionic radius (1.03 nm for Na + relative to 0.75 nm for Li + ) causes low cycling stability and rate capability or even complete electrochemical inactivity. Thus, the improvement of desirable anode materials for high-performance SIBs is urgently required for expanding their practical application scope. Recently, alloy-based materials have received considerable attention as SIB anode materials owing to their high theoretical capacities (Na-Si: 955 mAh·g −1 , Na-Ge: 368 mAh·g −1 , Na-Sn: 845 mAh·g −1 , Na-Pb: 486 mAh·g −1 ) [20]. However, similar to LIBs, the large volume change of these materials significantly restricts the long-term cycling of SIBs.
Ga-based materials, such as Ga oxide/sulfide anodes, have a large theoretical capacity (682-1591 mAh·g −1 ), innate self-healing capability, and a high tolerance against volume change [49][50][51][52]. Ga-based materials have recently emerged as potential electrode materials because of their unique self-healing properties based on the low melting temperature of Ga (29.9 • C). The intermediate liquid Ga formed during sodiation increases the tolerance of volume expansion of active materials, which significantly contributes to the cycling stability [53]. For instance, a composite including reduced graphene oxide and gallium oxide nanosheets (Ga 2 O 3 NSs/rGO) by Yang et al. provided a steady capacity of 555 mAh·g −1 at 0.1 A·g −1 [54], whereas a template-derived Ga 2 S 3 nanorod anode could obtain a discharge capacity of 476 mAh·g −1 at 0.4 A·g −1 [55]. Using in situ microscopy, Wu et al. investigated the self-healing properties of a liquid metal Ga-alloy during the charge/discharge process [56]. Considering the aforementioned advantages of Te and Ga, gallium telluride alloys (i.e., Ga x Te y ) are expected to be great candidate anode materials for SIBs.
In this work, a Ga 2 Te 3 -based composite electrode (Ga 2 Te 3 -TiO 2 -C) was successfully synthesized utilizing a simple solid-state high-energy ball milling (HEBM) method and studied as a potential SIB anode material. The feasibility of the Ga 2 Te 3 -TiO 2 -C anode for SIBs was investigated through galvanostatic measurements, differential capacity analysis, and electrochemical impedance spectroscopy (EIS). Furthermore, the reaction mechanism of Ga 2 Te 3 -TiO 2 -C anode during sodiation/desodiation was first investigated via ex situ X-ray diffraction (XRD) analysis. In addition, the best C content (10 wt.%) in the Ga 2 Te 3 -TiO 2 -C composite was derived through various electrochemical tests. The high cycling and rate performances of Ga 2 Te 3 -TiO 2 -C(10%) obtained in this study are superior or equivalent to those of the most recent chalcogenide-based electrodes in SIBs.

Material Characterization
The morphology and crystallinity of Ga 2 Te 3 -TiO 2 and Ga 2 Te 3 -TiO 2 -C were characterized by employing XRD (D/MAX-2200 Rigaku, Tokyo, Japan) with Cu Kα (λ = 1.54 A) radiation at a scan rate of 2 • ·min −1 , as well as energy-dispersive X-ray spectroscopy (EDXS), scanning electron microscopy (SEM, Hitachi S4700, Tokyo, Japan), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F, Tokyo, Japan). The mechanism of phase change state in the electrode during Na ion reaction was performed through ex situ XRD.

