Impacts of Surface Energy on Lithium Ion Intercalation Properties of

Department of Materials Science and Engineering, University of Washington, Seattle 98185-2120, Washington United States. ACS Appl. Mater. Interfaces ,...
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Impacts of Surface Energy on Lithium Ion Intercalation Properties of V2O5 Wenda Ma,† Changkun Zhang,† Chaofeng Liu,† Xihui Nan,† Haoyu Fu,† and Guozhong Cao*,†,‡ †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China Department of Materials Science and Engineering, University of Washington, Seattle 98185-2120, Washington United States

‡

S Supporting Information *

ABSTRACT: Oxygen vacancies have demonstrated to be one of the most effective ways to alter electrochemical performance of electrodes for lithium ion batteries, though there is little information how oxygen vacancies affect the electrochemical properties. Vanadium pentoxide (V2O5) cathode has been investigated to explore the relationship among oxygen vacancies, surface energy, and electrochemical properties. The hydrogen-treated V2O5 (H−V2O5) sample synthesized via thermal treatment under H2 atmosphere possesses a high surface energy (63 mJ m−2) as compared to that of pristine V2O5 (40 mJ m−2) and delivers a high reversible capacity of 273.4 mAh g−1 at a current density of 50 mA g−1, retaining 189.0 mAh g−1 when the current density increases to 2 A g−1. It also displays a capacity retention of 92% after 100 cycles at 150 mA g−1. The presence of surface oxygen vacancies increases surface energy and possibly serves as a nucleation center to facilitate phase transition during lithium ion intercalation and deintercalation processes. KEYWORDS: oxygen vacancies, surface energy, electrochemistry, vanadium pentoxide, surface energy analyzer

1. INTRODUCTION Lithium ion batteries (LIBs) have become essential for our modern life to keep us connected all the time while in constant motion and for use in portable electronic devices as well as hybrid and all-electric vehicles.1−3 To meet the demand for high energy and high power for the rapid advancement of new technologies, it is imperative to improve the performance of cathodes in the LIBs. Among several promising cathodes, orthorhombic vanadium pentoxide (V2O5) has a layered structure that has been extensively investigated due to its high energy density, low cost, and abundant sources.4−6 The theoretical capacity of V2O5 is approximately 294 mAh g−1 for two Li ion de-/intercalation, and the practically achievable capacity is very close to this theoretical one, which is much higher than that of other common cathodes such as LiFePO4 (170 mAh g−1), LiMn2O4 (148 mAh g−1), and Li3V2(PO4)3 (197 mAh g−1).7−9 However, V2O5 still suffers from slow diffusion of Li ion (10−12−10−13 cm2 s−1), low electrical conductivity (10−7−10−6 S cm−1), poor cycle performance, and thus inferior rate capability for commercial application.10−12 The introduction of oxygen vacancies (VO·· ) has been demonstrated to enhance the Li ion diffusion coefficient and electronic conductivity, leading to much improved lithium ion intercalation properties.13 Several studies have investigated the influences of V··O on lithium ion intercalation properties of various electrodes. Liu et al. displayed that the existence of V··O in hydrogen-treated TiO2 nanowires delivered a much higher reversible capacity.14 Xu et al. showed that MoO3−x exhibited a high reversible capacity due © 2016 American Chemical Society

to high Na ion diffusion coefficient and electric conductivity attributing to the incorporation of V··O.15 The positive impacts of V O·· have also been proven in MnO2 , SnO 2 , and Li4Ti5O12.16−18 VO·· are commonly created through three different methods: doping of low-valent cations through substitution, thermal treatment in reducing gas, and controlled synthesis.19,20 Low-valence cation doping could easily control and adjust the amount and distribution of V··O. With the respect to V2O5, for example, Ni doping in V2O5 maintained the stable crystal structure and induced the formation of tetravalent vanadium ions, V4+, and delivered a practical storage capacity of 294 mAh g−1 at the current of 1/6 C, which is equal to the theoretical capacity.21 Oxygen-deficient V2O5 nanosheets prepared by hydrogenation at 200 °C revealed that V··O in the O(2) position had a positive effect on Li ion diffusion.22,23 Mesoporous V2O5 nanosheets with 10% of V4+ and self-doped V4+-V2O5 nanoflakes with 17.4% V4+ delivered an approximate theoretical capacity with 147 and 293 mAh g−1 at the current of 100 mA g−1 in the voltage ranges of 2.5−4.0 V and 2.0−4.0 V (versus Li+/Li), respectively.11,12 Both V4+ and V··O play a vital role affecting the chemistry and electrochemical property of V2O5 and are believed to promote and catalyze the electrochemical reactions at the interface between electrode and electrolyte, enhance both mass and charge transport properties, Received: May 27, 2016 Accepted: July 11, 2016 Published: July 11, 2016 19542

