Advanced BaLi2Ti6O ... - ACS Publications

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Advanced BaLi2Ti6O14 Anode Fabricated via Lithium Site Substitution by Magnesium Xiaoting Lin, Shangshu Qian, Haoxiang Yu, Lei Yan, Peng Li, Yaoyao Wu, Nengbing Long, Miao Shui, and Jie Shu* Faculty of Materials Science and Chemical Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo 315211, Zhejiang Province, People’s Republic of China S Supporting Information *

ABSTRACT: Advanced Na- and Mg-doped BaLi2Ti6O14 anodes in the form of BaLi1.9M0.1Ti6O14 (M = Na, Mg) are successfully fabricated and evaluated as lithium storage materials for rechargeable lithium-ion batteries. The effects of Na- and Mgdopings on the crystal structure, surface morphology and electrochemical behavior are investigated for BaLi2Ti6O14. The results show that both Na and Mg elements are successfully introduced into the Li site, and they do not alter the basic structure of BaLi2Ti6O14. The resulting BaLi1.9M0.1Ti6O14 (M = Na, Mg) exhibit significant improvements on the electrochemical performance in terms of the rate capability and cycle performance. Especially for BaLi1.9Mg0.1Ti6O14, it can deliver an initial charge capacity of 111.7 mAh g−1 at 5C. After 200 cycles, it still can maintain a reversible capacity of 90.1 mAh g−1 with the capacity retention of 80.7%. The enhanced electrochemical properties can be attributed to the reduced particle size, decreased charge transfer resistance and enhanced ionic/electronic conductivity induced by Mg doping. Besides, in situ X-ray diffraction also reveals that BaLi1.9Mg0.1Ti6O14 has high structural stability and reversibility during charge/discharge process. KEYWORDS: BaLi2Ti6O14, Mg doping, Anode material, Rate performance

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INTRODUCTION Rechargeable lithium-ion batteries, a fast developing technology in energy storage field, are the dominant power source for a wide range of portable electronic devices, communication facilities and stationary energy storage systems, and they have been also considered as the most advanced energy storage system for future electric vehicles and hybrid electric vehicles.1−3 Unfortunately, the current lithium-ion batteries using graphitebased anodes always suffer from serious safety problems when high charge/discharge rates are required. Therefore, a worldwide effort has been made to search for high performance anode materials so as to meet the demand of next generation lithiumion batteries.4−6 BaLi2Ti6O14, an important member of Ti-based materials, is postulated to be a promising candidate to replace graphite.7−9 It possesses several unique advantages, such as long and stable potential plateau, enhanced safety, nontoxicity and abundant titanium dioxide raw materials. Furthermore, BaLi2Ti6O14 anode with a theoretical capacity of 242 mAh g−1 has excellent Li+ insertion and extraction reversibility and displays a negligible structure change during charge and discharge processes. Despite the above-mentioned merits, BaLi2Ti6O14 has rather low electronic conductivity and Li+ diffusion coefficient, which make it suffer from the problem of poor rate capability. To overcome the disadvantage and further improve the rate © XXXX American Chemical Society

capability of BaLi2Ti6O14, lots of improved routes have been developed, such as synthesis of nanosized particles,10,11 coating with conductive carbon,12,13 morphology control14−16 and substitution of Li, Ti or O by other elements.17−19 Among these different methods, doping is an advantageous route because this method has a direct impact on the structure and stability of electrode materials during lithium-ion intercalation and deintercalation.20−25 For instance, the electronic conductivity of Li1.95Al0.05Na2Ti6O14 synthesized by M.M. Lao can be improved remarkably, and it also exhibits a relatively good cycling stability and rate performance.26 Furthermore, dopings of various cations, such as K+, Na+, Mg2+, Ca2+, Cu2+, Sn2+, Zn2+, La3+ and Al3+,27−38 are proved to be an effective way to improve the electrochemical performance of Li4Ti5O12. However, to the best of our knowledge, no study about metal ions doped BaLi2Ti6O14 as an anode material has been reported. From a commercial perspective, the solid-state reaction is the most attractive method due to the simple synthesis route and low cost. In the present work, BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 are successfully prepared via a simple solidstate method. The effects of Na, Mg doping on the modification Received: May 25, 2016 Revised: July 31, 2016

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DOI: 10.1021/acssuschemeng.6b01137 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering of surface morphology and electrochemical property of BaLi2Ti6O14 are explored in detail. Compared with other samples, Mg-doped BaLi2Ti6O14 exhibits remarkable improvement in Li storage performance, in terms of the rate capability and cyclic stability, which provides a new choice for the development of high rate performance electrode for rechargeable lithium-ion batteries.

