Article pubs.acs.org/JPCC
Structure and Electrochemistry of Vanadium-Modified LiFePO4 Jian Hong,† Xiao-Liang Wang,† Qi Wang,† Fredrick Omenya,‡ Natasha A. Chernova,‡ M. Stanley Whittingham,‡ and Jason Graetz*,† †
Brookhaven National Laboratory, Upton, New York 11973, United States State University of New York at Binghamton, Binghamton, New York 13902-6000, United States
‡
S Supporting Information *
ABSTRACT: Doping LiFePO4 with vanadium has proven to enhance electrochemical performance, but the underlying reasons for this improvement are not well understood. To better comprehend the relationships between the electrochemical performance, crystal structure, and surface carbon layer, we prepared vanadium-modified LiFePO4 by three different methods. The electrochemical performance of each sample was determined via a series of cycling studies, the detailed crystal structures of the doped samples were identified by X-ray diffraction and absorption spectroscopy, and the surface carbon coating was examined by high resolution transmission electron microscopy. In V-modified LiFePO4 prepared by a modified solid-state reaction, the vanadium is present in an impurity phase at the surface, which improves conductivity but has only a slight improvement in the electrochemical properties. The V-modified LiFePO4 samples prepared by the conventional solid-state reaction method and a solution method revealed that the vanadium was substituted into the lattice occupying iron sites in the FeO6 octahedron. This structural modification improves the cycling rate performance by increasing the Li+ effective cross-sectional area of the LiO6 octahedral face and thereby reducing the bottleneck for Li+ migration. In addition, analysis of the carbon coating revealed that the material prepared by the solution method forms a uniform carbon coating with a thin, well-ordered interface between the LiFePO4 and the carbon. The surface properties improve the electronic and ionic conductivities (with respect to the other samples), resulting in a high rate capability (87 mAh g−1 at 50 C).
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INTRODUCTION
over the years, vanadium-doped LiFePO4 exhibits the best overall electrochemical performance.10−13 Although there is general agreement that the addition of V to LiFePO4 improves electrochemical performance, there is little consensus on exactly what the V is doing to lead to this enhancement. In the polyanion system, V is stable over a wide range of oxidation states and can coexist with other transition metal ions.10−12,14−16 V is believed to substitute for Fe on octahedral sites forming a VO6 group. In a recent paper, we reported that in a solid-state reaction V will substitute (up to 10 mol %) resulting in a linear decrease in cell volume.11 We have also noted that vanadium might enter into a polyanion and occupy the tetrahedral P site, forming a VO43‑ group.13 Jin et al. showed that adding V engenders the formation of conductive V2O3 with no change in the unit cell volume at levels up to 10 mol % V, indicating no substitution of V into the lattice. However, even in this case where there is no substitution, there is an improvement in the electrochemical performance due to the increase in electronic conductivity.17 In view of the differences in these results, it seems likely that the location of V and its impact on the structure (if any) is highly dependent
With our increasing reliance on new and intermittent forms of energy (e.g., solar and wind), energy-storage devices, such as batteries, will play an ever more important role in improving our utilization and boosting efficiency.1 Li-ion batteries, with their high energy and power density, are currently being utilized in portable electronics, electric vehicles, and larger scale energy storage applications, but further improvements are needed. As an example, LixCoO2 is a commonly used cathode material in commercial lithium-ion batteries with a capacity of 140 mAh g−1 over a typical cycling range of 0.5 ≤ x ≤ 1.2 However, the high cost, limited abundance of Co, and safety issues have led to a search for alternative