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Chem. Mater. 2002, 14, 4430-4433
New Battery Cathode Materials: Synthesis, Characterization, and Electrochemical Performance of M1-xV3O8-yFz‚nH2O (M ) NH4, K) Charlie C. Torardi* and C. Roger Miao DuPont Company, Central Research and Development, Experimental Station, Wilmington, Delaware 19880-0356 Received June 5, 2002. Revised Manuscript Received August 13, 2002
The aqueous synthesis and electrochemical properties of the new nanocrystalline material M1-xV3O8-yFz‚nH2O (M ) NH4, K; x ≈ 0.0-0.2; y ≈ 0.0-0.2; z ≈ 0.1-0.2; n ≈ 1) are described. This material is easily prepared by precipitation from aqueous HF-acidified ammonium or potassium vanadate solutions. M1-xV3O8-yFz‚nH2O has been characterized by X-ray powder diffraction, electron microscopy, TGA, chemical analyses, and electrochemical studies. The proposed atomic structure is related to that of (NH4)2V6O16, with V6O16 layers separated by layers of hydrated ammonium cations. The M ) NH4 material exhibits a high Li capacity with good reversibility within the first few charge-discharge cycles. The lithium capacity of an electrode composed of (NH4)0.9V3O7.9F0.1‚nH2O/EPDM/carbon (88/4/8) is 409 (mA h)/g (C/80 rate), and the energy density is ∼1000 (W h)/kg (128-µm-thick cathode, 4-1.5 V versus Li metal anode). The HF concentration, starting vanadate salt, and temperature are important parameters in the synthesis of M1-xV3O8-yFz‚nH2O, with respect to achieving the optimum Li-ion capacity and phase purity. Precipitating M1-xV3O8-yFz‚nH2O in the presence of carbon black gives a composite that maintains the Li capacity and lowers the electrochemical-cell polarization.
Introduction
Experimental Section
Recently, we reported the aqueous synthesis and electrochemical properties of nanocrystalline MxV2O5Ay‚ nH2O from acidified vanadate solutions.1 In this material, M is a cation from the starting vanadate salt (e.g., M ) NH4, Li, or Mg), and A is the anion from the mineral acid (H2SO4, HNO3, or HCl). It is made in high yield and with reproducible composition and lithiuminsertion capacity. The structure and electrochemical properties of MxV2O5Ay‚nH2O are similar to those of xerogel- and aerogel-synthesized V2O5‚nH2O.2 For example, fibrous V2O5‚nH2O xerogel has a lithium capacity of ∼340 (mA h)/g at a C/55 rate,3 and layered MxV2O5Ay‚ nH2O exhibits a capacity of ∼380 (mA h)/g at C/80.1 Compared to the xerogel/aerogel methods, acid precipitation is a simpler and cheaper synthesis procedure, and it yields a product having basically the same electrochemical performance. When the strong mineral acid is replaced with aqueous HF, a different structure and new compositions are obtained. Here, we describe the synthesis, characterization, and electrochemical lithium insertion/removal for this new nanocrystalline material, M1-xV3O8-yFz‚nH2O with M ) NH4 and K.
Synthesis. The general synthesis process for M1-xV3O8-yFz‚nH2O consists of preparing an aqueous ammonium or potassium vanadate salt solution, heating it at ∼70 °C (NH4+) or to a boil (K+), and precipitating the product with aqueous hydrofluoric acid. The vanadium concentration is typically in the range of 0.1-1 M. As an alternative, V2O5 can be combined with aqueous ammonium or potassium hydroxide to form the corresponding vanadate solution. Hydrofluoric acid is added to provide an acid proton-to-vanadium ratio (designated the H/V ratio) in the range of about 1.5:1-3:1. Following washing, drying, grinding, and sieving (-200 mesh), the nominally MV3O8‚nH2O powder is ready for incorporation into an electrode. Other vanadate salts studied were those with M ) Li, Na, Rb, Cs, and Mg, but these starting materials did not give clean M1-xV3O8-yFz‚nH2O products. The results are discussed below. As an example synthesis for M ) NH4, 20.1 g of NH4VO3 was added to ∼850 mL of deionized H2O with stirring by a Teflon-coated magnetic stirring bar in a 1-L Pyrex beaker. The contents of the beaker were heated to the boiling point to form a solution. After the solution had been cooled to room temperature, dilute HF, made by combining 13.5 mL of concentrated HF with 30 mL of H2O, was added to the stirring solution. The H/V ratio was 2.25:1. The contents of the beaker were heated to about 70 °C and stirred at this temperature for 6 h, at which point the beaker was removed from the heat and stirring was discontinued. The orange-brown solids were allowed to settle for 10 min. About 800 mL of supernatant liquid was decanted. The pH of the supernatant liquid, measured with multicolor strip pH paper, was about 2-3. The decanted supernatant liquid was pale yellow in color, indicating the presence of some unprecipitated vanadium. About 850 mL of fresh deionized H2O was added to the precipitate in the beaker, which was then slurried with the water by stirring for about 1 min. The pH of the supernatant wash liquid was about 3. The product was
* Corresponding author: C. C. Torardi, DuPont Company, Central Research and Development, Experimental Station, Wilmington, DE 19880-0356. Phone: (302)695-2236. Fax: (302)695-1664. E-mail:
[email protected]. (1) Torardi, C. C.; Miao, C. R.; Lewittes, M. E.; Li, Z. J. Solid State Chem. 2001, 163, 93. (2) Owens, B. B.; Passerini, S.; Smyrl, W. H. Electrochim. Acta 1999, 45, 215. (3) Giorgetti, M.; Passerini, S.; Smyrl, W. H.; Mukerjee, S.; Yang, X. Q.; McBreen, J. J. Electrochem. Soc. 1999, 146, 2387.
