Three-Dimensional Chestnut-Like Architecture Assembled from

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Three-Dimensional Chestnut-like Architecture Assembled from NaTi3O6(OH)·2H2O@N-Doped Carbon Nanosheets with Enhanced Sodium Storage Properties Chunjin Wu, Zheng Zhang, Yi Tang, Zuguang Yang, Yongchun Li, Benhe Zhong, Zhen-Guo Wu, Xiaodong Guo, and Shi Xue Dou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17293 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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ACS Applied Materials & Interfaces

Three-Dimensional Chestnut-like Architecture Assembled from NaTi3O6(OH)·2H2O@N-Doped Carbon Nanosheets with Enhanced Sodium Storage Properties Chunjin Wu†, Zheng Zhang‡, Yi Tang§, Zuguang Yang†, Yongchun Li †, Benhe Zhong†, Zhen-guo Wu*,†, Xiaodong Guo*,†, Shi-xue Dou# †School

of Chemical Engineering, Sichuan University, Chengdu, 610065, PR China College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 PR China §National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610065, PR China #Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522 (Australia) KEYWORDS: sodium-ion batteries, sodium titanates, 2D nanosheets, 3D nanospheres, self-sacrificed template method. ‡

ABSTRACT: The application of sodium titanates anodes of low cost, feasible operate voltage and non-toxicity were severely hindered by the inferior cycling stability and poor rate capability. Here, three-dimensional chestnut-like NaTi3O6(OH)·2H2O@N-doped carbon nanospheres (NTOH @CN) with loose crystal structure were prepared by selfsacrificed template method. The nanospheres were composed of nanosheets and linked with nanowires, which interweaved to construct a meshwork structure. The growth mechanism of unique 3D NTOH @CN nanospheres was investigated by tracking the synthesis process of different hydrothermal durations. The rate performances of 3D NTOH @CN were superior to that of NaTi3O6(OH)·2H2O irregular spheres assembled from nanosheets (3D-NTOH) and NaTi3O6(OH)·2H2O nanosheets (2D-NTOH). Excellent cycle and rate performance were obtained due to the open crystal structure, unique 3D nanosphere morphology with short diffusion paths, N-doped carbon surrounding and the solid solution reaction. In addition, the reaction mechanism, morphology change and dynamics research during the sodium insertion/desertion process have been carefully studied. Based on varied ex-situ analysis, the irreversible metallic titanium formation and the excellent structural stability of nanospheres morphology has been evidenced. And the pseudocapacitive phenomenon was also detected and effectively enhanced Na+ ions storage capability. The systematical and comprehensive study provide a holograph for the design and synthesis of sodium titanates nanostructure.

1.INTRODUCTION Sodium-ion batteries (SIBs) become more attractive due to the inexhaustible resources and similar chemical and physical properties to that of Lithium-ion batteries (LIBs) 1-7. Due to low cost, feasible voltage plateaus, non-toxicity and environmental friendliness, sodium titanates have been considered to be a promising anode considering the recent reported anode materials 8-12. However, the unsatisfactory rate capability and insufficient conductivity of sodium titantes seriously hindered their practical application. To promote Na+ diffusion and achieve excellent rate performance of sodium titanates, two effective strategies were proposed. To realize fast Na+ transportation, one is searching loose crystal structure with enlarged interlayer space. Wang et al. had reported that NaTi3O6(OH)·2H2O was composed up of corrugated layers of (Ti6O14)4- units by linking edge- and corner-shared TiO6 octahedrons and the sodium titanates array delivered excellent cycling stability and superior Nastorage properties13. Such open structure possesses both layer and tunnel features and facilitates Na+ insertion/desertion. The other one is constructing unique nanostructures to shorten Na+/ electron diffusion length. Some unique morphologies have been reported, including 3D Na2Ti3O7 hollow spheres14, 3D Na2Ti3O7 micro-flowers15, 3D spider-web architecture and urchin-like ultrafine nanowires16, 17. Besides crystal structure modulation and morphology design, N-doped carbon additive was also considered as an effective method to enhance the electroconductibility. Qiao et al. gave a report that the storage sodium capability for Na2Ti3O7 with N-doped carbon was superior to that of bare

