High Energy Density Polyanion Electrode Material: LiVPO4O1–xFx (x

May 15, 2017 - ... Ministry of Science, ICT & Future Planning (NRF-2015M2A2A6A01044985). ..... Yang , X. Q.; Huang , X. J. Trip lite LiFeSO4F as catho...
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High Energy Density Polyanion Electrode Material: LiVPO4O1−xFx (x ≈ 0.25) with Tavorite Structure Minkyung Kim,† Seongsu Lee,‡ and Byoungwoo Kang*,† †

Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea ‡ Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong-gu, Daejeon 305-600, Republic of Korea S Supporting Information *

ABSTRACT: Polyanion compounds as an electrode material in lithium-ion battery are of great interest due to superior structural and thermal stability, but they have relatively low energy density. Here, we report high energy density electrode material, LiVPO4O1−xFx (x ≈ 0.25) with tavorite structure that is first synthesized by singlestep solid-state reaction and that can use more than 1.6 electrons during charging/ discharging process. The doping of F instead of O into LiVPO4O was confirmed with a structural solid-solution measured by neutron powder diffraction and with a mixed valence state of V measured by X-ray absorption near edge spectroscopy. The resulting material has distinct and beneficial electrochemical properties compared to both LiVPO4F and LiVPO4O. LiVPO4O0.75F0.25 has higher average operating voltage and better electrochemical activity than LiVPO4O and shows higher operating voltage (∼2.4 V) than LiVPO4F (∼1.8 V). As a result, LiVPO4O0.75F0.25 can achieve high specific capacity of 260 mA h/g in the voltage range from 2.0−4.8 V and thereby can deliver 820 W h/kg, which is relatively high energy density achieved in known polyanion compounds. material even though the kinetics are fast;14 LiVPO4O has poor electrochemical activity and poor cycle capacity retention15,21−23 even though the two redox voltages, ∼3.95 V for V4+/V5+ and ∼2.3 V for V4+/V3+, are appropriate use as a positive electrode material.15,16,24 To achieve high energy density in tavorite compounds that contain V, reasonable average voltage with a large capacity is required. To exploit two electrons in LiVPO4F or LiVPO4O as a positive electrode material, strategies must be developed to improve the poor electrochemical activity of LiVPO4O or to increase the low redox voltage of V2+/V3+ in LiVPO4F. As one of the strategies, lithium vanadium oxyfluorophosphate compounds25,26 (LiVPO4O1−xFx) can be suggested as having advantages on electrochemical performance over LiVPO4O and LiVPO4F. In this study, LiVPO4O1−xFx (x ≈ 0.25) (LiVPO4O1−xFx; nominal formula LiVPO4O0.75F0.25) has been synthesized by single-step solid-state reaction for the first time. Incorporation of F into LiVPO4O can form a solid-solution phase (LiVPO4O0.75F0.25) due to its structural similarity with LiVPO4F. Neutron diffraction and X-ray absorption spectroscopy (XAS) were carried out on the LiVPO4O0.75F0.25 sample to understand its structure and the oxidation state of V. Since V has a mixed valence state in LiVPO4O0.75F0.25, its local structure of LiVPO4O0.75F0.25 varies from LiVPO4O or LiVPO4F. Besides,

