Operando Structural and Electrochemical Investigation of Li1.5V3O8

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Operando Structural and Electrochemical Investigation of Li1.5V3O8 Nanorods in Li-ion Batteries Partheeban Thamodaran, Thangaian Kesavan, Murugan Vivekanantha, Baskar Senthilkumar, Prabeer Barpanda, and Manickam Sasidharan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01915 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018

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ACS Applied Energy Materials

Operando Structural and Electrochemical Investigation of Li1.5V3O8 Nanorods in Li-ion Batteries Partheeban Thamodaran,† Thangaian Kesavan,† Murugan Vivekanantha,†# Baskar Senthilkumar,‡ Prabeer Barpanda,‡ and Manickam Sasidharan† †SRM

Research Institute and Department of Chemistry, SRM Institute of Science and Technology, Chennai 603203, Tamil Nadu, India. #Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Chennai 603203, Tamil Nadu, India. ‡Faraday Materials Laboratory, Materials Research Center, Indian Institute of Science, Bangalore 560012, India.

ABSTRACT We report facile solvothermal synthesis of submicron sized rod-like Li1.5V3O8 crystals using ethylene glycol/ water as reacting media. The crystal structure and morphology of resulting compound were characterized by Rietveld refinement, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Xray photoelectron spectroscopy (XPS), and thermal analysis (TG/DTA). Rietveld analysis confirms monoclinic Li1.5V3O8 crystals with P21/m symmetry having monodispersed 5 µm long and 500 nm thick rod-like morphology. As cathode in Li-ion batteries (LIBs), Li1.5V3O8 nanorods deliver a reversible discharge capacity of ~239 mAh.g1 in the voltage window of 2.04.0 V (vs. Li/Li+) at 0.1 C rate after 50 cycles. Li1.5V3O8 nanorods retain an impressive discharge capacity of ~161 mAh.g1 after 250 cycles at 1C rate. Operando (in-situ) XRD investigation of Li1.5V3O8 during electrochemical (dis)charging confirms the phase transformations from a Lipoor -phase (Li1) via Lirich -phase (Li2.5) to a phase (Li4). Lowtemperature performance evaluation of Li1.5V3O8 cathode exhibits less than 50% of discharge capacity achieved at 25 °C. Evaluation of dis(charge) behavior over different temperatures suggest that charge transfer resistance (Rct) 1

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plays a crucial role in determining Liion diffusivity vis-à-vis specific capacity at lowtemperature. KEYWORDS: Li-ion battery, Cathode, Li1.5V3O8, In-situ XRD, Low-temperature performance. 1. INTRODUCTION Significant effort has been made on the development of cathode materials for secondary lithium-ion batteries (LIBs) to meet the demands of diverse range of applications from microchips to electric and hybrid electric vehicles (EV/HEV).13 Although LIBs have received great attention in both fundamental and applied research fields, performance enhancement is somewhat bottlenecked by limitation in cathode materials. In spite of tremendous growth of traditional cathode materials for LIBs (such as LiCoO2, LiNiO2, LiMn2O4), intensive efforts are still underway to further advance energy and power density. Since the energy density of a battery is determined by its operating voltage and specific capacity, much attention has been paid to develop high voltage inexpensive cathode materials. In this context, several Nirich layered oxides, Lirich layered oxides, highvoltage spinels, and polyanionic compounds have been investigated.26 Though cathode materials such as LiFePO4 (olivine phase), LiNi1/3Co1/3Mn1/3O2, LiNi0.5Co0.2Mn0.3O2, and LiNi0.8Co0.15Al0.05O2 have attained commercial success,710 they are still plagued with limited energy density and cycle life impeding their largescale applications.11,12 Thus, the design of appropriate electrode material is of prime importance to realize superior energy density to outperform other available cathode materials. Nanomaterials play an important role in electrochemical energy storage and conversion technologies, especially, lithiumion batteries (LIBs) to improve the energy density and cycle 2


