Niobium-Doped Titanosilicate Sitinakite Anode with Low Working

Jan 21, 2019 - Titanosilicate Sitinakite compound with an ideal formula of Na1.68H0.32Ti2O3SiO4·1.76H2O (NTSO) has been employed as a low ...
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Niobium-Doped Titanosilicate Sitinakite Anode with Low Working Potential and High Rate for Sodium-Ion Batteries Yao Liu,# Dong Yang,# Renhe Wang, Jingyuan Liu, Duan Bin, Haifeng Zhu, Kun Liu, Jianhang Huang, Yong-Gang Wang, and Yong-Yao Xia* Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Songhu Road 2205, Shanghai 200438, China

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV DE BARCELONA on 02/03/19. For personal use only.

S Supporting Information *

ABSTRACT: Titanosilicate sitinakite compound with an ideal formula of Na1.68H0.32Ti2O3SiO4·1.76H2O (NTSO) has been employed as a low intercalation potential anode for rechargable sodium ion batteries (SIBs), which exhibit low intercalation potential, low cost, and environmental friendliness. However, the NTSO suffers from shortcomings such as low electronic conductivity, which restricts its electrochemical performances. In this work, cation Nb-doped NTSO compounds have been synthesized and investigated systematically. The powder X-ray diffraction coupling with Rietveld refinement has been used to prove that the Nb has been successfully doped into the crystalline framework of NTSO. The electrochemical performances of Nb-doped NTSO (Na1.68H0.32(Ti1−xNbx)2O3SiO4·1.76H2O, 0 ≤ x ≤ 0.2) has been evaluated in SIBs. Compared to performances of pristine material, those of 10% Nb-doped samples exhibits enhanced electrochemical performances, which shows a stable capcaity of 124 mA h g−1 at a current density of 0.05 A g−1 and 55 mA h g−1 under a current density of 2.0 A g−1. First-principles calculations proved the formation of impurity bands and decrease of conductivity effective mass after Nb doped into the crystalline framework of NTSO with improvement in the electronic conductivity. These findings indicate that the cation doping is an effective way to modify the electrochemical performances of NTSO as an anode in SIBs. KEYWORDS: Titanosilicate sitinakite, Anode, Sodium-ion batteries, Low working potential, Nb doping, First-principles calculations

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INTRODUCTION Sodium-ion batteries (SIBs) are considered as a potential alternative for current lithium rechargeable batteries because of the abundance of sodium elements in the earth’s crust.1,2 To date, many cathode materials have been discovered, such as layered oxides (including O3, P2, O3′, P2′-type),3−5 polyanionic compounds (NaFePO4, Na3V2(PO4)3),6,7 and Prussian blue (such as KMnFe(CN)6).8 But the current anode materials are still limited for it large-scale applications because of the poor performances. Carbon-based anode materials, including hard carbon and soft carbon, have been intensity investigated.9 Soft carbon usually suffers from low volumetric energy density because of the low tap density.10 The insertion/extraction potential of sodium ions for hard carbon is close to zero V vs Na/Na+, resulting in safety issues in practical applications. Alloy-based anode materials usually suffer from large volumetric expansion, resulting in poor cycle performance.11 Recently, Ti-based anode materials have been intensely researched because of the safety working potential of Ti4+/Ti3+ redox during cycles.12−14 In addition, most Ti-based anode materials are low in cost (raw materials), easily prepared, nontoxic, and environmentally friendly. Increasing the electro© XXXX American Chemical Society

chemical potential could solve the safety issue at the expense of reducing the gravimetric energy density. Therefore, it is important to find suitable electrochemical potential Ti-based anodes in SIBs. Very recently, our group has been focusing on sodium or lithium titanosilicate compounds as a new type of anode material for sodium (or lithium)-ion batteries.15−17 The Na1.68H0.32Ti2O3SiO4·1.76H2O (NTSO) with tunnel structure could decrease the migration obstacle for large radii sodium ions during charge/discharge process. This new type of anode material shows the lowest electrochemical potential compared to most other Ti-based materials in SIBs. However, the electrical conductivity of this type of material is usually very low, which restricts its electrochemical performances in SIBs. On the basis of the above discussion, finding an effective way to enhance the intrinsic electrical conductivity of NTSO is necessary and meaningful. Among previous reports, enhancing the intrinsic electrical conductivity of electrode materials usually included carbon coating or cations doping process.18,19 Although the carbon coating is a conventional effective way to Received: December 3, 2018 Revised: January 10, 2019 Published: January 21, 2019 A

