Mo6+ Doping in Li3VO4 Anode for Li-Ion Batteries: Significantly

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Mo6+ doping in Li3VO4 anode for Li-ion batteries: significantly improve the reversible capacity and rate performance Youzhong Dong, He Duan, Kyu-sung Park, and Yanming Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06459 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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ACS Applied Materials & Interfaces

Mo6+ doping in Li3VO4 anode for Li-ion batteries: significantly improve the reversible capacity and rate performance Youzhong Dong1 *, He Duan2, Kyu-sung Park3, Yanming Zhao1 1

Department of Physics, South China University of Technology, Guangzhou, 510640, P. R. China

2

School of Physics and Optoelectronic Engineering, Guangdong University of Technology,

Guangzhou, 510006, P. R. China 3

Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States

_________________________________________ * Corresponding author. Fax: +86 20 87112837. E-mail address: [email protected] (Youzhong Dong)

Abstract Consider the almost insulator for pure Li3VO4 with a band gap of 3.77 eV, in order to significantly improve the electrical conductivity, the novel Li3V1-xMoxO4 (x = 0.00, 0.01, 0.02, 0.05 and 0.10) anode materials were prepared successfully by simple sol-gel method. Our calculations show that, by substitute Mo6+ for V5+, the extra electron occupied the V 3p empty orbital and caused the Fermi level shift up into the conduction band, where the Mo-doped Li3VO4 presents electrical conductor. The V/I curve measurements show that, by Mo doping in V site, the electronic conductivity of the Li3VO4 was increased by five orders of magnitude. And thence the polarization was obviously reduced. EIS measurement results indicated that by Mo-doping a higher lithium diffusion coefficient can be obtained. The significantly increased electronic conductivity combined the higher lithium diffusion coefficient leads to an

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obvious improvement in reversible capacity and rate performance for the Mo-doped Li3VO4. The resulting Li3V1-xMoxO4 (x = 0.01) material exhibited the excellent rate capability. At a high rate 5C, a big discharge capacity of the initial discharge capacity 439 mAh/g can be obtained which is higher than that of pure Li3VO4 (only 166 mAh/g) and after 100 cycles the mean capacity fade is only 0.06% per cycle. Keywords: Mo-doping; anode materials; Li3VO4; rate capability; first-principles calculations; band gap.

1. Introduction The increased-demand of rechargeable lithium-ion batteries (LIBs) with high-rates, large-capacity and good-safety for applications in electric vehicles and grid-scale energy storage systems has required people to develop more advanced electrode materials. At present, graphite is the most widely used anode materials for LIBs. However, the low specific capacity (theoretical capacity 372 mAh/g for LiC6) and the relatively poor safety caused by the low potential (only about 0.2 V vs. Li+/Li) limited its large-scale application in the future. Therefore, to find the alternative anode materials with high rate capacity and excellent cyclability, especially for good safety, are required1-4. Recently, Li3VO4, as a new candidate anode material for LIBs, is gaining interest based on the high specific capacity and safety. The crystal structure of the Li3VO4 is Orthorhombic with the space group of Pnm21 and lattice parameters a = 6.3290 Å, b = 5.4475 Å and c = 4.9501 Å5, which is isostructural to that of β-Li3PO46. Li3VO4


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compounds possess a framework structure formed by regular array of corner-shared VO4 tetrahedra and LiO4 tetrahedrons. Due to the existing of many empty, the Li+ can reversibly insert and extract from the framework structure. By in-situ XRD technology, Shigang Sun et al. observed the reversible phase transformation during the discharge/charge process where 2 Li were reversibly inserted/extracted (ca. 400 mAh/g)7. The cooresponding theory caculations also shows that 2 Li can be easily inserted in Li3VO4 to form a stable Li5VO4 with only 4% volume expansion8. But for more Li insertion, the reaction mechanism is still not very clear. X. Yang et al. synthesized the Li3VO4/C by sol-gel and the reversible specific capacity of 540 mAh/g (around 2.7 Li) was obtained at small current density 9. By GITT technology, Zhiyong Liang et al. get a reversible discharge/charge capacity of 590 mAh/g and calculated that the maximum amount of lithium insertion for Li3VO4 is 3 5. However, it is still very difficult to close to the theoretical capacity by gereral galvanostatic method. In addition, the rate performance is very poor and severe capacity fading is found especially at a large current density, which is not satisfied for future applications. The main reason is due to the low electronic conductivity of the Li3VO4 material. Li3VO4 possess high ionic conductivity which offen used as fast ions conductor. However, due to the wide band gap 10, Li3VO4 is almost electronic insulator. The very low electronic conductivity often caused the large resistance polarization, and thus resulting in a low reversible capacity and poor rate capability for Li3VO411. In order to overcome these problomes, various strategies have been developed including reducing



