Preparation, Characterization, and Lithium Intercalation Behavior of

Article pubs.acs.org/JPCC

Preparation, Characterization, and Lithium Intercalation Behavior of LiVO3 Cathode Material for Lithium-Ion Batteries Zheng Huang,† Liufei Cao,† Liang Chen,† Yafei Kuang,*,† Haihui Zhou,† Chaopeng Fu,† and Zhongxue Chen*,†,‡ †

College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China School of Power and Mechanical Engineering, Wuhan University, Wuhan, Hubei 430072, China

‡

ABSTRACT: A variety of LiVO3 compounds (LVO-350, LVO-400, LVO-450, and LVO-500) have been synthesized via a resorcinol-formalin assisted sol−gel method. The LiVO3 cathodes calcined at different temperatures are characterized by XRD, SEM, and galvanostatic charge/discharge to investigate the effects of annealing on the structure, morphology, and electrochemical characteristics. The results show that both crystallinity and morphology play important roles in the Li-storage properties of LiVO3; the sample annealed at 450 °C (LVO-450) has high crystallinity, moderated particle size, and thus exhibits the best electrochemical performance among the four electrodes. Furthermore, the Li-ion intercalation/deintercalation mechanism for LiVO3 is investigated by ex situ XRD, TEM, Raman spectroscopy, and XPS. The characterizations on the pristine and cycled LVO-450 reveal that the initial lithium insertion/extraction reaction of LiVO3 based on V5+/V4+ redox couple undergoes a successive structural transition: monoclinic LiVO3 transforms into cubic Li2VO3 upon lithium intercalation, followed by the lithiated Li2VO3 transforms into a more stable phase which has a similar crystalline structure to monoclinic LiV3O8 upon initial lithium deintercalation. Importantly, the observations in this work may provide valuable information for the design of LiVO3 and other high-capacity vanadium-based cathode materials for lithium-ion batteries.

1. INTRODUCTION

method but also the morphology greatly affects the electrochemical properties of LiV3O8.17 In this context, other alternative Li−V−O systems were intensively explored in recent years.18 Li2−xVO3 is one of the new high-capacity cathode materials for LIBs, which was first proposed by Pralong et al. in 2012; a specific capacity of 253 mAh g−1 and a specific energy density of 632 Wh kg−1 were obtained for this cathode.19 They also demonstrated that the lithium insertion/extraction process in Li2‑xVO3 only involves a single V5+/V4+ redox couple, the structure of lithiated phase Li2VO3 is quite stable and can deintercalate up to one lithium cation per formula reversibility;19 this characteristic probably enables Li2VO3 to have good cycling stability. Inspired by their work, Jian and co-workers successively synthesized two LiVO3 samples by combustion and solid-state method, respectively; both samples can deliver a high discharge capacity close to 300 mAh g−1.20,21 Kosova et al. synthesized LiVO3 by mechanochemically assisted solid state method and investigated the effect of annealing temperature on the structure and electrochemistry of LiVO3.22 Recently, our group prepared nanoplatestacked baguette-like LiVO3 by a template method, and the

Vanadium oxides have been regarded as an important class of materials with various attractive physical and chemical properties due to their diverse oxidation states and V−O coordination numbers.1,2 This diversity leads to the formation of a variety of vanadium-based compounds with open-framework layered structure, which can afford sufficient interspace for lithium insertion and extraction.3−5 Therefore, V2O5, LiV3O8, LiV2O5, Li3V2(PO4)3 and many other derivatives were extensively investigated as potential cathode materials for lithium-ion batteries (LIBs).6−11 That research demonstrated that vanadium-based materials may serve as high-capacity cathodes to replace traditional capacity-limited transition metal oxides. Among these vanadium compounds, lithium trivanadium oxide (LiV3O8) was studied mostly due to its structural stability and extremely high specific capacity.12,13 It has been reported that the electrochemical performance of LiV3O8 is largely influenced by the synthetic method and morphology.14,15 Therefore, several strategies were pursued in order to achieve high performance LiV3O8 cathode, including nanoarchitecture building, cation doping, and surface modifications. Although a reversible capacity as high as 388 mAh g−1 was obtained,16 the cycling performance of LiV3O8 is still far from practical applications. Further works showed that not only the synthetic © 2016 American Chemical Society

