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A Robust Strategy for Crafting Li5Cr7Ti6O25@CeO2 Composites as High-Performance Anode Material for Lithium-Ion Battery Jie Mei, Ting-Feng Yi, Xin-Yuan Li, Yan-Rong Zhu, Ying Xie, and Chao-Feng Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04457 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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ACS Applied Materials & Interfaces

A Robust Strategy for Crafting Li5Cr7Ti6O25@CeO2 Composites as High-Performance Anode Material for Lithium-Ion Battery Jie Mei, a Ting-Feng Yi, a,* Xin-Yuan Li, a Yan-Rong Zhu, a Ying Xie,b,* Chao-Feng Zhang c,*

a

School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan,

Anhui 243002, People’s Republic of China b

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of

Chemistry and Materials Science, Heilongjiang University, Harbin 150080, People’s Republic of China c

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, Anhui

230009, People’s Republic of China

1

ACS Paragon Plus Environment


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ABSTRACT

A facile strategy was developed to prepare Li5Cr7Ti6O25@CeO2 composites as high-performance anode material. XRD and Rietveld refinement results show that the CeO2 coating does not alter the structure of Li5Cr7Ti6O25, but increases the lattice parameter. SEM indicates that all samples have similar morphologies with a homogeneous particle distribution in the range of 100–500 nm. EDS mapping and HRTEM prove that CeO2 layer is successfully formed a coating layer on a surface of Li5Cr7Ti6O25 particles, and supply a good conductive connection between the Li5Cr7Ti6O25

particles.

The

electrochemical

characterization

reveals

that

Li5Cr7Ti6O25@CeO2 (3 wt %) electrode shows the highest reversibility of the insertion and deinsertion behavior of Li ion, the smallest electrochemical polarization, the best lithium-ion mobility among all electrodes, and a better electrochemical activity than the pristine one. Therefore, Li5Cr7Ti6O25@CeO2 (3 wt %) electrode indicates the highest delithiation and lithiation capacities at each rate. At 5 C charge-discharge rate, the pristine Li5Cr7Ti6O25 only delivers an initial delithiation capacity of about 94.7 mAh g-1, and the delithiation capacity merely achieves 87.4 mAh g-1 even after 100 cycles. However, Li5Cr7Ti6O25@CeO2 (3 wt %) delivers an initial delithiation capacity of 107.5 mAh·g-1, and the delithiation capacity also reaches 100.5 mAh g-1 even after 100 cycles. The cerium dioxide modification is a direct and efficient approach to improve the delithiation and lithiation capacities and cycle property of Li5Cr7Ti6O25 at large current densities. KEYWORDS Lithium-ion battery, Anode, Li5Cr7Ti6O25, CeO2, Rate performance

2


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1. INTRODUCTION Li-ion battery has been considered as outstanding electrochemical energy storage system as environmentally friendly products applied to electric vehicles or portable devices due to its own merits, such as large operating voltage, excellent cycling performance and high energy density.

1-3

Unfortunately, the safety restricts the

large-scale applications of Li-ion batteries as power battery. The safety of LIBs often determined by anode materials. The commercial carbon negative materials suffer from the severe safety problems due to the dendritic lithium growth on the carbonous negative electrode surface when large charge current density is required. 4 Developing noncarbonaceous anode materials with high safety has been viewed as one of the most important research trends for LIBs. Spinel lithium titanium oxide (Li4Ti5O12) anode was considered as one hopeful alternative to carbonous anode because of its zero-strain insertion property and high insertion voltage plateau (about 1.55 V, vs. Li0/Li+), which prevents the formation of solid-electrolyte interphase (SEI) film and lithium dendrites.

5,6

Unfortunately, the plow electronic conductivity of Li4Ti5O12

impedes the large-scale applications. The traditional solution is to focus on the doping, 7-12

surface coating, 13-18 or preparing nano materials with novel morphologies. 19-22

The doping and surface modification have been regarded as effective methods to enhance electrochemical properties of Li4Ti5O12. For example, Capsoni et al. 23 found that chromium (Cr) doping improved the conductivity of Li4Ti5O12. Unfortunately, the ions doping often decreases the theoretical capacity of spinel Li4Ti5O12 because of the reduction of active Ti or Li. Therefore, exploring novel Li4Ti5O12-based anode

3



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materials is a necessary work in the field of power lithium-ion battery. In 2000, Ohzuku found a new anode material LiCrTiO4 with a flat potential at 1.5 V, a theoretical capacity of 155 mAh g-1 and higher electronic conductivity than Li4Ti5O12, 24

which has an analogous crystal structure with Li4Ti5O12. Unfortunately, the

LiCrTiO4 anodes synthesized by solid-state method usually shows poor kinetic performance. 25 As we know, the content of lithium in the earth’s crust is only about 0.0065%, but the content of chromium reaches 0.01%. Obviously, the content of chromium is 1.54 times as big as lithium. Hence, it is necessary to develop low-lithium Li4Ti5O12-based anode material. Recently, our group reported a novel Cr, Ti-based compound Li5Cr7Ti6O25 with an acceptable electrochemical performance synthesized by conventional sol-gel method, but the expensive tetrabutyl titanate, acetate and nitrate were used as raw materials.

