Effect of Sodium-Site Doping on Enhancing the Lithium Storage

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Effect of Sodium-Site Doping on Enhancing the Lithium Storage Performance of Sodium Lithium Titanate Pengfei Wang,†,‡ Shangshu Qian,† Ting-Feng Yi,*,‡ Haoxiang Yu,† Lei Yan,† Peng Li,† Xiaoting Lin,† Miao Shui,† and Jie Shu*,† †

Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang Province People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan 243002, Anhui Province People’s Republic of China S Supporting Information *

ABSTRACT: Via Li+, Cu2+, Y3+, Ce4+, and Nb5+ dopings, a series of Na-site-substituted Na1.9M0.1Li2Ti6O14 are prepared and evaluated as lithium storage host materials. Structural and electrochemical analyses suggest that Na-site substitution by high-valent metal ions can effectively enhance the ionic and electronic conductivities of Na2Li2Ti6O14. As a result, Cu2+-, Y3+-, Ce4+-, and Nb5+-doped samples reveal better electrochemical performance than bare Na2Li2Ti6O14, especially for Na1.9Nb0.1Li2Ti6O14, which can deliver the highest reversible charge capacity of 259.4 mAh g−1 at 100 mA g−1 among all samples. Even when cycled at higher rates, Na1.9Nb0.1Li2Ti6O14 still can maintain excellent lithium storage capability with the reversible charge capacities of 242.9 mAh g−1 at 700 mA g−1, 216.4 mAh g−1 at 900 mA g−1, and 190.5 mAh g−1 at 1100 mA g−1. In addition, ex situ and in situ observations demonstrate that the zero-strain characteristic should also be responsible for the outstanding lithium storage capability of Na1.9Nb0.1Li2Ti6O14. All of this evidence indicates that Na1.9Nb0.1Li2Ti6O14 is a high-performance anode material for rechargeable lithium ion batteries. KEYWORDS: sodium lithium titanate, sodium-site doping, lithium storage capability, electrochemical property, structural evolution

1. INTRODUCTION As the global energy crisis and environmental problems unfold, green and sustainable energy sources have become increasingly important.1 Among the currently available energy storage systems, lithium ion batteries have received intense attention from both the academic and industrial communities as the main power sources in hybrid electric vehicles, plug-in hybrid electric vehicles, and full electric vehicles due to their high energy density, high working potential, and long cycle life.2−6 As a key component in lithium ion batteries, anode material has become a research hotspot in recent years. Today graphite is being widely used in commercial lithium ion batteries as anode material.7,8 With increasing high-power requirements, the low safety of graphitic carbon material induced by the possible formation of lithium dendrite at low potential restricts the further development of these batteries. Therefore, enormous attention has been focused on finding safe alternatives to replace the conventional graphite anode. Among the proposed anode materials, titanates with high structural and thermal stabilities have been demonstrated as promising anode candidates in high-power lithium ion batteries, such as LiMTiO4 (M = V, Cr),9,10 Li4Ti5O12,11−19 and Li2MTi3O8 (M = Zn, Co, Mg, Mn).20−26 Recently, a new kind of ternary titanium complex oxide with the composition of © 2016 American Chemical Society

MLi2Ti6O14 (M = Ba, Sr, 2Na) was successfully synthesized and used as anode material for lithium ion batteries.27−39 The theoretical capacity of MLi2Ti6O14 is about 260−280 mAh g−1 if all six Ti4+ ions in the structure are totally reduced to Ti3+. Additionally, MLi2Ti6O14 has a higher tap density than Li4Ti5O12.31 As a result, lithium ion batteries built with MLi2Ti6O14 anode can achieve a higher volume energy density. Compared with SrLi2Ti6O14 (1.40 V) and BaLi2Ti6O14 (1.41 V), Na2Li2Ti6O14 exhibits a lower operating potential of about 1.25 V, which allows for the possibility of fabricating highperformance lithium ion batteries with Na2Li2Ti6O14. Unfortunately, Na2Li2Ti6O14 also reveals the disadvantages of poor electronic conductivity and low lithium ion diffusion coefficient, which always result in poor electrochemical performance at high charge−discharge rates. To solve the problems referred to above, some effective methods are proposed to improve the lithium storage performance of Na2Li2Ti6O14, including coating32,40 and doping.28,41,42 Doping has been especially widely demonstrated as an effective way to improve the intrinsic electronic/ionic conductivities of various titanates for lithium Received: January 31, 2016 Accepted: April 7, 2016 Published: April 7, 2016 10302

DOI: 10.1021/acsami.6b01293 ACS Appl. Mater. Interfaces 2016, 8, 10302−10314

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration for the synthesis process, (b) XRD patterns of Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+), (c) Rietveld refinement profiles of Na2Li2Ti6O14, and (d) Rietveld refinement profile of Nb-doped Na2Li2Ti6O14.