DOI: 10.1021/acssuschemeng.8b06326 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

film was 3.0 mg cm−2. The reference and counter electrodes are sodium discs (AR, Sigma-Aldrich Co.) with a size of 12 mm diameter and 1 mm thickness. The electrolyte consists of 1 M sodium perchlorate (NaClO4, AR, Sigma-Aldrich Co.) dissolved in propylene carbonate (PC) and ethylene carbonate (EC) (EC and PC were purchased from Shanghai Xiaoyuan Energy Co. Ltd., China) in a volume ratio of 1:1. The separator is a glass microfiber supplied from Whatman Company with type grade of GF/C (diameter of 16 mm). The electrochemical evaluation used a 2016-type coin cell, which was fabricated in the glovebox (Mikrouna Co., Ltd., China, H2O and O2 < 1 ppm). Alternate current impedance spectroscopy (EIS) tests were employed on the Princeton electrochemical workstation at the frequency from 105 to 10−2 Hz (PARSTAT MC-500, USA) and amplitude of ±5 mV. The cell (three-electrode customized cell) was discharged at 0.5 V and maintained the stabile open potential before EIS measurements. The galvanostatic discharge−charge and EIS tests were carried out at 25 °C. Calculation Methodology. The first-principles calculations in this work were carried out using SIESTA package.22 A gradientcorrected functional known as PBEsol with a rotational-invariant formulation of Hubbard U correction was implemented.23,24 The nuclei and core electrons were described with Troullier-Martins norm-conserving pseudopotentials and the mesh cutoff of 300 Ry was adopted for charge densities and potentials.25 Double-ζ plus polarization basis sets were employed in the expansion of the Kohn−Sham orbitals and the orbital-confining cutoff radii were determined from an energy shift of 0.01 eV and the real space. The Brillouin zone integration was performed on a Γ-centered 3 × 3 × 2 kpoint mesh using Fermi smearing with the electronic temperature of 300 K. The structures were fully relaxed until the maximal force on each atom was less than 0.04 eV·Å−1 and maximum stress was less than 1 GPa. The value of Ueff for Ti 3d orbitals was obtained from a self-consistent approach in the study of NTSO. In the study of 10% Nb-doped NTSO, the value of Ueff for Nb 4d orbitals was obtained from the same approach and the value of Ueff for Ti 3d orbitals was kept fixed.26 The conductivity effective masses were calculated by using finite difference method on three-point stencil.27

improve the electronic conductivity of electrodes, hightemperature treatments is inevitable during the syntheses process. On the basis of our previous research, NTSO would decompose when the sintered temperature is up to 450 °C.15 Therefore, cation doping was employed to modify the performances of NTSO anode. In this work, Nb-doped NTSO samples were synthesized by using the hydrothermal method. The crystal structures have been analyzed through X-ray diffraction (XRD) coupling with Rietveld refinement. The results indicated that the Nb were successfully doped into the crystal lattice of NTSO. The crystal structure was not changed with doping contents controlled at less than 10%. Subsequently, the Nb-doped samples were applied as anode material in SIBs, which shows outstanding rate capability and long cycle life. In addition, the firstprinciples calculations were employed to prove the appearance of impurity bands and the band gap obviously decreased after Nb doping into the crystalline framework, resulting in improvement of the electronic conductivity of samples.

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EXPERIMENTAL SECTION

Material Synthesis. All chemicals are analytical grade and were used without further purification. The samples were prepared by using hydrothermal method. Tetrabutyltitanate was first mixed with tetraethyl orthosilicate (16 mmol) in a 1:1 mole ratio of Ti:Si in a glass beaker (Sigma-Aldrich Co.). The mixture solution was added into 6 mol L−1 NaOH aqueous solution (26 mL) under intense magnetic stirring for 1 h, and then ammonium niobate oxalate hydrate with different contents (5%, 10%, and 20% mole ratio, Sigma-Aldrich Co.) was added. The solution was transferred into a 100 mL Teflonlined pressure vessel. The Teflon bomb was then placed into a stainless-steel vessel, sealed, then placed inside an oven, and heated at 170 °C for 8 days. The powder was collected by filtration, washed using mixed deionized water and ethanol (1:1 in volume) three times, and then dried at 110 °C. For comparison, pure NTSO was synthesized using the same process without adding niobate compound. Materials Characterizations. The crystal structure was characterized using a Bruker D8-Advance (Germany) diffractometer equipped with Cu Kα radiation source (with a constant wavelength of 1.5406 Å), with a step size of 0.02°/step and maintain 1 s for every step. The refinements were carried out using the TOPAS software.20 The instrumental parameters including the zero-shift background are refined preferentially. The crystal parameters, such as unit cell parameters, atomic site occupanices, and so on, are refined subsequently. The original parameters were adopted from a previous report.21 The valence of samples were characterized using X-ray photoelectron spectroscopy (XPS) tests, which were carried out on a PHI-5300C (PerkinElmer, USA) equipped with Mg Kα radiation source (hν = 1253.6 eV). The C 1s (284.6 eV) was applied as calibration of binding energies and XPS data were processed using the XPSPEAK41 program. The morphology of the samples was characterized through field emission-scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan) and high-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F20 S-Twin, UAS). The inductively coupled plasma (ICP) emission spectrometer measurement was employed to determine the mole ratio of Ti:Nb in the doped sample. The samples were dissolved in the hydrochloric acid for ICP measurements. Electrochemical Measurements. The working electrodes were mixed with Nb-doped NTSO, super P, and binder (sodium carboxymethyl cellulose, supplied by Sigma-Aldrich Co.) in the ratio of 8:1:1, using deionized water as solvent. Cu foil was used as current collector. The slurry was cast onto the Cu foil and dried in a vacuum oven for 12 h under 80 °C. The electrode films were punched into discs with diameter of 12 mm after pressing and were transferred to a glovebox immediately. The typical mass loading of the electrode

