Solvothermal synthesis and electrochemical behavior of nanocrystalline cubic Li–Ti–O oxides with cationic disorder

Solid State Ionics 176 (2005) 1877 – 1885 www.elsevier.com/locate/ssi

Solvothermal synthesis and electrochemical behavior of nanocrystalline cubic Li–Ti–O oxides with cationic disorder Dina Fattakhova a, Valery Petrykin b,1, Jirˇ´ı Brus c, Tereza Kostla´nova´ a, Jirˇ´ı Deˇde*ek a, Petr Krtil a,* a

J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ 18223, Prague, Czech Republic b Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, Japan c Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovskeho nam. 2, CZ 16206, Prague, Czech Republic Received 26 July 2004; received in revised form 13 May 2005; accepted 25 May 2005

Abstract Electrochemically active Li – Ti – O oxides of approximate composition LiTi2O4 + d were prepared by solvothermal reaction between TiO2 and lithium hydroxide in water or ethanol at temperatures below 200 -C. This reaction proceeds via dissolution – precipitation mechanism and its course is not sensitive to crystal structure of TiO2 used in synthesis. The product of the hydrothermal process has cubic structure derived from that of spinel (Fd3m), with O atoms in 32e sites, Li in 8a sites and Ti atoms randomly distributed between 16c and 16d positions. The materials prepared in ethanol contain Ti in the 16d position only; on the other hand, Li is distributed between both types of available tetrahedral sites 8a and 8b. All prepared materials are active for Li insertion. Li entering the material during reduction process is preferentially located in 8b position. The presence of Li tetrahedrally coordinated positions indicates that Li insertion leads to a formation of a metastable structure stabilized probably by the nanocrystalline character of the material. D 2005 Elsevier B.V. All rights reserved. Keywords: Solvothermal synthesis; Li insertion; Nanocrystalline spinel

The insertion behavior of the ternary Li –Ti– O oxides has been investigated in connection with their possible applications as electrode materials in lithium/lithium ion batteries [1 –5]. Studies presented so far have been focused on the behavior of Li– Ti– O spinels (Fd3m) (e.g., LiTi2O4 or Li4Ti5O12) [1– 3] and orthorhombic ramsdellite (Pbnm) (Li2Ti3O7) [4,5]. Insertion characteristics, such as specific capacity, diffusion coefficients or charge transfer kinetics [2,3] were determined for both materials by a classical electrochemical approach. The actual mechanism of the Li insertion into Li –Ti– O spinels was studied by diffraction methods [2,3] and 6Li magic angle spinning (MAS) NMR spectroscopy [6]. * Corresponding author. E-mail address: [email protected] (P. Krtil). 1 Present address: Institute of Multidisciplinary Research and Advanced Materials, Tohoku University, Sendai, Japan. 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.05.013

Relatively low number of studies devoted to ternary oxides in comparison with these on the insertion behavior of titanium dioxide can be attributed to relative lack of convenient synthetic routes to prepare active phases in a given ternary system. Spinels and ramsdellites are usually prepared from the oxide (hydroxide) mixtures of corresponding stoichiometry by solid-state reaction at temperatures exceeding 500 -C [2,3] and 1100 -C [4,5,7], respectively. Low-temperature synthesis of electrochemically active Li – Ti –O spinels has not been reported until recently [8]. An alternative route to spinel by annealing hydrothermally prepared precursor was also reported [9]. The hydrothermal processing proved to be useful synthetic instrument in titanium chemistry. It was used to prepare for instance SrTiO3[10], BaTiO3[11], or to control the crystal growth and particle shape in the case of lead titanate [12]. A growth in tetramethylammonium containing solutions at temperatures between 90 and 270 -C was also

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used to control the size distribution, particle shape and selforganization of mesoscopic TiO2 (anatase) [13]. This paper describes the solvothermal preparation of nanocrystalline cubic Li– Ti– O phases with cationic disorder and their electrochemical activity. In particular, we describe the structure of the prepared materials and effect of the used solvent on the structure of the reaction product. The results of conventional electrochemical study combined with 6Li MAS NMR spectroscopy are used to qualify the effect of the cationic disorder on the electrochemical activity towards Li insertion and on the mechanism of the insertion process.