cathode materials that are safe, abundant, and cheap. Currently, LiFePO4 is one of the best alternative cathodes for high-power lithium ion batteries, and it has been widely investigated in recent years.3−7 LiFePO4 exhibits good thermal stability, a large storage capacity of 170 mAh·g−1, and high abundance and is environmentally friendly, but it has one major disadvantage: it suffers from a low electronic conductivity.7 Carbon coatings and isovalent ion substitution (V3+, Mn2+, Co2+, Ni2+, Mg2+, Zn2+, and Ca2+) have proven to be effective methods for improving electronic conductivity and overall electrochemical performance.3,8,9 Of all of the elements doped into LiFePO4 © 2012 American Chemical Society
Received: July 12, 2012 Revised: September 4, 2012 Published: September 11, 2012 20787
dx.doi.org/10.1021/jp306936t | J. Phys. Chem. C 2012, 116, 20787−20793
The Journal of Physical Chemistry C
Article
corresponding to 170 mAh g−1) over a voltage range of 2.0−4.2 V. The phase composition and the crystal structure of the assynthesized samples were determined by high-resolution synchrotron powder XRD data, acquired at a wavelength of 0.7744 Å, at beamline X14A of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. Rietveld refinement was performed to determine the lattice parameters and site occupancies for the as-synthesized samples using the GSAS/EXPGUI package. X-ray absorption spectroscopy experiments were carried out at beamlines X3B, X18A, and X19A of NSLS. The energy of the synchrotron ring was 2.8 GeV and the ring current was in the range from 110 to 300 mA. XAS scans were performed ex-situ at room temperature. Fe K-edge (7112 eV) measurements were performed in transmission mode, whereas V K-edge (5460 eV) spectra were acquired in fluorescence mode mode using a Vortex 4-element Si drift detector, due to the dilute concentration. Fe and V metal foils were measured as reference samples for X-ray energy calibration and energy alignment. The IFEFFIT package was used to process and analyze data.19 The morphology and particle sizes were obtained by high-resolution transmission spectroscopy using a JEOL 2100F microscope.
upon the precursor, the preparation method, and the synthesis temperature.11 To better understand how the synthesis affects the unit cell parameters and crystal structure, we generated three samples of V-doped LiFePO4 using the conventional solid-state reaction (LFP-CSS), a solution method (LFP-SM), and a modified solid-state reaction (LFP-MSS). An undoped sample was also prepared using the conventional solid-state reaction. X-ray diffraction (XRD) was used to determine the long-range structure and refinements were performed to determine the atomic occupancies (e.g., % V on Fe sites). X-ray absorption spectroscopy (XAS) was used to examine the local structure around the Fe and V atoms. The surface carbon coating was examined by high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS) was used to determine the elemental distributions.
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EXPERIMENTAL METHODS V-modified LiFePO4 prepared by the solution method (LFPSM) used the following precursors: LiOH·2H 2 O, Fe(NO3)3·9H2O, NH4H2PO4, NH4VO3, citric acid, with partially oxidized carbon particles as a nucleating agent. The precursors used in the conventional solid-state synthesis were Li2CO3, FeC2O4·2H2O, NH4H2PO4, NH4VO3, and citric acid. The precursors used in the modified solid-state reaction were Li2CO3, FePO4·2H2O, NH4VO3, and citric acid (the molar ratio of Li/Fe/P/V was 1:1:1:0.005). An undoped sample of LiFePO4 (undoped LFP-CSS) was also prepared by the conventional solid-state synthesis using Li2CO3, FeC2O4·2H2O, NH4H2PO4, and citric acid. For the LFP-SM sample, partially oxidized carbon black was prepared by treating Super P with concentrated HNO3/H2SO4 (∼1:1 in volume) at 75 °C for 12 h, thereafter washing it with deionized water, followed by drying overnight at 80 °C in a vacuum oven.18 The LFP-SM sample was prepared by first mixing together the precursors along with some partially oxidized carbon black. Water was added to obtain a 0.01 molar concentration