DOI: 10.1021/acsami.6b06359 ACS Appl. Mater. Interfaces 2016, 8, 19542−19549

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Diagram Illustrating of the Experimental Approach of Reacting V2O5 with Hydrogena

a Scheme shows creation of oxygen vacancies on the surface (left to middle) and the distribution and location of oxygen vacancies in V2O5 crystal structure, as suggested by Peng et al.22

temperature, NA is Avogadro’s number, and a is the crosssectional area of the probe molecule.27,28 If the probes use different n-alkane molecules and the dispersive interaction only occurs, then the work of adhesion can be shown as follows

and buffer the internal mechanical stress induced by the volume variation when cycling.11,12,21,22,24−26 In the present investigation, a surface energy analyzer (SEA) was used to measure the surface energy of V2O5 powder with surface V··O. As is shown in Scheme 1, the surface of V2O5 particles through H2 thermal treatment is covered with V··O for surface energy measurement. 1.1. Surface Energy Analyzer Based on Inverse Gas Chromatography. The SEA is based on inverse gas chromatography (IGC), which is a versatile, sensitive, powerful technique capable of determining the surface parameters of sample.27 In IGC, the column is packed with the solid powder, and a single gas or vapor (probe molecules) is injected into the column.28 The method is based on the physical adsorption time of the gas probe molecule on solid surface. From the retention time of the known probe molecules, the surface energy of solid sample is calculated. Compared with traditional IGC methods, the SEA with IGC method is more accurate because SEA delivers different molecule probes with the same surface coverage whereas the traditional IGC uses the same gas volume for all kinds of probes.29 For the IGC method, surface energy is divided into two parts: dispersive component, γds , and polar component, γSP s , for calculation. The principles of the IGC have been widely described in literature.27,28 Briefly reiterating here, the retention of the gas probe molecules is quantified by the net retention time tN and net retention volume VN. VN is the surfacecontaining volume of gas probes on solid sample surface pushed by inert carrier gas through the chromatographic column, which can be calculated by the following equation. VN =

j T F(t R − t0) m 273.15

Wa = W ad = 2 γsd γld

γds

where and are, respectively, the dispersive components of the surface energy of the solid sample and probe. If we combine eqs 2 and 3, then eq 4 can be concluded.31 RT ln VN = 2NAa γld γsd + Const

(4)

From eq 4, it can be seen that RT ln VN ∝ a γld so that a series of nonpolar n-alkane using as probes can plot a line with RT ln VN versus a γld . The dispersive component of surface energy will calculate through the slope of line shown in eq 5. ⎛ slope ⎞2 γsd = ⎜ ⎟ ⎝ 2NA ⎠

(5)

This method is the Schultz method for obtaining dispersive surface energy. For the polar component of surface energy, the specific contributions are calculated through the specific free energy of adsorption ΔGSP that is the vertical distance between the polar probe and the dispersive probe reference line plot. The specific component of surface energy is divided by acid (Lewis acceptor), γ+s , and base (Lewis donor), γ−s , parameters provided by van Oss.29 This method is exhibited by eq 6. −ΔGSP = NAa2( γl+γs− +

(1)

where tR is retention time of the injected gas probe molecules and t0 is the mobile phase (methane) dead time. This subtraction obtains the net retention time tN. F is the exit flow rate at 1 atm and 273.15 K, and T is the column temperature. m is the mass of the sample, and j is the James− Martin gas correction factor.30 VN relies only on the sample−probe interactions at infinite dilution conditions, so the free energy of adsorption of the probes, ΔGa, can be written as follows. −ΔGa = RT ln VN + Const = NAaWA

(3)

γdl

γl−γs+ )

(6)

Theoretically speaking, a pair of acidic and basic probes such as dichloromethane and ethyl acetate is just needed so as to obtain the specific component of the surface energy by means of Owens and Wendt approach using in eq 7 along with the van Oss method.32 γsSP = 2 γs+γs−

(7)

In this study, the surface energy of the H−V2O5 was measured, and the relationship among V··O, surface energy, and electrochemical performance was discussed to prove that increases of surface energy are caused by the redundant V··O instead of thermodynamics reasoning.