carbon14. And according to our previous study, amino acid could serve as both morphology controlling agent and carbon/ nitrogen sources18,19. Unfortunately, the growth mechanism of morphology evolution was still unclear and further investigated. Based on the above consideration, 3D chestnut-like NaTi3O6(OH)·2H2O@N-doped carbon nanospheres assembled from nanosheets was successfully synthesized by selfsacrificed template method. The growth process of NTOH @CN nanospheres was tracked and discussed. The NTOH @CN electrodes showed the best cycle performance amongst the previous reports with an ultra-high capacity retention (75.32%) after 1600 cycles at the current density of 1000 mA g-1. The rate performances of 3D NTOH @CN were superior to that of 3D NTOH irregular spheres and 2D-NTOH nanosheets. Moreover, the reaction mechanism, morphology evolution and reaction kinetics of NTOH @CN electrodes material during the discharge/charge process were also comprehensively studied. 2. EXPERIMENTAL SEGMENT 2.1 NTOH@CN Preparation NTOH @CN was synthesized by L-tryptophan-assisted template-free hydrothermal method. Sodium hydroxide, Ltryptophan and tetrabutyl titanate could be used as the sodium source, carbon and nitrogen additive and titanium source, respectively. 0.2 g L-tryptophan was put into the blended solution of 90 ml water and 1 ml ethanol, stirring for 2 h. Then, 1 g tetrabutyl titanate was explored to be dissolved into 10 ml ethanol with uniformly blending for 10 min, which was gradually dropped into the mixed solution with L-

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tryptophan. The reaction was proceeding for 5 h. The yellow powder was obtained after centrifugation and air-drying. The collected powder was again dispersed into 2.5 M, 100 ml NaOH solution by ultrasonication, which was poured into 100 ml reaction autoclave and transferred to the oven at 160 ˚C for different hours. Reaching to the room temperature, the solution with white powder was washed to be neutral and the sample was then freeze-dried. The final NTOH @CN product was obtained. NTOH was synthesized without using Ltryptophan and other procedures were same. 2D-NTOH was prepared by the dismemberment of NTOH @CN when the hydrothermal reaction reached for 18 h.

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became strong, which belonged to the peaks of NTOH @CN. And it was worth noting that two obvious impurity peaks at ca. 17.68˚ and 23.89˚ were observed for 18 h and 8 h, respectively. Based on the above results, the reaction duration of 12 h was selected as the optimized conditions.

2.2 Characterization and Electrochemical Measurements The morphologies and phase structures of NaTi3O6(OH)·2H2O samples were characterized by transmission electron microscopy (TEM, JEOL 2100F), emission scanning electron microscopy (SEM, HITACHI S-4800) and powder X-ray diffraction (XRD, Panalytical EMPYREAN) measurements, respectively. TEM-electronic differential scanning (TEM-EDS) mappings were conducted with an TME equipped with an energy dispersive spectroscopy analyzer. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) were operated to evaluated the valence state change. Raman (Thermo DXRxi) and Fourier-transform infrared (FTIR, Bruker R200-L spectrophotometer) analysis were explored to detect the change of band. High angle annular dark field (HAADF) was determined by Talos f200s. The thermogravimetric analysis (TGA, NETZSCH STA 409) was performed under the N2 condition. Autosorb-1 MP Automated Physisorption Analyzer confirmed the specific surface area of NTOH@CN sample. The carbon and nitrogen content were analyzed by EA 3000.