26

The molar ratio is only 1:2.6

between the Li and transition metal (Cr+Ti) in Li5Cr7Ti6O25, but molar ratio is 1:1.25 between the Li and transition metal (Ti) in Li4Ti5O12. Hence, we can expect that Li5Cr7Ti6O25 has lower cost than the Li4Ti5O12. Based on the one-electron transfer between Ti4+ and Ti3+ ions, the theoretical capacity of Li4Ti5O12 cycled between 3.0 and 0.0 V (vs. Li/Li+) is about 293 mAh g–1. However, according to our previous work, 26

the theoretical capacity of Li5Cr7Ti6O25 cycled between 3.0 and 0.0 V (vs. Li/Li+) is

about 320 mAh g–1 based on the one-electron transfer between Ti4+ and Ti3+, Cr2+ and Cr3+ ions. The reversible rate capacity of Li5Cr7Ti6O25 is still inferior to Li4Ti5O12 prepared by sol-gel method, but the cycling stability of Li5Cr7Ti6O25 is as excellent as the Li4Ti5O12 prepared by soft chemical method. In fact, the inferior reversible rate

4


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capacity is because Li5Cr7Ti6O25 (147 mAh g–1) has low theoretical capacity than that of Li4Ti5O12 (175 mAh g–1) cycled between 3.0 and 1.0 V (vs. Li/Li+). However, the sol-gel method or other soft chemical synthesis methods, such as hydrothermal method, combustion method, spray drying process, etc. also limits the commercial application of Li4Ti5O12 due to the high synthesis cost. The cycling stability and reversible rate capacity of Li4Ti5O12 prepared by solid state method are also bad because of its inherent feature, such as low electronic conductivity. Hence, it can be expected that the cycling stability and reversible rate capacity of modified Li5Cr7Ti6O25 prepared by solid state method with relative low cost are better than Li4Ti5O12 prepared by the same one. Fortunately, our group reported that CeO2 coating enhanced the cycling stability and rate capacity of Li4Ti5O12 anode and LiNi0.5Mn1.5O4 cathode because CeO2 can form a wonderful electrical contact between the oxide and electrode material, and then accelerated the electron transfer from oxide to the supported electrode material. 27,28 Yang and Zhang et al. 29,30 also found that the CeO2 coating can enhance the electrochemical properties of Li4Ti5O12 materials. Hence, the Li5Cr7Ti6O25@CeO2 composites as a promising anode material for LIBs were prepared by simple solid state method with low cost used cheap carbonate (Li2CO3) and oxides (Cr2O3, TiO2) as raw materials in this work. We found that the CeO2-coated Li5Cr7Ti6O25 materials exhibited remarkable improvements in cycling stability, rate capacity conductivity and lithium ion diffusion ability.

2. EXPERIMENTAL 5



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Li5Cr7Ti6O25 powders were compound via a solid-state method with analytically pure reagent. The synthetic route is listed in Figure 1, and the details of the synthetic process is given in the Supporting Information (SI). The physical properties of the pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 materials were characterized by XRD (X-ray diffraction), Rietveld refinements, SEM (scanning electron microscopy), HRTEM (high-resolution transmission electron microscopy), SAED (selected area electron diffraction) and EDS mapping, respectively. The electrochemical properties were evaluated by CV (cyclic voltammetry), EIS (electrochemical impedance spectroscopy) and charge–discharge tests, respectively. The detailed characterizations and assembly of button cell (CR 2025) are described in detail in Supporting Information (SI).

Figure 1. Synthetic route of Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 composites

3. RESULTS AND DISCUSSION From the XRD patterns of Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 samples given in Figure 2a, it can be found that all the sharp diffraction peaks correspond to the standard diffraction peaks of Li5Cr7Ti6O25. Moreover, no obvious impurity peaks are 6