(2)

ΔGa is also related to the work of adhesion (WA) as shown in eq 2, where R and T are universal gas constant and absolute 19543

DOI: 10.1021/acsami.6b06359 ACS Appl. Mater. Interfaces 2016, 8, 19542−19549

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ACS Applied Materials & Interfaces

2. RESULTS AND DISCUSSION From the TGA curve of V2O5 gel shown in Figure S1a, there is no mass loss at temperatures above 350 °C, indicating that the crystal water has been removed completely. XRD patterns of pristine V2O5, H−V2O5-350 °C, and H−V2O5-400 °C samples are shown in Figure 1. In Figure 1a, H−V2O5-350 °C displays a

detectable morphology difference between H−V2O5 and pristine V2O5 samples as shown in Figure S3. The valence states of vanadium and oxygen in H−V2O5 and pristine V2O5 samples were investigated by XPS measurement. In Figure 2a,b, the V 2p3/2 peak spectra of H−V2O5 and pristine V2O5 demonstrate two peaks at 517.25 and 515.95 eV, which are attributed to the V5+ and V4+ components of the samples, respectively (reference by C 1s of 284.8 eV).21,33 The presence of V4+ suggests the presence of V··O, so as to maintain the electroneutrality. The molar ratio (V4+/(V4+ + V5+)) of H− V2O5 was estimated to be 6.3%, which is twice that of pristine V2O5 (3.1%). A small quantity of V··O was also found in pristine V2O5, which is in a good agreement with literature data.21,24 In Figure 2c,d, the spectra of O 1s core peak of V−O−V bond is 530.01 eV in V2O5.34,35 There are also characteristic peaks between 530 and 535 eV, in which the peaks of 533.50, 532.14, and 530.78 eV correspond to C−O, V−OH, and CO bonds, respectively, where the hydroxyl groups have absorption on the surface and the peaks of C−O and CO stem possibly from contamination of organics on the surface.36−39 Hydroxyl groups also occurred on the surface of hydrogen-treated TiO2.14 However, in Figure 2c,d, the areal proportion of V−OH peaks of H−V2O5 is 23% of total peaks (O 1s), which is slightly higher than that (20%) of pristine V2O5. This reveals that more functionalized hydroxyl groups were synthesized after annealing in H2. This result indicates that the increasing amount of V··O may be accompanied by an increasing amount of hydroxyl groups. The surface energy of the samples was measured by SEA. The n-alkanes such as nonane, octane, heptane, and hexane serving as dispersive probes were injected successively at 10% surface coverage for the calculation of dispersive surface energy. The polar component of surface energy was examined using ethyl acetate and dichloromethane as the polar probes. The column dead time was obtained using methane as inert probe, and the carrier gas was helium. The dispersive component was calculated using the Schultz method, and the specific polar component was obtained using van Oss approach.29,31 The specific surface area of the samples was determined by means of nitrogen sorption isotherms and calculated using the Brunauer−Emmett−Teller (BET) method (Figure S4), which is required for the SEA measurement. The patterns of the IGC method are shown in Figure 3a,b, and the surface energy results shown in Table 1 are calculated by eqs 4−7, where the γd is dispersive surface energy, γab is acid−base specific polar surface energy, and γt is the total surface energy. The total surface energy of H−V2O5 (63 mJ m−2) is seen to be 57% larger than that of pristine V2O5 (40 mJ m−2), which coincides with 50.8% more V··O on the surface of H−V2O5. The results suggest that the surface energy might be closely related to the amount of surface V··O. Both dispersive (58.76 mJ m−2) and polar (4.21 mJ m−2) surface energy of H−V2O5 are higher than those of pristine V2O5 (38.32 and 1.68 mJ m−2) because of the introduction of surface V··O. The dispersive surface energy corresponds to the combined impact of London, Debye, and Keesen forces, corresponding to the respective instant force of nonpolar dipoles, induction force of nonpolar dipoles and polar dipoles, and permanent force of polar dipoles. The formation of V··O improves both London and Debye forces, resulting in an increased dispersive surface energy. The increased polar surface energy in H−V2O5 is ascribed to stronger adhesion between sample surface and polar probes. The hydroxyl groups on the surface detected by XPS may play an important role to