1. INTRODUCTION Materials used in a positive electrode have a critical effect on the energy density of lithium-ion batteries (LIBs).1 Polyanion compounds are very promising candidates for such uses2 due to their superior structural and thermal stability as a result of strong covalent bonds.3 Especially, LiFePO4 has very high rate capability4−6 as well as superior structural and thermal stability.7 Tavorite-structure compounds, LiM(TO4)X (M, metal; T, pblock element; X, O, OH, F) can be a promising electrode material because two Li ions can be exchanged in the structure depending on transition metals. Tavorite-structured compounds such as LiFeSO4F,8−11 LiVPO4F,12−14 and LiVPO4O15−19 are of great potential as positive electrode materials due to their high operating voltage, structural stability, and structural advantage as a fast Li diffuser.20 Among them, V-based compounds have an advantage with respect to energy density because they can exchange more than one electron due to the range oxidation states of V from 2+ to 5+. For example, tavorite-LiVPO4F has two redox operating potentials, ∼4.2 V (vs Li+/Li0) for the V3+/V4+ redox couple and 1.8 V (vs Li+/Li0) for the V2+/V3+ redox couple. Similarly, tavorite-LiVPO4O can extract two electrons at 3.95 V (vs Li +/Li0) for V4+/V5+ and ∼2.3 V (vs Li+/Li0) for V3+/V4+. When all two-redox reactions (two electrons) are utilized, they can achieve very high specific capacities, 312 mA h/g (LiVPO4F) and 318 mA h/g (LiVPO4O). However, utilization of the two redox couples in each of these two compounds is limited. In LiVPO4F, the redox potential of V2+/V3+ (vs Li+/Li0) is only ∼1.8 V, which is too low to be exploited as a positive electrode © 2017 American Chemical Society

Received: January 11, 2017 Revised: May 15, 2017 Published: May 15, 2017 4690

DOI: 10.1021/acs.chemmater.7b00124 Chem. Mater. 2017, 29, 4690−4699

Article

Chemistry of Materials

(PTFE) = 70:25:5 (wt %). Active material and carbon were hand-mixed in a mortar for 10 min, and then binder and the mixture were blended. Then the electrode was dried at 120 °C for 12 h in vacuum. The electrode was cut into a film disk of 8 mm diameter, and the loading density of the electrode was approximately 2.38−2.9 mg/cm2. The halfcells were assembled in a glovebox in Ar atmosphere. Li metal was used as the counter electrode and the reference electrode, and a microporous polypropylene film (Celgard 2400) was used as a separator. The electrolyte was 1 M LiPF6 dissolved in a mixture of EC and DEC (1:1 v/ v). The electrochemical test was performed using Maccor at room temperature (RT).

we clearly demonstrate that the synthesized phase, LiVPO4O0.75F0.25, could achieve high capacity in the voltage range from 2−4.8 V with reasonable operating voltage range for a positive electrode material and high energy density of 820 W h/ kg experimentally. The incorporation of F in the LiVPO4O can have a beneficial effect on the working potential and electrochemical activity. The working potentials (∼4.0 and 2.4 V) of LiVPO4O0.75F0.25 show higher than those of LiVPO4O due to the inductive effect induced by the incorporation of F instead of O.27 Additionally, the two working potentials appear above 2 V unlike LiVPO4F has ∼1.8 V of one working potential, which is too low to use for cathode. Moreover, the vanadium in LiVPO4O0.75F0.25 has a mixed valence state, so its electronic conductivity can be increased; this trait can improve its electrochemical activity compared to that of LiVPO4O. Therefore, the material, LiVPO4O0.75F0.25 synthesized by simple solid-state reaction, could be a promising positive electrode material when further optimizations by minimizing the particle size and performing a carbon coating are achieved.

3. RESULTS 3.1. Synthesis of LiVPO4O0.75F0.25. To introduce both O and F into the V-containing tavorite structure, vanadium oxide precursor was used without any reducing agent. LiF was used as a source of Li and F. The precursors (V2O5, NH4H2PO4, and LiF) were mixing in DI water. During the mixing process, V2O5 and NH4H2PO4 were first reacted to form the intermediate phase, (NH4) (VO2)2(PO4)(H2O)3 (Figure S1). The oxidation state of V in the intermediate phase still remained at +5, the same as V in V2O5. The rest of the phosphate was reacted with water to form (NH4)5P3O10 and H3PO3 in XRD measurement (Figure S1). In contrast to reactions of V2O5 and NH4H2PO4, LiF did not react with either water or other precursors in part because LiF has a low solubility in water. The reacted mix of precursors with LiF was annealed at 700 °C under Ar atmosphere for 1 h. After annealing, the resulting material showed a tavorite structure without any secondary phases such as Li3V2(PO4)3 and V2O3 (Figure 1a). The XRD pattern is similar to that of either

