Design and Synthesis of Layered Na2Ti3O7 and Tunnel Na2Ti6O13 Hybrid Structures with Enhanced Electrochemical Behavior for Sodium‐Ion Batteries

Abstract A novel complementary approach for promising anode materials is proposed. Sodium titanates with layered Na2Ti3O7 and tunnel Na2Ti6O13 hybrid structure are presented, fabricated, and characterized. The hybrid sample exhibits excellent cycling stability and superior rate performance by the inhibition of layered phase transformation and synergetic effect. The structural evolution, reaction mechanism, and reaction dynamics of hybrid electrodes during the sodium insertion/desertion process are carefully investigated. In situ synchrotron X‐ray powder diffraction (SXRD) characterization is performed and the result indicates that Na+ inserts into tunnel structure with occurring solid solution reaction and intercalates into Na2Ti3O7 structure with appearing a phase transition in a low voltage. The reaction dynamics reveals that sodium ion diffusion of tunnel Na2Ti6O13 is faster than that of layered Na2Ti3O7. The synergetic complementary properties are significantly conductive to enhance electrochemical behavior of hybrid structure. This study provides a promising candidate anode for advanced sodium ion batteries (SIBs).


Experimental Section
Materials: All the reagents used in the present study were obtained from Sinopharm and employed without further purification.
Synthesis of layered and tunnel hybrid material (NNTO): NNTO was prepared by a facile hydrothermal method. Anatase TiO 2 (0.5 g) was well dispersed into NaOH (2 M, 100 ml) solution by magnetic stirring about 40 minutes. Then the mixture solution was transferred into 150 ml Teflon-lined autoclave. The autoclave was maintained at 180°C for different hours in an oven and was taken out from the oven after cooling down to room temperature. The white precipitant could be obtained by centrifugation and washed to about pH 7 with deionized water. The obtained powder was dried at 80°C for overnight in a vacuum oven. Finally, this product was firstly sintered at 500°C for 6 h with subsequent heat treatment at different temperatures for 12 h. Heating rate of the total process was 5°C/min.