Figure 1. (a) XRD patterns of pristine V2O5, H−V2O5-350 °C ,and H−V2O5-400 °C, respectively, in the 2θ range of 14−41°. (b) XRD patterns of V2O5 characteristic peak at (010) peak for pristine and H− V2O5-350 °C. A slight shift of H−V2O5-350 °C to the larger angle can be observed.

pure V2O5 pattern without any impure peak compared with the standard pristine V2O5 pattern (Powder Diffraction File (PDF) no. 01-072-0433, Joint Committee on Powder Diffraction Standards (JCPDS), 2004), whereas the H−V2O5-400 °C sample possibly has a trace amount of V6O13 phase with characteristic peaks at 2θ = 14.80, 25.35, 26.84, and 30.14° (PDF no. 00-043-1050, JCPDS, 2004) and also VO2 phase with characteristic peak at 2θ = 27.86° (also seen in Figure S2, PDF no. 00-009-0142, JCPDS, 2004). Hence, 350 °C was considered and chosen as an appropriate temperature for surface modification of V2O5. In Figure 1b, a little shift of the (010) peak of H−V2O5-350 °C to a large angle (Δ2θ = 0.013°), suggests a reduction of lattice volume, possibly due to the presence of V··O.22 Although the shift value of 0.013° may not be accurate, the shift is certain as reflected by the symmetries of both peaks. The pristine yellow V2O5 changed to greenish-yellow H− V2O5 after hydrogenation. The greenish-yellow color indicates the existence of V4+.12 Figure S1b shows the photographs of pristine V2O5 and H−V2O5 films, and their UV−vis absorption spectra using dilute slurry of samples demonstrates that both H−V2O5 and pristine V2O5 have similar absorption though H− V2O5 has a greenish-yellow color. SEM images reveal no 19544

DOI: 10.1021/acsami.6b06359 ACS Appl. Mater. Interfaces 2016, 8, 19542−19549

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Figure 2. X-ray photoelectron spectroscopy (XPS) spectra for V 2p3/2 of (a) H−V2O5 and (b) pristine V2O5; O 1s of (c) H−V2O5 and (d) pristine V2O5.

Table 1. Surface Energy of H−V2O5 and Pristine V2O5 Samples Using in Schultz Method sample

BET specific surface area (m2 g−1)

γd (mJ m−2)

γab (mJ m−2)

γt (mJ m−2)

V2O5 H−V2O5

6.70 7.05

38.32 58.76

1.68 4.21

40.00 62.97

thermal treatment of the sample in hydrogen can lead to a highenergy metastable state, whereas a pretreatment of the samples in O2 may make sample more robust so as to maintain such a metastable state for an extended period of time. The electrochemical properties and lithium ion storage performance of H−V2O5 and pristine V2O5 electrodes were carried out using coin half-cells with lithium metal foil as the anode. Figure 4a presents the third cycle of the cyclic voltammetry (CV) curves of the H−V2O5 and pristine V2O5 electrodes at a scan rate of 0.1 mV s−1 in the voltage range of 2.0−4.0 V (versus Li/Li+). Three main reduction peaks at the potential of 3.45, 3.19, and 2.34 V for the pristine V2O5 belong to the phase transitions α/ε, ε/δ, and δ/γ, respectively.41−43 Three related anodic peaks are 3.56, 3.34, and 2.54 V and pairs of additional redox peaks at 3.56/3.67 V can be ascribed to phase transition γ/γ′.42,43 There is a pair of split redox peaks for both H−V2O5 and pristine V2O5 at 2.42/2.48 V and 2.41/2.52 V, which may be attributed to the tetravalent vanadium ions and the attendant V··O.12 The gap between redox peaks of the H−V2O5 electrode is narrower than that of pristine V2O5 shown in Table 2, implying that reduction of the polarization for H−V2O5 involved both decrease of the resistance and increase of Li ion diffusion.14 The second cycle and all of three cycles of the CV curves of the H−V2O5 and pristine V2O5 electrodes are respectively depicted in Figure S5a,b. In Figure S5b, both the second and third cycles of H−V2O5 coincide, implying the reversibility and stability of cycling performance. The 10th cycle of galvanostatic charging−discharging profiles of two samples are shown in Figure 4b, in which a voltage plateau of H−V2O5 electrode is flat in approximate agreement with the