Received: December 28, 2015 Revised: January 27, 2016 Published: January 28, 2016 3242

DOI: 10.1021/acs.jpcc.5b12666 J. Phys. Chem. C 2016, 120, 3242−3249

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The Journal of Physical Chemistry C

was also carried out with the three-electrode cell at the scan rate of 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was recorded using the impedance measuring unit (IM 6e, Zahner) with oscillation amplitude of 10 mV in the frequency range of 100 kHz to 0.01 Hz.

obtained novel morphological cathode demonstrated high capacity and superior cycling stability.23 However, the abovementioned works only focused on the synthetic optimization or morphology tailoring of LiVO3 electrode; none of them clearly revealed the lithium ion insertion/extraction mechanism and detailed structural transformation of LiVO3 during the charge− discharge process. In this work, we synthesized a variety of LiVO3 compounds by a resorcinol-formalin assisted sol−gel method. To investigate the annealing effect on the cathodes, XRD, SEM, and galvanostatic charge/discharge were used to characterize their physical and electrochemical properties. Further, the lithium intercalation/deintercalation behavior and structural evolutions of LiVO3 during charge and discharge were studied by ex situ XRD, ex situ TEM, Raman spectra, and XPS measurements conducted on the pristine and cycled samples.

3. RESULTS AND DISCUSSION The resorcinol-formalin assisted sol−gel method used in this work involves the synthesis of gel precursors and the pyrolysis of the precursor into the required oxide structure. The TG curve of the gel precursor in Figure 1a shows two main steps of

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The LiVO3 samples were prepared by a resorcinol-formalin assisted sol−gel method. First, 0.01 mol of CH3COOLi (>99%) was dissolved in ethanol at 60 °C. Later, 0.01 mol of NH4VO3 and oxalic acid were added and stirred. After the salts were completely dissolved, 0.16 g of resorcinol and 0.2 mL of formalin were added successively. Then, the solution was stirred vigorously at 80 °C until all the ethanol evaporated. The formed gel was dried at 100 °C in a vacuum oven overnight, then calcined at 350, 400, 450, and 500 °C for 10 h in air, respectively, the as obtained products were named as LVO-350, LVO-400, LVO-450, and LVO-500. 2.2. Morphology and Structural Characterization. Xray diffraction (XRD) of these samples was performed with a Bruker D8 advanced powder X-ray diffractometer with Cu Kα radiation. To obtain XRD patterns of the cycled electrodes, the test cells were disassembled in an argon-filled glovebox, and the electrodes were taken out. The decomposition processes of the polymer precursor during heat treatment were characterized by thermogravimetric (TG) on a model WCT−1A thermobalance (Beijing Optical Instrument Factory) at temperature range of 25−800 °C with a heating rate of 5 °C/min in air. The morphologies of the samples were observed with scanning electron microscopy (SEM, Histachi S-4800) and transmission electron microscopy (TEM, JEM-2100F). Raman spectroscopic analysis was performed with a Labram-010 Raman microspectroscopy (JY, FRA) system utilizing a 514.5 nm incident radiation in the range from 40 to 2000 cm−1. X-ray photoelectron spectroscopy measurements were carried out with an ESCALAB250 XPS spectrometer. 2.3. Electrochemical Measurements. The cathode used in this work was prepared by mixing 70 wt % LiVO3 powders, 20 wt % Super P, and 10 wt % PVDF together and dissolving the electrode mixture into N-methyl-2-pyrrolidone (NMP) to make slurry and then coating the slurry onto Al foil. The electrode was dried at 100 °C overnight in vacuum oven. The charge/discharge measurements were performed in a coin cell, the electrode and lithium foil (counter electrode) was separated by a microporous membrane (Celgard 2400), and the electrode was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethylene methyl carbonate (EMC) (1:1:1 by weight). 2016 type coin cells were assembled in an argon-filled glovebox and galvanostatically charged and discharged within the voltage range of 1.0 to 3.5 V at room temperature. Cyclic voltammetric measurement