Electrochemical Measurements
To assess the electrochemical performance of the electrode, the coin-typed cells were assembled in an Ar-filled glovebox with Na foil acting as the counter electrode and polyethylene as the separation membrane. The SIB electrolyte was 1 M NaClO 4 in propylene carbonate/ethylene carbonate (1:1 by v/v) with 5% fluoroethylene carbonate. An electrode was prepared using a 7.0:1.5:1.5 (w/w) combination of the active material, conductive carbon (Super-P, 99.9%, Alfa Aesar), and poly(acrylic acid) (PAA, Mw 450,000, Sigma Aldrich) binder. Then, electrodes were coated on Cu foil using a doctor blade and dried overnight in a vacuum oven at 70 • C (a typical mass loading of 1.0−1.5 mg on a Cu foil diameter of 12.5 mm). Using a battery-testing device (WBCS3000, WonATech, Seoul, Korea), the electrochemical performance of Ga 2 Te 3 -TiO 2 -C was assessed. For a voltage range of 0.01-2.5 V (vs. Na/Na + ), the galvanostatic charge-discharge (GCD) profile was investigated. Using cyclic voltammetry (CV) measurements with a scanning rate of 0.1 mV·s −1 , the electrochemical reactions of the electrodes with Na + were evaluated. A battery cycler (WBCS3000, WonATech) was used to measure the rate capability at current densities of 0.1, 0.5, 1, 3, 5, and 10 A·g −1 . The EIS was measured using a ZIVE MP1 (WonaTech) analyzer in the frequency range of 100 kHz-100 mHz at an AC amplitude of 10 mV.

Results and Discussion
The XRD results of the as-prepared Ga 2 Te 3 -TiO 2 following the HEBM process are shown in Figure 1a. The peaks at 26.  Figure S1) [57]. The absence of impurity peaks for the precursor components (Ga 2 O 3 , Ti, and Te) in Ga 2 Te 3 -TiO 2 indicated the complete conversion of the raw materials to the target product via a solid-state reaction. Nevertheless, the existence of small diffraction peaks near 2θ = 22.9 • , 35.1 • , and 37.5 • were associated with the formation of GaTe (PDF#33-0571) and Ga 2 Te 5 (PDF#45-0954), which are different forms of gallium telluride alloy (as shown in Figure S2). Given the advantages of gallium telluride alloy (i.e., Ga x Te y ), they are also expected to be good candidate anode materials for SIBs; thus, the existence of different forms of minor Ga x Te y (namely, Ga 2 Te 5 and GaTe) did not negatively affect the electrochemical performance of the Ga 2 Te 3 -TiO 2 -C composite (capacity still in the range of 682−1591 mAh·g −1 ). The EDXS showed that the four component elements of the composite in the electrode and the relative content of elements were reasonable with the stoichiometric ratio of Ga 2 Te 3 and TiO 2 (Figure 1b), indicating that the Ga 2 Te 3 -based composite was synthesized successfully.  Morphological and structural analyses of Ga2Te3-TiO2-C(10%) were conducted, including SEM, HRTEM, and EDXS, as indicated in Figure 2. According to SEM images ( Figure 2a,b), the Ga2Te3-TiO2-C(10%) particle size ranged from sub-micrometers to a few micrometers. The HRTEM images (Figures 2c and S3) revealed crystalline lattice spacings of 0.340, 0.294, and 0.208 nm, which corresponded to the (111), (200), and (220) crystal planes of Ga2Te3, respectively, and 0.311 nm, which corresponded to the (002) plane of TiO2. Additionally, amorphous C, which was anticipated to serve as a buffering network for the active material, formed surrounding Ga2Te3 and TiO2. The scanning transmission electron microscopy image with EDXS mapping analysis (Figures 2d and S4) showed a uniform distribution of each element (Ga, Te, Ti, O, and C) in the Ga2Te3-TiO2-C(10%). Furthermore, the SEM-EDXS analysis results (Figures S5 and S6) of Ga2Te3-TiO2 with different content of C showed C concentrations almost identical to their theoretical values. Additionally, the stoichiometric ratio of the constituent elements was nearly identical to the theoretical values, according to a quantitative