Morphology and phase analysis of NNTO:
The morphology and crystallographic properties of the as-prepared samples were characterized by field emission scanning electron microscopy (SEM, HITACHI S-4800), transmission electron microscopy (TEM, JEOL 2100F), synchrotron radiation XRD (SXRD) at ALBA's beamline BL04-MSPD and powder X-ray diffraction (XRD, Panalytical EMPYREAN, Cu Kα radiation). The SXRD and XRD data was refined by Rietveld method using PDXL software (Rigaku Co., Ltd., PDXL 2.1) and FullProf program.
In situ X-ray characterization: In situ X-ray synchrotron diffraction (In situ XRD) measurement was operated at beamline P02.1 at the synchrotron diffraction instrument PETRA III (DESY, Hamburg). Detailed study of beamline P02.1 had been given by Herklotz et al. in 2013. [1] A 16-inch 2D flat panel detector of XRD 1621 N ES Series (PerkinElmer) with 2048 ⅹ 2048 pixels and a pixel size of 200 um was used for recording the diffraction patterns. [2] In situ XRD data was collected in the 2θ range from 1.02° to 42.8°. The electrochemical test was performed during the 1 st discharge down to 0.01 V and subsequent charge up to 2.5 V, then 2 nd discharge down to 0.118 V and stopped. The XRD patterns were record one time every 10 mins interval. The cell battery was statically set for one hour when the 1 st discharge process was finished.
Eletrochemical characterization of NNTO: The Electrodes were made by spreading a mixture of active material (hybrid materials), acetylene black and CMC binder with a weight ratio of 75: 15: 10 on copper foil current collectors, which were dried at 80°C for 12h in a vacuum oven. Electrochemical performances of the electrodes were evaluated by coin cells (type CR2025) assembled in an argon-filled glove box with O 2 and H 2 O levels <0.5 ppm. For the half-cells preparation, sodium foil was used as the counter electrode and glass fiber (GF/D, Whatman) was used as the separator. The electrolyte was 1 M NaClO 4 and a mixture of ethylene carbonate/ propylene carbonate (EC: PC=1:1 v/v) with 2 wt % Fluoroethylene carbonate (FEC) (purchased from Fosai New Materials Co., Ltd., Jiangsu, China). The mass loading of active materials was around 2 mg cm -2 . The sodium cells were galvanostatically discharged and charged on a battery test system (Neware BTS-610) in a voltage range from 2.5 to 0.01 V (versus Na/Na + ). Cyclic Voltammetry tests were performed on an electrochemical workstation (LK 9805) in the voltage range of 2.5 to 0.01 V at a scan rate of 0.2 mV s -1 . The galvanostatic intermittent titration technique (GITT) tests were performed on a CT2001A LANHE electrochemical workstation. Electrochemical impedance spectroscopy (EIS) measurements were carried out by Zennium IM6 electrochemical workstation. Above all electrochemical measurements were conducted at 25°C.  To further clarify that the electrochemical performance of the hybrid structures could be superior to that of the pure Na 2 Ti 3 O 7 phase and Na 2 Ti 6 O 13 phase, the single Na 2 Ti 3 O 7 and Na 2 Ti 6 O 13 was fabricated by the transitional solid phase method. As shown in Figure S5, XRD analysis indicated that the single phase of sodium titanates was synthesized without impurity phase. In addition, the cycling performance of single phase was detected at a current density of 20 mA g -1 . The result indicated that the large capacity decay was observed before 20 cycles for single Na 2 Ti 3 O 7 phase and the excellent cycling performance was given after 20 cycles. As showed in Figure S5 c, the capacity fluctuation of single Na 2 Ti 6 O 13 phase occurred with the poor cycling performance. Figure S5. a) XRD patterns of single phase of Na 2 Ti 3 O 7 and Na 2 Ti 6 O 13 ; b)the cycling performance of Na 2 Ti 3 O 7 electrode at 20 mA g -1 ; c) the cycling performance of Na 2 Ti 6 O 13 electrode at 20 mA g -1 ; To further explain why capacity increase occurred during the initial stage, CV measurements of different cycled NNTO samples at various scan rates ranging from 0.2 to 0.6 mV s -1 were performed. The percent of capacitive/diffusion-controlled contribution of cycled NNTO at different charge states was presented in Figure S6. Based on CV curves of different charge states in Figure S6 (a, c, e), it was evidently observed that there were no obvious redox characteristic peaks of tunnel Na 2 Ti 6 O 13 of 10 th cycled sample as compared to the 1 st cycled sample. And the redox peaks of layered Na 2 Ti 3 O 7 located at around 0.11 V in Figure S5c was evidently seen, which was in good agreement with charge-discharge curves. With the cycling increase, the redox peak intensity of layered Na 2 Ti 3 O 7 at the 100 th charge state became weaker and the redox peak intensity of tunnel Na 2 Ti 6 O 13 enhanced. The larger area space under CV profiles at the 10 th charge state was much bigger as compared to the 1 st and 100 th cycled samples, indicating the better Na-storage. To confirm the surface/diffusion controlled contribution, the power law model is performed by the following equation： (S1) Where a and b are adjustable values. [3,4] The b value of 0.5 means that the current is controlled by semi-infinite linear diffusion and the b value of 1 represents that the current is controlled by surface-controlled. As depicted in Figure S7a, the b value of the 1 st cycle (0.791) exhibits that the Na-storage is mainly controlled from surface-controlled process. The b values of 0.568 and 0.572 at the 10 th and 100 th charge state suggests that the diffusioncontrolled Na-storage occupies significant roles. Another analysis is also employed to confirm the percentage of surface/diffusion-controlled contribution and the equation is expressed as below: The above equation could be rearranged into another way: Here and could represent capacitive and diffusion-controlled contribution.
[5] Figure   S7b shows the relationship of i/ν 1/2 vs ν 1/2 at different charge states. As portrayed in Figure S6 (b, d, f), the diffusion-controlled Na-storage is continuously improved and exhibits a linear increasing trend and reaches a maximum value at the 100 th charge state as compared to that at the 1 st and 10 th cycled sample. This reason is that more active material takes part in redox reaction with more electrolyte penetrating from the outer surface to the bulk. The decay of capacity of 100 th cycled sample may be related with the larger electrode polarization and the structural destruction of layered Na 2 Ti 3 O 7 from volume effect and the structural reconstruction/relaxation. [6,7]