Rapid Lithiation and Delithiation Property of V-Doped Li2ZnTi3O8 as

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Rapid Lithiation and De-lithiation property of V-doped Li2ZnTi3O8 as Anode Material for Lithium-ion Battery Ting-Feng Yi, Jin-Zhu Wu, Jing Yuan, Yanrong Zhu, and Pengfei Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00505 • Publication Date (Web): 16 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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ACS Sustainable Chemistry & Engineering

Rapid Lithiation and De-lithiation property of V-doped Li2ZnTi3O8 as Anode Material for Lithium-ion Battery Ting-Feng Yi *, Jin-Zhu Wu, Jing Yuan, Yan-Rong Zhu, Peng-Fei Wang School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan, Anhui 243002, PR China



Corresponding author: Ting-Feng Yi

Telephone number: 86-555-2311807 Fax number: 86-555-2311552 E-mail: [email protected] (Dr. Ting-Feng Yi)

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ACS Paragon Plus Environment


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Abstract Li2-xVxZnTi3O8 (x=0, 0.05, 0.1 and 0.15) anode (negative electrode) materials were successfully synthesized by a facile solid-state reaction method, and the structure, morphology and electrochemical property of the Li2-xVxZnTi3O8 materials were investigated. Li2-xVxZnTi3O8 (x=0 and 0.05) samples show the pure phase structure with P4332 space group, but several rutile TiO2 peaks are founded in Li2-xVxZnTi3O8 (x≥0.1). V-doping does not change the electrochemical reaction mechanism and destroy the structure of Li2ZnTi3O8. All samples show a size distribution under 1 μm, but the V-doped powders show a narrower particle size distribution and less agglomeration than those of pristine Li2ZnTi3 O8. Electrochemical kinetics results reveal that the V-doped Li2ZnTi3O8 samples display higher reversibility, larger lithium diffusion coefficients and lower charge transfer resistances than those of pristine Li2 ZnTi3O8. Galvanostatic electrochemical tests show that the Li1.95V0.05ZnTi3O8/Li half cell delivers the highest lithiation capacities at various rates (213.3, 171.2, 132.5, 84.7 mA h g−1 at 0.2 C, 1 C, 2 C and 5 C rates, respectively), whereas the Li2 ZnTi3O8/Li half cell delivers much less lithiation capacities at all rates (184.5, 129.5, 107.3 and 24 mA h g−1 at 0.2 C, 1 C, 2 C and 5 C rates, respectively). The simple preparation process, low preparation cost, excellent cycling stability and wide voltage range make the Li1.95V0.05ZnTi3 O8 exhibit a potential commercial application in the future. Keywords:

Lithium-ion

battery;

Anode;

Vanadium-doping;

Li2 ZnTi3O8

2


Rate

capacity;

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■ INTRODUCTION Lithium-ion batteries (LIBs) were widely used as power batteries for electric vehicles (EVs), hybrid electric vehicles (HEVs) and plug in hybrid electric vehicles (PHEVs) due to lot of advantages, such as small volume, light weight, high voltage and energy density. 1 The properties of anode material have a great effect on the power density and safety of a battery. Carbonaceous material was the first to be used as anode of large-scale LIBs. However, it shows a low voltage of about 0.1 V (vs. Li0/Li+).2 Hence, the formation of highly reactive metallic lithium dendrites often results in the short circuits when quick charging mode is used. This safety issues limit its use in power battery field. Recently, spinel Li4 Ti5O12 has been discovered as a kind of promising anode materials for LIBs in commercial applications because of the high potential plateau at 1.5 V (vs. Li0/Li+), good thermal stability and structure stability during the lithiation / de-lithiation process. 3-5 Unfortunately, the poor conductivity and lithium migration ability of Li4Ti5O12 has been main obstacles to its application. 6-9

Hence, developing new anode materials used for LIBs have been attracted by

researchers because of increasing demands for energy density and power density of LIBs. More recently, spinel structure Li2ZnTi3O8 (P4332 space group) material has been considered as a promising substitute material for Li4Ti5O12 since the first used as anode of LIBs.

