Ba0.9La0.1Li2Ti6O14: Advanced lithium storage material for lithium-ion batteries

Electrochimica Acta 232 (2017) 132–141

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Ba0.9La0.1Li2Ti6O14: Advanced lithium storage material for lithium-ion batteries Haoxiang Yu1, Minghe Luo1, Shangshu Qian, Lei Yan, Peng Li, Hua Lan, Nengbing Long, Miao Shui, 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

A R T I C L E I N F O

Article history: Received 20 October 2016 Received in revised form 4 January 2017 Accepted 22 February 2017 Available online 24 February 2017 Keyword: BaLi2Ti6O14 Metal doping Anode material Lithium storage material In-situ observation

A B S T R A C T

Metal doping is an effective way to improve the electrochemical properties of titanates. In this work, Basite substituted Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) are synthesized to fabricate high performance titanate anode for lithium-ion batteries. Electrochemical evaluations reveal that introducing metal doping at Basite can result in higher ionic/electronic conductivity. As a result, Ba0.9M0.1Li2Ti6O14 exhibits enhanced lithium storage capability. Especially for Ba0.9La0.1Li2Ti6O14, it shows the best electrochemical performance with a high reversible charge capacity of 151.3 mAh g
1 and high capacity retention of 94.21% at a current density of 100 mA g
1. For comparison, the pristine BaLi2Ti6O14 only exhibits a reversible charge capacity of 128.5 mAh g
1 with the capacity retention of 80.06% after 100 cycles. Further, the lithium storage process is investigated in detail by in-situ structural observation, which reveals a maximum volume expansion of 1.9% for Ba0.9La0.1Li2Ti6O14 during charge-discharge cycle. It shows that Ba0.9La0.1Li2Ti6O14 is a possible material with high structural reversibility for lithium storage. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction As a kind of lithium-ion insertion material, graphite offers the excellent performance as anode in rechargeable lithium-ion batteries (LIBs), such as low cost, non-toxicity, and high energy density. However, there is still some safety issues associated with the carbonaceous anode due to its low working potential (0.1 V vs. Li+/Li) [1–3]. To fulfill the stringent requirements for high power application, considerable efforts from electrochemists and battery researchers have been devoted to search for safer alternative anode materials which not only exhibit outstanding electrochemical performance but also have high safety over the past decades [4–7]. Among numerous explored anode materials, titanate has been deemed as a promising candidate of anode material for LIBs because it shows excellent cycle life and higher operating potential compared to graphite [8–30]. And as a significant member of titanate series, spinel oxide Li4Ti5O12 has attracted plenty of attention [7,12,23–25]. However, only a few literatures report other members of titanate materials as anodes for LIBs which have the similar electrochemical property with that of Li4Ti5O12.

* Corresponding author. Tel.: +86-574-87600787, fax: +86-574-87609987. E-mail address: [email protected] (J. Shu). These authors contributed equally to this work.

1

http://dx.doi.org/10.1016/j.electacta.2017.02.134 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

Nowadays, the research interest for titanate materials (besides Li4Ti5O12) has been renewed as the demand for high-effective electrochemical energy storage devices. In recent years, MLi2Ti6O14 (M = Sr, Ba, 2Na) has been regarded as new lithium-ion insertion anodes in the family of lithium-ion conducting titanate materials for LIBs [13–21,27–39]. From their lithium insertion behaviors, Na2Li2Ti6O14 can reversibly intercalate two lithium ions per formula while SrLi2Ti6O14 and BaLi2Ti6O14 are capable of inserting nearly four lithium ions per unit [15–17]. And from their practical electrochemical properties, Na2Li2Ti6O14 can present an initial charge capacity of 89.9 mAh g
1 and the reversible charge capacity is only capable of maintaining at about 80.0 mAh g
1 after 40 cycles [38]. For comparison, the electrochemical results of MLi2Ti6O14 (M = Sr, Ba) show that these two host materials can provide a higher practical lithium storage capacity of about 140.0 mAh g
1 after 40 cycles [16,17]. Furthermore, the area-specific impedance of Li/SrLi2Ti6O14 cell is much lower than that of Li/Li4Ti5O12 cell [17], indicating that MLi2Ti6O14 (M = Sr, Ba) may be promising lithium storage materials for rechargeable LIBs. However, MLi2Ti6O14 (M = Sr, Ba, 2Na) also is an insulator that exhibits the disadvantage with low electronic conductivity and low lithium-ion diffusion coefficient, which can lead to poor electrochemical properties, especially at high rates. To solve these problems mentioned above, different approaches are applied to enhance the lithium storage

H. Yu et al. / Electrochimica Acta 232 (2017) 132–141

a

b

Ba0.9K0.1Li2Ti6O14

Ba0.9Zn0.1Li2Ti6O14

Intensity (a.u.)

