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Effects of Pre-inserted Na-ions on Li-ion Electrochemical Intercalation Properties of VO 2

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Xinyuan Li, Chaofeng Liu, Changkun Zhang, Haoyu Fu, Xihui Nan, Wenda Ma, Zhuoyu Li, Kan Wang, Hai-Bo Wu, and Guozhong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08052 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 1, 2016

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ACS Applied Materials & Interfaces

Effects of Pre-inserted Na-ions on Li-ion Electrochemical Intercalation Properties of V2O5 Xinyuan Li1,2, Chaofeng Liu1, Changkun Zhang1, Haoyu Fu1, Xihui Nan1, Wenda Ma1, Zhuoyu Li1, Kan Wang1, Haibo Wu2, and Guozhong Cao1,3,* 1

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, National Center for Nanoscience and Technology (NCNST), Beijing, 100083, China 2 College of Life and Environmental Science, Minzu University of China, Beijing, 100081, China 3 Department of Materials Science and Engineering, University of Washington, Seattle, Washington, 98195, USA

Keywords: V2O5, Na pre-insertion, Cathode, Electrochemical intercalation, Lithium ion battery

Abstract Na-pre-inserted V2O5 samples, NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02), were synthesized through sol-gel and freeze-drying route and subsequent calcination. X-ray diffraction (XRD) results showed that all the synthesized materials have typical orthorhombic structure without impurity phases. The lattice parameters were refined via Rietveld refinement method, and the results suggested that the lattice parameters of pre-inserted sample increased in comparison with the pristine V2O5. X-ray photoelectron spectroscopy (XPS) measurements demonstrated that the V4+ concentration in the Na-pre-inserted V2O5 samples gradually increased with the increasing amount of sodium. Both results from XRD and XPS strongly suggested Na-ions indeed enter the interlamination position in V2O5 crystal to expand the channels for Li-ion migration. NaxV2O5 samples exhibited improved electrochemical properties compared with the pristine V2O5. Amongst all of samples, Na0.01V2O5 1

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delivered the highest reversible specific capacity, best cycling stability and excellent rate capability. The analysis and discussion on ion diffusion revealed that the pre-inserted Na-ions benefited the mobility of Li-ion to improve the rate capabilities of electrodes.

Introduction The shortage of fossil fuels and the huge energy demand have been driving the rapid progress on advanced technologies corresponding to energy harvest and storage.1 Lithium ion batteries are the reliable choices for the large-scale energy storage due to their high energy density, the stable cyclability and excellent rate capability, as well as good safety concerns.2-3 As a result, lithium ion batteries play an increasingly important role in powering electric vehicles, smart portable electronics, wearable electronic devices, and implantable medical devices in modern society,4-6 However, cathode material, one of important components in lithium ion battery, still confront with some challenges, such as the limited lifetime, the relatively low power output and the worrying safety issues.7-9 Vanadium oxides are the classic materials with layered structures and have distinct advantages as a cathode on safety, specific capacity and cost compared with commercial cathode LiCoO2.10-11 Especially, vanadium pentoxide (V2O5) has been attracting extensive attention as a practical cathode candidate for lithium ion batteries.12-16 The layered V2O5 consists of square-pyramidal VO5 units that connect each other through sharing edges. This unique crystal structure renders it the reversibly Li-ion insertion or extraction. The

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theoretical capacity of V2O5 reaches 294 mAh g−1 with two Li-ion insertion/extraction, which is higher than those of other cathode materials, such as LiMn2O4 (145 mAh g-1)17 and commercial cathode LiCoO2 (140 mAh g-1).18 However, the unstable structure, and low ion diffusion coefficient restrict its practical applications.19-21 Three approaches are extensively used to improve the performances of cathode materials: composites, nanostructures and doping.7,

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For example, the uniform polypyrrole