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EXPERIMENTAL SECTION

Na-, Mg-doped and undoped BaLi2Ti6O14 were synthesized by a conventional solid-state method. BaCO3 (99.5%, Aladdin), Li2CO3 (99.5%, Aladdin), TiO2 (5−10 nm, 99.5%, Aladdin), Na2CO3 (99.5%, Aladdin) and MgO (99.5%, Aladdin) were analytical grade and used as raw materials to prepare BaLi2Ti6O14, BaLi1.9Na 0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14. In the synthesis of pristine BaLi2Ti6O14, the precursors were mixed at a predetermined molar ratio of Li:Ba:Ti = 2.04:1:6. For Na- or Mg-doped BaLi2Ti6O14 samples, the molar ratio of Li:Na(Mg):Ba:Ti is 1.94:0.1:1:6. Here, 2.0% excessive Li source was added to compensate for the Li volatilization during the high temperature calcination process. First, stoichiometric amounts of starting reagents were mixed by planetary ball milling in ethanol for 10 h to form the precursor. The obtained precursor slurry was dried at 80 °C to evaporate ethanol, then progressively heated up to 600 °C to decompose the carbonate salts and finally calcined at 950 °C for 10 h in argon atmosphere. The crystal structures of as-prepared samples were identified by Bruker D8 Focus X-ray diffraction (XRD) instrument operating at 40 kV and 40 mA using Cu Kα radiation (λ = 0.154 06 nm). The surface morphologies and lattice structures of samples were examined using a field emission scanning electron microscopy (FE-SEM, Hitachi, S4800) equipped with energy dispersive spectroscopy (EDS) and a transmission electron microscopy (TEM, JEOL 2100F) equipped with selected area electron diffraction (SAED). The valence state of elements in samples was investigated by PHI-Quantum 2000 X-ray photoelectron spectroscopy (XPS) instrument. The electrochemical performances of samples were evaluated by using CR2032 coin-type cells. The working electrodes were first prepared by mixing of 80 wt % as-prepared sample as active material, 10 wt % carbon black as conductive additive and 10 wt % polyvinylidene difluoride as binder, and N-methyl-2-pyrrolidone as solvent. Next, the homegeneous slurry of the mixture was coated uniformly onto a copper foil and dried at 100 °C for 12 h in a vacuum oven. After that, the film coated copper foil was cut into disks with a diameter of 15 mm. The average mass loading of active material is about 3.0 mg for the disks. In a typical coin-type cell, the as-prepared disk was used as the working electrode and lithium foil was provided as the counter electrode separated by a Whatman glass fiber. The electrolyte was 1 mol L−1 LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1, v/v). The coin-type cells were assembled in an argon-filled glovebox, where both moisture and oxygen levels were kept at less than 1 ppm. For electrochemical measurements, charge−discharge behavior and rate performance of coin-type cells were measured by multichannel Land CT2001A battery test system between 0.5 and 2.0 V (vs Li+/Li). Cyclic voltammogram (CV) was performed at a scan rate of 0.1 mV s−1 from 0.5 to 3.0 V on a CHI 1000B electrochemical workstation at room temperature. Electrochemical impedance spectroscopy (EIS) patterns were carried out to characterize the interfacial resistances of anode over the frequency range from 10−2 to 105 Hz on a Bio-Logic VSP-300 electrochemical workstation.

Figure 1. (a) XRD patterns and (b) crystal structure of BaLi1.9M0.1Ti6O14 (M = Li, Na, Mg).

doping of Na or Mg does not alter the basic crystal structure of BaLi2Ti6O14. The purity characteristics of as-prepared samples can also be confirmed by the XRD Rietveld refinement results of BaLi2Ti6O14 and BaLi1.9M0.1Ti6O14 (M = Na, Mg) as displayed in Figure S1 (Supporting Information), and the refined results are totally summarized in Table 1. The calculated data reveal that the Table 1. Refined Lattice Parameters for BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 sample

a (Å)

b (Å)

c (Å)

V (Å3)

