Enhanced lithium storage performance of V₂O₅ with oxygen vacancy

Abstract

Orthorhombic phase V₂O₅ nanosheets with a high V⁴⁺ content (V-V₂O₅) have been fabricated via a facile sol–gel method and freeze-drying technology followed with a vacuum annealing process. XPS tests demonstrated that the content of V⁴⁺ in the as-prepared V-V₂O₅ sample was 7.4%, much higher than that (4.7%) in the V₂O₅ sample annealed in air. Compared with the V₂O₅ annealed in air, the V-V₂O₅ sample exhibited better cycling stability, higher lithium storage activity, and smaller electrochemical reaction resistance when evaluated as a cathode active material for lithium ion batteries. For example, the specific capacity of the V-V₂O₅ and V₂O₅ electrodes after 100 cycles at 200 mA g⁻¹ are 224.7 and 199.2 mA h g⁻¹, respectively; after 200 cycles at 3 A g⁻¹ are 150 and 136.7 mA h g⁻¹, respectively.

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1. Introduction

Orthorhombic vanadium pentoxide (V₂O₅) has been widely studied in lithium ion battery cathode materials because of its typical lamellar crystal structure.¹ Compared with conventional lithium ion battery cathode materials, V₂O₅ has much higher theoretical capacity (294 mA h g⁻¹, when storing two Li⁺) than other materials. V₂O₅ also has the advantages of abundant resources, low cost, relatively simple preparation process and good safety.^2–4 However, the drawbacks such as structure instability, low electronic and ionic conductivity and slow electrochemical kinetics drastically reduce its cycling stability and rate capability, which has become the main restrictions to its practical application in the cathode materials of lithium ion batteries.^5,6 To overcome these drawbacks, numerous efforts have been carried out. Some researches mainly concentrate on modifier materials such as various nanostructures V₂O₅,^7,8 aliovalent-ion doped V₂O₅,^6,9 V₂O₅ coated with carbon materials or other compounds.^10,11 These modifiers showed enhanced electrochemical kinetics, specific capacity and rate performance than the pristine V₂O₅ due to their small size and large surface area, which could greatly increase the contact area between actives materials and electrolyte, shorten the diffusion pathways of Li⁺, and relax the mechanical stress associated Li⁺ intercalation/deintercalation. However, it should be noted that these modifications are complex, difficult to control and costly.

The heat treatment atmosphere has great influence on the valence state of V element in V₂O₅. Generally, when being heat treated in an oxidation atmosphere (such as air and O₂ atmosphere), the V⁴⁺/V⁵⁺ ratio and O vacancies in the final V₂O₅ product are very small; when being heat treated in inert gas (such as Ar atmosphere), and reduction atmospheres (such as H₂ atmosphere), the V⁴⁺/V⁵⁺ ratio and O vacancies in the final V₂O₅ product are high, and the V₂O₅ phase can even transform to VO₂ phase in strong reduction atmosphere. These O vacancies in V₂O₅ can provide more active sites for the embedding of Li⁺, and increase the specific discharge capacity of the materials. O vacancies can also increase the electronic and ionic conductivity of the materials and promote the lithium storage kinetics.^12–14 However, the preparation process of V₂O₅ with O vacancy is complex and consumptive, and the electrochemical lithium storage capacity is not satisfactory. It is found that under the condition of oxygen depletion, oxygen can easily escape from the lattice due to the breakage of the V=O bond, resulting in the formation of multiple O vacancies in V₂O₅ materials.

Herein, we prepared orthorhombic phase V₂O₅ nanosheets with a high V⁴⁺ content and O vacancies (denoted as V-V₂O₅) by a facile sol–gel method combined with freeze drying technique followed with annealing in vacuum. The microstructure of V-V₂O₅ was analyzed by physical characterizations, and the electrochemical properties of V-V₂O₅ were evaluated as potential cathode materials for lithium ion batteries (LIBs). The results show that the electrode material has good lithium storage activity, cyclic stability and high current discharge performance.