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found form the XRD patterns, indicating that the CeO2 coating does not alter the structure of Li5Cr7Ti6O25. Whereas a few impurity peaks are found form the XRD patterns of CeO2-coated Li5Cr7Ti6O25, as given in Figure 2a, and these can be attributed to CeO2. To further figure out the structure of the CeO2-coated Li5Cr7Ti6O25, Rietveld refinement is carried out using the crystallographic data of spinel as the primary data as given in Figure 2(b-e), and the refinement model is also given in Figure 2f. The Rietveld refinement results can verify that Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 composites have an analogous crystal structure with LiCrTiO4, and a successful synthesis of highly purified Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 is achieved by solid state method. From the refined results, it shows that the lattice parameters of the Li5Cr7Ti6O25@CeO2 (0, 3, 5 and 10 wt %) are 8.3140, 8.3211, 8.3293 and 8.3421 Å, respectively. This reveals that few Ce4+ ions enter the crystal lattice of Li5Cr7Ti6O25, and then increase the lattice parameter. The reason is that Ce4+ ion (0.87 Å) has larger ionic radius than Ti4+ (0.605 Å) and Cr3+ (0.615 Å) ions. Figure 3 shows the SEM images of pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 samples. From the SEM pictures, it can be found that the morphologies of all powders are similar, and all specimens indicate homogeneous particle distribution in the range of 100–400 nm. The high-resolution image of Li5Cr7Ti6O25@CeO2 (5 wt %) powders is given in Figure 4a, and it can be found that the particle size of the sample is about 150 nm. It is reasonable to believe that the influence of grain size on the electrochemical performance of Li5Cr7Ti6O25 could be excluded, and the difference of electrochemical property may be primarily ascribed to the CeO2 coating.

7



Figure 4 shows the EDS mapping result of Li5Cr7Ti6O25@CeO2 (5 wt %) powders. It can be confirmed the existence of Cr, Ti, Ce and O elements in Li5Cr7Ti6O25@CeO2, and a uniform distribution of all elements can be found on the surface of Li5Cr7Ti6O25 after coating. (a) * CeO2

(b)

Li5Cr7Ti6O25(Sim) Li5Cr7Ti6O25(Exp)

Intensity / a.u.

*

Difference Bragg Position

(iv)

*

Intensity / a.u.

*

(iii) (ii)

Li5Cr7Ti6O25

(i) 10

20

30

(c)

40

50

2 Theta / degree

60

70

80

10

20

30

40

50

60

70

80

(d)

Li5Cr7Ti6O25-5% CeO2(Sim)

Li5Cr7Ti6O25-3% CeO2(Exp)

Li5Cr7Ti6O25-5% CeO2(Exp)

Difference

Difference Bragg Position

Intensity / a.u.

Intensity / a.u.

Bragg Position

Li5Cr7Ti6O25 CeO2

Li5Cr7Ti6O25 CeO2

10

20

30

40

50

60

70

80

90

10

2 Theta / degree (e)

90

2 Theta / degree


20

30

40

50

60

70

80

90

2 Theta / degree


(f)

Li5Cr7Ti6O25-10% CeO2(Exp) Difference Bragg Position

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Li5Cr7Ti6O25 CeO2

10

20

30

40

50

60

70

80

90

2 Theta / degree

Figure 2. (a) XRD patterns of all samples (i) Li5Cr7Ti6O25, (ii) Li5Cr7Ti6O25@CeO2 (3 wt %), (iii) Li5Cr7Ti6O25@CeO2 (5 wt %), (iv) Li5Cr7Ti6O25@CeO2 (10 wt %), Rietveld refinements of (b) Li5Cr7Ti6O25, (c) Li5Cr7Ti6O25@CeO2 (3 wt %), (d) Li5Cr7Ti6O25@CeO2 (5 wt %) and (e)

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Li5Cr7Ti6O25@CeO2 (10 wt %), (f) refinement model

Figure 3. SEM images of (a) Li5Cr7Ti6O25, (b) Li5Cr7Ti6O25@CeO2 (3 wt %), (c) Li5Cr7Ti6O25@CeO2 (5 wt %), and (d) Li5Cr7Ti6O25@CeO2 (10 wt %).

The more detailed microstructures of all of the Li5Cr7Ti6O25@CeO2 (5 wt %) sample are further investigated by HRTEM and SAED as presented in Figure 5. It can be found that the Li5Cr7Ti6O25 particles are wrapped by CeO2 coating layer with a thickness of about 2-3 nm (Figure 5a, b). The lattice spacing of 0.4892, 0.2663 and 0.2077 nm in HRTEM images can be indexed to the (111), (311) and (400) planes of Li5Cr7Ti6O25.31 The lattice spacing of 0.1968 nm in the Figure 5a corresponds correspond with the d-spacing of the (220) crystal face of the coated CeO2. 32 The results are in accordance with the XRD results mentioned in Figure 2a and the previous works. Hence, we can confirm that CeO2 layer is successfully formed a

