TiO2 Hollow Spheres With Flower-Like SnO2 Shell as Anodes for Lithium-Ion Batteries

SnO2 is a promising anode material for lithium-ion batteries due to its high theoretical specific capacity and low operation voltage. However, its poor cycling performance hinders its commercial application. In order to improve the cycling stability of SnO2 electrodes, novel flower-like SnO2/TiO2 hollow spheres were prepared by facile hydrothermal method using carbon spheres as templates. Their flower-like shell and mesoporous structure highlighted a large specific surface area and excellent ion migration performance. Their TiO2 hollow sphere matrix and 2D SnO2 nano-flakes ensured good cycle stability. The electrochemical measurements indicated that novel flower-like SnO2/TiO2 hollow spheres delivered a high specific capacity, low irreversible capacity loss and superior rate performance. After 1,000 cycles at current densities of 200 mA g−1, the capacity of the flower-like SnO2/TiO2 hollow spheres was still maintained at 720 mAh g−1. Their rate capacity reached 486 mAh g−1 when the current densities gradually increase to 2,000 mA g−1.


INTRODUCTION
As one of the most promising energy storage devices, rechargeable lithium ion batteries (LIBs) have been widely used in smart phones, computers, electric vehicles and other portable electronic devices due to their long cycle life and high energy density (Lee et al., 2009;Scrosati et al., 2011;Tian et al., 2017;Fan et al., 2019). However, graphite, as a commercial anode material for LIBs, has gradually lost its competitiveness in practical application given it poor rate property and low theoretical capacity (372 mAh g −1 ) (Li et al., 2014;Zhang J. et al., 2019). Compared with graphite, metal oxide anode materials have attracted much attention because of their high theoretical specific capacities (Chen et al., 2015;Jiang et al., 2019;Li et al., 2019;Liu et al., 2019;. Among metal oxides, tin dioxide (SnO 2 ) was expected to be a promising anode material for LIBs given its high theoretical specific capacity (781 mAh g −1 ) and low operation voltage (Ao et al., 2020;Liu Q. et al., 2020). Unfortunately, SnO 2 can not be applied as a LIB anode material alone due to the poor cycling performance resulted from the large volume variation during lithium ion insertion and extraction, resulting in crushing, and structural disintegration (Zhu et al., 2014;Cheong et al., 2017;Liu Q. et al., 2020). In order to improve the cycling stability of SnO 2 electrodes, various nanostructured SnO 2 materials have been developed (Yang et al., 2011;Ji et al., 2013;Jean et al., 2017;Mao et al., 2021;Shen et al., 2021). Among them, the hollow spheres were deemed to be an ideal structure of the anode materials for LIBs due to their low density, high surface-to-volume ratio, isotropic physical properties and structural stability (Miao et al., 2016;Wu et al., 2019). However, due to the limitation of dynamics, the pure SnO 2 hollow spheres are still easy to be broken during the long period of charging and discharging, especially at a high current density (Tian et al., 2019). Composite with other materials was considered to be an effective way to alleviated the structural disintegration of SnO 2 and improve its electrode cycling stability (Wu et al., 2015;Wu K. et al., 2018;Tian et al., 2019). In recent years, although the rate capacity of TiO 2 is relatively low (∼170 mAh g −1 ), TiO 2 was widely recommended as the anode composite matrix material for lithium-ion batteries given its high operating voltage, low price, and less volume change during the lithium-ion insertion and extraction processes (Luo et al., 2016;Li et al., 2017;Tian et al., 2018;Xu et al., 2020). In addition, the reasonably arranged two-dimensional (2D) singlelayer nano-flakes endow the electrodes good cycling stability due to their large effective contact area and high tolerance to volume variations (Wang et al., 2011;Fan et al., 2020). Therefore, the specific capacity and the cycling stability of SnO 2 -based electrode materials were expected to be further improved by vertical selfassembly of 2D SnO 2 nano-flakes on the surface of TiO 2 hollow spheres. However, how to vertically assemble 2D SnO 2 nanoflakes on the surface of TiO 2 hollow spheres to obtain uniform hollow composite spheres was still a major challenge given the aggregation characteristics of nanoparticles and their dependence on the synthetic environment.
In this study, three-dimensional (3D) hierarchical flowerlike SnO 2 /TiO 2 hollow composite spheres were prepared by a facile two-step hydrothermal synthesis. The flower-like SnO 2 shell imparted these hierarchical hollow spheres a large specific surface area and high specific capacity. The TiO 2 hollow matrix supported the composite products free from collapse during charging and discharging. Their hollow structure and the existence of 2D SnO 2 nano-flakes made these hollow composite spheres more stable to cycle, even at high current density.