2. Materials and methods

2.1 Synthesis of V-V₂O₅

All chemical reagents were of analytical purity and used without further purification. The synthesis of V-V₂O₅ is as following: firstly, commercialized V₂O₅ powder (C-V₂O₅) was added into de-ionized (DI) water and 30 wt% H₂O₂ to obtain a solution with a V₂O₅ concentration of 0.3 M and n(H₂O₂) : n(V) = 8 : 1. The resulting solution was stirred for 15 min at room temperature. Sonicated for 10 min, the obtained solution was diluted with DI to C_V = 0.056 M. And then the solution was sonicated for about 1 h until it turned into brownish red V₂O₅ gel. The gel was stored overnight and diluted with DI water and fully stirred to form a brick-red coloured, transparent sol with C_V of 0.028 M. Secondly, this solution was pre-frozen in a refrigerator for 1 day and then the solvent (water) in the frozen sample was removed using a freeze dryer (FD-1A-50, Boyikang Corp., Beijing, China) under a vacuum at 50 °C for 2 days to get the V₂O₅ precursor. Thirdly, the V₂O₅ precursor was annealed in vacuum at 400 °C for 1 h and then annealed at 400 °C in the air for 1 h to form V₂O₅ containing O vacancy (denoted as V-V₂O₅). For comparison, the V₂O₅ precursor was also annealed in air for 2 h and the final product is denoted as V₂O₅.

2.2 Material characterizations

The crystal structure of the prepared samples was determined on X'Pert³ diffractometer (PANalytical, Netherlands) with a Cu K_α as radiation source (λ = 1.54056 °A) in a 2θ range of 10–70°. The morphology of the prepared samples was characterized on a field-emission scanning electron microscopy (FE-SEM) (Hitachi, SU-5000) and transmission electron microscope (TEM, JEOL JEM-2100F). The chemical composition and the valence state of V in the prepared samples were analyzed by X-ray photoelectron spectroscopy (XPS) spectrometer (ESCALAB 250Xi) using monochromic Al K_α excitation.

2.3 Electrochemical measurements

The electrochemical performances of the samples were tested using CR 2016 coin-type cell with metallic lithium as the anode and polypropylene (PP) film as the separator. The cathodes were fabricated by mixing V-V₂O₅, acetylene black, and poly(vinyldifluoride) (PVDF) with a weight ratio of 7 : 2 : 1 in n-methyl-2-pyrrolidone (NMP) solvent. The resulting slurry was then uniformly spread on an aluminium foil current collector. The cathodes were dried at 80 °C for 12 h in an oven and then punched into small disks with a diameter of 15 mm. The thickness of the electrode was about 15 μm and the mass loading of the active material was about 1.0 mg cm⁻². The electrolyte was 1 M LiPF6 in EC/DMC/DEC (1 : 1 : 1 by weight). The cells were galvanostatically charged and discharged between 2.0 and 4.0 V (vs. Li/Li⁺) using LANDCT2001A battery tester at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out on an electrochemical workstation (CHI 760). The CV test was performed between 2.0 and 4.0 V at a scan rate of 0.2 mV s⁻¹. The EIS was measured was performed in the frequency range from 0.01 Hz to 100 kHz at the open circuit voltage (OCV) after given discharge/charge cycles with 5 mV voltage amplitude.

3 Results and discussions

3.1 Structure characterization

Fig. 1 presents the XRD patterns of the pure V₂O₅ and V-V₂O₅ samples. Both pure V₂O₅ and V-V₂O₅ samples show a single orthorhombic V₂O₅ phase (JCPDS card no. 41-1426) without detectable secondary phase. The peaks around 2theta = 15.49°, 20.35°, 26.23° and 31.09° correspond to (200), (001), (101), (110) diffraction peak of the orthogonal phase V₂O₅. By careful observation, it can be found that there is obvious difference of the relative intensities of (001), (110) and (011) diffraction peaks for the two samples. For V-V₂O₅, the intensity ratio between (001) and (110) diffractions and the ratio between (001) and (011) diffractions are 6.2 and 3.3, respectively, which are smaller than those (9.5 and 4.3) of pure V₂O₅. This indicates that the V-V₂O₅ exposed more facets than the pure V₂O₅. The lattice constants of samples were calculated based on the XRD patterns in Fig. 1 by jade software. For V₂O₅, calculated lattices constants are a = 11.484 Å, b = 3.556 Å, c = 4.357 Å; for V-V₂O₅, the calculated lattice constants are a = 11.513 Å, b = 3.564 Å, c = 4.372 Å. It can be seen that the V-V₂O₅ shows slight lattice expansions as compared to that of V₂O₅. It is known that radius of V⁴⁺ is larger than that of V⁵⁺. Therefore, the lattice expansion can be attributed to the increased V⁴⁺ content in V-V₂O₅ sample (Fig. 3).