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life13-15. Of late, vanadium based oxides and phosphates have garnered wide attention as cathode candidates for LIBs owing to their high specific capacity, ease of material processing, and affordability.16,17 In particular, V2O5, VO2, and LiV3O8 have been studied as cathode materials in LIBs delivering high specific capacity compared to the wellknown LiFePO4 and LiCoO2.1821 Advantageously, the vanadium based oxides show high chemical stability and superior safety characteristics without releasing oxygen from the lattice unlike cobalt or nickel oxides due to positioning of V4+/5+ and V3+/4+ redox couples well above the 2p band of O2. Among various vanadium oxides, the lithium trivanadate (Li1+XV3O8, x = 0.10.5) has gained much attention on account of its crystal structure.22 It has a lamellar monoclinic structure with P21/m space group where (V3O8)(1+x) units run along the bc direction in the layered structure and stacked one above the other along the aaxis. The trivanadate unit structurally consists of octahedral VO6 and distorted trigonal bipyramidal VO5 units interconnected by cornersharing oxygen atoms to form VO layered structures consisting of interstitial octahedral (Oh) and tetrahedral (Td) sites for Li ion occupation in between the layers. Different from other layered materials where the weak van der Waals forces provide structural stability, in Li1+XV3O8, immobile Li+ located in the octahedral (Oh) sites provide requite binding to hold the layers together. The excess Li corresponding to the amount x occupies the tetrahedral sites (Td) between the layers and participates in the (de)intercalation processes.23,24 Despite its structural advantages, electrochemical properties such as reversible capacity and cycling behavior often rely on synthesis conditions. Till date, several methods such as solidstate reaction,25,26 hydrothermal,27 microwave synthesis,28 spray pyrolysis,29 solgel method,3032 and rheological phase reaction33 approaches have been employed to prepare Li1+xV3O8 with improved electrochemical properties 3



such as reversible capacity, cycle life, and rate capability. However, majority of research efforts resulted in Li1+xV3O8 with x = 0.10.2, where the amount of Li ions situated at the Td sites available for Li (de)intercalation would be low. Recently, Zhu et al.34 have reported a lowtemperature synthesis of lithiumrich Li1.5V3O8 nanosheets which exhibited good capacity and cycling stability. The high charge/discharge capacities and cycle life stability are ascribed to the nanosheet morphology with more active sites and shortened Li+ ion diffusion path length. While these prior studies reveal general trend of electrochemical oxidation/reduction of LiV3O8, detailed analysis at various de(lithiation) states is not much explored. With most studies of LiV3O8 focusing on improving the specific capacity by refining the synthesis method, the focus on phase transformation and related structural stability are not well documented. For instance, insight into the structural stability during electrochemical cycling and reaction mechanism in LiV3O8 during (de)lithiation processes can shed light on the practical applicability. In this context, the structural evolution of LiV3O8 has been investigated by exsitu as well as insitu XRD to predict the potential at which either single phase or twophase transition occurs ( phase transition).3537 Furthermore, temperature dependence electrochemical behavior of LiV3O8 is another critical parameter worthwhile to be investigated. However, so far most of electrochemical studies were performed at ambient room temperature but its performances either at hightemperature or subzero is scarcely explored. Recently, the effect of temperature on (dis)charge properties of LiFePO4 were reported and reveal temperature dependence Li+ ion transportation kinetics, and structural/phase change.38,39 Herein, we report a simple onepot solvothermal method using ethylene glycol/water solvent system in presence of polyvinyl pyrolidine (PVP) additive to synthesize Li1.5V3O8 with rod like morphology. Its physical and 4


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electrochemical properties were studied synergizing suites of analytical techniques. Structural and phase transformation properties of the Li1.5V3O8 were analyzed using insitu XRD during (dis)charge processes. Electrochemical (de)intercalation studies at different temperature were also performed between 5 ºC and 55 ºC to get insights on capacity behavior, phase transformation, and electrode structural stability.

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. In a typical synthesis of Li1.5V3O8 nanorods, ammonium metavanadate (3 mmol) and 0.1g of polyvinyl pyrolidine (PVP) were dissolved in a mixture of deionized water and ethylene glycol (1:2) solvents followed by stirring for 20 min (Scheme 1). When the entire solution turned to pale yellow, it was transferred into a Teflonlined stainless autoclave and was heated at 180 °C for 20 h. After solvothermal reaction, the resultant black solid product was recovered from the vessel, centrifuged, and washed thoroughly with deionized water followed by ethanol washing, and finally air dried in an oven at 80 °C for 12 h. Lithiation of the resultant black solid product was accomplished by solidstate mixing of stoichiometric amounts of lithium hydroxide (LiOH.xH2O) followed by calcination at 600 °C for 6 h under ambient condition. 2.2. Materials Characterization. The phase purity of assynthesized Li1.5V3O8 was confirmed by powder Xray diffraction studies (XRD) using Bruker (D8 Advance, Da Vinci) analytical instruments with Cu kα radiation source (1.54 Å). The measurement was recorded at 2θ  1080° at a scan speed of 3⁰/min with step size of 0.04. Rietveld refinement was conducted using GSAS program with the EXPGUI graphical interface. Surface morphology was observed 5