Figure 1. (a) Illustration of the growth mechanism of asprepared 3D NTOH @CN nanospheres; (b-e) TEM images of different reaction time; The morphology evolution of 3D NTOH @CN nanospheres is schematically illustrated in Figure 1a. Firstly, uniform TiO2 spheres coating L-tryptophan was obtained and served as the precursor. Secondly, 3D hollow NTOH @CN nanospheres composed of nanosheets were prepared via the reaction between TiO2 and NaOH. With the reaction progress, hollow nanospheres became much denser. After 18 h, 3D NTOH @CN nanospheres were broken to 2D-NTOH nanosheets. Spherical TiO2 powder with a diameter of 500-600 nm could be observed at the initial stage (Figure 1b). An amorphous coating layer of ~30 nm on the outer surface of TiO2 sphere may be ascribed to L-tryptophan thin film, which generated through the condensation reaction between -COOH of Ltryptophan and -OH of Ti(OH)x from the hydrolysis of tetrabutyl titanate14. When TiO2 particles were contacted with NaOH solution, sodium titanates nanosheets and nanowires would be formed by a dissolution-recrystallization process. Figure S2a-d clearly presented that the residual TiO2 closely attached to the surrounding of formed NTOH @CN material. Figure 1c-e and Figure 2b described the TEM images of NTOH @CN after reaction for 2, 8, 12, 18 h, respectively. When the reaction proceeded for 2 h, a small amount of NTOH @CN nanosheets formed and directionally arranged to accumulated 3D hollow nanospheres. By HRTEM analysis, the results suggested that the interlayer spacing of 5.79 Å and 1.89 Å were corresponded to (-202) lattice plane of NTOH @CN and (200) lattice plane of TiO2, respectively (inset of Figure 1c). More TiO2 component was converted into NTOH @CN nanosheets with the reaction development, which produced much denser hollow nanospheres (Figure 1d and Figure 2b). When the reaction reached 18 h, 3D nanospheres were broken and formed 2D nanosheets (Figure 1e). The SEM images of NTOH irregular spheres were given in Figure S2e-f.

The electrochemical performances of NaTi3O6(OH)·2H2O electrodes were measured by half-cells. The half cells were finished in a glove-box. Oxygen and water content were lower than 0.5 ppm. The component proportion of active material, acetylene black and CMC binder was 75: 15: 10. The slurry on copper foil was dried at 80 ˚C for 12 h in a vacuum oven. Sodium foil was the counter electrode and glass fiber (Whatman GF/D) was the separator. The electrolyte was prepared from 1M NaClO4 which was dissolved in ethylene carbonate (EC)/propylene carbonate (PC) at a volume ration of 1:1 with 2 wt % fluoroethylene carbonate (FEC). The mass loading of electrode for active materials was close to 2.5 mg cm-2. To study reaction mechanism and morphology evolution, the cells were disassembled. The cycled electrodes were cleaned using DMC solvent for several times, which was dried overnight. The battery tester (Neware BTS-610) was conducted to assess the electrochemical performance between 2.5 and 0.01 V (vs Na+/Na). The electrochemical workstation (LK 9805) was used to test cyclic voltammetry (CV) at a scan rate of 0.2 mV s-1. The electrochemical workstation (Zennium IM6) determined Electrochemical impedance spectroscopy (EIS) measurements. All electrochemical measurements were at the room temperature. 3. RESULTS AND DISCUSSION The crystal phase evolution during hydrothermal process was tracked by according XRD patterns at different reaction time (Figure S1). The as-prepared TiO2 from tetrabutyl titanate were existed in amorphous type. The characteristic peaks of NTOH @CN around 28.42˚, 48.4˚ and 62.09˚ with low crystallinity appeared after reaction for 2 h. With the reaction development, the intensity of the peaks at 25.62˚ and 38.76˚

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The crystallinity evolution of TiO2 powder to 3D NTOH @CN nanospheres was also confirmed by Raman spectroscopy and FTIR spectra. As depicted in Figure S3a, these peaks located at 600.22, 757.79, 865.10 cm-1 referred the Ti-O bending and stretching vibration of TiO220. And these peaks intensity evidently decreased with reaction developing, which indicated the consumption of TiO2 component. The new appeared peaks at 280.13, 447.62, 700.38, 907.72 cm-1 after reaction for 2 h demonstrated the formation of NTOH @CN, which was consistent with XRD and FTIR analysis. Two bands located at 280.13 and 447.62 cm-1 was corresponding to Na-O-Ti stretching vibration21, 22. And Raman peaks at 700.38 was ascribed to Ti-O-H bond 9 and short Ti-O bond22, 23, respectively.

proceeding.

