Article pubs.acs.org/JPCC
Significantly Improved Electrochemical Performance in Li3V2(PO4)3/C Promoted by SiO2 Coating for Lithium-Ion Batteries Lu-Lu Zhang,†,‡,§ Gan Liang,*,§ Gang Peng,‡ Feng Zou,† Yun-Hui Huang,*,† Mark C. Croft,⊥ and Alexander Ignatov⊥ †
School of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mold Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China ‡ College of Mechanical and Material Engineering, Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China § Department of Physics, Sam Houston State University, Huntsville, Texas 77341, United States ⊥ Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, United States and NSLS, Brookhaven National Laboratory, Upton, New York 11973, United States ABSTRACT: Li3V2(PO4)3/C (LVP/C) coated with various amounts of SiO2 has been synthesized, and the effect of surface modification by SiO2 on the performance of LVP has been systematically investigated with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), magnetic susceptibility, Raman spectroscopy, and electrochemical measurements. Based on the analysis of XRD, XPS, and TEM, it is confirmed that SiO2 coating on the surface of LVP particles does not change the monoclinic structure of LVP. The XAS, XPS, and magnetic susceptibility results indicate that the valence of V in both the pristine and SiO2-coated LVP are close to +3. Furthermore, our results reveal that the electrochemical performance of LVP/ C can be significantly improved by the SiO2 coating, which is due to the enhanced structural stability and reduced charge-transfer resistance.
1. INTRODUCTION Owing to the growing global energy crisis and “global warming”, lithium-ion batteries have attracted ever increasing attention. The cathode materials play an important role in lithium-ion batteries, because not only it is the key to improving the electrochemical performance of the whole cell, but its cost also occupies a large proportion of the whole cell. Up to now, besides LiCoO2, other cathode materials such as LiFePO4, LiMn2O4, and LiNi1/3Co1/3Mn1/3O2 have also been successfully brought to the market. In recent years, monoclinic Li3V2(PO4)3 (LVP) has received extensive attention because of its high theoretical capacity (197 mAh g−1) and high operating voltage (up to 4.0 V).1−5 However, like other lithium transition metal phosphates, the low electronic conductivity of LVP greatly affects its electrochemical performance especially at high rate, and thus prevents its large-scale application in electric vehicles (EVs) and hybrid electric vehicles (HEVs).3 Much effort has been made to improve the electronic conductivity of LVP by carbon coating,6−14 metallic ion doping (Fe, La, Cr, Sc, Mg, K, Co, Al, Ti, Mn, Na, Nb, etc.),4,15−26 and surface modification (NbOPO4, MgO, etc.).5,27 LVP can extract all the three Li+ ions in the unit cell when charged to 4.8 V, exploiting the V3+/V4+ and V4+/V5+ redox couples.3,4 However, the capacity drops rapidly at such a high voltage (4.8 V) due to electrolyte decomposition and vanadium dissolution in the © 2012 American Chemical Society
electrolyte. Surface modification can efficiently reduce capacity fading, because the protective layer can prevent the direct contact between LVP and the electrolyte solution, thus alleviating vanadium dissolution in the electrolyte and improving the structural stability of LVP. In the past, for example, SiO2 modification has been proved to be an effective method in improving the electrochemical performance of cathode materials such as LiNi0.5Mn1.5O4,28 LiMn2O4,29 and LiFePO4,30 etc. because of significant improvement in structural stability, Li-ion conductivity, and capacity retention. In this work, a series of SiO2-modified Li3V2(PO4)3/C composites were prepared, and the effects of SiO2 on the physicochemical properties and electrochemical performance have been systematically investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), magnetic susceptibility, Raman spectrometry, and electrochemical measurements. Received: February 3, 2012 Revised: May 17, 2012 Published: May 22, 2012 12401