2. EXPERIMENTAL SECTION 2.1. Synthesis of LiVPO4O0.75F0.25. It was synthesized by singlestep solid-state reaction. LiF, V2O5, and NH4H2PO4 were used as starting precursors. All precursors were mixed and ball-milled in DI water for >6 h. Then the mixture was dried at 120 °C. Dried powder was pelletized and annealed at 700 °C for 1 h under Ar atmosphere (ramping time: 3 h) in a covered alumina crucible. 2.2. Neutron Diffraction. The neutron diffraction data of LiVPO4F were collected in HANARO at the Korea Atomic Energy Research Institute. Wavelength was 1.8347 Å, and the scan was from 10° to 150° in steps 0.05°. The data were collected at room temperature (RT). 2.3. X-ray Diffraction. X-ray diffraction (XRD) measurement was measured to characterize the structure of samples. Powder XRD analysis was performed with RIGAKU D/MAX-2500/PC equipped with Cu− Kα radiation. Data were collected with a step width of 1° or 2° per 1 min (depending on the sample) from 10° to 60° at 40 kV and 100 mA. 2.4. Scanning Electron Microscope (SEM). Field-emission scanning electron microscope (FE-SEM, Philips electron optics B.V, XL30S FEG) was performed to characterize the size and morphology of particles. Energy-dispersive X-ray spectroscopy (EDS) was used to detect the amount of F in the samples synthesized with F. 2.5. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). To determine chemical composition of Li, P, and V, ICP-AES (Spectro ARCOS EOP) was performed. 2.6. Ion Chromatography (IC). To determine fluorine composition in the sample, IC was conducted (Dionex ICS-5000+Mitsubishi AQF-2100H) by two methods. The first one was liquid ion chromatography. The fluorine was first filtered by fluorine distilling apparatus. Attenuated fluorine liquid was obtained by analyzing fluorine by IC. Also, the amount of fluorine was confirmed with combustion ion chromatography (CIC) that was analyzed with bare solid powder. Both experiments were conducted at RT. 2.7. X-ray Absorption Near Edge Spectroscopy (XANES). To characterize the V oxidation state of the samples, XANES analysis was conducted. V K-edge X-ray absorption data were taken in transmission at the 7D and 1D beamlines of the Pohang Accelerator Laboratory (PAL, 3.0 GeV, Korea) using a Si (111) double-crystal monochromator to monochromatize the X-ray photon energy. The spectra were recorded at RT under He atmosphere (7D) and vacuum (1D) and calibrated using a standard V metal foil. The XANES data were analyzed using the IFEFFIT software package (ATEHNA program).28 The preedge background was removed by fitting the pre-edge region with a straight line and subtracting the extrapolated values from the entire spectrum. Zero of energy was taken at 5465 eV. 2.8. Electrochemical Test. Swagelok cells were used for electrochemical cell tests. For fabrication of the positive electrode, the ratio of the composite was active material/carbon black (Super P)/binder

Figure 1. XRD patterns of the samples prepared in different solvents. (a) Ball-milling in DI water and (b) ball-milling in acetone. Inset: SEM image of the sample prepared in DI water.

LiVPO4F or LiVPO4O. Formation of the intermediate phase such as (NH4) (VO2)2(PO4)(H2O)3 during ball-milling in water solvent is a critical step to obtain a highly pure compound with tavorite structure. Otherwise (e.g., ball-milling in acetone), the intermediate phase did not form. Without the formation of this intermediate phase in ball-milling process, Li3V2(PO4)3 and V2O3 phases easily formed along with the tavorite phase (Figure 1b). Considering that Li3V2(PO4)3 and V2O3 can form due to the loss of F from LiF during a heat treatment,29 the intermediate phase may help to suppress this loss and favor formation of tavorite structure because the intermediate phase may directly react with LiF. The formation of the intermediate phase can affect a reaction pathway that helps to stabilize the tavorite phase. It should be noted that highly pure tavorite LiVPO4O0.75F0.25 is 4691

