Chem. Mater. 2004, 16, 4867-4869
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Li1+rV3O8 Gel and Xerogel: a New Insight Matthieu Dubarry,† Joe¨l Gaubicher,*,† Dominique Guyomard,† Nathalie Steunou,‡ and Jacques Livage‡ Institut des Mate´ riaux Jean Rouxel, 2 rue de la Houssinie` re, BP 32229, 44322 Nantes Cedex 3, France, and Laboratoire de Chimie de la Matie` re Condense´ e, Universite´ Pierre et Marie Curie-Paris VI, 4 place Jussieu, 75252 Paris Cedex 05 Received May 25, 2004 Revised Manuscript Received September 14, 2004 Lithium vanadium oxide, Li1+RV3O8 (R ) 0.1-0.2) has been extensively studied during the past 20 years for its attractive electrochemical properties in rechargeable lithium batteries.1-3 It provides a fair energy density and a good capacity retention.4,5 Two routes have been mainly used to synthesize these oxides: solid-state reactions2,3,5 and sol-gel syntheses.4,6 In the latter case, a gellike intermediate is prepared via the condensation of solute precursors in aqueous solution. The obtained xerogel has been described as a poorly crystallized layered hydrate Li1+RV3O8‚nH2O. The chemical nature of the gel and xerogel has never been studied and they were assumed to be single-phase materials. Very close materials were also obtained via reaction of Li1+RV3O8 in water.7-9 According to the drying procedure, different hydrated phases (related to different interlayer distances)9 or even mixed phases4,8 were obtained. Neither the structure nor the water content of these xerogels or hydrates has been carefully characterized. The transformation mechanism of the xerogels/hydrates to crystallized anhydrous Li1+RV3O8 upon heating also was never investigated. The lithium insertion behavior of Li1+RV3O8 strongly depends on the firing temperature of the xerogel.4-6,10 Samples prepared upon heating at 580 °C exhibit a stable capacity of 180 mAh/g upon cycling,5 whereas those heated at 350 °C exhibit a larger initial capacity (280 mAh/g) that decreases rapidly upon cycling.5,10 Understanding the chemical nature of the gel precursor * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 0033 2 40 37 39 32. Fax: 0033 2 40 37 39 95. † Institut des Mate ´ riaux Jean Rouxel. ‡ Universite ´ Pierre et Marie Curie-Paris VI. (1) Guyomard, D. In New Trends in Electrochemical Technology: Energy Storage Systems in Electronics; Osaka, T., Matta, D., Eds.; Gordon & Breach: Philadelphia, PA, 2000; Ch. 9, p 253. (2) Nassau, K.; Murphy, D. W. J. Non-Cryst. Solids 1981, 44, 297. (3) Pistoia, G.; Panero, S.; Tocci, M.; Moshtev, R. V.; Manev, V. Solid State Ionics 1984, 13, 311. (4) West, K.; Zachau-Christainsen, B.; Skaarup, S.; Saidi, M. Y.; Barker, J.; Olsen, I. I.; Pynenburg, R.; Koksbang, R. J. Electrochem. Soc. 1996, 143, 820. (5) Jouanneau, S.; Le Gal La Salle, A.; Verbaere, A.; Guyomard, D.; Deschamps, M.; Lascaud, S. Solid State Ionics. In press. (6) Pistoia, G.; Pasquali, M.; Wang, G.; Li, L. J. Electrochem. Soc. 1990, 137, 2365. (7) Scho¨llhorn, R.; Klein-Reesink, F.; Reimold, R. J. Chem. Soc. Chem. Commun. 1979, 398. (8) Manev, V.; Momchilov, A.; Nassalevska, A.; Pistoia, G.; Pasquali, M. J. Power Sources 1995, 54, 501. (9) Kumagai, N.; Yu, A. J. Electrochem. Soc. 1997, 144, 830. (10) Jouanneau, S.; Le Gal La Salle, A.; Verbaere, A.; Deschamps, M.; Lascaud, S.; Guyomard, D. J. Mater. Chem. 2003, 13, 921.
