Article pubs.acs.org/cm
Why Substitution Enhances the Reactivity of LiFePO4 Fredrick Omenya,† Natasha A. Chernova,† Ruibo Zhang,† Jin Fang,† Yiqing Huang,† Fred Cohen,† Nathaniel Dobrzynski,† Sanjaya Senanayake,‡ Wenqian Xu,‡ and M. Stanley Whittingham*,†,§ †
Chemistry and Materials, Binghamton University, Binghamton, New York 13902-6000, United States Chemistry Program, Brookhaven National Laboratory, Upton, New York 11973, United States § Northeastern Center for Chemical Energy Storage, Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States ‡
ABSTRACT: The impact of substitution at the Fe site in LiFePO4 on reaction pathway, kinetics, and crystallographic changes upon electrochemical delithiation has been determined. Substitution was found by X-ray diffraction to reduce the lattice mismatch between the Li-rich and the Li-poor phases of the substituted samples as compared to the unsubstituted one. Substitution was also found, by monitoring the 200 reflection peaks of both the triphylite and heterosite phases, to increase the composition width of the single phase formed on lithium removal, Li 1 − x FePO 4 . A single phase was observed as high as x = 0.15 in Li1 − xFe0.85V0.1PO4, whereas LiFePO4 at the same state of charge and of similar particle size show the existence of two phases. In addition, the temperature at which a single phase is observed for the composition range 0 ≤ x ≤ 1 is decreased from slightly above 300 °C to ca. 200 °C. This increased single-phase-like behavior explains the enhanced kinetics of substituted LiFePO4 and is consistent with a pseudosingle-phase reaction mechanism. KEYWORDS: Olivine, lithium battery, vanadium substitution, reaction mechanism
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increase in electronic conductivity by about 8 orders of magnitude and improved rate capability upon doping LiFePO4 with low level of aliovalent dopant, below 1 at. % at the Li site. The conductivity increase was later attributed to the formation of a conductive coating of Fe2P, which is formed at higher temperatures, above 700 °C.9 The improved rate capability upon substitution at the Li site has also been questioned because, in olivine compounds, Li ion is known to move along 1D tunnels and the presence of immobile aliovalent substituting ion at the Li site would impede the Li diffusion thus decreasing the electrochemical performance.10,11 However, Meethong et al. in a series of papers12 have shown that metal substitution on the Li site indeed enhances the rate behavior. In their materials, the lattice volume increases with metal substitution. In this article, we discuss the impact on the reaction mechanism of vanadium doping on the Fe site, which in contrast results in a decrease in the lattice volume. We recently showed13 that vanadium is substituted on the Fe site as V3+ and that the amount of vanadium incorporated decreases with increasing formation temperature. Keeping the synthesis temperature at 550 °C, we were able to substitute over 10% of vanadium at the Fe site of LiFePO4 without coformation of any impurity phases and keeping uniform particle size. At higher temperatures above 600 °C, the aliovalent ions are partially
t has been well established that substitution enhances the electrochemical performance of LiFePO4 as cathode material in Li-ion batteries, especially at high current densities.1 However, the reaction mechanism enabling fast kinetics of LiFePO4, an electronic insulator and poor ionic conductor, is still a subject of debate. Currently, two main models of LiFePO4 delithiation reaction have been proposed. The domino-cascade model2 suggests a two-phase reaction with Li being removed from the channels along the b axis and the LiαFePO4/Li1 − βFePO4 reaction front moving rapidly along the a axis. Here, LiαFePO4 and Li1 − βFePO4 are limited solid solutions, the extent of which increases with decrease of nanoparticle size3 and at elevated temperatures.4 Malik et al.5 argue that the nucleation energy required to initiate the twophase transformation is too high, inconsistent with fast kinetics. They predict, based on first-principle calculations, a metastable solid solution path at just 30 mV overpotential, consistent with the earlier experimental finding of 20 mV by Dreyer et al.6 Both models predict that the equilibrium state is two-phase, with no coexistence of LiαFePO4 and Li1 − βFePO4 within a single nanocrystal, consistent with experimental findings. In contrast, in crystals of several micrometers in size a reaction front between the LiFePO4 and FePO4 phases is observed together often with cracking of the crystal.7 The reasons why the substitution promotes the delithiation and lithiation processes are even less understood. The initial work by Chung et al.8 spurred a lot of interest on the possibility and the role of aliovalent doping of LiFePO4 by reporting an © 2012 American Chemical Society
Received: October 9, 2012 Revised: December 5, 2012 Published: December 6, 2012 85
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Figure 1. SEM images of (a) LiFePO4 and (b) LiFe0.85V0.1PO4.
