Nanostructured Mn-Doped V2O5 Cathode Material Fabricated from

Oct 22, 2015 - We chose to synthesize a layered vanadium(III) precursor host, vanadium jarosite, in order to make Mn-doped V2O5. An analog of vanadium...
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Nanostructured Mn-Doped V2O5 Cathode Material Fabricated from Layered Vanadium Jarosite Hongmei Zeng,*,†,‡,§ Deyu Liu,†,§ Yichi Zhang,† Kimberly A. See,† Young-Si Jun,† Guang Wu,† Jeffrey A. Gerbec,⊥ Xiulei Ji,¶ and Galen D. Stucky*,†,⊥ †

Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States College of Chemistry, Sichuan University, Chengdu, 610064, China ⊥ Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, California 93106-5150, United States ¶ Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, United States ‡

S Supporting Information *

ABSTRACT: We propose a nanostructured Mn-doped V2O5 lithium-ion battery cathode material that facilitates cathodic charge transport. The synthesis strategy uses a layered compound, vanadium(III) jarosite, as the precursor, in which the Mn2+ ions are doped uniformly between the vanadium oxide crystal layers. Through a two-step transformation, the vanadium jarosite was converted into Mn2+-doped V2O5. The resulting aliovalent doping of the larger Mn cations in the modified V2O5 structure increases the cell volume, which facilitates diffusion of Li+ ions, and introduces oxygen vacancies that improve the electronic conductivity. Comparison of the electrochemical performance in Li-ion batteries of undoped and the Mn2+-doped V2O5 hierarchical structure made from layered vanadium jarosite confirms that the Mn-doping improves ion transport to give a high cathodic columbic capacity (253 mAhg−1 at 1C, 86% of the theoretical value, 294 mAhg−1) and excellent cycling stability.

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INTRODUCTION Rechargeable lithium-ion batteries (LIBs) are very attractive for use in portable electronics, electric vehicles, and grid-scale energy storage systems due to their high energy density, high power density, and long cycle-life.1−3 Orthorhombic vanadium oxide (V2O5) has been intensively studied as a cathode material because of its high theoretical capacity (294 mAhg−1, or 271 mAhg−1 for the fully intercalated form Li2V2O5) compared to other cathode materials such as LiCoO2, LiMn2O4, and LiFePO4.4−7 However, the use of V2O5 as a cathode is plagued by its low electrical conductivity, poor structural stability upon cycling, and slow lithium ion diffusion kinetics in its structure.8 Various strategies have been developed to improve the electrochemical performance of V2O5 including nanostructuring the orthorhombic V2O5,9,10 integrating V2O5 with carbon materials,11−15 and doping V2O5 crystal lattice with other transition metals.16−19 There is still ample room to improve the performance,7−20 which according to recently reported electrode performance data is typically about 240 mAhg−1, well below the theoretical capacity of 294 mAhg−1. A doping step of atomic species within or between the neutral slabs of vanadium oxide crystal lattice structure following the routine bulk V2O5 preparation may improve the electrochemical performance of V2O5.16−19 However, this requires complicated chemistry with multichemical steps during the synthesis.23 The sol−gel chemistry route for preparing microscopically, and uniformly, doped V2O5 is also difficult because of the different chemical behaviors of the precursors. © XXXX American Chemical Society

The selection of suitable precursors is a key step in the design and preparation of functional materials. To date, most of the synthetically doped V2O5 electrodes are made using routine commercialized precursors, which also produce bulk intrinsic V2O5 among the doped phases, so that the overall performance is not able to satisfactorily attain the desired requirements of a good LIB cathode material.21,22 Therefore, there is a need for a simple and effective method to uniformly incorporate charged dopants into the V2O5 lattice. To address this challenge, we propose using a single precursor to synthesize the desired doped-oxide cathode. The key step is the design and synthesis of a phase or compound that can host different ions. We chose to synthesize a layered vanadium(III) precursor host, vanadium jarosite, in order to make Mn-doped V 2 O 5 . An analog of vanadium jarosite, iron jarosite KFe3(OH)6(SO4)2, was discovered in 1852. Iron jarosite has a layered structure that can host a variety of cations and has attracted considerable attention for its resulting interesting magnetic properties.24,25 We report the synthesis of vanadium jarosite AV3(OH)6(SO4)2, where A = H3O+ (called intrinsic or undoped in this work), Na+, K+, Mn2+, etc., and its use to make a uniformly Mn-doped V2O5 that has significantly improved electrochemical performance as a LIB cathode. Received: July 23, 2015 Revised: September 17, 2015