BaLi2Ti6O14 BaLi1.9Na0.1Ti6O14 BaLi1.9Mg0.1Ti6O14

16.5597 16.6004 16.5620

11.2580 11.2855 11.2582

11.5720 11.6027 11.5807

2157.3575 2173.6941 2159.3177

lattice parameters of Na- and Mg-doped samples increase compared with the pristine BaLi2Ti6O14. The crystal structure of BaLi1.9M0.1Ti6O14 (M= Na, Mg) is depicted in Figure 1b, which is built by edge- and corner-sharing TiO6 octahedra forming layers parallel to the (100) plane. Consecutive layers are linked by sharing common corners along the (100) direction. Lithium atoms in tetrahedral coordination occupy vacancies of the TiO6 octahedra framework, and strontium atoms lie in 11-coordinated sites between two successive layers. Within this structure, additional lithium ions can take the interstitial space during the electrochemical insertion process as depicted in Figure 1b. Also, the structure representation of the BaLi1.9M0.1Ti6O14 (M= Na, Mg) shows that the lithium atoms at 16g sites are partially substituted by sodium or magnesium atoms. To detect the existence of various elements and their valence states of BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14, the XPS spectra are shown in Figure 2. From Figure 2a,b, two XPS signals from Ti element in the IV oxidation state are observed at 458.69 and 464.57 eV, which are assigned to Ti 2p3/2 and 2p1/2 peaks,

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RESULTS AND DISCUSSION The XRD patterns of as-synthesized BaLi2 Ti 6 O 14 , BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 are shown in Figure 1a. All diffraction peaks of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 can be well indexed to the structure of standard BaLi2Ti6O14 with space group Cmca (No. 64). No obvious impurity peaks can be detected, indicating that low dose B


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Figure 2. XPS spectra of (a) Na, (b) Ti element in BaLi1.9Na0.1Ti6O14 and (c) Mg, (d) Ti element in BaLi1.9Mg0.1Ti6O14.

respectively. The peak of Na 1s is also identified at 1072.0 eV, which reflects the presence of Na element in BaLi1.9Na0.1Ti6O14. For the BaLi1.9Mg0.1Ti6O14 material, a remarkable signal located at the binding energy of 1303.3 eV is found, which can be assigned to the existence of Mg2+ in the structure. Due to different valence state between Li+ and Mg2+, replacing Li+ with Mg2+ will lead to the formation of Ti3+.39 The presence of Ti3+ in BaLi1.9Mg0.1Ti6O14 lattice is supported by the appearance of a small peak at around 459.5 eV, corresponding to the binding energy of Ti3+ (Figure 2d). Therefore, the lattice constants (a, b and c) of three samples in Table 1 are postulated to be influenced by (1) the different ionic radius of Li+ (0.76 Å), Na+ (1.02 Å) and Mg2+ (0.72 Å),29,40 (2) the valence transition from Ti4+ (0.605 Å) to Ti3+ (0.67 Å) for charge compensation. It is obvious that the lattice parameters of BaLi1.9Na0.1Ti6O14 are larger than those of pristine BaLi2Ti6O14, which can be ascribed to the larger radii of the doped sodium ions. Although the ionic radius of Mg2+ (0.72 Å) is smaller than Li+ (0.76 Å), slight volume expansion can still be observed for BaLi1.9Mg0.1Ti6O14. This may be attributed to the larger electrostatic repulsive force of high valence of Mg2+ in 16g sites than Li+ in the same sites. As a result, volume expansion is also observed for BaLi1.9Mg0.1Ti6O14. Based on the lithium ion transportation kinetics, the enlarged lattice constant is beneficial for fast lithium-ion transfer and good electrochemical property. To confirm the effects of doping on the surface morphology of BaLi2Ti6O14, the pristine BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and Li1.9Mg0.1Ti6O14 are investigated by SEM and EDS. As can be seen in Figure 3, the bare BaLi2Ti6O14 has disordered morphology composed of agglomerated particles with much wider size distribution from 0.5 to 2.0 μm, while the Na and Mgdoped BaLi2Ti6O14 samples exhibit smaller particle size and better particle distribution. In agreement with the result from XPS measurement, the elemental mapping and EDS images presented in Figure 3e-h and S2 (Supporting Information) also confirmed the successful introduction of Na and Mg elements into the structure of BaLi1.9Na0.1Ti6O14 and Li1.9Mg0.1Ti6O14