DOI: 10.1021/acs.chemmater.7b00124 Chem. Mater. 2017, 29, 4690−4699

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Chemistry of Materials

LiVPO4O and to lower energy when it has V3+ in LiVPO4F (Figure 2a). The absorption energy of the V K-edge in the LiVPO4O0.75F0.25 sample lies between that of LiVPO4F and that of LiVPO4O; this result indicates that the LiVPO4O0.75F0.25 sample has a mixed oxidation state of V3+ and V4+. Also, the mixed valence state of V in the LiVPO4O0.75F0.25 sample indicates that the F could be incorporated into the bulk; this conclusion is consistent with the result of the chemical analyses. In the pre-edge region, the intensity of the peak also reveals important information about local structure. The intensity of the pre-edge peak depends on local geometry and local distortion of the site.32 Usually, the intensity of the pre-edge peak in distorted geometry increases compared to that in perfect symmetry.32 LiVPO4O has higher intensity of the pre-edge peak than LiVPO4F (Figure 2a); this observation indicates that VO6 octahedra are more distorted in LiVPO4O than in LiVPO4F.24 In the LiVPO4O0.75F0.25 sample, the intensity of the pre-edge peak of V was higher than that of LiVPO4F but lower than that of LiVPO4O. This result suggests that the V octahedral sites in the LiVPO4O0.75F0.25 sample are more distorted than those of LiVPO4F but less distorted than those of LiVPO4O. Closer examination of the spectrum of LiVPO4F shows multiplet structure in the pre-edge peak region (Figure 2b, first peak of green line). The splitting in the 1s → 3d transition is usually shown in V2O3 because the d levels of V3+ ions in the octahedral environment split into t2g and eg sets as a result of crystal-field splitting.16,32 Unlikely the pre-edge peak of LiVPO4F, the LiVPO4O0.75F0.25 sample shows a sharp single pre-edge peak (Figure 2b). This difference can indicate that V local structure is quite different in LiVPO4O0.75F0.25 compared to LiVPO4F and LiVPO4O. The derivative of the normalized V k-edge peak of LiVPO4O0.75F0.25 was fitted by linear combination using the reference data, LiVPO4F and LiVPO4O. The fitted peak of the pre-edge peak in the LiVPO4O0.75F0.25 was well-matched (Figure S3); the result shows 70 wt % LiVPO4O and 30 wt % LiVPO4F; this result matches with the result of above chemical analysis. 3.3. Crystal Structure of the LiVPO4O0.75F0.25 Sample. To understand structural features of the LiVPO4O0.75F0.25, neutron powder diffraction was measured and then Reitveld refinement was performed (Figure 3a). Refinement analysis was conducted with both LiVPO4F and LiVPO4O structure information. However, the R-factor of refinement analysis based on the LiVPO4F structure, which has one Li site per unit cell,24 was smaller than that based on the LiVPO4O. Therefore, the refinement result based on LiVPO4F structure was used and F sites of LiVPO4F were replaced with both O and F (stoichiometric ratio O/F = 0.75:0.25). Structural parameters obtained from the refinement (Table 1a) yielded lattice parameters of the sample LiVPO4O0.75F0.25: a = 5.14747(19) Å, b = 5.27584(20) Å, c = 7.16107(27) Å, α = 107.02846(200)°, β = 107.29683(200)°, and γ = 98.90382(232)°. The unit cell volume of the LiVPO4O0.75F0.25 was 171.196(0.011) Å3 (V/Z = 85.6 Å3), which is much smaller than that of LiVPO4F (174.36 Å3, V/Z = 87.2 Å3) but a little larger than that of LiVPO4O (V/Z 85.45 Å 3 ). 24 Lattice parameters (a, b, and c) of the LiVPO4O0.75F0.25 also are shorter than those of LiVPO4F.24 Because an O ion has smaller ionic radius than an F ion, the reduced unit-cell volume is consistent with the incorporation of O into LiVPO4F. On the basis of the refinement results, crystal structure of the LiVPO4O0.75F0.25 was reconstructed (Figure 3). The 3D framework of the LiVPO4O0.75F0.25 is similar to that of LiVPO4F (Figure 3b). V atoms sit in octahedral sites that consist of both F