2. EXPERIMENTAL SECTION

ion batteries. In the previous work, the doped ions, including W6+,43 V5+,44 Ti4+,42 La3+,45 Cr3+,46 Ni2+,47 Ag+,48 Ta5+,49 Nb5+,50 F−,51 Br−,43 and N3−,52,53 have been widely used to improve the lithium storage capability of titanates. However, few reports undertake a comparative study on the effect that doping with metal ions of different valences has on enhancing the electrochemical properties of Na2Li2Ti6O14. In this paper, Na2Li2Ti6O14 and its Na-site-substituted products are synthesized by a simple solid-state method and used as anode materials in lithium ion batteries. The microstructure, morphology, conductivity, and electrochemical characteristics of as-prepared Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) are thoroughly investigated and compared with each other. The results show that Cu2+-, Y3+-, Ce4+-, and Nb5+-doped Na2Li2Ti6O14 can deliver better electronic/ionic conductivities and electrochemical properties over bare Na2Li2Ti6O14 and Li+-doped product. Nb5+-doped Na2Li2Ti6O14 shows the best cycle and rate performances among all the as-prepared samples. Moreover, in situ and ex situ techniques are also proposed to study the lithiation− delithiation mechanism in Na1.9Nb0.1Li2Ti6O14 during electrochemical cycles.

2.1. Synthesis of Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14. Here, Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) were synthesized by a traditional solid-state method and all chemicals used in the experiments were analytical reagent. For bare Na2Li2Ti6O14 preparation, CH3COOLi·2H2O (99.5%, Aladdin), CH3COONa·3H2O (99.5%, Aladdin), and TiO2 (99.5%, Aladdin) were mixed by planetary ball milling in ethanol for 12 h to form the precursor. The molar ratio of Li:Na:Ti was 1:1:3 in the precursor. The obtained precursor slurry was dried in an oven at 80 °C for 12 h and then ground into fine powders. After that, the as-formed powders were placed in an alumina boat and calcined at 800 °C for 10 h under an air atmosphere to obtain the bare Na2Li2Ti6O14. In the preparation of precursors for Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+), a 5.0% molar ratio of CH3COONa·3H2O was replaced by CH3COOLi·2H2O (99.5%, Aladdin), (CH 3 COO) 2 Cu·H 2 O (99.5%, Aladdin), (CH3COO)3Y·4H2O (99.5%, Aladdin), (NH4)2Ce(NO3)6 (99.0%, Aladdin), or Nb(HC2O4)5·6H2O (99.5%, Aladdin) before planetary ball milling, respectively. The calcination process of Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) is the same as for the pure sample. The detailed preparation process of Na 2 Li 2 Ti 6 O 1 4 and Na1.9M0.1Li2Ti6O14 is shown in Figure 1a. 2.2. Materials Characterization. The phase identification and crystallinity analysis of pure Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 were characterized by Bruker D8 Focus X-ray diffraction (XRD, X-ray 10303


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ACS Applied Materials & Interfaces diffractometer equipped with Cu Kα radiation, λ = 1.5406 Å) with the scattering angle range of 10°−80°. In situ XRD observation was conducted on the same instrument based on the homemade in situ cell and technique. The surface morphology and particle size of six samples were observed by scanning electron microscopy (SEM) (Hitachi S4800). The fine crystal structure of Na1.9Nb0.1Li2Ti6O14 was analyzed by high-resolution transmission electron microscopy (HRTEM) (JEOL JEM-2010) and corresponding selected area electron diffraction (SAED). X-ray photoelectron (XPS) spectra were recorded on a Shimadzu Axis Ultra spectrometer with a Mg Kα excitation source. 2.3. Electrochemical Measurements. The electrochemical performances of the as-prepared products were evaluated by CR2032 coin-type cells. The working electrodes were prepared by blending the as-prepared active material, carbon black, and polyvinylidene fluoride at a weight ratio of 8:1:1 in 1-methyl-2pyrrolidinone. The above slurry was then coated onto a Cu foil and dried at 120 °C in a vacuum oven for 12 h. Each working electrode was cut into a disck with a diameter of 15 mm before use. Galvanostatic charge−discharge measurements were measured by using CR2032 coin-type cells on a multichannel Land CT2001A battery test system. Cell assembly was carried out in an Ar-filled glovebox by using lithium foil as the counter electrode and Celgard2400 polypropylene as the separator. In a typical half-cell, the electrolyte was a 1 mol L−1 solution of LiPF6 dissolved in ethylene carbonatedimethyl carbonate (1:1 in volume). Cyclic voltammetry was performed at a scan rate of 0.1 mV s−1 from 0.0 to 3.0 V on a computer-controlled CHI 1000B electrochemical workstation at room temperature. Electrochemical impedance spectroscopy (EIS) patterns were recorded on a CHI 660D electrochemical workstation with a frequency range of 10−2−105 Hz by using a three-electrode system. In the above electrochemical measurements, all the working electrodes were selected with the same mass of 2.0 mg for active material.

Table 1. Lattice Parameters of the As-Obtained Samples sample

a (Å)

b (Å)

c (Å)

V (Å3)

Na2Li2Ti6O14 Na1.9Li2.1Ti6O14 Na1.9Cu0.1Li2Ti6O14 Na1.9Y0.1Li2Ti6O14 Na1.9Ce0.1Li2Ti6O14 Na1.9Nb0.1Li2Ti6O14