samples. The TEM images of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and Li1.9Mg0.1Ti6O14 are shown in Figure 4a,d,g. It can be found that a smaller grain size is obtained in the BaLi1.9Na0.1Ti6O14 and Li1.9Mg0.1Ti6O14 samples compared with BaLi2Ti6O14, which is in good agreement with the SEM pictures. In addition, the clear lattice fringes in HRTEM images and the well-defined spots in SAED patterns reveal high crystallinity of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and Li1.9Mg0.1Ti6O14. All these evidence further prove that Na and Mg elements are successfully introduced into the structure of BaLi2Ti6O14 via a facile solid state reaction. Cyclic voltammograms of the electrodes of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and Li1.9Mg0.1Ti6O14 at a scan rate of 0.1 mV s−1 are shown in Figure 5a. It can be observed that all the investigated electrodes have similar redox peaks at around 1.49/ 1.36 V and 1.19/1.16 V, implying that Na and Mg dopings do not change the electrochemical reaction process of BaLi2Ti6O14. Besides, the obvious irreversible peak appears at around 0.73 V can be attributed to the reduction decomposition reaction of the organic electrolyte. With a close observation, it is clear that the potential difference between the main anodic and cathodic peaks for BaLi1.9Mg0.1Ti6O14 is lower than other two samples, suggesting the lower redox polarization of BaLi1.9Mg0.1Ti6O14. Such differences may be contributed to the enhanced electronic conductivity from the charge compensation (Ti4+ → Ti3+) with a substitution of divalent Mg ions for monovalent Li ions in the structure. Figure 5b,c shows a comparison of charge/discharge curves and cycling properties between BaLi 2 Ti 6 O 14 and BaLi1.9M0.1Ti6O14 (M = Na, Mg) at a current density of 100 mA g−1 (0.5C). For the discharge process, the curves show a clear plateau at 1.40 V, followed by two gradual slopes in the potential range of 0.5−1.0 V and 1.0−1.4 V, demonstrating that the intercalation of Li+ into BaLi2Ti6O14 is performed in stages. During charging, two small platforms appear in the working windows of 0.5−1.0 V and 1.0−1.4 V, and a main delithiation plateau at 1.44 V can be observed. As shown in Figure S3 C


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Figure 3. SEM images of (a, b) BaLi2Ti6O14, (c) BaLi1.9Na0.1Ti6O14 and (d) BaLi1.9Mg0.1Ti6O14. (e) EDS spectrum and (f) element mapping images of Na, Ba, Ti and O of BaLi1.9Na0.1Ti6O14, (g) EDS spectrum and (h) element mapping images for Mg, Ba, Ti and O of BaLi1.9 Mg0.1Ti6O14.

the 100th cycle with the capacity retention of 91.8%, while the reversible capacity of BaLi2Ti6O14 fades to 94.3 mAh g−1 with a poor capacity retention of 61.1% along with the repeated charge/ discharge process, which indicate the advantage of Mg doping for BaLi2Ti6O14. To understand the improved electrochemical performance after doping, EIS measurements on BaLi 2 Ti 6 O 14 , BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 electrodes are investigated before cycles. As shown in Figure 5d, the plots of three samples exhibit the commonness which consists of a depressed semicircle at high frequency region and a straight line in low frequency region. The depressed semicircle observed at high frequency region is attributed to the contact/solution resistance and charge transfer resistance, while the line at the low frequency region, namely the Warburg impedance, is attributed to the diffusion of lithium-ion within the bulk of materials. By using the

(Supporting Information), a large irreversible capacity loss can be observed in three as-samples during the first discharge/charge cycle. As the electrochemical reaction of BaLi2Ti6O14 with Li is a highly reversible process,9 the low first cycle Coulombic efficiency can be ascribed to the formation of a solid electrolyte interphase (SEI) layer on the BaLi1.9M0.1Ti6O14 (M = Li, Na, Mg) surface and possible Li trapping at the anode. In the subsequent cycles, the lithium storage capacity of BaLi2Ti6O14 and BaLi1.9M0.1Ti6O14 are mainly based on the reversible redox reactions between trivalent titanium ions (Ti3+) and tetravalent titanium ions (Ti4+). After 50 cycles, the reversible specific capacity of BaLi1.9Mg0.1Ti6O14 is 146.4 mAh g−1, which is much higher than that of BaLi2Ti6O14 (110.7 mAh g−1) and BaLi1.9Na0.1Ti6O14 (125.2 mAh g−1). As shown in Figure 5c and Table S1 (Supporting Information), the BaLi1.9Mg0.1Ti6O14 can remain the reversible specific capacity of 139.6 mAh g−1 in D