1. Experimental 1.1. Synthesis Nanocrystalline anatase (Bayer PKP 5538) as well as nanocrystalline mixture of rutile and anatase (P25, Degussa) were used in syntheses. The other chemicals in all experiments were of p.a. grade and were obtained from Fluka. Reactions were carried out in poly(tetrafluoroethylene) (PTFE)-lined stainless steel autoclaves containing 15 ml of 0.6 M LiOH solution; pH of the solution was adjusted to 14 using sodium or lithium hydroxide solution. The effect of the solvent on the structure and behavior of the prepared cubic phases was studied by using Millipore Milli-Q deionized water and ethanol with variable water content. Amount of titanium dioxide used in reaction mixture was such as to achieve preset Ti:Li molar ratio in the range between 2:1 and 1:5. All syntheses were carried out at temperatures between 120 and 200 -C. 1.2. Material characterization Crystallinity and phase purity of prepared samples were checked by powder X-ray diffraction using Siemens D8 Advance powder X-ray diffractometer and CuKa radiation. For the structure refinement of the prepared material by Rietveld method, the XRD pattern was collected in the step scanning mode with the step size of 0.02- and accumulation time of 10 s. The refinement has been carried out using the GSAS software package [14]. The distribution and coordination of lithium were determined using 6Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The NMR data were collected at ambient temperature using single pulse experiment with a Bruker Avance 500 MHz Wide Bore spectrometer at 73.61 MHz with 1 M aqueous solution of LiCl as a reference. Typical spectra were recorded by averaging 256 scans with k/2 (1.8 As) or k/6 (0.6 As) pulse with 2 s delay. The samples were spun at frequency of 10 kHz. The shape of the NMR spectra showed negligible dependence on pulse length and discussion of results is restricted to spectra recorded with the pulse length of k/2 only. Crystal morphology and size distribution were examined by field-emission SEM S-4500 (Hitachi). Specific

surface area of prepared materials was determined from the Kr adsorption isotherms on ACCUSORB 2100E instrument (Micromeritics). The samples were degassed at 180 –190 -C for at least 12 h until pressure of 10 4 Pa was attained prior each specific surface area measurement. Thermal analysis of samples was performed on NETZSCH STA 409 TG/DTA apparatus complemented with mass spectrometer QMS 403/4 (Balzers) allowing for qualitative analysis of gases evolved during heating. 1.3. Electrochemical characterization The electrochemical activity of prepared phases was examined by cyclic voltammetry in three-electrode arrangement with Li –Ti –O working electrode and Li counter and reference electrodes. Electrochemical experiments were carried out using PAR263A potentiostat in 1 M solution of Li(CF3SO2)2N (Aldrich) in 1:1 (w/w) mixture of ethylene carbonate (EC) and dimethoxyethane (DME) (both Aldrich). Li – Ti – O electrodes were prepared by mixing powder samples with carbon black (mass fraction of carbon black was about 15%) in dimethylpyrrolidone. Resulting suspension was cast in thin layer on conducting glass (Asahi) substrate and dried at 130 -C in air. The mechanism of the Li insertion was studied using 6Li MAS NMR on samples partially reduced by reaction with stoichiometric amount of butyllithium (BuLi, Fluka) in hexane (p.a. quality Aldrich).