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the particle size of Li3VO4 by solution-based method12, coating carbon on the surface of Li3VO4,13 changing the particle morphology14,

15

, and preparation of carbon

nanotube/ Li3VO4 and graphene/ Li3VO4 composites16,

17

. Although above those

methods improved the electrochemical performance in some degrees, the lattice electronic conductivity within the crystal of the Li3VO4 has not been improved, a big polarization still exist and the high rate performance remains a big challenging at present. Doping is an effective method to improve electrode materials, especially for those materials with wide band gap18, 19. Our provious experimental results showed that doping Mg2+ in Li site can improve the electronic conductivity of the Li3VO4. However, the inactive Mg2+ occupied Li site may hinder the migration of lithium ions and then affect the electrochemical property. Considering the wide band gap of Li3VO4, here, we propose to replace V5+ with Mo6+ to affect the electronic structure (d0 electronic configuration of Li3VO4), increase the concentration of the electronic charge carriers and form an n-type semiconductor, and then significantly improve the electrinic conductivity. At the same time, Mo6+ occupied V site will not block the transport of lithium ions. Our theoretical calculation shows that Mo6+ on the V5+ site, one extra electron was intrduced. The extra electron occupied the V 3p empty orbital and caused the Fermi level shift up into the conduction band, where the Mo-doped Li3VO4 present an electrical conductor. Electronic conductivity measurement shows that doping Mo6+ to V5+ site significantly increased the electronic conductivity of the Li3VO4 by a factor of ~ 106. The resulting materials, Li3V(1-x)Mo xO4 (x = 0.01) with


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high electronic, ionic conductivity and fast Li-ion diffusion, show a high specific capacity with little polarization and good rate performance.

2. Experimental The Li3V1-xMoxO4 (x = 0.00, 0.01, 0.02, 0.05 and 0.10) samples were all prepared by a simple sol-gel method, where citric acid was used as chelating agent and carbon sources. Firstly, required amounts of CH3COOLi⋅2H2O was added into the deionized water to form a transparent solution. Then, V2O5, MoO3 and citric acid were added into the solution in turns under continuous stirring at 50 °C for 0.5 h to form an orange solution. The solution was heated to 80 °C in a water bath with continuous stirring to remove the excess water. After drying at 80°C in an air oven, the obtaind gray gel was ground with and preheated at 350 °C for 5h in an Ar atomosphere to yield the precursor. Finally, the obtained precursor was reground again and calcined at 650 °C for 5 h under a stream of a mixture of 5%H2 + 95%Ar in a seald tube furnace to get Li3V1-xMoxO4 samples. X-ray diffraction (XRD) patterns were recorded at room temperature by Philips X’pert PRO with a scainning range from 10° to 90° and a step size of 0.02°. Rietveld refinements were performed by Fullprof 2000 software package. The chemical binding state and electronic structure were conducted on an XPS spectrometer (ESCALAB210) with Al Kα radiation. For the electronic conductivity measurement, the pellets were prepared by die-pressing the powder samples and firing at 600 °C for 5h under a stream of a mixture of 5%H2 + 95%Ar. The size of the pellets was around 0.8 cm in diameter and



