Chemical and Electrochemical Lithiation of LiVOPO4 Cathodes for

Article pubs.acs.org/cm

Chemical and Electrochemical Lithiation of LiVOPO4 Cathodes for Lithium-Ion Batteries Katharine L. Harrison,† Craig A. Bridges,‡ Carlo U. Segre,§ C. Daniel Varnado Jr.,∥ Danielle Applestone,⊥ Christopher W. Bielawski,∥ Mariappan Parans Paranthaman,‡ and Arumugam Manthiram*,† †

Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, United States Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Physics & CSRRI, Illinois Institute of Technology, Chicago, Illinois 60616, United States ∥ Department of Chemistry and ⊥ Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, United States ‡

S Supporting Information *

ABSTRACT: The theoretical capacity of LiVOPO4 could be increased from 159 to 318 mAh/g with the insertion of a second Li+ ion into the lattice to form Li2VOPO4, significantly enhancing the energy density of lithium-ion batteries. The phase changes accompanying the second Li+ insertion into α-LiVOPO4 and β-LiVOPO4 are presented here at various degrees of lithiation, employing both electrochemical and chemical lithiation. Inductively coupled plasma, X-ray absorption spectroscopy, and Fourier transform infrared spectroscopy measurements indicate that a composition of Li2VOPO4 can be realized with an oxidation state of V3+ by the chemical lithiation process. The accompanying structural changes are evidenced by X-ray and neutron powder diffraction. Spectroscopic and diffraction data collected with the chemically lithiated samples as well as diffraction data on the electrochemically lithiated samples reveal that a significant amount of lithium can be inserted into α-LiVOPO4 before a phase change occurs. In contrast, lithiation of β-LiVOPO4 is more consistent with the formation of a two-phase mixture throughout most of the lithiation range. The phases observed with the ambient-temperature lithiation processes presented here are significantly different from those reported in the literature.

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the ability to reversibly insert/extract more than one Li+ ion per transition metal ion. One such material is LiVOPO4, which offers the ability to reversibly insert/extract two Li+ ions per vanadium ion, involving two voltage plateaus around 4 and 2 V with a theoretical capacity of 318 mAh/g. Even with an extraction/insertion of one Li+ ion, LiVOPO4 offers higher energy density than LiFePO4, with a theoretical capacity of 159 mAh/g at 4 V. However, relatively few studies have focused on understanding and optimizing the insertion/extraction of two Li+ ions into/from LiVOPO4, compared to the large body of literature for LiFePO4.

INTRODUCTION

There has been considerable interest recently in developing alternative cathode materials that are safer and less expensive than the currently used layered Li[Mn,Ni,Co]O2 for lithiumion batteries, particularly for large-scale applications like electric vehicles and grid energy storage. Following the initial investigation of the polyanion cathodes Fe 2 (SO 4 ) 3 , Fe2(MoO4)3, and Fe2(WO4)3 by Manthiram and Goodenough,1,2 many polyanion cathodes have since then been investigated as potential candidates. Among them, olivine LiFePO4 is the most extensively investigated cathode, but its energy density is limited due to the relatively low operating voltage of 3.45 V, reversible extraction/insertion of only one Li+ ion per transition metal ion, and the low packing density of the LiFePO4 nanoparticles. Very few polyanion cathodes exhibit © 2014 American Chemical Society

Received: May 2, 2014 Published: May 20, 2014 3849

dx.doi.org/10.1021/cm501588j | Chem. Mater. 2014, 26, 3849−3861

Chemistry of Materials

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lithiation process. Finally, galvanostatic intermittent titration technique (GITT) measurements are employed to understand how the equilibrium potential changes with lithium insertion. Although the second voltage plateau occurs below 2.5 V, which may be too low to be ideal for some applications that require high voltages, the second voltage plateau can be helpful for determining the state of charge with certainty and preventing overdischarge even if the extra capacity is not used directly.11,29

Delithiated VOPO4 forms in seven polymorphic modifications but could be used only with lithiated anode materials.3−10 In contrast, LiVOPO4 forms in three polymorphic modifications: triclinic (α), orthorhombic (β), and tetragonal (α1).9,11−26 A few studies have demonstrated that a second Li+ ion can be inserted into LiVOPO4. For example, Pozas et al.27 synthesized orthorhombic Li1.6VOPO4 by a reaction of αVOPO4 with LiNO3·H2O. The Whittingham group showed that ε-VOPO4 can be synthesized by heating monoclinic H 2 VOPO 4 . 28−33 Because the structures of ε-VOPO 4 , H2VOPO4, and α-LiVOPO4 are closely related, they hypothesized that a second Li+ ion could be inserted electrochemically into ε-VOPO4. They found that ∼1.6 Li ions (discharge capacity of ∼250 mAh/g) could be inserted into ε-VOPO4 electrochemically with the insertion of the second Li occurring between 2.0 and 2.5 V. They report that the second Li+ ion insertion is accompanied by a large lattice parameter increase, but the details regarding the structural change and X-ray diffraction (XRD) patterns have not been reported. Similarly, Ren et al.21,34 recently reported that α-LiVOPO4 and βLiVOPO4 could be cycled as anode materials. They proposed that the first lithium inserts to form Li2VOPO4. Further reaction with lithium involves an irreversible phase separation into V metal and Li3PO4, which on charging undergoes the reversible reaction, V + Li3PO4 → VPO4 + 3Li+ + 3e−. Davis et al.35 conducted a nuclear magnetic resonance (NMR) spectroscopy study on Li2VOPO4 which was synthesized from αLiVOPO4 by chemical lithiation, but no diffraction or elemental analyses were provided for the Li2VOPO4 phase. Perhaps the most detailed study of the second lithium insertion process was provided by Allen et al.11 They inserted additional lithium into α-LiVOPO4 and β-LiVOPO4 electrochemically to obtain α-Li1.76VOPO4 and β-Li1.47VOPO4. Detailed X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy measurements indicate that the short V−O (vanadyl) bond in LiVOPO4 increased in length with increasing Li+ insertion as the V4+ ion is reduced to V3+, as is expected because V3+ does not form VO bonds. Due to the bonding changes that occur as V4+ transitions to V3+, there should be a structural rearrangement with the insertion of the second Li+ ion, but the rearrangement has not been detailed in the literature. There is a diffraction database entry for αLi1.75VOPO4, but the reference for the data is an ICDD Grant in Aid report with no freely accessible details.36 In essence, although several reports suggest that it is possible to insert more than one lithium ion into α-LiVOPO4 and βLiVOPO4, none of the studies detail the phase transformation that occurs and it is unclear how much lithium can actually be inserted. In order to develop an understanding of the structural and phase evolution, we present here a systematic investigation of chemical and electrochemical insertion of lithium into both α-LiVOPO4 and β-LiVOPO4. The pristine, single lithiated samples are chemically lithiated to Li1.5VOPO4 and Li2VOPO4 and electrochemically lithiated at steps of 0.1 Li + in Li1+xVOPO4 with 0 ≤ x ≤ 1. The chemically lithiated products (Li/V = 1.5 and 2) are investigated with inductively coupled plasma (ICP) analysis, Fourier transform infrared spectroscopy (FTIR), X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and neutron powder diffraction (NPD). The pristine LiVOPO4 samples are also discharged in pouch cells and studied with ex situ XRD measurements to reveal the structural changes that occur at various stages during the