(PPy) coating on V2O5 can effectively prevent the dissolution of vanadium ions and enhance the reversible redox reaction in the charge/discharge process, resulting in the improved reversibility and cycling behavior.23 Besides, V2O5 cathode materials have been prepared with designed nanostructures exhibiting a better electrochemical performance due to their small size and large surface area that may increase the contact area between active material and electrolyte and shorten lithium ion diffusion pathway. 12, 24-26 Meanwhile, Metal ions doping is also a facile and effective method for modifying the performances of cathode materials,27 no exception in V2O5, such as Ag,28 Cu,29 Mn,30 Fe,31 Tb,32 Ti,33 Al,34-35 Cr,36 Ni,37 or Sn38 doped V2O5. For example, Ni doped V2O5 as the cathode material for lithium ion battery not only exhibits excellent lithium storage properties, but also presents an improved wettability for organic electrolyte.39-40 Sn-doped V2O5 shows faster kinetics and stable cyclability in comparison with pure V2O5 owing to the presence of more low valence state V4+ and the surface and bulk defects in Sn-doped V2O5.38 Therefore, selecting a suitable ion for doping V2O5 cathode material may effectively tune the electrochemical properties of V2O5. Sodium ion, an alkali ion, possesses a larger atomic radius of 116 pm within

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the six-coordination, while Li ion has a smaller size of 90 pm at the same circumstance. Na-doping has been applied in some cathode materials, such as Na-doped Li3V2(PO4)3 cathode materials optimize the particle size and morphology, as well as result in a higher electronic conductivity of 6.74 × 10−3 S cm−1 than the pristine Li3V2(PO4)3 of 2.16 × 10−4 S cm−1. 41 Na-doped LiFePO4 leads to a lower polarization and higher lithium-ion diffusion coefficient from 1.21 × 10-16 to 1.06×10-14 cm2 s-1, which can effectively improve the kinetic performances in the electrochemical reaction.42 However, few exploration on electrochemical properties of Na-doped V2O5 was carried out, albeit Na doped V2O5 was used as thermoelectric sensors due to their high Seebeck coefficient and an improved electronic conductivity by a factor of ~104.43 In this study, NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples were prepared via sol-gel and freeze-drying routine followed with annealing, and their electrochemical properties were investigated. Meanwhile, XRD patterns of samples were refined via Rietveld refinement methods to obtain the information on crystal lattice under the help of XPS results by quantitative analyses and a possible explanation for the enhanced lithium ion diffusion coefficients is discussed.

Experimental Section Synthesis NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples were synthesized via the sol-gel method. Stoichiometric amounts of V2O5, NaNO3 and H2O2 were employed as the raw materials. First, V2O5 and H2O2 were dissolved in appropriate amount of deionized water with magnetic stirring. After a clear orange solution formed, stoichiometric amounts of NaNO3 was added to the solution while stirring at room 4


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temperature to form NaxV2O5 precursors for 30 minutes, and then the solution slowly became a sol. The color of the sol changed from orange to deep red during the stirring process. The aging was completed at 60 °C in an air oven until the sol transformed to gel. The gel was freeze-dried by a freezer dryer at -82 °C. Then, the dried gel powder was annealed at 400 °C for 0.5 h in air.

Structural Characterization The crystalline structure of samples were identified on a X’ Pert3 diffractometer (PANalytical, Netherlands) with a Cu-Kα radiation source (λ = 1.54056 Å) over the range of 10° to 70° (2θ) with a step size of 0.013°, the accelerating voltage and current were 40 kV and 40 mA, respectively. The unit cell parameters were refined by the Rietveld method using the Fullprof program. The microstructures of samples were conducted by a cold field emission scanning electron microscope (FESEM, HITACHI SU8200) with the working voltage and current of 5 kV and 10 µA, respectively. X-ray photoelectron spectroscopy (XPS) analyses were conducted on a K-Alpha 1063 instrument using monochromated Al-Ka as the X-ray source. XPS data analysis was carried out using the Service Physics ESCA 2000-A analysis program (Service Physics, Bend, OR).