through fieldemission scanning electron microscopy (FESEM) at 10 kV using ZEISS electron microscopy. TEM micrographs were obtained using highresolution transmission electron microscopy (HRTEM) using JEOL JEM2010 instrument operated at 200 kV. Xray photoelectron spectroscopy (XPS) was recorded to identify the chemical states of the Li1.5V3O8 using 1032 instrument with Al-alpha as the Xray source. For insitu XRD, data were collected continuously during (dis)charging studies of Li1.5V3O8 at C/20 current rate. Temperaturedependent electrochemical study was conducted using humidity chamber (ESPEC, SH222, Korea). 2.3. Cell Fabrication and Electrochemical Measurements. Electrochemical performance of Li1.5V3O8 was evaluated by constructing CR2032 coin-type halfcells assembled inside an argon filled MBraun glove box maintained at oxygen and moisture contents less than 0.5 ppm. The working electrode was prepared by thoroughly mixing the powders of active material Li1.5V3O8 and conductive super P (SP) carbon in the weight ratio of 80:20 for about 30 min. Later on, the slurry was prepared by adding 0.1 mL of the freshly prepared carbomethoxycellulose 3 % (CMC) solution to the thoroughly mixed active material. The obtained slurry was uniformly coated on an Alfoil using doctor blade technique and then dried in vacuum at 120 ℃ for 12 h. The dried working electrode was cut into circular disks and was used as working electrode along with glass microfiber (Whatman 47 mm) as separator, and pure Li metal foil as reference electrode. Then, few drops of the electrolyte solution were added to the coin cell assembly and then sealed inside the glove box. The electrolyte solution was prepared by dissolving lithium hexaflurophosphate (LiPF6) in 1:1 mixture of ethylene carbonate and diethylene carbonate (EC:DEC) solvents. Electrochemical characterization techniques like cyclic voltammetry and galvanostatic charge/discharge tests were performed using Biologic instrument 6


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BCS 810 at various current densities within the potential range 2.04.0 V (vs Li/Li+). Cyclic voltammetry study was carried at 0.2 mV/s scan rate and galvanostatic impedance measurements were recorded with frequency range from 100 kHz to 10 mHz at 10 mA amplitude. 3. RESULTS AND DISCUSSION Materials Characterization. The LiV3O8 phase formation was initially evaluated using TG/DTA by mixing LiOH.xH2O (5% excess) with solvothermally synthesized black powder (Figure S1, Supporting Information). The first weight loss peak observed below 100 °C is ascribed to the physisorbed water molecules (8% loss), while the 5% loss between 100 and 240 °C corresponds to decomposition of adsorbed organics. The straight-line feature of TG curve after 240 °C indicates the formation of crystalline Li1.5V3O8 and the observed growth pattern in the present study is in good agreement with previous report.40 The phase formation and crystalline nature of Li3V6O18 (Li1.5V3O8) were further verified by Rietveld analysis (Figure 1). For clarity only main hkl planes are indexed; the reflections at 2θ value of 13.88, 23.27, 27.60, 28.32, 30.7, 40.80, and 50.58 correspond to (001), (300), (202), (111), (301) (103), and (020) planes, respectively, that can be readily indexed to layered monoclinic crystal structure of space group P21/m (JCPDS 86-2421). The observed sharp peaks indicate high crystallinity of monoclinic Li1.5V3O8. The lattice parameter values were calculated to be a = 12.006(x) Å, b = 3.6003(x) Å, c = 6.666(x) Å, α = 90.00, β = 107.72(x)°, γ = 90.00, unit cell volume = 274.498(x) Å3. The structural parameters values are given in supporting information Table S1. The Li1.5V3O8 crystal structure essentially consists of V3O8 layers formed by connecting VO6 octahedra and VO5 trigonal pyramidal units through corner sharing of oxygen atom (Figure 1) which are stacked along the aaxis comprising (Oh) and (Td) sites. In a typical Li1+XV3O8 7