Figure 1. (a) TG curve of the gel precursor. (b) XRD patterns of LVO-350, LVO-400, LVO-450, and LVO-500.

mass loss, the initial weight loss of ∼11% from room temperature to 250 °C was caused by removal of physically absorbed water. Subsequently, a significant weight loss of 47.2% between 250 and 380 °C is attributed to the pyrolysis of the gel precursor.24 There is no obvious weight loss in the TG curve when the temperature rises above 400 °C, indicating that the final product LiVO3 has been formed below this temperature. Therefore, the gel precursor is calcined at four selected temperatures (350, 400, 450, and 500 °C) in this work. Figure 1b shows the XRD patterns of the LiVO3 powders pyrolyzed from the gel precursor and calcined at 350, 400, 450, and 500 °C. As can be seen, all the samples show a similar welldefined XRD pattern which can be indexed to monoclinic LiVO3 (JCPDS Card no.: 33-0835, space group: C2/c). The 3243


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Figure 2. SEM images of (a) LVO-350, (b) LVO-400, (c) LVO-450, and (d) LVO-500.

Figure 3. (a) Cyclic voltammograms of LVO-450 between 1.0 and 3.5 V recorded at a sweep rate of 0.1 mV s−1, (b) typical charge/discharge curves of LVO-450 at a current density of 50 mA g−1 (C/6), and (c and d) cycling performances and the corresponding Coulombic efficiencies of LVO350, LVO-400, LVO-450, and LVO-500 at C/6.

reported values.19,22,23 In addition, the XRD peaks of the LiVO3 powders become sharper and stronger as calcination temper-

Rietveld refined cell parameters of a = 10.146 Å, b = 8.415 Å, c = 5.875 Å, and β = 110.45° for LVO-450 are close to previously 3244


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The Journal of Physical Chemistry C ature increases from 350 to 500 °C, suggesting that the material has an improved crystallinity at higher calcination temperature. Obviously, the crystallite size and crystallinity of the samples may affect their Li-storage properties. The SEM images of the LVO3 calcined at different temperatures are shown in Figure 2. It can be seen in Figure 2a that LVO-350 is composed of uniformly distributed nanoparticles with an average size of 100−200 nm. In this work, a resorcinol-formalin assisted sol−gel method was employed to synthesize LiVO3. During the synthesis process, the metallic Li and V ions are tightly bound by the hydrophilic groups in the polymeric chain. This immobilization of metallic ions favors preventing aggregation of the LiVO3 nanoparticles and ensuring uniform distribution.25−27 When the annealing temperature was increased to 400 °C, the particle size of LVO400 grows to 400−500 nm (Figure 2b). As the temperature was further increased to 450 °C, the uniformly distributed nanoparticles tend to aggregate, and thus a sudden increase in particle size was observed for LVO-450 in Figure 2c. The particle agglomeration was more obvious for LVO-500 (Figure 2d); therefore, we can see that the particle size of LVO-450 and LVO-500 both locate in the range of 1−3 μm. The Li-ion insertion-extraction properties of LiVO3 were initially studied by cyclic voltammetry (CV). Figure 3a shows the CV curve of LVO-450 conducted at a scan rate of 0.1 mV s−1 in the voltage range of 1.0 to 3.5 V (vs Li/Li+). During the cathodic scan, six main reduction peaks around 2.88, 2.82, 2.73, 2.55, 2.31, and 1.53 V were clearly observed, corresponding to the lithium intercalation process in LiVO3. Besides, several small peaks also appeared in the curves, suggesting a complex multistep reduction of V5+ during initial discharge process. In the subsequent anodic scan, six oxidation peaks at 2.17, 2.53, 2.71, 2.79, 2.86, and 2.91 V can also be detected. It is worth noting that the reduction peaks in the voltage range of 2.4−1.0 V for the initial cycle are quite different from those for the second and third cycles, indicating that LiVO3 may undergo a structural transition during the first lithium insertion process. These observations are quite consistent with previous reports.22,23 Interestingly, little change of current or potential for the redox peaks is visible in subsequent scans, implying a highly reversible lithium intercalation/deintercalation process and superior structural stability of LiVO3 cathode upon cycling. Figure 3b shows the typical charge/discharge curves of LVO450 at a current density of 50 mA g−1 (C/6). As is seen, the initial discharge curve displays a low voltage plateau at around 1.7 V, implying a biphasic reaction. After the first discharge, the curves for both charge and discharge processes become sloped, indicative of a monophasic mechanism. Those observations are in good agreement with the CV results and previously reported works.19,23 Interestingly, although the discharge capacity of LiVO3 decreases gradually, the average discharge voltage still keeps unchanged, indicating the LiVO3 cathode can maintain good structural stability upon repeated cycling. Figure 3c shows the cycling performance of LVO-350, LVO400, LVO-450, and LVO-500 at a current density of 50 mA g−1. It can be seen that LVO-350 electrode delivered an extremely high initial discharge capacity of 351 mAh g−1, whereas only 28.8% of its initial capacity can be retained after 100 cycles. As a comparison, the LVO-400 electrode exhibited a slightly lower initial capacity of 345 mAh g−1, but with much higher capacity retention of 58.0%. Obviously, LiVO3 sample calcined at 350 °C has a low crystallinity and small particle size, which may facilitate lithium insertion/extraction reaction and thus lead to a