16.478 16.469 16.472 16.473 16.476 16.465

5.735 5.721 5.728 5.733 5.726 5.717

11.221 11.202 11.201 11.218 11.236 11.205

1060.399 1055.443 1056.832 1059.425 1060.022 1054.731

Nb 5+ dopings induce the slight lattice shrinkage of Na2Li2Ti6O14. In addition, the Rietveld refinement results also confirm that Na2Li2Ti6O14 can offer vacant sites like 8e, 4a, and 4b sites in the orthorhombic framework for reversible Li storage. The SEM, elemental mapping, and EDS images of Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) are presented in Figure 2. As seen from SEM images, it is clear that the solid-state-prepared Na2Li2Ti6O14 powders show homogeneous particle distribution from 200 to 400 nm without serious agglomeration. After Na-site substitution by Li+, Cu2+, and Y3+, the Na1.9M0.1Li2Ti6O14 products display similar irregular particle shape and size compared with those of undoped Na2Li2Ti6O14. However, Ce4+ doping induces the particle aggregation of as-obtained product, which exhibits an average particle size of 200−800 nm. In contrast, Na1.9Nb0.1Li2Ti6O14 displays the smallest particle size (100−250 nm) among all the as-prepared samples. The reduced particle size may be beneficial for rapid lithium ion transportation in the sample. In addition, the elemental mapping and EDS images presented in Figures 2 and S2 (Supporting Information) also indicate that Cu2+, Y3+, Ce4+, and Nb5+ ions are successfully introduced into the structure of Na2Li2Ti6O14 after Na-site substitution, which is consistent with the identification of high-purity phases for Na1.9M0.1Li2Ti6O14 as described in Figure 1. Figure 3a shows the typical cyclic voltammograms (CVs) of Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) recorded at a scan rate of 0.1 mV s−1. For a better comparison, all the working electrodes used the same mass of active materials (2.0 mg). It is obvious that one main pair of characteristic redox peaks can be observed at about 1.18 V (cathodic peak) and 1.37 V (anodic peak) for all the six samples. During the cathodic scan, the peak current of the reduction peak at about 1.18 V has a different value for Cu2+-, Y 3+ -, Ce 4+ -, or Nb 5+ -doped samples (0.41 mA for Na1.9Cu0.1Li2Ti6O14, 0.43 mA for Na1.9Y0.1Li2Ti6O14, 0.33 mA for Na1.9Ce0.1Li2Ti6O14, and 0.46 mA for Na1.9Nb0.1Li2Ti6O14), which is much higher than that of bare Na2Li2Ti6O14 (0.25 mA) and Na1.9Li2.1Ti6O14 (0.28 mA). In the reverse anodic process, Na1.9Nb0.1Li2Ti6O14 also shows the highest peak current of 0.44 mA at about 1.37 V, followed by that of Na1.9Y0.1Li2Ti6O14 (0.39 mA), Na1.9Cu0.1Li2Ti6O14 (0.38 mA), Na1.9Ce0.1Li2Ti6O14 (0.36 mA), Na1.9Li2.1Ti6O14 (0.31 mA), and bare Na2Li2Ti6O14 (0.28 mA). Furthermore, the smaller potential difference between the anodic and cathodic peaks of Na1.9Y0.1Li2Ti6O14 (0.18 V) and Na1.9Nb0.1Li2Ti6O14 (0.18 V) shows their lower electrochemical polarization than that of Na1.9Cu0.1Li2Ti6O14 (0.19 V), Na1.9Ce0.1Li2Ti6O14 (0.20 V), Na2Li2Ti6O14 (0.20 V), and Na1.9Li2.1Ti6O14 (0.20 V). All of this evidence suggests that Na1.9Nb0.1Li2Ti6O14 may have better kinetic features over other samples.

3. RESULTS AND DISCUSSION The XRD patterns of Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) calcined at 800 °C are shown in Figure 1b. All the diffraction peaks observed for the prepared sample can be assigned to the expected reflections of an orthorhombic structure with space group Fmmm (69) (JCPDS card No. 52-0690), which is in good accordance with the XRD results of former reports about Na2Li2Ti6O14.27−32,42 No obvious impurity phases can be found in the Li+-, Cu2+-, Y3+-, Ce4+-, and Nb5+-doped products, suggesting that Li+, Cu2+, Y3+, Ce4+, and Nb5+ ions can totally enter the lattices of Na2Li2Ti6O14 without destruction of the basic crystal structure. The successful substitution by Li+, Cu2+, Y3+, Ce4+, Nb5+ at Na sites is contributed to the smaller radii of Li+ (0.68 Å), Cu2+ (0.72 Å), Y3+ (0.89 Å), Ce4+ (0.92 Å), and Nb5+ (0.69 Å) than Na+ (0.97 Å). The high-purity characteristics of as-prepared samples can also be confirmed by the XRD Rietveld refinement results of Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) as displayed in Figures 1c,d and S1 (Supporting Information). It is clearly seen that all the observed peaks are well-consistent with the standard Bragg positions in the XRD Rietveld refinement graphs of Na1.9M0.1Li2Ti6O14. Moreover, the refined lattice parameters for Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 are presented in Table 1. It can be found that the lattice parameters a, b, c, and V of bare Na2Li2Ti6O14 are 16.478 Å, 5.735 Å, 11.221 Å, and 1060.399 Å3, which are in agreement with the standard values of Na2Li2Ti6O14 (JCPDS card No. 52-0690). After doping, Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) products provide the lattice volumes of 1055.443, 1056.832, 1059.425, 1060.022, and 1054.731 Å3, respectively. Due to the smaller radius compared to Na+, it is obvious that Li+, Cu2+, Y3+, Ce4+, 10304