As to FTIR analysis, the peaks at 1383.11 and 744.04 cm-1 corresponded to Ti-O modes23, 24. As the reaction progressed, the above peaks vanished and indicated the exhaustion of TiO2. Meanwhile, the intensified peaks at 900.10 cm-1 (Na-OTi bond)22 and 462.80 cm-1 (Ti-O bonds in TiO6 octahedra21) inferred the NTOH @CN formation. The above results confirmed that the consumption of TiO2 component and the gradually formation of NTOH @CN with the reaction

Figure 2. (a) XRD patterns of NTOH @CN sample of hydrothermal reaction for 12 h; TEM images: low magnification (b); (c) high magnification (inset of SAED image); (d) HRTEM image; (e-f) FIB result;(g) HAADF images; EDS mapping: (h) C; (i) N; (j) Na; (k) Ti; (l) O. Figure 2a showed XRD pattern of NTOH @CN after reaction for 12 h, which was analyzed in-depth and in good agreement with some previous reports17. The simulated XRD pattern was obtained by diamond software. The peaks located at ca. 24.8 ˚, 25.6˚, 28.4˚, 48.4˚ were indexed to the (-111), (-603), (111), (020) planes. Among them, (-111) plane was also corresponding to the lattice plane of tunnel Na2Ti6O13 (JCPDS No. 01-073-1398). And the (111) and (020) planes also belonged to layered Na2Ti3O7 (JCPDS No. 00-01-1329). As illustrated in Figure S4a, layered Na2Ti3O7 and tunnel Na2Ti6O13 structures were composed of TiO6 octahedrons, which were linked with edge or corner. In terms of NTOH @CN phase, TiO6 octahedrons were connected via edge- and



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corner-sharing to form corrugated layers of (Ti6O14)4- units. The NTOH @CN component could be regarded as an intermediate between layered Na2Ti3O7 and tunnel Na2Ti6O13, which was needed to further be verified11. To further demonstrate the component of as-prepared material, EDS, TG, EA measurements were performed. The atomic ratio of Na/Ti was 1:3.46, which suggested the existence of a small amount of TiO2 (Table S1) and agreed with HRTEM results. EA analysis showed that the percentage content of C and N was 1.28% and 0.31 %, respectively (Table S2). Figure S5b revealed a weight loss of 13.75 %, which indicated that 1 mol NTOH @CN contained ca. 2 mol H2O. Figure 2b showed the low magnification TEM morphology of the as-prepared NTOH @CN material with uniformly dispersed 3D chestnut-like nanospheres morphology of 400500 nm. The chestnut-like nanospheres were linked with nanowires, which disorderly interweaved and constructed a network structure. As displayed in Figure S4b, 3D chestnutlike architecture exhibited a large surface area (150.56 m2 g-1), which was much larger than that of the previous reports13. The pore size distribution curve supported by Barrett-JoynerHalenda (BJH) displayed that the size of pores varied during the scale of 3-5 nm (micropores).

Figure 3. (a) CV curves for the initial three cycles; cycling performance of different electrodes at the room temperature: (b) 200 mA g-1; (c) 400 mA g-1; d) rate performances; (e) cycling performance of NTOH @CN sample at 1000 mA g-1; (b) safety measurement at 50 ˚C at 200 mA g-1;