10.1021/cm020620u CCC: $22.00 © 2002 American Chemical Society Published on Web 09/21/2002
High Lithium Capacity M1-xV3O8-yFz‚nH2O
Figure 1. Characteristic X-ray powder diffraction pattern for M1-xV3O8-yFz‚nH2O. collected on filter paper by suction filtration and dried under an infrared lamp for several hours to give 11.5 g of material. An X-ray powder diffraction pattern showed only the broad lines characteristic of the nominal NH4V3O8‚nH2O composition prepared by our HF-precipitation method (Figure 1). The composition (NH4)0.9V3O7.9F0.1‚0.9H2O was calculated from the results of thermogravimetric and chemical analyses and the assumptions that vanadium is pentavalent and that all fluorine substitutes for framework oxygen. Nitrogen was determined by Kjeldahl titration, vanadium by inductively coupled plasma (ICP), and fluorine by gravimetric-titrimetric reaction with CeCl3. TGA, in combination with the nitrogen analysis, was used to determine the amount of water. (In the TGA for M ) NH4, the end of water loss overlaps with the start of ammonium-ion loss. The latter appears to be only a thermal event with no redox chemistry between ammonium and the pentavalent vanadium.) An additional 2.1 g of solid was formed from the original decanted supernatant liquid after it was allowed to stand for 3 days at room temperature. An XPD pattern was identical to that of the first crop. The composition (NH4)0.9V3O7.9F0.2‚0.9H2O was obtained from chemical and TGA analyses. A yield of 76% was calculated from the total product recovered. The sieved powder (-200 mesh) of (NH4)0.9V3O7.9F0.1‚0.9H2O was cast into an electrode sheet (128 µm thick) from which test disks were punched. The disks were transferred to a drybox, assembled into coin cells, and tested as described below. Because the electrode disks were transferred to the drybox in a heated (70 °C) copper block, the water content was lowered, i.e., n < 0.9. We estimate n ≈ 0.1-0.4 for the astested electrode by analogy with MxV2O5Ay‚nH2O coin-cell electrodes.1 The discharge capacity was found to be 409 (mA h)/g at a discharge rate of ∼ C/80. X-ray Powder Diffraction. Room-temperature X-ray powder diffraction data were obtained using a Philips APD3720 or X’Pert PW/3040 powder diffractometer with a theta compensating slit, graphite monochromator, and Cu radiation. Electrochemical Measurements. To make electrodes of the M ) NH4 and K products, 1.5000 g of -200 mesh sieved powder was combined with 0.1364 g of Super P carbon black (MMM S.A. Carbon, Brussels, Belgium) and 1.709 g of a 4 wt % solution of Nordel EPDM (DuPont, ethylene propylene diene monomer) rubber in cyclohexane. An additional 4 mL of cyclohexane was added to improve the flow. The mixture was shaken in a capped glass vial for 15 min on a mechanical shaker to form a cathode paste. The cathode paste was spread onto a sheet of Teflon FEP (DuPont) and drawn down to form a film using a doctor blade having a 15-mil gap. The dried film, consisting of 88 wt % active powder, 4 wt % binder, and 8 wt % carbon black, was hot-pressed through a calender roller between Kapton polyimide sheets (DuPont) under 20 psi at 110 °C. The thickness of the films was typically 115-140 µm. The consolidated electrode films were examined as cathodes against Li metal anodes in coin cells. LiPF6 in EC/DMC (ethylene carbonate/dimethyl carbonate from E. M. Industries) served as the electrolyte solution. A glass fiber separator was used between the electrodes. Disks of cathode (1.40 cm2), anode (∼1.33 cm2), and separator (2.54 cm2) were cut with punches and placed in a heated copper block at 70 °C while pumping