dx.doi.org/10.1021/jp301127r | J. Phys. Chem. C 2012, 116, 12401−12408
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2. EXPERIMENTAL SECTION Li3V2(PO4)3/C composite was prepared by a two-step solidstate process as described elsewhere.5 Tetraethyoxysilane (TEOS) was used as silica source. SiO 2 -modified Li3V2(PO4)3/C samples were prepared in a several step process. Li3V2(PO4)3/C powders were first dispersed in a progressive amount of solutions of 40 μL TEOS/mL ethanol solution by ultrasonic method for 2 h, with the concentration of TEOS being 1, 2, and 3 wt % of Li3V2(PO4)3/C powders, respectively. Then, the mixture was stirred by a magnetic force stirrer until dry. Subsequently, the precursor was calcined at 600 °C for 5 h in nitrogen atmosphere and then cooled down slowly to room temperature. To exclude the possible effect of heat treatment, LVP/C prepared without TEOS was also sintered at 600 °C for 5 h in nitrogen atmosphere. Hereafter, the Li3V2(PO4)3/C composites prepared with 0, 1, 2, and 3 wt % TEOS will be referred as pristine LVP/C, LVP/C-1Si, LVP/ C-2Si, and LVP/C-3Si, respectively. The amount of residual carbon in the final products is approximately 3.5 wt % as determined by carbon−sulfur analyzer (CS600, LECO, U.S.). X-ray diffraction patterns were obtained using an X’Pert Pro diffractometer with Cu-Kα radiation (λ = 1.5406 Å) (XRD, X’Pert Pro, PANalytical B.V.). The morphology was obtained with a field-scanning electron microscope (FSEM, Sirion 200, Holland) coupled with an energy dispersive X-ray (EDX) detector and a transmission electron microscope (TEM, JEM2100, JEOL). The carbon coating on pristine LVP/C and SiO2modified LVP/C powders was examined by Raman spectroscopy (VERTEX 70, Bruker). Vanadium concentration dissolved in the electrolyte was tested by inductively coupled plasma mass spectrometer (ICP-MS, ELANDRC-e, Perkin-Elmer). The oxidation states of V and Si at the surface and in the interior of pristine LVP/C and LVP/C-2Si samples were studied by X-ray photoelectron spectroscopy (XPS, PHI Quantera, U-P) measurement. V K-edge X-ray absorption spectroscopy (XAS) data were taken in fluorescence and transmission mode at beamline X-19A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The energy resolution (ΔE/E) of the X-19A beamline was 2 × 10−4, corresponding to about 1.1 eV at V Kedge. The XAS spectra presented in this paper were background subtracted and normalized to unity in the continuum region about 100 eV above the edge. The magnetization measurements were carried from 5 to 300 K in a magnetic field of 500 Oe with a superconducting quantuminterference device (SQUID) magnetometer (Model MPMS, Quantum Design, San Diego, CA). Gelatin capsules were used as the sample containers for the powder samples. Electrochemical measurements were carried out with CR2032 coin cells. The working electrodes were prepared by roll milling a mixture of 75 wt % active materials, 20 wt % carbon black, and 5 wt % PTFE in N-methylpyrrolidinone. The slurry of the mixture was coated onto an aluminum foil (20 μm in thickness) using an automatic film-coating equipment. The resulting film was dried under an infrared light to remove volatile solvent, punched into discs with diameter of 14 mm, and then pressed under a pressure of 6 MPa. After drying at 120 °C for 12 h in vacuum, the discs were transferred into an argon-filled glovebox (Super 1220/750, Mikrouna) and assembled as working electrodes in coin cells using Celgard 2400 as the separator and lithium foil as counter and reference electrodes. The electrolyte was 1 mol L−1 LiPF6 in a mixed
solvent of ethylene carbonate and dimethyl carbonate (EC/ DMC, 1:1 by volume). The cells were cycled at different Crates between 3.0 and 4.8 V on a cell testing instrument (LAND CT2001A, China). Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) measurements were performed on an electrochemical working station (PARSTAT 2273, Princeton Applied Research, U.S.). CV curves were monitored at a scanning rate of 0.05 mV s−1 within a voltage range of 3.3−4.8 V, and EIS spectra were obtained over a frequency range between 1 mHz and 100 kHz.