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0.06 cm in thickness. Then the silver conductor paste was coated on both sides to form the blocking electrodes. The electronic conductivity measurement was measured by linear voltage scanning method at a scanning rate of 1 mV/s from 0 to 0.2 V. For the electrochemical measurement, the eletrodes were prepared with the as-prepared samples, acetylene black and polyvinylidiene ﬂuoride (PVDF) binder at a weight ratio of 85:10:5. The slurry was coated onto copper foil current collector, dried at 50 °C for 24h and pressed (5 MPa) respectively. Then, the electrodes fabricated were dried again at 90 °C for 12h in a vacuum and cut into disc with a diameter of 14 mm using a punch. The mass load of the active materials were about 5 mg. 2032 coin cells were assembled in a high-purity argon-filled glove box filled with the lithium metal foil as the counter electrode, Celgard® 2320 as a separator, and 1M LiPF6 EC:DMC (1:1 vol.%) electrolyte. The electrochemical performance was evaluated on a Land battery tester system (Wnhan, China) at room temperature. Cyclic voltammetry (CV) was measured on electrochemical workstation (Autolab PGSTAT100 type) between 0.1 and 3.0 V under a constant current (0.05 mVs-1). Electrochemical impedance spectroscopy (EIS) was also measured on the same electrochemical workstation at open circuit voltage state for the fresh electrodes in the frequency range 1 MHz - 0.01 Hz with an ac voltage of 5 mV. The First principle calculations were performed using Vienna ab initio simulation soft package (VASP). Where, the projector augment wave pseudopotentials were adopted to describe the interactions of electrons. The valence configuration of Li, V, O and Mo was 2s1, 3d34s2, 2s22p4 and 4d54s1, respectively. For exchange and correlation,


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the Perdew-Burke-Ernzerof (PBE) exchange-correlation functional with the generalized gradient approximation was adopted. The energy cutoff for plane-wave set is 800 eV and the monkhorst-Pack k-point was 1×3×3. Geometries were optimized for all atoms (corresponding residual force ≤ 0.003 eV/Å). The calculated lattice parameters for Li3VO4 are a = 6.3334, b = 5.4522, and c = 4.9741 Å respectively, in good agreement with our experimental results (see refined lattice parameters). For defected simulation calculation, a 1×3×3 supercell was used, where the supercell contained 54 atoms of Li, 17 atoms of V, 1 atoms of Mo and 72 atoms of O.

3. Results and discussion The XRD of the Li3V(1-x)Mo xO4 samples were shown in Fig. 1, where the x varies from 0.00 to 0.10. From this figure we can see that, for the small Mo-doping contents (x ≤ 0.02), only a single Li3VO4 phase was observed which indicated that Mo6+ substitued sucessfully for V5+ to form a Li3V(1-x)Mo xO4 solid solution phase. However, for the higher Mo-doping contents (x ≥0.5), due to different valent states and ionic radius of Mo6+ (0.410 Å) and V5+ (0.355 Å), a small amount of Li2MoO4 (the violet dashed position in Fig. 1) can be observed as impurity phase. In order to further explore the structural information of Mo-doping samples, rietveld refinement were performed for all Li3V(1-x)Mo xO4 samples. For sigle phase samples, the structural parameter of Li3VO4 phase was adopted as the initial model where the space group is Pmn21. For the impurity phase samples, the two-phase model was performed and the selected initial models derive from Li3VO4: space group



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Pmn21 and Li2MoO4: space group R-3. For all samples, the reasonably small Rwp factor (< 10 %) indicated the accuracy of the selected model and the reliability of the refined results. Fig. 2 show the typical refinement results for Li3V(1-x)Mo xO4 compounds with x = 0.01 (single-phase) and x = 0.05 (two-phase). Because the occupied site of the doping atoms has a strong influence on the electrochemical performance of the electrode materials, some doped atoms often occupy the Li site and block the migration path of Li+. In order to determine whether doped-Mo occupy the Li site in our samples, we substituted some Mo atoms on the Li sites in the initial model and refined the occupancy and/or isotropic temperature parameters for Li and Mo atoms. The obviously higher Rwp or negative values of thermal parameters for Mo and Li atoms indicated that the doped Mo atoms only occupied the V 2a site in our samples (see the inset of Fig. 2(a)). The corresponding structural parameters of Li3V(1-x)Mo xO4 (x=0.02) from refinement results were listed in Table 1. The main reason why Mo occupies the V-site rather than the Li-site can be attributed to the different ionic radius and ionic valance state. In tetrahedral coordination, the ionic radius of Mo6+ (0.410 Å) is similar to that of V5+ (0.355 Å) but is significantly smaller than that of Li+ (0.590 Å).20 In addition, the difference of the valuence state between Mo6+ and Li+ is significantly larger than that of Mo6+ and V5+. So, Mo occupied the V site more easily for the forming of the stable and single Li3VO4 phase. However, due to the slightly diffferent ionic radius of Mo6+ and V5+ in tetrahedral coordination, a varying lattice parameters can be observed (see Table. 2). As expected, the lattice parameters a, b, c and unit-cell volum of the Li3VO4 were increased by substitute


