A 3.4 V Layered VOPO4 Cathode for Na-Ion Batteries - Chemistry of

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A 3.4 V Layered VOPO4 Cathode for Na-Ion Batteries Guang He, Wang Hay Kan, and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute, University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Vanadyl phostate (VOPO4) with a layered structure was successfully prepared by chemical delithiation of the tetragonal αI-LiVOPO4 at room temperature. X-ray diffraction (XRD) studies show the resulting VOPO4 has a crystal structure similar to αI-LiVOPO4. The V4+/V5+ redox couple exhibits a discharge plateau around 3.5 V vs Na/Na+ in sodium cells. The sample offers a capacity of ∼110 mAh g−1 (0.67 Na+) without any conductive coating and ∼150 mAh g−1 (0.90 Na+) with a preliminary optimization with reduced graphene oxide (rGO) nanosheets. The electrochemically sodiated VOPO4 electrodes show XRD patterns similar to the chemically sodiated VOPO4, indicating the formation of the same product by the two sodiation processes. Bond valence sum (BVS) map calculations suggest that sodium ions likely take the sites between two adjacent VOPO4 sheets. Rietveld refinement of the chemically sodiated Na0.8VOPO4 indicate that the sodium ions could take the 8j sites similar to the lithium ions in αILiVOPO4. This new layered VOPO4 is a competitive cathode candidate among the polyanion cathodes for sodium-ion batteries. for Na/Na+ and Li/Li+ vs SHE). Thus, the development of suitable electrode materials with high capacity, high operating voltage, and acceptable rate capability is the primary challenge for NIBs. First attempts on NIB cathodes were focused on layered sodium transition-metal oxides (NaMO2 with M = Co, Fe, Mn, Cr, ...),7,8 similar to the beginning of LIBs. In recent years, polyanion cathodes have been extensively studied, in particular vanadium-based materials. This is because the vanadium redox couples V3+/V4+ and/or V4+/V5+ could provide a high voltage of above 3.0 V when combined with the PO4 groups and thereby improve the energy density. Furthermore, some alkali vanadium phosphates are sodium superionic conductors (NASICON) that allow fast sodium-ion diffusion in the structure,9 for example, Na3V2(PO4)3. The NSICON Na3V2(PO4)3 crystallizes in a rhombohedral structure with the space group R-3C.10−12 The large open framework of this structure provides sufficient space for fast sodium transfer. The V3+/V4+ couple offers a theoretical discharge capacity of 117 mAh g−1 with a well-defined flat plateau at ∼3.3 V.12 Excellent cyclability and rate capability have been reported by different groups with highly tailored NASICON-type materials.1,13−17 The sodium insertion/extraction mechanism and the detailed structures of Na3V2(PO4)3 has also been discussed.18,19 In spite of the great achievement, the energy density of Na3V2(PO4)3 is limited to 320 mAh g−1 (at ∼3.9 V and ∼2.3 V vs Li/Li+).29−32 In essence, VOPO4 is an attractive cathode candidate in NIBs as well. First of all, VOPO4 has a theoretical capacity of 165 mAh g−1 in both NIBs and LIBs. In addition, the voltage of a sodium compound is typically 0.4−0.5 V lower than that of its lithium counterpart in LIBs.33 Given the high potential of VOPO4 in LIBs, the operating voltage of VOPO4 cathode in NIBs is expected to be 3.4−3.5 V, which gives a noticeably high theoretical energy density of ∼570 Wh kg−1. Nonetheless, studies on VOPO4/ NaVOPO4 systems are still rare. Goodenough’s group explored the monoclinic NaVOPO4 as the cathode in NIBs.34 The large particle size associated with the three-dimensional (3D) structure was not favorable for the diffusion of large sodium ions. As a result, the capacity of the pristine NaVOPO4 before the ball-milling treatment was only 30 mAh g−1, accompanied by large irreversible capacities. Among the various polymorphs of VOPO4, only αI-VOPO4 has a layered structure, which may facilitate fast Na+-ion diffusion. This material was traditionally prepared by a dehydration of α-VOPO4·2H2O,35 but the process has to be handled very carefully to avoid any other forms of VOPO4 as side products. The αI-VOPO4 crystallizes in tetragonal symmetry with the space group P4/n. It has a stacking of VOPO4 sheets along the c direction. In each VOPO4 plane, PO4 and distorted VO5 polyhedra are alternatively arranged by a corner-sharing of the oxygen to form [VOPO4]∞ in the a−b plane. The large space between two adjacent VOPO4 planes is expected to be sufficient to accommodate both lithium and sodium ions. Herein, we report for the first time, the electrochemical properties of layered VOPO4 as a cathode material in NIBs. The layered αI-VOPO4 is prepared through the chemical delithiation of αI-LiVOPO4. In sodium-ion cells, the VOPO4 cathode delivers a reversible capacity of 110 mAh g−1, most of which are contributed by a long plateau at 3.4−3.5 V. By incorporating reduced graphene oxide (rGO) into the cathode, the capacity is increased to 150 mAh g−1, equivalent to 0.9 sodium ion per formula, due to the enhancement in electrical conductivity. Both the electrochemically and chemically sodiated products exhibit similar XRD patterns, implying that the same structure is adopted during both the sodiation processes. Bond valence sum maps of VOPO4 and Rietveld refinement with the XRD data of the chemically sodiated NaVOPO4 show that sodium ions are likely to occupy the 8j