consistently synthesized when the ball-milled precursors in water solvent were used. Particle size and morphology of the LiVPO4O0.75F0.25 sample were observed using a scanning electron microscope (SEM) (Figure 1a, inset). The resulting material shows a faceted morphology with average particle size of 2 μm. 3.2. Characterization of the LiVPO4O0.75F0.25. To characterize the stoichiometry of the LiVPO4O0.75F0.25, chemical composition of elements was measured using inductive-coupled plasma (ICP) for Li, V, and P, ion chromatography (IC), combustion ion chromatography (CIC), and energy dispersive X-ray spectroscopy (EDS) for F. The chemical ratio of oxygen was calculated by assuming the tavorite phase, LiVPO4O1−xFx (Figure S2). The chemical composition ratio of the resulting material was 1:1:1:∼0.25 for Li/V/P/F, so the nominal composition of the resulting material with the tavorite structure was LiVPO4O0.75F0.25. Even though F was added in the exact mole percent (LiF), the F can easily be lost during heat treatment at high temperature14,29 and can be partially replaced by O. Taking the chemical ratio into account, the oxidation state of vanadium is a mix of 3+ and 4+. To measure the V oxidation state in the F doped sample, LiVPO4O0.75F0.25, X-ray absorption near edge spectroscopy (XANES) was performed. Two reference samples, LiVPO4F for V3+ and LiVPO4O for V4+, were measured together. XANES data (Figure 2a) show that vanadium has a different oxidation state in LiVPO4O0.75F0.25 than in the two reference samples. In the absorption edge region in XANES data, the position of the absorption edge is related to the oxidation state of the absorbing atoms; as the oxidation state of the absorbing atoms increases, the absorption edge shifts to higher energy.30−32 The absorption energy of V K-edge shifts to higher energy when it has V4+ in

Figure 2. (a) Normalized vanadium(V) K-edge of XANES spectra of LiVPO4F, LiVPO4O0.75F0.25, and LiVPO4O. (b) Derivative plots of the V K-edge spectra of LiVPO4F, LiVPO4O0.75F0.25, and LiVPO4O. 4692

DOI: 10.1021/acs.chemmater.7b00124 Chem. Mater. 2017, 29, 4690−4699

Article

Chemistry of Materials

Figure 3. (a) Neutron powder diffraction pattern and Rietveld refinement of the sample, LiVPO4O0.75F0.25. (b) Closed-packed Vanadium octahedra in structure (yellow box); (c) three-dimensional connected octahedral and tetrahedral frameworks; (d) local structure of Li in the LiVPO4O0.75F0.25 and bond lengths surrounding Li ions; (e) V−O (F) lengths and angle of LiVPO4O0.75F0.25 chains in the structure. Observed (red dots), calculated (black line), difference (green line), and Bragg position (blue line) plots. Representation of the crystal structure of the LiVPO4O0.75F0.25 from Rietveld refinement of neutron diffraction pattern.

Table 1. (a) Structural Parameters Obtained from Rietveld Refinement of Neutron Powder Diffraction Data and (b) Bond Lengths [Å] of V−O(F) in Two VO5.75F0.25 Octahedra

and O (VO5O0.75F0.25). The LiVPO4O0.75F0.25 octahedron has staggered configuration along in the c direction, as in LiVPO4O and LiVPO4F (yellow box in Figure 3b).24 P atoms sit in tetrahedral sites composed of four O atoms. Four VO6 octahedra are connected to one PO4 tetrahedron by a shared O, so a 3D framework forms (Figure 3c). Even though the framework of the LiVPO4O0.75F0.25 is similar to that of LiVPO4F, the local structure of the LiVPO4O0.75F0.25 is changed due to the incorporation of O. Each Li ion is surrounded by six O atoms, and the average bond length of Li−O is 2.206 Å (Figure 3d), which is a little longer than that of LiVPO4F (2.24, 2.19, 2.31, 1.90, 1.90 Å; average d: 2.108 Å). Typically, in V phosphates, the local environments around V atoms are differentiated depending on their oxidation state.33 For example, V3+ lies in regular octahedral sites, and V4+ lies in distorted octahedral sites, square pyramids, or distorted tetrahedra. Therefore, the distortion and bond lengths (V−O/ F) of vanadium octahedra were separately investigated (Table