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Li2 ZnTi3O8 can be described as the spinel join of

Li4Ti5O12–Zn2TiO4,

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which exhibits good cycling stability. Li2 ZnTi3O8 exhibits a

lithiation voltage plateau at about 0.5 V (vs. Li0/Li+), 12 which can avoid the formation of dendritic lithium growing. However, the low conductivity and poor lithium

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diffusion coefficient impede its practical application. Hence, many efforts have been devoted to modify Li2 ZnTi3O8 using an alien ion (such as Ag+,13 Ni2+,14 Cu2+,15 Al3+16) , or by coating a other phases with high conductivity (such as carbon-based materials,

17,18

LiCoO2

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), or reducing the diameter of particles.

20

However,

carbon-based material coating or nanocrystallization technology often reduces the volumetric specific energy of the battery due to the low powder tap density. Doping has been considered as an effective ways to enhance electronic conductivity and improve the intrinsic properties of electrode materials as a matter of fact. From previous work, it can be found that V5+ ion doping is a quite effective to stabilize the structure of Ti-based materials, thus improving the capacity and cycling performance. 21

Fast lithiation and de-lithiation are important requirements for the application in

HEVs or PHEVs.22 Simultaneously, an excellent cycling stability with a fast lithiation and de-lithiation process is beneficial to the commercialization of LIBs as power batteries.

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Moreover, previous reports indicates that V5+ ion doping is an effective

way to increase the electrochemical properties of TiO2 anode, and Li2 FeSiO4 cathodes

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24

LiFePO4 cathode 25

, and so on. However, up to now, no investigation on the

fast lithiation and de-lithiation property of V-doped Li2ZnTi3O8 (Li2-xVxZnTi3 O8). Many routes were used to synthesize Li2 ZnTi3 O8, such as solid-state method, sol-gel method,

15,18

27

and molten-salt synthesis, 28 and so on. Obviously, the synthesis

method mentioned above is complicated costly, and then goes against commercial applications. However, the solid-state synthesis route with simple technique and low synthesis cost shows a great commercial application. In this study, V-doped

4


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Li2 ZnTi3O8 anode materials have been prepared by a simple solid-state method, and the electrodes exhibit significantly increased fast lithiation and de-lithiation performances.

■ EXPERIMENTAL SECTION Material Synthesis. Li2-xVxZnTi3O8 (x=0, 0.05, 0.1 and 0.15) powders were prepared by a solid-state method. Li2CO3, anatase-phase TiO2, V2O5 and Zn(CH3COO)2·2H2O were used as the raw materials at a proper molar ratio, and then mixed and ball-milled. The mixed raw materials were sintered at 800 °C for 6 h in air, and then cooled to room temperature. At last, the cooled powder were ball-milled (alcohol as the medium) about 0.5 h to obtain the final Li2-xVxZnTi3O8 powders. Battery preparation. A CR2025 coin-cell assembly was used to test the electrochemical performance of Li2-xVxZnTi3O8 materials. The slurry includes the Li2-xVxZnTi3O8 powders (80%), super P conductive carbon (10%), and the binder (10 wt% polyvinylidene fluoride, dissolved in N-methyl-2-pyrrolidone). A half LIB was assembled by using the metallic lithium as counter electrode, Li2-xVxZnTi3O8 coated on the Cu foil as the working electrode, and porous polypropylene film (Celgard 2300) as the separator. 1 M LiPF6 in ethylene carbonate/ dimethyl carbonate (1:1 in volume) was used as electrolyte. Each electrode was cut into discs with a diameter of 14 mm Materials characterization and electrochemical tests. The crystal structures of all samples were identified by X-ray diffraction (XRD) with Cu Ka radiation. The morphology and particle size of all samples were observed by using scanning electron microscopy (SEM, SU8000). A Quanta-450 scanning electron microscope (SEM) was

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used to image the materials and to collect EDS maps of as-prepared powders. Raman spectroscopy that performed on a SPEX-1403 Raman Spectrometer at room temperature with Ar+ laser of 488 nm excitation. A CHI 1000C electrochemical workstation was used to carry out cyclic voltammetry (CV) test, and the voltage range is between 0.05 and 3 V, and the scanning rate is 0.5 mV s-1. The electrochemical impedance spectroscopy (EIS) of the cells were carried out in the frequency range from 0.01 Hz to 10 kHzz with an AC amplitude of 5 mV using Princeton P4000 electrochemical working station. The lithiation and de-lithiation tests of the cells were performed on a Land Battery Test System (Wuhan Jinnuo, China) in the potential range between 0.05 and 3.0 V (vs. Li/Li+) at different current densities. The lithiation rate and de-lithiation rate are the same.

■ RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of Li2-xVxZnTi3O8 (x=0, 0.05, 0.1 and 0.15) samples. It can be found that the major diffraction peaks of all materials correspond to the standard diffraction peaks of spinel Li2 ZnTi3O8 with P4332 space group (JCPDS#44-1037). This indicates that the V-doping does not change the spinel structure of Li2ZnTi3O8. With increasing of V content, little weak impuritypeaks assigned to rutile TiO2 are detected, indicating that the amount of vanadium ions should be appropriately introduced to avoid TiO2 impurity. It is logical to conclude that the conductivity of Li2 ZnTi3O8 can be improved by a doping of Li+ site by V5+ ion due to the increased electron concentration.

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Under the condition of oxidation,

the lattice defects may compensate the vanadium for extra charge. Hence, the Li

6


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vacancy can accomplish the charge compensation, and then it can be expected that the V-doped Li2ZnTi3O8 samples have higher conductivity than that of pristine one.

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In addition, it can be found there is an obvious shift of the peaks on the XRD patterns because of the doping of vanadium. To observe the peak position variation clearly, the strongest (311) plane is magnified and is given in Figure 1 (right). It can be seen that the diffraction peak of (311) plane slightly shifts to a higher angle due to the vanadium doping. This reveals that the lattice parameter of Li2-xVxZnTi3O8 (x=0.05, 0.1 and 0.15) is slightly lower than the undoped Li2 ZnTi3O8. The reason may be that Li+ ion (0.76 Å 31) has a larger ionic radius than that of V5+ (0.54Å 31) ion considering the coordination numbers. However, the lattice parameter does not decrease with the increasing the content of V. Interestingly, the lattice parameter of Li2-xVxZnTi3O8 (x=0.1) is slightly higher than that of Li2-xVxZnTi3 O8 (x=0.05, 0.15). When the doped content x≥0.1, the rutile TiO2 is detected. This indicates that the charge compensation does not be only achieved by a lithium vacancy, and some Ti4+ can be reduced to Ti3+ in order to maintain electric neutrality. As we know, the Ti3+ ion (0.67 Å larger ionic radius than that of Ti3+ (0.605 Å

31

31

) has a

) ion. Hence, Li2-xVxZnTi3O8 (x=0.1)

show a increased lattice parameter compared with pristine Li2-xVxZnTi3O8 (x=0.05). However, V5+ ion has the smallest ionic radius after all, and then Li2-xVxZnTi3O8 (x=0.15) show a reduced lattice parameter compared with pristine Li2-xVxZnTi3 O8 (x=0.1). There is an obvious (2 2 0) peak (at 2θ≈30.2°) in Li2-xVxZnTi3O8 as plotted in Figure 1. This can be ascribed to the presence of heavy metal ion (such as Zn2+ and V5+) in tetrahedral (8c) sites of the spinel-type structure.

7


As we know, Zn2+ and Li+


were distributed over tetrahedral (8c) site with occupancy of 0.5 and 0.5 in Li2 ZnTi3O8 compound, respectively. The octahedral (12d) sites are occupied by Ti4+ and oxygen occupy on 8c and 24e sites. In Li2-xVxZnTi3O8 (x=0.05, 0.1, 0.15) compounds, Rietveld refinement result as given in Figure S1 of the Supporting Information (SI) shows that V5+ ions replace some Li+ ions from tetrahedral (8c). As a result of this, the Li+ amount of tetrahedral occupancy is decreased with V5+ substitutions. With increasing V5+ substitution, few V5+ can be observed to occupy octahedral (4b) sites.

* rutileTiO *

*

x=0.15

*

*

x=0.1

(311)

2

Intensity / a.u.

