Li3V2(PO4) - American Chemical Society

May 7, 2014 - (EDX) and high-resolution transmission electron microscopy. (HR-TEM ... patterns of LiFePO4 (at x = 0) and Li3V2(PO4)3 (at x = 1) are ...
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Li3V2(PO4)3 Addition to the Olivine Phase: Understanding the Effect in Electrochemical Performance Sudeep Sarkar and Sagar Mitra* Electrochemical Energy Laboratory, Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India S Supporting Information *

ABSTRACT: Several chemical compositions of (1 − x)LiFePO4− xLi3V3(PO4)3 with Li3V2(PO4)3 decorated LiFePO4 morphology are synthesized via modified solid-state synthesis. The current study is undertaken to establish a relation between composite formation and their electrochemistry as an excellent cathode material. A detailed physical and structural investigation revealed the formation of a Li3V2(PO4)3 decorated LiFePO4 composite. To investigate the electrochemical performance of LiFePO4−Li3V2(PO4)3 composites, a series of compositions are prepared with end member of LiFePO4 and Li3V2(PO4)3. A specific composition of 0.97LiFePO4−0.03Li3V3(PO4)3 shows a high reversible capacity of ∼163.8 mAh g−1 at 1 C current rate in the potential window of 2−4.5 V. The present study could provide a great physical insight into the stability of solid electrolyte interface and subsequent decrease in charge transfer resistance of composite materials and exhibits an excellent electrode kinetics and electrochemical stability compared to pristine LiFePO4. Electrode kinetics is studied at 100% depth of discharge at open circuit potential and continuous charge−discharge cycle by the electrochemical impedance spectroscopy technique. It is observed that the improvement of electrochemical performance is mainly controlled by the Li3V2(PO4)3 phase because of its unique three-dimensional open structure. Finally, understanding the physical and electrochemical behavior of the (1 − x)LiFePO4−xLi3V3(PO4)3 composite electrode is studied that could be useful to design new cathode materials further for lithium ion batteries.

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ions in Fe or Li sites is impossible.33,34 But, in practice, vanadium substitution in the Fe site is an attractive pathway to create better electrode material, and this will be discussed in detail in the present work. To improve LiFePO4 electrode kinetics, vanadium incorporation in the Fe site was studied by Omenya et al.,34 Hong et al.,35 Harrison et al.,36 Yang et al.,37 and many others. Zhang et al.38 has revealed that the improvement in the electrochemical properties could be due to expansion of lattice volume and Li−O bond length that allows faster lithium ion movement. Moreover, Omenya et al.,34 Hong et al.,35 and Harrison et al.36 have suggested that only 10% of vanadium could be accommodated in the LiFePO4 host structure and a higher percentage of vanadium leads to the formation of Li3V2(PO4)3 as an impurity phase at high temperature. Interestingly, Zhao et al. has claimed vanadium doping in LiFePO4 is not possible even at 1% substitution level with theoretical and experimental evidence.39 However, recent studies show 20% vanadium can be incorporated using a lowtemperature synthesis method into the olivine structure.36 The observed enhancement in electrochemical activity may be due

INTRODUCTION The lithium iron phosphate based cathode has been a comprehensively studied electrode for lithium ion batteries due to its excellent thermal stability, low cost, and environmental compatibility.1,2 The triphylite LiFePO4 espouses the olivine-type structure, where Li+ and Fe2+ occupy half of the octahedral sites and one-eighth of the tetrahedral sites, respectively. This peculiar distribution of Li+ and Fe2+ within the octahedral sites creates a special impact on both electronic and ionic conductivity. In the LiFePO4 unit cell, FeO6 octahedra share corners between each other and make electronic delocalization more difficult; as a result, the pristine LiFePO4 phase become more of an electronic insulator compared to the oxide counterpart (LiFeO2). Therefore, lithium insertion/deinsertion in LiFePO4 is more difficult at high rates without the use of any conductive carbon coating. Recent developments in electrode fabrication suggest that nanosized LiFePO4 particles allow faster lithium ion diffusion due to a shorter diffusion path, whereas to improve the electronic conductivity, two major approaches have been introduced in various reports including (1) coating of conductive carbon particles or other phases and (2) multivalent cation doping into the Fe site.3−32 Simulation studies show favorable divalent substitution at the Fe site but at the same time show substitution of supervalent © 2014 American Chemical Society

Received: January 12, 2014 Revised: May 3, 2014 Published: May 7, 2014 11512

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was stirred with 400 rpm at 120 °C for 3 h to form a sticky blue color gel. The gel was dried in a vacuum oven to remove excess water at 65 °C. The obtained powder was crushed and heated at 100 °C in air for 1 h to form vanadyl oxalate (VOC2O4· nH2O).44,45 Second, the preparation procedure of Li3V2(PO4)3 is the same as that of LiFePO4, where lithium nitrate, asprepared vanadyl oxalate, and diammonium hydrogen phosphate were ultrasonicated in acetone medium for 1 h. The molar ratio of Li, V, and P was taken as 3:2:3. The asprepared sample was annealed at 800 °C for 6 h in 5% H2−95% N2 atmosphere to get crystalline Li3V2(PO4)3. Formation of Composite (1 − x)LiFePO4−xLi3V2(PO4)3 (Where x = 0.01, 0.03, 0.10, 0.15, and 0.20). An asprepared ultrasonicated sample of Li3V2(PO4)3 was added to LiFePO4 solution with a desired weight ratio. The sample was then dried overnight in an oven at 65 °C and further heat treated at 650 °C for 6 h in 5% H2−95% N2 atmosphere to form a composite of (1 − x)LiFePO4−xLi3V2(PO4)3 with varying x value. The choice of composite formation temperature is based on the fact that the purpose of this study is to form a composite of Li3V2(PO4)3−LiFePO4 that is only possible at high temperature,36 as low-temperature synthesis can accommodate vanadium in the olivine host matrix36 that is deliberately avoided in the present case. Material Characterization. The powder XRD (Philips X’pert X-ray diffractometer) using Cu Kα was employed to identify the phase of the prepared composite material within 15−50° 2θ value with a scan rate of 0.5° min−1. Carbon content was quantified by carbon hydrogen nitrogen analysis (CHN) (Flash EA 1112 series, Thermo Finnigan). XPS data was collected with a Multi Lab Thermo VG scientific spectrometer using a concentric hemispherical analyzer (CHA). The X-ray source was a microfocused monochromatic Al Kα. FT-IR spectroscopy in transmittance mode (MAGNA 550, Nicolet Instrument Co.) was employed to understand the metal− oxygen bond information and their vibrational modes in the composite. FT-IR spectroscopy in ATR mode (MAGNA 550, Nicolet Instrument Co.) was also used to characterize the composite material at 100% depth of discharge (DOD) for various cycles in ex situ mode. For ex situ measurements, electrodes were dissembled from the cell and washed with dimethylformamide (DMF) in an argon-filled glovebox and dried well prior to the FT-IR studies. Field emission gun scanning electron microscopy (FEG-SEM, JSM-7600F, and Carl-Zeiss, Ultra-55) with energy dispersive X-ray spectroscopy (EDX) and high-resolution transmission electron microscopy (HR-TEM, Jeol-2100F) were employed to study the microstructure. Electrochemical Performance. Electrochemical performance was carried out using a Swagelok-type half cell configuration with composite material as the working electrode and lithium foil as the counter/reference electrode separated by borosilicate glass fiber (GF/D, Whatman) as separator soaked in LP-30 electrolyte (Merck). The working electrode comprises 70% active material, 20% carbon (including in situ carbon and ex situ carbon black, C-65, Timcal), and 10% PVDF (Sigma− Aldrich) as binder. Electrode material mentioned above was blended with N-methyl pyrrolidone (NMP) solvent and was cast on aluminum foil. The aluminum foil was then dried overnight at 65 °C, and the electrode was cut onto a circular disc of 10 mm or 12 mm diameter for further use. Charge/ discharge and electrochemical impedance spectroscopy (EIS) experiments with composite materials were performed in a