1b). In each octahedron in the LiVPO4O0.75F0.25, the V−O bond lengths were from 1.93−1.96 Å and distortion of the VO6 octahedra was Δ = 4.7 × 10−5; the V−F bond lengths were from 1.90−1.99 Å, and the distortion was Δ = 3.6 × 10−4. For comparison, in LiVPO4F, the narrow range of V−O distances from 1.96−1.98 Å and V−F distances of 1.98 Å reduced the distortions (V−O, Δ = 3.98 × 10−5; V−F, Δ = 2.28 × 10−5),24 whereas LiVPO4O has severely distorted VO6 octahedra (V−O, Δ = 7.63 × 10−3; V−F, 5.51 × 10−3).24 The VO6 octahedra of the LiVPO4O0.75F0.25 are more distorted than those of LiVPO4F but far less distorted than those of LiVPO4O. Thus, by comparing the distortion of the LiVPO4O0.75F0.25 with that of LiVPO4F and LiVPO4O, the V octahedral structure of the LiVPO4O0.75F0.25 is intermediate between LiVPO4F and LiVPO4O, depending on the oxidation state of V. The two octahedra that are connected along the chain direction (c-direction) alternate (Figure 3e). The angle of V−F−V (or V−O−V) is 137.6°, which is larger than that of V−F−V (131.8°) in LiVPO4F and a little smaller than those of 4693

DOI: 10.1021/acs.chemmater.7b00124 Chem. Mater. 2017, 29, 4690−4699

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Chemistry of Materials

Figure 4. (a) GITT voltage curve of the LiVPO4O0.75F0.25 sample obtained from 2.5−4.5 V. Inset is GITT plot for LiVPO4O. (b) Comparison of polarization-capacity plot for the two samples, LiVPO4O0.75F0.25 and LiVPO4O from GITT test, (c) Open circuit voltage (OCV) of the LiVPO4O0.75F0.25. (d) Differential capacity plot of OCV for (1) LiVPO4O, (2) LiVPO4O0.75F0.25, and (3) LiVPO4F. (e) Voltage profiles of cycle retention test at C/10 from 2.5 to 4.5 V. (f) Cycle retention for 40 cycles at C/10 from 4.5 to 2.5 V.

times lower than that of LiVPO4O; this difference means that the replacement of O by F in the LiVPO4O0.75F0.25 helps to improve electrochemical activity. Also, the open circuit voltage (OCV) of the LiVPO4O0.75F0.25 (Figure 4c) is flat and does not have a voltage step behavior in the middle of charge process unlikely LiVPO4F that has a voltage step at Li∼2/3VPO4F.24 Furthermore, the differential capacity plot (Figure 4d) clearly shows that redox potential of the LiVPO4O0.75F0.25 is ∼4.06 V (vs Li+/Li0) for charge and ∼3.97 V (vs Li+/Li0) for discharge and is distinct from that of LiVPO4F and LiVPO4O. The difference between the voltage curves in the charge process and distinct redox potential in the LiVPO4O0.75F0.25 indicate that the F can be incorporated to the bulk of LiVPO4O resulting in a solid-solution phase of both LiVPO 4 O and LiVPO 4 F rather than a composite. If LiVPO4O0.75F0.25 was the composite of both phases, the redox potential of the LiVPO4O0.75F0.25 would be separated into two potentials, ∼4.2 V for LiVPO4F and ∼3.95 V for LiVPO4O in charge process. However, the LiVPO4O0.75F0.25 shows only one redox potential at ∼4 V in charge process that is a higher than that of LiVPO4O and lower than that of LiVPO4F. When the two redox couples (V3+/V4+ and V4+/V5) in a voltage window range from 2.5−4.5 V are in operation, the capacity retention was improved (Figure 4f). Discharge capacity started at 96 mA h/g and increased to ∼108 mA h/g during 20 cycles. Capacity fading started at the 35th cycle, and only 91 mA h/g was delivered at the 40th (last) cycle. The voltage profiles (Figure 4e) show larger discharge capacity than charge capacity; this difference may be the result of higher activity of the redox reaction of V3+/V4+ in discharge process compared to that of V4+/5+ in charge process. Even though the LiVPO4O0.75F0.25 did not have any residual carbon and had large average particle size of 2 μm, electrochemical activity of the LiVPO4O0.75F0.25 was significantly improved compared to LiVPO4O by replacing O with F.