Intensity (a.u.)

BaLi2Ti6O14

133

Ba0.9La0.1Li2Ti6O14

BaLi2Ti6O14

10

20

30 2 Theta (deg.)

40

50

10

20

30 2 Theta (deg.)

40

50

Fig. 1. The XRD patterns of BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) powders obtained at 950  C.

capability of MLi2Ti6O14 (M = Sr, Ba, 2Na). One of these involves the preparation of MLi2Ti6O14 anode via element substitution, such as the Li/Ti-site substitution in Na2Li2Ti6O14 by Na+ [22], Mg2+ [22], Al3+ [14,21], Zr4+ [22]. It suggests that the presence of the suitable doping atoms in the structure can effectively improve the electrochemical performances of MLi2Ti6O14. In this context, BaLi2Ti6O14 and its Ba-site substitution products are obtained by a simple high temperature solid-state method. Their structural, morphological and electrochemical properties of as-prepared Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) are investigated and compared before and after doping. The results exhibit that the dopings of K+ and La3+ in the structure can effectively improve the electronic/ionic conductivities of BaLi2Ti6O14. As a result, Ba0.9M0.1Li2Ti6O14 (M = K, La) anodes show enhanced charge/discharge performances compared to bare BaLi2Ti6O14 and Ba0.9Zn0.1Li2Ti6O14. 2. Experimental

Ba-site substitution products were investigated by using the Hitachi SU-70 scanning electron microscope (SEM) equipped with an Oxford Inca-300 energy dispersive spectroscopy (EDS). 2.2. Electrochemical evaluation The testing CR2032 coin-type cell was composed of lithium metal foil as counter electrode, Whatman glass fiber as separator, and the as-prepared material as working electrode. Here, the working electrodes with a diameter of 15 mm consisted of active material (80 wt.%), carbon black conductive additive (10 wt.%) and polyvinylidene difluoride binder (10 wt.%). The CR2032 coin-type cells were cycled on multichannel LANHE CT2001A battery test system. Cyclic voltammetry (CV) was performed on a CHI1000B electrochemical workstation. Electrochemical impedance spectra (EIS) were carried out on a Bio-Logic VSP-300 electrochemical workstation. Here, three-electrode systems were used to conduct CV and EIS experiments, in which both the counter and reference electrodes are lithium metal foils.

2.1. Material preparation and characterization 3. Results and discussion In this work, pristine BaLi2Ti6O14 was synthesized by a traditional high temperature solid-state method. All the chemical reagents used in this experiment were of analytical-reagent grade. First of all, stoichiometric amounts of BaCO3, Li2CO3, and TiO2 were mixed by planetary ball-milling in ethanol for 12 hours. After ballmilling, the obtained mixture was dried at 80  C to evaporate the needless ethanol. And then the mixture was pretreated at 600  C for 4 hours to drive off CO2 and calcined at 950  C for 10 hours in air to obtain the final samples. As the preparation of Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La), it took La2O3, ZnO, or KNO3 as the source of Mdoping before ball-milling. The following preparation steps were the same as the pristine sample. The phase purity and composition of as-obtained samples were characterized by Bruker D8 Focus X-ray diffraction (XRD) equipped with a Cu-Ka radiation diffractometer (l = 1.5406 Å). In-situ structural evolutions of Ba0.9M0.1Li2Ti6O14 were characterized by using the same XRD instrument as described in our previous work [40,41]. For in-situ XRD investigation, homemade in-situ cell was fabricated by using Be disk as X-ray window. The surface morphology and elemental distribution of BaLi2Ti6O14 and its Table 1 The refined lattice parameters of BaLi2Ti6O14, Ba0.9K0.1Li2Ti6O14, Ba0.9Zn0.1Li2Ti6O14 and Ba0.9La0.1Li2Ti6O14. Sample

a (Å)

b (Å)

c (Å)

V (Å3)