to an increase in electronic conductivity of the active material by the addition of vanadium.37 Without going to the argument of vanadium doping into the Fe site, one could develop another strategy to enhance the electrochemical performance of LiFePO4 by simply forming composite with Li3V2(PO4)3. The ionic diffusivity of LiFePO4 can be enhanced by the addition of Li3V2(PO4)3 due to faster lithium ion migration in Li3V2(PO4)3 (Dlithium = 10−10 to 10−9 cm2 s−1)40 compared to LiFePO4 (Dlithium = 10−16 to 10−14 cm2 s−1).41 Wang et al. has reported various compositions of a LiFePO4−Li3V2(PO4)3 composite where enhancement in conductivity of LiFePO4 by adding Li3V2(PO4)342 phase was observed. As per their report, the 8LiFePO4−2Li3V2(PO4)3/C composite exhibited first discharge capacity of 151 mAh g−1 at 1 C rate and 129 and 123 mAh g−1 at 3 and 5 C, respectively.42 Further, Zhong et al. demonstrated the electronic conductivity and lithium diffusion coefficient of 10−3 S cm−1 and ∼10−10 cm2 s −1 , respectively, using a composition of 9LiFePO 4 − Li3V2(PO4)3/C prepared by the sol−gel process.43 The present study focuses on the preparation of various Li3V2(PO4)3−LiFePO4 composites and their electrochemical performance. Few literature on Li3V2(PO4)3−LiFePO4 composites is available; however, the reason for the enhancement of electrochemical performance is not well understood by the addition of Li3V2(PO4)3 to LiFePO4. In the present study, LiFePO4 and Li3V2(PO4)3 were separately prepared, and the composite was later synthesized. It is noteworthy to mention here that the present work emphasizes the formation of Li3V2(PO4)3−LiFePO4 composite material and the electrochemical properties along with unfold the physiochemical understanding. Several experimental techniques that include vibration spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), combined with electrochemical methods were performed to understand the phenomena behind the enhanced performance. To determine the electrode−electrolyte interface during cycling, a systematic investigation using electrochemical impedance spectroscopy and ex situ Fourier transform IR (FT-IR) spectroscopy was performed. All experimental evidence indicates the formation of a composite with a faster lithium ion diffusion due to the addition of Li3V2(PO4)3 to the LiFePO4 phase.

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EXPERIMENTAL SECTION Preparation of LiFePO4. LiFePO4 was prepared by a modified solid-state method followed by annealing in a H2−N2 atmosphere at 650 °C. Starting materials for LiFePO4 synthesis are lithium nitrate (LiNO3, 99%, Sigma−Aldrich), iron oxalate dihydrate (FeC2O4·2H2O, 99%, Sigma−Aldrich), and diammonium hydrogen phosphate ((NH4)2HPO4, 99.9%, Merck, India). The above sources of Li, Fe, and P were taken in 1:1:1 molar ratio in a test tube containing acetone (99%, Sigma−Aldrich) and ultrasonicated for 1 h. Sucrose (C12H22O11, 99%, Merck, India) as a carbon source was added in the solution (10 wt %). The ultrasonicated sample was dried overnight in an oven and then annealed at 650 °C for 6 h in 5% H2−95% N2 atmosphere to get crystalline LiFePO4. Preparation of Li3V2(PO4)3. For Li3V2(PO4)3 preparation, a two-step method was adopted. At first, 0.3 molar oxalic acid dihydrate (H2C2O4·2H2O, 99%, Merck India) was dissolved in 50 mL of water, and 0.1 molar vanadium oxide (V2O5, 99%, Sigma−Aldrich) was added in the above solution. The solution 11513

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Figure 1. (a) X-ray diffraction patterns of the (1 − x)LiFePO4−xLi3V2(PO4)3 (x is varied from 0 to 1) composites and (b) zoomed portion of part (a). (Blue and red dots are indicating the peaks of the LiFePO4 and Li3V2(PO4)3 phases, respectively.)

Biologic VMP-3 battery testing unit at 20 °C. The cyclic voltammetry (CV) experiment with the composite was performed in a Biologic VMP-3 at a scan rate of 0.1 mV s−1. Conductivity of the as-prepared sample was tested in a Biologic VMP-3 battery testing unit by applying a frequency of 1 MHz to 1 Hz with current perturbation of 5 mA at 25 ± 2 °C. For the conductivity test, a film of the composite material was prepared by adding 97% active material, 3% PVDF−HFP (Sigma−Aldrich) and few drops of dimethyl phthalate (DMP, Sigma−Aldrich) as plasticizer in acetone. The mixture was stirred at 400 rpm for 2 h, and a viscous slurry of the above composition was cast onto a glass substrate and dried overnight at 65 °C. The overnight dried sample was dissolved in ethanol for 3 h to remove DMP to obtain a uniform free-standing film of the composite material (30 μm thickness), which was further cut into 3.5 cm × 3.0 cm for conductivity measurement.