Figure 3. Plots of RT ln VN vs a γld used to calculate dispersive part of surface energy and specific interaction parameters of ethyl acetate and dichloromethane with (a) H−V2O5 sample and (b) pristine V2O5 sample.

introduce the hydrogen-bond interaction that is a main factor for polar surface energy,32 and the addition of hydroxyl groups can lead to the enhancement of polar surface energy, which was also found with organic solvent and Si−OH groups.40 The 19545

DOI: 10.1021/acsami.6b06359 ACS Appl. Mater. Interfaces 2016, 8, 19542−19549

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Figure 4. (a) Third cycle of the CV curves of the H−V2O5 and pristine V2O5 electrodes at a scan rate of 0.1 mV s−1 in the voltage range of 2.0−4.0 V (versus Li/Li+). (b) Tenth cycle of galvanostatic charging−discharging profiles of the H−V2O5 and pristine V2O5 electrodes in the voltage range of 2.0−4.0 V (versus Li/Li+). (c) Rate capability of H−V2O5 and pristine V2O5 electrodes. (d) Cycling performance and Coulombic efficiency of H− V2O5 and pristine V2O5 electrodes at discharge current density of 150 mA g−1.

Table 2. Redox Peaks of CV Curves and Gap between Redox Peaks for H−V2O5 and V2O5 redox peaks

α/ε (V)

gap (V)

ε/δ (V)

gap (V)

δ/γ (V)

gap (V)

γ/γ′ (V)

gap (V)

V2O5 H−V2O5

3.45/3.56 3.46/3.54

0.11 0.08

3.19/3.34 3.20/3.34

0.15 0.14

2.34/2.54 2.36/2.51

0.20 0.15

3.56/3.67 3.57/3.66

0.11 0.09

the surface of sample enhances its electrochemical activity and maintains a metastable high-energy state in order to reduce the Gibbs free energy of phase transformation and develop the availability of active material. The cycling performances of the H−V2O5 and pristine V2O5 samples were also examined at the current density of 150 mA g−1 for 100 cycles (Figure 4d). Figure 4d shows that the H−V2O5 sample has a stable discharge capacity less than 0.08% decay per cycle over 100 cycles. H−V2O5 exhibits a discharge capacity of 232.4 mAh g−1 after 100 cycles, maintaining a capacity retention of 92% from its cycle 3 capacity (252.3 mAh g−1), whereas the capacity of pristine V2O5 sample has a capacity retention of 81%. Additionally, the presence of hydroxyl groups on the surface of sample might develop the performance of electrode,45 and addition of OH molecules can improve the wettability of separator and electrolyte resulting in the better property of electrode.46 Also, using hydroxyl groups as precursor to form V··O consumes lower binding energy than forming V··O directly, and the specific oriented V··O (O(2)) will be synthesized.22,47 Thus, the hydroxyl groups might be beneficial to the performance of electrode. The electrochemical impedance spectra (EIS) of H−V2O5 and pristine V2O5 after three cycles have been measured and are shown in Figure 5a. The Nyquist plots consist of the compressed semicircle that attributes to the charge transfer resistance (Rct) and constant-phase element (CPE, referring to the double-layer capacitance).20,48 In the low-frequency area, a slope line at approximately 45° ascribes to the Warburg