analysis of the EDXS result. In the EDS spectrum of G2Te3-TiO2, Te was determined to be 25%. However, in the presence of 10% carbon content, the distribution of Te gradually decreased (specifically to 18%, as shown in EDS spectrum of Ga2Te3-TiO2-C(10%) in Figure S5); thus, the distribution of Te content in the TEM elemental mapping was relatively less. Morphological and structural analyses of Ga 2 Te 3 -TiO 2 -C(10%) were conducted, including SEM, HRTEM, and EDXS, as indicated in Figure 2. According to SEM images ( Figure 2a,b), the Ga 2 Te 3 -TiO 2 -C(10%) particle size ranged from sub-micrometers to a few micrometers. The HRTEM images (Figures 2c and S3) revealed crystalline lattice spacings of 0.340, 0.294, and 0.208 nm, which corresponded to the (111), (200), and (220) crystal planes of Ga 2 Te 3 , respectively, and 0.311 nm, which corresponded to the (002) plane of TiO 2 . Additionally, amorphous C, which was anticipated to serve as a buffering network for the active material, formed surrounding Ga 2 Te 3 and TiO 2 . The scanning transmission electron microscopy image with EDXS mapping analysis (Figures 2d and S4) showed a uniform distribution of each element (Ga, Te, Ti, O, and C) in the Ga 2 Te 3 -TiO 2 -C(10%). Furthermore, the SEM-EDXS analysis results ( Figures S5 and S6) of Ga 2 Te 3 -TiO 2 with different content of C showed C concentrations almost identical to their theoretical values. Additionally, the stoichiometric ratio of the constituent elements was nearly identical to the theoretical values, according to a quantitative analysis of the EDXS result. In the EDS spectrum of G 2 Te 3 -TiO 2 , Te was determined to be 25%. However, in the presence of 10% carbon content, the distribution of Te gradually decreased (specifically to 18%, as shown in EDS spectrum of Ga 2 Te 3 -TiO 2 -C(10%) in Figure S5); thus, the distribution of Te content in the TEM elemental mapping was relatively less.
The Na ion storage characteristics of the Ga 2 Te 3 -TiO 2 -C electrode were studied using a half-cell form with Na metal as the counter electrode ( Figure 3). The GCD voltage profiles of Ga 2 Te 3 -TiO 2 -C(10%), Ga 2 Te 3 -TiO 2 -C(20%), and Ga 2 Te 3 -TiO 2 -C(30%) for SIBs are shown in Figure 3a and Figures S7. The first discharge/charge capacities of Ga 2 Te 3 -TiO 2 -C(10%), Ga 2 Te 3 -TiO 2 -C(20%), and Ga 2 Te 3 -TiO 2 -C(30%) were 599/414, 550/357, and 462/279 mAh·g −1 , respectively, which corresponded to initial coulombic efficiencies (ICEs) of 69.1%, 64.9%, and 60.4%, respectively. The poor reversibility between the first and second cycles was because of the SEI layer formation in the first cycle. This poor reversion was well documented in previous studies [58][59][60]. The large capacity difference between the first and second cycle indicated the large irreversible capacity contribution from the SEI layer. However, the reversibility of the electrode (Ga 2 Te 3 -TiO 2 -C (10%) was rapidly enhanced after the second cycle, which could be confirmed by the change in coulombic efficiency (Table S6). According to the EDXS results ( Figure S5) and computed theoretical capacities of the separate components (Table S2), the capacity contributions of C and TiO 2 to Ga 2 Te 3 -TiO 2 -C(10%) were estimated to be 13% and 22%, respectively. Furthermore, the roles of C and active material were examined (as shown in Figure S8). Ga 2 Te 3 -TiO 2 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 (TiO 2 -C) showed very low electrochemical efficiency, close to the theoretical capacity (116 mAh·g −1 ) (Table S2). The low-capacity contribution of the TiO 2 -C (~35%) indicated its main role as a buffering matrix. Due to interfacial Na ion storage and electrolyte breakdown, the measured capacities of Ga 2 Te 3 -TiO 2 -C(10%) and Ga 2 Te 3 -TiO 2 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 Ga 2 Te 3 -TiO 2 -C for the SIBs was studied using the CE (Table S4) and DCP test of the first 300 cycles ( Figure S9). The CE of Ga 2 Te 3 -TiO 2 -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 Ga 2 Te 3 -TiO 2 -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 Ga 2 Te 3 -TiO 2 -C(20%) (323.8 mAh·g −1 ) and Ga 2 Te 3 -TiO 2 -C(30%) (264.9 mAh·g −1 ) ( Figure 3b). As shown in Figure S10, although some aggregated particles were observed, the Ga 2 Te 3 -TiO 2 -C(10%) electrode morphology was generally well maintained after 300 cycles. This is because of the presence of TiO 2 -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 Ga 2 Te 3 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 Ga 2 Te 3 -TiO 2 -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 Ga 2 Te 3 -TiO 2 -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 Ga 2 Te 3 -TiO 2 -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 Ga 2 Te 3 -TiO 2 -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 Ga 2 Te 3 -TiO 2 -C(10%) electrode was slightly higher (69.2%) than that of the Ga 2 Te 3 -TiO 2 -C(20%) (ICE = 64.8%) and Ga 2 Te 3 -TiO 2 -C(30%) electrodes (ICE = 60.5%). Then, after 10 cycles, the CE of the Ga 2 Te 3 -TiO 2 -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 Ga 2 Te 3 -TiO 2 -C(10%) electrode showed a high degree of sodiation/desodiation reversibility. The CV curves of the Ga 2 Te 3 -TiO 2 -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 Ga 2 Te 3 to form Na 2 Te and Ga. The reaction between Ga and Na to generate NaGa 4 was attributed to being responsible for the peak at 0.52 V. Thus, Na 2 Te and NaGa 4 were the final products after the discharge step was complete. In the charge step, two oxidation peaks were noticed at 0.92 and 1.72 V. The first peak was the result of Na being completely excluded, turning NaGa 4 into Ga. Then, Ga intruded into Na 2 Te to form Ga 2 Te 3 when the anode was charged to 1.72 V. The ex situ investigations concern a thorough analysis of this phase transition. After the second cycle, the curves nearly overlapped, indicating the excellent stability and reversibility of Ga 2 Te 3 -TiO 2 -C(10%). The CV curves of Ga 2 Te 3 -TiO 2 -C(20%) and Ga 2 Te 3 -TiO 2 -C(30%) were almost identical to that of Ga 2 Te 3 -TiO 2 -C(10%), with a similar level of cyclic stability after the second cycle ( Figure S16). In addition, the electrochemical performance of Ga 2 Te 3 -TiO 2 was examined ( Figure S17). The GCD profiles of Ga 2 Te 3 -TiO 2 presented initial charge/discharge capacities of 606/426 mAh·g −1 , corresponding to an ICE of 70.2%, which is higher than that of the Ga 2 Te 3 -based composite with different C contents. Despite this high ICE of Ga 2 Te 3 -TiO 2 , the capacity gradually decreased with the increase in cycle number, and reached 309 mAh·g −1 after 30 cycles, with a capacity retention of 67%. This is much lower than the Ga 2 Te 3 -TiO 2 electrode with various carbon contents. In addition, the CV curves did not overlap in the first five cycles. Therefore, the presence of C clearly stabilized the electrode structure, leading to the enhanced electrochemical performance. The rate performances ( The Na ion storage characteristics of the Ga2Te3-TiO2-C electrode were studied using a half-cell form with Na metal as the counter electrode ( Figure 3). The GCD voltage profiles of Ga2Te3-TiO2-C(10%), Ga2Te3-TiO2-C(20%), and Ga2Te3-TiO2-C(30%) for SIBs are shown in Figures 3a and S7. The first discharge/charge capacities of Ga2Te3-TiO2-C(10%) Ga2Te3-TiO2-C(20%), and Ga2Te3-TiO2-C(30%) were 599/414, 550/357, and 462/279 mAh·g −1 , respectively, which corresponded to initial coulombic efficiencies (ICEs) of 69.1%, 64.9%, and 60.4%, respectively. The poor reversibility between the first and second cycles was because of the SEI layer formation in the first cycle. This poor reversion was well documented