ejected from the olivine structure. 13 These vanadium substituted compounds show improved rate performance,13 consistent with previous reports. In this article, we investigate how and why such substitution affects the reaction mechanism and thermodynamics.
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have been reported to change the olivine two-phase reaction pathway.3,15 Therefore, we studied a series of samples synthesized by the same solid-state method and producing samples of similar particle sizes (between 40 to 80 nm) independent of vanadium content (Figure 1). XRD data confirms olivine phase and absence of any crystalline impurity upon vanadium substitution at the Fe site when using reagent stoichiometries as in LiVyFe1−3/2y PO4. To investigate how such substitution affects the reaction mechanism, first we established a base case by taking ex-situ XRD patterns of pure single-phase unsubstituted LiFePO4 at different stages of electrochemical delithiation. The XRD patterns of LixFePO4 at x = 0.25 and x = 0.5 are two-phase mixtures evidenced by the coexistence of the Li-rich and Lipoor phases in both composition as shown in Figure 2. These
EXPERIMENTAL SECTION
Pristine LiFePO4 was synthesized by solid-state reactions of lithium carbonate (Li2CO3), iron(II) oxalate dihydrate (FeC2O4·2H2O) and ammonium dihydrogen phosphate (NH4H2PO4) in the molar ratio of 1:1:1. In the synthesis of the vanadium substituted LiFePO4, ammonium vanadate was used as the vanadium source, the stochiometric composition was controlled as per the targeted product composition; detailed synthesis procedure can be found in ref 13. X-ray diffraction (XRD) patterns of pristine and electrochemically delithiated materials were taken at beamline X7B, National Synchrotron Light Source, Brookhaven National Laboratory with an average wavelength of 0.3196 Å. Rietveld refinements of XRD data were performed using the GSAS/EXPGUI package.14 The cathode materials for the electrochemical tests were prepared by mixing the active material: carbon black: polyvinylidene fluoride (PVDF) in the ratio of 85:9:6 in 1-methyl-2-pyrrolidinone solvent. The slurry was then cast onto an Al foil current collector and dried. The punched electrodes had an area of 1.2 cm2 with a maximized mass, for the ex-situ experiments, of about 8−12 mg of active material. The 2325-type coin cells were assembled in a He-filled glovebox using lithium foil (Aldrich, thickness 23 μm) as the counter and reference electrode, and Celgard 2400 as the separator. LiPF6 (1 M) in ethylene carbonate (EC) and dimethyl carbonate (DMC) (LP30 from Industries) in a 1:1 volume ratio was used as electrolyte. The cells were tested using a VMP2 multichannel potentiostat (Biologic). The ex-situ cells with a loading of 8−12 mg of active material were charged at slow current densities (64−96 μA/cm2, corresponding to C/20) to different states of charge, the upper charge cutoff voltage was set at 4.3 V. The cells were then disassembled in the glovebox and the electrodes were soaked in dimethyl carbonate to dissolve all of the electrolytes. In the galvanostatic intermittent titration technique (GITT) experiment, the cells were charged with a current density of 30−35 μA/cm2 corresponding to C/20 for 2 h followed by open circuit relaxation for 24 h. This was repeated until a cutoff voltage of 4.2 V was reached followed by a discharge to 2 V under the same setting conditions.