More detailed morphology features of the as-prepared NTOH @CN could be identified from the high magnification TEM image (Figure 2c). And NTOH @CN nanospheres were constructed by two dimensional (2D) nanosheets. SAED analysis (inset of Figure 2c) again validated the presence of TiO2, which was in accordance with the XRD and HRTEM result. The interlattice spacing of 0.996 Å was corresponding to the (323) plane of TiO2 (JCPDS No. 00-021-1272). The spacing of 1.8792 Å belonged to the (020) plane of NTOH @CN. As to HRTEM image (Figure 2d), the interlattice spacing of 3.733 Å corresponded to the (400) plane of NTOH @CN and the interplanar spacing (1.89 Å) was indexed to (200) lattice plane of TiO2, which was in good agreement with the result of SAED investigation. Focused Ion Beam (FIB) technology was carried out to detect whether 3D NTOH @CN nanospheres were hollow or not (Figure 2e-f). Figure 2e shows that numerous small holes were directionally arranged, which indicated the orientated rank of nanosheets. By linear scanning for spheres, the result (Figure 2f) showed that C, N, Na, Ti, O elements homogeneously distributed inside the nanospheres. The EDS elemental mapping results proved the same result (Figure 2g-l).

The cyclic voltammograms (CV) curves (Figure 3a) was carried out to detect the voltage platform. A broad cathodic peak of 1st cycle at 0.98 V disappeared in the subsequent process, which was related with the formation of SEI film and side reaction of the electrolyte. The anodic peak approached to a lower voltage (0.56 V) as compared to the oxidation peak in the 1st cycle, which referred a small polarization and predicted an excellent cycling performance. Figure 3b-c displayed cycling performance of different electrodes at different current densities. The result indicated that the cycling stability of NTOH @CN and 3D-NTOH samples was superior to that of 2D-NTOH electrodes at the different current densities. Figure 3d showed that the rate performances of three different samples. It was evident that the N-doped carbon modification for NaTi3O6(OH)·2H2O was beneficial to improve rate performances at high current density. Even at 1000 mA g-1, 61.58 mAh g-1 could still be retained over 1600 cycles. Moreover, NTOH @CN electrodes delivered a capacity of 100. 46 mAh g-1 with 98.79 % coulombic efficiency at 200 mA g-1 at 50 ˚ C. The excellent stability at both room temperature and 50 ˚C suggests a good environmental adaptation. Figure S6 depicted the galvanostatic charge-discharge curves of NTOH @CN at 20 mA g-1. The discharge and charge capacities were 391.13 and 141.69 mAh g-1, respectively. The declining slope of dischargecharge profiles implied that a solid solution reaction happened in the electrochemical process.


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ACS Applied Materials & Interfaces retained after the first cycle. Even after a prolonged cycling, the TEM and HRTEM images revealed that the entire nanospheres morphology was unchanged, which was in accordance with SEM analysis. For the ex-situ FTIR spectra of different cycled NTOH @CN half-cells (Figure S8a), the peaks at 3429.62 and 1639.66 cm-1 were ascribed to the O-H groups vibrations23. The decrease of bands intensity at 2360.21 cm-1 represents the expansion of Ti-O bond distances29. The bending and stretching vibrations of band at 1384.73 cm-1 was related with the alkali-oxygen vibrations30. And the C-O bending and stretching vibration at 1121.95 cm-1 was corresponding to the XPS result24, 27. The peaks located at 875.36 and 615.59 cm-1 demonstrated the TiO vibrations in (Ti6O14)4- units31. The D band for the disordered carbon atoms) and G band with sp2- hybridized graphitic carbon atoms were found from ex-situ Raman analysis (Figure S8b) 32-35.