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Mo6+ for V5+ which will be favor to improve the mobility of Li-ion. Table 1. Structural parameters of Li3V(1-x)Mo xO4 (x=0.02) from refinement results Atoms

Wyckoff site x

y

z

Occupancy

Li1

2a

0

0.6676(5)

0.5083(4)

0.50

Li2

4b

0.2479(6)

0.1682(3)

0.4598(4)

1.00

V

2a

0

0.3291(3)

0

0.49

Mo

2a

0

0.3291(3)

0

0.01

O1

2a

0

0.3314(4)

0.6187(3)

0.50

O2

2a

0

0.6354(5)

0.0656(2)

0.50

O3

4b

0.1833(3)

0.0654(4)

1.00

0.2334(5)

Table 2. Refined lattice parameters of the single phase Li3V(1-x)Mo xO4 samples, here α = β = γ = 90o for all samples V(Å3)

Samples

a (Å)

b(Å)

c(Å)

x = 0.00

6.3231(8)

5.4447(6)

4.9478(7)

170.339(9)

x = 0.01

6.3267(5)

5.4468(8)

4.9493(7)

170.558(7)

x = 0.02

6.3286(7)

5.4480(4)

4.9501(8)

170.671(8)

The valance states of Mo in Mo-doped Li3VO4 sample was examined by XPS. Fig. 3(a) shows the corresponding XPS survey spectrum of Mo-doped Li3VO4 indicating that the materials consist of mainly Li, V and O along with the relatively smaller contribution of Mo, which is in accordance with the expected result. Fig. 3(b) shows



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the high resolution of Mo 3d spectra after fitting. The fitted doublet peaks observed at binding energies of 232.94 and 236.12 eV, with a 3.18 eV spin-orbit splitting, can be attributed to Mo6+ 3d5/2 and Mo6+ 3d3/2, respectively, which agree well with the previously reported

21-23

. The XPS results show that the oxidation state of Mo in

Mo-doped Li3VO4 sample is Mo6+. In addition, the Vanadium valance states change of the Li3VO4 samples are also analysized by XPS before and after Mo-doping. The experimental values and the fitting curves of XPS of V2p for Li3V(1-x)Mo xO4 (x = 0.00 and 0.02) samples are shown in Fig. 4. From the fitting results we can see that for the pure Li3VO4 sample the two typial intensity peaks at 517.99 and 517.67 eV, corresponding to the V 2p3/2 and V 2p1/2 of V5+, indicated that the valance state of vanadium is +5. But for the Mo-doped sample, in addition to the V 2p3/2 and V 2p1/2 of V5+, the less intensity peak at 517.02 eV which can be ascribed to V 2p3/2 of V4+ indicated the existence of a small amount of V4+ in our sample. It is easy to understand the appearance of the mixed valance state of vanadium (V4+ and V5+) in Mo-doped Li3VO4 sample. Substitute V5+ with higher valance state Mo6+, in order to keep the charge balance, there some amount of V4+ will be formed. EPR spectroscopy is a sensitive tool to indentify the valance states of element through the fine structure of the EPR spectra. For getting more detailed valance information of V and Mo, an EPR measurement at lower temperature (liquid nitrogen or helium temperature) will be considered to do in our future study. Electronic conductivity and ionic conductivity play a key role in improving the electorchemical performance of the electrodes materials, especially when the