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EXPERIMENTAL SECTION

Synthesis of LiVOPO4. Although the α-LiVOPO4 polymorph was synthesized and characterized as described elsewhere by us with a microwave-assisted solvothermal method,37 the method is briefly summarized here. V2O5 (Alfa Aesar, 0.0909 g), oxalic acid (Fisher, 0.1891 g), and phosphoric acid (Fisher, 85%, 0.5765 g) were dissolved in 11.25 mL of water and stirred overnight until the solutions became transparent. The stoichiometry and expected products of this reaction are summarized in reaction 1 as V2O5 + 3H 2C2O4 → 2VOC2 O4 + 3H 2O + 2CO2

(1)

Afterward, lithium hydroxide monohydrate (Fisher, 0.2098 g) was added and the solution was stirred for an additional hour. The precursors were added according to the molar ratio of Li:V:P = 5:1:5. Finally, 3.75 mL of ethanol was added before transferring the resulting solution to 100 mL polytetrafluoroethylene (PTFE) microwave reaction vessels. The vessels were inserted into ceramic liners and sealed such that autogenous pressure was generated during the reaction. The vessels were secured on a rotor, which was placed on a turntable in a microwave reaction system (Anton Paar Synthos 3000). The temperature was ramped to 230 °C at a power of 600 W and then held at 230 °C for 10 min. The overall reaction that occurs is then VOC2 O4 + LiOH + H3PO4 → α‐LiVOPO4 + H 2O + H 2 + 2CO2

(2)

β-LiVOPO4 was synthesized by a sol−gel method as described in more detail elsewhere.22,38,39 Briefly, a typical batch involved dissolving V2O5 (Alfa Aesar, 1.2125 g) and oxalic acid (Fisher, 2.5214 g) in ∼80 mL of water according to reaction 1 above. The solution was stirred at 70 °C until the solution turned blue. Lithium nitrate (Acros Organics, 0.9193 g) and ammonium dihydrogen phosphate (Acros Organics, 1.5337 g) were then added, and the resulting solution was stirred for 4 h. The solution was then dried in an air oven at 100 °C until a green powder was formed. The powder was heated in air at 300 °C for 4 h and then at 500 °C for 4 h. The stoichiometry and expected products of this reaction are summarized in reaction 3 as

VOC2 O4 + LiNO3 + NH4H 2PO4 → β‐LiVOPO4 + 2CO + 2NO + 3H 2O

(3)

Synthesis of the Li3V2(PO4)3 sample used as a standard for XAS measurements has been described elsewhere.39 Chemical Lithiation. Chemical lithiation was performed with nbutyllithium (Acros, 1.6 M in hexanes) and standard Schlenk techniques under an atmosphere of dry nitrogen at ambient temperature. Hexanes (Fischer) was dried and degassed by a Vacuum Atmospheres Company solvent purification system (model number 103991-0319). To prepare the α-Li2VOPO4 and β-Li2VOPO4 samples, respectively, 0.263 g (1.56 mmol) of pristine α-LiVOPO4 and βLiVOPO4 were added to each of two flame-dried 50 mL Schlenk flasks (one for each polymorph) equipped with stir bars. Similarly, to prepare α-Li1.5VOPO4 and β-Li1.5VOPO4, respectively, 0.27 g (1.6 mmol) of pristine α-LiVOPO4 and β-LiVOPO4 were added to each of two flame-dried 50 mL Schlenk flasks. The flasks were sealed with rubber septa and then evacuated and backfilled with dry nitrogen. Hexanes (10 mL for the Li1.5VOPO4 product and 20 mL for the Li2VOPO4 product) was added to the respective flasks with a syringe. Then, 0.5 and 1.0 mL (0.8 and 1.6 mmol, 1.6 M in hexanes) of n3850



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butyllithium were added in one portion to the flasks at ambient temperature to obtain the Li1.5VOPO4 and Li2VOPO4 products, respectively. It should be noted that slightly less LiVOPO4 was added to the flasks for the Li2VOPO4 samples such that there would be a slight excess of n-butyllithium (1.60 versus stoichiometric 1.56 mmol or 2.6% excess of the stoichiometric amount); 1.0 mL of nbutyllithium was still used since it could be more conveniently measured with a syringe. Larger batches of α-Li2VOPO4 and βLi2VOPO4 samples were also prepared for NPD measurements. For the large batches, 1.89 g of α-LiVOPO4 and β-LiVOPO4 were added to each of the two flame-dried 250 mL Schlenk flasks. Then, 200 mL of hexanes was added to each flask, followed by 7.0 mL (11.2 mmol, 1.6 M in hexanes) of n-butyllithium. No excess n-butyllithium was used for these batches. Adding the n-butyllithium resulted in rapid color changes of the solid from light green to varying shades of grayish green/blue. The colors of the α-LiVOPO4 dispersion before and after adding nbutyllithium are shown in Figure 1. The dispersions were then stirred

better powder averaging, the samples were spun at 60 Hz during data collection. Data were collected over a range from 0.5 to 49.995° 2θ. NPD data were collected at 300 K on the POWGEN beamline43 at the Spallation Neutron Source (SNS) on samples sealed in 8 mm vanadium cans and at a central wavelength of 2.665 Å (range from 1.1038 to 5.6923 Å). Diffraction data were analyzed with Rietveld refinement44 using GSAS/EXPGUI45 and TOPAS, and indexing was carried out with the GSAS-II and TOPAS software packages. Electrochemical Characterization. Electrodes were prepared by grinding 70 wt % active material with 15 wt % acetylene black (Denki Kagaku Kogyo Kabushiki Kaisha, Japan) and 15 wt % teflonated acetylene black (TAB) in a mortar and pestle. TAB consists of equal parts polytetrafluoroethylene (PTFE) and acetylene black. The resulting composites were rolled into thin sheets and cut into 1.27 cm2 area circles with a punch. They were then pressed to Al current collectors. The electrodes consisted of approximately 8−10 mg of active material and were dried overnight in a vacuum oven at 115 °C before constructing pouch cells. Aluminized barrier liner (Bemis Shield Pack) was used as the pouch material with Li metal anodes pressed to Cu metal current collectors. The electrodes were separated by one layer each of polypropylene (cathode side) and blown microfiber separators (anode side). The cells were filled with 1:1 (v/v) ethylene carbonate (EC)/diethyl carbonate (DEC) as the electrolyte with LiPF6 salt and were impulse sealed in an argon filled glovebox. To obtain good electrical contact, the cells were compressed in vises consisting of two plastic plates screwed together with foam separators in between the plates. For the ex situ XRD measurements, the cells were discharged at a rate of C/100 to various states of discharge and charge on Arbin battery cyclers. The cells were transferred to an argon filled glovebox and cut open to retrieve the electrodes. Pouch cells were used due to the ease of opening them for ex situ measurements without shorting the cells. The retrieved electrodes were washed with DEC, dried, and then sealed between tapes and Kapton film (Chemplex number 442). They were secured to a glass slide for XRD measurements with a Rigaku Ultima (IV) X-ray diffractometer. GITT measurements were carried out with CR2032 coin cells prepared similar to the pouch cells, but with smaller electrodes (0.64 cm2 area). The Li1+xVOPO4 coin cells were discharged in 0.05 Li increments (0 ≤ x ≤ 1) at a C/100 rate and then held at open circuit for 48 h after each step.