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RESULTS AND DISCUSSION

Corresponding powder XRD patterns of these samples are shown in Figure 1 (0%, 5%, 10%, and 20% Nb-doped NTSO). All diffraction peaks observed were assigned to expect reflections of the NTSO structure (JCPDS file no. 47-0519). The results indicated that the crystal structure stayed similar to

Figure 1. X-ray diffraction (XRD) patterns of pristine NTSO and 5%, 10%, and 20% Nb-doped compounds; the magnified view of 2θ range from 10.5° to 12.5° (100) peak of NTSO and Nb-doped NTSO, using graphite as the interior label. B

DOI: 10.1021/acssuschemeng.8b06326 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. (a, b) Crystal structure analysis of NTSO and 10% Nb-doped NTSO: Rietveld refinement analysis of powder XRD.

and χ2 demonstrate our refinements are acceptable. The results indicated the crystal structure is tetragonal phase without any impurities. Lattice parameters of a(b) and c are 7.81438 and 11.98212 Å for NTSO and 7.817494 and 11.99068 Å for 10% Nb-doped NTSO. The volume of the unit cell expands from 731.68 to 732.79 Å3, which is ascribed to a larger radius of Nb than of Ti. The results of Rietveld refinement further proved that Nb has been successfully doped into the crystal framework of NTSO. The FE-SEM images for different contents of Nb-doped compounds are shown in Figure 3a−d. The morphology of the samples is similar to that of the Nb contents less than 10%, displaying regular micrometer-size cuboids. When the doping content is near 20%, the morphology of the sample changed obviously, presenting irregular morphology. It might form some other phases with increasing contents of Nb. The results matched well with the XRD analyses aforementioned. We focused on investigating the microstructure of 10% Nb-doped NTSO in the following research. The different resolution of TEM images for 10% Nb-doped NTSO and corresponding selected area electron diffraction pattern (SAED) are shown in Figure 4 (TEM images of pristine NTSO are shown in our previous paper15). Figure 4a−c shows the TEM image of 10% Nb-doped NTSO, which shows the micrometer-sized cuboids with approximately 0.1 × 0.2 × 0.4 μm in dimension. The SADE (Figure 4e) pattern proved that these cuboids are single-crystal in nature, same as pure NTSO reported previously. Figure 4d shows the HRTEM images. The interplanar distances between the neighboring lattice fringes can be measured as 7.7 Å, corresponding to the [001] facet. Figure 4f−k shows the super-EDS elemental mapping. The elements for Na, Ti, Si, O, and Nb distributed homogeneously. Especially, the Nb homogeneous distribution further proved that Nb was doped into the framework structure (Figure 4k). The energy spectrum of each element and percentage of selected atoms (Ti, Nb) are shown in Figure 4n. The mole atom ratio for Ti and Nb is close to 10, indicating the contents of Nb are very close to the nominal composition of 10%. For better understanding, the formula for 10% Nb-doped NTSO was written as Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O. The amount of Nb was further proved using ICP analysis. The result indicates that the Ti:Nb mole ratio is 4.31:0.45, equaling 9.5% of Nb in the doped sample, close to the theoretical value. The electrochemical performances of as-prepared Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O was evaluated in SIBs, which is shown in Figure 5. The initial discharge specific