that then was transferred into a sealed container. After holding it at 80 °C for 12 h, the precursor was dried in an oven. The precursor was pressed into pellets and sintered at 350 °C for 5 h and at 700 °C for another 5 h. For LFP-CSS, LFP-MSS, and undoped LFP-CSS, all of the precursors were ball milled with isopropyl alcohol and then dried in an inert atmosphere at room temperature. Thereafter they were pressed into pellets and sintered at 350 °C for 5 h and then at 700 °C for another 5 h under Ar/H2 in a tube furnace to obtain the final products. Cathodes for the electrochemical tests were prepared by mixing 80% active material, 12% carbon black, and 8% polyvinylidene fluoride (PVDF) with the solvent 1-methyl-2pyrrolidinone (NMP). The slurry was cast onto an Al foil current collector before drying. The dried electrodes (with an area of 1.2 cm2 and containing ∼5 mg of material) were assembled in 2025-type coin cells in an Ar-filled glovebox with pure lithium foil (Aldrich, thickness 23 μm) as the counter- and reference electrode. Celgard 2030 was used as the separator and LiPF6 (1M) in a 1:1 volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) (from Ferro company) was used as the electrolyte. The cells were tested using an Arbin multichannel cycler. The galvanostatic charge/discharge experiments were performed at current densities corresponding to rates of 0.1, 0.5, 1, 5, 10, 20, 30, and 50C rates (1C
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RESULTS AND DISCUSSION Electrochemistry. Figure 1 illustrates the voltage profiles from the four half cells prepared with LFP-SM, LFP-CSS, LFP-
Figure 1. Charge/discharge profiles (second cycle) from LiFePO4 samples (LFP-SM, LFP-CSS, LFP-MSS, and undoped LFP-CSS).
MSS, and undoped LFP-CSS cycled at room temperature using a C/10 rate (determined from the theoretical capacity of 170 mAh g−1). The undoped LFP-CSS shows a high polarization, which leads to separated charge/discharge plateaus and a curved discharge tail. On the contrary, the three V-modified samples (LFP-SM, LFP-CSS, and LFP-MSS) exhibit voltage profiles with smooth plateaus at 3.46 (charge) and 3.37 V (discharge) as well as sharp and vertical tails. The difference between the charge and discharge plateaus is as low as 100 mV, indicating that the polarization of the V-modified samples is low. This is likely due to the higher electronic conductivity (∼10−6 S/cm), which is approximately an order of magnitude greater than the conductivity measured for the unmodified sample (∼10−7 S/cm) at a pressure of 2 bar. It is worth noting that when V is substituted into the structure (e.g., at Fe sites) 20788
dx.doi.org/10.1021/jp306936t | J. Phys. Chem. C 2012, 116, 20787−20793
The Journal of Physical Chemistry C
Article
This technique is typically used to study the chemical diffusion coefficient in single-phase systems by following the voltage after a titration current pulse; it can also provide useful information in two-phase systems. In LiFePO4, the cell voltage returns to its initial state after a current pulse, but the rate at which this occurs yields important information about the electrode kinetics. Figure 3 shows the titration curves for V-modified
the theoretical capacity is slightly reduced since some of the Li must be removed to keep the charge balanced (e.g., Li1‑xFe1‑xVxPO4). The theoretical capacity of LiFePO4 with 5% V (in a trivalent oxidation state) is 161.5 mAh g−1.11 The discharge capacities for the three V-modified samples were 153.5 (LFP-SM), 145.5 (LFP-CSS), and 140.4 mAh g−1 (LFEMSS) and the capacity of the undoped sample was 130.3 mAh g−1 (undoped LFP-CSS). Among them, the undoped LFP-CSS exhibited the least capacity, while the material prepared using the solution method exhibited the best electrochemical performance with a capacity of 153.5 mAh g−1 at a C/10 rate. A comparison of the electrochemical performance for the three V-modified samples is summarized in Figure 2a. LiFePO4
Figure 3. Galvanostatic intermittent titration (GITT) of LiFePO4 prepared by the solution method. The quasi equilibrium state of charge and discharge was ∼20 mV.