Figure 1. Powder XRD of samples: (a) xerogel, (b) D-SC, and (c) D-LC.
and the chemical processes that lead from the gel to Li1+RV3O8 oxide might suggest new directions to improve its electrochemical properties. The aim of this communication is to report characterization of the nature of the gellike Li1+RV3O8‚nH2O precursor, to give key points of its transformation mechanism to Li1+RV3O8 oxide, and to explain some subsequent characteristics of this latter compound. Li1.1V3O8‚nH2O gels have been prepared as described elsewhere.4,6 The V2O5 powder is mixed in water with a stoichiometric amount of LiOH‚H2O (Li/V ) 0.37) and heated at 50 °C under nitrogen for 24-36 h. A gelatinous precipitate (noted GP) is obtained only for initial vanadium concentrations ranging from 0.75 to 2.25 mol/ L. It is made of an aqueous solution trapped within the porous solid. Centrifugation was then performed at 6400 r/s for 1 h to remove most of the solution. A red precipitate (solid component, noted SC) and a yellow solution (liquid component, noted LC) were obtained. Liquid 51V NMR shows that LC contains [V10O28]6- as expected at pH 4.2. SC and LC were dried overnight at 90 °C along with the pristine gelatinous precipitate. Dried compounds are called D-SC and D-LC, respectively. Room-temperature XRD data were collected in BraggBrentano geometry with a Siemens D5000 diffractometer equipped with a MOXTEK detector, whereas a PSD detector was used for thermodiffractometry experiments. The xerogel XRD pattern (Figure 1a) actually exhibits two distinct components. The first one corresponds to D-SC (Figure 1b) while the other one corresponds to D-LC (Figure 1c). The only difference between the two diagrams is the position of the first peak, which is related to the interlayer distance.4,7 Aging GP for nine months led to a well-crystallized D-SC compound for which the XRD diagram shows close similarity with that of lamellar hewettite group compounds such as CaV6O16‚3H2O (details will be published elsewhere). Corresponding compounds D-SC and D-LC have been noted Li1.1V3O8‚xH2O (0.5 < x < 3.3) (Figure 1b) and Li1.1V3O8‚yH2O (0.5 < y < 1) (Figure 1c), respectively. These results point out both the diphasic nature of Li1+RV3O8‚nH2O xerogels and the structural relationship of each of the components with that of the hewettite family. Upon heating at 580 °C for 10 h, samples that arise either from LC or SC both give pure Li1.1V3O8, as shown by XRD, but with quite different grain size (Figure 2). LC leads to large rodlike particles (Figure 2c) about 10 times longer than those obtained from SC (Figure 2b)
10.1021/cm0491734 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/19/2004
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Figure 2. SEM images showing the morphology of samples heated 10 h at 580 °C: (a) from xerogel, (b) from D-SC, and (c) from D-LC.
Figure 3. Thermodiffractograms of (a) xerogel, (b) D-SC, and (c) D-LC.
(10 µm vs 1 µm). A mixture of small and large particles is observed for the Li1.1V3O8 powder obtained upon heating GP (Figure 2a). We believe that this bimodal character of the grain size should always be observed upon firing a gelatinous precursor of Li1+RV3O8. The formation of the crystalline anhydrous Li1.1V3O8 compound was followed by thermodiffractometry (Figure 3). Two phase transformations are observed. The first one, noted 1, occurs between 120 and 180 °C and corresponds to the partial dehydration of Li1.1V3O8‚xH2O into Li1.1V3O8‚yH2O. The second one, noted 2, corresponds to the transformation of Li1.1V3O8‚yH2O into Li1.1V3O8. It starts at 180 °C and ends up at 250 °C. Separate thermodiffractometry of D-SC (b) and D-LC (c) shows that the peak widths are much larger when obtained from the solid part. As a matter of fact, from single line profile analysis (integral breath method implemented in WinPlotR11), and for a given temperature, Li1.1V3O8 crystallites are approximately four times smaller in the direction perpendicular to the interlayer space for compounds that come from the solid part. This is in agreement with the bimodal grain size observed from SEM and can be ascribed to specificities (kinetics (11) Roisnel, T.; Rodriguez-Carvajal, J. Mater. Sci. Forum 2001, 378-381, 118.