that of the Nb contents of 5% and 10%. When the doping content increased to 20%, the compound introduced some uncertain phases with the appearance of new diffraction peaks. The magnified 2θ from 10.5° to 12.5° is shown in the inset of Figure 1, which indicated that the diffraction peak of Nbdoped materials shifts to low degrees. According to Shannon’s radii,29 the diameter of Nb5+ cation is approximately 0.62 Å and the diameter of cation Ti4+ is approximately 0.56 Å. Theoretically speaking, it could cause an increase in interplanar spacing and unit cell volume with parts of Ti4+ replaced with Nb5+. Binding states of Nb-doped NTSO (10%) were investigated by XPS (Figures S1−S4, Supporting Information), using the XPSPEAK41 program to process data. The binding energy located at 458.1 and 463.8 eV corresponds to Ti 2p3/2 and 2p1/2, respectively. The spin−orbit splitting was calculated at about 5.7 eV, indicating the Ti4+ oxidation state of NTSO. The characteristic for Ti3+ appeared for the Nb-doped sample. The binding energy of Nb 3d3/2 and 3d5/2 located at 209.9 and 207.2 eV, corresponding to that of Nb5+.30 To further prove that Nb was successfully doped into the crystal framework, we compared the crystal parameters of the pristine sample to those of 10% Nb-doped NTSO based on Rietveld refinement, using TOPAS software.20 Rietveld refinement of powder XRD patterns are shown in Figure 2. The experimental, calculated, and difference curves are labeled with different colors. The crystallographic data for NTSO and 10% Nb-doped NTSO are summarized in Table 1. The reasonably small values of Rwp, Rp, Table 1. Crystallographic Data for NTSO and 10% NbDoped NTSO Obtained from the Rietveld Refinement XRD Patternsa radiation source crystal system space group samples lattice parameters cell volume Rwp Rp χ2

powder X-ray (Cu Kα, λ = 1.5406 Å) tetragonal

powder X-ray (Cu Kα, λ = 1.5406 Å) tetragonal

P42/mcm (no. 132) P42/mcm (no. 132) pristine NTSO 10% Nb-doped NTSO a(b) = 7.81438, c = 11.98212 a(b) = 7.817494, (Å), α(β,γ) = 90° c = 11.99068 (Å), α(β,γ) = 90° 731.68 Å3 732.79 Å3 8.13% 12.26% 5.83% 8.17% 1.39 1.49

Rwp = ∑ω(I0 − IC)2/∑ [ωI02])1/2. Rp = ∑|I0 − IC|/∑Ic. χ2 = ∑ω(I0 − IC)2/∑(Nobs − Nvar.).28 a

C

DOI: 10.1021/acssuschemeng.8b06326 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. (a−d) FE-SEM images of prepared compounds: pristine NTSO and 5%, 10%, and 20% Nb(V) doping.

Figure 4. (a−e) TEM images of 10% Nb-doped NTSO under different resolution with the corresponding HRTEM images and SAED pattern; (f− k) super-EDS elemental mapping; (n) energy spectrum of each element and percentage of selected atoms (Ti, Nb).

capacity was 210 mA h g−1 and the reversible capacity was 121 mA h g−1 under current of 0.05 A g−1 (Figure 5a), corresponding to the 57% of initial Coulombic efficiency. The reversible capacity of 124 mA h g−1 was obtained and kept a cycle stability of 95 mA h g−1 after 200 cycles (Figure S5). The rate performance evaluated under different current density is illustrated in Figure 5b,c. The discharge specific capacities were 124, 100, 91, 85, 81, 78, 73, 70, 63, and 54 mA h g−1 under current of 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 1.0, and 2.0 A g−1, respectively. It can demonstrate that the rate preformance of Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O ob-

viously improved compared to that of pristine NTSO (Figure 5d).15 Figure 5g shows the cycle life at current of 1.0 A g−1. The specific capacity was 63 mA h g−1 and maintained at 58 mA h g−1 after 1100 cycles. The retention of capacity was up to 92% with capacity loss only 0.007% for each cycle. The electrochemical performances of 5% and 20% Nb-doped NTSO are shown in Figures S6−S9, for which the discharge specific capacities were lower than that of the 10% Nb-doped sample. As we can see from Table S1, the Nb-doping titanosilicate sitinakite compound shows the lowest intercalation potential compared to most Ti-based anode materials, but D

DOI: 10.1021/acssuschemeng.8b06326 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 5. Electrochemical performances of Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O: (a) charge/dicharge curves of Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4· 1.76H2O under current density of 0.05 A g−1; (b) charge discharge curves under different current density; (c) cycle performance under different current densities (0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 1.0, and 2.0 A g−1); (d) discharge specific capacity under different current densities, pristine, and Nb-doped materials; (e) EIS for Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O and NTSO; (f) linear fitting of Z′ at high frequency vs ω−1/2; (g) cycle preformance under current density of 1.0 A g−1.