Figure 2. Ex situ synchrotron diffraction patterns of unsubstituted Li1 − xFePO4 at different states of charge: x = 0.0, 0.25, 0.5, and 0.94. The insert shows the magnified section of the 200 reflection peaks, in the rectangular box, of the triphylite and heterosite phases.
two phases do not show any observable shift in the diffraction peak positions upon electrochemical delithiation at different states of charge, as characterized by the 200 reflection peaks at around 3.56° and 3.74°, shown in the insert of Figure 2. As shown in Table 1, the Rietveld refinement of the pristine and the delithiated samples are consistent with the above results where only a small change in the lattice parameters is observed for the triphylite (T) or heterosite (H) phases at different states of charge, thus supporting the small lithium solubility. Using Vegard’s law our data suggests a value of 3% for β. This is
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RESULTS AND DISCUSSION For a systematic investigation of the effect of vanadium substitution on reaction mechanism of LiFePO4, it is necessary to avoid any discrepancies that may arise from synthesis methods, because different synthesis methods might result in different particle sizes and defect chemistries, factors which 86
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Table 1. Lattice Parameters of LiFePO4 and LiFe0.85V0.1PO4 sample LiFePO4_pristine LiFePO4_50% Charged LiFePO4_94% charged LiF0.85V0.1PO4_pristine LiF0.85V0.1PO4_50% charged LiF0.85V0.1PO4_87% charged
T T H H T H H
a (Å)
b (Å)
c (Å)
V (Å3)
Rwp (%)
10.3239(7) 10.3096(6) 9.8348(6) 9.8347(3) 10.3056(4) 10.2521(4) 9.8725(6) 9.8464(4)
6.0051(4) 5.9990(3) 5.8051(3) 5.8034(2) 5.9940(2) 5.9714(6) 5.8183(6) 5.8003(2)
4.6938(3) 4.6970(3) 4.7826(3) 4.7834(2) 4.6944(1) 4.7099(6) 4.7801(6) 4.7820(2)
291.00(2) 290.51(3) 273.05(2) 273.01(1) 289.99(1) 288.34(5) 274.57(5) 273.11(2)
5.5 2.3 2.5 2.6 3.3 2.6
consistent with the work of Yamada et al.,16 where a small Li solubility range is reported upon delithiation of LiFePO4 forming intermediate phases LiαFePO4 and Li1 − βFePO4 where α = 0.032 and β = 0.038. After establishing the two-phase behavior of LixFePO4 with the narrow solubility limits, we investigated the crystallographic changes accompanying the electrochemical delithiation of LiFe0.85V0.1PO4. Upon electrochemical delithiation of this compound at x = 0.25 and 0.5 two olivine phases, the triphylite and heterosite, coexist, whereas at a higher state of charge, x = 0.87, only a single heterosite phase is observed (Figure 3). Comparing the diffraction peaks for the
Figure 4. Diffraction patterns of (a) Li 0.5 FePO 4 and (b) Li0.5Fe0.85V0.1PO4, insert shows magnified 200 diffraction peaks of triphylite and heterosite phases.