structure, unlike immobile octahedral Li+ sites (1 Li+), the excess Li+ at tetrahedral sites gains much importance due to its participation in (de)intercalation processes. Therefore, it is envisioned that Li1.5V3O8 with better lattice parameter values would be expected to show good electrochemical performance. In order to further probe surface chemistry and structural features of Li1.5V3O8, FTIR spectrum was recorded (Figure S2, Supporting Information). The absorption band at 1005 cm1 is attributed to V=O stretching frequency, while the bands at 814 and 686 cm1 represent VOV stretching vibrations. The absorption peaks at 510 cm1 could be ascribed to either VO or VO bending vibrations according to literature report.41 FESEM and HRTEM (Figure 2ad) techniques were employed to get further insight on the morphological and textural details of Li1.5V3O8. Lowmagnification image (Figure 2a) displays monodispersed rodlike Li1.5V3O8 crystals with average length 5m with thickness of about 500 nm (Figure 2b). To gain information on morphological evolution of Li1.5V3O8, control synthesis experiments in the absence of the PVP have been performed but resulted in rather irregular structures with mixture of rods and sheets structures with appreciable stacking faults as shown in Figure 2c,d. From the observed results, it is envisioned that NH functionality of PVP polymer backbone effectively mediates the precursor concentration during nucleation and growth events to obtain uniformly distributed nanorods of Li1.5V3O8. Such morphology control was also realized using polyethylene glycol (PEG) as capping agent to produce uniform sheet morphology of LiV3O8.42 Morphological evaluation with HRTEM (Figure 3) further confirms the rodlike Li1.5V3O8 crystals with size feature of about 500 nm thickness and 5 m length (Figure 3a,b). Moreover, highresolution TEM image (Figure 3c) exhibit interfringe spacing value of 3.14 8


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nm corresponding to a crystal plane of (111) plane which is in good agreement with dspacing value calculated through the Xray diffraction studies. The selected area electron diffraction (SAED) of Li1.5V3O8 (Figure 3d) explicitly shows diffraction spots corresponding to the monoclinic structure with high crystallinity, further corroborating the Xray diffraction results. The X-ray photoelectron spectroscopy (XPS) experiments have been performed to identify the chemical environments of elements present in the Li1.5V3O8 nanorods. The XPS survey spectrum of Li1.5V3O8 shows (Figure 4a) distinct signals for elements such as Li (1s), V (2p) and O (2s). The Figure 4b corresponding to V (2p) is further deconvoluted into four peaks; two major peaks at 517 and 522 eV are ascribed to V5+ and two minor peaks appearing at 419 and 424 eV are attributed to V+4 oxidation states. Furthermore, XPS peak intensity ratio suggests existence of less V+4 compared to V+5. The appearance of O (2s) peak (Figure 4c) at 530.1 eV clearly confirms the presence oxygen in the Li1.5V3O8 material.43 Electrochemical studies We investigated the electrochemical properties of Li1.5V3O8 as cathode material in lithiumion batteries using cyclic voltammetry (CV), galvanostatic charge/discharges, and electrochemical impedance spectroscopy (EIS) techniques. To investigate Li+ (de)intercalation mechanism, CV traces of Li1.5V3O8 electrode were measured at a scan rate of 0.2 mV/s within the potential window of 2.04.0 V (vs. Li/Li+) (Figure 5a). The cathodic and anodic peaks represent the Li+ ion insertion and extraction processes, respectively. During the cathodic sweep, three prominent reduction peaks at 2.78 V, 2.45 V and 3.40 V (weak) are observed. The peaks at 2.78 (broad) and 3.40 V are due to singlephase reaction with lithium ions occupying empty tetrahedral sites.44,45 While the peak at 2.45 V is ascribed to lithium occupancy in octahedral sites formed upon intercalation during which a twophase transition from Li3V3O8 to Li4V3O8 9