3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the as-obtained samples. All patterns are well indexed as monoclinic structure of LVP
Figure 1. XRD patterns of the as-prepared samples: (a) pristine LVP/ C, (b) LVP/C-1Si, (c) LVP/C-2Si, and (d) LVP/C-3Si.
with a space group of P21/n, indicating that SiO2 incorporation does not change the monoclinic structure of LVP. Moreover, no diffraction peaks from carbon and silica-containing compounds are observed, indicating that the pyrolytic carbon from glucose and silica-containing compounds from TEOS is in amorphous form or its content is too low to be detected. Pyrolytic carbon can act not only as a conductor to improve the electronic conductivity of LVP but also as a reductive agent to reduce V5+ to V3+. To investigate the oxidation states of V and Si, and to check whether SiO2 remains only on the surface of LVP particles, V 2p3/2 and/or Si 2p XPS core levels for pristine LVP/C and LVP/C-2Si samples at the surface and in the interior (at the ∼60 nm depth) were measured and are illustrated in Figure 2. The scale of binding energy (BE) is referenced by setting the BE of C1s to 284.5 eV. Figures 2a2 and b2 show a V 2p3/2 main peak at ∼516.8 eV, indicating that the oxidation state of V is nearly trivalent for these two samples. Thus, it seems that the SiO2 incorporation in LVP/C does not change the oxidation state of V ions. Noting that, for the two samples, the intensity of the V 2p3/2 main peak at the surface is lower than that in the interior because of the coating layer. The pristine material shows no Si XPS signal and cuts out a zero signal figure (Figure 2a3). Compared with the Si 2p XPS core levels for pristine LVP/C, the spectrum for LVP/C-2Si sample shows an obvious Si 2p main peak at 103.4 eV (Figure 2b3), This result indicates that the valence of Si in the sample is +4, similar to that of SiO2 in nano-SiO2-coated LiMn2O4 cathode materials.29 Furthermore, Figure 2b3 shows that the intensity of the Si 2p peak for the LVP/C-2Si sample is much weaker in the interior than at the surface of the sample particles, indicating that the incorporated SiO2 basically remained on the surface of the 12402
dx.doi.org/10.1021/jp301127r | J. Phys. Chem. C 2012, 116, 12401−12408
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around 5470 eV) of V compounds are too complicated to interpret due to the stronger d/p hybridization that accompanies the larger spatial extent of the V 3d-states. Figure 4 shows the SEM images of pristine LVP/C and LVP/ C-2Si samples. The particles for both samples present irregular
Figure 2. XPS spectra of some typical samples: (a) pristine LVP/C and (b) LVP/C-2Si.
LVP particles rather than entering deep into the interior of the particles. In Figure 3 the V K-edge XAS spectra for the pristine LVP/C and LVP/C-2Si samples are shown along with the spectra of
Figure 4. SEM images of the as-prepared samples: (a) pristine LVP/C and (b) LVP/C-2Si.
granular shapes with some agglomeration and with a wide sizedistribution ranging from ∼10 nm to 1.0 μm. There is no significant difference between the SEM images of pristine LVP/ C and LVP/C-2Si samples. The EDX spectrum, in the inset of Figure 4, clearly evidences the existence of silica. In order to investigate the distribution of the component elements in the particles, the elemental mapping of V, P, O, C and Si species in LVP/C-2Si sample was explored by EDX. As displayed in Figure 5, all the elements (V, P, O, C, and Si) show homogeneous distribution, which demonstrates that these elements are uniformly distributed in the LVP/C-2Si sample. The morphology and structure of pristine LVP/C and LVP/ C-2Si powders were further confirmed by TEM and HRTEM (Figure 6). Combined with the results of XRD and XPS, it is reasonable to believe that pristine LVP/C particles are coated with amorphous carbon, and LVP/C-2Si particles are wrapped and/or connected with a mixed layer combined by amorphous carbon and SiO2. The thickness of the layer is about 3−5 nm
Figure 3. XAS spectra of pristine LVP/C, LVP/C-2Si, and reference compounds.
some related reference compounds. The formal valence values for V are +3 for standards LaVO3, Eu2VO4, and La2FeVO6; +4 for VO2, La2NiVO6, and Sr3V2O7; and +5 for Sr3V2O8, respectively. It can be seen that the edge energy (defined as the energy at inflection point of the rising part of the edge) increases from about 5480 to 5484 eV with the increase of valence from +3 to +5. The values of the edge energy for LVP/ C and LVP/C-2Si are 5479.8 and 5480.2 eV, respectively, almost identical to those for the trivalent V reference compounds. This result reveals that the valence of V in LVP/ C and LVP/C-2Si specimen should be very close to +3, which also verifies that SiO2 incorporation in LVP/C does not change the valence state of V ions. The pre-edge features (located at 12403
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Figure 5. EDX mapping of LVP/C-2Si powders.