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Figure 2. SEM, elemental mapping, and EDS images of (a, b, c) Na2Li2Ti6O14, (d, e, f) Na1.9Li2.1Ti6O14, (g, h, i) Na1.9Cu0.1Li2Ti6O14, (j, k, l) Na1.9Y0.1Li2Ti6O14, (m, n, o) Na1.9Ce0.1Li2Ti6O14, (p, q, r) Na1.9Nb0.1Li2Ti6O14, repectively.

for Na1.9Cu0.1Li2Ti6O14, 97.6 mAh g−1 for Na1.9Y0.1Li2Ti6O14, and 82.2 mAh g−1 for Na1.9Ce0.1Li2Ti6O14). Therefore, Nb5+ doping may be more suitable for Na2Li2Ti6O14 to improve its lithium storage properties. The cycling performance of bare Na 2 Li 2Ti6 O14 and Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) samples between 0.0 and 3.0 V is shown in Figure 3c. It compares the lithium storage capabilities of bare Na2Li2Ti6O14 and the other five doped products at a current density of 100 mA g−1. It is clear that Na1.9Nb0.1Li2Ti6O14 shows higher specific capacity and better reversibility than the other samples. As we can seen, Na1.9Nb0.1Li2Ti6O14 exhibits the highest initial charge capacity of 259.4 mAh g−1. In contrast, the bare and Li+-, Cu2+-, Y3+-, and Ce4+-doped Na2Li2Ti6O14 show initial charge capacities of 216.2, 213.7, 233.2, 248.6, and 223.2 mAh g−1, respectively.

Figure 3b presents the galvanostatic charge−discharge curves of Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) tested in a potential range from 0.0 to 3.0 V at a current density of 100 mA g−1. From the galvanostatic charge− discharge curves, it can be clearly observed that long discharge and charge plateaus are located at about 1.18 and 1.37 V for all the six samples, respectively, which are in agreement with the oxidation and reduction peaks in CVs and dQ/dV curves as presented in Figures 3a and S3 (Supporting Information), respectively. In addition, long slopes can also be observed below 1.0 V for all samples, suggesting further lithiation in Na1.9M0.1Li2Ti6O14. It can be found that Na1.9Nb0.1Li2Ti6O14 shows the highest charge-plateau specific capacity, as high as 105.3 mAh g−1, among all six samples (77.1 mAh g−1 for Na2Li2Ti6O14, 80.0 mAh g−1 for Na1.9Li2.1Ti6O14, 86.5 mAh g−1 10305


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Figure 3. (a) The cyclic voltammograms, (b) charge−discharge curves, (c) cycling properties, (d) EIS patterns, (e) graph of Zre plotted against ω−1/2 at the low-frequency region, and (f) lithium ion chemical diffusion coefficient of Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14. (g) High rate charge− discharge curves, (h) cycling properties, and (i) Coulombic efficiency of Na1.9Nb0.1Li2Ti6O14.

line in the low-frequency region. By using an equivalent circuit, all the EIS spectra are simulated and the calculated data are listed in Table 2. In the equivalent circuit, Rs is the solution/ contact resistance, Rct denotes the charge transfer resistance, CPE is the constant phase element, and Wo is the Warburg diffusion impedance. The solution/contact resistances (Rs), as listed in Table 2, are calculated as 13.54 Ω for Na2Li2Ti6O14, 9.21 Ω for Na1.9Li2.1Ti6O14, 6.98 Ω for Na1.9Cu0.1Li2Ti6O14, 3.57 Ω for Na1.9Y0.1Li2Ti6O14, 8.43 Ω for Na1.9Ce0.1Li2Ti6O14,

After 50 cycles, Na1.9Nb0.1Li2Ti6O14 can still keep a reversible charge capacity of 245.7 mAh g−1, with a capacity retention of 94.7%. For comparison, the reversible charge capacities at the 50th cycle are 189.2 mAh g−1 for Na2Li2Ti6O14, 198.4 mAh g−1 for Na1.9Li2.1Ti6O14, 217.7 mAh g−1 for Na1.9Cu0.1Li2Ti6O14, 233.8 mAh g−1 for Na1.9Y0.1Li2Ti6O14, and 206.6 mAh g−1 for Na1.9Ce0.1Li2Ti6O14 with the corresponding capacity retentions of 87.5%, 92.8%, 93.3%, 94.0%, and 92.6%, respectively. The above results show that Na-site substitution by Li+, Cu2+, Y3+, Ce4+, and Nb5+ can improve the lithium storage capability of Na2Li2Ti6O14. Nb5+ doping, especially, may be the most effective way to enhance the electrochemical property of Na2Li2Ti6O14 among all the five dopants. In order to understand the improved electrochemical performance through Na-site doping, EIS measurements before cycles are investigated on bare Na 2 Li 2 Ti 6 O 14 and Na1.9M0.1Li2Ti6O14 (M = Li+, Cu2+, Y3+, Ce4+, Nb5+) samples in a frequency range of 10−2−105 Hz. As revealed in Figure 3d, the EIS patterns show that each curve is composed of a depressed semicircle in the high-frequency region and a straight

Table 2. Calculated Electrochemical Parameters from EIS Patterns

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sample

Rs (Ω)

Rct (Ω)

DLi (cm2 s‑1)