occurs.45 The minor peak at 2.2 V in the cathodic sweep is related to the slower insertion kinetic step where the singlephase transition corresponding to the Li4V3O8 phase takes place. On the other hand, two major oxidation peaks at 2.6 and 2.95 V and a minor peak at 3.71 V were evident in the reverse anodic sweep due to Li+ deintercalation from the (Td) and (Oh)sites of host Li1.5V3O8 structure. Interestingly, with increasing cycle numbers, the redox couples overlap with each other at the respective potentials with negligible changes in the current produced. Thus, high reversibility and facile Li+ diffusivity in the host Li1.5V3O8 upon (de)intercalation is evident from the observed anodic and cathodic peaks. The galvanostatic chargedischarge behavior of Li1.5V3O8 nanorods are recorded over a potential range of 2.04.0 V (vs Li/Li+) at a rate of 0.1C (Figure 5b). The welldefined plateaus located at about 2.78, 2.45, and 2.20 V in the discharge traces are correlated to the singlephase Li+ insertion process, the twophase transformation from Li3V3O8 to Li4V3O8, and the slower kinetic Li+ insertion process, respectively.46 The prominent plateau features observed in (dis)charge profiles corroborate well with CV results of Li1.5V3O8. Reversible discharge capacity of 239 mAh.g1 (against theoretical capacity of 275.5 mAh. g1) was achieved after 50 cycles at 0.1 C rate in the potential window of 2.04.0 V (Figure 5c) and the overall electrochemical reaction involves a two-phase transformation from Li1.5V3O8 to Li4V3O8. The first cycle Coulombic efficiency was found to be 87% mainly due to commencement of initial discharge at OCV potential of 3V. However, the Coulombic efficiency was found to be nearly 100% after first cycle indicating high stability of the electrode. The charge/discharge capacity of Li1.5V3O8 electrode shows an increasing trend in the specific capacity from 239 to 244 mAh.g1 till 8th cycles which can be ascribed to slow activation process within the micron-sized electrode 10


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materials. The excellent Coulombic efficiency and the minimal capacity loss in the initial cycles reveal that the irreversible electrolyte decomposition and other parasitic reactions are minimal, which justifies the use of nanorod Li1.5V3O8 material as cathode. Encouraged by the enhanced specific capacity and cycling performance, the Li1.5V3O8 electrode was studied for power capability by subjecting to different current densities of 0.05 C, 0.1 C, 0.5C, 1 C, 3 C, and 5 C rates (Figure 5d). The corresponding dischargecharge profiles of electrodes at various current densities are also shown in Figure 5e. At lower current rate of 0.05 C, the electrode exhibited stable reversible capacity of 271 mAh.g1. Upon increasing discharge/charge rates from 0.1 C to 0.5C, 1C, 3C and 5C, reversible discharge capacities are maintained, respectively at 234, 190, 163, 110 and 83 mAh.g1. Similar to other electrode materials, Li1.5V3O8 electrode also furnish low specific capacity at high current rate owing to faster diffusion kinetics of Li+ ions but the electrode regains its original capacity of 189, 233, 259 mAh.g1 on lowering the current rate back to 0.5 C, 0.1C and 0.05C respectively. The stability of the Li1.5V3O8 electrodes has also been studied by subjecting to prolonged cycles as shown in Figure 5g. At a current rate of 1 C, the Li1.5V3O8 constructed electrode delivered a discharge capacity of 161 mAh.g1 with > 99% Coulombic efficiency till 250 charge/discharge cycles indicating high electrode stability. Over 400 cycles, the electrode exhibits a slight capacity fading of about 0.02 % loss per cycle and finally delivered 103 mAh.g1. The decrease in Coulombic efficiency of 0.02 % is almost negligible over the initial 250 charge/ discharge cycles. The enhanced charge/discharge performance and stability of nanorod Li1.5V3O8 electrode are attributed to onedimensional shape which facilitates better Li+ diffusion/electron conduction via facile percolation of the electrolyte solution providing stable electrode/electrolyte interface. 11