Fig. 1 XRD patterns of the V-V₂O₅ and V₂O₅ samples.

Fig. 2 gives the SEM and TEM images of the pure V₂O₅ and V-V₂O₅ samples. It can be seen that both V-V₂O₅ and V₂O₅ are 2D sheet-like morphologies. By observing the high magnification SEM and TEM images, the surface of the V-V₂O₅ has a lot of deeper ravines and the surface fluctuates greatly (Fig. 2c, g and h), while V₂O₅ is a flat sheet with cracks attributed to the mechanical force in the sample preparation process (Fig. 2f, j and k). Compared with pure V₂O₅, the coarse surface of V-V₂O₅ is more effective to inhibit the stacking of sheets, store more electrolyte, offer larger material–electrolyte contact area, and relax the mechanical strain generated upon the lithium ion intercalation/deintercalation cycles. The lattice fringe (inset of Fig. 2i and l) of about 4.36 Å can be assigned to the (001) plane of orthorhombic V₂O₅, consistent with the XRD patterns shown in Fig. 1.

Fig. 2 FESEM images of the V-V₂O₅ (a–c) and V₂O₅ (d–f) samples; TEM images of the V-V₂O₅ (g–i) and V₂O₅ (j–l) samples.

Fig. 3 XPS spectra on V2p_3/2 of (a) V-V₂O₅ and (b) V₂O₅ samples.

XPS was carried out to investigate the oxidation state of vanadium and O vacancies in V-V₂O₅ and V₂O₅ samples. The V2p_3/2 spectra shown in Fig. 3 compose of two components locating at the binding energy values of 517.6 and 516.3 eV, which are associated with two formal oxidation degrees, V⁵⁺ and V⁴⁺.^15,16 From the area ratio of the fitted spectrum of V⁵⁺ and V⁴⁺, the molar ratios of V⁴⁺/(V⁴⁺ + V⁵⁺) in V-V₂O₅ and V₂O₅ are 7.4% and 4.7%, respectively. According to charge neutrality of system, the formation of one O vacancy should corresponds the generation of two V⁴⁺. Thus, the O vacancies concentrations in V-V₂O₅ and V₂O₅ are calculated to be 3.7% and 2.35%, respectively. The increased of V⁴⁺ content in V-V₂O₅ can be attribute to the loss of lattice O in V₂O₅ during the annealing process in oxygen depletion condition (vacuum). The present of oxygen vacancies could leave more open voids, which will provide more migration paths for the fast Li⁺ diffusion and facilitate the reversible phase transitions of V₂O₅ during the Li⁺ insertion/extraction process.^12,17 Furthermore, it has been suggested that the mixed valence V⁴⁺/V⁵⁺ could improve the electrical conductivity of the material due to the synergic activity in V₂O₅ materials.