Synthesis of Carbon Sphere Templates
In a typical experiment, 89 g anhydrous glucose was dissolved in 250 mL deionized water and stirred at room temperature for 30 min. After stirring, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 180 • C for 5 h, and then cooled to room temperature. After washing with deionized water and anhydrous ethanol, the products were dried at 80 • C for 8 h. The black brown products obtained were the carbon sphere templates.

Synthesis of TiO 2 Hollow Microspheres
Carbon spheres were firstly synthesized by glucose hydrothermal method. 0.6 g carbon spheres were added to the prepared 30 mL titanium tetrachloride solution for ultrasonic dispersion for 15 min. The mixture was stirred at room temperature for 6 h, and then centrifuged and washed with acetic acid. The obtained products were placed in an oven at 80 • C for 8 h. Then, they were annealed in air at 500 • C for 3 h. After cooling to the room temperature, TiO 2 hollow spheres were obtained.

Synthesis of 3D Flower-Like SnO 2 /TiO 2 Hollow Spheres
In a typical procedure, 0.5 g SnCl 4 ·5H 2 O, 0.33 g NaOH, 0.74 g CTAB, and 30 mL deionized water were firstly mixed in a 50 mL glass tube reactor. Then 0.6 g the as-prepared TiO 2 hollow microspheres were dipped into the mixed solution, dispersed by ultrasonic for 15 min, and stirred at room temperature for 6 h. The mixture was transferred to a 50 mL Teflon-lined autoclave and heated at 120 • C for 12 h. The products were centrifuged and washed with deionized water and absolute ethanol for at least three times. After dried in an oven at 80 • C for 6-10 h, they were annealed at 500 • C for 3 h.

Structure and Morphology Characterization
The structure of the obtained hollow composite spheres were determined by X-ray powder diffraction (XRD, Panalytical X' Pert, Holland) with Cu-Ka radiation (λ = 1.5418 Å). The morphologies of the obtained products were examined by Scanning Electron Microscopy (SEM, JSM-7000F, Japan) and Transmission Electron Microscopy (TEM, TitanX 60e300, USA). A Scanning Electron Energy-dispersive X-ray spectrometer (EDS) was attached to the TEM to analyze the composition of the specimens. The surface elemental electronic states and chemical bonds of materials were analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5600 ESCA). The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area of the products using nitrogen adsorption-desorption isotherm obtained by Micrometics Tristar 3000 system.

Electrochemical Measurements
Using lithium-metal foil as the cathode, the CR 2032 coin cells were assembled in an argon-filled glove box with <0.5 ppm of water and oxygen. The cathode was made of a coating containing 80% active material, 10% super-P-Li carbon black and 10% polyvinylidene fluoride (PVDF). 1 M LiPF6 was dissolved in ethylene carbonate and diethyl carbonate with a volume ratio of 1:1 as electrolyte. The cells were galvanostatically charged and discharged on Neware-CT3008 battery tester. Cyclic voltammetry (CV) was conducted on a PARSTAT 4000 electrochemical workstation at a scan rate of 0.2 mV s −1 in the range of 3.0-0.01 V (vs. Li/Li + ). Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 100 kHz−0.01 Hz with an ac perturbation voltage of 5 mV.