Na2Li2Ti6O14 Na1.9Li2.1Ti6O14 Na1.9Cu0.1Li2Ti6O14 Na1.9Y0.1Li2Ti6O14 Na1.9Ce0.1Li2Ti6O14 Na1.9Nb0.1Li2Ti6O14

13.54 9.21 6.98 3.57 8.43 1.25

328.93 259.42 175.59 79.92 234.65 27.96

1.89 2.96 7.98 1.37 6.76 2.44

× × × × × ×

10−15 10−15 10−15 10−14 10−15 10−14


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Figure 4. TEM, SAED, and HRTEM images for (a, b, c) Na1.9Nb0.1Li2Ti6O14 before discharge, (d, e, f) discharged to 0.0 V, and (g, h, i) charged to 3.0 V, respectively.

where R is the gas constant (8.314 J mol−1 K−1), T is the absolute temperature (298 K), n is the number of electrons transferred in the half-reaction for the redox couple, F is the Faraday constant (96485 C mol−1), A is the geometrical electrode area (1.77 cm2), CLi is the concentration of lithium ion in the solid, and σ is the Warburg factor, which can be obtained from the slope of the Zre−ω−1/2 line in Figure 3e. As previously reported,15,16 the relationship between the real part of impedance (Zre) and ω is governed by eq 2:

and 1.25 Ω for Na1.9Nb0.1Li2Ti6O14. In addition, it can also be clearly observed that Na1.9Nb0.1Li2Ti6O14 shows the lowest charge transfer resistance of 27.96 Ω among all the six samples (328.93 Ω for Na2Li2Ti6O14, 259.42 Ω for Na1.9Li2.1Ti6O14, 175.59 Ω for Na 1 . 9 Cu 0 . 1 Li 2 Ti 6 O 1 4 , 79.92 Ω for Na1.9Y0.1Li2Ti6O14, and 234.65 Ω for Na1.9Ce0.1Li2Ti6O14). The above calculated electrochemical data reveal that both Y3+ and Nb5+ doping can more effectively improve the electronic conductivity of Na2Li2Ti6O14 compared to Li+, Cu2+, and Ce4+ doping. This indicates that the partial replacement of monovalent Na+ by high-valent Mz+ (Y3+, Ce4+, Nb5+) over low-valent Mz+ (Li+, Cu2+) in the structure leads to a reduction of many more Ti cations from Ti 4 + to Ti 3 + in Na 1.9 M0.1Li2Ti 6O14. As a result, Na 1.9Y0.1Li 2Ti6O 14 and Na1.9Nb0.1Li2Ti6O14 exhibit higher electronic conductivity over bare Na2Li2Ti6O14 and other doped samples. Although Ce4+ is a tetravalent ion, the large size of Na1.9Ce0.1Li2Ti6O14 leads to poor electronic contact between particles and low specific surface area for charge transfer. Thus, both Rs and Rct of Na1.9Ce0.1Li2Ti6O14 are lower than those of Na1.9Cu0.1Li2Ti6O14 and Na1.9Nb0.1Li2Ti6O14. As is well-known, the lithium ion diffusion behavior in a solid can be evaluated by the straight line of the EIS pattern in the low-frequency region. Here, the lithium ion diffusion coefficient (DLi) for Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 can be calculated according to eq 1 DLi +

⎞2 1⎛ RT ⎟ = ⎜ 2 2 2 ⎝ n F AC Liσ ⎠

Zre = R ct + R s + σω−1/2

(2)

On the basis of the eqs 1 and 2, the final calculated values of DLi are given in Figure 3f and Table 2. It can be found that Na1.9Nb0.1Li2Ti6O14 exhibits the largest lithium diffusion coefficient (2.44 × 10−14 cm2 s−1) among all samples, indicating that Nb5+ doping is the most effective method to improve the ionic and electronic conductivities of Na2Li2Ti6O14. To provide more information about the electrochemical performances of Na2Li2Ti6O14 after Nb5+ doping, the rapid charge−discharge behaviors are recorded at higher rates (>500 mA g −1 ). The high rate charge−discharge curves of Na1.9Nb0.1Li2Ti6O14 cycled at the current densities of 700, 900, and 1100 mA g−1 are shown in Figure 3g, which reveal similar charge−discharge behaviors as the curves recorded at low rate (100 mA g−1). Figure 3i exhibits the cyclic performances of Na1.9Nb0.1Li2Ti6O14 at high rates. It can be found that Na1.9Nb0.1Li2Ti6O14 can deliver the initial charge capacities of 242.9 mAh g−1 at 700 mA g−1, 216.4 mAh g−1 at 900 mA g−1, and 190.5 mAh g−1 at 1100 mA g−1. After 200 cycles, it still can maintain the reversible capacities of 206.3

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Figure 5. XPS spectra of Ti and Nb elements for (a, b) Na1.9Nb0.1Li2Ti6O14 before discharge, (c, d) discharged to 0.0 V, and (e, f) charged to 3.0 V, respectively.