2. Results and discussion Solvothermal treatment of titanium dioxide in Li+ alkaline media led to formation of white solid products. The white colour of the resulting materials indicates that the produced oxide contains only tetravalent Ti. The powder received directly from the solvothermal process is a mixture of a ternary Li –Ti –O oxide with carbonates (lithium or sodium) which are formed during the processing due to the contact with ambient atmosphere. Carbonates cannot be removed along with excess OH during filtration and more robust washing and repetitive centrifugation needs to be implemented to decrease the carbonate content below detection limits of either X-ray diffraction or 6Li NMR spectroscopy. It ought to be noted that, sometimes, the traces of carbonates remain preserved even after centrifugation probably due to its encapsulation in material’s pores. The titania is completely converted to a ternary oxide if the Li:Ti molar ratio in the reaction mixture exceeds 2:1. When the Li:Ti ratio in the reaction mixture drops below 2, one observes a recrystallization of titania from anatase to rutile polymorph [15] rather than a formation of ternary oxide. The rate of the reaction is sufficiently fast above 150 -C, and at 200 -C, one can obtain complete conversion in 90 min. The chemical analysis shows that, regardless of the presence of sodium hydroxide, the product of the solvothermal processing contains only Li, Ti and O. The chemical composition of

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2.1. Structural characterization

Intensity

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b LiO.5TiO2 Lambda 1.5406 A. L–S cycle 230

Hist 1 Obsd. and Diff. Profiles

Counts 0.0

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XRD patterns of the solvothermal reaction products are presented in Fig. 2. An overall weak intensity of the diffraction patterns can be explained by small particle size of the prepared

a

X10E 3 2.0

the product is not affected by reaction temperature nor by Li:Ti ratio in the reaction mixture. The chemical composition of prepared materials is characterized by molar ratio Li:Ti of 0.48 T 0.01 in the case of synthesis in water and 0.55 T 0.01 in the case of the synthesis in ethanol. Similar ratio of lithium to titanium is found, e.g., in LiTi2O4, which represents a limiting composition of the Li – Ti –O spinel [16]. The solvothermal reaction leads to the same reaction products regardless whether rutile or anatase powders were used as starting material. The reaction therefore proceeds most likely via dissolution – precipitation mechanism. The solvothermal reaction products are nanocrystalline powders of cubic shape (see Fig. 1). Prepared materials show relatively narrow distribution of particle size with the average size of ca. 50 nm. This particle size value is consistent to the specific surface area of 52 m2/g determined by BET measurements.

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Fig. 2. (a) X-ray diffraction patterns of solvothermal reaction products prepared in water at 200 -C (1), in ethanol at 120 -C (2), in ethanol at 150 -C (3) and in ethanol at 200 -C (4). The materials were prepared from mixture of Li:Ti ratio 5:1, the duration of synthesis was 12 h. (b) Rietveld analysis of the X-ray diffraction pattern of the solvothermal reaction prepared in water.

material and by generally low scattering intensity of the Li– Ti–O oxides. 2.2. Material prepared in water

Fig. 1. SEM image of the product of hydrothermal synthesis before (a) and after (b) annealing.

In the case of the reaction in water (see pattern (a) in Fig. 2), we observed three resolved broad peaks corresponding to ˚ (2h = 43.8-), 1.48 A ˚ (2h = 63.5-) and 1.21 d spacing of 2.11 A ˚ A (2h = 80.4-). The observed diffraction pattern is not affected by the reaction temperature. Essential feature of the material prepared in water is pronounced diffuse scattering with the maximum at d equal to approximately ˚ (2h approximately 18.5-). This diffraction pattern 4.80 A does not give satisfactory match with any of the Li –Ti –O oxides listed in the PDF database. The main diffraction peaks may be, nevertheless, indexed as belonging to cubic LiTiO2 ˚ , Fm3m) [17]. This oxide should posses a Frock (a å 4.14 A salt_ structure with a random distribution of Li and Ti in the