BaLi2Ti6O14 Ba0.9K0.1Li2Ti6O14 Ba0.9Zn0.1Li2Ti6O14 Ba0.9La0.1Li2Ti6O14

16.54173 16.54528 16.55701 16.55716

11.23980 11.24641 11.24941 11.24686

11.55010 11.54394 11.54868 11.55203

2147.46 2148.04 2151.02 2151.17

Fig. 1 presents the XRD patterns of BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) powders obtained at 950  C. As viewed from Fig. 1a, this pattern can be indexed to the orthorhombic BaLi2Ti6O14 phase (ICSD card No. 152568). And no impurity peak can also be observed in Fig. 1b by XRD, which means a successful Ba-site substitution by M (M = K, Zn, La) in BaLi2Ti6O14. Meanwhile, the successful doping of K, Zn and La are also confirmed by inductively coupled plasma-atomic emission spectrometry (ICPAES) characterizations, the results are shown in Table S1. Each characteristic diffraction peak is sharp and observable, which indicates the good crystallinity of as-received products obtained from high-temperature sintering process. The space group of MLi2Ti6O14 is Cmca and the refined lattice parameters of BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) are shown in Table 1 and Fig. S1. These Rietveld refinement results are in accordance with the previous reports [15–17,32]. It can also be found that the lattice parameters show no obvious change after Basite doping, which indicates that K, Zn and La are successfully introduced in the structure of BaLi2Ti6O14. Fig. 2 depicts the SEM, EDS and mapping graphs of the pristine BaLi2Ti6O14 and its M (M = K, Zn, La) doped BaLi2Ti6O14 products. As observed, all the samples are well-crystallized with the particle size in the range of 450 to 900 nm. For the pristine BaLi2Ti6O14, it shows homogeneous particle distribution with the average size of 750–900 nm as shown in Fig. 2. After Ba-site substitution by K, Zn or La, Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) present irregular particle shape with the average size decreasing to the value between 450 and 650 nm. It indicates that M (M = K, Zn, La) doping slightly

134

H. Yu et al. / Electrochimica Acta 232 (2017) 132–141

Fig. 2. The SEM, EDS and mapping graphs of the pristine BaLi2Ti6O14 and its M-doped BaLi2Ti6O14 (M = K, Zn and La).

after La doping. In contrast, Ba0.9Zn0.1Li2Ti6O14 and Ba0.9K0.1Li2Ti6O14 does not show obvious changes of redox polarization due to the inexistence of Ti4+/Ti3+ couple after doping. The two redox peaks suggests that the lithiation/delithiation process of these electrodes have occurred in two regions. Of particular significance is that the remaining curves are nearly overlap except the initial cycle, indicating good reversibility, excellent cyclability, superb kinetics and high cyclic efficiency for lithium ions insertion/ extraction at the electrodes [4]. As viewed from Fig. 3a–d, all four samples show a lower operating potential against Li4Ti5O12, which may be beneficial to improve the energy density of the LIBs. Fig. 3e–h display the typical initial three discharge/charge profiles of BaLi2Ti6O14 (Fig. 3e), Ba0.9La0.1Li2Ti6O14 (Fig. 3f), Ba0.9Zn0.1Li2Ti6O14 (Fig. 3g), Ba0.9K0.1Li2Ti6O14 (Fig. 3h), respectively. The galvanostatic discharge and charge tests are obtained between 0.5 and 3.0 V at a current density of 100 mA g
1. Seen from Fig. 3e–h, two lithiation/delithiation plateaus are shown at approximately 1.35 and 1.15 V during the 2nd and 3rd process, which are in good accordance with the data obtained from CVs. The initial charge capacities of BaLi2Ti6O14, Ba0.9La0.1Li2Ti6O14, Ba0.9Zn0.1Li2Ti6O14 and Ba0.9K0.1Li2Ti6O14 are 160.5, 160.6, 161.6 and 163.1 mAh g
1, respectively. It suggests that four lithium ions per formula can be reversibly inserted and extracted according to the redox reactions of Ti4+/Ti3+ couples, which is consistent with the electrochemical result as reported by Belharouak [17]. The impedance spectra of BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) before the initial discharge process measured

reduces the particle size of BaLi2Ti6O14, which may contribute to their electrochemical performance. The energy dispersive X-ray spectra and elemental mappings of Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) show that each doping element is well-distributed in the asprepared sample, which further confirms the Ba-site successful substitution by K, Zn and La in the structure. The mappings of other elements in Ba0.9M0.1Li2Ti6O14 are also presented in Fig. S2. Cyclic voltammetry data of four samples shown in Fig. 3a–d are obtained in the range from 0.5 to 3.0 V at a scan rate of 0.1 mV s
1. It compares the cyclic voltammograms of pristine BaLi2Ti6O14 and other three M-doped BaLi2Ti6O14 (M = K, Zn, La). The data reveal that the reduction process of the initial cycle is slightly different from the following cycles. It may be attributed to the polarization of coin-type cell [15]. In addition, all the curves display a weak irreversible reduction peak at 0.65 V during the first discharge process. It might be related to the decomposition of electrolyte with the formation of the solid-electrolyte interface (SEI) [15]. Seen from Fig. 3a–d, each curve exhibits two pairs of main redox peaks during the 2nd and 3rd cycle, which are located at 1.49/ 1.36 V and 1.24/1.16 V for BaLi2Ti6O14, 1.48/1.36 V and 1.18/1.15 V for Ba0.9La0.1Li2Ti6O14, 1.51/1.35 V and 1.25/1.14 V for Ba0.9Zn0.1Li2Ti6O14, 1.53/1.35 V and 1.25/1.16 V for Ba0.9K0.1Li2Ti6O14, respectively. These results are also in good agreement with the previous report [16]. By estimating the polarization effect of all samples, Ba0.9La0.1Li2Ti6O14 shows lower redox polarization (0.16 and 0.11 V) compared with the other materials, suggesting the reduction of Ti cations from Ti4+ to Ti3+ in the structure for charge compensation

st

1 cycle 2nd cycle 3rd cycle

-0.3 -0.6

0.0 st

1 cycle 2nd cycle 3rd cycle

-0.2 -0.4

0.0 st

-0.3

1 cycle 2nd cycle 3rd cycle

-0.6

3.0

3.5

BaLi2Ti6O14

0.5

f

3.0

1.5 2.0 Potential (V)