Cell Fabrication and Electrochemical Characterization The electrochemical performances of all samples were studied using 2032 coin cells. To prepare the working electrode, the slurry contained a mixture of active materials [NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02)], carbon black and carboxy-methyl cellulose (CMC) at a weight ratio of 70:20:10 was bladed on a Al foil and then subjected to thermal treatment at 120 °C with 12 h in vacuum. The mass loading of the active material on each electrode disk was 1.0-2.0 mg cm-2. The commercial electrolyte (LBC 305-01, CAPCHEM), a polypropylene film (Celgard 2400) as separator and lithium metal foil as counter electrodes were used in the cells. All cells were assembled in an argon-filled glovebox in which both the content of oxygen and water are below 1 ppm. 5



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Galvanostatic charge-discharge tests of all cells were carried out on a Land CT2001A system (Wuhan, China) with various current densities. The operating voltage window for NaxV2O5 is 2.0-4.0 V (vs. Li/Li+). Cyclic voltammograms (CVs) were conducted on a Solartron SI 1287 at the scanning rate of 0.1 mV s-1 for characterizing the redox reaction of electrode materials in the charge/discharge process. Electrochemical impedance spectroscopy (EIS) were performed at 3.4 V using the Solartron 1287A in conjunction with a Solartron 1260A impedance analyzer over the frequency range from 100 kHz to 0.1 Hz and the AC amplitude was 10.0 mV. The current densities for the half cells were calculated on the mass of active materials of electrodes. All electrochemical measurements were carried out at room temperature.

Results and discussion X-ray diffraction was performed on NaxV2O5 samples and XRD patterns are presented in Figure 1 (a). All the characteristic peaks of the samples are indexed to the orthorhombic V2O5 (JCPDS card no. 41-1426) without detectable impurity phase at the first glance. With the increased Na content, the crystal structure of V2O5 remains intact and no impurity phase can be detected in the samples that has the content of Na ＜ 2%. An enlarged peak (001) in XRD patterns of NaxV2O5 samples are shown in Figure 1 (b), it revealed that the peak (001) in samples shifts toward lower degrees with the increase of sodium content, the diffraction peak (001) in NaxV2O5 samples shift monotonically from 20.38 to 20.32, 20.30 and 20.29°, respectively, indicating that sodium ions caused an increase in interplanar distance of V2O5. There is a big shift between Na-V2O5 and V2O5, but very little shift with different content of Na, suggesting the introduction of Na form an intercalation compound. Meanwhile, the layered structures of V2O5 indeed correspond to the (001) lattice plane44. The other two peaks at high angles also be shown in Figure S1, the peaks (420) and (710) in samples also shift toward lower degrees with the increase of sodium content, implying the shifts are consistent in all peaks. Therefore, In order to obtain details on the 6


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structure of NaxV2O5 samples, Rietveld refinements were performed with the standard orthorhombic structure and can be acceptable because the calculated curves are fitted well with the measured results as shown in Figure 1 (c-f). The Rp and Rwp are two key factors to weigh the acceptability of the refinement results. Rp is the profile factor related to the residual error directly calculated by the model structure of XRD spectrum with the experimental data. Rwp is the weighted profile factor that increases the weight of a particular location on the basis of Rp. It is acceptable that the value of Rp and Rwp below 15% and the smaller the better under normal circumstances. The Rp and Rwp in Table 1 imply that the phases fitting are acceptable in all samples. More importantly, the lattice parameters and the unit cell volumes are summarized in Table 1, the lattice parameters (a, b, c and volume) gradually increase, which coincide with the results suggested in Figure 1 (b) and Figure S1, indicating that Na ions enter into the lattice of V2O5 indeed. This trend in lattice parameters also agree well with other Na-doped cathodes.42, 45 Differences in lattice parameters among NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02,) samples are listed in Table S 1, it is found that the expansion at different orientations (a, b, c) exhibit the distinct increments, lattice parameter along c-axis get an order of magnitude bigger than of the increments along a and b, implying that Na ions do not occupied vanadium sites, or else it will results in an homogeneous expansion along each orientation in the crystal lattice of V2O5. It is not easy to form the substitutional solid solution because the difference in ionic radius between Na+ (116 pm) and V5+ (68 pm) exceeds ~50.0% that is larger than the limiting value of 30% in substitutional compounds. The difference is defined as follows: 46 ∆= (R1-R2)/R2

(1)

where R1 is the radius of the bigger ion (Na+) and R2 is the smaller one (V5+). Na ions just enter the interlamination of V2O5 to constitute an interstitial solid solution rather than replace the position of vanadium.46 Besides, the chemical valences in both ions are also discrepant, interplanar Na ions with one charge result in a tiny expansion in the lattice parameters a and b. In addition, the radius ratio between Na+ and O2- (128 pm) is 0.875 that far exceeds the maximum value of 0.732 for an octahedral 7



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coordination.46 Besides, the electronegativity of sodium and oxygen are 0.93 and 3.44 respectively. On the facts described above, sodium ions can be introduced into the interplanar sites of vanadium pentoxide to form an interstitial solid solution.