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metal positions, which ought to be octahedrally coordinated by oxygen. In such a case, the diffuse scattering peaks should be attributed to an amorphous phase, which most likely has a metal – oxygen framework resembling that in the Li –Ti –O spinel. This hypothesis (i.e., major phase of rock salt type and amorphous by-product) is not, however, supported by 6Li MAS NMR spectra. The 6Li MAS NMR spectrum of the material shows a single peak located at 0.33 T 0.02 ppm (see curve (a) in Fig. 3). The presence of a single 6Li signal demonstrates that lithium coordination is highly uniform as can be expected of crystalline material rather than from a mixture containing both crystalline and amorphous phases. The half-width of the NMR peak was about 36 Hz (i.e., ca. 0.5 ppm). This value agrees well with that reported for single coordinated Li in the literature [18]. The position of the 6 Li NMR signal can be assigned to a Li– Ti– O phase, since the position of 6Li in Li2CO3, which also may be present, should be located at 0.15 ppm [18]. The NMR signal appears at the position which can be according to the literature data assigned to tetrahedrally coordinated Li in Li – Ti–O oxide. Since the rock salt structural model excludes the tetrahedral coordination of Li, one may expect that actual structure of the material can be rather derived from a spinel arrangement. In fact, the observed position is in a good agreement with signal generated by Li in 8a position of a Li – Ti –O spinel [6,18]. The apparent deviation of the experimental diffraction pattern from that expected for spinel, i.e., diffuse scattering in the range corresponding to (111) reflection and missing (311) reflection can be attributed most likely to a disorder in Ti– O framework. The refinement of the structure using the common Rietveld method is, however, difficult. This can be ascribed to a complex background and anisotropic broad-

c

ening of the diffraction peaks. Although the Rietveld analysis could not be carried out in usual way (to fit the observed diffraction pattern one would need number of fitting variables exceeding the number of observed X-ray reflections), one can qualify main structural features responsible for weak intensities of (111) and (311) reflections. Extraction of structural factors by Le Bail method discloses extra electronic density in the 16c positions, which are vacant in common spinel structure. At the same time, one can simulate the diffraction pattern similar to the experimental one assuming a disorder in the Ti sublattice with approximately equal occupancies of Ti in 16d (fully occupied in ordered LiTi2O4 spinel) and 16c (vacant in LiTi2O4) positions. For the actual refinement shown in Fig. 2b LiTi2O4 spinel as the starting structural model has been used. To minimize the number of the parameters of the refinement we have selected manually the background far from the reflection positions, these points were fitted with the shifted Chebyschev polynomial; the parameters of the background fit were kept constant during the refinement. In a similar way, the parameters of the pseudo-Voigt profile function have been estimated independently and kept constant during the refinement. Only lattice parameter a, Ti occupancy in 16c and 16d sites and oxygen coordinates varied in the refinement procedure. The oxygen coordinates were fixed during the final circle of the refinement; Ti thermal parameters were refined assuming that thermal parameters of Ti in both 16c and 16d positions are equal. Under these conditions, the Rietveld analysis converged giving the refined lattice ˚ and the Ti occupancy of 16c and parameter a = 8.277 A 16d sites of 0.495 and 0.505, respectively. As follows from Fig. 2b, the final structural model obtained in Rietveld refinement reproduces the positions as well as relative intensity of the diffraction peaks (Rwp = 13.68%, Rp = 9.98%, R(Fsq) = 4.91%, m2 = 1.421). To verify the result, the intensity extraction method has been changed from Rietveld type to model biased (F(calc) weighted) and the refinement yielded essentially the same result. No parameter damping or Marquardt algorithm has been used during the refinement. The proposed structural model is visualized in Fig. 4. 2.3. Material prepared in ethanol

b

a 2

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0

-1

-2

Li shift [ppm] Fig. 3. 6Li MAS NMR spectra of the product of solvothermal reaction in water (a), ethanol (b). Curve (c) shows the material prepared in ethanol after annealing at 400 -C. Synthesis conditions correspond to experiments shown in Fig. 2 (a) and (d). The line was added to guide the eye.