2.5

3.0

Ba0.9K0.1Li2Ti6O14

2.5

Potential (V)

2.5

1.0

2.0 1.5 1.0 0.5 50

100 150 200 250 Capacity (mAh g-1)

300

350

g

3.0

1.5 2.0 Potential (V)

2.5

Ba0.9Zn0.1Li2Ti6O14

1.5 1.0

-0.9

50

100 150 200 250 Capacity (mAh g-1)

300

350

0.5

1.0

h

3.0

1.5 2.0 Potential (V)

2.5

3.0

Ba0.9La0.1Li2Ti6O14

2.5 2.0

1st cycle 2nd cycle 3rd cycle

1.5 1.0 0.5

0

1st cycle 2nd cycle 3rd cycle

-0.6

3.0

2.0

1st cycle 2nd cycle 3rd cycle

0.5 0

1.0

2.5

2.0

1st cycle 2nd cycle 3rd cycle

0.5

0.0

-0.3

Potential (V)

e

3.0

1.5 2.0 2.5 Potential (V)

Potential (V)

1.0

Ba0.9La0.1Li2Ti6O14

0.3

-0.9 0.5

d

0.6

Ba0.9Zn0.1Li2Ti6O14

0.3 Current (mA)

Current (mA)

0.0

c

0.6

Ba0.9K0.1Li2Ti6O14

0.2

Potential (V)

Current (mA)

b

0.4

BaLi2Ti6O14

0.3

Current (mA)

a

0.6

1st cycle 2nd cycle 3rd cycle

1.5 1.0 0.5

0

50

100 150 200 250 Capacity (mAh g-1)

300

350

0

50

100 150 200 250 Capacity (mAh g-1)

300

350

Fig. 3. Cyclic voltammograms and charge-discharge curves of (a, e) BaLi2Ti6O14, (b, f) Ba0.9K0.1Li2Ti6O14, (c, g) Ba0.9Zn0.1Li2Ti6O14 and (d, h) Ba0.9La0.1Li2Ti6O14.

H. Yu et al. / Electrochimica Acta 232 (2017) 132–141

5000

a

1000

BaLi2Ti6O14

800

135

BaLi2Ti6O14

b

Ba0.9K0.1Li2Ti6O14 Ba0.9Zn0.1Li2Ti6O14 Ba0.9La0.1Li2Ti6O14

4000

Ba0.9K0.1Li2Ti6O14 Ba0.9Zn0.1Li2Ti6O14

-Z'' (Ω)

Z' (Ω)

Ba0.9La0.1Li2Ti6O14

600

400

3000

2000

200

1000

OCV

0 0

400

600 Z' (Ω)

800

1000

1.5

100 mA g-1

c

Capacity (mAh g )

150

2.0

BaLi2Ti6O14 Ba0.9K0.1Li2Ti6O14

3.5

4.0

mA g-1

100

-1

100

2.5 3.0 ω-0.5 (Hz-0.5)

d

200

-1

Capacity (mAh g )

200

200

200

300

150

100 400

500

600

700

BaLi2Ti6O14

100

Ba0.9K0.1Li2Ti6O14

Ba0.9Zn0.1Li2Ti6O14

Ba0.9Zn0.1Li2Ti6O14

Ba0.9La0.1Li2Ti6O14

Ba0.9La0.1Li2Ti6O14

50 50

0

20

80

100

0

20

40

60 80 100 Cycle number

120

140

160

500 mA g-1

e

200 Capacity (mAh g-1)

40 60 Cycle number

150 100

BaLi2Ti6O14 Ba0.9K0.1Li2Ti6O14

50

Ba0.9Zn0.1Li2Ti6O14 Ba0.9La0.1Li2Ti6O14 0

100

200

300

Cycle number Fig. 4. (a) EIS spectra, (b) corresponding Z’ versus v
0.5 curves, (c) cycle performance and (d) rate capability of BaLi2Ti6O14, Ba0.9K0.1Li2Ti6O14, Ba0.9Zn0.1Li2Ti6O14 and Ba0.9La0.1Li2Ti6O14; (e) high rate capability of BaLi2Ti6O14, Ba0.9K0.1Li2Ti6O14, Ba0.9Zn0.1Li2Ti6O14 and Ba0.9La0.1Li2Ti6O14.