for paramagnetic materials. The χ−1 data for the pristine LVP/ C sample can be fit to the Curie−Weiss law χ(T) = C/(T − θp) in the whole temperature range of 5 K ≤ T ≤ 300 K, with the Curie constant C = 1.662 emu·K/(Oe·mol) and the paramagnetic Curie temperature θp = −22 K. The value of the effective moment, μeff, estimated from the formula μeff = (8C)1/2 is 3.65 μB per formula unit (f.u.) for Li3V2(PO4)3, or μeff = 2.58 μB per V ion. This value is very close to the theoretical value of μeff = 2.83 μB for V3+ ions, indicating that the valences of the V ions in LVP are indeed close to +3. This result is consistent with our XPS and XAS results. The negative value of θp suggests that the Weiss molecular field due to the magnetic interaction between V ions is antiferromagnetic in character. It is seen from Figure 7 that the M(T) curve for LVP/C-2Si is below that for the pristine LVP/C in the whole temperature range, consistent with the fact that the LVP/C particles in the LVP/C-2Si sample are coated with SiO2 layers. Since it is
(Figure 6d), which is favorable in enhancing the conductivity and forming an efficient barrier to prevent the direct contact with the electrolyte solution.31 Tested by ICP-MS, the vanadium concentrations in the electrolyte are 176.583 and 17.191 μg L−1, respectively, for the pristine LVP/C and LVP/ C-2Si cells after 50 cycles. Obviously, the SiO2 coating layer greatly alleviates vanadium dissolution in the electrolyte over cycling, thus improving the structural stability of LVP.32 Furthermore, both the fast Fourier transform (FFT) pattern in the insert of Figure 6b and the clear crystal planes (with a dspacing of 0.407 nm corresponding to the (012) planes) in Figure 6d clearly confirm the monoclinic LVP structure. In the present work, we carried out the first study on the magnetic properties of the LVP-based materials. Figure 7 shows the magnetization M(T) and reciprocal magnetic susceptibility χ−1(T) curves for the pristine LVP/C and LVP/C-2Si samples. The M(T) curves for both samples display the typical behavior 12404
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Figure 8, it is apparent that the CV curves of the pristine LVP/ C and SiO2-modified LVP/C electrodes are very similar. There
Figure 8. CV profiles of the as-prepared samples at a slow scanning rate of 0.05 mV s−1 and in a potential window of 3.3−4.8 V (vs Li+/ Li).
are four oxidation peaks around 3.65, 3.71, 4.13, and 4.57 V, respectively, corresponding to the extraction of the three lithium ions from LVP by a sequence of phase transition processes between the single phase of LixV2(PO4)3 (x = 3.0, 2.5, 2.0, 1.0, and 0),34 and three reduction peaks around 3.94, 3.62, 3.55 V, respectively, attributed to the reinsertion of the three lithium ions in LVP. These transitions are characterized by a solid solution behavior (the initial reinsertion of lithium ion in LVP) and two subsequent Li+-insertion processes (Li2V2(PO4)3 → Li2.5V2(PO4)3 → Li3V2(PO4)3). The Li+ ions extraction/reinsertion is associated with V3+/V4+ and V4+/V5+ redox couples. According to the Randles Sevcik equation, Ip = 2.69 × 105n3/2AD1/2v1/2C (Ip is the CV peak current, n is the number of electrons involved in the redox process, A is the electrode area, D is the Li+ diffusion coefficient, v is the potential scan rate, and C is the shuttle concentration),35 the stronger the CV current, the larger the Li+ diffusion coefficient. From Figure 8, it is clearly seen that the currents of four oxidation peaks are stronger than those of three reduction peaks, indicating that extraction of Li+ ions from LVP is faster than reinsertion into LVP. Noting that, with increasing silica content (