Figure 1. (a)XRD patterns of NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples and (b) Enlarged (001) peak of XRD patterns for NaxV2O5 samples, Rietveld refinement results for NaxV2O5 samples: (c) x= 0.00; (d) x= 0.005; (e) x= 0.01; (f) x= 0.02.

Table 1. The lattice parameters and the unit cell volumes of NaxV2O5 samples.

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Samples

a(Å)

b(Å)

c(Å)

V(Å3)

Rwp(%)

Rp(%)

Na0.00V2O5 Na0.005V2O5 Na0.01V2O5 Na0.02V2O5

11.50496 11.50564 11.50595 11.50665

3.56163 3.56218 3.56279 3.56355

4.37205 4.37436 4.37646 4.37756

179.1509 179.2838 179.4055 179.4998

9.99 9.53 8.59 10.9

7.24 7.17 6.25 8.21

Rwp : The weighted profile factor that increases the weight of a particular location on the basis of Rp. Rp : The profile factor related to the residual error directly calculated by the model structure of XRD spectrum with the experimental data. (It is acceptable that the value of Rp and Rwp below 15% and the smaller the better under normal circumstances) Figure 2 a-d show the microstructure of NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) compounds after calcination. Comparison in the different samples demonstrates the negligible effects of Na-content on the homogeneity and particle size. All samples are piled with corrugated sheet structure, these sheets morphology could be related to the formation of V2O5 as previously reported.12 Samples prefer to grow along one orientation in the process of crystal growth because the initial nuclei have discrepant surface energy on different crystal planes.47 The facet, such as (111), possesses the higher surface energy that easily adsorbs component ions and grow preferentially. Corrugated sheet structure clearly observed in NaxV2O5 (x = 0.00, 0.005, 0.01) compounds, indicating that tiny sodium ions do not have an effect on crystal growth, However, Na0.02V2O5 exhibits a different morphology and the corrugated sheets disappeared owing to the formed impurity affects the preferential growth in the synthesis process.

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Figure 2. SEM micrographs of the NaxV2O5 samples with (a) x=0.00, (b) x=0.005, (c) x=0.01 and (d) x=0.02 where the scale bar in each image is equivalent to 1 µm.

X-ray photoelectron spectroscopy (XPS) measurements were carried out on the NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) compounds to investigate the oxidation state of vanadium, and the results are shown in Figure 3. Both the V 2p 3/2 peak spectra in the pure V2O5 and Na pre-inserted V2O5 are consisted of two components located at 517.2 and 515.9 eV, respectively. These binding energy values associate with oxidation degree of V5+ and V4+, respectively.16 The molar ratio of V4+/(V4+ + V5+) for NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples intuitively reflected in Table 2 are 3.92%, 4.62% , 6.22% and 6.26% respectively. With the increase of the sodium content, the V4+ concentration in the Na-doped V2O5 sample is gradually increased, suggesting that the partial reduction of the pentavalent ions to tetravalent ions in the Na pre-inserted V2O5 sample. From the quantitative analyses, the introduction of Na+ caused equal molarity V5+ reduced to V4+ within the margin of error: V2O5 + xe- + xNa+ = NaxV2O5 10


(2)

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Therefore, Na+ is just caused the formation of equal V4+ rather than substitution of vanadium resulting in reducing the amount of V5+. These agree well with the Rietveld results, confirming again that Na+ enter the interlamination of V2O5. For the Na pre-inserted V2O5 samples, Na element is shown with the broad photoelectron peaks at binding energies of 1071.3 eV (Na 1s ) (Figure S2), This agrees well with the reported data of Na 1s in Na1.25+xV3O8 (with x < 0, = 0, and > 0),48 the area ratio of Na (1s) / V(2p) for NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) compounds are 0, 0.007 , 0.011 and 0.017 respectively, indicating that sodium element indeed inserted in V2O5 with an increasing trend. Pre-insertion of Na ions has two impacts on vanadium pentoxides: enlarged the distance between adjacent layers (as reflected by the increased c-constant) beneficial the subsequent Li-ion diffusion, and reduced some pentavalent vanadium ions to tetravalent vanadium ions that would improve the electronic conductivity as well as catalyze the intercalation reactions. Other large cations may also improve the intercalation properties; however, they may not be easy to be inserted between two adjacent layers. Some cations can take substitutional sites, which also demonstrated an appreciable improvement in intercalation properties.49 So pre-insertion of other cations (large or small) is an excellent topic for further research and but has a lot of challenges.