Figure 4. Ex situ electrochemical performance test at the different charge states: (a) C 1s spectrum of XPS analysis; (b) Ti 2p spectrum of f XPS analysis; (c-e) Ex-situ SEM images at different charge states; (f-h) Ex-situ TEM images at different charge states (dotted circle: acetylene black and dotted line block: NTOH @CN); XPS analysis was operated to confirm the valence state at different charge states. The C 1s spectrum (Figure 4a) showed some characteristic peaks at 284.5, 284.8, 285.8, 288.0, 288.7 eV, which were assigned to sp2-C, C-O, C-C, C-N, C=O, groups, respectively1, 21, 25, 26. The result further confirmed the existence of C and N elements, which was in good agreement with the EDS analysis. And the C-O group derived from the electrolyte/NTOH @CN interfacial reaction was also observed27. Figure 4b showed Ti 2p spectrum result at different charge states. The Ti3+ peaks were obviously seen, which denoted that Ti4+/Ti3+ redox reaction happened. In addition, this result showed that Ti0 would be irreversibility formed from the reduction of Ti4+ (Table S3). Wu et al. had reported that sodium ions seemed to partly reduce TiO2 because of the sufficient lattice strain28. Our previous study also found that Ti4+ could be reduced to Ti0 in the Na2Ti6O13 host structure21. Ex-situ SEM and TEM (Figure 4c-e, Figure S7a-c and Figure 4fh) were conducted to get more detailed morphology evolution. According to the low and high magnification SEM images at the different cycled charge states, NTOH @CN electrodes still remained the morphology of 3D chestnut-like nanospheres with the cycling progress. The result revealed that the NTOH @CN material maintained excellent structural stability without distinct structural changes after a prolonged cycling. Based on TEM images, NTOH @CN material and acetylene black were homogeneously mixed together. It was noteworthy that a thin SEI film layer of 2 nm could be obviously observed, which was attached to outside surface of the NTOH @CN electrodes. The TEM results also validated that the morphology of 3D chestnut-like nanospheres was

Figure 5. (a) EIS curves for different cycled NTOH @CN electrode material; (b) the profiles displaying the relation of Z’ and ω-1/2; (c) CV profiles of NTOH @CN electrodes at different scan rates for fresh half-cell; (d) capacitive/diffusioncontrolled contribution for various scan rates. To further explore the reaction kinetics of NTOH @CN electrodes, electrochemical impedance spectroscopy (EIS) was performed. As presented in Figure 5a and Figure S9a, the Nyquist curves for the 1st, 15th and 30th NTOH @CN electrodes were drawn from 10 mHz to 100 kHz. A semicircle and an inclined straight were observed at the high and low frequency region for all the curves. Rs, Rsei and Rct represented the electrolyte resistance, SEI film resistance, and charge transfer impedance, respectively. Zw was the Warburg impedance. The equivalent circuit (inset of Figure 5a) was obtained from the previous reports21, 36. The fitting results (Table S4) indicated that the value of Rs had no obvious change. Because of the large surface area and more SEI film formation, the Rsei value reached 96.19 Ω for 1st cycled electrode. The Rsei values of 15th and 30th cycled sample were 17.05 Ω and 41.12 Ω, respectively, which were much smaller than that of 1st cycled sample. Xiaoling Cui et al.37 founded that the chemical degradation of SEI film was induced by the deposition of transition metal compounds on the anode electrode. And the irreversible reaction occurred in the electrochemical process. When the accumulation about metallic titanium exceeded a



special value, Ti may catalyze and promote the degradation of generated SEI film, which resulted in the reduction of Rsei value. Besides, the Rct value evidently decreased from 1st cycle (85 Ω) to 15th cycle (45.05 Ω) and 30th cycle (41.58 Ω). That denoted the improved Na+ diffusion in the NTOH @CN host structure. According to the following equation, sodium ion diffusion coefficient (D Na+) was calculated.

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scan rates was quantified. It is noticeable that the pseudocapacitance contribution occupied the dominant character at different sweep rates. And the surface capacitive contribution at 0.2 mV s-1 was about 76.3 % from the quantitative analysis. Further rose to 0.6 mV s-1, the capacitive value reached to 84.7 %. The high pseudocapacitance proportion effectively enhanced Na+ ions storage capability.