LiVPO4O (137.08° and 138.60°). However, P−O distances of PO4 tetrahedral sites in the LiVPO4O0.75F0.25 range from 1.51− 1.54 Å, which are very similar to those in LiVPO4F. V oxidation states were evaluated by bond valence sum (BVS) calculation, and the neutron refinement results were +3.75 and +3.79. These values match well with the calculated V oxidation state (+3.75) that is obtained from the chemical composition. On the basis of structural features of the LiVPO4O0.75F0.25, the structure of the LiVPO4O0.75F0.25 can be the solid-solution phase of both LiVPO4F and LiVPO4O rather than a composite. Furthermore, structural features of the LiVPO4O0.75F0.25 indirectly indicate that the F can be incorporated into the bulk. 3.4. Electrochemical Properties of the LiVPO4O0.75F0.25. To evaluate the electrochemical properties of the LiVPO4O0.75F0.25, a half-cell test was performed at room temperature. Theoretical capacity was calculated based on the following reaction: LiV(III, V)PO4 O0.75F0.25 → V(IV, V)PO4 O0.75F0.25 + Li+ + e− Theoretical capacity: 158 mA h/g

Galvanostatic intermittent titration technique (GITT) analysis was performed on the LiVPO4O0.75F0.25 to quantify its quasithermodynamic electrochemical property (Figure 4a). The cell was charged at C/50 for 30 min and then rested for 2 h. Specific capacities, ∼152.44 mA h/g for charge and 122.41 mA h/g for discharge in GITT teas, were obtained. Taking these capacities into account, both V3+/V4+ and V4+/V5+ redox couples in the LiVPO4O0.75F0.25 could contribute to the achieved capacity. This possibility will be explained in detail with XANES data during charge/discharge process. Moreover, the LiVPO4O0.75F0.25 has much lower polarization than that of LiVPO4O (Figure 4b). LiVPO4O shows much higher polarization (83 mV) than LiVPO4O0.75F0.25 even at the smallest polarization point (Figure 4b). The average polarization of the LiVPO4O0.75F0.25 is ∼1/64694

DOI: 10.1021/acs.chemmater.7b00124 Chem. Mater. 2017, 29, 4690−4699

Article

Chemistry of Materials

Figure 5. (a) Normalized V k-edge for ex-situ XANES measurement of the LiVPO4O0.75F0.25 electrode prepared at a charging process. (b) Derivative of normalized V k-edge XANES data of the charge process. (c) Normalized V k-edge XANES data of the electrode prepared at a discharging process. (d) Derivative of normalized V k-edge XANES data of the discharge process. Inset: Voltage profile of the LiVPO4O0.75F0.25 sample at C/20 and points on the voltage represent the position where the cells were disassembled for ex-situ measurements of XANES.

Figure 6. (a) Voltage profiles of LiVPO4O0.75F0.25 at C/20 from 2 to 3 V (starting from the discharge process). (b) Cycle retention of LiVPO4O0.75F0.25 at C/20 at low voltage range. (c) Differential capacity plot of LiVPO4O and of LiVPO4O0.75F0.25 at C/20. (d) Voltage profiles of cycle retention test at C/ 20 with a voltage window from 2−4.8 V (first to sixth cycles). (e) Cycle retention of the LiVPO4O0.75F0.25 sample for 15 cycles at C/20 from 2 to 4.8 V. (f) Comparison of actually achieved energy densities in known polyanion positive electrode materials as reported in the literature (Li2FeP2O7,35 Li3V2(PO4)3,34 Li1.1NaVPO4.8F0.2,36 Li2FeSiO4,37 LiFeBO3,38 LiVOPO4,15 Li2MnSiO4,39 Mo0.05V0.95OPO4.19