In contrast to the reaction in water, the XRD pattern of the material prepared in ethanol shows pronounced temperature dependence (see patterns (b –d)) of Fig. 2). While the XRD patterns of materials prepared at low temperatures (t < 150 -C) resemble that of the product of reaction in water. The XRD patterns of the product of reaction at higher temperatures (t > 150 -C) bear closer resemblance to that of a spinel. They contain, besides of the diffraction peaks observed also for the hydrothermal reaction product, well-resolved peaks at d of 4.80 and ˚ (2h of approximately 18.5 and 35.1-). Both peaks 2.56 A

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2.5. Material prepared in water

can be indexed assuming cubic face centered unit cell. ˚ corresponding to (111) reflection is The peak at 4.81 A superimposed on a broad diffuse peak similar to that observed for the hydrothermal reaction product. This can be rationalized by the fact that the ethanol used in synthesis was not absolute and contained about 4% (V/V) of water. While the X-ray diffraction indicates significantly smaller disorder in Ti sublattice compared to the product of the reaction in water, the 6Li MAS NMR indicates pronounced disorder of lithium as can be confirmed by presence of two different Li signals (see curve (b) in Fig. 3) located at ca. 0.32 and 0.72 ppm. The comparison of integral intensities for both signals indicates that the majority of the lithium enters the tetrahedrally coordinated site represented by signal at 0.32 ppm. The shifted position of the second lithium signal reflects weaker shielding of the Li nucleus. It can mean that the apparent Li ‘‘coordination number’’ is lower than 4, i.e., the arrangement of oxygen atoms surrounding Li is almost planar due to the strong distortion of the coordination polyhedron, or that there is a distortion caused by cation –cation interaction. The later possibility seems to be more consistent with the observed 6Li NMR spectrum. Placement of Li in, e.g., 8b position should provide the 6Li NMR signal at more positive values of chemical shift due to face sharing of coordination tetrahedron of Li with TiO6 octahedra [6]. The signal of tetrahedrally coordinated Li in 8a position should then remain unaffected.

2.6. Material prepared in ethanol In this case, the observed change of the XRD pattern upon annealing is less significant. Initial endothermic

* *

Intensity

Fig. 4. Structural model of the material prepared by solvothermal reaction in water. The inset shows the site assignment. The structure was drawn ˚. assuming Fd3m symmetry and unit cell parameter of 8.40A

In the case of material prepared in water, the annealing leads to formation of well-resolved peaks corresponding to (111) and (311) reflection of a spinel in place of the diffuse scattering maxima. This process is accompanied with a loss of about 9% of sample’s weight in two distinctive steps. In the first step (t < 200 -C), the material loses mainly water adsorbed on the sample surface (see Fig 6a and b). This initial water removal is accompanied with an endothermic process on the DTA curve with a maximum at 150 -C. The second step proceeds in the temperature range between 200 and 400 -C and is also characterized by release of water from the sample. The overall heat balance of the process is, on the other hand, exothermic. The materials mass remains practically constant upon further heating, although low levels of CO2 due to decomposition of traces of carbonates were detected at temperatures above 300 -C. The abundance of CO2 levels detected by mass spectroscopy is two orders of magnitude lower, than that of water. The observed thermal behavior suggests that the material contains certain amount of crystalline water or protons in the structure. The presence of the protons in the structure seems to be realistic due to geometric reasons. The protons leave the structure during the annealing to temperatures above 200 -C. This process proceeds along with ordering of the Ti sublattice. The annealing has no effect on the NMR spectra. Such behavior is consistent with the proposed structure since the disorder is restricted to Ti– O framework.

2.4. Thermal behavior The annealing of the prepared powders to 250– 300 -C leads to significant changes in XRD pattern, or, in the case of materials made in ethanol, in NMR spectra (see Figs. 3 and 5). In both cases, the structural changes triggered by annealing are accompanied with mass decrease of the material.

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2θ Fig. 5. X-ray diffraction patterns of solvothermal reaction product annealed at 200 -C (a), 300 -C (b), 400 -(c) and 500 -C (d) for 4 h. Stars mark the diffraction pattern of Pt holder.