redox peak in the CV curve in Figure 4a, and comparison of the two plateaus exhibits H−V2O5 has an energy density larger than that of pristine V2O5. The galvanostatic charging−discharging profiles of H−V2O5 and pristine V2O5 electrode over cycles 2− 100 in the voltage range of 2.0−4.0 V (versus Li/Li+) are shown in Figure S6a,b. In Figure S6a,b, with increasing cycles the plateau potential of H−V2O5 remains almost unchanged; in contrast, the gap between charging−discharging plateaus of pristine V2O5 is becoming larger resulting in enhanced polarization. The comparisons of cycles 2 and 50 of the profiles of H−V2O5 and pristine V2O5 electrodes are presented in Figure S6c,d, respectively. Figure 4c shows the rate capability of H−V2O5 and pristine V2O5 electrodes. The discharge capacity of H−V2O5 electrode is 273, 256, 245, 225, 207, and 189 mAh g−1 at the rates of 0.05, 0.1, 0.2, 0.5, 1, and 2 A g−1, respectively. The discharge capacities of the H−V2O5 sample are higher than those of pristine V2O5 sample at all current densities because the existing of high-energy V··O points can reduce the nucleation energy of phase transition. The discharge capacity of H−V2O5 can retain its original value when the current density returns to 0.05 A g−1, but the discharge capacity of the pristine V2O5 sample cannot reach the initial value distinctly, indicating that the high-energy state enhances the rate capability and reversibility of electrode. V··O on the surface can buffer the volume change during Li ion de/intercalation as well as enhance the electrical conductivity and Li ion diffusion by means of importing high-energy nucleation points so that V··O can develop the rate capability of the electrode.24,44 V··O on 19546

DOI: 10.1021/acsami.6b06359 ACS Appl. Mater. Interfaces 2016, 8, 19542−19549

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ACS Applied Materials & Interfaces

coefficient of H−V2O5 are attributed, at least in part, to the much increased amount of V··O.

3. CONCLUSIONS Surface oxygen vacancies, V··O in V2O5, were introduced through thermal treatment in H2 for a short period of time, accompanied the formation of tetravalent cations, V4+. The hydrogen-treated V2O5 ensures the favorable cycling stability and excellent rate capability with the initial discharge capacity 273.4 mAh g−1 at a current density of 50 mA g−1 and 189.0 mAh g−1 at 2 A g−1. The H−V2O5 sample possessed a stable discharge capacity with 0.08% decay per cycle at a current density of 150 mA g−1. V··O and/or V4+ were found to increase the surface energy, catalyze the lithium ion intercalation reactions, and facilitate phase transition during charging− discharging cycles. The increased hydroxyl groups on the surface of H−V2O5 might also benefit the performance of electrode, which remains a task for future exploration. 4. EXPERIMENTAL SECTION 4.1. Sample Preparation. A dilute solution of V2O5·nH2O was synthesized via a facile method reported by Liu et al.26 Briefly, 0.5 g of V2O5 powder (TianJin, FuChemical, >99.0%) was dissolved in a mixture of 7.5 mL of deionized water and 2.5 mL of 30% H2O2. The solution was sonicated for 30 min to get a yellow-brown gel and then dried at 80 °C for 1 h. After aging for 12 h, the sample was thermally treated under different gas at 350 °C. For H−V2O5-350 °C (H− V2O5), the sample was first treated in O2 for 1 h, followed by 10 min in Ar with flow rate of 200 sccm, then in H2 for 15 min, and slowly cooled down in Ar. For V2O5, the sample was treated only in O2 for 1.5 h for comparison. Also, with the same dehydration temperature (350 °C), another sample labeled as H−V2O5-400 °C was also treated under the same conditions as H−V2O5-350 °C but at 400 °C for surface treatment in H2. 4.2. Material Characterization. The crystallographic features of the samples were detected by X-ray diffraction (XRD) on a Marcogroup diffractometer (MXP21 VAHF) with Cu Kα radiation (λ = 1.5418 Å) in the range of 10−50°. The morphology of samples was characterized by the field-emission scanning electron microscopy (Nova NanoSEM 450, Czech Republic). The surface area of samples was examined by means of nitrogen sorption isotherms using Brunauer−Emmett−Teller method (ASAP 2020 HD88, USA). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA) was used to examine the elements and their bonding/ valence states on the sample surface with Al KR radiation. The binding energy of XPS was corrected by adjusting the C 1s photoelectron peak at 284.8 eV for the reference. Thermal gravity analysis (TGA) was done on a Simultaneous Thermal Analyzer (STA 449F3, NETZSCH, Germany) from 20 to 450 °C in O2 atmosphere with a heating rate of 5 °C min−1. The samples were also characterized by ultraviolet−visible spectroscopy (UV-3600, SHIMAZU, Japan) with a wavelength ranging from 350 to 800 nm. The surface energy of samples was obtained by surface energy analyzer (SEA, Surface Measurement Systems, UK) with all the measurements conducted at 40 °C. 4.3. Electrochemical Property and Li Ion Intercalation Measurement. The electrochemical properties of samples were analyzed using R2032 coin-type cells. For preparation of electrode, a slurry was formed by active material, conductive acetylene black, and poly(vinylidene fluoride) with a molar ratio of 7:2:1 in N-methyl-2pyrrolidone covered Al foil as current collector with around 150 μm thickness. The electrode was punched into round disk with a diameter of 8 mm after vacuum drying at 120 °C for 12 h. Cells were assembled in a glovebox (MIKROUNA, China) filled with argon, and Li metal as reference electrode was used as anode. The separator used was Celgard 2400 (Celgard, LLC Corp., USA), and the electrolyte was LBC305-01 (CAPCHEM, China). The galvanostatic discharging/ charging process was obtained by using LAND CT2001A system