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x=0.05

x=0 15

30

45

60

7535.4 35.6 35.8 36.0

2 / 

2 / 

Figure 1 XRD patterns of the Li2-xVxZnTi3O8 (x=0, 0.05, 0.1, 0.15) powders Figure 2 shows the SEM images of the Li2-xVxZnTi3O8 (x=0, 0.05, 0.1 and 0.15) powders. All samples show a size distribution under 1 μm. It is obvious that the undoepd Li2ZnTi3 O8 has a wide size distribution and an obvious particle agglomeration. However, Li2-xVxZnTi3O8 (x=0.05, 0.1 and 0.15) powders show narrow particle size distribution and an unconspicuous agglomeration. The 8


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phenomenon reveals that V-doping can inhibit the particle agglomeration during high temperature sintering. A good dispersion and less agglomeration may make full contact between Li2-xVxZnTi3O8 powders and electrolyte, and then contribute to the diffusion and transmission of lithium ions.

(a)

(b)

(c)

(d)

Figure 2 SEM images of the Li2-xVxZnTi3O8 powders(a) x=0, (b) x=0.05, (c) x=0.1, (d) x=0.15 In addition, in order to prove the V-doped anode materials homogeneously, EDS mapping of Li2-xVxZnTi3O8 (x=0.05) material has been given in Figure 3. It can be found that the presence of vanadium, zinc, titanium, and oxygen, respectively. It can be seen that not only transition metal elements including zinc and titanium are homogeneously distribution on the surface of the particles, but also vanadium atoms are uniformly distributed in Li2-xVxZnTi3O8 (x=0.05) electrode material. Therefore, it can be concluded that V has been doped in Li2 ZnTi3O8 anode material successfully

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and homogeneously. The EDS profile of Li2-xVxZnTi3O8 (x=0.1) is performed and shown in Figure S2 of the Supporting Information (SI). The atom ratio of V:Zn:Ti:O is 0.1:1.0:3.2:6.7, revealing that a small amount of Ti3+ exists in Li2-xVxZnTi3O8 (x=0.1) in order to maintain electric neutrality. These clearly confirm the presence of vanadium, zinc, titanium, and oxygen, and vanadium has entered the spinel crystal lattice. In this work, the raw material prepared Li2-xVxZnTi3O8 of V source is V2O5, and the synthesis gas atmosphere is air. It is therefore reasonable to infer that the valence state of V is +5 in Li2-xVxZnTi3O8 powders. The results mentioned above can be compared V-doed lithium titanium spinel (Li4Ti4.9V0.1O12).32 (a)

(b)

(c)

(e)

(d)

Figure 3. EDX mapping images of Li2-xVxZnTi3O8 (x=0.05) electrode material (a) SEM image, (b) O element, (c) Ti element, (d) V element and (e) Zn element

Figure 4 shows the Raman spectroscopy of Li2 ZnTi3O8 and Li1.95V0.05ZnTi3O8 materials. As plotted in Figure 4, the Raman spectrum of Li2ZnTi3O8 between 200 and 800 cm−1 indicates the peaks at 232, 264, 354, 402, 441, 521, 655, 717 (721) cm-1. The mode observed at 402 cm−1 can be assigned to the Zn–O symmetric stretching vibrations in Li2ZnTi3 O8 and Li1.95V0.05ZnTi3O8 compounds. The band at 441 cm−1 10


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corresponds to the stretching vibrations of the Li–O bonds in LiO4 tetrahedra.

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The

strongest feature at 402 cm−1 assigned to A1g mode of ZnO4 tetrahedra. 14 The higher frequency band at 717 cm−1 is assigned to the symmetric stretching vibrations of the Ti–O bonds in TiO6 octahedral groups. It can be found that there is a blue shift from 717 to 721 cm−1 in Raman band corresponding to the Ti–O vibration due to the doping vanadium ions, and then enhance the cation-oxygen bonding.

16

It has been

reported that increase of sharpness of A1g mode could decrease conductivity of LixMn2O4 (0.1

Rapid Lithiation and Delithiation Property of V-Doped Li2ZnTi3O8 as

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