Figure 1. Photographs of the LiVOPO4 products before and after chemical lithiation. The color change was immediate upon the addition of n-butyllithium. for 48 h under nitrogen, at which time the solids were allowed to settle and the supernatant was removed by a syringe. The residual solids were washed twice with hexanes and then dried under vacuum. The samples were then transferred to an argon filled glovebox for storage. Pictures of the powders before and after lithiation are shown in Figure 1, indicating the clear color changes in the products. Materials Characterization. The pristine α-LiVOPO4 and βLiVOPO4 samples and all of the chemically and electrochemically lithiated samples were characterized by XRD on a Rigaku Ultima IV Xray diffractometer with filtered Cu Kα radiation. A PerkinElmer BX spectrometer was used to obtain FTIR spectra. Pellets for FTIR analysis were prepared by grinding and pressing samples with dried KBr powder. ICP analyses were performed with a Varian 715-ES ICP optical emission spectrometer. Four standards were prepared for each element by diluting concentrated commercial ICP standards. Errors between calibration standard data points and the calibration curve were always less than 2−3% and repeatability between measurements of the same sample was within 1−2%. V K-edge X-ray absorption data were taken in transmission at the MRCAT (Sector 10, Advanced Photon Source (APS)) bending magnet beamline at Argonne National Laboratory. A total of 5 mg of the finely powdered samples were ground with boron nitride, were pressed into 7 mm-diameter pellets of less than 1 mm overall thickness, and then were encapsulated in 64 μm Kapton tape. The X-ray energy was selected by a water cooled Si(111) monochromator with a 50% detuned second crystal for the elimination of harmonics. All samples are referenced to a V metal foil downstream of the sample. The 20 cm-long ion chambers contained flowing gas mixtures tuned to obtain 10% absorption in Io and 80% absorption in It and Iref. Absorption spectra were aligned, merged, and normalized with Athena.40,41 EXAFS spectra were fit with Artemis.40,41 Powder XRD data were collected at beamline 11-BM42 at the APS at 300 K, with a wavelength of 0.413961 Å and a step size of 0.001°. The instrument has a resolution of ΔQ/Q = 1.7 × 10−4, corresponding to a peak fwhm at the measurement energy of ∼0.005° 2θ. Powder samples were contained within 0.8 mm Kapton capillary tubes, and for

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RESULTS AND DISCUSSION Chemical Lithiation. α-LiVOPO4 and β-LiVOPO4 were chemically lithiated with a sufficient quantity of n-butyllithium to synthesize nominally “Li1.5VOPO4” and “Li2VOPO4”. ICP data, shown in Table 1, confirm that the lithium content is close to the intended nominal values. As noted in the Experimental Section, the chemically lithiated “Li2VOPO4” products were Table 1. Elemental Analysis of Pristine and Chemically Lithiated LiVOPO4a intended sample pristine α-LiVOPO4 α-Li1.5VOPO4 α-Li2VOPO4 pristine β-LiVOPO4 β-Li1.5VOPO4 β-Li2VOPO4 β-Li2VOPO4b

batch size

Li/P

V/P

formula

estimated V oxidation state

−

0.97

0.98

Li0.97(VO)0.98PO4

4.07+

small large −

1.49 1.94 0.99

0.97 0.97 1.00

Li1.49(VO)0.97PO4 Li1.94(VO)0.97PO4 Li0.99(VO)1.00PO4

3.56+ 3.09+ 4.01+

small large small

1.53 1.87 2.06

1.01 0.99 1.00

Li1.53(VO)1.01PO4 Li1.87(VO)0.99PO4 Li2.06(VO)1.00PO4

3.46+ 3.14+ 2.94+

Errors in the ICP results are ∼2−3%. bSample prepared with a slight excess of n-butyllithium. a

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prepared in small and large batches. The lithium content for the α-“Li2VOPO4” was repeatable between three different chemical lithiation attempts (two small batches and one large batch), so elemental analysis is shown only for the large batch. The lithium content varied more for the β-“Li2VOPO4” sample. The Li/P ratio was 2.06 in the smaller batch prepared by chemical lithiation with a slight excess n-butyllithium but was only 1.87 for the large batch without any excess n-butyllithium. Thus, the deficiency in Li for the β-“Li2VOPO4” sample prepared in the large batch is likely not an intrinsic limit to lithiation but rather likely results from error in weighing the LiVOPO4 or small inconsistencies in the amount of n-butyllithium, which is difficult to measure accurately with a syringe, or from errors in the concentration of the n-butyllithium precursor. It is unclear whether the Li/P ratio is higher than 2 for the small batch because the excess n-butyllithium was not completely washed out of the sample or if it is because more than 2 Li can be inserted into β-LiVOPO4. Note that ICP error analysis is in the Experimental Section. The estimated oxidation states of V were also calculated based on charge balance, assuming that all of the lithium present is incorporated into the structure, which may not be the case, especially for the β-“Li2VOPO4” sample showing Li/P = 2.06. Henceforth, to avoid confusion regarding the compositions, the samples will be referred to by their ICP-determined elemental ratios shown in the fourth column labeled “Formula” of Table 1. To confirm that lithiation truly occurred and was accompanied by a change in V oxidation state, XANES measurements were taken on the LiVOPO4 samples before and after lithiation. The large batches of fully chemically lithiated materials corresponding to α-Li1.94(VO)0.97PO4 and βLi2.06(VO)1.00PO4 were analyzed here. Figure 2a compares the Li1+xVOPO4 data to Li3V2(PO4)3, which serves as a V3+

standard. The spectra for the LiVOPO4 samples before lithiation and the Li3V2(PO4)3 standard are consistent with the literature.11,39,46,47 The pre-edge feature arises because of the 1s → 3d transition for V which is possible due to V 3d/V 4p and O 2p mixing. The large pre-edge peak evident in the LiVOPO4 samples occurs because the VO bond in LiVOPO4 leads to a significant VO6 octahedral distortion. Conversely, the Li3V2(PO4)3 sample has many more symmetric octahedra, so the pre-edge peak is less prominent. The double peak exhibited by the Li3V2(PO4)3 sample reflects the crystal-field splitting of the V 3d orbitals into t2g and eg sets.11,18,39 While α-LiVOPO4 shows two small pre-edge peaks similar in intensity to those of Li3V2(PO4)3, β-LiVOPO4 has a very different pre-edge structure, more like a superposition of the two small peaks and the large single peak of the unlithiated material. The edge positions in the XANES data are identified as the second peak in the derivative of the XANES spectra (Figure 2a, inset) and correlate with the oxidation state of a material, with smaller oxidation states shifting to lower binding energies. It is clear that the pre-edge features and edge positions for the chemically lithiated samples agree well with the Li3V2(PO4)3 standard, indicating that the oxidation state is close to V3+. The obvious changes in the XANES signatures between the pristine and chemically lithiated Li1+xVOPO4 samples show that the additional lithium present after chemical lithiation is associated with the expected oxidation state shift from V4+ to V3+ with chemical lithiation to nominally Li2VOPO4. The oxidation state shift suggests the extra lithium from chemical lithiation is present in the Li1+xVOPO4 samples rather than as an impurity. However, while the α-LiVOPO4 shows a single peak in the derivative at an energy slightly lower than that of Li3V2(PO4)3, the β-LiVOPO4 sample has two peaks, the second one at the same energy as the unlithiated material. Figure 2b shows the k3-weighted Fourier Transform of the EXAFS for all four samples. The position of the first peak is indicative of the distances of the V−O local environment. In the case of the alpha phase, chemical lithiation results in a significant shift of the peak to longer distances, while for the beta phase, the peak simply broadens with lithiation and has a significantly reduced intensity. Using a model similar to that of Allen et al.11 to fit the EXAFS spectra confirms that the short VO distance observed in both the pristine samples is absent in the lithiated samples (see the Supporting Information for details) and that all V−O bond lengths are increased, consistent with a V4+ to V3+ transition. Because V in LiVOPO4 transitions to V3+ when the second lithium is inserted, the VO bond should no longer be present after lithiation to Li2VOPO4. To evidence the reduction of V and the vanishing of the VO bond, FTIR spectra are presented in Figure 3 for the pristine and chemically lithiated samples. The pristine FTIR spectra match literature data, and a stretching frequency consistent with a VO bond was observed near 900 cm−1 in both the samples.25,48 Although VO bond stretching frequencies generally occur at higher wavenumbers, near 1000 cm−1, there are many examples of vanadium-based phosphates in the literature that exhibit VO stretching frequencies near 900 cm−1, as is the case for LiVOPO4.24,25,49−52 Additionally, Figure 3 clearly shows that the VO stretching frequency peak shifts to lower wavenumbers (weaker, more ionic bonds) with increasing Li content for α-Li1.49(VO)0.97PO4 and α-Li 1 . 9 4 (VO) 0 . 9 7 PO 4 compared to pristine α-

Figure 2. V-edge (a) XANES data with smoothed derivative in the inset and (b) EXAFS data for Li1+xVOPO4 samples before and after chemical lithiation compared to Li3V2(PO4)3. 3852



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nism involving a phase close to β-Li0.99(VO)1.00PO4 with VO and a phase close to β-Li2VOPO4 without a VO bond and with little solid solution between the two phases. This is consistent with the observations of two peaks in the XANES derivative, the superposition of pre-edge peaks, and the absence of a shift in the first shell EXAFS peak in lithiated βLi1.87VOPO4. Previous ex situ EXAFS measurements have indicated that the VO bond in β-Li1+xVOPO4 increases in length with lithium content for 1.16 ≤ (1 + x) ≤ 1.47, which is inconsistent with our FTIR results presented for 0.99 ≤ (1 + x) ≤ 2.06.11 However, their pristine sample was nonstoichiometric βLi1.16(VO)0.92PO4, which may affect the lithiation process. Furthermore, our fitting procedure as detailed in the Supporting Information shows that a simplified model consisting of four equatorial oxygen and two axial oxygen at different distances is inadequate to completely describe the local environment of the pristine samples. Therefore, simplified models like those used in this work as well as in Allen et al.11 can only be indicative of changes in the local environment upon lithiation and should not be used to identify V−O bond distances. XRD patterns of the chemically lithiated samples shown in Figure 4 reveal clear changes in the XRD patterns after

Figure 3. FTIR spectra of α-LiVOPO4 and β-LiVOPO4 samples before and after chemical lithiation.

Li0.97(VO)0.98PO4. This result is consistent with the reduction of V4+ to V3+ because V3+ should form weaker bonds with oxygen than V4+ and short VO bonds do not form with V3+. The shifting of the VO stretching frequency with increasing lithium content suggests that in both of the chemically lithiated α-LiVOPO4 samples, mixed-valent V3+/4+ exists, which is consistent with the longer V−O bonds being present, and lithium insertion has occurred by a single-phase mechanism. If lithium insertion occurs instead by a two-phase reaction mechanism such that all the extra lithium is accommodated only as a second phase in the α-Li1.49(VO)0.97PO4 and αLi1.94(VO)0.97PO4 samples, then the VO stretching frequencies would be in the same locations as in the pristine sample and the peaks would simply change intensity. The VO stretching frequency almost disappears for the fully lithiated sample, further confirming that vanadium has been reduced to V3+ and the sample has been chemically lithiated. A relatively small signal consistent with a VO stretching frequency remains for the α-Li1.94(VO)0.97PO4 sample, which is consistent with the elemental analysis results that show a Li content of only 1.94 (Table 1). The FTIR data agree with our EXAFS analysis showing that the V−O bond increases from αLi0.97(VO)0.98PO4 to α-Li1.94(VO)0.97PO4 as well as with previous EXAFS measurements.11 In contrast to the α-Li 1 . 4 9 (VO) 0 . 9 7 PO 4 and αLi1.94(VO)0.97PO4 samples, which show a shift in the VO stretching frequency with increasing lithium content, the βLi1.53(VO)1.01PO4 and β-Li1.87(VO)0.99PO4 samples show VO stretching frequencies at approximately the same wavenumber as that of the pristine β-Li0.99(VO)1.00PO4, and the βLi2.06(VO)1.00PO4 sample does not show any VO stretching (Figure 3). However, the intensity of the VO stretching frequency decreases for the β-Li1.53(VO)1.01PO4 and βLi1.87(VO)0.99PO4 samples relative to that of pristine βLi0.99(VO)1.00PO4, indicating that the VO bond has the same length but the species containing the VO bond decreases in concentration. The lack of a strong shift in the V O stretching frequency reflects a two-phase reaction mecha-

Figure 4. XRD patterns of α-LiVOPO4 and β-LiVOPO4 samples before and after chemical lithiation compared to the patterns in the XRD database.

chemical lithiation. It should be noted that these patterns are different from the patterns in XRD databases for β-Li1.6VOPO4 and α-Li1.75VOPO4 (although not at exactly the same lithium contents, which will be addressed later by comparison to electrochemically lithiated samples). The database pattern for the α-Li1.75VOPO4 sample, synthesized by electrochemical 3853



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with little solid solution between β-Li0.99(VO)1.00PO4 and βLi2.06(VO)1.00PO4. The β-Li1.87(VO)0.99PO4 sample shows a behavior in between that of β-Li1.53(VO)1.01PO4 and βLi2.06(VO)1.00PO4. These observations are consistent with our spectroscopy results, suggesting a two-phase mixture for Li1.53(VO)1.01PO4 (FTIR) and β-Li1.87(VO)0.99PO4 (FTIR and XAS). Moreover, these observations imply that a new phase forms to accommodate the extra lithium (Li/V > 1) during chemical lithiation, and this new lithium-rich phase consists of V3+ with no VO bond. Electrochemical Lithiation and Structural Analysis. Because the XRD patterns of the chemically lithiated materials do not clearly resemble previously published patterns for Li1+xVOPO4 samples hosting more than one lithium ion per formula unit, the structural transformation that occurs during electrochemical lithiation was compared to the chemically lithiated products. Thus, pouch cells of pristine α-LiVOPO4 and β-LiVOPO4 were constructed and discharged at a rate of C/100 to various states of discharge for ex situ XRD measurements. The discharge curves are shown in Figure 6, and the states of discharge that were accessed for ex situ XRD are indicated.

lithiation, is very similar to the pristine pattern for αLiVOPO4.36 The database pattern for β-Li1.6VOPO4, synthesized by mixing precursors in acetone and then heating under nitrogen, does not resemble the pristine or chemically lithiated products presented here,27 indicating that a different phase forms through our chemical lithiation than by these direct synthesis processes. The phases formed in the present work have not been previously presented in the literature. Several observations can be gleaned by examining the XRD patterns in the low-angle region (Figure 5). The α-

Figure 5. XRD patterns in a small range of angles for (a) pristine αLi0.97(VO)0.98PO4, (b) chemically lithiated α-Li1.49(VO)0.97PO4, (c) chemically lithiated α-Li 1 .9 4 (VO) 0 . 97 PO 4 , (d) pristine βLi0.99(VO)1.00PO4, (e) chemically lithiated β-Li1.53(VO)1.01PO4, (f) chemically lithiated β- Li1.87(VO)0.99PO4, and (g) chemically lithiated Li2.06(VO)1.00PO4. The vertical dashed lines refer to the prominent peaks in the end member for easily comparing the peak shifts. Figure 6. Discharge curves at a C/100 rate for α-LiVOPO4 and βLiVOPO4, with the states of discharge used for ex situ XRD indicated (note changes in scale).

Li1.49(VO)0.97PO4 sample exhibits many of the same peaks as the pristine α-Li0.97(VO)0.98PO4 pattern, but several peaks in the α-Li1.49(VO)0.97PO4 pattern are shifted, missing, or new relative to the pristine α-Li0.97(VO)0.98PO4 pattern (most notably the peaks at 2θ ≈ 22.5°, 27.3°, 29.6°, and 34.5°). Similarly, the peaks in α-Li1.49(VO)0.97PO4 do not align well with the peaks for α-Li1.94(VO)0.97PO4. It is clear that the αLi1.49(VO)0.97PO4 sample is not simply a mixture of the αLi0.97(VO)0.98PO4 and the α-Li1.94(VO)0.97PO4 patterns. The peak shifts suggest solid solution behavior between the αLi0.97(VO)0.98PO4 and the α-Li1.49(VO)0.97PO4 phases or between these phases and intermediate phases. This observation corroborates our FTIR and EXAFS data and previous EXAFS11 data which suggest that the short V−O bond weakens with increasing lithiation (consistent with a solid solution mechanism), indicating that at least some lithium is inserted into the α-Li0.97(VO)0.98PO4 structure instead of or in accompaniment with the formation of a new phase. Conversely, the β-Li1.53(VO)1.01PO4 pattern displays all of the β-Li0.99(VO)1.00PO4 peaks at the same angles as well as the β-Li2.06(VO)1.00PO4 peaks with some small shifts for a few peaks. Thus, the β-Li1.53(VO)1.01PO4 sample is a two-phase mixture consisting of the pristine and fully lithiated samples

Because the α-LiVOPO4 curve was still somewhat flat at 2 V (which has been used as a potential cutoff in previous studies),11,29 the cells were also cycled down to 1.8 V corresponding to 1.92 Li+. Just above 1.8 V, the slope of the curve changes such that it is clear that discharging to lower potential would not increase the capacity significantly. Because the ex situ measurements were undertaken for comparison to the chemically lithiated α-Li1.94(VO)0.97PO4 product (1.94 Li+), discharging down to 1.8 V with a lithium content of 1.92 per formula unit was deemed reasonable. The discharge profile of α-LiVOPO4 shows that there is a slightly sloping plateau region (up to about 1.4 Li+ ions), followed by an obvious continually sloping region. By the Gibbs phase rule, the plateau region implies a two-phase mixture up to 1.4 Li+ ions and then a single phase after that. However, because Figure 6 shows the discharge curves under load, it is difficult to determine whether the initial slightly sloping region is really a single or two-phase region. To better understand the relationship between the shapes of the discharge curves and the phases present (two-phase versus single-phase), GITT 3854



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The capacity for the second lithium ion insertion into the βLiVOPO4 polymorph occurs at slightly lower voltage (2.2 V versus 2.4 V) than that for the α polymorph, so the cells were discharged down to a lower voltage, as shown in Figure 6. Discharging to 1.8 V led to a capacity equivalent to ∼1.8 Li+ ions per formula unit. The discharge curve was still fairly flat at 1.8 V, so electrodes were also discharged down to 1.6 V, resulting in a significantly sloping discharge curve and a capacity corresponding to ∼β-Li2.11VOPO4, which is similar to the chemical lithiation results with β-Li2.06(VO)1.00PO4. The electrochemical and chemical lithiation results for βLi2VOPO4 show slightly higher lithium content than expected, which could mean that more than two lithium ions can be incorporated into the new phase that forms upon lithiation of pristine β-LiVOPO4, especially because both the chemical and the electrochemical lithiation results show similar values. However, this result could also imply that there is a lithiumrich impurity phase present or could evidence side reactions during electrochemical lithiation. The chemical lithiation products were washed with hexanes to remove excess leftover n-butyllithium, but it is also possible that a small amount of impurity is present. Finally, lithium content slightly higher than two could indicate that the initial oxidation state of V in the pristine sample is slightly higher than 4+ with a slightly lower lithium content than is implied by the formula β-LiVOPO4, although the elemental analysis suggested ratios consistent with β-LiVOPO4 within error and no detectable impurities are present in the XRD pattern. Note that the initial open circuit potential for the pristine LiVOPO4 samples was always close to 3 V, consistent with close to a 4+ oxidation state. Ren et al.34 showed that discharging β-LiVOPO4 down to 0.01 V resulted in irreversible decomposition of β-LiVOPO4 into Li 2O, V metal, Li3 PO 4, and VPO 3. Their cyclic voltammetry curves revealed a continuous discharge peak between about 1.6 and 2.1 V (presumably for V 3+/4+ corresponding to the insertion of second lithium into βLiVOPO4) and then a flat region until approximately 1.1 V, after which there are a series of other peaks. This result agrees with the assumption made here that the lithiation of βLiVOPO4 occurs down to about 1.6 V and the material irreversibly decomposes only at lower voltages. As will be discussed in more detail below, discharging β-LiVOPO4 down to 1.6 V was reversible and pristine β-VOPO4 was recovered upon subsequent charging to 4.5 V. Analysis of the GITT data (Figure 7) reveals that the overpotential for the β polymorph is much larger than that for the α polymorph, as indicated by the relatively large changes between the measurements under load and the measurements after rest at open circuit. GITT measurements for the β polymorph show a strongly sloping region for a very small range between x = 0 and x = 0.1 Li+ ions inserted in Li1+xVOPO4 and then an almost flat region (two-phase) between x ∼ 0.1 and x ∼ 0.6 Li+ ions inserted. There is much larger overpotential as the formula approaches Li2VOPO4 (for x > 0.6) and slightly more slope to the open-circuit voltage measurements. This result could imply a possible single-phase region for x > 0.6. However, examination of the open-circuit potential versus time (Supporting Information Figure S1) reveals that 48 h was not sufficient relaxation time for the spikes shown in Figure 7 to represent a true equilibrium potential, implying that even the region corresponding to x > 0.6 may have been flat given longer relaxation times. Therefore, the exact extent of the two-phase

measurements were made by inserting Li+ ions in 0.05 increments into Li1+xVOPO4 at a C/100 rate and then allowing the cells to rest at open circuit for 48 h before inserting the next 0.05 Li+ ions. The results are shown in Figure 7, where the

Figure 7. GITT curves at a C/100 rate for α-LiVOPO4 and βLiVOPO4, with a 48 h rest between each 0.1 Li increment (note changes in scale).

relatively flat regions are potential measurements under load at various states of discharge and the spikes in the data indicate the change in potential over the 48 h relaxation period at open circuit. Thus, the peaks of the spikes roughly represent the open-circuit voltage (assuming full relaxation occurs after 48 h). The open-circuit voltage measurements for the α polymorph show a strongly sloping region between 0 and ∼0.25 Li+ ions inserted, a very slightly sloping region between ∼0.25 and ∼0.5 Li+ ions inserted, and another strongly sloping region between ∼0.5 and ∼0.92 Li+ ions inserted. The strongly sloping regions represent clear solid solution behavior. The relatively flat region between ∼0.25 and ∼0.5 Li+ ions inserted reflects a small region where two phases are present. The occurrence of a significant structural change somewhere before α-Li1.5VOPO4 is in agreement with the chemically lithiated XRD data in Figures 4 and 5, which show a different pattern (accompanied by relatively large shifts in peak position) for α-Li1.49(VO)0.97PO4 compared to that for pristine α-Li0.97(VO)0.98PO4. Although open-circuit voltage measurements in the GITT discharge curve between 0.25 and 0.5 Li+ inserted are slightly sloping, a closer look at the open-circuit voltage versus time shows that 48 h is not sufficient to allow the system to completely relax to the equilibrium value (see Supporting Information Figure S1); thus, the slight slope likely results from insufficient relaxation time and this region is likely a two-phase region. At the point in the discharge curve that would correspond approximately to α-Li1.5VOPO4, the curve is transitioning from two phases to a single phase, so one would expect that the emerging new phase (approximately αLi1.5VOPO4) would have a mixture of V3+ and V4+ ions. Diffraction results presented later suggest that this structural transformation to a second phase may occur without a change in symmetry (i.e., P1̅ is retained) but rather with a significant change in lattice parameters and distortion of the framework to accommodate additional lithium. This result is consistent with the shifts in the VO bond stretching frequencies in the FTIR measurements (Figure 3). 3855



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LiVOPO4 sample much more closely than the α-Li1.7VOPO4 or α-Li1.8VOPO4 samples shown here. Furthermore, it is helpful to compare the chemically and electrochemically lithiated samples to ascertain whether they display the same structural transformation. Supporting Information Figure S3 shows such a comparison for the α-Li1.5VOPO4 and α-Li2VOPO4 materials. It is clear that the chemically and electrochemically lithiated materials match well, which indicates that the structural transformation occurring during electrochemical lithiation is replicated by chemical lithiation. High quality synchrotron powder X-ray diffraction data and neutron diffraction data have been collected to determine the structure of Li2VOPO4 polymorphs. The investigation of αLi2VOPO4 was largely made with the assumption of phase purity, based upon the cyclability of the X-ray diffraction results. The large batch of β-Li2VOPO4 (“Li1.87(VO)0.99PO4”) synthesized for structural analysis is known to be lithium deficient and is not composed of a single end member βLi2VOPO4-like phase (see Table 1 and Figures 4 and 5). As a result, the primary focus has been on the analysis of αLi2VOPO4. Initially, attempts were made to determine the lattice parameters of the α-Li2VOPO4 phase using XRD data and the indexing routines in the programs GSAS-II and TOPAS. Several sets of lattice parameters were identified, and representative examples are given in Supporting Information Table S2. To assist in identifying likely unit cells, a comparison of related Li−M−P−X is presented in Supporting Information Table S3; on the basis of related compounds, lithium transition metal phosphate compounds tend to crystallize with 2 or 4 formula units (f.u.) per unit cell, and with densities in the range of ∼2.8−3.5 g/mL. For the M:P ratio of 1:1, the density is typically ∼3.1−3.4 g/mL. Therefore, potential unit cells were evaluated by calculating the theoretical density for a given volume and number of formula units. The results in Supporting Information Table S2 suggest that, to obtain a reasonably good fit to the data, while maintaining a relatively small cell typical of known Li−M−P−X phases (with 2 to 8 f.u./cell), a P1 or P1̅ space group must be used. This is not unexpected given that the parent compound α-LiVOPO4 crystallizes in the P1̅ space group, and intercalation should occur through a largely topotactic process. Attempts were made to solve the structure ab initio using several different cells in triclinic space groups and using both the XRD and NPD data simultaneously, but chemically acceptable results were not obtained. Given that the same basic structural features are present in several related compounds, attempts were made to fit the data based upon models derived from known structures. Several materials are known to crystallize in the Tavorite-type structure of AM(BO4)X, where A is from Group 1 or 2, M is a metal, B is typically S or P in battery materials, and the anion X is generally oxide, hydroxide, or fluoride (the mineral Tavorite is LiFe(PO4)(OH)).53 Examples of Li-ion battery related materials include LiMOPO4 (M = V, Ti), LiMPO4F (M = V, Fe, Ti), and LiMSO4F (M = Fe, Mn, Co, Ni), with several other possibilities known or suggested on the basis of computational results.53 Furthermore, structures have been reported for Li2VPO4F54 and Li2FePO4F55 prepared through ion exchange from Na 2 FePO 4 F and for isostructural Li2CoPO4F56 and Li2NiPO4F.57 Generally, these single and doubly lithiated compounds contain corner-sharing chains of octahedrally coordinated vanadium cations that are connected via PO4 tetrahedra to four neighboring chains, as shown in Supporting Information Figure S4a−d. The Li2MPO4F (M =

region is unclear from the GITT data, but longer relaxation tests were time prohibitive (the data corresponding to the GITT measurements shown in Figure 7 took ∼45 days to complete). Regardless, the GITT measurements suggest that the potential is relatively constant for the β polymorph over a larger range than for the α polymorph, indicating a larger range of two-phase behavior. This agrees well with the XRD, XAS, and FTIR data which suggest that the β polymorph is a twophase system throughout much of the insertion of the second Li+ ion. To further examine the structural changes accompanying insertion of a second Li+ ion into LiVOPO4, ex situ XRD measurements were conducted and the corresponding results are summarized in Supporting Information Figures S2 (full range) and Figure 8 (low angle region). The peak shifts upon

Figure 8. Ex situ XRD patterns of α-LiVOPO4 at various states of discharge, detailing the low-angle region. The vertical dashed lines refer to the prominent peaks in the end member for easily comparing the peak shifts.

lithiation of up to x = 0.3 agrees with GITT data, indicating a solid solution in this region. New peaks that arise when x = 0.4 also agree with GITT measurements, suggesting a two-phase region between ∼1.3 and 1.5 Li+ ions. Finally, the GITT measurements, indicating a single phase for samples with ≥1.5 Li+ ions, are also consistent with ex situ XRD measurements. A database pattern for α-Li1.75VOPO4 is shown for a comparison, and it again appears quite different from the electrochemically lithiated samples examined in this study. To reiterate, this database pattern resembles that of the pristine α3856



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Table 2. Lattice Parameters, Densities, and Rietveld Fit Quality for P1̅ Models of α-Li2VOPO4 space groupa

Rwp

Rp

GOF

volume (Å3)

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

Γ (deg)

f.u./cell

density (g/mL)

P1̅ P1̅ P1(̅ fix)

9.8 9.0 9.3

13.0 11.8 12.7

3.8 3.5 3.6

352.06 703.98 703.23

7.0961 7.0949 7.0748

7.8110 7.8119 7.8156

7.1012 14.1998 14.2054

90.17 90.19 90.15

116.55 116.54 116.44

90.72 90.70 90.78

4 8 8

3.316 3.317 3.321

a

For the sample labeled “fix”, the lattice parameters were fixed during the Rietveld fit.

Figure 9. Structural plots of (a) LiVOPO4 in comparison with the (b) relaxed model of α-Li2VOPO4 with doubled c axis. There are differences in the Li site occupancy (which are all half occupied in the doubly lithiated model), the tilting of the polyhedra with respect to each other, and the position of the V and P cations within their polyhedra. Rietveld fits of the powder (c) X-ray diffraction and (b) neutron diffraction data based on the P1̅ models of α-Li2VOPO4. Plots are for the doubled c-axis cell with fixed lattice parameters in Supporting Information Table S3.