3.2 Electrochemical performances

Fig. 4 show the CV profiles of the first and the fifth cycles at a scan rate of 0.2 mV s⁻¹ for the pure V₂O₅ and V-V₂O₅ electrodes, respectively. As seen from Fig. 4a, for pure V₂O₅ and V-V₂O₅ in the first cycle, three reduction peaks at 3.35, 3.10 and 2.20 V are observed, which correspond to the α/ε, ε/δ and δ/γ phase transitions, respectively. During the Li⁺ deintercalation process, four oxidation peaks are observed for two samples. The peak at 2.51 V corresponds to the γ/δ phase transition, the peak at 3.24 and 3.35 V correspond to the δ/ε phase transition, and the peak at 3.47 V corresponds to the ε/α phase transition.^18–20 However, after the first intercalation process, the irreversible γ′ phase of V₂O₅ is indicated by the emergence of oxidation peak at 3.65 V and reduction peaks broadening at 2.22 V.^19,21 After 50 cycles in Fig. 4b, the reduction peak currents of the V-V₂O₅ and V₂O₅ decrease at 3.16 V while increase at 3.54 V in the γ′ intercalation process; simultaneously, the two oxidation peaks corresponding to δ/ε phase transition decrease at 3.24 and 3.35 V while increase at 3.65 V. Additionally, the peak area of V-V₂O₅ is always greater than that of pure V₂O₅, which indicates that the V-V₂O₅ sample obtained by vacuum annealing has a higher lithium storage activity.

Fig. 4 The (a) first and (b) fifth cycles of CV curves of V-V₂O₅ and V₂O₅ electrodes at a scan rate of 0.2 mV s⁻¹.

Fig. 5a gives the cycling response of the V-V₂O₅ and pure V₂O₅ electrodes at a current density of 200 mA g⁻¹. Obviously, V-V₂O₅ electrode exhibits higher discharge capacity than the pure V₂O₅ electrode. The maximum discharge specific capacities of V-V₂O₅ and V₂O₅ samples are 230.2 and 256.6 mA h g⁻¹, respectively. The discharge specific capacities after 50 cycles are 213.1 and 237.9 mA h g⁻¹, and after 100 cycles are 199.2 and 224.7 mA h g⁻¹, respectively. The intercalation/deintercalation lithium capacity of V-V₂O₅ is always higher than that of V₂O₅. This change of discharge capacity is in accordance with the CV results (Fig. 4). The capacity retention of V-V₂O₅ electrode at 100^th cycle is 87.6%, superior to that reported in the literatures on the hydrogenated V₂O₅ prepared by the H₂ reduction (84% at 100 mA g⁻¹ and 100^th cycle),¹³ on the V₂O₅ nanorods prepared by the electrostatic spinning method (43% at 50 mA g⁻¹ and 50^th cycle)²² and on the carbon coated V₂O₅ nanoparticles (82% at 29.4 mA g⁻¹ and 30^th cycle),¹⁰ indicating an excellent capacity retention capability.

Fig. 5 (a) Cycling performance and coulombic efficiencies of the V-V₂O₅ and V₂O₅ electrodes at a current density of 200 mA g⁻¹; (b) differential specific capacity plots of the V-V₂O₅ at different cycle number at a current density of 200 mA g⁻¹, cycling performance of V-V₂O₅ and V₂O₅ at a current density of (c) 1 A g⁻¹ and (d) 3 A g⁻¹, respectively.

Fig. 5b presents the differential capacity corresponding to the cyclic performance of V-V₂O₅ in Fig. 5a. During the first discharge, the three peaks at 3.37, 3.2 and 2.32 V correspond to the α/ε, ε/δ and δ/γ phase transitions, respectively, in accordance with the reduction peaks in CV curve (Fig. 4a); during the charging process, the peak at 2.45 V corresponds to the γ/δ phase transition, the peak at 3.23 and 3.30 V correspond to the δ/ε phase transition, and the peak at 3.43 V corresponds to the ε/α phase transition, in accordance with the oxidation peaks in CV curve (Fig. 4a). With the increase of charge/discharge the phase transition between ε phase and δ phase decreases gradually, while the intensities of peaks at 3.57 V upon the lithium ion intercalation cycle and at 3.65 V upon the lithium ion deintercalation cycle are gradually increased. This observation is in good agreement with the CV results.