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Figure 3. XPS spectra of high-resolution scan on the V 2p 3/2 peaks performed on the (a) Na0.00V2O5 (b) Na0.005V2O5 (c) Na0.01V2O5 (d) Na0.02V2O5

Table 2. The molar ratio of V4+/(V4+ + V5+) for NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples.

The molar ratio

The difference in The sodium content

Samples (%) Na0.00V2O5 Na0.005V2O5 Na0.01V2O5 Na0.02V2O5

(%)

3.92 4.62 5.22 6.26

0 0.7 1.3 2.34

(%) 0 0.5 1.0 2.0

The charge-discharge curves of the NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples at 0.1 A g-1 rate in the voltage range of 2.0-4.0 V are shown in Figure 4 (a). From the charge curves in the first cycle, the curve profiles are different between Na 12


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pre-inserted and no inserted samples, and discharge plateau of Na pre-inserted samples are unconspicuous. NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples deliver the discharge specific capacity of 147.8, 211.3, 253 and 230.6 mAh g-1 at the current density of 0.1 A g-1, respectively, which indicate Na pre-inserting improve the specific capacity to some extent. The difference between charge and discharge capacities of the NaxV2O5 (x = 0.01, 0.02) at the first cycle attributes to the intercalated lithium ions do not deintercalate completely, the irreversible capacity approximates to -8.5 and -3.5 mAh g-1, respectively. There also be a strange phenomenon that charge capacities of Na0.00V2O5 and Na0.005V2O5 are higher than their discharge capacity, 49.7 and 10.6 mAh g-1, respectively. On the view of chemical state of cation, vanadium ions in V2O5 take on oxidation states of V4+ or V5+.6e, 23 the low concentration of V4+ in V2O5 comes from the unintentional dopants and/or incomplete oxidation. Intercalation of lithium ion with the reduction of vanadium from V5+ to V4+ takes place in the discharge process, and deintercalation of lithium ion with the oxidation of vanadium from V4+ to V5+ occurs in the process of charge. Therefore, it is speculated that the low concentration of V4+ in the pristine V2O5 happen to be oxidized at the first charge process. This phenomenon agrees well with those given in several reports.12, 50 Besides, the first three charge-discharge curves of the NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples at 0.1 A g-1 rate in the voltage range of 2.0–4.0 V are shown in Figure S3 (a-d). It is obvious that the charging curves at the second and third cycles almost overlap, indicative of a stable state in the electrode materials and surface state during cycling. However, the second discharge curves are different from the first discharge curves with the increased capacities of +52.3, +13.5, -12.7 and -6 mAh g-1, respectively, indicating that electrolyte gradually diffused into the large space between V2O5 sheets in the first cycle. Consequently, the electrolyte between the sheets will facilitate the electrochemical reaction, leading to the increase of capacity in the subsequent cycles.51 Meanwhile, the introduction of Na ions may accelerate the process of electrolyte diffusion. The third charge-discharge curve of the NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples at 0.1 A g-1 rate in the voltage range of 2.0-4.0 V are shown in Figure 4(b). Good reversible two plateau regions can be observed at all the 13



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samples. The discharge/charge plateaus agree well with the redox peaks shown in the CV curve (Figure 4 (c)).