9



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coating on a surface of Li5Cr7Ti6O25 particles, and supply a good conductive connection between the Li5Cr7Ti6O25 particles and CeO2 layer. The SAED pattern of Li5Cr7Ti6O25 sample as shown in Figure 5d shows that the commutative white and dark rings around Li5Cr7Ti6O25 crystals, indicating a variety of diffraction planes, which correspond to the (511), (111) and (311) crystal planes of Li5Cr7Ti6O25 crystal. The result is also in accordance with the XRD characterization. (a)

(b)

(c)

(d)

(e)

(f)

Figure 4. (a) TEM, (b) SEM and (c, d, e, f) elemental mapping images of Li5Cr7Ti6O25@CeO2 (5

10


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wt %) powder (a)

(b)

(c)

(d)

Figure 5. (a, b, c) HR-TEM images and (d) SAED pattern of Li5Cr7Ti6O25@CeO2 (5 wt %) powder

Figure

6a

indicates

the

CV

curves

of

pristine

Li5Cr7Ti6O25

and

Li5Cr7Ti6O25@CeO2 cells at a scan rate of 0.2 mV s-1 in the range of 1.0~2.5 V. A couple of redox peaks between 1.4 and 1.6 V can be found, which corresponds to the reversible transformation between Ti4+ and Ti3+. The difference between oxidation potential and reduction potential

reflects

the

intercalation

and

deintercalation reversibility of Li ions. 33 The ΔE values of Li5Cr7Ti6O25@CeO2 (0, 3, 5 and 10 wt %) are 174, 167, 187 and 175 mV, respectively. Obviously, Li5Cr7Ti6O25@CeO2 (3 wt %) shows the highest reversibility due to the minimal potential difference among all samples.

11



1.599 1.605

0.3

500

0.0 1.43 1.425

-0.3 -0.6

1.42

2100

1.2

1.4

400

300

1400

100

0

1.425

1.6

1.8

2.0

2.2

2.4

0

100

200

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400

500

Z' /

0

700

Potential / V 3000

200

700 0

1.0

(i) (ii) (iii) (iv)

(b)

2800

- Z'' / ohm

Current / mA

0.6

3500

(i) (ii) (iii) (iv)

1.612

- Z'' / 

1.587

(a)

1400

2100

2800

3500

Z' / ohm

(c)

2400

Zre / ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1800

1200

(i) (ii) (iii) (iv)

600

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-0.5

Figure 6. (a) CV curves, (b) EIS patterns and (c) corresponding correlation between Z' and ω-0.5 at the low frequency range of (i) Li5Cr7Ti6O25, (ii) Li5Cr7Ti6O25@CeO2 (3 wt %), (iii) Li5Cr7Ti6O25@CeO2 (5 wt %), and (iv) Li5Cr7Ti6O25@CeO2 (10 wt %).

The EIS patterns of pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 cells before cycling at open circuit potential are shown in Figure 6b. A depressed semicircle at the high frequency range and a straight line at the low frequency range can be found from the Nyquist plots of all samples. The equivalent circuit applied to fit the Nyquist plots is shown in Figure S1, and the fitted values are listed in Table S1. It is obvious that all coated samples have a smaller charge transfer resistance than the pristine Li5Cr7Ti6O25, indicating that CeO2 coating is beneficial for the improvement of the charge

12


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transportation between the Li5Cr7Ti6O25 electrode and electrolyte interface.

34

The

solution resistance caused by the electrolyte can be reflected by Rs, and the Li5Cr7Ti6O25@CeO2 electrodes are apparently smaller than the pristine Li5Cr7Ti6O25. The result reveals that CeO2 modification reduces the solution resistances between Li5Cr7Ti6O25 and electrolyte. An interesting thing is that the line (W) at the low frequency region deviates from 45°. Before the EIS test, the half cells were placed about 24 h in order to make the electrode be wetted by the electrolyte. During this process, the trace sediment on the surface of electrode unavoidably slightly grows at the rest time because of the electrode etching by acidic species (HF) in the electrolyte. Hence, the electrode surface of the half-cell is rough, and then leads to an inhomogeneous ion concentration on the surface of the electrode. Hence, this inhomogeneity of ion concentration results in a deviation of Warburg component. This result is consistent with the results of the reported Li4Ti5O12

10,18,28

and

Li5Cr7Ti6O25.31 The DLi (Li-ion diffusion coefficient) value of electrode material is obtained from the Warburg factor (σ) obtained from the slope of Zre vs ω−0.5 as given in Figure 6c. The calculated equations are as follows: 35-37 Z re  Rct  Rs  



1 2

R 2T 2 DLi  2 4 4 2 2 2 A n F CLi

(1)

(2)

The nomenclature of the above equations is listed in the Supporting Information, and the calculated results are given in Table S1. Li5Cr7Ti6O25@CeO2 (3 and 5 wt %) electrodes have higher lithium ion diffusion coefficient than that of pristine one, 13