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Figure 4. TEM, HRTEM and SAED images of (a, b, c) BaLi2Ti6O14, (d, e, f) BaLi1.9Na0.1Ti6O14 and (g, h, i) BaLi1.9Mg0.1Ti6O14.

that of BaLi 2 Ti 6 O 14 (1.11 × 10 −16 cm 2 s −1 ) and BaLi1.9Na0.1Ti6O14 (3.84 × 10−16 cm2 s−1). On the basis of the above results, it is known that performing Li-site Mg substitution improves the Li+ diffusion rate and reduces the charge transfer resistance in BaLi2Ti6O14. As a result, BaLi1.9Mg0.1Ti6O14 exhibits much better rate performance compared with the BaLi2Ti6O14 and BaLi1.9Na0.1Ti6O14. The rate capabilities of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 are evaluated by charge/discharge at the rates from 1C to 5C. As shown in Figure 5f, the reversible specific capacity of BaLi2Ti6O14 drops significantly as the increase of charge/discharge rate. It can only deliver a reversible capacity of 136.5 mAh g−1 at 1C, 106.4 mAh g−1 at 2C, 89.8 mAh g−1 at 3C, 81.0 mAh g−1 at 4C and 74.8 mAh g−1 at 5C, respectively. After being doped by Na or Mg, the rate properties of BaLi1.9M0.1Ti6O14 show obvious improvement. When discharging at relatively low rates such as 2C and 4C, the reversible capacities of BaLi1.9Na0.1Ti6O14 are 113.6 and 98.1 mAh g−1, respectively. In contrast, the corresponding capacity values for BaLi1.9Mg0.1Ti6O14 are 129.3 mAh g−1 at 2C and 115.2 mAh g−1 at 4C, respectively. Even cycled at 5C, BaLi1.9Mg0.1Ti6O14 can

equivalent circuit as inset in Figure 5d, EIS spectra are simulated and the calculated data are listed in Table 2. The charge transfer resistances (Rct) of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 are 110.9, 69.57 and 20.65 Ω, respectively. This result indicates that the charge transfer at the electrolyte/ electrode interface is greatly improved after Na and Mg dopings. Furthermore, Li+ ion diffusion coefficient (DLi+) can be calculated from the plots in the low frequency region based on eqs 1 and 2:

DLi =

(RT )2 2(An2 F 2C Liσ )2

Z′ = R ct + R s + σω−1/2

(1) (2)

Where Z′, ω, R, T, A, F and CLi are the real part of impedance, angular frequency, gas constant, absolute temperature, surface area of the electrode, Faraday constant and molar concentration of Li+ ions in solid, respectively. Besides, σ is the Warburg factor, which can be obtained from the slope of the Z′−ω−1/2 lines shown in Figure 5e.41,42 The DLi+ of BaLi1.9Mg0.1Ti6O14 is calculated to be 6.33 × 10−16cm2 s−1, which is much higher than E


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Figure 5. (a) Cyclic voltammogram curves of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 recorded at 0.1 mV s−1; (b) charge/discharge curves and (c) cycling properties of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 at a current density of 100 mA g−1 (0.5C); (d) electrochemical impedance spectra; (e) graph of Z′ plotted against ω−1/2 at low frequency region for BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14; (f) rate performance of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14, and BaLi1.9Mg0.1Ti6O14 at rates from 1C to 5C.