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multivalent ions were involved for the electrode materis in charge-discharge process24. Li3VO4 material is an ions conductor with a high ionic conductivity, but due to the wide band gap, it is almost electronic insulator. In charge-discharge process, the oxidation and reduction reaction occur between trivalent vanadium ions and pentavalant vanadium ions, there three electrons were included in this reaction. Clearly, the significantly improved electronic conductivity will have an important effect on the improving the electrochemical porformance of Li3VO4 materials. Here, the effect of Mo-doping on the electronic conductivity of Li3VO4 has been investigated. Fig. 5 shows the V/I curves of the single phase Li3V(1-x)Mo xO4 ( x = 0.00, 0.01 and 0.02) samples measured by linear voltage scanning method. There, an obvious linear relationship between voltage and current can be observed for all samples. The different slop for every sample reflected the effect of the Mo-doping amount on the electronic conductivity of the Li3VO4 material. The variation of electronic conductivity with the doping amount is shown in the inset of Fig. 5. As expected, by substitution the V5+ with the supervalant Mo6+ cations, the electronic conductivity of Li3VO4 was significantly increased. There, for the pure Li3VO4, the electronic conductivity is only 1.66 x 10-10 S cm-1, but for the Li3V(1-x)Mo xO4 samples, the value reach to 1.94 x 10-5 S cm-1 for x = 0.01 and 3.25 x 10-7 S cm-1 for x = 0.01 respectively. Especially for the Li3V(1-x)Mo xO4 with x = 0.01, the electronic conductivity was increased by five orders of magnitude, equaling to that of Li2Mn2O4 (~ 10-5 S cm-1) which currently used in the lithium storage cathodes

25

. The significantly increased



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electronic conductivity will be very helpful in improving the electrochemical properties of Li3VO4 material. In order to further clarify the effect of Mo-doping on the electronic property of Li3VO4, the first principle calculation based on the DFT was used to calculate the band structure and DOS (density of states) of Li3VO4 using the Vienna ab initio simulation package (VASP). The energetically optimized lattice models of pure and Mo-doped Li3VO4 are present in Fig. 6. The crystal structure of the pure Li3VO4 belongs to the orthorhombic crystalline system with the Pmn21 space group. There, three Li ions occupied two Wyckoff sites (4b and 2a sites). V occupied in 2a site is four coordinated by four oxygen atoms forming a stable VO4 tetrahedron. The corresponding lowest energy structure is shown in Fig. 6(a). For Mo-doped Li3VO4, after optimization of lattice parameters, the lowest energy structure is shown in Fig. 6(b), where Mo occupied the V 2a site. The calculated band structure of the pure Li3VO4 crystal is shown in Fig. 7. Obviously, the pure Li3VO4 crystal is an insulater with a calculated band gap of 3.77 eV which is in good agreement with experimental measurement results 26. The partial density of states of pure Li3VO4 show that the 2p orbitals of O have the dominant contribution to the vanlance band whereas the conduction band is mainly composed of the 3d orbitals of V. Clearly, for the pure Li3VO4, the valance band edge mainly consists of O atoms, wheres the conduction band edge is mainly composed of V atoms. So, the shift of the conduction edge can be easily achieved by doping the supervalant cation in V site. Fig.8 shows the band structure and partial density of states of Mo-doped Li3VO4,


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compared with the band structure of pure Li3VO4 supercell. As the Mo atom has one more electron than V, it adds one extra electron to the system. The extra electron occupied the V 3p empty orbital and caused the Fermi level shift up into the conduction band. Therefore, Mo-doped Li3VO4 present the characteristic of metal. Remarkably, Mo-doped Li3VO4 is an electrical conductor, where Mo has an important role as donor. The results are in good agreement with our experimental results where the electronic conductivity the Mo-doped Li3VO4 was increased by five orders of magnitude. Because our samples were synthesized by sol-gel methode where citric acid was used as chelating agent. To accurately estimate the electrochemical property of the materials, the residual carbon on the samples surface caused by the decomposition of the citric acid throughout the reaction is determined by a simple thermal analysis experiment method 27. From the inset of Fig. 9 we can see that in inert atmosphere (N2) the Li3VO4 is very stable. There are both no exothermic or endothermic peaks in the DTA curve and no weight loss in the TG curve. But in the air atmosphere, the TDA curve of the same sample exhibit an obvious exothermic peak at 385°C which can be attribited to the burning of carbon. In addition, an obvious weight loss (5.62%) can be observed in the TG curve between 320 °C and 420 °C. Based on the above results, the weight loss should be attribited to the oxidization of the residual carbon on the Li3VO4 to carbon dioxide

27

. So we determined that the final amount of carbon is

around 5.62% for Li3VO4 sample. The electrochemical performance of the Li3V(1-x)Mo xO4 samples (x = 0.00 and