To get more insights into the capacity fading after 250 cycles, the electrochemical stability of the materials was investigated by correlating the charge/discharge cycles at different intervals with electrochemical impedance spectra (EIS). The EIS measurements were performed in the frequency range of 100 kHz to 10 mHz for fresh electrode, as well as at the end of 50th, 100th, 200th, and 400th cycles at 1C rate (Figure 5f). The EIS spectra display compressed semicircle at highfrequency region and a Warburg line feature at lowfrequency region ascribed charge transfer resistance (Rct) at electrodeelectrolyte interface and Li+ ion diffusion in Warburg region (Zw), respectively. There is no much increase in charge transfer resistance (Rct) till 200 cycles but further cycling the electrode up to 400 cycles show an increase in Rct owing to dissolution of vanadium vis-à-vis decrease in the electrode conductivity as reflected from the capacity fading. However, the low frequency Warburg line mostly inclined at 45° suggested an improved Li+ diffusion behavior and similar Warburg line features were observed at different charge/discharge cycles. In-situ XRD studies Insitu XRD was employed to get more insight into different phase transformation behavior of Li1.5V3O8 nanorods. The cumulative insitu diffraction data (Figure 6) suggests that Li1.5V3O8 belongs to the Lipoor alpha phase (α), Li2.5V3O8 belongs to Lirich alpha phase (α), and Li3.8V3O8 belongs to Lirich beta phase (β). These specific lithiated states of XRD patterns are shown in Figure S3. From the obtained XRD patterns, the peak shift observed is attributed to the phase transformation from Lipoor αphase to Lirich rock salt βphase during discharging and the reverse behavior while charging. Further these phase transformations can be explained as follow. 12


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Stage I: L1.5V3O8 to Li2.5V3O8 at discharging; the expanded view of diffractograms at different region of Li1.5V3O8 nanorods are displayed in Figure 6b, where (001) peak at 13.88°, (202) peak at 27.58°, and (002) peak at 28.0° were shifted towards higher angle; whereas (111) reflection at 28.3° moved to lower angle during the discharging. These results indicate a shrinkage of interlayers along the ‘a’ axis, while expand along the bc axis owing to electrostatic attraction between Li+ ions and V3O8– layers owing to accommodation of more lithium in the interlayers during discharge. During lithiation from Li1.5 to Li2.5, the (001) peak at 13.88° moves toward 14.31° and (202) peak at 27.58° to 28.58°, accompanied by (111) reflection at 28.3° shifting to lower angle 27.82°. Furthermore, the (202) and (111) peaks underwent cross over initially, which simultaneously shifts back to their respective diffraction angles and such cross over features were also observed by Zhang et al.34 for Li1.1V3O8. These XRD peak shift features are attributed to the phase transformation from Lipoor αphase to the Lirich αphase (Li1.5V3O8 to Li2.5V3O8). Stage II: L2.5V3O8 to Li3.6V3O8 at discharging; In stage II, when discharge potential reaches 2.6 V, the (001) peak intensity decreases with emergence of new peak at 14.95° with concomitant intensity reduction of (202), and (111) reflections. Meanwhile, new peak appears at 27.05° corresponding to the βphase in the Li2.5, as shown in Figure 6b. On further discharge up to 2.0 V, intensities of (001), (202) and (111) disappear with gradual appearance of reflections corresponding to βphase. This confirms monoclinic layered Lirich αphase (Li2.5V3O8) to disordered rock salt Lirich βphase (Li3.8V3O8) transformation. Stage III: L3.6V3O8 to Li2.5V3O8 at charging; In stage III, insitu XRD reflections of Li1.5V3O8 are reversed while charging between 2.04.0 V (vs. Li/Li+) as shown in Figure 6a. 13



During initial phase, the intensity of (100) and (111) peaks corresponding to βphase (Li3.8V3O8) gradually decreases. On further charging, (100), (111), and (202) reflections at 14.33°, 27.8°, and 28.67° respectively, pertaining to αphase appeared with gradual increase in peak intensity with simultaneous complete disappearance of reflections corresponding to βphase marking β α phase reversal. More importantly, (100) reflection shift gradually from 14.33° to 14.08° and (111) peak to 27.8°, (202) peak at 28.67° and finally crossed over to its respective original position, indicating the formation of Lirich αphase with Li2.5 state (Figure 6b). Stage IV: L2.5V3O8 to Li1.3V3O8 at charging; On subsequent delithiation (discharging), the peaks corresponding to Lipoor αphase (Li1.3V3O8) increase its intensity and reverse back to its original position of Lipoor αphase. Finally, 0.2 Li comes from host crystal lattice of Li1.5V3O8 which is beneficial for capacity enhancement. Effect of temperature on the capacity behavior of Li1.5V3O8 The influence of temperature over the electrochemical behavior of Li1.5V3O8 nanorods has been examined by galvanostatic charge/discharge studies at 1C rate. Reversible capacity at 5 °C, 25 °C, and 55 °C were recorded to be 74 mAh.g1, 160 mAh.g1 and 224 mAh.g1 respectively (Figure. 7a). Cycling stability of Li1.5V3O8 nanorods based electrodes at 55 °C, 25 °C and 5 °C over 100 cycles (Figure 7b) indicated that, at 55 °C the electrode delivered higher capacity albeit with poor Coulombic efficiency. As predicted, at 25 °C and 5 °C, the Li1.5V3O8 electrode delivered a stable capacity up to 100 cycles. Usually the capacity behavior and the stability of electrodes at different temperature mainly depend on Liion diffusivity within electrolyte solution, structural stability, and interfacial SEI layer robustness. The calculated Li 14