Chem. Mater., Vol. 14, No. 10, 2002 4431 into a drybox. The cathode disks contained ∼0.01 g of active material. In a dry helium atmosphere, electrolyte solution was added to the cathode and separator pieces, which were stacked, along with the Li and a spacer, into a coin-cell pan and sealed under pressure using the 2325 Coin Cell Crimper System manufactured by the National Research Council of Canada. Electrochemical measurements were performed with a Maccor series 4000 tester (Maccor, Inc., Tulsa, OK) and version 3.0 (SP1) software. The coin cells were tested over the range of 4-1.5 V. The measurement of discharge capacity began with a constant current of 0.5 mA. When the voltage reached 1.5 V, the discharge mode was changed to constant voltage, and the current was allowed to slowly decay to 1/10 of the original value, i.e., 0.05 mA. This constant-voltage portion of the discharge, which allows the cell to nearly reach equilibrium, has the effect of reducing the potential drop from the current so that the remaining cell polarization is principally due to the overpotential required for lithium insertion into the cathode material. The discharge capacity is the integrated charge transfer during both the constant-current and constantvoltage portions of the discharge. After discharge, the cell was charged by reversing the above process, using a constant current of 0.5 mA until a maximum voltage of 4.0 V was reached. Again, this voltage was held constant until the voltage decayed to 1/10 of the initial charge current, 0.05 mA. The reversible fraction is the ratio of the first discharge capacity to the first charge capacity. Cycling experiments were performed by repeating the same protocol until sufficient cycles were obtained to predict the cell life (or until a short circuit terminated the test). The end of life criterion was 80% of the capacity of the second cycle of the cycle test.
Results and Discussion Synthesis and Electrochemical Properties. The aqueous synthesis of nanocrystalline M1-xV3O8-yFz‚nH2O (M ) NH4 or K; x ≈ 0.0-0.2; y ≈ 0.0-0.2; z ≈ 0.1-0.2; n ≈ 1) is easily performed by precipitation from HFacidified vanadate solutions. Apparently, the weaker acid (HF) results in the formation of a significantly different soluble vanadium precursor species, compared to the strong mineral acids that ultimately produce MxV2O5Ay‚nH2O.1 The orange-brown color of the title compositions indicates that vanadium is in the +5 oxidation state. Values for x and z were determined experimentally by TGA (for M ) NH4) and chemical analyses on products synthesized with H/V ratios of 1.5:1-3:1. TGA showed that n is ∼1 at ambient temperature and humidity and that water is reversibly removed and inserted up to ∼200 °C. For y, we propose the range 0.0-0.2. If fluoride substitutes for framework oxygen, then y will be greater than zero, but if all fluoride is located elsewhere in the structure, e.g., as HF, then y ) 0 when x ) 0. The M ) NH4 material is prepared cleanly at ∼70 °C in the H/V ratio range 1.5:1-3:1. At higher temperatures, an impurity of (NH4)2V6O16 results, and lower temperatures reduce the product yield. At lower H/V values (e.g., 1:1), the reaction temperature must be lowered to avoid formation of (NH4)2V6O16, and at higher H/V values (e.g., 4:1), MxV2O5Fy‚nH2O is precipitated. For M ) K, running the reaction at the boiling point is preferred. Vanadate salts with M ) Li, Na, Rb, Cs, and Mg gave mixed-phase or different products at room temperature and at boiling with HF at H/V ) 2.5. Lithium, sodium, and magnesium vanadates gave only MxV2O5Fy‚nH2O, rubidium resulted in a considerable amount of Rb2V6O16
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Figure 2. SEM image of a (NH4)1-xV3O8-yFz‚nH2O + 8% carbon composite showing 50-nm-thick crystals of vanadate and 25-nm particles of carbon.