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0.01) was measured using coin cells by a galvanostatic discharge/charge method. Fig. 10 shows the typical discharge/charge curves of both samples for the first two and tenth cycles at 0.1C rate (corresponding to 59 mA/g) in the potential range of 0.1- 3.0 V. For the pure Li3VO4 sample, the low charge/discharge capacities and obviously capacity loss can be observed, where the discharge capacities are 635.5, 504.8, and 437.5 mAh/g for the first, second and tenth cycles respectively and the capacity loss from the second cycle to tenth cycle is 67.3mAh/g. However, for Mo-doped sample, due to the significantly increased electronic conductivity, the obviously improved electrochemical properties were obtained. The discharge capicity for the first, second and tenth cycles are 705.8, 557.5 and 543.7mAh/g respectively. Except for the first cycle, almost no capacity loss can be observed in whether discharge curves or the charge curves. However, for the Mo-doped sample, a big irreversible discharge capacity loss for the first cycle can be still observed. The irreversible discharge capacity loss can be attributed to the side reactions such as formation of solid electrolyte interface (SEI) film and decomposition of electrolyte which has been observed by several groups

28, 29

. The significant capacity loss may be reduced by

coating a thin uniform polymer layer on the surface of the sample and it will be studied in our future study. In additon, for Mo-doped Li3VO4 sample, a reduced polarization is also observed based on the plateau voltage difference of the discharge and charge curves for every cycle that can be refected more clearly from the CV test results. Fig. 11 exhibits the CV curves of pure Li3VO4 and Mo-doped Li3VO4 over a


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voltage range of 0.1-3V at a scan rate of 0.05 mVs-1 in the first four cycles. For Mo-doped Li3VO4 sample, the intensities of all redox peaks for every cycle are larger than the intensities of corresponding redox peaks of pure Li3VO4 sample, suggesting that the lithium diffusion rate can be increased by V-site Mo-doping

30

. More

important, a significant small separation between the oxidation and the reduction peaks can be observed by Mo-doping. During the cathodic sweep, compared with the Li3VO4, the corresponding reduction peaks of Mo-doped Li3VO4 for all cycles shift to higher voltage value. However, in the anodic sweep process, the shift direction of the oxidation peaks is opposite. For Mo-doped Li3VO4, the corresponding oxidation peaks for all cycles shift to lower voltage value. So, a significantly reduced voltage difference between the oxidation peak and the reduction peaks can be observed for the Mo-doped Li3VO4. The voltage difference between the oxidation peak and the reduction often reflected the polarization degree of the electrode. Obviously, due to the significant increase of the electronic conductivity, Mo-doping is favorable for reducing the polarization of Li3VO4 and thence increased the electrochemical reversibility of the electrode that is in agreement with the charge/discharge measure results. Electronic conductivity and ionic conductivity play a key role in improving the rate performance of the electrodes materials, especially when the multivalent ions were involved for the electrode materis in charge-discharge process 24. Li3VO4 material is an ions conductor with a high ionic conductivity, but due to the band gap, it is almost electronic insulator. In charge-discharge process, the oxidation and reduction reaction



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occur between trivalent vanadium ions and pentavalant vanadium ions, there three electrons were included in this reaction. So, the significantly improved electronic conductivity will have an important effect on the improving the rate porformance of Li3VO4 materials. Fig. 12 shows the cycle performance of pure and Mo-doped Li3VO4 material at diferent discharge rate (C/10, C/2, 1C, 2C and 5C). As expected, due to obviously increased electronic conductivity (about five orders of magnitude), both the discharge capacity and cycle capability of Mo-doped Li3VO4 have been significantly improved at all current rates, especially for higher rates. At C/10 discharge rate, except for the first cycle, the discharge capacities for the second and the tenth cycles of the pure Li3VO4 are 506 and 438 mAh/g respectively and after 9 cycles, the capacity retention is only 86.6%. However, for the Mo-doped Li3VO4, the discharge capacities for the second and the tenth cycles are 559 and 542 mAh/g respectively, the capacity retention reach to 97%. Especially for the higher rate (5C), the discharge capacity for the initial cycle of the pure Li3VO4 is only 166 mAh/g, but for Mo-doped Li3VO4, the discharge capacity is 439 mAh/g, and after 100 cycles, the discharge capacity is still as high as 413 mAh/g. The capacity retention of Mo-doped Li3VO4 at 5C rate is 94% and the mean capacity fade is only 0.06% per cycle. Doping elements often prevent the diffusion of lithium ions in the electrode material. In order to investigate the effect of Mo-doping on the electrode kinetics of Li3VO4, electrochemical impedance spectroscopy was employed to caculate the diffusion coefficient of lithium ions of electrode materials. Fig. 13 shows the Nyquist plots of the pure and Mo-doped Li3VO4 samples. The corresponding lithium ions diffusion