high discharge capacity. However, low crystallinity and small particle size also bring unstable structure and interface, which may accelerate the dissolution of vanadium during cycling, finally resulting in poor cycling performance. In this regard, when the annealing temperature was increased to 450 and 500 °C, the initial discharge capacity for the obtained LVO-450 and LVO-500 electrodes drops to 320 and 300 mAh g−1, the capacity retention after 100 cycles was enhanced to 67.2% and 66.8%, respectively, showing good cycling stability. Further, the Coulombic efficiencies of LVO-450 and LVO-500 electrodes both stayed above 98% in 100 cycles (Figure 3d), which are higher than that of LVO-400 (∼95%) and LVO-350 (∼90%), indicating that the lithium intercalation/deintercalation process is highly reversible for LVO-450 and LVO-500 electrodes. Electrochemical impedance spectroscopy (EIS) was employed to further investigate the cathode behavior of LiVO3 electrode. Figure 4 shows the EIS of the pristine and cycled

Figure 4. Electrochemical impedance spectra of LVO-450 at different cycles.

(1st, 2nd, 10th, 50th, and 100th) LVO-450 electrodes. As can be seen, all the Nyquist plots consisted of two parts. One is the semicircle at high frequency region, corresponding to the charge transfer resistance at the electrolyte−electrode interface, and the other is a sloping line at low frequency, which relates to the lithium diffusion Warburg impedance within the electrode. All EIS data are simulated by using the equivalent circuit shown in the inset of Figure 4. In this equivalent circuit, Rs and Rct represent the ohmic resistance and the charge transfer resistance, respectively, whereas Cdl stands for the doublelayer capacitance and Zw refers to the Warburg impedance. The calculated parameter values from the EIS experimental data are listed in Table 1. As can be seen, compared to the data recorded before cycling, there is a sharp decrease in Rct for the Table 1. Equivalent Circuit Parameters Obtained from Simulation of EIS Experimental Data in Figure 4

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different cycles

Rs (Ω)

Rct (Ω)

fresh 1st cycle 2nd cycle 10th cycle 50th cycle 100th cycle

19.37 13.33 15.03 15.39 13.57 10.38

239.5 46.82 55.85 82.42 118.4 180.0 DOI: 10.1021/acs.jpcc.5b12666 J. Phys. Chem. C 2016, 120, 3242−3249