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take advantage of its very high melting point (1940 °C) for maintaining the nano/microstructure. The 900 °C vacuum annealed sample is confirmed as being a pure V2O3 phase (JCPDS card 01-085-1411, which indicates a complete conversion with no side reactions (Figure S2a). Afterward, the flake-shaped V2O3 is converted to the orthorhombic V2O5 structure, also with a flake morphology, at 400 °C in air. The diffraction pattern indicates that the product is structurally phase-pure V2O5 (JCPDS card 41-1426) implying a complete oxidative transformation of the lattice structure (Figure S2b). X-ray photoelectron spectroscopy (XPS) revealed that the material has oxygen vacancies and a high concentration of defects with a composition of V5+1.68V4+0.32O4.84 (Figure S3a). SEM images in Figure 2 show the morphologies of the precursor and the as-prepared vanadium oxide in the first and

Vanadium jarosite is a V3+-based matrix (Figure 1 inset) and has an ability, analogous to that of iron jarosite, to host multiple

Figure 1. TGA−MS curves for undoped (H−V) jarosite.

cations in the interlayers.26 The guest ion concentrations can be varied over a wide range by replacing them with the interlayer H3O+. After the kinetically controlled pyrolysis and oxidation of the precursor, a series of products including doped V2O3, nonstoichiometric VO2/V2O5 oxides, and fully oxidized V2O5 can be obtained. These products have a flake-like hierarchical morphology, because of the layered vanadium jarosite structure, and retain this shape with processing. More specifically, we demonstrate a synthesis of the vanadium jarosite with Mndoping via a microwave-assisted solvothermal reaction.27,28 This process is fast and efficient in comparison with the hydrothermal processes that have been used previously for other jarosite phase materials.26 The final V2O5 product has homogeneous Mn-doping in the lattice without any detectable phase separation of Mn oxides. The doped material shows superior electrochemical performance in a LIB half-cell. A comparison of electrochemical performance between the undoped and Mn-doped V2O5 indicates that the Mn doping facilitates the Li+ ion transfer, resulting in a high capacity (253 ± 3 mAhg−1 at 1C) with good cycling stability.

Figure 2. SEM images of (a) Mn−V jarosite, (b) Mn-doped V2O3, and (c) Mn-doped V2O5. (d) EDX element maps of V, O, and Mn of MnxV2−xO5 (x = 0.31). Insets are zoom-in images of (a), (b), and (c).

RESULTS AND DISCUSSION The crystal structure of undoped and Mn-doped vanadium jarosite (labeled H−V and Mn−V jarosite, respectively) is confirmed by X-ray diffraction (Supporting Information Figure S1a). All the diffraction peaks of the samples can be indexed as a vanadium jarosite phase with minor shift due to the ion size difference between H3O+ and Mn2+ (JCPDS card 27-1379). There is no evidence of manganese oxide or other species in the Mn−V jarosite sample. Figure S1b presents the EDX spectrum of the Mn−V jarosite that confirms the presence of Mn in the Mn-doped vanadium jarosite structure. TGA−MS analysis was performed on H−V jarosite in a N2 atmosphere to study the pyrolysis of the precursor. Figure 1 shows the decomposition profile as a function of temperature. There are several processes including dehydration, release of sulfur dioxide, and recrystallization. During pyrolysis, the precursor is transformed to the rhombohedral V2O3 structure. We chose a two-step procedure for the V2O5 material in order to maintain the morphology of the precursor, in accordance with the thermal stabilities of the different vanadium oxides. The first step, vacuum annealing at 900 °C, decomposes the precursor to V2O3 with a corundum structure allowing us to

second steps. Mn−V jarosite exhibits the morphology of randomly assembled flakes with a desert-rose shape (Figure 2a). The thickness of each flake is approximately 100−250 nm, which is similar to that of the undoped vanadium jarosite (Figure S4b). Compared to the undoped vanadium jarosite, the Mn-doped material is slightly more uniform, which is most likely the result of some chemical restructuring during microcrystal growth. After the first step, the as-prepared Mndoped V2O3 maintains the flake morphology with a rough surface (Figure 2b). It is noteworthy that the Mn-doped V2O3 has a hierarchical structure: from the primary crystal grain platelets that make up flakes and then to the desert-rose morphology (Figure 2). Further processing into Mn-doped V2O5 similarly results in a rougher morphology compared to the vanadium jarosite precursor (Figure 2c). Moreover, EDX elemental mapping of MnxV2−xO5−1.5x (x = 0.31) validated the uniform Mn dispersion (Figure 2d). After annealing in air, Mn-doped V2O5 still has a uniform Mn substitution at the microscale, with Mn ions predominantly in the host vanadium oxide lattice as revealed by the X-ray diffraction data (Figure 3a). The doping level can be