mAh g−1 at 700 mA g−1, 186.6 mAh g−1 at 900 mA g−1, and 157.1 mAh g−1 at 1100 mA g−1, with the capacity retentions of 84.9%, 86.2%, and 82.5%, respectively. These results indicate that Na1.9Nb0.1Li2Ti6O14 has good cycling stability upon longterm high rate charge−discharge cycles. Furthermore, the cycling Coulombic efficiency can be calculated as the average values of 99.2% at 700 mA g−1, 99.0% at 900 mA g−1, and 98.3% at 1100 mA g−1, as shown in Figure 3h. This further demonstrates the excellent electrochemical performance of Na1.9Nb0.1Li2Ti6O14 as an anode material for lithium ion batteries. In order to study the lithium storage process of Na1.9Nb0.1Li2Ti6O14 during charge−discharge process, ex situ TEM, ex situ XPS, and in situ XRD are performed between 0.0 and 3.0 V, as displayed in Figures 4−6. As shown in Figure 4a,d,g, the particle size of Na1.9Nb0.1Li2Ti6O14 is about 100− 300 nm, which is consistent with the result of SEM images in Figure 2. It is obvious that the interplanar distances of 1.427,

2.001, and 3.075 Å in the SAED image (Figure 4b) and 4.8846 Å in HRTEM image (Figure 4c) can be indexed to the (040), (024), (113), and (111) planes of Na1.9Nb0.1Li2Ti6O14, respectively. After a full lithiation process to 0.0 V, the particles of lithiated Na1.9Nb0.1Li2Ti6O14 remain stable without any breakdown, as displayed in Figure 4d. With a careful observation, it can be found that the detected d-spacings of the specific planes for lithiated sample are 1.474, 2.046, 3.108, and 4.9706 Å in SAED and HRTEM images, as presented inFigure 4e,f. This means that the insertion of Li+ ions leads to a slight expansion of the lattice volume, which is similar to the structural evolution of lithiated Li4Ti5O12.54 After a recharge process to 3.0 V, the structure of the delithiated product can be stably maintained, as observed in Figure 4g−i. The fringe spacings are 1.433, 2.06, 3.078, and 4.9215 Å in SAED and HRTEM images, which are close to the d-values of pristine Na1.9Nb0.1Li2Ti6O14. This indicates that a reversible shrinkage of lattices for Na1.9Nb0.1Li2Ti6O14 takes place during the 10308


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Figure 6. In situ XRD patterns and selected in situ XRD patterns of Na1.9Nb0.1Li2Ti6O14 between 0.0 and 3.0 V.

lithium storage capacity in Na1.9Nb0.1Li2Ti6O14 totally contributed to the redox of the Ti3+/Ti4+ couple. Moreover, the presence of trace Ti4+ in Figure 5c is also in accordance with the reversible lithium storage value of 5.59 Li per formula in the structure of Na1.9Nb0.1Li2Ti6O14, which is lower than the theoretical maximum lithium storage capacity of 6.0 Li per formula. Upon a recharge process to 3.0 V, the oxidation from the Ti3+ state to the Ti4+ state can be observed, as shown in Figure 5e. At the same time, no valence variation of Nb5+ during the delithiation process is also found in Figure 5f. This suggests that the electrochemical reaction for Na1.9Nb0.1Li2Ti6O14 is a reversible evolution, which is in agreement with the results of electrochemical measurement and in situ TEM. To present a detailed lithium storage mechanism in Na1.9Nb0.1Li2Ti6O14 during the charge−discharge process, in situ XRD observation is conducted between 0.0 and 3.0 V, as displayed in Figure 6. Compared with the powder XRD pattern in Figure 1, it is obvious that the relative intensity of featured diffraction peaks for Na1.9Nb0.1Li2Ti6O14 is greatly reduced by the metallic Be disk used as the X-ray transmission window in the homemade in situ XRD cell. A seen from the in situ XRD patterns in Figure 6, the first black bold curve is recorded before the charge−discharge cycle. All the Bragg positions in the initial XRD pattern show the featured reflections of the orthorhombic phase and are indexed to the Na1.9Nb0.1Li2Ti6O14 sample (JCPDS card No. 52-0690), which is in agreement with the Rietveld refinement result as presented in Figure 1d. In addition, the thin black, red, and blue curves in Figure 6 refer to the discharge, charge, and potentiostatic charge processes, respectively. It is obvious that the evolutions of (002), (111),

delithiation process. The results of ex situ TEM analysis of the structural transformation prove the high structural stability and reversibility of Na1.9Nb0.1Li2Ti6O14 as a lithium storage anode material. To investigate the redox changes during cycles, an ex situ XPS technique is used to detect the probable valence evolutions in Na1.9Nb0.1Li2Ti6O14. Here, the XPS spectra of the pristine Na1.9Nb0.1Li2Ti6O14 and its lithiated/delithiated samples are displayed in Figure 5. For comparison, the XPS spectra of bare Na2Li2Ti6O14 are also presented in Figure S4 (Supporting Information). As shown in Figure 5a, the binding energy of Ti 2p3/2 and Ti 2p1/2 can be clearly observed at 458.6 and 464.3 eV, respectively, indicating that the Ti element mainly exists as the chemical state of Ti4+ in the Na1.9Nb0.1Li2Ti6O14.55 In addition, two main XPS peaks centered at 207.0 and 209.8 eV in Figure 5b are attributed to the Nb 3d5/2 and Nb 3d3/2 spin− orbit doublet for Nb5+, respectively. Besides, a weak XPS peak is also detected at 459.7 eV, which is assigned to the appearance of trace Ti3+ in the as-synthesized sample. This indicates that the successful substitution of Na+ by Nb5+ in the structure induces a reduction of some Ti cations from Ti4+ to Ti3+ in Na1.9Nb0.1Li2Ti6O14. Thus, Na1.9M0.1Li2Ti6O14 (M = Cu2+, Y3+, Ce4+, Nb5+) samples show higher conductivity than the bare Na2Li2Ti6O14, which further proves the enhanced electrochemical properties as presented in Figure 3. When the Na1.9Nb0.1Li2Ti6O14 is discharged to 0.0 V, it is obvious that the XPS peak at 459.7 eV becomes the main signal and the signal of Ti4+ becomes weak (Figure 5c), which is assigned to the reduction of Ti4+ to Ti3+ during lithiation process. Meanwhile, the two featured peaks of Nb5+ show no changes after full lithiation, as presented in Figure 5d. This indicates that the 10309