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The Journal of Physical Chemistry C electrode after the first cycle, implying a typical activation process.23 Subsequently, the value of Rct increases gradually from the first cycle to the 100th cycle, probably because the continuous dissolution of vanadium during cycling may destroy the stability of the electrode/electrolyte interface. This result may account for the continuous capacity fade of LVO-450 upon cycling in Figure 3c. To gain further insight into the lithium intercalation/ deintercalation mechanisms operating for the LiVO3 cathode, ex situ XRD, ex situ TEM, Raman, and XPS analysis were conducted at different depths of charge and discharge (marked in Figure 5a). Figure 5b shows the corresponding ex situ XRD

worth noting that the XRD reflections of the Li1+xVO3 intermediates are much weaker and broader than those of pristine sample, indicating the discharged product has a very small crystal size or exists in a low crystalline state. During the initial charge process, the characteristic peak of the cubic Li2VO3 phase at 15.4° moves toward lower angle gradually. The main XRD reflection of the delithiated product situates at 13.9°, which overlaps closely with the (110) peak of the pristine LiVO3. However, the characteristic main peaks of LiVO3 phase did not reappear, suggesting the cubic Li2VO3 transformed into a more stable phase after the initial cycle. This finding is in good agreement with our previous research,23 but is quite different from Kosova’s work;22 we presumed that this phase may be indexed to a newly formed LiVO3 which has a similar crystalline structure to monoclinic LiV3O8 (JCPDS Card no.: 35-0437). It is interesting to note that a small peak emerged at 11.8° when the cell was discharged to 2.7 V, and this peak existed along the first cycle. After exact match searching in the standard XRD database (ICSD), we found this peak did not belong to any Li−V−O compounds. Manev and his co-workers have demonstrated that the introduction of small amounts of inorganic compounds, such as H2O, CO2 and NH3, into the crystal structure of Li−V−O compounds may enlarge the interlayer spacing, and lead to the shift of lattice parameter a to lower angles.28 In this work, the intercalation of F− and CO32−based small molecules into Li2‑xVO3 during cycling may also account for the presence of the new peak at lower angle. TEM images taken from the pristine and cycled LVO-450 are shown in Figure 6. The high-resolution TEM (HRTEM) image in Figure 6b shows two legible lattice fringes with basal distances of 0.23 and 0.15 nm, corresponding to the (311) and (−204) planes of monoclinic LiVO3 (JCPDS Card no.: 330835). The corresponding selected-area electron diffraction (SAED) pattern (the upper right inset) from the inside to the outside can be indexed to the (311) and (440) planes of monoclinic LiVO3, respectively. Compared to the uncycled sample (Figure 6a), the LVO-450 electrode after the initial cycle can still retain its original morphology without any collapse (Figure 6c). However, the HRTEM image in Figure 6d indicates that the cycled electrode has lower crystallinity due to “electrochemical grinding effect”. In Figure 6d, several sets of lattice fringes with distances of 0.24 and 0.21 nm are still visible but do not belong to any planes of LiVO3. Although we have not solved the exact crystal structure of cycled LiVO3, the above results still further demonstrate that monoclinic LiVO3 will transform into a new phase after cycling. Figure 7 shows the Raman spectra (in the range of 100− 1100 cm−1) of LVO-450 at different depths of initial charge and discharge. It has been demonstrated that LiVO3 has a C2h factor group symmetry of the unit-cell, and 42 internal and 15 external vibration modes exist in the LiVO3 structure with a C2/c space group.29,30 In the Raman spectrum of pristine LiVO3, several peaks are observed in the low frequency region of 120−260 cm−1 which correspond to the translational mode of the VO43− groups.31 It has been proved that these peaks are strongly related with the layered structure,32,33 from the Raman spectra recorded at different lithiation and delithiation states, we can find these peaks always existed, indicating the electrode can maintain its layered structure during charge and discharge. The peaks located in the medium frequency region of 300−500 cm−1 and 500−650 cm−1 are assigned to the bending vibrations of the triply coordinated oxygen (V3−O) bonds and the bridging V−O−V bonds (doubly coordinated oxygen).34,35