The third-cycle cyclic voltammograms of NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples were measured at a scanning rate of 0.1 mV s−1 within the voltage range of 2.0-4.0 V vs Li/Li+ at room temperature (Figure 4 (c)). The symmetrical feature of the CV curve suggests a good reversibility in the cycling process. For Na0.00V2O5, three intensive reduction peaks located at 3.41, 3.21, and 2.31 V correspond to the phase transitions α / ε, ε / δ, and δ / γ, respectively.11 At the same time three obvious oxidation peaks appear during the anodic scanning, at 2.50, 3.39, and 3.46 V, respectively. Apart from these peaks, a pair of obvious redox peaks appears in the high potential region (3.55 V/3.65 V) can be ascribed to the irreversible phase transition of the γ / γ ′system.52 For the NaxV2O5 (x = 0.005, 0.01, 0.02) sample, the shape of the CV curve is similar to that of Na0.00V2O5. However, as the introduction of sodium content, the CV curves present some changes. Taking Na0.01V2O5 electrode as an example, the reduction peaks shift toward 3.40 V, 3.20 V and 2.30 V, respectively. The potential differences between oxidation and reduction peaks reflect the polarization of the electrode. More importantly, new peaks appear in Na pre-inserted samples, such as the redox peaks at 2.91/2.98 V stems from the redox reactions in NaxV2O5 (x = 0.005, 0.01, 0.02) sample, which imply different reactions during the electrochemical processes. Of particular concern is Na0.005V2O5 sample different from other Na pre-inserted sample, it does not exist peak around here, similar to Na0.00V2O5, which may attribute to the lower sodium content does not induce the new electrode reaction. Meanwhile, the introduction of sodium result in the phase transition of the γ / γ ′system disappeared.

Figure 4 (d) shows the rate capability of NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02), and the discharge capacity of the pristine sample (Na0.00V2O5) at 0.1 A g-1, 0.2 A g-1, 0.4 A g-1, 0.8 A g-1, 1.6 A g-1 and 0.1 A g-1 in succession are 206.0, 192.3, 178.3, 158.6, 134.9 and 194.7 mAh g−1, which is lower than that of the Na pre-inserted samples. 14


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Na0.01V2O5 displays the best performance, its discharge capacities are 243.1, 231.2, 210.1, 185.2, 154.6, and 224.6 mAh g−1, respectively. It is obvious that sodium pre-inserted samples play a good performance in rate capability compared with un-inserted V2O5. Na pre-inserted is substantially beneficial to the rate performance may be the reason that the sodium ions acted as pillars in V2O5 structures, as suggested in Li1.1Ni0.2Co0.3Mn0.4O2

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or LiFePO4.54 This kind of pillars in the V2O5

framework can provide large space for the movement of lithium ions, which agrees with XRD pattern results of increasing the unit cell volume of V2O5 with Na content. Pre-insertion of Na ion enlarges the distance between adjacent layers beneficial the diffusion of lithium ions during the intercalation and deintercalation process and provide more space to host lithium ions with lowered insertion energy. The pre-inserted Na ions would also reduce some pentavalent vanadium ions to tetravalent; such tetravalent vanadium cations may promote or catalyze the intercalation reaction.55 Reduction of pentavalent vanadium cations to tetravalent is accompanied to an appreciable change of ionic radius, so the host crystal lattice would be distorted resulting in possibly reduced mass transport. Reduction of pentavalent vanadium cations to tetravalent also means less valence change available to accommodate the insertion of lithium ions. So the promotion and enhancement of Li-ion intercalation would be balanced or over run by the counter impacts that reduce the Li-ion intercalation, 0.01 might be this magic number for this system.

As shown in Figure 4 (e), obviously, the reversible discharge capacity and cycling stability for the V2O5 samples are lower than those of Na pre-inserted V2O5. It indicates that Na pre-inserting is beneficial to the reversible intercalation and deintercalation of Li+, so it enhances the specific capacity reversibility of the V2O5. Na0.01V2O5 exhibits the highest reversible discharge capacity and excellent cycling stability. It shows an initial reversible capacity of 216.2 mAh g-1 at 0.4 A g-1 within the range of working potential between 2.0-4.0 V. The reversible capacity can keep at 191.4 mAh g-1 after 100 cycles, corresponding to 88.5 % of the initial charge capacity. The coulombic efficiencies for NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) electrodes are 15



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constant at approximately 100% throughout the overall cycling process, the intercalation of lithium do not deintercalation completely at first correspond to an abnormal phenomenon that coulombic efficiencies reach to about 110%.