RESULTS AND DISCUSSION
The synthesis process of 3D hierarchical flower-like SnO 2 /TiO 2 hollow composite spheres is schematically illustrated in Figure 1a. TiO 2 coatings were formed on the surface of the carbon spheres by direct hydrolysis of titanium tetrachloride. TiO 2 hollow spheres were obtained by the calcination of the above hydrolyzed products. Subsequently, ultrathin SnO 2 nano-flakes were assembled on the surface of TiO 2 hollow spheres under hydrothermal conditions to form 3D hierarchical flower-like SnO 2 /TiO 2 hollow composite spheres. Here, CTAB, as one of amphiphilic chemicals, not only acted as a dispersant, but also was easy to adhere to crystal surface due to its double layer structure . Therefore, with the help of CTAB, the SnO 2 nanoparticles tend to grow along a specific crystallographic plane during the hydrothermal process, forming SnO 2 nano-flakes. Figure 1b shows the SEM images of the pure carbon spheres with smooth surface. The average diameter of carbon spheres was about 600 nm. The SEM image of the semiclosed spheres indicates that the TiO 2 spheres clearly exhibited a hollow structure (Figure 1c). They were similar to carbon spheres in diameter. Figure 1d displays the SEM images of 3D hierarchical flower-like SnO 2 /TiO 2 hollow composite spheres. The 2D SnO 2 flakes were vertically assembled on the surface of the TiO 2 hollow spheres to form a flower-like shell. The magnified image of the semi-closed spheres shows that the SnO 2 /TiO 2 composite spheres presented a significant hollow architecture and their flower-like shell was assembled from SnO 2 nano-flakes with a thickness of about 25 nm (Supplementary Figure 1a). This flower-like hollow structure facilitates the insertion and extraction of lithium-ions due to its expanded specific surface area (Emamdoust and Shayesteh, 2018). Supplementary Figure 1b shows that the TiO 2 spheres were semi-coated with a thin SnO 2 shell after incubation for 2 h. When the reaction time was extended to 18 h, thick SnO 2 nanoflakes were compactly assembled on the surface of TiO 2 spheres (Supplementary Figure 1c), increasing the risk of pulverization of the active materials during the insertion and extraction of lithium-ions (Gogotsi and Simon, 2011;Li et al., 2017). Figure 2a presents the TEM images of TiO 2 hollow spheres. Their centers were in sharp contrast to their edges, showing a distinctly hollow structure. Their relative lattice fringe spacing was 0.352 nm, corresponding to the (101) diffraction planes of rutile TiO 2 (Figure 2b) (Nguyen et al., 2020). The EDX mapping indicates that Ti and O elements were contained in the TiO 2 hollow spheres (Figure 2c). After the second hydrothermal reaction, SnO 2 nano-flakes were vertically assembled on the surface of TiO 2 hollow spheres (Figure 2d). The flowerlike SnO 2 /TiO 2 products still maintained a distinctly hollow structure. The lattice fringe spacing of flower-like shell was 0.335 nm, matching with the (101) diffraction planes of SnO 2 (Figure 2e) . The EDX mapping shows that the flower-like SnO 2 /TiO 2 hollow spheres were composed of O, Ti and Sn elements (Figure 2f). The Sn element evenly distributed in the flower-like shell of the final hollow spheres, confirming their remarkable core-shell architecture.  Figure 3A shows the XRD pattern of TiO 2 and the flowerlike SnO 2 /TiO 2 hollow spheres. The diffraction peaks of the pure TiO 2 hollow spheres matched well with the standard peaks of rutile crystal phase (JCPDS No. 21-1276). In addition to the diffraction peaks for rutile TiO 2, the diffraction peaks of the flower-like SnO 2 /TiO 2 hollow spheres were composed of SnO 2 (JCPDS NO. 41-1445). The chemical states of the flower-like SnO 2 /TiO 2 hollow spheres were further investigated by using XPS. The XPS survey spectrum of the flower-like SnO 2 /TiO 2 hollow spheres contained C, Sn, O, Ti elements ( Figure 3B). Figure 3C plots the high-resolution XPS spectrum of O 1s. The peaks at 530.7 in the spectrum of O 1s ( Figure 3E) represented oxides formed by O 2− composed of Ti and Zn. As shown in Figure 3D, the Sn 3d spectrum had two enhancement peaks at 487.1 and 495.3 eV, corresponding to the Sn 4+ 3d 5/2 and Sn 4+ 3d 3/2 binding energies of SnO 2 , respectively Sun et al., 2020). The Ti 2p spectrum in Figure 3E had two characteristic peaks at 459.5 and 465.3eV, which were attributed to Ti 4+ 2p 3/2 and Ti 4+ 2p 1/2 , respectively . The XPS results indicate that the obtained products were a mixture of TiO 2 and SnO 2 , which was consistent with the SEM, TEM and XRD results.