from 10 mHz–100 kHz is presented in Fig. 4a. Each spectrum shows the commonness which has a depressed semicircle in the high-frequency region and a straight line in the low-frequency region. This characteristic represents a typical behavior of block-electrode measured at open circuit voltage (OCV) state. Thus, the featured electrochemical parameters can be calculated by an equivalent circuit model inserted in Fig. 4a. The depressed semicircle is related to the charge transfer resistance (Rct) and the straight line is assigned to the lithium ion chemical diffusion in the samples. By using the equivalent circuit, all the impedance spectra are simulated and the obtained data are listed in Table 2. In the equivalent circuit model, Re means

Table 2 The electrochemical parameters calculated from EIS spectra of BaLi2Ti6O14, Ba0.9K0.1Li2Ti6O14, Ba0.9Zn0.1Li2Ti6O14 and Ba0.9La0.1Li2Ti6O14 by an equivalent circuit. Sample

Re (V)

Rct (V)

DLi (cm2 s
1)

BaLi2Ti6O14 Ba0.9K0.1Li2Ti6O14 Ba0.9Zn0.1Li2Ti6O14 Ba0.9La0.1Li2Ti6O14

8.289 7.541 11.08 9.663

243.9 241.3 207.6 111.1

2.748  10
15 1.903  10
15 7.493  10
15 1.494  10
14

136

H. Yu et al. / Electrochimica Acta 232 (2017) 132–141

2

a

1 0

0.2, 0.4, 0.8, 1.6, 3 mV s-1

-1

BaLi2Ti6O14

-2 0.5

1.0

1.5

2.0

2.5

following equation [42,43]: ðRTÞ2

DLi ¼

In this equation, R, T, A, n, F and C are constants, and s is the Warburg factor, which can be received according to the following equation: Z0 ¼ Rct þ Re þ sv
1=2

2

b

1 0

0.2, 0.4, 0.8, 1.6, 3 mV s-1

-1

Ba0.9K0.1Li2Ti6O14

-2

3.0

0.5

1.0

0

0.2, 0.4, 0.8, 1.6, 3 mV s-1

Ba0.9Zn0.1Li2Ti6O14 0.5

1.0

1.5

2.0

2.5

Current density (× 10-3 A kg-1)

Current density (× 10-3 A kg-1)

1

-2

1.5

2.0

2.5

3.0

Potential (V)

c

-1

ð2Þ

Here, v is the angular frequency. Fig. 4b depicts the graph of Z’ versus v
1/2 in the low frequency region for BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La). Based on the calculation, it can be seen that the lithium ion diffusion coefficient almost increases after metal doping (Table 2), and the lithium ion diffusion coefficient of Ba0.9La0.1Li2Ti6O14 is the highest one. It indicates that the most effective way to improve the electrochemical property of BaLi2Ti6O14 is La doping.

Potential (V)

2

ð1Þ

2ðAn2 F2 CLi s Þ2

Current density (× 10-3 A kg-1)

Current density (× 10-3 A kg-1)

solution and contact ohmic resistance, Rct represents charge transfer resistance. CPE denotes constant phase element and W equals to Warburg diffusion impedance. Among the simulated parameters, it is obvious that Rct is the dominate element for the kinetic process on these electrodes. As viewed in Fig. 4a and Table 2, it is clear that Ba0.9M0.1Li2Ti6O14 (M = La, Zn, K) display a lower Rct value than that of BaLi2Ti6O14. And W of the metal doping materials also show smaller values than that of pristine material. These results indicate that Ba-site doping may be beneficial to the conductivity of BaLi2Ti6O14. From Table 2, it is obvious that the values of charge transfer resistance and Warburg diffusion impedance for Ba0.9La0.1Li2Ti6O14 are the smallest among all the as-prepared products. This electrochemical result reveals that Ba0.9La0.1Li2Ti6O14 may have better kinetic characteristics than other samples. To investigate the change of ionic conductivity after Ba-site doping, the lithium ion diffusion coefficient (DLi) in BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 can be calculated from the

2

d

1 0

0.2, 0.4, 0.8, 1.6, 3 mV s-1

-1

Ba0.9La0.1Li2Ti6O14

-2

3.0

0.5

Potential (V)

1.0

1.5

2.0

2.5

3.0

Potential (V) 30

ip (× 10-3 A kg-1)

1.8

e

1.2

BaLi2Ti6O14

0.6

Ba0.9K0.1Li2Ti6O14

-0.5

Ba0.9Zn0.1Li2Ti6O14 Ba0.9La0.1Li2Ti6O14

-1.0

C

-1.5 -2.0 0.01

0.02

0.03

υ (V

0.04 0.5

-0.5

s )

0.05

0.0

DLi-Cathodic peak

f

A DLi (× 10-15 cm2 s-1)

2.4

DLi-Anodic peak

20

10

0

Pristine

K

Zn

La

Metal dopant

Fig. 5. Cyclic voltammograms curves of (a) BaLi2Ti6O14, (b) Ba0.9K0.1Li2Ti6O14, (c) Ba0.9Zn0.1Li2Ti6O14 and (d) Ba0.9La0.1Li2Ti6O14 collected at different scan rates (0.2, 0.4, 0.8, 1.6, and 3.0 mV s
1); (e) the relationship between the square root of scan rate and the peak current density; (f) the corresponding lithium ion diffusion coefficient.