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indicating that the former has higher electrochemical activity during cycling. However, the Li5Cr7Ti6O25@CeO2 (10wt %) electrode shows the smaller lithium ion diffusion coefficient than that of pristine one. The reason may be that the thick CeO2 coating layer on the surface of Li5Cr7Ti6O25 impedes the initial migration of lithium ions, and then decreases the initial Li-ion diffusion coefficient. Whereas, the charge transfer resistance is reduced because of the large electronic conductivity of CeO2. These phenomena display results to the reported Li0.33La0.56TiO3 and TiN-coated spinel Li4Ti5O12 anodes. 38,39 Among the four samples, Li5Cr7Ti6O25@CeO2 (3 wt %) has the highest the lithium ion diffusion. From the above findings, the coated amount of CeO2 must be properly optimized to improve Li-ion migration and gain an enhanced rate performance. Figure 7a indicates the first charge/discharge curves of pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 cells at 0.2C rate between 1.0 and 2.5 V. The initial delithiation (lithiation) capacities of Li5Cr7Ti6O25@CeO2 (0, 3, 5 and 10 wt %) cells are 121.3 (148.2), 131.1 (188.3), 93.7 (126.9) and 97.6 (142.8) mAh g-1, respectively. Li5Cr7Ti6O25@CeO2 (3 wt %) shows the highest charge (delithiation) and discharge (lithiation) capacity among all samples. The voltage differences (ΔE) between delithiation plateau and lithiation plateau of all cells reflect the electrode polarization. 40

As given in Figure 7b, it can be found the voltage differences (ΔE) of

Li5Cr7Ti6O25@CeO2 (0, 3, 5 and 10 wt %) cells are about 21.7, 11.4, 14.8 and 12.8 mV, respectively. Obviously, the CeO2 coated Li5Cr7Ti6O25 electrodes show lower voltage difference than the pristine one, indicating small electrode polarization.

14


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Interestingly, Li5Cr7Ti6O25@CeO2 (3 wt %) shows the smallest voltage difference, and then indicates the lowest electrode polarization among all samples. This result is uniform with the consequences of CVs. 2.8

(a)

(i) (ii) (iii) (iv

(b)

(i) (ii) (iii) (iv

1.52

Voltage / V

Voltage / V

2.4

2.0

1.6

E4E1E3 E2

1.50

1.48

1.2

0

30

60

90

120

150

180

0

30

-1 Specific capacity / mAh g

-1

-1

1C

0.2 C 5C

(i) (ii) (iii) (iv)

40

20

40

Discharge Capacity / mAh g

0.5 C

0.2 C

0

120

150

(d)

3C

0

90

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60

-1 Specific capacity / mAh g

(c)

Charge Capacity / mAh g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

80

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0.2 C

0.5 C

1C

120

5C

80

(i) (ii) (iii) (iv)

40 0

0.2 C

3C

0

20

Cycle number

40

60

80

100

Cycle number

Figure 7. (a) Initial delithiation (charge) and lithiation (discharge) curves at 0.2 C rates, (b) voltage plateau difference (ΔE) between delithiation and lithiation from Figure 7a, (c) rate performance of delithiation

and

(d)

rate

performance

of

lithiation

for

pristine

Li5Cr7Ti6O25

and

Li5Cr7Ti6O25@CeO2 electrodes for (i) 0 CeO2, (ii) 3 wt.% CeO2, (iii) 5 wt.% CeO2, and (iv) 10 wt.% CeO2. Both the charge rate and discharge rate are completely the same.

Figure 7 c and d give the rate performance curves of charge (delithiation) and

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discharge (lithiation) capacity for pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 cells at different current densities between 1.0 and 2.5 V. Both the charge rate and discharge rate are completely the same. It is obvious that Li5Cr7Ti6O25@CeO2 (3 wt %) reveals the best rate performance of reversible charge (delithiation) capacities (131.1, 129.2, 126, 116, 104.5 mAh·g-1 at 0.2, 0.5, 1, 3 and 5 C, respectively) among all samples. When the current density returns to 0.2 C, the reversible delithiation capacity also maintains 126.2 mAh·g-1, and shows remarkable restorability. Nevertheless, the pure Li5Cr7Ti6O25 delivers comparatively low rate performance of reversible charge (delithiation) capacities (121.3, 109.6, 101.2, 85.3 and 78.1 mAh g-1 at 0.2, 0.5, 1, 3 C and 5 C, respectively). The pristine Li5Cr7Ti6O25 also shows remarkable restorability, but the reversible delithiation capacity is only 126.2 mAh·g-1 when the current density returns to 0.2 C. The interesting phenomenon is that not all coated samples show a high reversible capacity. It can be found that Li5Cr7Ti6O25@CeO2 (5 and 10 wt %) anodes have lower discharge/charge capacities than pristine Li5Cr7Ti6O25. It is likely to because CeO2 does not have electrochemical activity, and it can reduce the discharge/charge capacity of the material. Table S2 gives the comparison of discharge capacity between Li5Cr7Ti6O25@CeO2 (3 wt %) in this work and previously reported Li4Ti5O12 electrodes. From Table S2, although Li5Cr7Ti6O25 has lower theoretical capacity than that of Li4Ti5O12 cycled between 3.0 and 1.0 V (vs. Li/Li+), it can be noted that such excellent rate capacity of Li5Cr7Ti6O25 with less CeO2 content (3 wt%) is superior to most reported data of Li4Ti5O12, 41-45 F-doped Li4Ti5O12, 46 Mg-doped Li4Ti5O12,