Nitrogen adsorption-desorption measurements further verified the interface advantages of the flower-like SnO 2 /TiO 2 hollow spheres. Figure 3F shows the nitrogen adsorptiondesorption isotherm of the flower-like SnO 2 /TiO 2 hollow spheres had a type IV isotherm, indicating that the final products were the mesoporous structure. The specific surface area of the flower-like SnO 2 /TiO 2 hollow spheres was calculated to be about 138.5 m 2 g −1 . The pore diameter was mainly distributed between 1 and 8 nm. Their mesoporous structure and larger specific surface area facilitated the insertion and extraction of lithium ions, resulting in a large practical capacity of the final electrode Zhong et al., 2020).
The electrochemical properties of the flower-like SnO 2 /TiO 2 hollow spheres were tested by using lithium foils as counter and reference electrode. The typical CV curves of 3D flower-like SnO 2 /TiO 2 hollow spheres were plotted in Figure 4A. In the first cycle, there were two cathodic current peaks at about 0.74 and 1.65 V. The cathodic peak at about 0.74 V corresponded to Sn reduction of SnO 2 (Wu N. et al., 2018). The cathodic peak around 1.65 V was caused by the SEI layers generated on the surface of the flower-like SnO 2 /TiO 2 hollow spheres. This peak disappeared in the subsequent cycles given the stable formation of SEI layers. In the anodic process, three obvious peaks appeared at about 0.6, 1.2, and 2.1V, respectively. The oxidation peak at about 0.6 V could be attributed to the dealloying process of Li x Sn. The peak appearing around 1.2 V was due to the partially reversible reaction between Sn and SnO 2 . The redox peak at about 2.0V was originated from interaction between lithium ions and TiO 2 (Jeun et al., 2013;Yuan et al., 2015;Liu Q. et al., 2020). The CV curves overlapped well in the subsequent cycles, indicating that the electrochemical reaction had good reversibility. The above electrochemical reaction mechanism were described as follows: xLi + + TiO 2 + xe − = 2Li x TiO 2 (0 ≤ x ≤ 1)  Figure 4B presents the charge-discharge voltage profiles of the flower-like SnO 2 /TiO 2 hollow spheres at current densities of 200 mA g −1 . The flower-like SnO 2 /TiO 2 hollow spheres delivered an initial discharge/charge capacity of 1,372/856 mAh g −1 .
The initial capacity loss was mainly caused by the incomplete conversion reaction and the formation of the SIE layer. The charge-discharge profiles of the second and third curves overlapped well, revealing that the flower-like SnO 2 /TiO 2 hollow sphere electrodes had good cycling stability. This was consistent with the CV test results. Figure 4C shows the cycling performance of the flower-like SnO 2 /TiO 2 hollow spheres, TiO 2 hollow spheres and SnO 2 nanoparticles at current densities of 200 mA g −1 . Compared to the pure TiO 2 hollow spheres and SnO 2 nanoparticles, the flower-like SnO 2 /TiO 2 hollow spheres inherited not only large specific capacity of SnO 2 , but also the good cycling stability of TiO 2 . After 1,000 cycles, their discharge capacity was still maintained at about 720 mAh g −1 . In contrast, the capacity of pure SnO 2 nanoparticles decayed rapidly to after 40 cycles. Although the cycling performance of TiO 2 hollow spheres was stable, their capacity was low. Figure 4D shows the rate performance of the above-mentioned electrodes in the range of 100-2,000 mA g −1 . The flower-like SnO 2 /TiO 2 hollow spheres maintained a stable discharge capacity of 887, 826,723, 574, and 486 mAh g −1 at a high current density of 100, 200, 500, 1,600, and 2,000 mA g −1 , respectively. When the current density returned to the 100 mA g −1 , the stable discharge