Figure 5. (a) Initial galvanostatic discharge−charge curves of LVO450 at 50 mA g−1; the solid circles represent the different cell voltages: discharge to 2.7, 2.4, 1.6, and 1.0 V and charge to 1.7, 2.1, 2.7, and 3.5 V; (b) the corresponding ex situ XRD patterns at different depths of discharge and charge.

patterns of LVO-450. As can be seen, when the cell was discharged to 2.7 V, most of the main peaks for the pristine sample disappeared, whereas, the peak located at 13.9° still remained. The position of this peak gradually shifted to higher angle as the depth of discharge increased, the final lithiated product can be well refined with a Fd3m ̅ cubic structure and assigned to rock salt Li2VO3 with lattice parameter value of a = 8.137 Å. This observation demonstrates that the lithium intercalation in LiVO3 experiences a two-phase reaction. It is 3246


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Figure 6. TEM image, HRTEM image, and SAED pattern (inset) of (a, b) pristine and (c, d) cycled LVO-450 (1st).

Li1+xVO3, the main peaks in this region shift back to lower frequency but are quite different from those for the pristine sample. These observations further demonstrate the irreversible structural rearrangement of monoclinic LiVO3 to cubic Li2VO3 upon the initial discharge process, and the transformation of Li2VO3 to a more stable LiVO3 phase upon the initial charge process. Furthermore, on comparing the Raman features of LiVO3 and lithiated Li1+xVO3, we find that some bands, which have a singlet form in the spectrum of LiVO3, are clearly 2-fold split in the spectrum of Li1+xVO3. This split should be attributed to the nonequivalent character of the ladders in the lattice of Li1+xVO3,31 which induce two kinds of vanadium environments (denoted as peak L and peak H). Interestingly, the intensity ratio IL/IH of these two peaks increases gradually as the x in the Li1+xVO3 increases, we presume that peak L and H are related to the V4+ and V5+ species respectively in the Li−V−O compounds. Obviously, the variation of the V4+/V5+ valence ratio is consistent with the discharge and charge process of Li1+xVO3. The valence changes of vanadium in Li1+xVO3 during discharge and charge are further studied by X-ray photoelectron spectroscopy (XPS). Figure 8 shows the binding energy of V 2p signals in the pristine LiVO3, lithiated phase, and delithiated phase. As is seen in Figure 8a, two peaks emerged at 517.3 and 525.0 eV for the pristine LiVO3 should be assigned to the V 2p3/2 and V 2p1/2 of V5+ ion, respectively.36,37 Meanwhile, the atomic ratio of [V]/[O] calculated from the XPS measurement is approximately 1/3, which agrees well with the stoichiometry of LiVO3. After initial discharge and charge, the shape and symmetry of these two peaks for the V 2p electrons are quite different. The peak at lower energy side for the lithiated phase (Figure 8b) can be fitted with two peaks located at 516.8 and 517.8 eV, which are assigned to the 2p3/2 electrons of V4+ and V5+ ions, respectively;38 likewise, two peaks at 523.8 and 525.0 eV corresponding to the 2p1/2 electrons of V4+ and V5+ ions can be resolved from the peak at higher energy side.39,40 The existence of V5+ ions after the initial discharge may be attributed to high hygroscopicity of lithiated product.22 In addition, both V 2p3/2 and V 2p1/2 peaks of the delithiated phase (Figure 8c) can also resolve into two peaks relating to the

Figure 7. Raman spectra of LVO-450 at different depths of initial discharge and charge.