46

Fe-doped Li4Ti5O12,

47

graphene-coated Li4Ti5O12,

16


48

carbon-coated

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Li4Ti5O12,

49

and Li2TiO3-coated Li4Ti5O12

pristine Li4Ti5O12 prepared by sol-gel,

51

50

prepared by solid-state method and

glycine-nitrate auto-combustion,

52

and

high-energy ball milling assisted solid-state 53 methods in the literatures.

120 90 60

(i) (ii) (iii) (iv)

30 0

0

20

40

60

80

100

Cycle number

75 (i) (ii) (iii) (iv)

50 25

0

20

40

60

80

Discharge capacity / mAh g-1

5C charge-discharge rate

100

0


150 120 90

(i) (ii) (iii) (iv)

60 30 0

0

20

40

60

80

100

Cycle number

(c)

125

(b)

180


Discharge capacity / mAh g-1

Charge Capacity / mAh g-1

150 (a)

Charge capacity / mAh g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

(d)

125

5 C charge-discharge rate

100 75 (i) (ii) (iii) (iv)

50 25 0

0

20

40

60

80

100

Cycle number

Cycle number

Figure 8. Cycling performance of pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 electrodes (a) delithiation at 1 C rate, (b) lithiation at 1 C charge-discharge rate, (c) delithiation at 5 C rate and (d) lithiation at 5 C rate. (i) 0 CeO2, (ii) 3 wt.% CeO2, (iii) 5 wt.% CeO2, and (iv) 10 wt.% CeO2. Both the charge rate and discharge rate are completely the same.

As power battery electrode material for HEVs or EVs, the high rate performance, especially the charge-discharge performance at high current density, is vital for the success of any commercial application.

54

Therefore, it is necessary to

17



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focus on the charge-discharge property of Li5Cr7Ti6O25@CeO2 materials at high current density. Figure 8 gives the cycle property of pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 cells at 1 C and 5 C rates between 1.0 and 2.5 V. Both the charge rate and discharge rate are completely the same. At 1 C rate, Li5Cr7Ti6O25@CeO2 (3 wt %) delivers an initial charge (delithiation) capacity of 118 mAh·g-1, and the capacity also reaches 113 mAh·g-1 even after 100 cycles. However, the pristine Li5Cr7Ti6O25 only delivers an initial charge (delithiation) capacity of about 101.7 mAh·g-1, and the capacity only maintains 99.5 mAh g-1 even after 100 cycles. The capacity difference between Li5Cr7Ti6O25@CeO2 (3 wt %) and pristine Li5Cr7Ti6O25 becomes to get bigger when the charge-discharge rates is further bigger. At 5 C charge-discharge rate, the pristine Li5Cr7Ti6O25 only delivers an initial charge (delithiation) capacity of about 94.7 mAh g-1, and the capacity only maintains 87.4 mAh g-1 even after 100 cycles. However, Li5Cr7Ti6O25@CeO2 (3 wt %) delivers an initial delithiation capacity of 107.5 mAh g-1, and the capacity also reaches 100.5 mAh g-1 even at the 100th cycles. Compared with the pristine Li5Cr7Ti6O25, Li5Cr7Ti6O25@CeO2 (3 wt %) delivers a higher discharge (lithiation) capacities at 1 C and 5 C rate. However, the Li5Cr7Ti6O25@CeO2 (5 and 10 wt %) electrodes deliver lower charge (delithiation) and discharge (lithiation) capacities than those of the pristine one. Based on the above electrochemical results, Li5Cr7Ti6O25@CeO2 (3 wt %) electrode shows the smallest potential difference (Figure 6a), the lowest voltage plateau difference (Figure 7b), and the largest Li-ion diffusion coefficient among all electrodes, and a smaller charge transfer resistance than the pristine one. These results