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coefficient can be calculated from the following equation: 31 ଵ

‫ܦ‬௅௜ = ଶ ቂቀ

௏೘

ிௌఙ

ቁ ቀ−

ୢா ୢ௫

ଶ

ቁቃ

Where: the Warburg factor σ can be obtained from the slope of Zre Vs. ω-1/2σ based on the linear fitting results (Fig. 13, inset). The calculated lithium ions diffusion coefficients are 7.66x10-8 and 6.32 x10-7 cm2 s-1 for pure Li3VO4 and Mo-doped Li3VO4, respectively. It is obviously that, due to the increasement of the unit-cell volum, the lithium ions diffusion coefficients of Mo-doped Li3VO4 is larger than that of pure Li3VO4 suggesting that doping Mo6+ in V-site will contribute to improving the electrochemical kinetics of lithium ions insertion/extraction. The increased lithium ions diffusion coefficients is another factor for the improving the rate performance of the Mo-doped Li3VO4. This result is in good agreement with rates cycle performance measurement results (Fig. 12).

4. Conclusions Li3V(1-x)Mo xO4 (x = 0.00, 0.01, 0.02, 0.05 and 0.10) samples were successfully synthesized by simple sol-gel method. XRD patterns show that, due to different valent states and ionic radius of Mo6+ (0.410 Å) and V5+ (0.355 Å), the single Li3V(1-x)MoxO4 solid solution phase can be abtained in a small Mo-doping content range (x ≤ 0.02) where, the doped Mo atoms only occupied the V 2a site. XPS analysis shows that, in order to keep the charge balance, substitute V5+ with higher valance state Mo6+, a small amount of V4+ can be found in Mo-doped smaples. Due to the wide band gap (around 3.77 eV), pure Li3VO4 is almost electronic insulator. Our calculated results show that by doping Mo6+ on the V5+ site, one extra electron was intrduced. The extra



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electron occupied the V 3p empty orbital and caused the Fermi level shift up into the conduction band, where Mo-doped Li3VO4 present the characteristic of metal. And the corresponding electronic conductivity of Li3VO4 was increased by five orders of magnitude. The significantly increased electronic conductivity reduces the polarization and thence increased the electrochemical reversibility of the electrode, where, for Mo-doped sample, except for the first cycle, almost no capacity loss can be observed in whether discharge curve or the charge curve. The EIS results show that, due to the increasement of the unit-cell volum, Mo-doping increased the lithium ions diffusion coefficients of Li3VO4. For pure Li3VO4, lithium ions diffusion coefficients are 7.66x10-8 cm2 s-1, but for the Mo-doped Li3VO4, the value is 6.32 x10-7 cm2 s-1. The rate performance results show that the significantly increased electronic conductivity combined the improved lithium diffusion coefficient lead to an obvious improvement in rate capacity and cycling stability for the Mo-doped Li3VO4. For Li3V1-xMoxO4 (x = 0.01) sample, at 0.1C rate, the discharge capacities for the second and the tenth cycles are 559 and 542 mAh/g respectively, the capacity retention reach to 97%. Even at a high rate 5C, a big discharge capacity of the initial discharge capacity 439 mAh/g can still be obtained which is higher than that of pure Li3VO4 (only 166 mAh/g) and after 100 cycles the mean capacity fade is only 0.06% per cycle.

Acknowledgements This work was supported by the Science and technology program of Guangzhou


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China (Grant No. 201707010069) and National Science Foundation of China (Grant No. 11404066 and No. 51172077). And the project sponsored by the Scientific Research Foundation for the returned overseas Chinese scholars, State Education Ministry.