In order to further study the cyclic stability of V-V₂O₅ under high current, the cyclic properties of V-V₂O₅ and V₂O₅ samples were compared at 1 and 3 A g⁻¹ current densities, respectively (Fig. 5c and d). At 1 A g⁻¹, the discharge specific capacity of V-V₂O₅ and V₂O₅ rises first and then decreases, and the maximum capacities are 218.4 and 200.6 mA h g⁻¹, respectively. The significantly improved capacity in the first few cycles during cycling measurement can be attributed the activation of electrode (the penetration of electrolyte and/or increase of active surface). The discharge specific capacities of V-V₂O₅ and V₂O₅ after 200 cycles are 189.3 and 172.4 mA h g⁻¹, respectively. When the current density increases to 3 A g⁻¹, the capacity of two electrodes increase gradually and remain stable. The maximum discharge specific capacities of V-V₂O₅ and V₂O₅ are 150 and 141.7 mA h g⁻¹, respectively. The excellent lithium storage performance of V-V₂O₅ could be ascribed to the vacuum annealing process. Under the condition of oxygen poor, V₂O₅ easily loses the O in the crystal structure and forms the O vacancies. The O vacancies can help reduce V⁵⁺ to V⁴⁺, which increases the electronic and ionic conductivity of the material and also provide additional embedding position for the lithium ion.

The Nyquist impedance spectra of V-V₂O₅ and V₂O₅ are shown in Fig. 6, measured at fully discharged state under 200 mA g⁻¹ in 100th cycles. The Nyquist plots were fitted using an analogical equivalent circuit. In this equivalent circuit, R_s corresponds to the equivalent series resistance (ESR) which contains all ohmic resistance due to the electrolyte and other parts of the system. Charge transfer resistance (R_ct) represents the charge-transfer impedance at the electrode/electrolyte interface, corresponding to the semicircles in high-medium frequency scope. W refers to the Warburg impedance, corresponding to the inclined line in the low-frequency scope. A constant phase element (CPE) is used in the equivalent circuit instead of a pure capacitance due to the inhomogeneous surface of the working electrode. The solid line represents the impedance calculated using the equivalent circuit and the error between the experimental and fitting data was less than 1%. Note that the depressed semicircles related to the charge transfer in the high-medium frequency region and the angled straight line corresponding to the low frequency range can be effectively simulated. The fitting parameters of the semicircle can be calculated from the corresponding equivalent circuit. The R_ct values of V-V₂O₅ and V₂O₅ are 213.1 and 320.6 Ω, respectively. It evidently implies that V-V₂O₅ possesses higher electrochemical reaction kinetics, which is ascribed to its O vacancies facilitating lithium insertion/extraction.

Fig. 6 Nyquist plots of V-V₂O₅ and V₂O₅ electrodes.

4 Conclusions

The orthorhombic phase V₂O₅ (V-V₂O₅) nanosheet with O vacancies was prepared by a facile sol–gel method combined with freeze drying technique followed with annealing in vacuum. XRD analysis revealed that the relative intensities of (110) and (011) diffraction peaks for V-V₂O₅ were enhanced, and V-V₂O₅ showed slight lattice expansions due to O vacancies. FESEM demonstrated that V-V₂O₅ was 2D sheet-like morphology with coarse surface. XPS tests indicated that O vacancies induced more V⁴⁺ in V₂O₅. When used as cathode material for Li-ion batteries, the V-V₂O₅ exhibited much enhanced rate capability and cycling stability as compared to the pure V₂O₅ counterpart. The superior lithium storage performance of the V-V₂O₅ could be ascribed to the following reasons: coarse surface improves the specific surface area of the nanosheet, and increases the effective contact area between electrode and electrolyte; the predominantly exposed (110) and (011) crystal planes of V-V₂O₅ provide channels for facile Li⁺ intercalation and deintercalation, which contributes to the enhanced rate capability; the increased low valence state V⁴⁺ may improve the conductivity of V-V₂O₅ and decrease electrochemical reaction resistance. Furthermore, the results indicated that the synergistic effect of V⁴⁺ and oxygen vacancy will improve the structure integrity and enhance the diffusion of Li ion. It enlightens us that adjusting the proportion of different valance state of metal elements in metallic oxides is a promising approach for improving their lithium storage for LIBs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the financial supports from the National Natural Science Foundation of China (No. 51664012, 51464009 and 51562006), Guangxi Natural Science Foundation of China (2017GXNSFAA198117 and 2015GXNSFGA139006) and Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials (EMFM20181117).

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