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reveal that Li5Cr7Ti6O25@CeO2 (3 wt %) electrode shows the highest reversibility of the intercalation/deintercalation behavior of Li ion, the smallest electrochemical polarization, the best lithium-ion mobility among all electrodes, and a better electrochemical activity than the pristine one. Therefore, Li5Cr7Ti6O25@CeO2 (3 wt %) electrode indicates the highest delithiation and lithiation capacities at each rate. In addition, the Li5Cr7Ti6O25@CeO2 (3 wt %) electrode shows a higher first cycle columbic efficiency (85.05 %) than the pristine one (82.93 %) at 5 C rate, and the coulombic efficiency of all samples is close to 100% after several cycles (Figure S2 of SI). The coulombic efficiency is charge capacity divided by the discharge capacity. It has been reported that the initial efficiency greater than 85% in a half cell is one of the most significant criteria for the industrially acceptable full battery design. 55 As shown in Figure S2, the initial efficiencies of Li5Cr7Ti6O25@CeO2 (0, 3, 5 and 10 wt %) are 83%, 85%, 79% and 87%, respectively. The initial coulombic efficiencies of pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 are close to 85%, and then indicate an industrially acceptable coulombic efficiency even as full battery.

Li5Cr7Ti6O25 based

anodes have the relatively low initial coulombic efficiency (less than 90%), and the reason is as follows: (1) The trace H2O from the commercial electrolyte could be adsorbed on the surface of electrode, and O2 may be generated due to the electrolytic reaction during the first Li+ intercalation and deintercalation process. Then, a certain amount of lithium was irreversibly consumed; 56 (2) the surface defects or voids are also change to irreversible insertion sites of lithium ions because of the sub-micron grade single particle size (Figure 3) and ball-milling during the synthesis process.

19


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Furthermore, the initial coulombic efficiency of Li5Cr7Ti6O25@CeO2 and the pristine one is obviously higher than that of Li4Ti5O12 with unique clew-like hierarchical structure composed of ultrafine nanowire units (LTO-NWs, 44%)58, Li4Ti5O12-TiO2 nanobelts (55.1%),

57

hierarchical structure Li4Ti5O12-TiO2 (59.3%),

59

the hard

carbon anodes (about 60%) and other novel anodes, such as Si nanowires (79%) ZnO/Ni/C composite hollow microspheres (61.4%) Si/C anodes (81.5%)

62

61

60,

, and Micro-sized nano-porous

. After several cycles, the coulombic efficiencies of all

Li5Cr7Ti6O25 based anodes are close to 100%. Like spinel Li4Ti5O12, Li5Cr7Ti6O25 based anodes have excellent electrochemical properties and simple synthesis routes, especially lower synthesis cost than Li4Ti5O12. Hence, the ample evidence presented enables us to reasonably conclude that Li5Cr7Ti6O25 based anodes also show an industrially acceptable coulombic efficiency even as full battery compared with the modified Li4Ti5O12 anodes. To fully discuss the effect of CeO2 on the electrochemical kinetic of Li5Cr7Ti6O25, EIS of pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 (3 wt %) at various states of charge (SOC) is compared. Figure S3 a and b of SI give EIS patterns and fitted Nyquist plots of Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 (3 wt %) after 20 cycles at 0.2 C rate charged to 2.5 V. Figure S5 a and b of SI give EIS patterns and fitted Nyquist plots of Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 (3 wt %) after 20 cycles at 0.2 C rate discharged to 1 V. As reported by He et al. 63 the SEI film can be formed on the surface of Li4Ti5O12 anode after a few cycles between 1 V and 3 V because of the

interface

reaction

between

the

electrolyte

20


and

Li4Ti5O12

anode.

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So, considering the interface reaction between the electrolyte and Li5Cr7Ti6O25, the equivalent circuit applied to fit the Nyquist plots is shown in Figure S4 of SI, and the fitted values are listed in Table S3 of SI. The Warburg factor (σ) obtained from the slope of Zre vs ω−0.5 as given in Figure S3c and Figure S5c of SI used to calculate the Li+ diffusion coefficients, and the calculated results are listed in Table S3 of SI. From Table S3 of SI, it can be found that the progressive penetration process of the electrolyte and the consecutive expansion and shrinkage of crystal cell reduce the charge transfer resistance of both samples after cycling. In addition, the charge transfer resistances at discharge state (fully lithiation) are less than at charge state (fully delithiation). The reason may be that content of Ti3+ ions of Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 (3 wt %) at lithiation state is higher than at delithiation state. The higher content of Ti3+ improves electron number, then increase electronic conductivity.64 However, the Li+ diffusion coefficients at discharge state (full lithiation) are less than at charge state (full delithiation). The reason may be that the reduction of potential difference and the lithium intercalation at lithiation state block the migration of lithium ion. It is obvious that Li5Cr7Ti6O25@CeO2 (3 wt %) sample have a smaller charge transfer resistance and higher lithium ion diffusion coefficient than the pristine Li5Cr7Ti6O25 after 20 cycles at full lithiation and delithiation states. These results reveal that the appropriate CeO2 coating improves the electrochemical activity of Li5Cr7Ti6O25 during cycling. Based on the preparation process and HRTEM test of Li5Cr7Ti6O25@CeO2 composites, it can be determined that Li5Cr7Ti6O25 powder is tightly wrapped by