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Figure 2. FT-IR spectra of (1 − x)LiFePO4−xLi3V2(PO4)3 composite materials.

RESULTS AND DISCUSSION Figure 1a shows XRD patterns of (1 − x)LiFePO4− xLi3V2(PO4)3 composite materials prepared by a modified solid-state technique followed by a sucrose-assisted carbothermal synthesis process. The XRD pattern of pure LiFePO4 (at x = 0) and pure Li3V2(PO4)3 (at x = 1) illustrates the ordered orthorhombic and monoclinic phase belonging to the space groups Pnma and P21/n, respectively. The diffraction patterns of LiFePO4 (at x = 0) and Li3V2(PO4)3 (at x = 1) are in well agreement with JCPDS card nos. 04-012-5179 and 01076-8442, respectively. The compositions between 0 < x < 1 in the (1 − x)LiFePO4−xLi3V2(PO4)3 composites have signature peaks resembling either the LiFePO4 or the Li3V2(PO4)3 phase. For x = 0.01 and 0.03 composition, the signature peaks are due to the Li3V2(PO4)3 phase occurring at a low diffraction angle; however, with a further increase in the Li 3 V 2 (PO 4 ) 3 concentration, the monoclinic phase of Li3V2(PO4)3 became more prominent (Figure 1b). The formation of the composite was prominent at a higher concentration of the Li3V2(PO4)3 phase. In order to investigate the local bonding environment around V and Fe of (1 − x)LiFePO4−xLi3V2(PO4)3 composites, FT-IR spectroscopy was used that is shown in Figure 2. Earlier, a detailed vibrational spectroscopy study on the LiFePO4 phase was discussed by Burba et al.,46 and we have utilized this report for our understanding. The different vibrational motions of metal−anion of the LiFePO4 phase are divided here into two distinct regions for discussion: (i) higher wavenumber region of

900−1150 cm−1, which corresponds to symmetric ν1 and antisymmetric ν3 stretching modes of PO43−; and (ii) lower wavenumber region of 400−650 cm−1 attributed to symmetric ν 2 and antisymmetric ν4 bending modes of the PO 43− group.46−48 Also, in this region (400−650 cm−1), lithium ion cage modes are expected to undergo translational vibration and can be further used to determine the nearest neighbor oxygen atoms of the phosphate group. For x = 0.01 and 0.03 compositions, the changes in the FT-IR pattern were more or less similar to the LiFePO4 olivine structure with minute changes in the lower frequency region. However, with further increase in Li3V2(PO4)3 concentration to LiFePO4 resulted in the change of oxygen environment for the phosphate group, which is prominent for x = 0.10. This change was consistent with further increase of the Li3V2(PO4)3 concentration. However, at the higher wavelength range 900−1150 cm−1, the peaks were broader with an increase in the Li3V2(PO4)3 concentration. The stretching vibrations of the P−O group in this region are originating from PO43− tetrahedral geometry.47,49 The high frequency band at ∼1053 cm−1 was due to phosphate ions that represents the symmetric stretching (P− O) corresponding to the stretching mode of phosphorus nonbridging oxygen and the symmetric and antisymmetric degenerate (O−P−O) bending modes arise from the doublet at 1200 and 1230 cm−1, respectively.47,49,50 The smaller shoulder appears at ∼1200 cm−1 for x = 0.03 composition, which was 11514

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attributed to antisymmetric degenerated of the PO 4 3− stretching mode, and it was observed that the intensity of this particular mode increases with increase in the Li3V2(PO4)3 concentration in LiFePO4. However, at lower concentration (for x = 0.01) of Li3V2(PO4)3, the peak at about 1200 cm−1 was not distinct in nature that may be attributed to the minor presence of the Li3V2(PO4)3 phase. Furthermore, the presence of a triplet at 633, 578, and 507 cm−1 was attributed to the stretching vibration of V−O bonds from the Li3V2(PO4)3 compound.47,49,50 For x = 0, the shoulder ∼493.7 cm−1 and 570.8 cm−1 were assigned to symmetric and antisymmetric bending modes of PO43−, whereas the band ∼642 cm−1 arises from an antisymmetric bending mode of PO43−. At the high frequency region, the broad shoulder ∼948.9 and 1064 cm−1 arise from symmetric and antisymmetric stretching modes of PO43−. The local structure/environment of PO43− changes by increasing concentration of Li3V2(PO4)3, but the FT-IR resolution is not enough to monitor such changes critically for x = 0.01 and 0.03 compositions. However, with further addition of Li3V2(PO4)3 (x = 0.10), the changes in local environment near the low frequency zone is drastic that mainly arises from stretching of V−O bonds of the Li3V2(PO4)3 compound. At x = 0.10, the broad peak at 1053 cm−1 was due to symmetric stretching of the P−O bond corresponding to the Li3V2(PO4)3 compound that again overlapped with the band arising due to the antisymmetric stretching mode of the P−O bonds of olivine−LiFePO4. Also, at the high frequency region, the closely spaced doublet at 1200 and 1230 cm−1 arises from symmetric and antisymmetric degenerate (O−P−O) bending modes of Li3V2(PO4)3 that was quite separated and distinguishable at a higher concentration of Li3V2(PO4)3. The detailed FT-IR results confirm the composite formation of (1 − x)LiFePO4−xLi3V2(PO4)3 that is consistent with the XRD results. Further, XPS (qualitative analysis) was used to confirm the oxidation states and investigate local environments of the elements in the composite. Figure 3 shows the XPS spectra of