Li0.5Fe0.85V0.1PO4 phases with the triphylite phase diffraction peaks being at higher two-theta values, whereas the heterosite phase diffraction peaks appear at lower two-theta values compared to that of Li0.5FePO4. The above observations are illustrated in the insert in Figure 4, which focuses on the 200diffraction peak, but however are not limited to this diffraction peak. The reduced lattice mismatch suggests a wider solid solution range for the substituted samples as further supported by the two-phase Rietveld refinements of Li0.5Fe0.85V0.1PO4 and Li0.5FePO4. The refinement results of the Li0.5Fe0.85V0.1PO4 sample, shown in Table 1, suggest a value of β of 12−13% and 13−15% for α, and give a volume of 288.34 Å3 for the triphylite phase and 274.57 Å3 for the heterosite phase resulting in a unit cell volume mismatch of ca. 4.8% as compared to the Li0.5FePO4 with a unit cell volume of change between the Lirich and Li-poor phases of 6.0%. Our findings do not confirm the recent theoretical calculations17 predicting an increase in the lattice mismatch due to an increase in the unit cell volume for the vanadium substituted LiFePO4 as compared to the pure system. According to that work, the increase in volume would facilitate Li diffusion due to a larger diffusion pathway. To confirm the extended solid solution range in vanadiumsubstituted compound, we examined XRD patterns over the initial stages of electrochemical lithium removal. The results show the existence of a single olivine phase for Li1 − xFe0.85V0.1PO4 for 0 < x < 0.15. The unsubstituted LiFePO4 does not show such extended solid solution, for instance, at x = 0.1, Li1 − xFePO4 shows the coexistence of two phases, the Li-rich and Li-poor, as exemplified by the 200 heterosite diffraction peak (arrow in Figure 5), whereas Li1 − xFe0.85V0.1PO4 (x = 0.1) shows only triphylite peaks. Because we compare substituted and pristine LiFePO4 of similar particle size, this is not a particle size effect but rather a
Figure 3. Ex situ synchrotron diffraction patterns XRD of Li1 − xFe0.85V0.1PO4 at different states of charge: x = 0.0, 0.25, 0.5, and 0.87. The insert shows the magnified section of the 200 reflection peaks of the triphylite and heterosite phases at x = 0.0, 0.5, and 0.87, the lines mark the positions of the peak centers.
LiFe0.85V0.1PO4 at different states of charge, x = 0.0, 0.5, and 0.87, the electrochemically delithiated samples show an unmistakable shift in the peak positions both for the Li-rich and Li-poor phases as compared to end or near-end members. The partially delithiated triphylite phase diffraction peaks shift to the higher two-theta angles as compared to the pristine phase, whereas the heterosite diffraction peaks shift to the lower angles as compared to the end member delithiated phase. The overall effect leads to a reduced lattice mismatch as compared to that of the delithiated LiFePO4. This feature is exemplified by the 200 reflection peaks, shown in the insert of Figure 3, but not only limited to the 200-diffraction peak, a feature not observed in LixFePO4 samples of similar particle sizes. The reduced lattice mismatch of the partially delithiated substituted sample is further illustrated by comparing the diffraction patterns of Li0.5Fe0.85V0.1PO4 and Li0.5FePO4 (Figure 4). Clearly, there is a reduced lattice mismatch of 87
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Figure 5. Diffraction patterns of (a) Li 0.9 FePO 4 and (b) Li0.9Fe0.85V0.1PO4, the insert shows the magnified 200 diffraction peaks region of the triphylite and heterosite phases.
substitution effect. We attribute the crystallographic evidence of the extended solid solution and reduced lattice mismatch between the two phases as the factors accounting for the improved rate capabilities observed for the vanadium substitution at the Fe site.13 These Li solubility ranges are higher than that reported for the size-dependent induced solid solution for the 40 nm particle size.3b The kinetics of the reaction of the LiFePO 4 and LiFe0.85V0.1PO4 were evaluated by galvanostatic intermittent titration (GITT). Even though it is designed for studying the chemical diffusion coefficient in solid solution systems, it can provide comparative information of the electrode kinetics of a two-phase system through the voltage relaxation time to equilibrium potential and through the magnitude of overpotential. Figure 6 shows the GITT results for two different samples, LiFePO4 and 10 mol. % vanadium substituted at the Fe site. From the results in parts a and b of Figure 6, the LiFe0.85V0.1PO4 exhibits a lower polarization, 0.03 versus 0.05 V, than LiFePO4 of similar particle sizes. The Fe site substituted sample also shows a narrower equilibrium potential hysteresis than the unsubstituted sample. Parts c and d of Figure 6 illustrate the duration the cell takes to relax to reach an equilibrium