The high-frequency Raman peaks above 800 cm−1 are attributed to the V−O stretching vibrations.22,31 When the cell was discharged, the Raman spectrum of LiVO3 exhibited several spectroscopic changes: the main peak at 155.9 cm−1 shifted to higher frequency centered at 167.7 cm−1, and the variation is relatively small for the translational mode. In comparison, the change is more obvious for the stretching mode. As can be seen in the enlarged image (right), the main peak at 945.9 cm−1 for the pristine sample gradually shifted to higher frequency along with the increasing depth of discharge. This evolution of Raman spectrum is associated with the steric VO repulsion.31 When lithium ion is intercalated into the crystal structure of LiVO3, the lattice parameters a and V (volume) decrease, as a result, the steric repulsion of VO increases, and accordingly the characteristic Raman peaks shift to higher frequency. When lithium ion is extracted from 3247


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Foundation of China (13JJ4004) and China Postdoctoral Science Foundation funded (2013M542463).

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(1) Liu, J.; Zhang, J.-G.; Yang, Z.; Lemmon, J. P.; Imhoff, C.; Graff, G. L.; Li, L.; Hu, J.; Wang, C.; Xiao, J.; et al. Materials Science and Materials Chemistry for Large Scale Electrochemical Energy Storage: From Transportation to Electrical Grid. Adv. Funct. Mater. 2013, 23, 929−946. (2) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577−3613. (3) Wang, Y.; Takahashi, K.; Lee, K. H.; Cao, G. Z. Nanostructured Vanadium Oxide Electrodes for Enhanced Lithium-Ion Intercalation. Adv. Funct. Mater. 2006, 16, 1133−1144. (4) Liu, J.; Xia, H.; Xue, D.; Lu, L. Double-Shelled Nanocapsules of V2O5-Based Composites as High-Performance Anode and Cathode Materials for Li Ion Batteries. J. Am. Chem. Soc. 2009, 131, 12086− 12087. (5) Gao, X.-P.; Yang, H.-X. Multi-electron Reaction Materials for High Energy Density Batteries. Energy Environ. Sci. 2010, 3, 174−189. (6) Chen, J.-J.; Symes, M. D.; Fan, S.-C.; Zheng, M.-S.; Miras, H. N.; Dong, Q.-F.; Cronin, L. High-Performance Polyoxometalate-Based Cathode Materials for Rechargeable Lithium-Ion Batteries. Adv. Mater. 2015, 27, 4649−4654. (7) Huang, H.; Yin, S. C.; Kerr, T.; Taylor, N.; Nazar, L. F. Nanostructured Composites: A High Capacity, Fast Rate Li3V2(PO4)3/Carbon Cathode for Rechargeable Lithium Batteries. Adv. Mater. 2002, 14, 1525−1528. (8) Semenenko, D. A.; Itkis, D. M.; Pomerantseva, E. A.; Goodilin, E. A.; Kulova, T. L.; Skundin, A. M.; Tretyakov, Y. D. LiV2O5 Nanobelts for High Capacity Lithium-Ion Battery Cathodes. Electrochem. Commun. 2010, 12, 1154−1157. (9) Liu, Q.; Li, Z. F.; Liu, Y.; Zhang, H.; Ren, Y.; Sun, C. J.; Lu, W.; Zhou, Y.; Stanciu, L.; Stach, E. A.; Xie, J. Graphene-Modified Nanostructured Vanadium Pentoxide Hybrids with Extraordinary Electrochemical Performance for Li-Ion Batteries. Nat. Commun. 2015, 6, 6127. (10) Xu, X.; Luo, Y.-Z.; Mai, L.-Q.; Zhao, Y.-L.; An, Q.-Y.; Xu, L.; Hu, F.; Zhang, L.; Zhang, Q.-J. Topotactically Synthesized Ultralong LiV3O8 Nanowire Cathode Materials for High-Rate and Long-Life Rechargeable Lithium Batteries. NPG Asia Mater. 2012, 4, e20. (11) Cao, L.; Chen, L.; Huang, Z.; Kuang, Y.; Zhou, H.; Chen, Z. NaV3O8 Nanoplates as a Lithium-Ion-Battery Cathode with Superior Rate Capability and Cycle Stability. ChemElectroChem 2016, 3, 122− 129. (12) Lee, J. H.; Lee, J. K.; Yoon, W. Y. Electrochemical Analysis of the Effect of Cr Coating the LiV3O8 Cathode in a Lithium Ion Battery with a Lithium Powder Anode. ACS Appl. Mater. Interfaces 2013, 5, 7058−7064. (13) Mo, R.; Du, Y.; Zhang, N.; Rooney, D.; Sun, K. In Situ Synthesis of LiV3O8 Nanorods on Graphene as High Rate-Performance Cathode Materials for Rechargeable Lithium Batteries. Chem. Commun. 2013, 49, 9143−9145. (14) Pan, A.; Liu, J.; Zhang, J.-G.; Cao, G.; Xu, W.; Nie, Z.; Xiao, J.; Choi, D.; Arey, B. W.; Wang, C.; et al. Template Free Synthesis of LiV3O8 Nanorods as a Cathode Material for High-Rate Secondary Lithium Batteries. J. Mater. Chem. 2011, 21, 1153−1161. (15) Pan, A.; Zhang, J.-G.; Cao, G.; Liang, S.; Wang, C.; Nie, Z.; Arey, B. W.; Xu, W.; Liu, D.; Xiao, J.; et al. Nanosheet-Structured LiV3O8 with High Capacity and Excellent Stability for High Energy Lithium Batteries. J. Mater. Chem. 2011, 21, 10077−10084. (16) Shi, Q.; Liu, J.; Hu, R.; Zeng, M.; Dai, M.; Zhu, M. An Amorphous Wrapped Nanorod LiV3O8 Electrode with Enhanced Performance for Lithium Ion Batteries. RSC Adv. 2012, 2, 7273−7278. (17) West, K.; Bachau-C, Z.; Skaarup, S.; Saidi, Y.; Barker, J.; Olsen, I. I.; Pynenburg, R.; Koksbang, R. Comparison of LiV3O8 Cathode Materials Prepared by Different Methods. J. Electrochem. Soc. 1996, 143, 820−825.