work. Figure 6 shows the in situ XRD results of BaLi1.9Mg0.1Ti6O14 during the first charge/discharge cycle. In addition to the BaLi2+xTi6O14 (0 ≤ x ≤ 6) phase, the reflections of the BeO are also detected, which is produced on the X-ray window of Be disk during previous work. The original red bold in situ XRD pattern shows a pure phase of BaLi1.9Mg0.1Ti6O14 before cycles. Taking a close look at the variation of the XRD curves, the (113), (132), (240), (800) and (044) peaks shift toward lower angle during discharging and return to the original positions along with the same deviation path during charging. Besides, some original peaks disappear along with the lithiation process, and then reappear after charging, such as (221), (131), (421) and (331) peaks. Similar with structural changes of BaLi2Ti6O14 during cycling,9 no evidence of the formation of a new phase is detected in the lithiated BaLi1.9Mg0.1Ti6O14 and the distortion of the crystal structure is negligible, which tells that the structural changes of BaLi1.9Mg0.1Ti6O14 is also a quasi-reversible process during lithiation and delithiation. To confirm further the structural stability at high rate, a long-term charge/discharge cycling behavior of BaLi1.9Mg0.1Ti6O14 is recorded at 5C as shown in Figure 7. It can be found that BaLi1.9Mg0.1Ti6O14 can deliver an initial charge capacity of 111.7 mAh g−1 at 5C. After 200 cycles, it still can maintain the reversible capacity of 90.1 mAh g−1 with the capacity retention of 80.7%. This result further proves that BaLi1.9Mg0.1Ti6O14 is an advanced anode material for lithium-ion batteries.

Table 2. Electrochemical Parameters of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 Calculated from EIS Patterns sample

Rs (Ω)

Rct (Ω)

DLi (cm2 s−1)

BaLi2Ti6O14 BaLi1.9Na0.1Ti6O14 BaLi1.9Mg0.1Ti6O14

5.57 5.44 4.88

110.9 69.57 20.65

1.11 × 10−16 3.84 × 10−16 6.33 × 10−16

still deliver a high reversible capacity of 111.6 mAh g−1. Figure S4 (Supporting Information) and Table S2 (Supporting Information) exhibit the detailed charge/discharge profiles and specific capacities of three samples at various charge/discharge rates. The excellent electrochemical performance of BaLi1.9Mg0.1Ti6O14 could be attributed to the favorable effects of Mg doping into BaLi2Ti6O14. First, smaller particle size can shorten the diffusion length of Li+, decrease the charge transfer resistance, and increase the contact area between electrode and electrolyte. Furthermore, the Mg doping process involves the reduction of partial titanium ions from Ti4+ to Ti3+ in the structure, and the presence of Ti4+/ Ti3+ couple can increase the electronic conductivity and decrease the electrode polarization of BaLi2Ti6O14 during the charge/ discharge cycles. To study further the Mg-doping effects on the structural changes of BaLi1.9Mg0.1Ti6O14 during Li+ insertion and extraction, homemade in situ XRD technique is used in this F


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Figure 6. In situ XRD patterns of BaLi1.9Mg0.1Ti6O14 in 0.5−3.0 V.

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CONCLUSIONS In this work, Na- and Mg-doped BaLi2Ti6O14 samples are successfully prepared via a solid-state reaction method. The XRD and SEM results show that both BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 are well crystallized single-phase products with narrow particle size distribution. Electrochemical tests show that BaLi1.9Mg0.1Ti6O14 has higher reversible capacity and better cycling stability compared with BaLi 2 Ti 6 O 14 and BaLi1.9Na0.1Ti6O14. A large capacity of 124.1 mAh g−1 can be kept

for BaLi1.9Mg0.1Ti6O14 after 100 cycles at a current density of 100 mA g−1 (0.5C). In contrast, BaLi2Ti6O14 and BaLi1.9Na0.1Ti6O14 can only deliver the reversible capacities of 94.5 and 116.7 mAh g−1 at 0.5C after 100 cycles, respectively. Furthermore, BaLi1.9Mg0.1Ti6O14 also shows outstanding rate property. This enhanced electrochemical performance should be contributed to the reduced particle size, improved electronic/ionic conductivity and decreased charge transfer resistance after Mg doping. Besides, in situ XRD result also confirms that BaLi1.9Mg0.1Ti6O14 G


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Figure 7. Cycling performance and corresponding Coulombic efficiency of BaLi1.9Mg0.1Ti6O14 recorded at 5C.

has stable host structure for lithium storage during lithiation and delithiation. The outstanding electrochemical properties of BaLi1.9Mg0.1Ti6O14 make it a promising anode material for power lithium-ion batteries.

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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.6b01137. Rietveld refinement profiles, atomic ratio, charge/ discharge curves and specific capacities of BaLi2Ti6O14, BaLi1.9Na0.1Ti6O14 and BaLi1.9Mg0.1Ti6O14 (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Jie Shu. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is sponsored by Ningbo Key Innovation Team (2014B81005), Ningbo Natural Science Foundation (2016A610068) and K.C. Wong Magna Fund in Ningbo University.

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REFERENCES

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