each for the LiFePO4 and Li3V2(PO4)3 phases. This peak was assigned to the phosphate ion species arising from the −PO43− ionic species, and any change in the phosphorus environment could be evident from a minor shift in the peak position at the higher binding energy side. Figure S1a (Supporting Information) shows the XPS survey scan of various composite materials. Fe 2p spectra for each sample splits into 2p1/2 and 2p3/2 due to the spin−orbit coupling. Figure S1a (Supporting Information) depicts XPS spectra of Fe 2p consisting of a major peak at ∼710.9 eV and a corresponding satellite peak at ∼724.4 eV for Fe 2p3/2 and Fe 2p1/2, respectively (individual scan for Fe, V, etc are not shown). These peaks are characteristic of the valence state of Fe2+ in LiFePO4. The formation of composite material is again confirmed from the qualitative XPS analysis. The binding energies of V 2p3/2 and V 2p1/2 centered at ∼517 and ∼524 eV are characteristic peaks of V3+ in Li3V2(PO4)3 that is consistent with previous results.51 Figure S1a (Supporting Information) shows the binding energy of the C 1s spectrum at 284.5 eV assigned to amorphous carbon with sp2 character ((C−C) bond). The deconvoluted spectra (Figure 3) of phosphorus into two distinct set of phases show the formation of composite between LiFePO4/Li3V2(PO4)3. Thus, vanadium forms separate species of the Li3V2(PO4)3 phase rather than doped into the Fe site of the LiFePO4 host matrix. Electrochemical Study. For pure LiFePO4, one lithium ion can be intercalated/deintercalated in the ∼3.4 V region due to the Fe2+/3+ redox couple, whereas Li3V2(PO4)3 can accommodate three lithium ions by reversible electrochemistry in the potential range between 3 and 4.8 V.34−38,52−55 Li3V2(PO4)3 electrochemistry is based on partial conversion of the V3+/4+ redox couple at ∼3.6 V/3.7 V, whereas complete conversion of V3+ to V4+ occurs at ∼4.1 V that leads to removal of two extra lithium ions from the lattice.34−38,52−55 Further removal of lithium ions from the host matrix is possible but at higher potential, ∼4.8 V region; however, this leads to a structural instability in the Li3V2(PO4)3 material. Thus, a strategy to achieve better electrochemical stability has been adopted where two lithium ions were removed from the Li3V2(PO4)3 matrix initially by charging to a peak potential of ∼4.5 V. Figure 4a−g illustrates the voltage profiles of (1 − x)LiFePO4−xLi3V2(PO4)3 composite materials (where x = 0, 0.01, 0.03, 0.10, 0.15, 0.20, and 1), with charge/discharge at 1 C current rate (1 h to remove complete lithium from the host matrix) in a potential window between 2.0 and 4.5 V. Figure 4a−f shows the unique feature of LiFePO4, a plateau at the ∼3.4 V region due to the Fe2+/3+ redox couple whereas the observed peaks (in the case of Figure 4d−g) at the ∼3.6−4.1 V region is due to the V3+/4+ redox couple that corresponds to the Li3V2(PO4)3 material. The initial discharge capacity of LiFePO4 (x = 0) was 138.1 mAh g−1, and with further increase in the Li3V2(PO4)3 component, the initial capacity of the composite material increases. Therefore, the effect of composite formation on electrochemical performance was evident from x = 0.03 (0.97LiFePO4−0.03Li3V2(PO4)3). At x = 0.03, the initial discharge capacity was found to be 163.8 mAh g−1 at 1 C current rate. As evident from the charge/discharge profile (Figure 4b, c), for x = 0.01 and 0.03, no plateau due to the V3+/4+ redox couple at 3.6, 3.7, and 4.1 V was observed that may be due to charge/discharge at the high current rate (1 C) process. However, the presence of the same plateau was observed for the V3+/4+ redox couple in charge/discharge at 0.1 C current rate (Figure 5a), and CV (Figure 5b−h) further

Figure 3. XPS of phosphorus (deconvoluted peaks) of composite materials with varying composition (qualitative analysis).

the P 2p component. All spectra of P 2p revealed a single peak centered ∼133 eV that corresponds to full width at halfmaximum (fwhm) of 2 eV, resulting in a split of P 2p into two components 2p3/2 and 2p1/2 due to spin−orbit coupling at ∼133.4 and ∼134.3 eV, respectively (for LiFePO4). However, with the incorporation of Li3V2(PO4)3, the peak of P was shifted toward higher binding energy, and for better understanding, the peaks are deconvoluted into two components 11515

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Figure 4. (a−g) Galvanostatic first charge/discharge profile of (1 − x)LiFePO4−xLi3V2(PO4)3 (where x = 0, 0.01, 0.03, 0.10, 0.15, 0.20, and 1.0) measured at 1 C at 20 ± 2 °C (active material used in the range 1.55−1.70 mg cm−2).

Li3V2(PO4)3 phase, that is, x = 1.0, there are three anodic plateaus at ∼3.6, 3.7, and 4.1 V that are due to partial oxidation of V3+ to V4+ and complete oxidation of V3+ to V4+, respectively. Similarly, three reversible cathodic peaks arise at ∼3.55, 3.64, and 4.03 V against Li/Li+ that leads to insertion/deinsertion of two lithium ions in the matrix. The insertion of two lithium ions in the host matrix delivers an initial discharge capacity of 133.7 mAh g−1 for pure Li3V2(PO4)3. The three plateaus appear in the charge/discharge curves of Li3V2(PO4)3 at ∼3.6, 3.7, and 4.1 V corresponding to the phase transformation of Li3V2(PO4)3 to LixV2(PO4)3 (where x = 2.5, 2, and 1). Figure S1b (Supporting Information) reflects the capacity contribution from LiFePO4 and Li3V2(PO4)3 in the composite material. It is well depicted in Figure S1b (Supporting Information) that the highest initial discharge capacity was observed at x = 0.03 and with further increase of the Li3V2(PO4)3 content the capacity decreases. However, at x = 0.03, there was no well-defined plateau for Li3V2(PO4)3, so actual contribution of Li2V3(PO4)3 was unknown (marked as “?” in Figure S1b, Supporting Information) that may be resulted due to the high rate charge/discharge experiment. Similarly, Figure S1c (Supporting Information) shows the discharge capacity of the composite material as a function of cycle number that shows excellent electrochemical stability up to 100 cycles. The cyclic stability and capacity retention in this composite material could be attributed to the formation of the LiFePO4 composite with a three-dimensional open structure Li3V2(PO4)3, which allows faster lithium kinetics and acts as a good additive for the LiFePO4 phase.