Figure 5. (a) Nyquist plots of H−V2O5 and pristine V2O5 electrodes after three cycles in the frequency range of 100 kHz to 0.1 Hz. Inset: equivalent circuit for fitting the EIS plots. (b) Relationship curves between Z′ and ω−1/2 in the low-frequency range.

impedance (W) that is connected to the Li ion diffusion. After fitting the second compressed semicircle with equivalent circuit, the Rct of H−V2O5 and pristine V2O5 are 80.09 and 108.30 Ω, respectively. The Rct of H−V2O5 is lower than that of pristine V2O5, indicating that V··O with high activation energy can develop the kinetics for the electron conduction and Li ion migration while preventing active material from dissolution in the electrolyte at the same time.26,49 The Li ion diffusion coefficient can be calculated by the following equations:12 Z′ = R s + R f + R ct + σwω−1/2 D Li+ =

R2T 2 2A2 n 4F 4C 2σw2

(8)

(9)

where Rs is the sum of resistance of electrolyte, electrode, and separator, Rf is resistance of other factors (probably surface resistance in this study), Rct is charge transfer resistance, ω is the angular frequency in the low frequency range, and σw is the Warburg impedance coefficient that is obtained from the slope of fitting lines in Figure 5b. R (8.314 J mol−1 K−1) is the gas constant, T (298 K) is the absolute temperature, A (0.50 cm2) is the area of the electrode, n (2) is the number of electron transfer per mole active material during Li ion de/intercalation process, F (96 500 C mol−1) is the Faraday constant, and C (1.0 × 10−3 mol cm−3) is the molar concentration of Li ions of electrolyte.20 The resistances of H−V2O5 and pristine V2O5 samples are calculated from the fitting process and eqs 8 and 9. On the basis of eq 9 and the fitting line in Figure 5b, the calculated Li ion diffusion coefficients of H−V2O5 and pristine V2O5 are 1.17 × 10−11 and 6.74 × 10−12 cm2 s−1, respectively. The Li ion diffusion coefficient of H−V2O5 is higher than that of pristine V2O5, demonstrating that activation effects of V··O can provide more facile Li ion diffusion among the active material.26,49 Both the low resistance and large Li ion diffusion 19547

DOI: 10.1021/acsami.6b06359 ACS Appl. Mater. Interfaces 2016, 8, 19542−19549

Research Article

ACS Applied Materials & Interfaces

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(Wuhan LAND electronics Co., Ltd., China). Both cyclic voltammetry (CV) at a scanning rate of 0.1 mV s−1 in the potential range from 4.0 to 2.0 V (vs Li/Li+) and EIS in the frequency range between 100 kHz to 0.1 Hz at the 3.6 V potential were performed by a Solartron device.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06359. Additional experimental results about TGA curve; UV− vis absorption spectra; XRD spectrum; SEM image; N2 adsorption−desorption isotherm; complementary CV curve; galvanostatic charging−discharging profiles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Thousands Talents” program for the pioneer 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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REFERENCES

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DOI: 10.1021/acsami.6b06359 ACS Appl. Mater. Interfaces 2016, 8, 19542−19549

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DOI: 10.1021/acsami.6b06359 ACS Appl. Mater. Interfaces 2016, 8, 19542−19549