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worthwhile to note that there is no manganese oxide crystalline phase detected in any samples of this series, though the MnV2O6 phase does exist with medium to high doping levels (M10 to M31). Although the effective Mn ion substitution into the V2O5 lattice is lower than the apparent number, the Mn atoms are well dispersed in the vanadium oxide material. According to the XRD refinement, the content of MnV2O6 is 20% for the material prepared with M31. According to the relationship between doping level and phase composition (Figure S6), we found that the saturation limit for Mn ions is around x = 0.10, where Mn ions can be incorporated into the V2O5 lattice without forming the undesired MnV2O6 phase. The electrochemical performances of the undoped V2O5 and M01 electrodes are shown in Figure 4. Cyclic voltammograms Figure 3. (a) Rietveld refined XRD patterns of the compounds. (b) Plot of the lattice volume versus doping level, x (MnxV2−xO5). (c) Plot of lattice parameters a, b, c versus x.

controlled by the Mn molar content in the precursor and is also confirmed by the atomic emission spectrometry (ICP−AES). The formula of Mn-doped V2O5 can be denoted as MnxV2−xO5 (x = 0.01, 0.03, ..., 0.31, labeled as M01, M03, ..., M31, respectively). Figure S5 shows the correlation in detail between the Mn/V molar ratio of the synthesis input and the jarosite product compositions. Mn atoms are incorporated into the material successfully due to the capability of vanadium jarosite to host other metal ions between its lattice layers. XPS is used to determine the valence state of Mn (Figure S3a, b) and the ratio of [V4+]/[V5+] in the final-stage oxide products. The Mn2p3/2 band (Figure S3c) can be fit with two peaks at 641.1 and 642.5 eV, which implies the coexistence of divalent and higher valent Mn.29,30 The V2p3/2 signal split into two peaks centered at 517.3 and 515.9 eV, which correspond to V5+ and V4+, respectively.31 The spectra indicate the ratios of [V4+]/([V4+] + [V5+]) in the undoped V2O5 and M01 are 0.16 and 0.15 respectively. The mixed-valence states in one crystal phase suggest the existence of a high concentration of defects, such as dislocations and vacancies, which may facilitate the charge transfer by creating additional dopant energy levels and a more open structure for intercalation. To further confirm the incorporation of Mn cations into the V2O5 lattice, we used Rietveld refinement to evaluate the change in V2O5 lattice parameters as a function of Mn content (Figure 3, Figure S6). The XRD patterns confirm that the predominating phase in a series of doped vanadium oxides is V2O5, Pmmn (Figure 3a, Figure S6). The powder XRD pattern was fit to the known structure of V2O5 (space group Pmmn) using a GSAS program. Main diffraction peaks from the undoped V2O5 and M01 are well modeled by the orthorhombic V2O5 structure without observable impurity phases. At 1% Mn doping, the lattice parameters of the V2O5 are not strongly affected. Figure 3b and Figure 3c show the change in lattice parameters and unit cell volume of V2O5 as the nominal doping concentration x increases. Lattice parameters and volumes exhibit a positive correlation with increasing Mn ion content that is in accordance with Vegard’s law. This dependence is due to the larger ionic radius of Mn2+ (83 pm) compared to V5+ (54 pm). In the case of the heavily doped sample, M31, diffraction peaks appear at 27.3°, 28.4°, 29.4°, and 38.6°, indicating the formation of monoclinic MnV2O6 (JCPDS card 35-0139), which is inert for the cathode of Li-ion battery.32 It is

Figure 4. Electrochemical performance of the MnxV2−xO5 electrode in the voltage range between 2.05 and 4.0 V (vs Li). (a) Cyclic voltammetry (CV) curves at a scan rate of 0.1 mVs−1. (b) Voltage profile of the first, second, and tenth cycles cycled at a rate of 300 mAg−1 (1C). (c) Rate performance of the undoped V2O5 and M01 electrodes; the orange line represents the theoretical specific capacity. (d) Cycling performance of the undoped V2O5 and M01 electrodes at a rate of 300 mAg−1. Error bars indicate the standard deviation of three replicate cells.