To confirm use of the two redox couples in the range from 3− 4.8 V, ex-situ XANES measurements were carried out on the

LiVPO4O0.75F0.25 electrodes that were charged and discharged at C/20 (Figure 5a, inset). As a delithiation proceeds (charging 4695

DOI: 10.1021/acs.chemmater.7b00124 Chem. Mater. 2017, 29, 4690−4699

Article

Chemistry of Materials

from 2 to 3 V at C/20 (Figure 6a). The cell delivered ∼110 mA h/g of capacity during first discharging process, but capacity quickly decayed as further cycles were proceeded (Figure 6b). By comparing the operating potential of the two samples, LiVPO4O and LiVPO4O0.75F0.25 at C/20 in a voltage window from 2 to 3 V (Figure 6c), the LiVPO4O0.75F0.25 shows higher average working potential at ∼2.4 V than both LiVPO4O (∼2.2 V) and LiVPO4F (∼1.8 V). The increase of the redox potential can be partly due to an inductive effect.27 In other words, the replacement of O with F in LiVPO4O can increase the ionic characteristic of the V−O(F) bonding that easily increases a redox potential. As a result, incorporation of F into LiVPO4O enables to increase in working potential that will increase energy density in a reasonable operating voltage window. Because the LiVPO4O0.75F0.25 can use two redox reactions, the cell was tested in wide voltage range from 2−4.8 V (Figure 6d). At C/20, 204 mA h/g of capacity was achieved during the first charge process. This large charge capacity in first charge process may be the result of unwanted side reactions such as electrolyte decomposition at high potential. At first discharge, 261 mAh/g of capacity was achieved from 2.0 to 4.8 V: ∼114 mAh/g of capacity from 3.0 to 4.8 V (first redox reaction) and ∼147 mAh/g of capacity from 2 to 3 V (second redox reaction). Given the theoretical capacity of the two redox reactions, 318 mAh/g, 82% of theoretical capacity of LiVPO4O0.75F0.25 could be delivered. This large delivered capacity indicates that almost 1.6 electrons can be reversibly exchanged in LiVPO4O0.75F0.25. However, the achieved capacity quickly decayed as the number of cycles are increased (Figure 6e). Possible reasons for this severe degradation may be fast degradation of the second redox reaction (Figure 6a); the voltage plateau at 2.4 V in both charge and discharge process decrease continuously and significantly. Considering that V2+ would be destabilized in distorted local environments induced by mixed chain along the oxygen and fluorine,25 the instability of V2+ at low voltage region can cause low activity of V2+/V3+ redox reaction leading to less available capacity than theoretical one and poor capacity retention. To fully utilize high energy density of the LiVPO4O0.75F0.25 over a wide range from 2.0−4.8 V, ways to improve poor capacity retention should be developed. Even though its poor capacity retention requires further optimizations, relatively high energy density, ∼820 W h/kg, compared to known polyanion compounds (Figure 6f) makes the LiVPO4O0.75F0.25 attractive as a positive electrode material for lithium-ion battery. Energy density of known polyanion compounds in Figure 6f is calculated based on an achievable energy density reported in literatures rather than their theoretical energy density. Especially, LiVPO 4 O 0.75 F 0.25 shows higher energy density than Li 3 V 2 (PO 4 ) 3 in the range from 2.0−4.8 V. Moreover, Li3V2(PO4)3 has a sever capacity fading problem when >1.5 Li ions can be exchanged with the use of V4+/V5+.34 However, the LiVPO4O0.75F0.25 shows relatively stable reaction in the region of V4+/V5+ at high voltage region (>3 V) even though second redox reaction (V4+/V3+ and V3+/V2+ redox reaction) at low voltage region (3 V) in the LiVPO4O0.75F0.25 is between that in LiVPO4O and that in LiVPO4F, but the potential of the second redox reaction at low voltage region (