The cells after first and fifth cycles with 0.05A g-1 at a state of charging to 4V where the deintercalation of lithium ions have been completed are dismantled in an argon-filled glovebox in which both the content of oxygen and water are below 1 ppm. The cathodes are cleaned by dimethyl carbonate and identified on a X’ Pert3 diffractometer (PANalytical, Netherlands) with a Cu-Kα radiation source (λ = 1.54056 Å) over the range of 10° to 50° (2θ) with a step size of 0.013°, the accelerating voltage and current were 40 kV and 40 mA, respectively. As shown in Figure 4 (f), The (001) diffraction peak in Na0.01V2O5 samples shift from 20.35 to 19.5°, and keep in 19.5° after several cycles, the shift of peak (001) is the intercalation of lithium ions do not completely extract. The invariability of peak (001) after several cycles with 0.05A g-1 at a state of charging to 4V imply the interlayer distance remains unchanged, so the pre-inserted sodium ions are still there. These are well with the coulombic efficiency as shown in Figure 4 (e).

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Figure 4. (a) Charge-discharge curves of NaxV2O5 (x =0 .00, 0.005, 0.01, 0.02) at 0.1A g-1 in the voltage range of 2.0-4.0 V, (b) The third charge-discharge curve of the NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples at 0.1 A g-1 rate in the voltage range of 2.0-4.0 V. (c) The third-cycle cyclic voltammetry profiles of NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) at a scanning rate of 0.1 mV s-1 in the voltage range of 2.0-4.0 V, and the specific values of redox peaks corresponds to redox reaction are shown in Table 3. (d) Rate performance of NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples at varied current densities (e) Cycle performance and coulombic efficiency of NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) at 0.4A g-1 and (f) Ex-situ XRD patterns obtained for Na0.01V2O5 at various cycles of charge.

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Table 3. The specific values from the third-cycle cyclic voltammetry profiles

Samples Na0.00V2O5 Na0.005V2O5 Na0.01V2O5 Na0.02V2O5

phase transitions [reduction/oxidation (V)] α/ε ε/δ δ/γ γ/γ′ 3.41/3.46 3.40/3.46 3.40/3.46 3.40/3.46

3.21/3.39 3.20/3.39 3.20/3.39 3.20/3.39

2.31/2.50 2.30/2.51 2.30/2.51 2.30/2.51

3.55/3.65 -

New peaks R/O (V) 2.91/2.98 2.90/2.98

Electrochemical impedance spectroscopy (EIS) measurement of NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) at the open circuit voltage (3.4 V) was carried out in the frequency range between 10 kHz and 0.1 Hz, all spectra are shown in Figure 5 (a). The curves include a semicircle in the high frequencies and followed by a straight line in the low frequencies. Equivalent circuit shown in the inset of Figure 5 (a) was used to discern the values of resistances, where Rs, Rf, Rct, CPE and ZW represent the resistance of the electrolyte, the resistance of the SEI film and charge transfer resistance, the double layer capacitance and the Warburg resistance, respectively. The arcs in high frequencies are assigned to the charge transfer resistance of electrode materials. As shown in the inset of Figure 5 (a), the pre-insertion of sodium ions would reduce some pentavalent vanadium cations to tetravalent cations as revealed by XPS analyses with the results shown in Figure 3 and Table 2, so that the electron hopping between tetra and pentavalent cations become possible. All samples show the charge transfer resistance from 40-52Ω , the specific values are shown in Table 3, which is far less than that of transition metal phosphates cathode materials45 and NMC cathode materials,56 revealing that V2O5 cathode materials still have a low charge transfer resistance in half cells. Such a low charge transfer resistance could be partly attributable to the tetravalent vanadium cations and oxygen vacancies presented in the pristine vanadium pentoxide as indicated in Figure 3 and Table 2, which have also been widely reported and accepted in literature.16 Meanwhile, the introduction of Na has no obvious effect on the charge transfer resistance of electrode materials within the margin of error.