D. Fattakhova et al. / Solid State Ionics 176 (2005) 1877 – 1885

0

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process of losing water is closely followed by an exothermic removal of carbonate traces at ca. 300 -C (see Fig. 7). The water removal observed in the temperature range 200 – 400 -C for samples prepared in water is in this case suppressed. The ordering processes connected with annealing are reflected by the 6Li MAS NMR spectra (see curve (c) in Fig. 3). The annealing leads to suppression of the NMR signal located at 0.72 ppm corresponding to Li in tetrahedrally coordinated 8b position. The 6Li MAS NMR spectrum of the annealed sample (see curve (c) in Fig. 3) contains single rather asymmetric peak with maximum at 0.3 ppm with slight broadening near 0.12 ppm. This change of the NMR spectrum can be explained by a ordering of Li between tetrahedrally coordinated 8a position and octahedrally coordinated position, probably of 16d type. Such a process conforms to the known fact that the Li in Li – Ti – O spinels is either tetrahedrally or octahedrally coordinated and the only composition which allows for complete accommodation of Li in tetrahedral position is characterized with Li:Ti ratio of 1:2. The annealing has a minor effect on the Ti– O framework.

109 Abundance m/z=18

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Fig. 7. (a) Thermal gravimetry (TG) (solid line) and differential thermal analysis (DTA) (dotted line) curves of the product of solvothermal synthesis in ethanol; heating rate of 5 -C/min in flowing. (b) H2O (solid line) and CO2 (dotted line) evolution mass signals of the product of solvothermal synthesis in ethanol. Data were acquired during TG/DTA experiment at heating rate of 5 -C/min in flowing air.

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2.7. Electrochemical behavior 0

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Temperature / °C Fig. 6. (a) Thermal gravimetry (TG) (solid line) and differential thermal analysis (DTA) (dotted line) curves of the product of hydrothermal synthesis; heating rate of 5 (C/min in flowing air. (b) H2O (solid line) and CO2 (dotted line) evolution mass signals of the product of hydrothermal synthesis in water. Data were acquired during TG/DTA experiment at heating rate of 5 -C/min in flowing air.

Despite the similarity of the structure between the solvothermally produced materials and ordered Li – Ti–O spinel, the electrochemical activity of the materials with cationic disorder significantly differs from that of the ordered spinel (see Fig. 8). The materials with cationic disorder give cyclic voltammograms with broad peaks which are located at more positive potentials with respect to those of ordered spinel. 2.8. Material prepared in water In the case of material prepared by reaction in water, the shape of the voltammogram changes with repetitive cycling. The first reduction of the pristine material proceeds apparently in two steps characterized by two cathodic peaks located at 1.70 and 1.45 V. The subsequent oxidation, on the other hand, proceeds apparently in a single step characterized by anodic peak located at 1.80 V. The more negative cathodic peak is missing in the subsequent cycles. Specific capacity calculated from the charge integrated during the

D. Fattakhova et al. / Solid State Ionics 176 (2005) 1877 – 1885

1.0

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towards Li insertion of oxides prepared in ethanol improves with increasing synthetic temperature. In all cases, we can characterize the Li insertion process by a single cathodic peak located at ca. 1.59 V and single anodic peak at 1.65 V. The specific capacity obtained from voltammetric experiments ranged typically between 118 and 128 mAh/g. The materials also show acceptable cycleability since the coulombic reversibility in the voltammetric experiments never decreased below 90%. The better cycling behavior of the ethanol prepared materials can be attributed to significantly lower disorder in Ti sublattice. 2.10. Mechanism of the Li insertion process

E [V] Fig. 8. Cyclic voltammograms of Li-Ti-O oxides prepared by solvothermal reaction in water (solid line) and ethanol (dashed line). Voltammetric behavior of ordered Li-Ti-O spinel annealed at 500 -C (dotted line) was added for comparison. The voltammograms were obtained in 1M Li(CF3SO2)2N in enthylene carbonate/dimethoxy ethane at polarization rate of 0.1 mV/s.

