Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 17430−17436
pubs.acs.org/IECR
In Operando Investigation of the Structural Evolution during Calcination and Corresponding Enhanced Performance of ThreeDimensional Na2Ti6O13@C−N Hierarchical Microflowers Chunjin Wu,† Chenguang Shi,‡ Lin Yang,† Zheng Zhang,‡ Yi Tang,§ Zuguang Yang,† Ruikai Yang,† Zhen-guo Wu,*,† Xiaodong Guo,*,† Ling Huang,‡ and Benhe Zhong† †
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
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‡
S Supporting Information *
ABSTRACT: Three-dimensional (3D) Na2Ti6O13 microflowers assembled from directionally arranged and closely interlinked one-dimensional N-doped carbon Na2Ti6O13 nanorods have been successfully prepared by a selfsacrificed template method followed by sintering in argon. In situ hightemperature X-ray powder diffraction (XRD) characterization was conducted to explore the phase transformation and structural evolution. The results indicated that the NaTi3O6(OH)·2H2O precursor was converted into the tunnel Na2Ti6O13 phase and the layered Na2Ti3O7 phase at the different temperatures. At a certain temperature, the tunnel and layered phases have a mutual transformation. The higher temperature was not conductive to the formation of pure phases. The crystal structural evolution has been given based on the synthesis process and in situ high temperature XRD patterns. When served as anode for SIBs, a capacity of ca. 30 mAh g−1 over 6000 continuous cycles was retained at 2000 mA g−1. Moreover, the structural and morphology integrity, reaction mechanism and pseudocapacitance behavior were also investigated.
1. INTRODUCTION In the past few years, room-temperature sodium-ion batteries (SIBs) have attracted much more attention and have been considered as a promising candidate for a large-scale energy storage system because of the low-cost and inexhaustible resources.1−4 The anode electrodes of SIBs have been expansively explored and made great progress. Of all various kinds of anode materials, sodium titanate materials with superior stability and environmental benignity, nontoxicity, and low cost have recently become a research focus.5−7 Because of the low work voltage platform and excellent cycling stability, Na2Ti6O13 (NTO) has been reported as a promising anode material during the current studied systems.8−10 Nevertheless, the low reversible capacity with limited Na-ion uptake and the limited rate capabilities with 2D open framework are unsatisfied, which seriously hinder their practical development.11,12 Continuous efforts have been undertaken to promote Na+ transfer and achieve superior rate capability. Nanoengineering technology as a common modified method could shorten iontransport distance and enhance the reaction dynamics.13−16 Cao et al.11 prepared one-dimensional NTO nanorods (1 D NTO) by annealing the NTO nanowires precursor, which retained 109 mAh g−1 after 2800 cycles even at 1 A g−1. The rational design of unique three-dimensional micro/nanostruc© 2018 American Chemical Society
tures have been considered as another effective method to improve the rate capability.7,17,18 The reasons are explained that 3 D morphology structures provide more active sites, high specific surface area, and short diffusion path, such as nanosheets-assembled 3D microflowers,7 3D spider-web architecture,13 and 3D hollow sphere.17 The pseudocapacitance effect often occurred with the large specific surface area and contributed a part of capacity due to more Na+ ion surface adsorption and exposed abundant channels. Besides nanoscale and structure design, carbon coating has been a commonly used approach to enhance conductivity.19 As compared to pure carbon additive, N-doped carbon might further promote ionic and electronic conductivity.20,21 Qiao et al.17 reported that Na2Ti3O7@N-doped carbon hollow spheres delivered 63 mAh g−1 after 1000 continuous cycles at 50 C. On the basis of our previous study, amino acid could serve as both morphologies controlling agent and carbon/nitrogen sources.21 On the basis of the above consideration, a novel work is proposed about the formation of 3D unique morphology of in situ doped nitrogen and carbon, which is constructed by 1D nanorods. Received: Revised: Accepted: Published: 17430
October 17, 2018 November 18, 2018 November 28, 2018 November 28, 2018 DOI: 10.1021/acs.iecr.8b05151 Ind. Eng. Chem. Res. 2018, 57, 17430−17436
Article
Industrial & Engineering Chemistry Research
cycled electrodes were cleaned using DMC solvent several times. 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 Elec-trochemical impedance spectroscopy (EIS) measurements. All electrochemical measurements were at the room temperature.
Here, 3D Na2Ti6O13 microflowers assembled from 1D Ndoped carbon Na2Ti6O13 nanorods (NTO@CN) have been successfully synthesized by a self-sacrificed template method followed by sintering in Argon. In situ high-temperature XRD result displayed that the NaTi3O6(OH)·2H2O precursor was converted into the tunnel Na2Ti6O13 phase and layered Na2Ti3O7 phase at different temperatures. At a certain temperature, the tunnel and layered phases have a mutual transformation. At 2000 mA g−1, NTO@CN electrode delivered 30 mAh g−1 after about 6000 continuous cycles. Moreover, the structural evolution with Na+ insertion/ desertion in the crystal lattice, morphology change of different cycled samples, reaction mechanism, as well as capacity fluctuation during the charge−discharge process were also investigated.
3. RESULTS AND DISCUSSION The preparation process of NTO@CN is schematically illustrated in Figure 1a. First, TiO2@L-tryptophan spheres
2. EXPERIMENTAL SECTION 2.1. NTOH@CN Preparation. NTO@CN was prepared by a self-sacrificed template method. Sodium hybroxide, tetrabutyl titanate, and L-tryptophan were respectively served as sodium source, titanium source, and carbon/nitrogen additive. Initially, 0.2 g of L-tryptophan was dispersed in the blended solution of 1 mL water and 90 mL ethanol. Then, 1 g of tetrabutyl titanate was explored to be dissolved in 10 mL of ethanol with blending for 10 min and dropped into the above mixed solution. After 5 h, the sample was obtained by centrifuging, washing, and airdrying. The collected yellow powder was dispersed in 100 mL of 2.5 M NaOH solution in a reaction autoclave. The solution mixture was heated at 160 °C for 12 h. When room temperature was reached, the white product was washed to be neutral and the sample was then freeze-dried. The precursor was calcined in a tube furnace at 500 °C for 3 h with a rate of 3 °C per min in argon. 2.2. Characterization and Electrochemical Measurements. The phase structure and morphology of the asprepared and cycled NTO@CN samples were characterized by XRD analysis, (analytical EMPYREAN), emission scanning electron microscopy (SEM, HITACHI S-4800), and transmission electron microscopy (TEM, JEOL 2100F). The XRD data were obtained using Cu Kα radiation in the 2θ range of 10−80°. X-ray photoelectron spectroscopy (XPS) analysis were operated by a Thermo ESCALAB 250XI instrument. Raman measurements (Thermo DXRxi) and FTIR instrument (Bruker R200-L spectrophotometer) detected the bending and stretching vibrations of band. High-angle annular dark field (HAADF) was performed by Talos f200s. In situ hightemperature XRD (Bruker D8 discover) was performed to explore the structural evolution during the calcination process. The electrochemical performances of NTO@CN electrode were tested by CR2025 half-cells, which were finished in a glovebox (O2 and H2O levels