sites in between two adjacent VOPO4 layers, similar to the case of lithium ions in αI-LiVOPO4.

2. EXPERIMENTAL SECTION Synthesis of αI-LiVOPO4. αI-LiVOPO4 was synthesized by a microwave-assisted solvothermal (MT-SW) method. Briefly, V2O5 and oxalic acid dehydrate (molar ratio 1:3) were first dissolved in deionized water at 60 °C to obtain a clear blue solution. Afterward, LiOH·H2O, phosphoric acid (85%), and ethanol were added in sequence under stirring (Li: V: P = 1.9:1:1 and H2O: EtOH = 1:1). The mixture was then transferred to polytetrafluoroethylene (PTFE) microwave reaction vessels. The solution in each vessel was ∼15 mL, in which the concentration of V was kept at 0.067 M. The microwave-assisted solvothermal reactions were run with a maximum temperature and pressure of of 220 °C and 50 bar, respectively. The overall reaction duration was about 45−50 min, including approximately 20−25 min of ramping time to the desired temperature/pressure. Finally, the vessels were cooled down and the products were collected and washed with water and acetone. The αI-LiVOPO4/rGO composite was prepared with similar procedures, except the addition of the GO dispersion into the precursor before the microwave reaction. Chemical Delithiation. The αI-LiVOPO4 and αI-LiVOPO4/rGO were delithiated by nitronium tetrafluoroborate (NO2BF4) in the predried acetonitrile medium under Ar atmosphere. The product was washed with acetonitrile twice after the delithiation and kept in vacuum oven at 150 °C for further use. All the delithiation process was performed under Ar as the layered VOPO4 is air/moisture-sensitive. Materials Characterizations. X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV X-ray diffractometer. Rietveld refinement with GSAS/EXPGUI was used to analyze the X-ray powder diffraction data for unit cell parameters, phase fractions, and atomic structural parameters. Elemental analyses were performed with a Varian 715-ES inductively coupled plasma (ICP) spectrometer. Scanning electron microscopy (SEM) images were obtained with a Hitachi JEOL S5500. Electrochemistry. The αI-VOPO4 and αI-VOPO4/rGO cathodes were casted in a glovebox with N-methyl-2pyrrolidone (NMP) solvent onto aluminum current collectors employing polyvinylidene fluoride (PVDF) as the binder. A typical electrode formulation for the αI-VOPO4 and αIVOPO4/rGO electrodes is as follows: αI-VOPO4:Super P:PVDF = 70:20:10 and αI-VOPO4/rGO:Super P:PVDF = 683

DOI: 10.1021/acs.chemmater.5b04605 Chem. Mater. 2016, 28, 682−688

Article

Chemistry of Materials 80:10:10. CR 2032 sodium coin cells were fabricated inside an Ar-filled glovebox with 1 M NaClO4 in propylene carbonate (PC) and ethylene carbonate (EC) (1:1 vol/vol) electrolyte, a metallic sodium negative electrode, and glass fiber separator. The active mass loading was ∼3.0 mg cm−2.