first cathodic scan is equal to 160 mAh/g. However, the observed specific capacity of the subsequent anodic scan is only 80mAh/g. The low coulombic reversibility improves to c.a. 90% in the subsequent cycles. 2.9. Material prepared in ethanol In the case of the material prepared in ethanol, we observed more favourable electrochemical behavior with respect to that of the material prepared in water. The activity

Intensity

b

Intensity

a

The information necessary to describe the mechanism of the Li insertion process cannot be gathered using electrochemical methods, but can be drawn from 6Li MAS NMR spectra (see Fig. 9) since sensitivity of 6Li NMR spectra towards local Li environment in Li –Ti –O oxides was well established [6,18,20]. The typical positions of 6Li in different local configurations are summarized in Table 1. The 6Li MAS NMR spectra of partially reduced oxides differ significantly from those of completely oxidized materials. Regardless of the degree of reduction, the NMR spectra of partially reduced materials contain four different Li signals located at 0.7, 0.2, 0.3 and 0.7 ppm. The observed peaks show good agreement with the signals reported for Li in 16c, 16d, 8a and 8b position of spinel framework, respectively [2]. The integral intensities of the signals corresponding to Li in different local environments are summarized in Table 2. In our analysis of NMR data, we

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Fig. 9. (a) 6Li MAS NMR spectra of partially reduced Li – T – O oxides prepared by reduction of oxide prepared in water from 30% (lower curve) and 80% (upper curve). (b) 6Li MAS NMR spectra of partially reduced Li – T – O oxides prepared by reduction of oxide prepared in ethanol from 30% (lower curve) and 80% (upper curve).

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Table 1 The chemical shifts of 6Li in MAS NMR spectra as a function of coordination number Compound

CN [LiO]x

Position type

Shift (ppm)

Reference

LiNO3 LiIO3 Li2SO4 LiOH Li2CO3 LiBO2 Li2B4O7 Li4Ti5O12

6 6 4 4 4 4 3 4 4 6 6

6b 2b 4e 2a 8f 4e 16b 8a 8b 16c 16d

1.2 0.1 0.6 +0.33 +1.2 +1.4 +0.3 0.3 0.9 0.7 0.2

[20] [20] [20] [20] [18] [18] [18] [18, 6] [6] [6] [18, 6]

follow the model formulated by Murphy et al. [19] which assumes that (a) the inserted lithium preferentially occupies a vacant lattice position, (b) the charge disbalance caused by reduction is compensated on local level and (c) the system attempts to minimize its energy by minimizing the face sharing of cationic coordination polyhedra.

degree of reduction. The presence of compensating cation in 8b position indicates that the reduction of the structure can be viewed as reduction of Ti in 16d positions since the 16d position represents the Ti in the closest approach with respect to 8b position. It is interesting to note that the fraction of Li in the 8b position of the reduced samples agrees well with amount of Li which should enter the insertion host from solution according to the stoichiometry of the samples. The experimental specific capacity does not contradict the proposed behavior since the above described structural model contains sufficient number of vacant 8b positions. The tendency of the material to relax to a state of lower energy upon reduction is supported by the presence of octahedrally coordinated Li in partially reduced oxides. This phase transition process involves transfer of Li from both types of tetrahedral positions and clearly favours filling the octahedral positions of 16d type. The fact that only smaller fraction of Li in reduced samples is located in octahedral sites indicates that the relaxation of the system to the state with minimum energy is hindered and proceeds only on local level. 2.12. Material prepared in ethanol