LiFePO4 prepared using the solution method (LFP-SM). The current pulses were 100 μA cm−2 for 2 h, followed by a 3 h voltage relaxation. The polarization (voltage difference between charge and discharge) for this sample is very low, but these curves also reveal that the Fe3+/Fe2+ redox couple is kinetically sluggish during the discharging process. Previous research on the mechanism of charge transport and the room temperature phase diagram of LixFePO4 has revealed an incomplete miscibility gap with small solid-solution compositional domains at 0 < x < α and 1 − β < x < 1.20 Normally, the two-phase region is between Li0.96FePO4 (α = 0.11) and Li0.11FePO4 (β = 0.04).21 Interestingly, the GITT data from V-modified LiFePO4 prepared by the solution method reveals a smaller solid solution region (0 < x < 0.032 and 0.90 < x < 0.95) and therefore, a larger two phase region extending from Li0.90Fe0.95V0.05PO4 to Li0.032.Fe0.95V0.05PO4. The two-phase region occurs over 91.8% of full capacity (158 mAh·g−1), whereas the two-phase region in pure LiFePO4 occurs over 85% of the full capacity (145 mAh g−1). The large two-phase region of LFP-SM improves the energy density of the battery when compared with undoped LiFePO4. X-ray Diffraction. The V-modified LiFePO4 was prepared by three different methods, each using a different set of reaction conditions and precursors, and therefore each sample exhibits a slightly different structure, morphology, and V distribution, which can be correlated to the electrochemical properties. To determine the phase purity and lattice parameters of the Vmodified samples, high-resolution synchrotron powder XRD patterns were acquired for undoped LFP-CSS and V-doped LFP-CSS, LFP-MSS, and LFP-SM. The diffraction patterns, shown in Figure 4, demonstrate a single orthorhombic structure with no observable peaks from impurities. It is noteworthy that impurity phases like Fe2P may form during synthesis, typically during prolonged, high temperature heating in a reducing
Figure 2. (a) Capacity of V-modified LiFePO4 (LFP-SM, LFP-CSS, and LFP-MSS) electrodes as a function of C-rate and (b) C-rate performance of V-modified LiFePO4 (LFP-SM).
prepared by the conventional solid state synthesis (LFP-CSS) has a capacity of 147 mAh g−1 at a rate of C/10, 123 mAh g−1 at 5C, and 100 mAh g−1 at 10C, as reported in a previous study.13 The capacity retention is even less in the LFP-MSS sample, especially at high rates. To further confirm the performance of LiFePO4/C synthesized by the solution method, we tested the material at different C-rates and the results are shown in Figure 2b. A capacity of ∼153.5 mAh g−1 was achieved at a low current rate (C/10). When the C-rate increases to 5C and 10C, the material has a capacity of 135 and 125 mAh g−1, respectively. At an extremely high rate of 50C, the electrode retains a capacity of 87 mAh g−1. The room temperature cycling stability of LFP-SM is excellent with littleto-no fade over 100 cycles at a rate of 1C (Figure 1S). The voltage profile and kinetics of the LFP-SM were investigated by galvanostatic intermittent titration (GITT). 20789
dx.doi.org/10.1021/jp306936t | J. Phys. Chem. C 2012, 116, 20787−20793
The Journal of Physical Chemistry C
Article
Figure 5. High-resolution synchrotron XRD pattern (obs), refinement (calc), background (bckgr), and difference (diff) from LiFePO4−SM.
Table 1. Unit Cell Parameters and the Refinement R Values for LFP-CSS, LFP-MSS, LFP-SM, and Undoped LFP-CSS
Figure 4. High-resolution synchrotron powder-XRD of V-doped and undoped LiFePO4.
environment, and may be difficult to detect by XRD.22 For the doped and undoped samples, all reflections were indexed on the basis of an olivine structure with the space group pnma.7 The LFP-SM shows a small peak at low angle (