𝑅2𝑇2

(1) Where gas constant, the absolute temperature (298.15 K), the surface area of NTOH @CN electrode, the number of electrons per units, the Faraday constant, the concentration of Na+, and the Warburg factor could be represented by R, A, n, F, C, and σ, respectively. By the following equation, the value of σ was calculated. Z′ = 𝑅𝑠 + 𝑅𝑐𝑡 +𝜎𝜔 ―1/2 (2) Where Z’ represents the imaginary impedance, Rs serves as the electrolyte resistance, Rct refers the charge transfer resistance, ω denotes the angular frequency of the low frequency, σ represented the slope of ω-1/2 vs Z’ profile (Figure 5b). The obtained values of DNa+ for different cycled samples (Table S5) indicated the improved Na+ transmission rate in the bulk with the cycle increasing. The slightly decreased DNa+ for 30th cycled electrode was possibly attributed to the increase of the electrode polarization. To evaluate the surface/diffusion-controlled ratio of NTOH @CN half-cell, CV measurements from 0.2 to 0.6 mV s-1 were conducted (Figure 5c). The result showed similar types and trend with broad anodic peaks. Intriguingly, the anodic peaks gradually moved toward the lower voltages, which demonstrated an excellent reaction reversibility. The power law model was employed to confirm the capacitive proportion of the whole stored charge. The following equation described the current density (i) as a function of scan rate (𝑣). i = a𝑣𝑏 (3) For this equation, a and b both are the changed parameters3841. To draw log i as a function of log 𝑣, the b value was obtained. In general, 0.5 for b value suggested a diffusion dominant behavior, the b value of 1.0 represented a capacitive process. The fitting result (Figure S8b) indicated that b value was quantified to 0.89, denoting that surface capacitive behavior occupies prominent character. The reason is that 3D NTOH @CN nanostructures constructed by 2D nanosheets provides a relatively large surface for Na+ adsorption. The surface adsorption leads to a pseudocapacitance effect and gives more pathways for Na+ from the surface to NTOH @CN bulk. The relationship of surface-controlled and diffusioncontrolled contribution was also expressed by another equation. 1 I(V) = 𝑘1𝑣 + 𝑘2𝑣2 1 (4) Where 𝑘1𝑣 and 𝑘2𝑣2 represents the current contribution of the capacitive effects and intercalation mechanism, respectively4245. 1 1 I(V)/𝑣2 = 𝑘1𝑣2 + 𝑘2 (5) D = 2𝐴2𝑛4𝐹4𝐶2𝜎2

Figure 6. Schematic illustration of structural advantage and excellent sodium storage Figure 6 proposed the reasonable mechanism for structural advantage and the outstanding electrochemical performance. The 3D chestnut-like nanostructures were constructed by numerous nanosheets, which directionally arranged and closely interlinked. The nanospheres were linked with nanowires, which disorderly interweaved and constructed a network structure. The unique 3D morphology constructed a good network of effective electron and ion transportation. And 2D nanosheets significantly shortened Na+ diffusion paths to realize fast conductive paths. Moreover, the 3D NTOH @CN nanospheres provides a large specific surface and abundant channels, which are conductive to fully contact between electrode and electrolyte. And the surface Na+ adsorption resulted in a pseudocapacitive phenomenon, which effectively boosted Na+ ions storage capability. At last, the corrugated layers of (Ti6O14)4- units provides a relatively large interlayer space, which was easy to insert/desert Na+ ions. The above analysis proposed a reasonable explanation for the excellent cycle and rate performance of NTOH @CN electrodes in the micro level. 4. CONCLUSION In summary, 3D chestnut-like NaTi3O6(OH)·2H2O@N-doped carbon nanospheres was prepared by self-sacrificed template method. The nanospheres were composed of nanosheets and linked with nanowires, which interweaved to construct a meshwork structure. The NTOH @CN anodes showed excellent cycle and rate performance due to the open crystal structure, unique 3D nanospheres with short diffusion paths and N-doped carbon surrounding. An ultra-long cycle performance with 62 mAh g-1 was retained over 1600 cycles at 1000 mA g-1. The rate performances of 3D NTOH @CN were superior to that of 3D NTOH irregular spheres and 2D-NTOH nanosheets. Furthermore, the reaction mechanism, morphology change and dynamics research during the sodium insertion/desertion process have been carefully detected via ex-situ measurements of XPS, SEM, TEM and FTIR. 3D NTOH @CN nanospheres assembled from nanosheets provided a new cognition for sodium titanates mateirals.