c0 is the concentration of sodium ions of NTSO (obtained according to the Rietveld refinement results), n is the number of transferred electrons per molecule (n = 2), R represents the gas constant, T is 298.15 K, and F represents the Faraday constant. The value of σ is the slope of Z′ versus ω−1/2. ω represents the frequency. The calculated values of DNa+ are 3 × 10−13 and 12 × 10−13 cm2 s−1 for Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O and NTSO, respectively. These results suggest that Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O exhibits the higher values of DNa+ compared to that of pristine NTSO. We further investiagted the electronic conductivity of Nbdoped NTSO through first-principles calculations. According to aforementioned XRD results, Na2Ti2O3SiO4·2H2O and Na2(Ti1.75Nb0.25)O3SiO4·H2O cells were implemented to simulate the NTSO and Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4· 1.76H2O, respectively, for the theoretical investigations. Figure 6a,b display the crystal structure. The values of Hubbard U are given in Figures S10 and S11, using a self-consistent procedure with a linear-response approach. Ueff,scf = 3.4 eV for Ti 3d orbitals and Ueff,scf = 2.8 eV for Nb 4d orbitals were then obtained, respectively. The calculated density of state (DOS) and band structure in the irreducible Brillouin zone are shown

higher than hard carbon, with long cycle life and excellent rate performance. To understand the enhanced rate performance of Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O, EIS testings were performed to compare the values of Rct (charge-transfer resistance) and DNa+ (sodium-ion diffusion coefficient) for NTSO and 10% Nb-doped NTSO. The expermental and simulated curves of Nyquist plots are displayed in Figure 5e. The equivalent circuit during the fitting process was typically similar to that found in previous reports (insert of Figure 5e). Rs is the physical resistance, CPE represents the constant phase angle element, and Zw is the Warburg resistance. The simulated results based on the equivalent circiut shows that the values of Rct are 132 and 334 Ω for Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O and NTSO, respectively. The values of DNa+ are calculated according to the results of Nyquist plots coupled with the following equations [(1) and (2)]:31 Z′ = σω−1/2 D Na+ =

R2T 2 2A n F c0 2σ 2 2 4 4

(1)

(2) E

DOI: 10.1021/acssuschemeng.8b06326 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. (a, b) Structure models used in DFT calculations of NTSO and 10% Nb NTSO; (c, d) density of states (DOS) for NTSO and 10% Nb NTSO; (e, f) calculated band structure in the irreducible Brillouin zone of NTSO and Nb-doped NTSO.

SIBs. In addition, these findings will be of great interest in both academic and industrial communities.

in Figure 6c−f, which illustrated that impurity bands appeared and the bandgap decreased from 2.93 to 1.38 eV after Nb doping. The values of conductivity effective mass of electrons are 4.52 me for NTSO and 3.79 me for Na1.68H0.32(Ti0.9Nb0.1)2O3SiO4·1.76H2O. These results reveal that electrons in the valence band can transfer more easily with introduction of Nb into the framework.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b06326. Electrochemical performance of other anode materials for sodium-ion batteries; XPS of materials, electrochemical test for other contents of Nb-doped NTSO, and linear response Uout calculations (PDF)

CONCLUSIONS

In summary, for the first time, different contents of niobiumdoped titanosilicate sitinakite compounds as a new type of anode material for SIBs were investigated. Introduction of some Nb into the NTSO framework led to a slight lattice expansion, which resulted in a significant increase in the speed of sodium-ion diffusion during intercalation/deintercalation process. In addition, first-principles calculations proved the formation of impurity bands at the Fermi level and narrowing of the band gap for Nb doping into the framework, which resulted in increasing of the electronic conductivity. Consequently, enhanced electrochemical performances in SIBs were achieved compared to that of pristine NTSO. Furthermore, NTSO anode operated at an average working voltage of 0.4 V versus Na+/Na, lower than most of the reported Ti-based anode materials. The present work may offer an effective way to improve the eletrochemical performance of a new type of titanosilicate sitinakite compound as anode in

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: 0086-21-51630318 (Y.Y.X.). ORCID

Yao Liu: 0000-0001-5514-1917 Duan Bin: 0000-0002-4142-9052 Yong-Gang Wang: 0000-0002-2447-4679 Yong-Yao Xia: 0000-0001-6379-9655 Author Contributions #

Yao Liu and Dong Yang contributed equally to this work.

Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acssuschemeng.8b06326 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2016YFB0901500) and the National Natural Science Foundation of China with Grant No. 21875045.

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DOI: 10.1021/acssuschemeng.8b06326 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX