Synthesis of LiVOPO4 by Emulsion Drying Method for Use as an

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Synthesis of LiVOPO4 by emulsion drying method for use as an anode material for rechargeable lithium batteries Jae-Sang Park, Nurulhuda Binti Mahadi, Hitoshi Yashiro, and Seung-Taek Myung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04586 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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ACS Applied Materials & Interfaces

Synthesis of LiVOPO4 by emulsion drying method for use as an anode material for rechargeable lithium batteries

Jae-Sang Park†, Nurulhuda Binti Mahadi†, Hitoshi Yashiro‡, Seung-Taek Myung†,*

†

Department of Nano Engineering, Sejong University, Seoul 143-747, South Korea

‡

Department of Chemistry and Bioengineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate

020-8551, Japan

Abstract Highly crystalline β-LiVOPO4 was synthesized from a water-in-oil emulsion. At 400 °C in ambient air, removal of the oil phase from the emulsion precipitates resulted in a poorly crystalline intermediate compound. On increasing the temperature to 750 °C under Ar, a single phase was formed. Rietveld refinement of the X-ray diffraction (XRD) data obtained from the product heated at 750 °C indicated that the product has an orthorhombic β-LiVOPO4 olivine structure with no impurities. Although the β-LiVOPO4 had an irreversible capacity in the first cycle, the electrode exhibited stable cyclability for 100 cycles, maintaining approximately 85.5% (573 mAh g-1) of the first charge capacity (670 mAh g-1). In addition, the β-LiVOPO4 electrode had a high capacity even at high rates: 601 mAh g-1 at 1C rate (670 mA g-1) and 373 mAh g-1 at 30C rates (20.1 A g-1). Consolidating the results from XRD, X-ray photoelectron spectroscopy, and time-of-flight secondary mass spectroscopy, we suggest that the electrochemical activity of the β-LiVOPO4 arises from the conversion reaction accompanied by the formation of Li2O and Li3PO4. In addition, the ion-conducting 1

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Li3PO4 contributes to high capacity delivery at high rates up to a C-rate of 30.

Keywords: Lithium vanadyl phosphate; Emulsion ; Anode ; Lithium ; Battery

1. Introduction Olivine, LiFePO4, is one of the most promising cathode materials for lithium ion batteries because of cost effectiveness and thermal stability compared to other cathode materials.1-5 The poor electric conductivity of LiFePO4 can be greatly improved by its coating with carbon,6-10 and the carbon-coated LiFePO4 can be operated even at temperatures below 0 °C.11 Kalaiselvi et al.12 have suggested that LiFePO4 can be used as an anode material, and Liu et al.13 attempted to use LiTi2(PO4)3 as an anode material. Both of these approaches, which used polyanionic systems as the anode materials, are interesting, despite the low capacities of the products. Graphite is the most commonly used anode material for lithium batteries, although its capacity must be further improved for use in high-capacity batteries. For this reason, many studies are being directed towards the use of conversion electrodes composed of metal oxides14-20 and metal fluorides21-25. Because the conversion reactions in these materials lead to the formation of metals via electrochemical reduction, the resulting capacity is exceptionally high. However, conversion electrodes typically suffer from poor cyclability because of the continuous rearrangement of the structure from oxide to metal during cycling. In addition, the formation of insulating Li2O hinders the kinetics of lithiation and increases the internal resistance. A decade ago, Barker et al.26 identified LiVOPO4 as an cathode material. The material has two crystalline phases: α-LiVOPO4 27 and β-LiVOPO4 28-30. The β-LiVOPO4, which show a V4+/5+ redox reaction, had a capacity of approximately 135 mAh g-1, which is higher than 2


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that of α-LiVOPO4 due to its structural similarity to olivine LiFePO4.26 This results in a high operation voltage of 4 V with a reasonable capacity.26 In addition, β-LiVOPO4 has been reported to be an anode material by Ren et al.31 They confirmed electrochemical activity of β-LiVOPO4 with a reversible capacity greater than 380 mAh g-1 although gradual capacity fade was seen during 30 cycles. However, α-LiVOPO4 was also present in their β-LiVOPO4. Thus, it is not clear whether the electrochemical reactions were solely related to the β-LiVOPO4 phase or whether the α-LiVOPO4 impurity also contributed to the anode performance. For these reasons, we attempted to synthesize single-phase β-LiVOPO4 using a water-in-oil type emulsion. Emulsion processing enables the homogeneous distribution of cations at the atomic level.5,32 Here, we report the phase evolution of the material from the initial dried emulsion to the material obtained after high-temperature calcination. The optimized single phase products were electrochemically tested as the anode in Li cells. In addition, we propose possible reactions that occur during charge and discharge.

2. Experimental β-LiVOPO4 was synthesized using a water-in-oil type emulsion. An aqueous solution was prepared by dissolving LiNO3 (Kanto) and NH4H2PO4 (Kanto) in distilled water at a molar ratio of Li:V:P = 1:1:1. To prepare the aqueous vanadium solution, V2O5 (Kanto) was completely dissolved in nitric acid. An emulsifying agent, Tween 85 ( surfactant, Kanto), was mixed with kerosene (Samchun) and stirred vigorously at 4000 rpm for 30 min. The aqueous solutions were then added to the oil mixture at a rate of five drops per second while the mixture was stirred at 1000 rpm by an impeller to produce a water-in-oil type emulsion.5,32 The synthesized emulsion was slowly dropped into hot kerosene (180 °C) to evaporate the water in the emulsions immediately, resulting in precipitation of a homogeneous mixture of 3



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metal ions and the surfactant on the bottom of the reaction vessel. The precipitates were calcined at various temperatures, and the produced powders were analyzed by X-ray diffractometry (XRD, Rigaku D-Max 2000). The collected XRD data were analyzed using the Rietveld refinement program Fullprof.33 The particle morphologies were observed using a scanning electron microscopy (SEM, Hitachi, S-4700). The electrochemical properties were measured in 2032-coin type cells, and electrodes were fabricated from a mixture of the prepared β-LiVOPO4 powders (85 wt.%), carbon black Super P (7.5 wt.%), and polyvinylidene fluoride (7.5 wt.%) in N-methylpyrrolidinone. The obtained slurry was then applied to a copper foil and dried in an oven at 80 °C for 1 h. The electrode was further dried overnight at 80 °C under vacuum. The electrolyte was 1 M LiPF6 in a 3:7 volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The fabricated cells were charged and discharged in a voltage range of 0 to 3 V while a constant current of 100 mA g-1 was applied at 25 °C. To investigate the structural changes during charge and discharge, we carefully disassembled the cells in an Ar-filled glove box, and the electrodes were washed with salt-free DMC overnight. Then, the electrodes were dried at 80 °C overnight in a vacuum oven. Ex situ XRD data were obtained using the rinsed electrodes. X-ray photoelectron spectroscopy (XPS, PHI5600, Perkin-Elmer, USA) measurements were performed in macro mode (3 mm × 3 mm) to obtain information on the chemical states of vanadium elements. The samples were first transferred into a hermetically sealed transfer chamber in the glove box and then transferred into the vacuum chamber of the XPS machine, preventing exposure to air or water for XPS measurement. To identify the presence of byproducts on the surface of the active materials after cycling, we analyzed the cycled electrodes using a time-of-flight secondary ion mass spectroscopy (ToF-SIMS, PHI TRIFT V nanoTOF, ULVAC-PHI, USA) 4


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surface analyzer equipped with a liquid Bi+ ion source and pulsed electron flooding. The measurement was done

at 10-9 Torr. During the analysis, the targets were bombarded by 10

keV Bi+ beams with a pulsed primary ion current varying from 0.3 to 0.5 pA. The total collection time was 100 s over a 300 µm × 300 µm area.

3. Results and discussion As soon as the water-in-oil type emulsion droplets dropped into the hot kerosene (180 °C), the water contained in the emulsion evaporated, leaving precipitates that are composed of the surfactant and metal ions. XRD patterns show the resulting phase evolution of the precipitates with increasing calcination temperature (Figure 1). The average oxidation state of vanadium in β-LiVOPO4 is 4+, and a reducing atmosphere is needed to reduce the initial pentavalent vanadium to a tetravalent state; to achieve this, we calcined the samples in air because combustion of the residual kerosene and surfactant creates an autogenous reducing atmosphere by forming CO and CO2 gases at temperatures below 400 °C. When the precipitates were burned at 400 °C for 30 min in air, the resultant solid was found to be poorly crystalline V2O5, as evidenced by the appearance of a broad amorphous peak between 15° and 35° (2θ) in the XRD pattern. The powders were black due to the presence of residual carbon derived from the combustion of the kerosene and surfactant. Pre-calcination was carried out at 500 °C for 4 h in air to remove any residual carbon (hereafter, we refer to this process as pre-calcination), followed by successive calcination at 550 °C for 2 h in air. As a result of the calcination, V2O5 was obtained as the minor phase with Li2VPO6 as the major phase. In Li2VPO6, the average oxidation state of vanadium is 5+. Because the average oxidation state of vanadium in β-LiVOPO4 is 4+, further heat treatment was carried out under an Ar atmosphere using the pre-calcined product to reduce the vanadium from 5+ to 4+. After 5



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calcination, the resulting XRD pattern indicates the formation of the β-LiVOPO4 phase albeit with a small amount of Li2VPO6 impurity, demonstrating that the reducing atmosphere is effective in reducing the vanadium from 5+ to 4+. Single phase β-LiVOPO4 was observed at 700 °C and was maintained up to 750 °C; however, at 800 °C, a new peak appeared at 13° (2θ), presumably due to the decomposition of the β-LiVOPO4 phase, and this impurity was found to be Li0.6V1.67O3.67 (JCPDS No.. 50-0230). Extending the calcining time at 750 °C did not further improve the crystallinity of the products. To analyze the crystal structure of the product calcined at 750 °C for 2 h in Ar, Rietveld refinement of the XRD data was carried out. During refinement, the structure was constrained to the orthorhombic Pnma space group (Figure 2a and Table. 1), and this choice of space group is supported by the good agreement of the measured and calculated patterns, indicating that β-LiVOPO4 was successfully prepared by the emulsion drying method. Unlike Ren et al.31, we found that the β-LiVOPO4 produced by the emulsion drying method did not contain impurities such as V2O5 and α-LiVOPO4. The structure of β-LiVOPO4, based on the results of Rietveld refinement, consists of VO6 octahedra linked together by PO4 tetrahedra (Figure 2b). The phosphate groups form additional bridges along the same chain, while the other two corners of the PO4 group are shared with the VO6 octahedra of two other chains, yielding a three-dimensional framework. Stacking of the chains generates empty channels in the direction parallel to the c-axis and tunnels along the [101] direction. Lithium ions fill the channels between the PO4 tetrahedra and VO6 octahedra. Furthermore, the calculated lattice parameters agree well with reported values.26,28−31 It is likely that the homogeneous mixing of cations in the emulsion contributes to the formation of single phase β-LiVOPO4. The resulting crystal structure of β-LiVOPO4 determined by Rietveld refinement is shown in Figure 2b. 6


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SEM observation was carried out for the β-LiVOPO4 sample calcined at 750 °C for 2 h under Ar (SI-Figure 1). After heat treatment at 550 °C in air, the β-LiVOPO4 sample did not form a composite with carbonaceous materials, as shown by the color of the sample, which was brown rather than black. The measured average particle size was found at less than 1 µm in diameter, and the particles have polygonal shapes. The as-synthesized LiVOPO4 calcined at 750 °C for 2 h under Ar was chosen for electrochemical test due to its high crystallinity compared to other samples. A constant current density of 100 mA g-1 at 25 °C (Figure 3) was applied during cycling. On discharge (reduction), the β-LiVOPO4 electrode delivered a large capacity of about 942 mAh g-1, exhibiting three distinct voltage plateaus at 1.7, 0.8, and 0.25 V (Figure 3a). The delivered capacity was approximately 670 mAh g-1 on charging (oxidation), showing a large irreversible capacity loss of 272 mAh g-1, i.e. 71% coulombic efficiency. The discharge and charge behaviors were not identical, the hysteresis can be ascribed to an irreversible capacity loss due to the additional conducting carbon materials34, reductive decomposition of the electrolyte35, and difficulty in reforming the original structure36,37. The irreversibility was significantly reduced from the second cycle onward, and the coulombic efficiency reached approximately 99% at the 10th cycle (Figures 3b and c). Although the delivered charge capacity decayed slowly with cycling, the delivered capacity was approximately 573 mAh g-1 at the 100th cycle, retaining 85.5% of the capacity at the first cycle. Both the achieved capacity and the retention are superior to those obtained by Ren et al.31. For instance, their β-LiVOPO4 electrode delivered a charge capacity of about 380 mAh g-1 at the first cycle and about 320 mAh g-1 at the 20th cycle. In addition, the rate capability of β-LiVOPO4 was measured at several currents on charging (Figures 4a and b). Before each charge, the electrode was discharged at a constant current of 100 mA g-1. The delivered capacity at 1C 7



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rate (670 mA g-1) was about 601 mAh g-1, and the electrode has a high charge capacity of about 373 mAh g-1 even at 30C rate (20.1 A g-1), showing approximately 62% of the capacity delivered at 1C rate. After testing at 30C rate, the charge capacity recovered to 652 mAh g-1 at 100 mA g-1. It is likely that the difference between the delivered capacities obtained by Ren et al.31 and those reported here is due to the homogeneity of the β-LiVOPO4 prepared by our method. In the present study, we employed an emulsion drying method to ensure the homogeneous distribution of cations such as Li, V, and P in the LiVOPO4 particles, which were found to be composed of highly crystalline β-LiVOPO4 with no impurities (Figure 2). Therefore, it is most likely that the homogeneity of the as-synthesized β-LiVOPO4 leads to the improved electrochemical performance. To understand the possible reasons for the irreversibility observed during the first cycle, we made ex situ XRD measurements of the electrode during the first discharge and charge (Figure 5a). The Ti current collector was used as an internal standard instead of Cu. At first, Li+ ions are inserted into the β-LiVOPO4 framework: LiVOPO4 + Li+ + e- Li2VOPO4 (reaction 1). For example, the formation of Li2VOPO4 (indicated as open circles in Figure 5b) yielded a minor phase, while β-LiVOPO4 was the major phase at 1.6 V. At 0.95 V, the phase transformation was almost complete, having formed the trigonal Li2VOPO4 phase that results from the insertion of Li+ ions into the β-LiVOPO4 structure followed by the reduction of V4+cations to V3+ cations. Assuming that the theoretical capacity of the β-LiVOPO4 is based on the above-mentioned insertion reaction, the delivered capacity of about 180 mAh g-1 is due to the insertion reaction that occurs in the voltage range of 3 to 0.95 V. As the reduction further progressed (0.3 V), amorphization of the crystal structure occurred until peaks arising from Li2VOPO4 were no longer observed in the XRD pattern. Meanwhile, peaks arising from new phases, such as Li3PO4 and VO, appeared. These observations suggest that, because there are no more available sites to accommodate Li+ ions in the crystal structure of 8


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Li2VOPO4, the lithiated Li2VOPO4 crystalline phase decomposes into Li3PO4 and VO phases; that is, structural reorganization due to the formation of the Li3PO4 phase occurs: Li2VOPO4 + Li+ + e- Li3PO4 + VO (reaction 2). As a result, peaks arising from the Li3PO4 phase appeared, although the peak intensities were low. The formed VO compound, which has low crystallinity, was found to have a simple cubic structure (Fm3m). At the end of discharge, at 0.01 V (Figure 5b), the relative intensities of the VO peaks at 38° and 45° (2θ) had decreased; furthermore, a new peak was observed at 42.5° (2θ), suggesting the formation of V metal following the conversion process; namely, VO + 2Li+ + 2e- Li2O + V (reaction 3). Recently, Ren et al.31 suggested that during discharge, the voltage plateau at 2.2 V results from the insertion of Li+ ions into β-LiVOPO4, although they did not report XRD data to support this. In our measurements, we found that Li+ ions were inserted into the β-LiVOPO4 structure to 0.95 V, as determined from the XRD patterns (Figure 5b). Furthermore, the Rietveld refinement indicates that the impurities, such as α-LiVOPO4, that were observed by Ren et al.31 are not present in our samples. This indicates that the insertion reaction in this voltage region is solely associated with Li+ insertion into the β-LiVOPO4 phase. Therefore, the reaction must progress via decomposition and conversion of VO to vanadium metal, accompanied by the formation of Li3PO4 and Li2O phases as byproducts. These conclusions differ from those of Ren et al.31. At the end of the charge, the resulting phase was amorphous, and no other peaks were found in the XRD pattern (Figures 5b−6). Meanwhile, peaks arising from Li3PO4 were slightly visible in the XRD pattern. Concerning the first irreversible capacity of about 272 mAh g-1 (Figure 3a), the incomplete recovery to the original LiVOPO4 structure can be related to the presence of Li3PO4. In metal oxide conversion reactions, the formation of insulating Li2O, which surrounds the active materials, impedes electron transfer. In contrast, 9



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in the LiVOPO4 system, Li3PO4, which is an ionic conductor (6.3 × 10-8 S cm-1)38, is formed, as observed in the ex situ XRD data (Figure 5). Therefore, the presence of Li3PO4 is responsible for the good reversibility, capacity retention (96% after the 6th cycle, Figure 3c), and high rate performance up to C-rates of 30C (373 mAh g-1, Figure 4) despite the formation of the insulating Li2O phase along with Li3PO4. The reactions mentioned above account for the irreversible reaction on the first discharge. To confirm the ex situ XRD results, we observed the outermost surfaces of the LiVOPO4 electrode during the first cycle using ToF-SIMS (Figures 6a and b) and XPS (Figure 6c). Because of the conversion process on discharge, we focused on two kinds of positive fragments, specifically, Li2O+ (m = 30.03), which arises from Li3PO4 and Li2O, and LiVP+ (m = 88.84), which arises from LiVOPO4, for the ToF-SIMS measurements. Compared to results obtained from the fresh electrode, a drastic increase in the relative intensity of the Li2O+ peak is notable at 0.01 V on discharge (Figure 6a). This result indicates that Li-O related compounds, such as Li2O and Li3PO4, were produced on discharge, and the abrupt increase in the intensity at 0.01 V supports proposed reactions 2 and 3. At the same time, further evidence for Li+ insertion was found in the ToF-SIMs spectrum for the electrode discharged to 0.95 V; in that spectrum, the intensity of the LiVP+ fragment (Figure 6b) was retained compared to the fresh electrode. In contrast, the relative intensity of the LiVP+ fragment was negligible when discharged to 0.01 V, indicating that the parent LiVOPO4 phase is no longer present due to the reorganization of the crystal structure following the conversion reaction. Therefore, the presence of Li2O+ and the absence of LiVP+ fragments demonstrate that the conversion process occurs via reactions 1 to 3 on discharge. Charging to 3 V, the intensity of Li2O+ fragment became lower, while that of the LiVP+ fragment reappeared, indicating the formation of Li-V-P-O network in the matrix despite the low degree of crystallinity. 10


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The main peak of V2O5 arising at the 2p3/2 orbital is found at 517.5 eV.38 Although the oxidation state of vanadium is lower (4+) in LiVOPO4 relative to that in V2O5, the binding energy of the as-synthesized LiVOPO4 was observed to have a higher value, 518 eV, due to the presence of PO4 covalent bonds in the compound (Figure 6c). Such an increase in the binding energy is usually observed when covalent bonds are present, as can be seen in the cases of Li2O (53 eV) and LiF (56 eV).39 In comparison with the as-synthesized LiVOPO4 and the electrode discharged to 0.95 V, there were no changes in the shapes of the spectra, but a shift of the binding energy toward lower energy supports the fact that the Li+ was inserted into the crystal structure of LiVOPO4, forming Li2V3+OPO4. When lithiated to 0.01 V, the resulting broad spectrum appears to be comprised of V2+O (514–516 eV) and vanadium metal (512–514 eV), in agreement with the results of ex situ XRD and ToF-SIMS. On charging to 3 V, the binding energy (517 eV) did not completely return to the original LiV4+OPO4 (518 eV) state, but the value is close to that of Li2V3+PO4 measured at 0.95 V on discharge, which is a result of the irreversible reaction. These data demonstrate that the first discharge is driven by Li+ insertion and conversion reactions and that the first charge is mainly related to the conversion reaction that results in the formation of amorphous Li1+xVOPO4 (x < 1), as summarized schematically in Figure 7. Even after 100 cycles, crystalline β-LiVOPO4 was not observed due to the gradual structural collapse that results from the repeated conversion reactions on cycling. Further experimental changes were made to reduce the irreversible capacity loss during the first cycle to enable utilization of the β-LiVOPO4 conversion electrode (Figure 8). Pre-lithiation is an effective way to reduce the irreversible first discharge capacity, and this method is favored for use in conversion systems.36,37 For this reason, pre-lithiation of the β-LiVOPO4 was conducted by direct contact with Li metal in the presence of the electrolyte for 30 min. The first discharge capacity was approximately 672 mAh g-1 at a current of 11



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100 mA g-1 (Figure 8), which is significantly lower than that of the capacity without pre-lithiation (942 mAh g-1). Under these test conditions, the pre-lithiated electrode showed good capacity retention after extensive cycling, namely, approximately 550 mAh g-1 at the 100th cycle, showing a similar electrode performance as the β-LiVOPO4 electrode (Figure 8).

4. Conclusions β-LiVOPO4 was synthesized for the first time using an emulsion drying method, which was found to be an effective way to prepare a homogeneous, phase-pure material. The optimum conditions were calcination at 750 °C for 2 h in Ar, resulting in the formation of a phase-pure and highly crystalline product. From the ex situ structural investigation, we found that β-LiVOPO4 underwent insertion-conversion reactions during the first discharge. On charging, the converted metal was transformed into Li1+xVOPO4 (x < 1), after which the conversion reaction was responsible for the continuous cycling behavior. Although an irreversible hysteresis was found in the first discharge, the cycling performance was stable upon cycling, showing a capacity of 573 mAh g-1 at the 100th cycle, and retaining 85.5% of capacity after 100 cycles. Furthermore, the electrode delivered a high charge capacity (373 mAh g-1) at 30C rate (20.1 A g-1), assisted by the ion-conducting Li3PO4 phase, which is a byproduct of the conversion reaction. Pre-lithiation of the electrode material was effective in minimizing the irreversible capacity of the first cycle. The high capacity and high rate performances suggest the feasibility of applications of β-LiVOPO4 as a negative electrode material for use in rechargeable lithium ion batteries.

ASSOCIATED CONTENT Supporting information 12


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SEM images of LiVOPO4 calcined at 750 oC for 2h in Ar: low and high magnification. “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * Tel: 82 2 3408 3454. Fax: 82 2 3408 4342. E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements The authors would like to thank Miwa Watanabe, Iwate University, for her assistance in the experimental work. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology of Korea (NRF-2014R1A2A1A11051197) and by the National Research Foundation of Korea funded by the Korean government (MEST) (NRF-2015M3D1A1069713). This work was also supported by the Human Resources Development of the Korean Institute of Energy Technology Evaluation and Planning (KETEP) grant, funded by the Korean government Ministry of Trade, Industry & Energy (No. 20154030200630).

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table. 1 Rietveld refinement data of LiVOPO4 calcined at 750 oC for 2h in air. Formula

LiVOPO4

Crystal system

Orthorhombic

Space group

Pnma 2

Atom

Site

x

y

z

g

Li

4b

0.5000(0)

0

0

1

B/Å 1.2

V

4c

0.3249(5)

0.25

0.7198(4)

1

0.7

P

4c

0.6234(6)

0.25

0.3748(9)

1

0.3

O1

4c

0.4527(16)

0.25

0.4892(14)

1

0.3

O2

8d

0.3706(6)

0.9421(12)

0.7546(9)

1

0.8

O3

4c

0.2939(15)

0.25

0.9988(13)

1

0.4

O3 4c Cell parameters

0.1198(14)

0.25 0.6505(14) a = 7.4569(2) Å

1

1.1

b = 6.2955(2) Å c = 7.1834(2) Å Distance

V-O1 = 1.9108(1) Å V-O2 = 1.9839(1) Å V-O3 = 2.0181(2) Å

Rwp

V-O3 = 1.6082(2) Å V-O3 = 2.3880(2) Å 10.1 %

Rp

8.06 %

19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity / A.U.


Page 20 of 28

o

800 C-2h in Ar o 750 C-2h in Ar o 700 C-2h in Ar o 650 C-2h in Ar o 550 C-6h in Ar o 500 C-4h in Air After Burn out

10

20

30

40

50

60

CuKα 2θ / degree Figure 1. Phase evolution of emulsion-dried Li-V-P-O precursor.

20


70

Page 21 of 28

(a)

Observed Calculated Difference Bragg peak position

201

142

115

124 323

402 232 412

033 040

203 302

131 213 400 230 004 410 303 322

220 221 122 311 013 113

121 202

111 200

101

3000

002

011

102

020

6000

211 112

9000

Intensity / Count

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

20

30

40

50

60

70

CuKα 2θ / degree

(b) (b)

z y

x

Figure 2. (a) Rietveld refinement result of XRD pattern of β-LiVOPO4 calcined at 750 oC for 2 h in Ar (b) and structural model based on the structural data obtained from the Rietveld refinement data (Table 1).

21



3.0

h t h h h t h 0 t t t t h 0 s t 0 0 0 0 1 5 1 3 5 7 1

(a) (b)

Voltage / V

2.5 2.0 1.5 1.0 0.5 0.0 0

300

600

900

0

300

600

900 -1

-1

Capacity / mAh g

Capacity / mAh g

100 800

80

600

60 Discharge capacity Charge capacity

400

Coulombic efficiency

40

200

(c) 0 0

10

20

30

40

50

60

70

80

90

20

0 100

Number of Cycles Figure 3. (a) First discharge and charge curve, (b) continuous discharge and charge curves, and (c) the corresponding cyclability and coulombic efficiency of β-LiVOPO4 calcined at 750 o C for 2 h in Ar.

22


Efficiency / %

-1

1000

Capacity / mAh g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

Page 23 of 28

3.0

(a)

Voltage / V

2.5 -1

2.0

Buffer, 100 mA g -1 0.2C, 134 mA g -1 1C, 670 mA g -1 2C, 1.34 A g -1 3C, 2.01 A g -1 5C, 3.35 A g -1 10C, 6.7 A g -1 20C, 13.4 A g -1 30C, 20.1 A g

1.5 1.0 0.5 0.0 0

100

200

300

400

500

600

700

800

-1

-1

Capacity / mAh g

Charge capacity / mAh g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800

(b)

700 600 500 400 300 200 100

0.2C 1C

2C

3C

5C

10C 20C 30C buffer

0 0

3

6

9

12

15

18

21

24

27

Number of cycles Figure 4. Rate capability of β-LiVOPO4; (a) charge curves at various current densities and (b) the corresponding capacity plots.

23


Fresh

3.0 2.5 2.0 1.5 1.0 0.5 0.0

Page 24 of 28

(a)

3V

1.8 V 1.6 V 0.95 V 0.01 V

0.3 V

0

200

400

600

-1

800

1000

Capacity / mAh g

(b)

Li2O V Li3PO4 VO Li2VOPO4

Ti Ti

CA Ti

Ti After 100 cycles

Intensity / A.U.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Voltage / V


3V 0.01 V 0.3 V

0.95 V 1.6 V

1.8 V Fresh

20

30 40 50 Cu Kα 2θ / degree

60

Figure 5 (a) First charge and discharge curves with black dots which indicate that XRD measurement were done and (b) ex situ XRD patterns of β-LiVOPO4 and after 100 cycles. Instead of Cu current collector, Ti mesh was used for differentiation of V metal formation during conversion, which overlaps with Cu. CA denotes added conducting agent, carbon black super P.

24


Page 25 of 28

(a)

LiVP+ : 88.93

(b)

Fresh

Fresh

Li2O+ : 30.02

0.95V

0.95V

30.02 88.93

0.01V

30.02

0.01V 88.93

30.02

29

3.0V

30

3.0V

88.93

31

88

89

m / z

90

m / z

(c) 517 eV

Intensity / A.U.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.0 V

515 eV 517 eV

0V

518 eV

0.95 V Fresh

526

524

522

520

518

516

514

512

Binding energy / eV Figure 6. ToF-SIMS data of β-LiVOPO4 during the first cycle: (a) Li2O+ and (b) LiVP+ fragments; (c) XPS spectra of β-LiVOPO4 during the first cycle.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Schematic illustration of electrochemical reaction of β-LiVOPO4.

26


Page 26 of 28

Page 27 of 28

+

3.0

Voltage / V vs. Li/Li

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5 1st 5th 10th 30th 50th 70th 100th

2.0 1.5 1.0 0.5 0.0 0

100

200

300

400

500

600

700

-1

Capacity / mAh g

Figure 8. Pre-lithiated LiVOPO4 electrode tested at 100 mA g-1 in the voltage range of 0 – 3 V at 25 oC.

27



Table of Content

-1

1000

Capacity / mAh g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

800 600

Discharge capacity Charge capacity

400 200 0 0

10

20

30

40

50

60

70

Number of Cycles

28


80

90

100

Synthesis of LiVOPO4 by Emulsion Drying Method for Use as an

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