and another unidentified phase, and cesium yielded only Cs2V6O16. Formation of the M2V6O16 compounds under the described synthesis conditions is favored by increasing size of the M cation, i.e., in the order K, NH4, Rb, Cs. A typical X-ray powder diffraction pattern for (NH4)1-xV3O8-yFz‚nH2O is shown in Figure 1. The broad diffraction peaks indicate an average crystallite size range of 25-60 nm. This is consistent with SEM images showing that the product contains porous agglomerates built from bundles of lathe-shaped crystallites (Figure 2). Individual crystallites appear to be up to 2500 nm in length but only 25-100 nm in thickness. The XPD pattern of the ammonium phase is very similar to that given for (NH4)2V6O16‚1.5H2O (JCPDSICDD card no. 51-0376). The latter compound was indexed as monoclinic (a ) 12.34 Å, b ) 3.59 Å, c ) 16.41 Å, β ) 93.3°), but structural details are not known.4 We propose that M1-xV3O8-yFz‚nH2O has a structure related to that of anhydrous (NH4)2V6O16,5 with water molecules inserted into the ammoniumcation layers that alternate with V6O16 layers. The lithium capacity of the M ) NH4 product is 35% greater than that of the M ) K material, e.g., 380-410 (mA h)/g for NH4 vs 280 (mA h)/g for K. Thus, although the products contain about the same amounts of vanadium and H2O, the discharge capacity is strongly influenced by the cation of the vanadium salt used in (4) Oka, Y.; Yao, T.; Sato, S.; Yamamoto, N. J. Solid State Chem. 1998, 140, 219. (5) Range, K.-J.; Eglmeier, C.; Heyns, A. M.; de Waal, D. Z. Naturforsch. B: Chem. Sci. 1990, 45, 31.
Figure 3. Two discharge-charge cycles for (NH4)1-xV3O8-yFz‚nH2O showing initial lithium capacity of 409 (mA h)/g between 4 and 1.5 V at ∼C/80. The constant-current section of the curve is the electrochemical behavior at ∼C/8 (330 (mA h)/g).
the synthesis, an intriguing observation also made for MxV2O5Ay‚nH2O.1 Figure 3 shows the lithium discharge-charge data as a function of voltage for M ) NH4. Lithium intercalates smoothly up to ∼130 (mA h)/g, or ∼0.5 electron per vanadium atom, at which point a structural and/or electronic transition occurs with a concomitant drop in voltage. The initial discharge of 409 (mA h)/g (C/80 rate at 1.5 V) corresponds to ∼1.6 lithium atoms per vanadium atom and an energy density estimate of ∼1000 (W h)/kg. High lithium uptake is attributed to the porous microstructure and the ∼50-nm-thick crystallites of the product (Figure 2). A large polarization is seen on charging, especially in the region where vanadium
High Lithium Capacity M1-xV3O8-yFz‚nH2O
is highly reduced. The “extra” capacity seen in Figure 3 at the end of the charge cycle at 4 V is most likely due to an oxidative degradation reaction between pentavalent vanadium and the solvent and/or binder, as seen for MxV2O5Ay‚nH2O.1 Lowering the operating Vmax to the initial open-circuit voltage of 3.75 V significantly decreases the extent of these oxidation-reduction side reactions. Reversibility is usually >85% for the first two discharge-charge cycles. (NH4)1-xV3O8-yFz‚nH2O Composite with Carbon. In one particular modification of the synthesis, carbon black (MMM Super P) was slurried into the ammonium vanadate solution prior to the addition of the hydrofluoric acid. A finely dispersed powder of carbon black with (NH4)1-xV3O8-yFz‚nH2O was obtained. The amount of carbon black in the product was ∼8 wt %. SEM showed the presence of the carbon (Figure 2) and revealed the microstructure to be similar to that of the carbon-free sample. The composite product exhibits 92% of the discharge capacity of the carbon-free material but with lower polarization evident in the 1.5-2.5 V range (Figure 4). The lower polarization of the composite product is due to improved electronic conductivity at the lower voltage. However, the polarization is still greater than that of MxV2O5Ay‚nH2O coated on carbon.1 Cycle Life. Vanadium oxide compounds are attractive as battery materials because they are easy to synthesize, have relatively low production costs, and good lithium capacity. The primary disadvantage is their poor cycle life. Presently, the cycle life (defined above) for M1-xV3O8-yFz‚nH2O is only 20-30 cycles (4-1.5 V). We discussed several factors that influence cycle life in MxV2O5Ay‚nH2O,1 and these same factors affect the cyclability of M1-xV3O8-yFz‚nH2O. Redox side reactions, occurring mainly at high V, and large struc-
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Figure 4. Two discharge-charge cycles for (NH4)1-xV3O8-yFz‚nH2O + carbon black composite showing a lithium capacity of 375 (mA h)/g between 4 and 1.5 V and lower polarization at lower voltage relative to that of the carbon-free material. The constant-current section of the curve is the electrochemical behavior at ∼C/8 (345 (mA h)/g).
tural changes, resulting from lithium intercalationdeintercalation, need to be eliminated or carefully restrained within the voltage range of interest. Clearly, more work is required to understand and control the cycle life in M1-xV3O8-yFz‚nH2O and other vanadium oxide cathode substances. Acknowledgment. We thank Mark Lewittes and Jay Majeski for the electrochemical cell data, Dennis Redmond and Catherine Foris for X-ray powder diffraction data, Ed Boyes and Keith Warrington for SEM images, and Art Moss for helpful discussions. CM020620U