confirms the presence of the Li3V2(PO4)3 phase along with LiFePO4. CV profiles of all the samples are shown in Figure 5b−h at scan rate of 0.1 mV s−1. Figure 5b shows the CV profile of pristine LiFePO4 where one anodic peak and one cathodic peak ∼3.5 and ∼3.35 V appears that corresponds to phase transformation between LiFePO4/FePO4. With addition of Li3V2(PO4)3 to the LiFePO4 (x = 0.01) phase a small peak at ∼4.1 V (inset of Figure 5c) was observed that is attributed to the V3+/4+ redox couple. However, the redox peaks of V3+/4+ at ∼3.6 and ∼3.7 V overlapped with the Fe2+/3+ redox couple of LiFePO4 at x = 0.01 and 0.03. With a further increase in the concentration of Li3V2(PO4)3, in LiFePO4 (x = 0.10), peaks at ∼3.6 and 3.7 V are well resolved that corresponds to the V3+/4+ redox couple. The peak at ∼4.1 V in anodic scan was distinctive in the entire composite sample, while in cathodic scan, the redox couple for V3+/4+ was observed at ∼4.0, 3.64, and 3.55 V. It is noteworthy to mention here that, with further increase of the Li3V2(PO4)3 content in the composite, a decrease in the initial discharge capacity was observed beyond x = 0.03. The first discharge capacities at x = 0.10, 0.15, and 0.20 compositions were 156.2, 154, and 143.8 mAh g−1 at 1 C rate, respectively. The observed trend in discharge capacity with the increase in the Li3V2(PO4)3 content in the composite was due to a decrease in the capacity contribution from LiFePO4 (higher theoretical storage capacity ∼169.8 mAh g−1). Thus, the highest capacity is achieved at x = 0.03 in (1 − x)LiFePO4− xLi3V2(PO4)3 at 1 C rate. At x = 0.10, 0.15, and 0.20 compositions, the V3+/4+ redox behavior was acute for the Li3V2(PO4)3 phase, and the results are in good agreement with the experimentally observed XRD and FT-IR data. For the pure 11516

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Figure 5. (a) Charge/discharge of (1 − x)LiFePO4−xLi3V2(PO4)3 composites (where x = 0.01, and 0.03) at 0.1 C current rate (inset: zoomed portion between shows plateau due to the V3+/4+ redox couple) and (b−h) CV profiles of (1 − x)LiFePO4−xLi3V2(PO4)3 composites (where x = 0, 0.01, 0.03, 0.10, 0.15, 0.20, and 1) at 0.1 mV s−1 scan rate in potential window between 2.0 and 4.5 V (active material used in the range 1.55−1.69 mg cm−2).

a nonuniform composite formation in 0.97LiFePO 4 − 0.03Li3V2(PO4)3 that could be due to the use of a trace amount of Li3V2(PO4)3. The diffraction planes were indexed according to JCPDS card nos. 04-012-5179 and 01-076-8442 for LiFePO4 and Li3V2(PO4)3, respectively. The SAED pattern in Figure 6d represents diffraction from LiFePO4 (d(301) = 2.76 Å (LiFePO4), d(311) = 2.51 Å (LiFePO4), d(321) = 2.03 Å (LiFePO4), and d(331) = 1.62 Å (LiFePO4)), and no peaks could be assigned to Li3V2(PO4)3 indicating nonuniform composite formation. However, detailed and through analysis of the 0.97LiFePO4−0.03Li3V2(PO4)3 sample shows the formation of composite with the presence of Li3V2(PO4)3 in minor quantity. The SAED pattern taken at the other region shows the signature of Li3V2(PO4)3 in the 0.97LiFePO4−0.03Li3V2(PO4)3 composite. The d(hkl) and Miller indexes were assigned for both

Morphological Study. Morphological characterizations of the composites were performed using FEG-SEM and HR-TEM instruments. Figure 6a−f shows FEG-SEM and high magnified images of 0.97LiFePO4−0.03Li3V2(PO4)3 composite material. The pebble-shaped particle growth with 30−100 nm diameter was observed from Figure 6b. Similarly, HR-TEM shown in Figure 6c, e shows the particle growth in pebble shape that was also confirmed by FEG-SEM. The HR-TEM imaging technique was further used to detect the thickness of carbon coating and a uniform carbon coating was observed in the 0.97LiFePO4− 0.03Li3V2(PO4)3 composite material as shown in Figure S1d (Supporting Information). The detailed micrograph study was performed to confirm the (1 − x)LiFePO4−xLi3V2(PO4)3 composite formation. Selected area electron diffraction (SAED) patterns were taken at different positions and show 11517

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Figure 6. (a, b) FEG-SEM, (c) HR-TEM (site 1), (d) SAED pattern for the position in part (c), (e) HR-TEM (site 2), and (f) SAED pattern for the position in part (e) of the 0.97LiFePO4−0.03Li3V2(PO4)3 composite.

LiFePO4 and Li3V2(PO4)3 (d(101) = 4.24 Å (LiFePO4), d(011) = 3.67 Å (LiFePO4), d(002) = 4.29 Å (Li3V2(PO4)3), and d(012) = 4.02 Å (Li3V2(PO4)3)) (Figure 6f). To validate the formation of (1 − x)LiFePO4−xLi3V2(PO4)3 composites, a detailed micrograph study on composition at x = 0.20 was performed and is illustrated in Figure 7a−d. The HR-TEM images (Figure 7b−d) reveal the formation of Li3V2(PO4)3 decorated around the LiFePO4 surface. However, some scattered LiFePO4 particles were observed occasionally around Li3V2(PO4)3. The HR-TEM images show the lattice fringes of LiFePO4 in the center and Li3V2(PO4)3 in its outer surface that further confirmed the composite formation. The d(hkl) value at spots 1, 2, and 3 are assigned and shown in Figure 7b−d. Figure 7e shows a schematic of a possible growth of (1 − x)LiFePO4− xLi3V2(PO4)3 composite formation for higher composition of Li3V2(PO4)3. The morphology of various compositions in (1 − x)LiFePO4−xLi3V2(PO4)3 is shown in Figure S2 (Supporting