(CV) of the M01 electrode vs a Li anode reveal three reversible redox reactions (Figure 4a) corresponding to the insertion and desertion of Li ions. The CV curve of the Mn-doped V2O5 is similar to that of the pure V2O5 (Figure S7).33 Subsequent CV cycles (the 10th cycle is shown) trace previous cycles well but pass slightly less current. There is no redox peak corresponding to Mn species, suggesting that the doped Mn is electrochemically inert within the potentials limits described here. Figure 4b shows the galvanostatic discharge−charge profiles of the first, second, and tenth cycles for Mn-doped V2O5 within the voltage range of 2.05−4.0 V vs Li+/Li. Consistent with the CV results, the three plateaus observed in the discharge curves at 3.35, 3.15, and 2.24 V indicate a multistep insertion process, which corresponds to the phase changes from α-V2O5 to ε-Li0.5V2O5, then to δ-LiV2O5, and finally to γ-Li2V2O5, respectively.34 A C

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Chemistry of Materials high specific discharge capacity of 253 mAhg−1 is obtained from the M01 electrode, which is 87% of the theoretical specific capacity for the insertion of two Li ions into V2O5 (294 mAhg−1). The discharge and charge plateaus generally remain stable over several cycles, which indicates that there is good structural reversibility for this Mn-doped V2O5 electrode. To probe the effect of Mn-doping on the electrochemical performance of the V2O5 cathode, the electrochemical behavior of the undoped V2O5 and M01 electrodes was studied with a series of charge−discharge rates (Figure 4c). Compared to previously reported data, our process further improves the rate performances of both the undoped V2O5 and M01 electrodes.35,36 The Mn-doping allows for better rate capability and higher capacity. The M01 cathode shows a significant improvement for capacity at all rates indicated (1C, 2C, and 5C) compared to the undoped V2O5. The M01 electrode exhibits initial discharge capacities of 251, 214, and 171 mAhg−1 at 1C, 2C, and 5C, respectively. The capacity recovers when cycled again at 1C to over 215 mAhg−1, which is about 16% higher than the undoped V2O5 electrode prepared using vanadium jarosite. An overdoped cathode shows mediocre electrochemical performance (Figure S8a), suggesting that the doping level needs to be carefully controlled to prevent the formation of MnV2O6, which compromises the electrochemical performance (Figure S8a). Furthermore, the M01 electrode exhibits significantly better capacity retention (201 mAhg−1 for discharge after 50 cycles) upon prolonged cycling at 1C with a high columbic efficiency that is close to 100% (Figure 4d). The capacity fades at an average rate of 0.40% per cycle. In contrast, the undoped V2O5 electrode delivers consistently lower specific capacities and exhibits a faster capacity fade at 0.60% per cycle. Electrochemical impedance spectroscopy (EIS) measurements were conducted to provide further information on the effect of Mn-doping on electrode kinetics. The Nyquist plots of two samples are shown in Figure S6c and d. The M01 electrode has a smaller charge transfer resistance (R2) than that of the undoped V2O5. Notably, both the ohmic resistance (RΩ) and lithium ion diffusion resistance (R1) of the M01 electrode are much lower than those of the undoped V2O5 electrode, which corresponds to improved lithium ion diffusion and better electrode kinetics.37 These low resistance values benefit the cyclability and rate performance of the M01 electrode. Considering the structural characterization, the performance difference between the undoped and Mn-doped V2O5 electrode can be attributed to the structural changes as a result of the Mndoping. The M01 electrode shows quite good cycling performances among state-of-art V2O5 cathode materials.7−20 The enhanced electrochemical performance of the M01 electrode is clearly related to the doping of Mn ions into the V2O5 crystal grains (30−50 nm size). Specifically, aliovalent doping of a larger cation increases the number of defects and the cell volume, which facilitates the diffusion of Li+ ions in the cathode material. The oxygen vacancies may also alleviate the stress and strain that accompanies Li-ion intercalation and deintercalation processes, thus improving the cycling stability and better rate capability.38 Again, with the flake-like jarosite precursor, even the undoped V2O5 has a notably larger specific capacity and better rate capability than that obtained using V2O5 obtained by traditional sol−gel or solvothermal chemistry.21,39,40 The improved electrochemical activity of the doped material is likely a result of both faster mass transfer kinetics afforded by

the layered, porous morphology and greater electronic conductivity that arises due to oxygen vacancies present as a result of the aliovalent doping strategy.22,41,42 For these reasons, vanadium jarosite is a preferred precursor for the preparation of doped V2O5 materials that exhibit good electrochemical performance.