1

𝑘1 and 𝑘2 at a special potential value were obtained by I(V)/𝑣2 1

as a function of 𝑣2 . The fitting line was plotted in Figure S8c. The different controlled proportion (Figure 5d) at various


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ACS Applied Materials & Interfaces [5]. Fang, Y. J.; Yu, X. Y.; Lou, X. W.; Formation of Hierarchical Cu-Doped CoSe2 Microboxes via Sequential Ion Exchange for High-Performance Sodium-Ion Batteries. Adv. Mater. 2018, 30, 1706668.

ASSOCIATED CONTENT Supporting Information XRD patterns of different hydrothermal reaction time; the result of Raman spectrum and FTIR spectra at the different reaction durations; EDS energy spectrum and TG curves; Na+ diffusion coefficients table; XRD analysis for different calcination temperatures; SEM images; the b value curve by

[6]. Hou, H. S.; Banks, C. E.; Jing, M. J.; Zhang, Y.; Ji, X. B.; Carbon Quantum Dots and Their Derivative 3D Porous Carbon Frameworks for Sodium Ion Batteries with Ultralong Cycle Life. Adv. Mater.2015, 27, 7861-7866.

𝟏

[7]. Zhao, G. G.; Zhang, Y.; Yang, L.; Jiang, Y. L.; Zhang, Y.; Hong, W. W.; Tian, Y.; Zhao, H. B.; Hu, J. G.; Zhou, L.; Hou, H. S.; Ji, X. B.; Mai, L. Q.; Nickel Chelate Derived NiS2 Decorated with Bifunctional Carbon: An Efficient Strategy to Promote Sodium Storage Performance. Adv. Funct. Mater. 2018, 28, 1803690.

log i as a function of log 𝒗 ; the straight line 𝐛𝐲 𝐈(𝐕)/𝒗𝟐 as a 𝟏

function of 𝒗𝟐.

AUTHOR INFORMATION

[8]. Ni, J.; Fu, S.; Wu, C.; Zhao, Y.; Maier, J.; Yu, Y.; Li, L., Superior Sodium Storage in Na2Ti3O7 Nanotube Arrays through Surface Engineering. Adv. Energy Mater. 2016, 6, 1502568.

Corresponding Author *E-mail: [email protected] (Z.-G. Wu) and [email protected] (X.-D. Guo)

[9]. Mei, Y.; Huang, Y.; Hu, X., Nanostructured Ti-based Anode Materials for Na-Ion Batteries. J. Mater. Chem. A 2016, 4, 12001-12013.

Author Contributions Funding Sources

[10]. Cao, K. Z.; Jiao, L. F.; Pang, W. K.; Liu, H. Q.; Zhou, T. F.; Guo, Z. P.; Wang, Y. J.; Yuan, H. T., Na2Ti6O13 Nanorods with Dominant Large Interlayer Spacing Exposed Facet for HighPerformance Na-Ion Batteries. Small. 2016, 12, 2991-2997.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

[11]. Wang, C. S.; Xi, Y.; Wang, M. J.; Zhang, C. S.; Wang, X.; Yang, Q.; Li, W. L.; Hu, C. G.; Zhang, D. Z.; Carbon-Modified Na2Ti3O7•2H2O Nanobelts as Redox Active Materials for HighPerformance Supercapacitor. Nano Energy, 2016, 28, 115123.

This work was supported by National Natural Science Foundation of China (No. 21506133), the Youth Foundation of Sichuan University (No. 2017SCU04A08) and the National Key Research and Development of China (grant No. 2017YFB0307504 and 2016YFD0200404). The research Foundation for the Postdoctoral Program of Sichuan University (No. 2017SCU12018, 2018SCU12045).

[12]. Liao, J. Y.; Manthiram, A.; High-Performance Na2Ti2O5 Nanowires Arrays Coated with VS2 Nanosheets for Sodium-Ion Storage. Nano Energy, 2015, 18, 20-27. [13]. Wang, X. F.; Li, Y. J.; Gao, Y. R.; Wang, Z. X.; Chen, L. Q., Additive-Free Sodium Titanate Nanotube Array as Advanced Electrode for Sodium Ion Batteries. Nano Energy 2015, 13, 687-692.

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