Information). The HR-TEM micrograph illustrates the formation of Li3V2(PO4)3 decorated LiFePO4 morphology for a higher content of Li3V2(PO4)3. For composition with a trace amount of Li3V2(PO4)3, no such morphology was observed. We can conclude from the present micrograph studies that the Li3V2(PO4)3 decorated LiFePO4 morphology in the composite is prominent with an increase in the Li3V2(PO4)3 content. The schematic of Li3V2(PO4)3 decorated LiFePO4 morphology is shown in Figure 8. At first, Li3V2(PO4)3 was added to the ultrasonicated LiFePO4 sample, where Li3V2(PO4)3 forms the outer layer around LiFePO4 and ceases further growth of the LiFePO4 particle thus forming Li3V2(PO4)3 decorated LiFePO4 morphology. To investigate further the morphology of LiFePO4− Li3V2(PO4)3, EDX line scan was performed with specific sample composition for x = 0.20, where the formation of Li3V2(PO4)3 decorated LiFePO4 particles was more prominent. 11518

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Figure 7. (a) HR-TEM, lattice pattern (b) for spot 1, (c) spot 2, and (d) spot 3 for the composition 0.80LiFePO4−0.20Li3V2(PO4)3 composite material, and (e) schematics of cross-sectional view of composite formation.

Figure 8. Schematic of (1 − x)LiFePO4−xLi3V2(PO4)3 composite formation.

Figure 9a−g shows EDX analysis along with the threedimensional schematic of possible growth of Li3V2(PO4)3 decorated LiFePO 4 for the sample 0.80LiFePO 4 − 0.20Li3V2(PO4)3. The summary of composition analysis of (1 − x)LiFePO4−xLi3V2(PO4)3 compositions performed by EDX analysis is given in Table S1 (Supporting Information). The carbon content of the sample was determined by CHN analysis. Table S2 (Supporting Information) shows the percentage of carbon in each composite sample. After a detailed inspection of the spectrum at x = 0.20 for Fe and V in particular, it can be concluded that the presence of V from Li3V2(PO4)3 was uniform at the surface of LiFePO4 and Fe from LiFePO4 at the core of the particle with some LiFePO4 particles scattered. However, as compared to Fe and V; the spectrum arises due to P and O (Figure 9b, c) being uniform because of contribution from both the species (LiFePO4 and Li3V2(PO4)3). Similarly, at a different section of the sample, elemental mapping was performed for x = 0.20 composition (Figure S3, Supporting Information). Here, at this point, we can conclude that, at a higher percentage of Li3V2(PO4)3 (starting at x = 0.10), the formation of such morphology between LiFePO4 and Li3V2(PO4)3 was observed. Electrochemical Impedance Spectroscopy Study. Figure 10a, b and Figure S4a−f (Supporting Information) show the EIS spectra of various compositions of the composite material. The EIS technique was used to study the kinetic

behavior in the composite material and to compare with the end member of the series. EIS was performed by applying voltage perturbation of 5 mV between the frequency range of 1 MHz to 0.1 mHz during the charge/discharge process at 1 C current rate. The Nyquist plot (Figure 10a and Figure S4a−f, Supporting Information) consists of depressed and overlapped semicircles at high and intermediate frequency range separated by different time constants and an inclined straight line at constant phase (∼45°) at the lower frequency region. In general, the high frequency semicircle is attributed to the resistance of surface films covered on the composite material in the electrolyte solution, whereas the medium to low frequency semicircle is attributed to charge transfer resistance, and a straight line at lower frequency is attributed to Warburg impedance associated with a diffusion limited process. From Figure 10a, it is evident that, with the increase of cycle number, the radius of the overlapped semicircle increases from OCV to cycle number 3 and that may be due to the formation of a solid−electrolyte interface (SEI) layer that increases during the charge/discharge process. However, at cycle number 10, there was substantial decrease in the radius of the high frequency semicircle that is attributed to the stability of the SEI layer as is evident from Figure S5a (Supporting Information). As seen from Figure S5a, b (Supporting Information), with the increase in the cycle number, Rsf and Rct for x = 0.03 decrease and get stabilized around the 10th cycle. At x = 0, from the 11519

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Figure 9. (a) FEG-SEM and EDX line scan, (b) three-dimensional schematic of Li3V2(PO4)3 decorated LiFePO4 morphology, EDX spectrum for (c) Fe, (d) V, (e) P, (f) O, and (g) C, and (h) high-resolution FEG-SEM for the composition 0.80LiFePO4−0.20Li3V2(PO4)3 composite material.

SEI was evident from Figure S5a (Supporting Information) that leads to superior electrochemical performance of the Li3V2(PO4)3 cathode and retains 93% of initial capacity for 100 cycles at 1 C current rate (Figure 4g and Figure S1c, Supporting Information). Hence, it can be concluded from this section that minor presence of Li3V2(PO4)3 in pristine LiFePO4 helps in decreasing the charge transfer resistance and to enhance the electrochemical performance of the LiFePO4 as a cathode material. Figure 10c, d shows and compares the Bode plots of the cell at various cycle numbers for the composition at x = 0.03. Figure 10c, d shows the relation between the frequency-real part of impedance (Z′) and the frequency-phase angle, respectively, which represents the general behavior of an equivalent circuit (Figure 11). It is indicated that the impedance between the frequencies 103 and 101 Hz remained almost constant for every cycle, while those at frequencies below 101 Hz start increasing slowly. The phase plot is more sensitive where the border peak

Nyquist plot of LiFePO4 (Figure S5a, Supporting Information), it is evident that the radius of the intermediate frequency semicircle increases with the increase in cycle number, which indicates an increase in charge transfer resistance, whereas resistance due to the SEI gets stabilized (Figure S5a, Supporting Information). As we know, the increase in Rct (“raise”) is not related to capacity fading but associated with power fade “rate decrease”.56 As the LiFePO4 sample delivered an initial capacity of 138.1 mAh g−1 and stabilized for 100 cycles without “capacity loss”, the “raise” in Rct is attributed to the fact that, at 1 C current rate, it fails to deliver full theoretical capacity (169.8 mAh g−1). However, with the introduction of Li3V2(PO4)3, the decrease in the semicircle radius indicates the formation of a stable SEI layer. This phenomenon is clearly evident from Figure S5a, b (Supporting Information) for the composition x = 1, that is, pristine Li3V2(PO4)3, where resistance due to surface film formation and charge transfer remain stable from cycle number 10 to 20. The stability of the 11520

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Figure 10. (a) EIS of the 0.97LiFePO4−0.03Li3V2(PO4)3 composite during various stages of cycling at OCV and various DOD (2 V), (b) enlarged portion of part (a) (dotted point: experimental curve; continuous line: fitted curve), and (c, d) Bode plot for the composition 0.97LiFePO4− 0.03Li3V2(PO4)3.