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CONCLUSIONS In summary, we have developed a facile preparation strategy to generate uniformly doped V2O5 as an active electrode material by making use of layered vanadium jarosite as the precursor. The two-step pyrolysis/oxidation process is an effective method that converts the vanadium jarosite to the desired MnxV2−xO5 product with well-maintained morphology. The Mn-doping expands the V2O5 lattice and introduces a considerable amount of oxygen vacancies, which benefits the electron transport in the crystal structure. The MnxV2−xO5 nanostructure exhibits enhanced cycling stability and rate capability as a cathode for lithium ion batteries. As a precursor synthesis species, vanadium jarosite has a flexible interlayer structure and good tolerance to various interstitial ions including transition metals and amines. With these advantages, the doping strategy based on jarosite can be extended to syntheses of other transition metal doped nanostructures.

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EXPERIMENTAL SECTION

Preparation of Vanadium Jarosite and Oxides. All the chemicals in this work were analytical grade and used directly without any further purification. The vanadium jarosite was prepared via a microwave-assisted solvothermal reaction of VOSO4·3H2O and glucose in an emulsion of water/hexyl alcohol. A typical synthesis was conducted as follows. First, 0.800 g of VOSO4·3H2O and 0.400 g of glucose were dissolved in 1.55 g of H2O as a blue solution, then 6.6 mL of 1-hexanol and 3.0 g of CTAB were added into the above solution and sonicated for several minutes. Afterward, the emulsion was transferred into the microwave reaction vial and sealed. The vial was processed under 195 °C for 1 h by a Biotage Initiator microwave system (normal absorption level, 600 rpm stirring, and fixed holding time). The vanadium jarosite was collected via centrifugation and washed with water and ethanol. The Mn-doped vanadium jarosite was prepared by the same process in the presence of Mn(Ac)2·4H2O. The vanadium jarosite was annealed at 900 °C under vacuum for 1 h with a heating rate of 5 °C min−1 to obtain the V2O3 powder. The further oxidation of V2O3 was performed in air at 400 °C for 1 h in order to transform V2O3 into V2O5. Materials Characterization. The TGA−MS thermal decomposition profile of (H3O)V3(OH)6SO4 powder was acquired in nitrogen (Mettler TGA/sDTA851e with ThermoStar 300 AMU mass spectrometer). The composition of the as-prepared materials was characterized by energy-dispersive X-ray spectroscopy (SEM integrated) and atomic emission spectrometry (ICP−AES, Thermo iCAP 6300). X-ray photoelectron spectroscopy (Kratos Axis Ultra) was performed with both the as-prepared undoped and Mn-doped V2O5. Phase analysis and determination of cell parameters at room temperature were obtained by a Phillips PANalytical X’Pert PRO diffractometer with Cu Kα radiation. Extraction of the peak positions, pattern indexing, and profile refinements were carried out using GSAS program. An FEI XL40 Sirion FEG digital scanning microscope was used for the morphological characterizations. Electrochemical Measurements. The test cathodes were prepared by mixing the active materials (V2O5), poly(vinylidene fluoride) binder, and Super P carbon (80:10:10, w/w/w) in cyclopentanone. The slurry was mixed thoroughly by sonication and subsequently drop-casted onto carbon-coated aluminum foil (the mass of the active material was about 1.2 mg and the diameter of the cathode was 15 mm). The coated electrodes were dried overnight at 65 °C. Electrochemical performance was tested in a CR2032 coin cell, D

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which was assembled in an argon-filled glovebox with the as-prepared cathode, metallic lithium anode, and 1 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate(EMC) (3:7 v/v) electrolyte. Cyclic voltammetry was performed in the potential range of 2.0−4.0 V (vs Li) at a scan rate of 0.1 mVs−1. Basic electrochemical tests were performed on a Bio-Logic VMP1 potentiostat. Galvanostatic discharge−charge experiments were conducted in the potential range of 2.05−4.0 V (vs Li). Electrochemical impedance spectroscopy (EIS) measurements were carried out at room temperature with the potential amplitude of 10 mV at the frequency range of 1000 kHz to 10 MHz.

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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/acs.chemmater.5b02840. Microwave reaction synthesis yield of vanadium jarosite, EDX spectra, additional XRD data, SEM images, and electrochemical impedance analysis (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

Authors H.Z. and D.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (21201125) and by the U.S. National Science Foundation (DMR 0805148). Fellowship support to K.A.S. from the ConvEne IGERT Program of the National Science Foundation (DGE 0801627) is gratefully acknowledged. This research made extensive use of the shared experimental facilities of the Materials Research Laboratory, supported by the MRSEC Program of the National Science Foundation (DMR 1121053), a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org). D.Y.L. was supported by the China Scholarship Council (2010631076) for doctoral study.

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REFERENCES

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DOI: 10.1021/acs.chemmater.5b02840 Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.chemmater.5b02840 Chem. Mater. XXXX, XXX, XXX−XXX