capacity of the flower-like SnO 2 /TiO 2 hollow spheres almost also restored. However, the SnO 2 electrodes almost lost function when the current density increased to 200 mA g −1 . According to the previous literature (Xia et al., 2016), the large reversible capacity of the flower-like SnO 2 /TiO 2 hollow spheres was mainly due to their large surface area, which offered more reactive sites for the interface between the active materials and the lithium ions. Compared with the previous literature (Table 1), the less volume change of thin 2D SnO 2 nano-flakes and TiO 2 hollow sphere matrix endowed the final electrodes an excellent cycling stability. Furthermore, the mesoporous architecture facilitated the diffusion of lithium ions and further improved the cycling performance of the flower-like SnO 2 /TiO 2 hollow spheres.
The superior cycling and rate performance of the flowerlike SnO 2 /TiO 2 hollow spheres was further verified by EIS measurements. Figure 5 and Supplementary Figure 2 illustrate the Nyquist plots of the test specimens before and after cycling. All of the Nyquist plots had a straight line in the low-frequency and a semicircle in the high-frequency region, respectively, presenting the lithium ion diffusion and the charge transfer process. A Randles equivalent circuit was inserted into Figure 5 to simulate the electrochemical system, where Rs was the ohmic resistance, CPE was the double-layer capacitance, R CT was the charge transfer resistance, and W was the Warburg impedance representing the solid-state diffusion of the lithium-ions in the active materials. The semicircle diameter of the 3D flowerlike SnO 2 /TiO 2 hollow spheres and TiO 2 hollow spheres was FIGURE 4 | (A) CV curves of 3D flower-like SnO 2 /TiO 2 hollow spheres, (B) Charge-discharge voltage profiles of 3D flower-like SnO 2 /TiO 2 hollow spheres, (C) Cycling performance of 3D flower-like SnO 2 /TiO 2 hollow spheres, TiO 2 hollow spheres and SnO 2 nanoparticles. (D) Rate performance of 3D flower-like SnO 2 /TiO 2 hollow spheres, TiO 2 hollow spheres, and SnO 2 nanoparticles. significantly smaller than that of SnO 2 nanoparticles. The charge transfer resistance of the 3D flower-like SnO 2 /TiO 2 hollow spheres, TiO 2 hollow spheres and SnO 2 nanoparticles were about 19.8, 31.6, and 242.5 , respectively. This indicates that the 3D flower-like SnO 2 /TiO 2 hollow spheres also had a lower charge transfer resistance than SnO 2 nanoparticles. After 100 cycles, the charge transfer resistance of the 3D flower-like SnO 2 /TiO 2 and TiO 2 hollow spheres exhibited a less increase, while that of FIGURE 5 | Impedance analysis of 3D flower-like SnO 2 /TiO 2 hollow spheres.
SnO 2 nanoparticles increased significantly after 15 cycles. The 3D flower-like SnO 2 /TiO 2 hollow spheres had a good cycling performance, which was attributed to the significant cycling stability of their TiO 2 hollow sphere matrix and 2D nano-flakes.

CONCLUSIONS
In summary, the flower-like SnO 2 /TiO 2 hollow spheres were successfully synthesized by facile hydrothermal methods with the help of carbon spheres. Their flower-like shell and mesoporous structure delivered a large specific surface area and excellent ion migration performance. The further electrochemical measurements demonstrated that the flower-like SnO 2 /TiO 2 hollow spheres exhibited a high specific capacity, low irreversible capacity loss, and superior rate performance. After 1,000 cycles at current densities of 200 mA g −1 , the capacity of the flower-like SnO 2 /TiO 2 hollow spheres was still maintained at 720 mAh g −1 . Their rate capacity reached 486 mAhg −1 when the current densities gradually increase to 2,000 mA g −1 .

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS
YW and ZZ conceived and designed the experiments. HZ supervised the research. YZ and XZ helped to synthesize the materials. XX and YW performed the synthesis and characterization, interpreted the data, and wrote the paper with help from ZZ. All authors contributed to the article and approved the submitted version.

FUNDING
This work was supported by National Natural Science