potential, the flat region. The substituted sample has much faster kinetics than the pure olivine phase as exemplified by the much shorter relaxation time it takes to reach the equilibrium potential as shown in Figure 6. The open circuit voltage of the unsubstituted Li1 − xFePO4 is slightly lower that of the Li1 − xFe0.85V0.1 at all states of charge. This is shown in part e of Figure 6 where the different cells reach an OCV after long relaxation time characterized by a flat potential. The effect of vanadium substitution of LiFePO4 on electrochemical performance was further demonstrated by the magnitude of overpotentials for cells charged at C/20, within the miscibility gap region. LiFePO4 has a charge overpotential of about 23 mV, the same as reported earlier by Dreyer et al.6 This overpotential is three times higher than that of the Fe-site vanadium substituted LiFePO4 with an overpotential of about 7.7 mV. These results suggest that the lower the overpotential the higher the rates of equilibration and reaction. This is also consistent with the pseudosolid-solution reaction mechanism proposed by Malik et al.5 To further understand the reason for the enhanced rate performance of the vanadium-substituted LiFePO4, we investigated the effect of temperature on the phase trans-
Figure 6. GITT charge discharge curves of the substituted at the Fe site and unsubstituted LiFePO4, a current corresponding to C/20 was applied to the cells for 2 h before a 24 h relaxation: (a) compares the overvoltage of Li1−xFePO4 and Li1−xFe0.85V0.1PO4, (b) the magnified dotted region of (a), (c), and (d) are the voltage−time graph for charge and discharge respectively, (e) the magnified single interval voltage−time graph.
formation. At room temperature in the miscibility gap region, both Li0.5FePO4 and Li0.5Fe0.85V0.1PO4 exist as two phases, the triphylite and the heterosite phases, on heating to higher temperatures these phases transform to single phase as a result of Li+ ions being able to diffuse from the lithium rich to lithium poor phase forming a single phase. For Li0.5FePO4, this single phase is reached at temperatures above 300 °C.4 On introducing 10 mol % vanadium into the structure, the solid solution temperature is lowered to about 200 °C for Li0.5Fe0.85V0.1PO4 this is about 100 °C less than the solid solution temperature for Li0.5FePO4 as shown in Figure 7. Five and 7.5 mol % gave intermediate values. The rectangular boxes indicate the peaks from different phases transforming into single peaks on solid solution formation. The transformation process for these substituted compositions starts at as low as 100 °C, which is much lower temperature than that observed for the unsubstituted counterpart in which the transformation starts at temperatures above 180 °C. This implies that less thermal energy is required for the transformation of the two phase mixture, a process that starts at temperature below 100 °C, by the Li+ ion diffusing from the lithium rich phase to lithium poor phase to a stable solid solution. These temperature studies support the hypothesis5 that a rather small energy is needed to disorder the lithium ions and allow their removal or insertion to occur by a single phase mechanism, LixFePO4, rather than by the movement of an interface between the two phases, Li1 − βFePO4 and LiαPO4. This energy is reduced by substitution on the iron site. 88
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REFERENCES
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Figure 7. In situ XRD patterns of (a) Li 0.5 FePO 4 , (b) Li0.5Fe0.85V0.1PO4 collected under He atmosphere, the rectangular boxes marked I, II, and III are peaks with observable transformations to track the transformation temperatures.
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CONCLUSIONS We have determined why vanadium substitution on the iron site in LiFePO4 increases the rate of lithium removal and insertion into this material. Substitution increases the singlephase regime LixFePO4 at both low and high values of x; it decreases the lattice mismatch between the lithium rich and the lithium poor phases; it decreases the transformation temperature for complete solid solution, and it decreases the overpotential for lithium disorder and reaction. Vanadium substitution also slightly increases the open circuit voltage, the free energy of reaction, of LiFePO4.
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is supported as part of the Northeastern Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0001294. Use of the National Synchrotron Light Source at Brookhaven National Laboratory is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. We also thank Professor Clare Grey, our NECCES intercalation thrust leader, for encouraging us to determine why substitution works in these materials. 89
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