H. Yu et al. / Electrochimica Acta 232 (2017) 132–141

Fig. 4c compares the charge-discharge cyclic performance of BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) ranged from 0.5 to 3.0 V at the current density of 100 mAh g
1. The initial charge capacities are 160.6, 161.6, 163.1 and 160.5 mAh g
1 for Ba0.9La0.1Li2Ti6O14, Ba0.9Zn0.1Li2Ti6O14, Ba0.9K0.1Li2Ti6O14 and BaLi2Ti6O14, respectively. After 100 cycles, Ba0.9La0.1Li2Ti6O14 can also show a reversible charge capacity of 151.3 mAh g
1 with high capacity retention of 94.21%, which is the highest charge capacity among four samples (146 mAh g
1 for Ba0.9Zn0.1Li2Ti6O14, 133.3 mAh g
1 for Ba0.9K0.1Li2Ti6O14 and 128.5 mAh g
1 for BaLi2Ti6O14). Clearly, Ba0.9La0.1Li2Ti6O14 electrode shows excellent cyclic performance. Rate performance is also carried out to investigate the evolutions of lithium storage capability of BaLi2Ti6O14 before and after Ba-site doping. Fig. 4d shows the de-intercalation capacities of the electrodes at different current densities ranged from 100 to 700 mAh g
1. It is clear that Ba0.9La0.1Li2Ti6O14 exhibits improved rate capability in comparison with other electrodes, which can deliver a charge capacity of 160.4 mAh g
1 at 100 mA g
1 (2nd cycle), 149.5 mAh g
1 at 200 mA g
1 (24th cycle), 145.9 mAh g
1 at 300 mA g
1 (44th cycle), 144 mAh g
1 at 400 mA g
1 (64th cycle), 142.5 mAh g
1 at 500 mA g
1 (84th cycle), 139.4 mAh g
1 at 600 mA g
1 (104th cycle), 138.2 mAh g
1 at 700 mA g
1 (124th cycle), respectively. In contrast, pristine BaLi2Ti6O14 exhibits the poorest rate capability. Furthermore, the rapid discharge-charge behaviors of all the samples at 500 mA g
1 are also investigated and the results are shown in Fig. 4e. It can be seen that Ba0.9La0.1Li2Ti6O14 electrode can deliver a high reversible capacity of 141 mAh g
1 after 300 cycles with showing only 0.04% capacity fading per cycle. For comparison, the reversible capacities for BaLi2Ti6O14, Ba0.9K0.1Li2Ti6O14, and Ba0.9Zn0.1Li2Ti6O14 are only 101.4, 104.1, and 125.5 mAh g
1 with the capacity retentions of 75.56%, 77.34%, and 83.33%, respectively. These results are in good agreement with cyclic performance (Fig. 4c). Therefore, the electrochemical properties of BaLi2Ti6O14 can be effectively enhanced by Ba-site doping, and in this work, the best way to improve the sample is La doping. In addition, the apparent chemical diffusion coefficients of lithium ions for BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) are also estimated by cyclic voltammetry experiments. Fig. 5a–5d presents the CV plots of BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) collected at different scan rates of 0.2, 0.4, 0.8, 1.6, and 3.0 mV s
1 between 0.5 and 3.0 V. With the increase of the scan rates, the peak currents (Ip) increase gradually. As seen from Fig. 5e and Fig. S3, the peak currents (Ip) display a linear relationship with

137

the square root of scanning rate (v0.5). This phenomenon indicates that the lithiation/delithiation reaction rate is diffusion-controlled. Therefore, the Randles-Sevchik equation for a semi-infinite diffusion of Li+ into BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) anodes can be used [44]. 0:5 Ip ¼ 0:4463F1:5 R
0:5 n1:5 T
1:5 ACLiþ D0:5 Liþ y