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Figure 7. Change of diffraction peak intensity upon lithiation/delithiation process for Na1.9Nb0.1Li2Ti6O14 in different 2θ ranges (a, e) 17°−19.5°, (b, f) 25.5°−29°, (c, g) 31.5°−33.8°, and (d, h) 42.5°−45.5°. Rietveld refinement patterns for (i) Na1.9Nb0.1Li2Ti6O14 before discharge, (j) with the 8e position fully occupied, (k) with the 8e and 4b positions fully occupied, and (l) with the 8e, 4b, and 4a positions fully occupied by lithium ions during the discharge process.

return along the same deviation path of the discharge process. This confirms that the structural transformation of Na1.9Nb0.1Li2Ti6O14 is a reversible process during lithiation and delithiation at 0.0−3.0 V. Figure 7 presents a detailed illustration for in situ XRD observation. As revealed in Figure 7a−h, the in situ evolution of relative intensity versus Bragg position also reveals that the diffraction peaks gradually shift to low angles upon lithiation and return along the same deviation path of discharge process during the reverse delithiation process. These results further indicate that the as-obtained

(202), (400), (311), (402), (113), (020), (511), (204), (022), (222), (422), (800), and (024) diffraction peaks can be observed in Figure 6 during the initial charge−discharge cycle. By tracking the changes of the (111), (202), (402), (113), (511), (204), (115), and (024) peaks, it can be found that the Bragg positions at 18.72°, 26.52°, 28.71°, 32.09°, 33.47°, 43.76°, and 44.99° migrate to the positions of 18.43°, 26.05°, 28.40°, 31.74°, 32.88°, 42.98°, and 44.71° after a full lithiation process, as the selected in situ XRD patterns show in Figure 6. During the reverse delithiation process, all the diffraction peaks 10310


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Figure 8. Lithiation models for Na1.9Nb0.1Li2Ti6O14: (a) before discharge, (b) with 8e site fully occupied, (c) with 8e and 4b sites fully occupied, (d) with 8e, 4b, and 4a sites fully occupied, (e) the three-dimensional spatial arrangement for all atoms after full lithiation, and (f) the evolutions of the lattice parameters (a, b, c, and V).

network of cross-linked TiO6 octahedra, LiO4 tetrahedra, and NaO11 (NbO11) polyhedra, in which 8e, 4a, and 4b vacant sites are available in the tunnels for lithium storage, as shown in Figure 8a. When the sample is discharged to 1.0 V, the inserted lithium ions can fully occupy the 8e positions in the structure and new LiO4 tetrahedra form in the interstice of the orthorhombic structure, as shown in Figure 8b, corresponding to 3.0 Li per formula accommodation in Na1.9Nb0.1Li2Ti6O14 to form Na1.9Nb0.1Li5Ti6O14 during the lithiation process. With a further discharge process from 1.0 to 0.5 V, another 1.5 Li per formula take the 4b positions (Figure 8c) to form new Li−O tetrahedra, which induce a phase transformation from Na1.9Nb0.1Li5Ti6O14 to Na1.9Nb0.1Li6.5Ti6O14. After a full lithiation process to 0.0 V, the short slope of the discharge curve should be attributed to 1.5 Li per formula occupation at 4a vacant positions to generate Na1.9Nb0.1Li8Ti6O14, as presented in Figure 8d. To observe the structure for fully

Na1.9Nb0.1Li2Ti6O14 has a stable host structure for reversible lithium storage, which is in good accordance with its excellent electrochemical performances in Figure 3. To make a thorough investigation of the structural evolution for Na1.9Nb0.1Li2Ti6O14, Rietveld refinements are performed for several selected in situ XRD patterns. As shown in Figures 7i−l and S5 (Supporting Information), four different lithiated XRD patterns are carefully refined according to the standard JCPDS card No. 52-0690. The diffraction peaks at 38.4° and 41.25° in the XRD patterns are attributed to characteristic peaks of BeO, which are produced from the electrochemical oxidation of the Be disk used as the X-ray transmission window during previous work. The detailed Rietveld refinement data of different lithiated/delithiated samples are given in Tables S1−S6 (Supporting Information). For the pristine sample, the Rietveld refinement result reveals that the crystal structure of Na1.9Nb0.1Li2Ti6O14 can be described as a three-dimensional 10311