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ion Diffusion coefficient (D) of Li1.5V3O8 nanorods was found to be D = 1.63×1013 cm2 s1, 1.25×1012 cm2 s1, 1.85×1012 cm2 s1 at 5 °C, 25 °C, and 55 °C, respectively. It is observed that the 5 °C tested cell showed low Li ion diffusivity compared to 25 °C and 55 °C, possibly due to freezing of electrolyte at low temperature; whereas higher diffusion coefficient value was observed at 55 °C due to high mobility of Li ion in the L1.5V3O8 electrode materials consistent with previously reported literature (22). Thus, higher temperature increases Liion diffusivity vis-a-vis capacity, but capacity loss is observed owing to instability of electrode materials and rapid SEI formation. At 5 °C, the electrode delivered lower capacity which could be ascertained to the low Li-ion diffusivity. The charge–discharge profiles at 55 °C for different cycles clearly indicate irreversible phase transformation and structure instability at higher temperature (Figure 7c). In order to further understand the electrochemical behavior of Li1.5V3O8 at different temperature, EIS analyses were carried out (Figure 7d) to correlate capacity behavior. It is seen at low temperature that, Rct (charge transfer resistance) increase results in lower specific capacity in contrary to the results at either 25 °C or 55 °C. In order to further explain the structural stability, comparison of dq/dv plots at different temperatures are plotted for the 1st and 100th cycles (Figure 8a,b). The first cycle dq/dv plot indicates three redox peaks appearing between 2.53.0V at 25 °C and 55 °C; whereas only one peak was observed at 5 °C and these three redox couples represent the phase transformation of Li1.5V3O8.41 At 5 °C, only one redox couple was observed with minor potential shift; on the other hand, the intensity of all these redox couples decrease for data acquired at 55 °C. Thus, the cell performed at 25 °C showed stable Li+ (de)insertion redox behavior (Figure 8b). Two new peaks at 2.1 V during discharge and 3.3 V 15



while charging indicate rapid irreversible structural changes occurring at 55 °C leading to faster capacity fading. 4. CONCLUSIONS Monodispersed submicron sized Li1.5V3O8 crystals were successfully synthesized by PVP-assisted solvothermal method using ethylene glycol solvent. Rietveld refinement confirmed the formation of monoclinic Li1.5V3O8 crystals with P21/m symmetry. Morphological evaluation with FESEM and HRTEM revealed uniform rodlike crystals with size features of 500 nm thickness having 5 µm length. The Li1.5V3O8 constructed electrode as cathode in LIBs delivered a reversible discharge capacity of ~239 mAh.g1 in the voltage window of 2.04.0V (vs Li/Li+) at 0.1 C after 50 cycles. At high current rate of 1C, the electrode delivered discharge capacity of 83 mAh.g1 after 400 cycles. Operando XRD study of Li1.5V3O8 during electrochemical dis(charge) processes proved the phase transformations from a Lipoor -phase (Li1) to Lirich -phase (Li2.5), and finally a phase (Li4). Investigation of dis(charge) behavior with temperature variation suggested that charge transfer resistance (Rct) played an important role in Liion diffusivity vis-à-vis specific capacity at low temperature. Li1.5V3O8 forms a robust cathode system for Li-ion batteries.

Associated Content Supporting Information Available: TG/DTA and FTIR spectral data of Li1.5V3O8, XRD patterns of Li1.5V3O8 at different lithiated states, Rietveld refinement data, and comparison of electrochemical data of Li1.5V3O8 with literature.