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At the same time the diffusion coefficient of lithium ions could be figured out from the low frequencies plots of EIS spectra (Figure 5 (b)) based on the following equations57 (3) (4) where R is the gas constant (8.314 J·mol-1 K-1); T is the absolute temperature (298K); A is the surface area of the electrode (0.50 cm2); n is the number of electrons per molecule during oxidation; F is the Faraday constant (96500 C); C is the concentration of lithium ion (1.0× 10-3 mol·cm-3); and σ is the Warburg factor which is related to Z’ obtained from the slope of the lines in Figure 5 (b). The selected low-frequency region ranges from 0.10 to 0.79 Hz. The diffusion coefficient values of NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) are found to be 1.28×10-14, 8.26×10-14, 9.66×10-14 and 4.02×10-14 cm2 s-1, respectively. The linear relations are shown in Table 4. It is obvious that the lithium ion diffusion coefficient increases due to the Na ion pre-insertion. This result clearly indicates that the lithium ion mobility in V2O5 can be effectively improved by a small amount of Na ion pre-insertion. The lithium ion diffusion coefficient in NaxV2O5 samples increases first, and then decreases with the further rising of Na content. Such a change may be attributed to the fact that a small quantity of Na pre-insertion enlarged the distance between adjacent layers as revealed by the XRD patterns (shown in Figure 1) and consequently opened extra space for lithium ion diffusion, which favor to increase the diffusion coefficient of lithium ion.42 However, sodium ions occupying the sites between V2O5 layers, also served as a lithium ion diffusion blocker because the electrostatic repulsion and steric hindrance, which would decrease the diffusion coefficient of lithium ions. Those two counteracting effects collectively determine the increase and decrease of the lithium ion diffusion coefficient in NaxV2O5.

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Figure 5. (a) EIS spectra of NaxV2O5 (x=0.00, 0.005, 0.01, 0.02) collected at 3.4 V in the frequency range between 0.1 Hz and 100 kHz with the amplitude of 5 mV, (b) Graph of Z’ plotted against ω-1/2 at low-frequency region for NaxV2O5 electrodes for x = 0.00, 0.005, 0.01 and 0.02. Table 4. The charge transfer resistance, the linear relation and the diffusion coefficient values of Li+ for NaxV2O5 (x = 0.00, 0.005, 0.01, 0.02) samples.

Samples

charge transfer resistance /Ω

Na0.00V2O5 Na0.005V2O5 Na0.01V2O5 Na0.02V2O5

43 40 52 45

linear relation X: ω-0.5 Y: Z'

diffusion coefficient of Li+

Y=829.73X-149.80 Y=327.29X-61.05 Y=302.67X-7.47 Y=469.20X-42.16

/ cm2 s-1 1.28 8.26 9.66 4.02

Conclusions Na-pre-inserted V2O5 samples were prepared by a simple sol-gel approach and freeze-drying method with the direct addition of sodium salt followed by an annealing process. The lattice parameters calculated from the XRD data were refined by Rietveld refinement methods, confirming that Na ions have successfully been inserted into the V2O5 lattice and Na-ions occupy the sites located at the interlamination of V2O5. V4+ concentration in the Na-pre-inserted V2O5 sample gradually increased with the increase of sodium content, which further proved Na-ions enter into the V2O5 lattice due to the electroneutrality. Compared with V2O5, the pre-inserted Na-ions 20


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materials, especially Na0.01V2O5, displayed excellent cycling stability and rate capabilities. After 100 cycles, the specific capacity of Na0.01V2O5 sample maintains 191.4 mAh g-1 that is higher than 169.3 mAh g−1 of the pure V2O5 sample at the same current density of 0.4 A g−1. Besides, Na0.01V2O5 sample also showed the highest lithium ion diffusion coefficient amongst all samples, suggesting that a small amount of Na can effectively expand the ion channels and improve the reactive kinetics of V2O5 owing to the bigger size of Na ion expanded the interplane spacing of the host. Na-pre-inserted V2O5 with improved electrochemical performances may be developed as a promising cathode material for lithium ion batteries in the practical applications.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. XRD patterns, Crystal constants table, XPS spectra, The first three charge– discharge curves of all samples (PDF)

Corresponding Author * E-mail: [email protected]

Acknowledgements This work was supported by the "Thousands Talents" program for the pioneer 21



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researcher and his innovation team, China. This work was also supported by the National Science Foundation of China (51374029) and the National Science Foundation (NSF, DMR-1505902), Program for New Century Excellent Talents in University (NCET-13-0668), Fundamental Research Funds for the Central Universities (FRF-TP-14-008C1) and China Postdoctoral Science Foundation (2015M570987).

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