in previous studies [58][59][60]. The large capacity difference between the first and second cycle indicated the large irreversible capacity contribution from the SEI layer. However, the reversibility of the electrode (Ga2Te3-TiO2-C (10%) was rapidly enhanced after the second cycle, which could be confirmed by the change in coulombic effi- The reaction mechanism during the first sodiation/desodiation process of the Ga 2 Te 3 -TiO 2 -C(10%) electrode was investigated using ex situ XRD (Figure 4a,b). Peaks corresponding to Na 2 Te and Ga were observed at a discharge voltage of 1.37 V (D: 1.37 V). When the electrode was fully discharged (D: 5 mV), NaGa 4 peaks were observed and Na 2 Te peaks remained. The NaGa 4 phase partly disappeared when the electrode was charged to 0.92 V (C: 0.92 V). In a charging state of 1.72 V, the Na 2 Te phase partly disappeared, Ga was observed, and NaGa 4 completely disappeared. Only the peaks corresponding to Ga 2 Te 3 were observed again when the electrode was fully charged to 2.5 V (C: 2.5 V). Ga 2 Te 3 undergoes the following structural changes during sodiation/desodiation: The reaction mechanism during the first sodiation/desodiation process of the Ga2Te3-TiO2-C(10%) electrode was investigated using ex situ XRD (Figure 4a,b). Peaks corresponding to Na2Te and Ga were observed at a discharge voltage of 1.37 V (D: 1.37 V). When the electrode was fully discharged (D: 5 mV), NaGa4 peaks were observed and Na2Te peaks remained. The NaGa4 phase partly disappeared when the electrode was charged to 0.92 V (C: 0.92V). In a charging state of 1.72 V, the Na2Te phase partly disappeared, Ga was observed, and NaGa4 completely disappeared. Only the peaks corresponding to Ga2Te3 were observed again when the electrode was fully charged to 2.5 V (C: 2.5 V). Ga2Te3 undergoes the following structural changes during sodiation/desodiation:  1st discharge It is noteworthy that, after the first cycle, the Ga2Te3 phase (major peaks at 53.8°, 69.4°, and 71.5°) was completely recovered without any impurity peaks, showing a highly reversible interaction of Ga2Te3 with Na ions. The alloying/dealloying and conversion mechanism of the Ga2Te3 electrode during charge/discharge is shown by the ex situ XRD results, as schematically depicted in Figure 4c. The reaction mechanism during the first sodiation/desodiation process of the Ga2Te3-TiO2-C(10%) electrode was investigated using ex situ XRD (Figure 4a,b). Peaks corresponding to Na2Te and Ga were observed at a discharge voltage of 1.37 V (D: 1.37 V). When the electrode was fully discharged (D: 5 mV), NaGa4 peaks were observed and Na2Te peaks remained. The NaGa4 phase partly disappeared when the electrode was charged to 0.92 V (C: 0.92V). In a charging state of 1.72 V, the Na2Te phase partly disappeared, Ga was observed, and NaGa4 completely disappeared. Only the peaks corresponding to Ga2Te3 were observed again when the electrode was fully charged to 2.5 V (C: 2.5 V). Ga2Te3 undergoes the following structural changes during sodiation/desodiation:  1st discharge It is noteworthy that, after the first cycle, the Ga2Te3 phase (major peaks at 53.8°, 69.4°, and 71.5°) was completely recovered without any impurity peaks, showing a highly reversible interaction of Ga2Te3 with Na ions. The alloying/dealloying and conversion mechanism of the Ga2Te3 electrode during charge/discharge is shown by the ex situ XRD results, as schematically depicted in Figure 4c. It is noteworthy that, after the first cycle, the Ga 2 Te 3 phase (major peaks at 53.8 • , 69.4 • , and 71.5 • ) was completely recovered without any impurity peaks, showing a highly reversible interaction of Ga 2 Te 3 with Na ions. The alloying/dealloying and conversion mechanism of the Ga 2 Te 3 electrode during charge/discharge is shown by the ex situ XRD results, as schematically depicted in Figure 4c.