Co, Ni) phases crystallize in a Pnma structure with edge-sharing MX6 octahedral chains connected to four neighboring chains (Supporting Information Figure S4e,f) through PO4 tetrahedra. Li−Li distances typically decrease on average from ∼3.4−3.9 Å in the LiMPO4X compounds, to ∼2.7−3.5 Å in Li2MPO4F (M = Co, Ni, Fe), while in the experimentally determined C2/c structure of Li2VPO4F, short ∼1.1 Å and ∼1.9 Å distances leads to 50% site occupation. Using the unit cells presented in Supporting Information Table S3, several potential cells based upon P1̅, Pnma, C2/c, and P4/nmm were compared against the diffraction data. Through Pawley refinements (i.e., without a crystal structure model58) of cells distorted from known LiyVPO4X (y = 1, 2; X = O, F) compounds, it was determined that a triclinic distortion of the C2/c cell of LiVPO4F provided a relatively good match to the synchrotron XRD data. Combined Rietveld refinements of XRD and NPD data were carried out on the basis of a P1̅ structure derived from the C2/c cell of LiVPO4F and from doubling of the c-axis of that cell (along the direction of the octahedral chains). The nondoubled cell is closely related to that of P1̅ LiVOPO4, but with the b and c axes inverted and significant changes in cell parameters. The phosphorus and vanadium atoms were held fixed at the relatively high symmetry positions derived from the C2/c LiVPO4F model due to

relatively large shifts during the early stages of minimization that led to poor quality fits. A penalty function was used to allow the positions of the oxygen and lithium to relax, while minimizing the likelihood of a nonphysical interatomic distance. Furthermore, a fourth order spherical harmonic peak broadening function was required to improve the fit of the XRD data, and the use of higher order did not provide further benefit for misfit reflections; this may indicate a relatively large microstrain that is difficult to model, that a larger unit cell is required, or the possibility that certain reflections are less affected by compositional inhomogeneity (i.e., little shift in particular d-spacings with cell changes during Li intercalation). There is a significant anisotropy in the peak width noted in the XRD pattern, with the (020) peak at ∼3.9 Å relatively sharp compared to the combined (200)/(002) peak at ∼3.16−3.17 Å. The latter peak exhibits a notable tailing to higher angle that may reflect compositional inhomogeneity. This anisotropy is less evident in the neutron diffraction data due to lower resolution and possibly that the sharper peaks contain a larger contribution from vanadium and are, therefore, weaker in the neutron data (the (020) peak is essentially absent in the NPD data but sharp and intense in the XRD data). The resulting Rietveld refinement fit statistics and lattice parameters are 3857



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to ∼117.2° reported for Li1.75VOPO4; the a axis length of 6.748 Å for α-LiVOPO4 increases significantly to ∼7.09 Å for the doubly lithiated composition, which explains the apparent disappearance of the strong (200) reflection of α-LiVOPO4 as a peak shift to ∼7.5° 2θ in Supporting Information Figure S5a; the angles in the doubly lithiated models presented here are more similar to that of C2/c Li2VFPO4 but still require the triclinic distortion to better match the diffraction data. An asymmetric broadening of several reflections significantly increases the difficulty in clearly indexing the unit cell, and attempts to obtain images from electron microscopy failed due to the extreme beam sensitivity of the compound. Despite the difficultly in establishing a definitive unit cell, the models presented here provide a relatively good fit to the NPD and XRD patterns and suggest that lithiation of α-LiVOPO4 is accompanied by an increase in the cell volume and number of formula units in the unit cell, in conjunction with subtle distortions in the structure to accommodate the lithium while maintaining the basic framework. Although structural refinement was not possible for the βLi2VOPO4 (“Li1.87(VO)0.99PO4”) sample due to lithium deficiency, some insight can be gained by examining ex situ XRD patterns at various states of discharge (Supporting Information Figure S7 and Figure 10) for the β-LiVOPO4 sample. Again, there are clear changes in the XRD patterns as

shown in Table 2, and plots of the fits are in Supporting Information Figure S5 and Figure 9c,d. Slightly better fits are obtained with the larger cell, which may reflect in part the additional variables present. The NPD data are not sensitive to the vanadium positions, and the fit reflects that the P, O, and Li atomic positions are well accounted for, whereas the XRD data are more sensitive in particular to V and P positions. Plots of the structure for the doubled cell are shown in Supporting Information Figure S6. The results for both doubled and nondoubled c-axis cells are suggestive of a structure with polyhedra tilted with respect to each other in a manner similar to P1̅ α-LiVOPO4 but with the phosphorus and vanadium ions at slightly higher symmetry positions and with additional Li sites (Figure 9a,b). On the basis of the oxygen displacements, it is likely that P and V are somewhat shifted from these positions onto lower symmetry sites, unlike the higher symmetry structure of C2/c Li2VFPO4, but the correlation encountered in the refinement may indicate a disordered displacement from the higher symmetry sites that is small and difficult to model with the available data. However, it is expected that the vanadium atoms will be closer to the center of their octahedra for the doubly lithiated cell with V3+, as opposed to the starting material with typically off-center V4+ in the VO6 octahedra. As shown in Supporting Information Figure S5 and Figure 9, the models provide a relatively good fit for both the XRD and the NPD data. Despite being a stronger scatterer with neutrons than X-rays, the lithium contribution to the neutron scattering remains sufficiently weak that it is difficult to define their positions. Whereas the C2/c structure has 2 Li, 1 V, 1 P, 2 O, and 1 F site, the doubled P1̅ cell has 16 Li, 6 V, 4 P, and 20 O positions. As in the C2/c structure, the Li sites are half occupied to accommodate the close proximity of some sites to each other. For some refinements, the V−O distances for some V sites contain relatively short ∼1.6 Å contacts, and V−Li distances of ∼2.1 and ∼2.4 Å are observed for two vanadium sites. Furthermore, it should be noted that six sites merged during the minimization to result in 10 partially occupied and 3 fully occupied sites, though this did not fully resolve the issue of somewhat shortened interatomic distances. Despite the difficulty in defining the position of 16 partially occupied sites, the doubled cell appeared to largely give a more chemically reasonable set of V−Li distances than the nondoubled cell. Overall, while a definitive structure solution is not possible on the basis of the fits provided, the results are suggestive that the structure of α-Li2VOPO4 is closely related to other Tavorite-type compounds but that the precise details of the local structural distortions are yet to be fully elucidated. The density has somewhat unexpectedly increased from P1̅ LiVOPO4; whereas an increase of ∼6% in volume might be expected on the basis of calculation53 and a ∼ 7.4% increase was observed from LiVPO4F to Li2VPO4F, the increase observed here is ∼2.6% for the doubled and nondoubled cells. With the increase in lithium content and the minimal volume increase, there is a slight apparent increase in density. As shown in Supporting Information Table S3, an increase in density is similarly reported for the P1̅ compound Li1.75VOPO4. The apparent difference in the patterns from the singly lithiated phase appears to relate in large part to the difference in lattice parameters: the nondoubled cell reported here (with b and c lattice parameters switched relative to P1̅ LiVOPO4) shows a decrease in the beta angle to ∼116.5°, rather than the increase

Figure 10. Ex situ XRD patterns of β-LiVOPO4 at various states of discharge, detailing the low-angle region. 3858



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and structures presented here are the results of lithiation of the respective LiVOPO4 polymorphs rather than due to structural decomposition and resulting impurity formation, but further work is required to precisely define the structures that form.

more lithium is inserted, but the changes are more subtle at lower states of discharge. At states of discharge ≥1.4 Li+ ions, there are not any obvious systematic shifts in the peak locations, again indicating that little additional lithium can be inserted into β-LiVOPO4 without a phase change occurring. Starting at around β-Li1.4VOPO4, though, a shoulder becomes visible on the peak around 26.5°, and as the lithium content increases further, several small new peaks arise. The ex situ XRD measurements show that the two-phase region continues until approximately β-Li1.9VOPO4, at which point the peaks from the pristine β-LiVOPO4 pattern are no longer visible. The long-range of two-phase behavior is consistent with the GITT, XAS, and FTIR data, which show that the β polymorph accommodates excess Li + by a two-phase mechanism throughout much of the lithiation range. Finally, it is worth pointing out that there is no similarity between the ex situ XRD pattern for β-Li1.6VOPO4 and the database pattern synthesized by mixing precursors in acetone and then firing in nitrogen.27 Clearly, the solid-state procedure in the literature does not result in the same phase as occurs during chemical or electrochemical lithiation of β-LiVOPO4 presented here. A comparison is shown between the chemically and electrochemically lithiated β-LiVOPO4 polymorph in Supporting Information Figure S8. The chemically and electrochemically lithiated beta phase samples generally agree quite well, again indicating that phase change occurs similarly for chemical and electrochemical lithiation processes. Finally, it is important to establish that the α-LiVOPO4 and β-LiVOPO4 structures remain stable after insertion of close to two lithium ions. After cycling the samples down to 1.8 and 1.6 V, respectively, the cells were then charged to 4.5 V and ex situ XRD patterns were collected. These patterns were compared to pristine LiVOPO4 cells charged to 4.5 V (without discharging first). The ex situ XRD results are shown in Figure 11, and it is