ð3Þ

The nomenclatures of the Randles-Sevchik equation are listed in the Supporting Information. Based on the Randles-Sevchik equation and the slope obtained from Fig. 5e, the lithium ion diffusion coefficients of BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) are calculated. The results are shown in Fig. 5f and Table S2. The lithium ion diffusion coefficient of Ba0.9La0.1Li2Ti6O14 is still the highest one. Compared with the data as shown in Fig. 4 and Table 2, the lithium ion diffusion coefficients obtained from cyclic voltammetry are close to the chemical diffusion coefficients calculated from EIS. Meanwhile, the chemical diffusion coefficients of lithium ions in BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) samples can also be confirmed by the galvanostatic intermittent titration (GITT) technique. Fig. S4a–S4d displays the GITT curves of BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) samples during the 4th cycles. Furthermore, the value of lithium ion diffusion coefficient for each sample is determined by applying the Fick’s second law. Hence, the lithium ion diffusion coefficient can be obtained by a simplified equation [44,45]: DLiþ ¼

4 mB Vm 2 DEs Þ ð ð Þ pt Mg A DEt

ðt 

L2 Þ DLiþ

ð4Þ

The nomenclatures of equation (4) are listed in the Supporting Information. On the base of equation (4), the lithium ion diffusion coefficients of BaLi2Ti6O14 and Ba0.9M0.1Li2Ti6O14 (M = K, Zn, La) from GITT are plotted in Fig. S4e. These results further validate the results obtained from the CV and EIS measurements. The insertion/extraction behavior of Ba0.9La0.1Li2Ti6O14 is further investigated by homemade in-situ XRD method. Here, an in-situ XRD observation is used to understand the phase transition during the first discharge/charge cycle and the results are shown in Fig. 6. Contour plot and wireframe of peak intensity versus 2u present the structural evolution process of Ba0.9La0.1Li2Ti6O14 at 90 mA g
1 (0.5C). It can be observed that the featured reflections appear at 18.96 , 20.43 , 25.49 , 27.92 , 28.80 , 30.042 , 39.48 , 43.70 and 44.95 in the in-situ XRD observation before discharging, corresponding to the (220), (221), (131), (023), (132), (223), (441), (800) and (044) planes of Li2BaTi6O14. The structural models

Fig. 6. Contour graphs of peak intensity and Bragg position for Ba0.9La0.1Li2Ti6O14 during in-situ XRD observation in the 2u range of (a, b) 17.6
21, (c, d) 25.1-30.5 , (e, f) 38.741.3 and (g, h) 42-45.5 .

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H. Yu et al. / Electrochimica Acta 232 (2017) 132–141

Fig. 7. The structural models for different hkl planes in Ba0.9La0.1Li2Ti6O14. (a) (220); (b) (221); (c) (131); (d) (023); (e) (132); (f) (223); (g) (441); (h) (800) and (i) (044).

for these planes are displayed in Fig. 7. These peaks are sensitive to the Li-storage process. In addition, another three reflections locating at 38.67, 41.32 and 44.13 are assigned to the beryllium oxide (JCPDS No. 78-1557) film on metallic Be X-ray transmission window in the in-situ cell. In the initial discharge process, the (220), (132), (800) and (044) reflections gradually shift to lower angle with the decrease of relative intensity. It should be noted that the (800) reflection migrates back to higher angle with a further discharge to 0.5 V. Although the (441) reflection also shifts to lower angle, its relative intensity gradually increases during the in-situ XRD cell discharged to 0.5 V. Besides, it can be observed that the (221), (131), (023) and (223) reflections disappear along with the process of lithiation. During the charge process, all the reflections return to the original state without obvious changes. It indicates that Ba0.9La0.1Li2Ti6O14 has stable structural and electrochemical reversibility. By processing the in-situ XRD patterns (Fig. S5), the evolution of the lattice parameters of Ba0.9La0.1Li2Ti6O14 can be calculated and received. Fig. 8 shows the lattice parameter evolution during the first cycle in the in-situ XRD cell and the lithium storage models for Ba0.9La0.1Li2Ti6O14 at different lithiated and delithiated states. As can be viewed from Fig. 8a, it is clear that the trend of lattice parameter retains a good symmetry during the whole dischargecharge cycle and the maximum expansion of the lattice volume

from 2156.88 to 2197.49 Å3 occurs after a lithiation process to 0.5 V. As a result, the overall change of lattice volume for Ba0.9La0.1Li2Ti6O14 is restricted to 1.9%. This result proves the good structural stability of Ba0.9La0.1Li2Ti6O14 during the discharge and charge process. This is also in accordance with the structural change of previous reported SrLi2Ti6O14 [31]. On the basis of the result of in-situ XRD investigation, lithium storage mechanism in Ba0.9La0.1Li2Ti6O14 can also be evaluated. For the pristine Ba0.9La0.1Li2Ti6O14, the available vacant sites suitable for accommodating lithium ions are the 4a, 4b, 8c and 8f positions in the structure [15]. When the in-situ XRD cell is discharged to 0.5 V, the available vacant sites for Ba0.9La0.1Li2Ti6O14 are fully occupied by lithium ions as exhibited in Fig. 8b. The process of intercalation/de-intercalation occurs in three regions (Fig. 8). In the first region, lithium ions that take the hexahedral 8c position are beginning to migrate upon delithiation. It takes approximately half an hour to deliver a capacity of about 40.3 mAh g
1, corresponding to one lithium ion per formula storage at 8c position. After Ba0.9La0.1Li2Ti6O14 is recharged to 1.5 V (Fig. 8c), the lithium ions at 4b and 4a sites are totally extracted at the second region. In this period, the 4a and 4b sites hold about one lithium ion per unit. With a full charge process to 3.0 V, about two lithium ions per formula are removed from 8f site. The detailed atom arrangements in Ba0.9La0.1Li2Ti6O14 at different lithiated/