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Acknowledgements The authors acknowledge the Ministry of New and Renewable Energy (MNRE), Govt. of India

for

financial

assistance

(No.31/03/2014-15/PVSE-R&D).

The

authors

thank

Nanotechnology Research Centre, SRMIST, for providing facility for FESEM analysis. BS and PB thanks the Shell Technology Center (STC) Bangalore for financial support.

Author Information * To whom all correspondence should be addressed, E-mail: [email protected]

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Figure Captions

Figure 1.

Rietveld refined X-ray diffraction pattern of the Li1.5V3O8.

Figure 2

FESEM images of Li1.5V3O8 with PVP (ab); and without PVP (cd).

Figure 3.

HRTEM images of the Li1.5V3O8 (ab); high-resolution TEM image with lattice fringes (c) and selected area electron diffraction (SAED) pattern (d).

Figure 4.

XPS spectra of Li1.5V3O8 nanorods: survey scan (a); V(2p) (b); and O (2s) (c).

Figure 5.

(a) Cyclic voltamograms at scan rate 0.2 mV/s; (b) Galvanostatic charge discharge profile at 0.1C rate between 2.0-.04 V (vs Li/Li+); (c) Cycle life and efficiency at 0.1 rate; (d) first cycle chargedischarge studies different C rates; (e) Rate performance with different C rates; (f) EIS spectrum of LiV3O8 and (g) Long cycle life performance of Li1.5V3O8 at high current rate (1C).

Figure 6.

Insitu XRD pattern of Li1.5V3O8: (a) overall scan from 1060° with time versus voltage plot, and (b) Different enlarged regions versus potential plots.

Figure 7.

Capacity profiles of Li1.5V3O8 at 5°C, 25°C, and 55°C: (a) first cycle charge/ discharge plots; (b) cycling stability; (c) Charge/discharge plot of Li1.5V3O8 at 55 °C; and (d) EIS studies at 5°C, 25°C and 55°C.

Figure 8.

dq/dv plots at three different temperatures (5°C, 25°C, and 55°C) for Li1.5V3O8: (a) 1st cycle comparison and (b) 100th cycle comparison.

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Scheme 1. Synthesis of Li1.5V3O8 under solvothermal conditions

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Figure 1. Rietveld refined X-ray diffraction pattern of the Li1.5V3O8.

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Figure 2. FESEM images of Li1.5V3O8 with PVP (ab); and without PVP (cd).

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Figure 3. HRTEM images of the Li1.5V3O8 (ab); high-resolution TEM image with lattice fringes (c) and selected area electron diffraction (SAED) pattern (d).

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Figure 4. XPS spectra of Li1.5V3O8 nanorods: survey scan (a); V(2p) (b); and O (2s) (c).

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Figure 5. (a) Cyclic voltamograms at scan rate 0.2 mV/s; (b) Galvanostatic charge discharge profile at 0.1C rate between 2.0-.04 V (vs Li/Li+); (c) Cycle life and efficiency at 0.1 rate; (d) first cycle chargedischarge studies different C rates; (e) Rate performance with different C rates; (f) EIS spectrum of LiV3O8 and (g) Long cycle life performance of Li1.5V3O8 at high current rate (1C).

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Figure 6. Insitu XRD pattern of Li1.5V3O8: (a) overall scan from 1060° with time versus voltage plot, and (b) Different enlarged regions versus potential plots

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Figure 7. Capacity profiles of Li1.5V3O8 at 5°C, 25°C, and 55°C: (a) first cycle charge/ discharge plots; (b) cycling stability; (c) Charge/discharge plot of Li1.5V3O8 at 55 °C; and (d) EIS studies at 5 °C, 25 °C and 55 °C.

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Figure 8. dq/dv plots at three different temperatures (5 °C, 25 °C, and 55 °C) for Li1.5V3O8: (a) 1st cycle comparison and (b) 100th cycle comparison.

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Graphical Abstract Operando Structural and Electrochemical Investigation of Li1.5V3O8 Nanorods in Li-ion Batteries Thamodaran Partheeban,† Thangaian Kesavan,† Murugan Vivekanantha,†# Baskar Senthilkumar,‡ Prabeer Barpanda,‡ and Manickam Sasidharan†

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Operando Structural and Electrochemical Investigation of Li1.5V3O8

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