3. RESULTS AND DISCUSSION The layered VOPO4 was prepared by chemical delithiation of αI-LiVOPO4 with nitronium tetrafluoroborate (NO2BF4). αILiVOPO4 was synthesized by a microwave-assisted solvothermal process. In Figure 1a, the inset is the scanning electron microscope (SEM) image of the layered VOPO4. The VOPO4 secondary particles consist of thin “nanoplates” that agglomerate to a “flower-ball.” This morphology is similar to that of the parent αI-LiVOPO4,36 indicating that the delithiation reaction did not cause severe morphological change. The large interface between the “nanoplates” and electrolyte is beneficial to charge and mass transfer. αI-LiVOPO4 crystallizes in a tetragonal symmetry with the space group of P4/nmm. Upon the extraction of lithium ions, the X-ray diffraction (XRD) pattern of the VOPO4 is almost identical to that of αI-LiVOPO4, except the systematic shift of the diffraction peaks to higher angles. The XRD pattern indicates the layered structure is maintained (Figure 1b) after delithiation. The peak shift is due to the contraction of the unit cell resulting from an oxidation of larger V4+ to smaller V5+ ions. αI-LiVOPO4 was first prepared by Dupré and co-workers by lithiating αI-VOPO4 or αII-VOPO4.35 In our experiment, the XRD pattern indicates that the structure of VOPO4 obtained by delithiating αI-LiVOPO4 has the α-VOSO4 symmetry with the space group P4/n, which is supposed to be the same as αIVOPO4.37 However, following it with heat-treatment of the VOPO4 in air resulted in the αII-VOPO4 phase at 700 °C (Figure S1), consistent with the conversion temperature from α-VOPO4·2H2O to αII-VOPO4.35 Cell parameters calculated from the Rietveld refinement of the VOPO4 are a = 6.219(1) Å, b = 6.219(1) Å, and c = 4.376(1) Å, slightly smaller than those for the αI-LiVOPO4. The compositions of both LiVOPO4 and VOPO4 were confirmed by ICP. Chemical sodiation with the layered VOPO4 was performed in acetonitrile with NaI as the sodium source and reducing agent. Figure S2 shows the color change of the sample during the sodiation process. The typical brown color from I2 appeared immediately upon the mixing of VOPO4 and NaI (50% excess), indicating a rapid redox reaction occurred in seconds. The brown color became much darker after about 2 min, but the yellow-green VOPO4 was still visible after a short time of rest. After 5 min, the VOPO4 color was completely shielded by the dark red/brown color of I2, even with a long rest time. The fast chemical sodiation reaction clearly shows the advantage of the 2D layered structure, which is beneficial for fast diffusion of sodium ions. In fact, another form of VOPO 4 , the orthorhombic β-VOPO4 that has a 3D structure, was also prepared and studied by chemical sodiation under the same condition, but the color change was much slower than that with the layered VOPO4 (Figure S3), suggesting it is more difficult to insert sodium ions into this structure. XRD patterns of the chemically sodiated products obtained with various amounts of NaI are presented in Figure 2. The compositions of the samples were confirmed with ICP analysis. The overall pattern of the product obtained with a small amount of NaI (NaI:VOPO4 = 0.3) looks similar to the pristine VOPO4, but the diffraction peaks become broader, implying

Figure 2. XRD patterns of the chemically sodiated VOPO4 obtained with various NaI:VOPO4 ratios. All the reactions were allowed for 24 h, except for the sample specified in the figure. The Na/P ratios were confirmed by ICP analysis.