Figure 11. Equivalent circuit model to represent the electrochemical half cell.

and Rct are electrolyte resistance, surface film resistance, and resistance due to charge transfer, respectively. Apparent Diffusion Coefficient Estimation. The Warburg impedance in the low frequency region in the Nyquist plot is associated with a diffusion process of lithium ions of an electrode material. The diffusion coefficient of lithium ions (Dlithium) can be calculated using the following relation (eq 1) at low frequency range:60−62

semicircular sections in the complex plane plot indicates two phenomenon occurring at two different time scales and overlapping with each other where Ohmic and capacitive reactions are comparable. The lower frequency region (0.1 Hz) resembles Warburg nature that is also conclusive from the Nyquist plot. The EIS experimental data was fitted with a Voigt-type analogue (Frumkin−Melik−Gaykazyan impedance) nonlinear series fit (inductance for OCV curve was neglected).57−59 Here, we use the similar circuit analysis to fit the experimental data; the Voigt-type components were a combination of series and parallel RC circuits that represents the migration of lithium ions through surface film, charge transfer, and solid-state diffusion (Warburg element). It is noteworthy to mention that constant phase element (CPE) was incorporated instead of pure capacitance (Csf, Cdl) where Re, Rsf,

D = R2T 2/(2n2A2 F 4c 2σ 2)

(1)

where c is the concentration of lithium ions in electrode material (mol cm−3) and σ is the Warburg factor (Ω s1/2), which can be estimated from the slope of the plot between Zre 1 versus ω− /2 at the lower frequency region. The calculated apparent diffusion coefficient value was calculated at OCV and 100% DOD for various cycle numbers 11521

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Figure 12. Ex situ FT-IR (ATR) spectra of (a) x = 0.03, (b) x = 0, and (c) x = 1.0 for (1 − x)LiFePO4−xLi3V2(PO4)3 composite material and (d) rate test of (1 − x)LiFePO4−xLi3V2(PO4)3 cathode material in the lithium half cell configuration at 20 ± 2 °C (active material used in the range 1.91−2.1 mg cm−2, first 51 cycles at 2 C charge/5 C discharge and followed by reversed the current rates).

conductivities of LiFePO4 with introduction of Li2V3(PO4)3, EIS measurements were carried out to determine the conductivity of the composite material. The equivalent circuit of mixed conductors developed by Jamnik65 was demonstrated with simplified parallel combination of electronic resistance and ionic resistance that is in series with a capacitor. According to the EIS analysis (Figure S5d, Supporting Information), intersection of the high frequency line with the real axis is the electronic and ionic resistance in parallel circuit and intersection of lower frequency line with real axis is the electronic resistance.65,66 The electronic and ionic conductivities of LiFePO4 are ∼7.93 × 10−7 and 1.08 × 10−5 S cm−1 (Table S3, Supporting Information), respectively. For Li3V2(PO4)3, the electronic and ionic conductivities were 3.494 × 10−5 and 1.85 × 10−5 S cm−1, respectively, which is quite low compared to the reported values.49,50 The variation of electronic conductivity may also be attributed to binder content in the free-standing film. Thus, it can be concluded from this section that the apparent conductivity of the composite was improved with the introduction of the Li3V2(PO4)3 phase in LiFePO4 material. To demonstrate the SEI layer formation and its stabilization, FT-IR study with the electrode material was employed for 0.97LiFePO4−0.03Li3V2(PO4)3, LiFePO4, and Li3V2(PO4)3 samples. FT-IR spectroscopy for 0.97LiFePO 4 − 0.03Li3V2(PO4)3, LiFePO4, and Li3V2(PO4)3 samples was performed in ATR mode that is shown in Figure 12a−c, respectively. The electrode from the cell was removed at the end of various discharge processes in an argon-filled glovebox

(Figure S5c, Supporting Information). It is evident from Figure S5c (Supporting Information) that, with an increase in Li2V3(PO4)3 content, the lithium ion diffusion coefficient value increases from ∼10−14 cm2 s−1 for LiFePO4 (x = 0) to ∼7.3 × 10−12 cm2 s−1 for Li2V3(PO4)3 (x = 1) and follows an increasing pattern from LiFePO4 to Li3V2(PO4)3. The value of the diffusion coefficient for pure olivine LiFePO4 is consistent with previous reports,41 but the value of the diffusion coefficient of Li2V3(PO4)3 differs with the reported value.63,64 The diffusion coefficient (Dlithium) value for Li2V3(PO4)3 as reported by Qiao et al. was ∼2.7 × 10−8 cm2 s−1,63 Huang et al. ∼10−9 cm2 s−1,64 and Du et al. ∼10−13 cm2 s−1.50 The variation in diffusion coefficient value arises mainly due to the fact that the experiments were performed at various onset potential of single/double phase regions. In this report, EIS experiments were performed at OCV and 3.0 V region that is the onset potential of a two phase region, so the estimated Dlithium value for the Li2V3(PO4)3 phase is appropriate to be designated as apparent rather than true value. However, for the pure LiFePO4 phase and composite materials, the experiments were performed at OCV and 2.0 V where a single-phase region exists which provides a true lithium ion diffusion coefficient value. Thus, it is evident from kinetic study that, with minor addition of Li2V3(PO4)3 to LiFePO4, lithium ion transport can be increased that results in a better rate capability for the composite material. Further, with the introduction of the Li2V3(PO4)3 phase in LiFePO4, there is possibility of variation in the intrinsic electronic and ionic conductivity of the parent material. To investigate the variation of electronic and ionic 11522