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lithium storage material among all the as-prepared samples. Electrochemical evaluations show that Na1.9Nb0.1Li2Ti6O14 can deliver the highest initial charge capacity of 259.4 mAh g−1 at a current density of 100 mA g−1 with the capacity retention of 94.7% after 50 cycles. Even when cycled at 700, 900, and 1100 mA g−1, it still can maintain the reversible capacities of 206.3, 186.6, and 157.1 mAh g−1 after 200 cycles, respectively. On the basis of the analyses of in situ XRD, ex situ TEM, and ex situ XPS observations, it is known that the electrochemical reaction in Na1.9Nb0.1Li2Ti6O14 during the charge−discharge process is quasireversible, which gives Na1.9Nb0.1Li2Ti6O14 the superior electrochemical performance needed for advanced lithium ion batteries.

lithiated sample, a three-dimensional spatial arrangement for different atoms is built, in which the 8e, 4b, and 4a sites are fully occupied by lithium ions, as shown in Figure 8e. Upon a reverse delithiation process to 3.0 V, the occupied lithium ions can reversibly be extracted from the 4a, 4b, and 8e sites in turn, leading to the regeneration of Na1.9Nb0.1Li2Ti6O14 at the end of charge process (see Rietveld refinement results for delithiated samples in the Supporting Information, Figure S5 and Table S6). On the basis of the Rietveld refinement results, it is known that the phase transformation of Na1.9Nb0.1Li2Ti6O14 is associated with a two-phase transition and two solid-solution reactions. The lithiation process at the flat working platform corresponds to a reversible two-phase transformation between Na1.9Nb0.1Li2Ti6O14 and Na1.9Nb0.1Li5Ti6O14. With further lithiation, there are two solid-solution processes (Na 1 . 9 Nb 0 . 1 Li 5 Ti 6 O 1 4 → Na 1 . 9 Nb 0 . 1 Li 6 . 5 Ti 6 O 1 4 → Na1.9Nb0.1Li8Ti6O14). The total structural evolution during the insertion/extraction process is presented in Figure S6 (Supporting Information). The reversibility of the lithium ion insertion/extraction in/out empty tetrahedral and octahedral vacant positions ensures the excellent cycling and rate properties of Na1.9Nb0.1Li2Ti6O14 as a high-performance anode material. In order to further deliver the structural changes during cycles, the evolutions of lattice parameters (a, b, c, and V) for Na1.9Nb0.1Li2Ti6O14 during lithiation and delithiation process are shown in Figure 8f. Here, x means the amount of inserted or extracted lithium ions per unit in Na1.9Nb0.1Li2+xTi6O14 during the charge−discharge process. Viewed from Figure 8f, it is clear that the lattice parameter a shows reverse evolution compared to the lattice parameters b and c. It increases from 16.4979 to 16.3789 Å with a lithiation of 5.59 Li per formula Na1.9Nb0.1Li2Ti6O14 and then reduces to 16.4679 Å after full delithiation. In contrast, lattice parameters b and c reveal an increase (from 5.7195 to 5.7491 Å for b and from 11.2438 to 11.3649 Å for c) during the discharge process and present a reverse decrease (from 5.7491 to 5.7191 Å for b and from 11.3649 to 11.2429 Å for c) upon charging. Thus, the resulting lattice volume of Na1.9Nb0.1Li2Ti6O14 increases slightly from 1060.9 to 1070.2 Å3 after a full lithiation process to 0.0 V. It is obvious that the whole volume expansion is only 0.87%, which is similar to that of zero-strain Li4Ti5O12.56 After a full delithiation, the lattice volume can return to the value of 1058.9 Å3. As seen from Figure 8f, it can also be found that the evolutions for all lattice parameters during lithiation and delithiation process reflect a quasimirror symmetry, suggesting that the structural transformation of Na1.9Nb0.1Li2Ti6O14 during charge−discharge process is quasireversible, which is in line with the results of ex situ TEM and ex situ XPS investigations. Therefore, Na1.9Nb0.1Li2Ti6O14 can be used as potential anode materials for rechargeable lithium ion batteries.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01293. Rietveld refinement profiles, elemental mapping images, differential capacity plots, XPS spectra, and lithiated/ delithiated structure of Na2Li2Ti6O14 and Na1.9M0.1Li2Ti6O14 (M = Li, Cu, Y, Ce, and Nb) and Rietveld refinement data and structural evolution of Na1.9Nb0.1Li2Ti6O14 during the charge−discharge cycle (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*T.-F.Y. e-mail: [email protected]. *J.S. e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is sponsored by National Natural Science Foundation of China (No. 51274002, 51404002) and Ningbo Key Innovation Team (2014B81005). The work is also supported by K.C. Wong Magna Fund of Ningbo University.

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REFERENCES

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4. CONCLUSIONS In this work, Li+-, Cu2+-, Y3+-, Ce4+-, and Nb5+-doped Na2Li2Ti6O14 products are successfully synthesized via Na-site substitution. Rietveld refinements indicate that Li+, Cu2+, Y3+, Ce4+, and Nb5+ ions totally enter the lattices of Na2Li2Ti6O14. As a result, Cu2+, Y3+, Ce4+, and Nb5+ dopings improve the ionic and electronic conductivities of Na2Li2Ti6O14 due to the partial reduction of Ti4+ to Ti3+ in the structure, which also leads to a decrease of the redox polarization for Na1.9M0.1Li2Ti6O14 during the lithiation−delithiation process. Nb5+ doping makes Na1.9Nb0.1Li2Ti6O14 become the best 10312


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Effect of Sodium-Site Doping on Enhancing the Lithium Storage

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