sodium intercalation at this stage is essentially a solid-solution process. As more NaI is added, most of the diffraction peaks of the Na0.5VOPO4 sample are significantly reduced in intensity, except the (020) peak. The (001) peak almost disappeared. Instead, a broad bump is present at 2θ = 15−20°. Further insertion of sodium ions with more NaI leads to a new diffraction peak at ∼16.5°. This peak is relatively sharp when the Na/P is 2.9 Å, formed between a vanadium atom with the oxygen from an adjacent layer. This distance is so long that it is essentially considered nonbonding with the nearest VO5 polydedron. This structural configuration is different from other vanadium/vanadyl compounds with 3D symmetries, where VO6/VO4F2 octahedra are formed.34,39,40 In view of the limitation of the X-ray data, further studies with Synchrotron and neutron diffraction techniques may be needed. Nevertheless, the preliminary studies imply that the intercalation chemistry in NaxVOPO4 should be analogous to the lithium counterpart. In sodium-ion cells, the first cycle shows a low capacity and large irreversible capacity (Figure S4), which possibly originate from the protons in the structure introduced during the delithiation process. Similar phenomenon has also been observed by us with the β-VOPO4 (obtained by delithiating β-LiVOPO4) in sodium ion cells,41 as well as the α-H2VOPO429 in lithium ion cells. Figure 4a shows the second charge−discharge profile at 0.05C rate at 4.3−1.5 V. Approximately 0.67 sodium (110 mAh g−1) ion could be 685

DOI: 10.1021/acs.chemmater.5b04605 Chem. Mater. 2016, 28, 682−688

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Chemistry of Materials

of discharge at a C/20 rate were examined by ex situ XRD (Figure 4d). Similar to the chemically sodiated NaxVOPO4 samples, the (001) diffraction peak first disappears for the electrode being discharged to 3.4 V, followed by the growth of the new diffraction peak at 17° upon successive discharge to lower potentials. The correlation of the XRD patterns reveals similar sodium intercalation process with comparable crystal chemistry for both the chemically and electrochemically sodiated products. Overall, the layered VOPO4 is favorable for fast sodium-ion diffusion, as evidenced by the rapid chemical sodiation. It offers a reversible capacity of ∼150 mAh g−1 in sodium-ion cells with rGO as a conductive additive in the electrode. Combined with the high operating voltage of 3.4 V, it is a promising cathode candidate among the various cathode materials for NIBs. Figure 5 illustrates the voltage−capacity−energy density comparison

Figure 4. Electrochemical performance of the layered VOPO4 electrodes: (a) a typical charge−discharge profile and CV (inset) between 1.5−4.3 V; (b) cycling stability at C/10 (16.5 mA g−1); (c) rate and cycling performance of the VOPO4/rGO composite; and (d) XRD pattern of the VOPO4 electrodes at different discharge depths.

reversibly inserted, most of which (∼0.5 Na+) are derived from a plateau at 3.4−3.5 V. At higher current rates of 0.1C, 0.2C and 0.5C rates, the plateau is slightly reduced by 0.05−0.1 V (Figure S5). The inset is the cyclic voltammetry (CV) curves on the first 7 cycles. The oxidation and reduction peak positions on the initial sweep are, respectively, at 3.8 and 3.4 V. The potential difference is slightly enlarged by ∼0.2 V on the following sweeps. The overall behavior of VOPO4 in sodium ion cells is accordance with that of the lithium counterpart. At a higher current rate of 0.1C, the VOPO4 cathode offers a stable capacity of ∼95 mAh g−1 over 250 cycles. The chemical sodiation showed fast ion diffusion within the layered structure. Hence, the slow charge transfer (low electronic conductivity) seems to be the primary obstacle in the electrochemical process. We have demonstrated that the incorporation of reduced graphene oxide (rGO) is beneficial to improve the electronic conductivity in the αI-LiVOPO4 electrodes.36 Therefore, by employing the αI-LiVOPO4/rGO composite as the precursor, VOPO4/rGO was prepared with a similar procedure. The morphology of the VOPO4/rGO was confirmed to be comparable to that of the bare VOPO4 (Figure S6). This cathode delivers a high capacity of ∼150 mAh g−1 at 0.05C rate, corresponding to the insertion of ∼0.9 sodium ion per formula. This great improvement is attributed to the rGO in the electrode. At higher rates of 0.1C and 0.2C, the capacities are still, respectively, >120 mAh g−1 and >100 mAh g−1, as shown in Figure 4c, further demonstrating the benefit of the conductive rGO in the electrodes. Future studies could focus on the microstructure optimization and electronic conductivity enhancement to increase the rate capability and capacity values further. It should be noted that the layered VOPO4 is a metastable phase. Too much extraction/insertion of large sodium ions may affect the stability of the structure. This is likely to account for the slow capacity decrease at 0.05C rate for the VOPO4/rGO electrode. On the other hand, the protons in the structure may play a role as well, as indicated by the slow increase in capacity in Figure 4b. Further studies are necessary to gain a better understanding of the intercalation mechanisms. Finally, the VOPO4 electrodes at different depths