Figure 8. XPS V 2p3/2 and V 2p1/2 core peaks of the (a) pristine LVO450, (b) lithiated phase, and (c) delithiated phase.

V4+ and V5+ ions, indicating not all the V4+ ions were oxidized to V5+ ions during the charge process. Nevertheless, the calculated atomic ratio of [V4+]/[V5+] in delithiated phase is much lower than that in lithiated phase, which further demonstrates the reversible lithium insertion/extraction reaction for LiVO3 cathode is based on the V5+/V4+ redox couple.

4. CONCLUSION In summary, we have prepared a variety of LiVO3 compounds by a resorcinol-formalin assisted sol−gel method. The physical and electrochemical characterizations are carried out by XRD, SEM, and galvanostatic charge/discharge to investigate the effects of annealing on the cathode materials. The LVO-350 sample (annealed at 350 °C) with nanoparticle size and low crystallinity exhibits the highest initial discharge capacity of 351 mAh g−1 but is however followed by a poor cycling performance. On the contrary, the LVO-450 sample with high crystallinity and moderate particle size delivers a slightly lower initial capacity of 320 mAh g−1, but with much higher capacity retention of 67.2% after 100 cycles. Ex situ XRD, ex situ TEM, and Raman spectroscopy characterizations on the pristine and cycled LVO-450 reveal that monoclinic LiVO3 transforms into cubic Li2VO3 during the initial discharge process, and subsequently, the lithiated Li2VO3 transforms into a more stable phase which has a similar crystalline structure to monoclinic LiV3O8 upon initial charge process. XPS measurements confirm the lithium insertion/extraction reactions of Li2‑xVO3 are based on the V5+/V4+ redox couple. Importantly, the observations in this work may provide valuable information for the design of LiVO3 and other high-capacity vanadiumbased cathode materials for lithium-ion batteries.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (No. 21303262), Hunan Provincial Natural Science 3248


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