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and washed with DMF and acetone to remove organic solvents from the electrolyte. Further, the washed electrode was dried overnight in an oven at 65 °C. In the case of electrode material aged for 24 h in electrolyte, the band arises in the region 900− 1150 cm−1 corresponding to symmetric ν1 and antisymmetric ν3 stretching of PO43− bands, whereas the vibrational band arising at ∼400−650 cm−1 was attributed to symmetric ν2 and antisymmetric ν4 bending modes of PO43−.46−48 Due to aging of electrode material for 24 h, there was slight formation of a SEI layer due to Li2CO3 at ∼836 and ∼1479 cm−1; however, no other peak due to the SEI layer was observed67−71 in the instrument resolution limit. The composition sample was analyzed at various stages of discharge cycle using ex situ FT-IR spectroscopy that shows the formation of a SEI layer in the material. The peak at 700 cm−1 belongs to P−F bonds of LixPFy that was formed due to interaction between Li-containing surfaces with LiPF6 solutions.67−69 The peaks appeared around 1645 cm−1, and 1080 cm−1 is attributed to the formation of ROCO2Li.69−71 The peak due to ROCO2Li (∼1645 cm−1) remains constant during various cycles of the discharge process.67−69 However, the peak intensity at ∼1080 cm−1 changes with cycle number, which enhances until the 3rd cycle and subsequently decreases after the 10th to 20th cycle. Similar phenomenon was observed in the case of the Li2CO3 peak where the peak at ∼ 1479 cm−1 remains constant whereas the peak at ∼836 cm−1 developed and subsequently decreased and remained constant upon further cycling, which indicates the stability of the SEI layer. In the case of LiFePO4 and Li3V2(PO4)3 electrodes, additional peaks at ∼1729, 1332, and 1155 cm−1 were attributed to polycarbonates,70,71 and the peak at ∼1400 cm−1 may be attributed to absorption of PVDF.70,71 The stability of the SEI plays a pivoting role in the electrochemical stability of the cathode material as observed from Figure 4 and Figure S1c (Supporting Information). Further, growth of the SEI on electrode material is well illustrated by FEG-SEM images as shown in Figure S6a−d (Supporting Information). Rate Performance. The rate test for all compositions was tested at high current rates such as 2 C charge and 5 C discharge rate for the first 51 cycles, and then, the current rate was reversed for the charge/discharge process (Figure 12d). The charge and discharge capacity of 0.97LiFePO 4 − 0.03Li3V2(PO4)3 was found to be maximum compared to all other compositions. The discharge capacity of 0.97LiFePO4− 0.03Li3V2(PO4)3 at 2 C and 5 C current rates was ∼150 mAh g−1, while with reversing the current rate, the charge and discharge capacities were found to be 130 and 150 mAh g−1, respectively. Figure 12d shows excellent capacity retention of the (1 − x)LiFePO4−xLi3V2(PO4)3 cathode material even at high rate charge/discharge simultaneously. The composite at x = 0.03 retains its capacity throughout 150 cycles with ∼100% Coulombic efficiency until the 78th cycle and 98% until the 150th cycle. In last, the superior performance of the presently studied cathode material is attributed to the addition of threedimensional open structure Li3V2(PO4)3 to LiFePO4, and with trace addition of Li3V2(PO4)3, kinetics and electrochemical performance have been enhanced significantly.

examined by XRD study, FT-IR spectroscopy, and XPS to validate the morphology, crystal structure, and local ionic environment. All characterization techniques proved the formation of the composite. Present results indicate the existence of oxidation plateaus at 3.6 , 3.7, and 4.1 V versus Li/Li+ of the V3+/4+ redox couple attributed to Li3V2(PO4)3 after complete removal of two lithium ions from the host matrix, whereas the oxidation plateau at ∼3.5 V corresponds to the Fe2+/3+ redox couple. It is noteworthy to mention here that the superior electrochemical performance of the (1 − x)LiFePO4−xLi3V2(PO4)3 composite material was a coupled phenomenon of unique morphology (Li3V2(PO4)3 was decorated on the surface of LiFePO4) that helps in stabilization of the SEI layer and decrease in Rct. The decrease in the radius of the high frequency semicircle is prominent with the addition of Li3V2(PO4)3. Similar observation was made in case of pristine LiFePO4 where the radius due to the high frequency semicircle decreases however the radius of the semicircle due to Rct increases that leads to unfavorable charge−transfer kinetics. The results also suggested that, due to the three-dimensional open structure of Li3V2(PO4)3, lithium ion kinetics is faster compared to other recently studied cathode materials. The best electrochemical performance of the composite cathode material was demonstrated at x = 0.03, with a first discharge capacity of 163.8 mAh g−1 at 1 C current rate. To demonstrate the robustness of the composite cathode material, a rate capability test was performed. At x = 0.03, the material delivered excellent capacity of about ∼150 mAh g−1 and Coulombic efficiency of 98−100% throughout cycling test under 2 C/5 C current rates. In brief, the present study not only demonstrates one of the best electrochemical results but also evolves as a strategy to understand the detailed mechanism that is responsible for superior electrochemical performance of composite materials. Thus, this strategy can help us to improve the electrochemical performance of less electronically conductive cathodes such as Li2FeSiO4, LiFeBO3, and so forth in the future and enrich the fundamental understanding of composite cathode materials.

CONCLUSIONS Li3V2(PO4)3 decorated LiFePO4/carbon composite was prepared by modified solid-state reaction and demonstrated as a high capacity and high voltage insertion-based cathode material. Further, composite of (1 − x)LiFePO4−xLi3V2(PO4)3 was

Author Contributions

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ASSOCIATED CONTENT

S Supporting Information *

XPS survey scan composite, contribution of capacity by LiFePO4 and Li3V2(PO4), discharge capacity of composite at 1 C current rate, and carbon coating for the sample 0.97LiFePO4−0.03Li3V2(PO4)3; HR-TEM of various composites; FEG-SEM and elemental mapping for the composition 0.80LiFePO4−0.20Li3V2(PO4)3; EIS of various compositions; variation of Rsf, Rct with cycle number, diffusion coefficient variation with cycle number, and conductive impedance of the composite; FEG-SEM image of electrode material of the 0.97 LiFePO4−0.03Li3V2(PO4)3/C composite; EDX analysis data; CHN analysis data; and electronic and ionic conductivity of the composite. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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*Phone: + 91-222576-7849; e-mail: [email protected]. The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest. 11523

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ACKNOWLEDGMENTS The authors are indebted to SAIF and Physics department, IIT Bombay, for their assistance with FT-IR, HR-TEM, FEG-SEM, and XPS analysis and Alok Mani Tripathi for HR-TEM experiment and analysis. This work has been supported by National Centre for Photovoltaic Research and Education (NCPRE) - Ministry of New and Renewable Energy, Govt. of India.

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