Figure 5. Average voltage (V) versus theoretical capacity (mAh g−1) and energy density (Wh kg−1) of various polyanion cathodes in NIBs. For vanadium-based cathodes, only a one-electron reaction is considered.

of the various polyanion cathodes for NIBs.12,33,39,42−47 With a high operating voltage and theoretical capacity, the layered VOPO4 as well as the sodiated product NaVOPO4 presented here has a competitive energy density among the various cathode materials for NIBs. Very recently, a highly reversible pseudocapacitance intercalation of Na ions in exfoliated VOPO4·2H2O nanostructure has been reported,48 implying that the practical performance of the layered VOPO4 and NaVOPO4 can potentially be considerably improved by structural optimization.

4. CONCLUSIONS Layered VOPO4 has been synthesized and demonstrated as a potential cathode material for sodium-ion batteries. A series of NaxVOPO4 samples has been readily prepared by chemical sodiation of layered VOPO4. Bond valence sum maps suggest that the sodium ions are located in the space between two adjacent VOPO4 layers, which is confirmed by the Rietveld refinement with the chemically sodiated Na0.8VOPO4. The VOPO4 cathode display a high reversible capacity of 110 mAh g−1, but it suffers from slow charge transfer resulting from low electronic conductivity. Interestingly, by incorporating rGO to improve the conductivity of the electrodes, the capacity could be increased to an impressive value of ∼150 mAh g−1. The high 686

DOI: 10.1021/acs.chemmater.5b04605 Chem. Mater. 2016, 28, 682−688

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Chemistry of Materials

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capacity along with the high operating voltage (3.4 V vs Na/ Na+) make it among one of the most promising polyanion cathode materials for sodium-ion batteries.

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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.5b04605. XRD patterns of the heat-treated VOPO4, digital photos of the chemical sodiation process with the layered VOPO4, digital photos of the chemical sodiation process with the β-VOPO4, initial charge−discharge profiles of the VOPO4 electrode, charge−discharge profiles of VOPO4 at various current rates, and SEM image of the VOPO4/rGO nanocomposite. (PDF)

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AUTHOR INFORMATION

Corresponding Author

* Phone: (512) 471-1791. Fax: 512-471-7681. E-mail: manth@ austin.utexas.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract no. DE- AC02-05CH11231, Subcontract no. 7000389 under the Batteries for Advanced Transportation Technologies (BATT) Program.

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ABBREVIATIONS LIBs, lithium-ion batteries; NIBs, sodium-ion batteries; MWST, microwave-assisted solvothermal; 1D, one-dimensional; 2D, two-dimensional; rGO, reduced graphene oxide; BVS, bond valence sum; PTFE, polytetrafluoroethylene; NMP, Nmethyl-2-pyrrolidone; PVDF, poly(vinylidene fluoride); XRD, X-ray diffraction; ICP, inductively coupled plasma; SEM, scanning electron microscopy; EC, ethylene carbonate; PC, propylene carbonate; NO2BF4, nitronium tetrafluoroborate; CV, cyclic voltammetry

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REFERENCES

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DOI: 10.1021/acs.chemmater.5b04605 Chem. Mater. 2016, 28, 682−688

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DOI: 10.1021/acs.chemmater.5b04605 Chem. Mater. 2016, 28, 682−688