2.11. Material prepared in water With respect to proposed occupancy of cationic positions (16c and 16d sites are half occupied by Ti and 8a sites are fully occupied by Li cations), one may expect the inserted Li to occupy either tetrahedrally coordinated positions (8b) or octahedrally coordinated positions (16c or 16d). Although the placement of Li into 8b position represents the closest approach between Li and the reduced Ti, the overall stability of such a structure should be disfavoured due to face sharing of [LiO4] with [TiO6] octahedra. A more favourable structure can be achieved should all the cations be located in octahedrally coordinated positions via a phase transition mechanism. As follows from Table 2, the Li insertion process leads to distribution of Li between octahedral and tetrahedral sites in the structure. The observed decrease of tetrahedrally coordinated Li fraction in partially reduced samples can be connected with transition of Li from 8a position into octahedrally coordinated positions. The NMR spectra indicate that the Li transfers preferentially into 16d position. The transfer of Li from tetrahedral positions is compensated by incorporation of Li into 8b position and the total occupancy of tetrahedrally coordinated positions increases with increasing

In the case of material prepared in ethanol, the reduction proceeds via similar mechanism as in the case of material prepared in water. As follows from Table 1, the lithium entering the material upon reduction is preferentially stored in position 8b. In contrast to the material prepared in water, the phase transition accommodating Li in octahedral positions is less hindered. The analysis of NMR spectra in this case gives less reliable Li occupancy of the different sites probably due to broadening of the observed signal connected with presence of trivalent titanium in partially reduced samples. It needs to be stressed that the reduced material with Li in 8b positions also represents a metastable structure with relatively high degree of cationic repulsions. The resulting structure is probably stabilized by nanocrystalline character of the materials [6].

3. Conclusions Nanocrystalline cubic Li– Ti– O oxides with composition of Li1+x Ti2 x O4+d (x between 0 and 0.1, d between 0.3 and 0.5) were prepared by solvothermal synthesis in water and

Table 2 Relative fractions of Li in available vacant sites in reduced samples Composition

Solvent

8a

8b

16c

16d

LiTi2O4+d Li1.3Ti2O4+d Li1.8Ti2O4+d Li1.1Ti1.9O4+d Li1.3Ti1.9O4+d Li1.9Ti1.9O4+d

Water Water Water Ethanol Ethanol Ethanol

1.000 0.400 T 0.002 0.354 T 0.014 0.850 T 0.040 0.352 T 0.004 0.159 T 0.025

– 0.167 T 0.005 0.287 T 0.009 0.150 T 0.030 0.357 T 0.003 0.546 T 0.059

– 0.128 T 0.003 0.036 T 0.008 – 0.259 T 0.004 0.222 T 0.018

– 0.306 T 0.005 0.321 T 0.019 – 0.035 T 0.007 0.079 T 0.037

The presented data were extracted from 6Li MAS NMR spectra.

D. Fattakhova et al. / Solid State Ionics 176 (2005) 1877 – 1885

ethanol. The prepared oxides have the spinel type structure (Fd3m) with pronounced disorder in Ti or Li sublattice. In the case of material synthesized in water, the disorder occurs in the Ti sublattice when Ti is evenly distributed between 16c and 16d positions. In the oxide prepared in ethanol, the disorder remains restricted to Li sublattice and the Li occupies both 8a and 8b sites. Both disordered nanophases yield a conventional spinel structure upon annealing above 200 -C, i.e., annealing at temperatures above 200 -C is essential for cation ordering of the originally metastable structure. Despite the cationic disorder, all prepared materials are active towards lithium insertion. The lithium entering the structure is preferentially accommodated in tetrahedral position 8b. The resulting structure of the reduced material is a metastable one and is stabilized probably by nanocrystalline character of the material.

Acknowledgement Funding for this project was provided by the Grant Agency of the Czech Republic under contract no 203/03/ 0823 and by the Ministry of Education Youth and Sports of the Czech Republic under contract ME533. The authors wish to thank Professor Masato Kakihana of the Tokyo Institute of Technology for the stimulating discussion. V.P. is grateful to Prof. M. Kakihana for the outstanding mentoring.

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