21



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cerium dioxide, and a Li5Cr7Ti6O25-CeO2 phase interface is formed. Hence, an interface model between Li5Cr7Ti6O25 and CeO2 can be constructed in Figure 9. Based on the crystal symmetry and the lattice parameters of both compounds, an excellent surfaces match between Li5Cr7Ti6O25 and CeO2 phase can be found, which are both along the [001] orientation. The surface vectors of Li5Cr7Ti6O25 is decrease to

2 b and 2

2 a, 2

2 c (a=b=c=8.3140 Å), and the lattice parameter of CeO2 is 5.41 Å. 2

Hence, the lattice mismatch between Li5Cr7Ti6O25 and CeO2 surfaces is merely 8%. Hence, it can be further proved that a stable and solid phase interface between Li5Cr7Ti6O25 and CeO2 can be formed, which supplies more places to store electrolyte and undergo the electrochemical reaction. It was reported that the internal adsorption of ions on the CeO2 surface could lead to the space-charge effect, 65 resulting in the incremental positive ion vacancy concentration at the CeO2 interface, and then forms a fine conductive interfacial layer between the Li5Cr7Ti6O25 and CeO2. The modification of CeO2 enhances the transmission efficiency of electrons and Li ions because of the wonderful electrical contact between Li5Cr7Ti6O25 and CeO2. Hence, the lithium-ion mobility, electrochemical activity and reversibility of the intercalation/deintercalation behavior of Li ion is enhanced, and the electrochemical polarization is reduced, and then the delithiation and lithiation capacities at high current densities and cycling stability are obviously improved due to the modification of CeO2. From the result discussed above, it can be deduced that the cerium dioxide modification is a direct and efficient approach to improve the delithiation and

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lithiation capacities and cycling performance of Li5Cr7Ti6O25 at high current densities.

Figure 9 Interface model between Li5Cr7Ti6O25 (LCTO) and CeO2.

4. CONCLUSIONS In this work, Li5Cr7Ti6O25@CeO2 (0, 3%, 5% and 10 wt %) composites were successfully prepared via a simple solid-state method. The CeO2 coating does not alter the structure of Li5Cr7Ti6O25, and the morphologies of all powders are similar with a homogeneous particle distribution in the range of 100–500 nm. The coated CeO2 layer supply a good conductive connection between the Li5Cr7Ti6O25 particles, and reduces the charge transfer resistance and electrode polarization of Li5Cr7Ti6O25. Moreover,

the

introduction

of

CeO2

improves

reversibility

of

the

intercalation/deintercalation behavior of Li ion, lithium ion diffusion coefficient and electrochemical activity of Li5Cr7Ti6O25. Li5Cr7Ti6O25@CeO2 (3 wt %) shows the highest delithiation (131.1, 129.2, 126, 116, 104.5 mAh·g-1 at 0.2, 0.5, 1, 3 and 5 C, respectively) and lithiation capacity among all samples. When the current density

23



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returns to 0.2 C, the reversible delithiation capacity also maintains 126.2 mAh·g-1, and shows remarkable restorability. The capacity difference between Li5Cr7Ti6O25@CeO2 (3 wt %) and pristine Li5Cr7Ti6O25 becomes to get bigger when the charge-discharge rates is further bigger. The cerium dioxide modification is a direct and efficient approach to improve the delithiation and lithiation capacities and cycling performance of Li5Cr7Ti6O25 at high current densities. ■ ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Material preparation of Li5Cr7Ti6O25 and CeO2-coated Li5Cr7Ti6O25, battery preparation, nomenclature for equations (1) and (2), coulombic efficiency curves of pristine Li5Cr7Ti6O25 and Li5Cr7Ti6O25@CeO2 electrodes at 5 C rate, equivalent circuit diagram used to fit EIS and fitted results of EIS. (PDF)

■AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Dr. Ting-Feng Yi); [email protected] (Dr. Ying Xie); [email protected] (Dr. Chao-Feng Zhang). Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation

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of China (nos. 51404002 and 51274002), Anhui Provincial Natural Science Foundation (no. 1508085MB25), and Anhui Provincial Science Fund for Excellent Young Scholars (no. gxyqZD2016066).

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J.

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of

Carbon

Microcapsules

Containing

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Robust Strategy for Crafting Li5Cr7Ti6O25@CeO2 ... - ACS Publications

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