H. Yu et al. / Electrochimica Acta 232 (2017) 132–141

a

Charge

Discharge

139

Potentiostatic charge

a (Å)

16.640 16.575 16.510

11.4 11.3

b (Å)

11.5

11.2 c (Å)

11.592 11.550 11.508

2180

Potential (V)

2160

Volume (Å3)

2200

3 2 1 0

39

78

117

156

195

234

273

312

Time (min) Fig. 8. (a) Lattice parameter evolution during in-situ observation and (b–e) the lithium storage models for Ba0.9La0.1Li2Ti6O14 at different lithiated and delithiated states.

Fig. 9. Schematic presentation of the relationship between (800) crystal plane and lattice parameter during charge process.

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H. Yu et al. / Electrochimica Acta 232 (2017) 132–141

delithiated states are also presented in Fig. S6. This process is quite different from that of previous reported Na2Li2Ti6O14 [14,21,27], in which all the 8f sites are fully taken by sodium atoms (Fig. 9). To confirm the relationship between lattice parameter and featured reflection, Fig. 8 depicts the variation of crystal structure upon recharging. In Fig. 8a, the (800) plane is highlighted by the blue plane, which is perpendicular to the a-axis (To make it easier to understand, all the atoms in the crystallite lattice have been removed, leaving only the insertion of lithium ions as shown in Fig. 8b). Thus, the change in the lattice parameter a during the recharging process can lead to the migration of the (800) plane. However, the changes of the lattice parameters b and c cannot induce the migration of the (800) plane due to its orthorhombic structure (a = b = g = 90 ). When lithium ions are extracted from the hexahedral 8c sites in the first region, the a-axis shrinks slightly, as shown in Fig. 8a, which results in the shifting of (800) plane to higher angle as shown in Fig. S7. After the hexahedral 8c sites are vacant, lithium ions begin to be extracted from the 4b and 4a sites. In this region, the a-axis expands obviously upon delithiation, which induces the moving of the (800) plane to lower angle. In the third region, the a-axis re-shrinks again with lithium ions extracting from the 8f sites. This makes the (800) plane migrate to the initial position. Therefore, the variation of lattice parameter is consistent with the shifting of featured reflections as shown in in-situ XRD observation. It further proves the structural stability and reversibility of Ba0.9La0.1Li2Ti6O14 as lithium host anode in rechargeable batteries. 4. Conclusion In summary, we successfully synthesize K, Zn and La-doped BaLi2Ti6O14 via a high temperature solid-state way in this report. As lithium storage anode, M-doped BaLi2Ti6O14 (M = K, Zn, La) show a higher reversible capacity, better rate property and superior cyclability than the pristine BaLi2Ti6O14. Compared with other samples, Ba0.9La0.1Li2Ti6O14 displays the highest lithium-ion diffusion coefficient and the lowest electrochemical resistance. As a result, Ba0.9La0.1Li2Ti6O14 presents the best lithium storage capability. It delivers an initial charge capacity of 160.6 mAh g
1 at a current density of 100 mA g
1. After 100 cycles, it displays a reversible capacity of 151.3 mAh g
1 with capacity retention of 94.21%. And the rate performance of Ba0.9La0.1Li2Ti6O14 also exhibits a high capacity of 138.2 mAh g
1 even at the current density of 700 mA g
1. In addition, the outstanding electrochemical property of Ba0.9La0.1Li2Ti6O14 is proved by in-situ XRD technique, which shows that Ba0.9La0.1Li2Ti6O14 is an advanced material with high structural reversibility for lithium storage. Acknowledgement This work is sponsored by National Natural Science Foundation of China (U1632114), Ningbo Key Innovation Team (2014B81005), Ningbo Natural Science Foundation (2016A610068) and K.C. Wong Magna Fund in Ningbo University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.02.134. References [1] H. Nishide, K. Oyaizu, Toward flexible batteries, Science 319 (2008) 737–738. [2] J. Alper, The battery: not yet a terminal case, Science 296 (2002) 1224–1226.

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