Communications
Figure 4. Specific capacity on cycling in the 3.7-2 V voltage range for samples fired for 10 h at 580 °C: (a) xerogel, (b) D-SC, and (c) D-LC.
and/or mechanisms) of the nucleation and growth processes. Electrochemical measurements were achieved with a Mac-Pile controller (Biologic, Claix, France) using standard Swagelok type cells. The composite electrode was coated on Al disks from a mix of 85% Li1.1V3O8, 10% C (Super P), and 5% PVDF. EC/DMC (2:1) + 1 M LiPF6 (Merck) was used as the electrolyte. Galvanostatic cyclings were obtained using a current that corresponds to the insertion-desinsertion of 1 Li per formula unit within 2.5 and 5 h, respectively, in the 3.7-2 V range. Figure 4 enables a comparison of the cycling behavior of the different compounds. They all exhibit a constant capacity upon cycling like most Li1+RV3O8 samples prepared at 580 °C.5 A significant discrepancy is observed, however, with respect to the discharge capacities: the capacity delivered from Li1.1V3O8 that comes from D-SC is roughly 190 mAh/g (Figure 4b), whereas that of larger grains arising from D-LC is lower (160 mAh/g) (Figure 4c). For comparison, an average value close to 180 mAh/g is obtained for Li1.1V3O8 obtained upon firing the xerogel (Figure 4a). To conclude, this paper shows that the gelatinous precipitate precursor of Li1+RV3O8, called gel in the literature, is actually a diphasic material made of a liquid-phase trapped in a porous solid. The solid red precipitate is an ill-crystallized hewettite-like hydrated phase Li1+RV3O8‚xH2O (0.5 < x < 3.3) that results from the condensation of solute precursors around pH 4. Such a pH is slightly higher than the point of zero charge (pH 2) so that a mixture of neutral [VO(OH)3(OH2)2]0 and anionic [VO(OH)4(OH2)]- precursors should be involved in the condensation process.12 Olation and oxolation reactions lead to a [V3O8]- network that precipitates in the presence of Li+ cations to give the hydrated oxide Li1+RV3O8‚xH2O. The yellow supernatant solution should correspond to solute decavanadate species that give hewettite-like Li1+RV3O8‚yH2O (0.5 < y < 1) upon drying at 90 °C. Decavanadates have been shown to transform into cyclic metavanadates [V4O12]4upon heating the aqueous solution.13 Layered solid phases are currently obtained upon thermohydrolysis around pH 7 and it has been assumed that anionic precursors such as [VO(OH)4(OH2)]- could be involved in the formation of the vanadium oxide network.12,14 In (12) Livage, J. Coord. Chem. Rev. 1998, 178-180, 999. (13) Bouhedja, L.; Steunou, N.; Maquet, J.; Livage, J. J. Solid State Chem. 2001, 162, 315. (14) Chirayil, T.; Zavalij, P. Y.; Whittingham, M. S. Chem. Mater. 1998, 10, 2629.
Communications
our case, neutral precursors should also be involved in the condensation processes leading to a solid network similar to that obtained upon gelation at room temperature. However, with the temperature being higher when the solid is formed at 90 °C from the solution the crystals should be larger. Li1+RV3O8 xerogels are thus biphasic hewettite-like hydrates Li1+RV3O8‚nH2O with different water content, and different grain and crystallite size. Crystallized Li1+RV3O8 compounds obtained after annealing above
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250 °C, then exhibit properties that arise from both parts, i.e., a bimodal grain size distribution and an intermediate capacity. Further work is going on to get a better insight of these mechanisms, the exact nature of the hydrates, the effect of both components on crystallite size as a function of temperature, and their influence on the electrochemical behavior. CM0491734