For the first, fifth, and 20th cycles, the EIS profiles of the Ga 2 Te 3 -TiO 2 -C(10%), Ga 2 Te 3 -TiO 2 -C(20%), and Ga 2 Te 3 -TiO 2 -C(30%) electrodes were obtained ( Figure 5). The simplified equivalent circuit shown in Figure 5d includes the electrolyte resistance (R b ), chargetransfer resistance (R ct ), SEI layer resistance (R SEI ), interfacial double-layer capacitance (C dl ), Warburg impedance (Z w ), and constant phase element (C PE ). The R ct at the electrodeelectrolyte interface is denoted by compressed semicircles in the mid-frequency region of the Nyquist plot. For all the electrodes, R ct gradually decreased as the cycle number increased from 1 to 20. Ga 2 Te 3 -TiO 2 -C(10%) exhibited the lowest value of R ct after 20 cycles (Table S8), demonstrating the most facile Na ion transportation, which led to the highest Na storage performance. 408, 374, 348, 321, and 318 mAh·g −1 , respectively (Figure 3e), which were considerably greater than those of Ga2Te3-TiO2-C(20%) and Ga2Te3-TiO2-C(30%). Surprisingly, even at 10 A·g −1 , Ga2Te3-TiO2-C(10%) had a CR of up to 96% (Figure 3f). Additionally, Ga2Te3-TiO2-C(10%) demonstrated a high rate performance when the discharge rate was reduced from 10 A·g −1 to 0.1 A·g −1 , resulting in high CR (99.3%).   For the first, fifth, and 20th cycles, the EIS profiles of the Ga2Te3-TiO2-C(10%), Ga2Te3-TiO2-C(20%), and Ga2Te3-TiO2-C(30%) electrodes were obtained ( Figure 5). The simplified equivalent circuit shown in Figure 5d includes the electrolyte resistance (Rb), charge-transfer resistance (Rct), SEI layer resistance (RSEI), interfacial double-layer capacitance (Cdl), Warburg impedance (Zw), and constant phase element (CPE). The Rct at the elec- trode-electrolyte interface is denoted by compressed semicircles in the mid-frequency region of the Nyquist plot. For all the electrodes, Rct gradually decreased as the cycle number increased from 1 to 20. Ga2Te3-TiO2-C(10%) exhibited the lowest value of Rct after 20 cycles (Table S8), demonstrating the most facile Na ion transportation, which led to the highest Na storage performance. Currently, there are only a few reports on Ga-based or Te-based anodes for SIBs. However, chalcogenide materials (In2S3, Sb2Se3, etc.) have high specific capacities when they undergo sequential conversion and alloying reactions owing to their unique properties. A comparison of the performances of Ga2Te3-TiO2-C(10%) and other chalcogenide materials demonstrated the high potential of the Ga2Te3-based composite electrodes for future applications (Table 1).  Currently, there are only a few reports on Ga-based or Te-based anodes for SIBs. However, chalcogenide materials (In 2 S 3 , Sb 2 Se 3 , etc.) have high specific capacities when they undergo sequential conversion and alloying reactions owing to their unique properties. A comparison of the performances of Ga 2 Te 3 -TiO 2 -C(10%) and other chalcogenide materials demonstrated the high potential of the Ga 2 Te 3 -based composite electrodes for future applications (Table 1).

Conclusions
We demonstrated a Ga 2 Te 3 -based composite as a prospective anode material for SIBs. The Ga 2 Te 3 -TiO 2 ·C(10%) anode achieved a high reversible capacity of 437 mAh·g −1 after 300 cycles at 0.1 A·g −1 , as well as a high rate capability (CR of 96% at 10 A·g −1 relative to 0.1 A·g −1 ). The nanoconfined Ga 2 Te 3 crystallites embedded within an electrically conductive TiO 2 -C hybrid matrix effectively accommodated the Ga 2 Te 3 particle volume variation and avoided the agglomeration of Ga during electrochemical reactions. In addition, Na ion diffusion kinetics and mechanical stability were enhanced by this beneficial morphology, thereby achieving high capacity and long-term cycling performance These findings offer a new direction toward the development of high-performance SIBs with long cycle lifetimes and expansion of the Ga-and Te-based materials in other electrochemical energy storage systems.

Conflicts of Interest:
The authors declare no conflict of interest.