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CONCLUSIONS While a second lithium ion has been shown to be inserted into LiVOPO4, increasing its theoretical capacity to 318 mAh/g (albeit half of that capacity occurs at potentials less than 2.5 V), the phase relationships during the lithiation process had not been detailed before in the literature. To address this deficiency, α-LiVOPO4 and β-LiVOPO4 were synthesized, and the lithium insertion process was investigated systematically with various techniques. The samples were chemically lithiated to obtain nominally α- and β-Li1.5VOPO4 as well as αand β-Li2VOPO4, and the samples were also electrochemically lithiated for a comparison. The XRD patterns show clear changes from the starting materials and are also significantly different from patterns described in the literature. FTIR, XAS, and ICP data indicate that the oxidation state shifts from V4+ for the pristine samples to V3+ for the chemically lithiated samples. XRD and FTIR data of the chemically lithiated phases reveal that lithiation into the α-LiVOPO4 polymorph proceeds in a manner such that some lithium (∼1.4 Li+) can be accommodated into the structure before a phase change occurs, after which there is another large single-phase region. Lithiation of the β-LiVOPO4 polymorph is more consistent with the formation of a two-phase mixture with a smaller range of solid solution between the two end members, viz., β-LiVOPO4 and β-Li2VOPO4. The phase changes that occur after partial lithiation may relate to the point at which close lithium− lithium contacts will occur without partial occupation of lithium sites, necessitating more significant changes in the framework. Understanding the lithiation process during discharge is critical for utilizing the full capacity of this material and to improve cycle life and rate capability. During the final stages of the review process for this manuscript, a “just accepted” manuscript by Bianchini et al.59 was partially published including a structural solution and in situ XRD measurement of the structural evolution from αLiVOPO4 to α-Li2VOPO4. Unfortunately, the “just accepted” version does not include figures, so it is difficult to compare to our manuscript. However, the structural solution for αLi2VOPO4 is available. We, therefore, simulated the patterns for α-Li1.5VOPO4, α-Li1.75VOPO4, and α-Li2VOPO4 on the basis of the lattice parameters for each phase and the structural model for α-Li2VOPO4, as is available in Bianchini et al.59 The results presented in Supporting Information Figures S10 and S11 show that none of these simulations can directly account for our data. Therefore, we have attempted to use the solution in Bianchini et al.59 to fit our X-ray and neutron diffraction data and compared the results directly with the results of the structural model derived from Li2VFPO4 that we have discussed (which includes partially occupied Li sites). The Rietveld refinement plots and crystal structure models are compared in Supporting Information Figures S12 and S13, and the refinement results are given in Supporting Information Tables S4−S7. We find that that while the solutions for α-Li2VOPO4 are comparable and are largely related by a simple transformation matrix, there is evidence in the diffraction data for differences in the samples. Furthermore, we present the possibility that there are partially occupied Li sites in close contact, in accordance with the structure of the closely related

Figure 11. Ex situ XRD patterns of LiVOPO4 after the pristine sample is charged to 4.5 V and after the fully discharged material is charged to 4.5 V.

clear that charging the fully discharged α-LiVOPO4 sample to 4.5 V results in the same VOPO4 phase as charging the pristine sample to 4.5 V (charge curves shown in Supporting Information Figure S9). The β-LiVOPO4 sample showed a similar trend; the ex situ XRD patterns obtained by charging the β-LiVOPO4 and α-LiVOPO4 polymorphs can be indexed to β-VOPO4 (orthorhombic) and η-VOPO4 (monoclinic), respectively. Thus, inserting 1.92 lithium ions into the αLiVOPO4 structure and 2.11 lithium ions into the β-LiVOPO4 structure does not cause an irreversible structural change. This structural reversibility supports the argument that the capacities 3859



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U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The authors thank Dr. Ashfia Huq for assistance with collection of NPD data at SNS and Dr. Ray Unocic for assistance with structural investigations by electron microscopy.

Li2VFPO4 phase. This is discussed in more detail in the Supporting Information. Furthermore, although the in situ XRD figures are unavailable in Bianchini et al.,59 we can surmise from the text that they report a two-phase mixture between αLiVOPO4 to α-Li1.5VOPO4 and more complicated transitions between α-Li1.5VOPO4 to α-Li2VOPO4. In contrast, we show clear evidence of solid solution regions in the GITT, FTIR, ex situ XRD, and XANES measurements of the α-Li1+xVOPO4 phase. The differences between their material and our material could be due to many factors. We use a low-temperature microwave-assisted solvothermal synthesis to obtain LiVOPO4 and then chemically lithiate it with n-butyllithium. Bianchini et al.59 use a high-temperature solid-state reaction to obtain LiVOPO4 and then chemically lithiate it with lithium aluminum hydride. These procedures likely lead to different defect structures and particle morphologies, which can certainly affect the lithiation process. Also, to decrease the particle size and improve the kinetics, Bianchini et al.59 extensively ball milled their sample with carbon. This process can change the defects and morphology of the material and can coat the material with carbon. These factors may change the reaction mechanism, especially for a material with poor reaction kinetics like LiVOPO4. Note that the chemical lithiation procedure is unlikely to be responsible for the differences between Bianchini et al.59 and our work. Both manuscripts show that consistent phases form during chemical and electrochemical lithiation, indicating that differences in the phase-change mechanism and the final end member phase is likely related to the initial pristine LiVOPO4 material. Similarly, differences are unlikely to result from in situ versus ex situ measurements because we see very similar XRD patterns from ex situ measurements and the chemically lithiated material.

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ASSOCIATED CONTENT

S Supporting Information *

More XRD and electrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (512) 471-1791. Fax: (512) 471-7681. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synthesis, basic phase analysis, and electrochemical characterization work at the University of Texas at Austin 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-AC0205CH11231, Subcontract No. 7000389, under the Batteries for Advanced Transportation Technologies (BATT) Program. The structural analysis work at Oak Ridge National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. Research at the SNS and CNMS-ShaRE facility was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Use of the Advanced Photon Source at Argonne National Laboratory (CUS) was supported by the 3860



Article

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more likely be caused by differences in the initial LiVOPO4 samples. We use very different synthesis methods and our unit cell volumes differ significantly (~1.2 Å3). This may be caused by the presence of defects arising from the lower temperature synthesis approach, as has been previously noted for solvothermal synthesis of related materials (e.g., LiFePO4).39 Regardless of the reason for differences in the phase change mechanisms, our data clearly supports a single-phase mechanism for 1.5 ≤ = 1 + x ≤ 1.92 in α-Li1+xVOPO4.

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NOTE ADDED IN PROOF During the proof process, Bianchini et al.'s full manuscript59 was released with figures so we could now compare our work to theirs more thoroughly. It is clear that their GITT curves are very different from ours as discussed in the Conclusion section. We surmised that this may be due to particle size effects, but they do not see a change in the plateau behavior with decreasing particle size (increasing ball mill time). Therefore, the differences between our data and their data may not be due to changes in particle size. Instead, the GITT differences may 3861


Chemical and Electrochemical Lithiation of LiVOPO4 Cathodes for

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