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(14) Zhang, C.; Song, H.; Liu, C.; Liu, Y.; Zhang, C.; Nan, X.; Cao, G. Fast and Reversible Li Ion Insertion in Carbon‐Encapsulated Li3VO4 as Anode for Lithium‐Ion Battery Adv. Funct. Mater. 2015, 25, 3497−3504 (15) Liu, J.; Lu, J.; Liang, S.; Liu, J.; Wang, W.; Lei, M.; Tang, S.; Yang, Q. Ultrathin Li3VO4 Nanoribbon/Graphene Sandwich-Like Nanostructures with Ultrahigh Lithium Ion Storage Properties Nano Energy 2015, 12, 709-724. (16) Li, Q.; Sheng, J.; Wei, Q.; An, Q.; Wei, X.; Zhang, P.; Mai, L. A Unique Hollow Li3VO4/Carbon Nanotube Composite Anode for High Rate Long-Life Lithium-Ion Batteries Nanoscale 2014, 6, 11072-11077 (17) Shi, Y.; Wang, J.; Chou, S.; Wexler, D.; Li, H.; Ozawa, K.; Liu, H.; Wu, Y. Hollow Structured Li3VO4 Wrapped with Graphene Nanosheets in Situ Prepared by a One-Pot Template-Free Method as an Anode for Lithium-Ion Batteries Nano Lett. 2013,13, 4715-4720 (18) Chung, Y.; Bloking, T.; Chiang, M. Electronically Conductive Phospho-Olivines as Lithium Storage Electrodes Nat. Mater. 2002, 2, 123-128 (19) Song, H.; Kim, T. A Mo-doped TiNb2O7 Anode for Lithium-ion Batteries with High Rate Capability due to Charge Redistribution Chem Commun. 2015, 51, 9849-9852 (20) Shannon, D. Revised Effective Ionic Radii and Systematic Studies of Interatomie Distances in Halides and Chaleogenides Acta Cryst. 1976, 32, 751. (21) Qin, P.; Fang, G.; Ke, W.; Cheng, F.; Zheng, Q.; Wan, J.; Lei, H.; Zhao, X. In Situ Growth of Double-layer MoO3/MoS2 Film from MoS2 for Hole-Transport Layers



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TOC Graphic


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Fig. 1 XRD patterns of Li3V(1-x)Mo xO4 (0≤x≤0.10) powder samples



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Fig. 2 the typical rietveld refinement results of Li3V(1-x)Mo xO4 samples: (a) x = 0.01for single phase model, (b) x = 0.05 for two-phase model. The positions occupied by dopedMo are presented in the inset.


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Fig. 3 XPS spectra of Li3V(1-x)MoxO4 (x=0.02) sample: (a) survey spectrum, (b) the high-resolution of Mo 3d spectra.



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Fig. 4 The XPS spectras of V 2p in Li3V(1-x)Mo xO4 samples: (a) x = 0.00; (b) x = 0.02


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Fig. 5 the V/I curves of the Li3V(1-x)Mo xO4 ( x = 0.00, 0.01 and 0.02) samples. The dependence of the electronic conductivity on the doping amount is presented in the inset.



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Fig. 6 Model systems and structural optimizations for (a) pure Li3VO4 and (b) Mo-doped Li3VO4.


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Fig. 7 Calculated electronic band dispersion and partial density of states of the perfect compound Li3VO4.



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Fig. 8 Calculated band structure and partial density of states of Mo-doped Li3VO4, as compared with the band structure of pure supercell: (a) the band structure of pure Li3VO4 supercell; (b) the band structure and (c) partial density of states of Mo-doped Li3VO4 unit cell.


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Fig. 9 The TD-DTA curves of Li3VO4 sample under air flow. Inset shows the TD-DTA curves of the same sample under high-purity N2 flow.



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Fig. 10 the typical discharge/charge curves of Li3V(1-x)Mo xO4 samples for the first two and tenth cycles: (a) x = 0.00; (b) x = 0.01


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Fig. 11 Cycle voltammograms of Li3VO4 (hollow dot curves) and Li3V0.99Mo0.01O4 (solid dot curves) in the initial four cycles.



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Fig. 12 the cycle performance of pure and Mo-doped Li3VO4 material at different discharge rate (C/10, C/2, 1C, 2C and 5C) at room temperature.


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Fig. 13 the EIS plots of the Li3VO4 and Mo-doped Li3VO4. The inset shows the linear fitting of the Zre Vs. ω-1/2 relationship.


Mo6+ Doping in Li3VO4 Anode for Li-Ion Batteries: Significantly

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