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Controllable formation and electrochemical properties of one-dimensional nanostructured spinel Li4Ti5O12
Electrochemistry Communications 7 (2005) 894–899 www.elsevier.com/locate/elecom
Controllable formation and electrochemical properties of one-dimensional nanostructured spinel Li4Ti5O12 Junrong Li a
a,b
, Zilong Tang a, Zhongtai Zhang
a,*
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Beijing Institute of Space Medico-engineering, Beijing 100094, PR China Received 3 May 2005; accepted 15 June 2005 Available online 5 August 2005
Abstract Novel spinel Li4Ti5O12 with nanotubes/nanowires morphology and high surface area has been prepared by a low temperature hydrothermal lithium ion exchange processing from hydrogen titanate nanotubes/nanowires precursors. The shape and morphology of spinel Li4Ti5O12 are controllable by varying the hydrogen titanate precursors (nanotube, nanowire, nanorod and nanobelt) from alkaline-hydrothermal approach. The crystal structure and morphology of the as-prepared lithium titanate nanotubes/nanowires have been investigated by TEM, HRTEM and XRD, respectively. The formation temperature of spinel Li4Ti5O12 nanotubes/nanowires is lower than that of bulk materials counterpart prepared by solid-state reaction or by sol–gel processing. The well reversible cyclic voltammetric results of both electrodes indicate enhanced electrochemical kinetics for lithium insertion. These novel onedimensional nanostructured materials may find promising applications in lithium ion batteries and electrochemical cells. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Spinel Li4Ti5O12; Nanotubes; Nanowires; Controllable formation; Hydrothermal ion exchange; Electrochemical Li-ion insertion
1. Introduction Cubic spinel oxide of lithium titanate has attracted attention as a superior electrode material for energy storage cells as a zero-strain lithium insertion hosts [1,2], because of the extremely small expansion and contraction during the charge and discharge processes (lithium ion insertion and extraction), respectively [3]. Recently, this cubic spinel Li4Ti5O12 also finds versatile applications in asymmetric hybrid electrochemical supercapacitors [4,5], electrochemical generators [6], all-solid-state lithium ion batteries [7,8] and room temperature Li metal batteries [9]. * Corresponding author. Tel.: +8610 6277 2623; fax: +8610 6278 3046. E-mail addresses: [email protected] (J. Li), zzt@tsinghua. edu.cn (Z. Zhang).
1388-2481/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2005.06.012
Regarding previous related work, the spinel oxide Li4Ti5O12 has been synthesized by a high-temperature solid-state reaction of TiO2 and Li2CO3 or LiOH at the temperature of 800–1000 °C [3]. Apparently, this synthetic route will give microcrystalline products. Nanocrystalline Li4Ti5O12 has been prepared by highenergy ball milling of the conventional microcrystalline spinel Li4Ti5O12 prepared by high temperature solidstate reaction [10,11]. The electrochemical performance of the milled product with particles around 600 nm in size, however, is not significantly different from that of the non-milled starting material. Amatucci et al. [12] reported on nanocrystalline Li4Ti5O12 exhibiting a very promising charging rate and cycling stability in a hybrid cell with a supercapacitor-like counter electrode. Sol–gel processing seems to be a viable way to prepare nanocrystalline spinel Li4Ti5O12; however, the expensive organo-metal precursors will prevent its massive
J. Li et al. / Electrochemistry Communications 7 (2005) 894–899
application in energy storage devices [13,14]. Nanocrystalline cubic spinel Li–Ti–O phases were prepared by low temperature hydrothermal reaction of TiO2 and LiOH in water at temperatures between 130 and 200 °C for 20 h, and this low temperature hydrothermal prepared spinel phases show enhanced charge transfer kinetics, in both heterogeneous charge transfer constants and lithium ion diffusion coefficients than corresponding values of conventional spinel Li4Ti5O12 derived from high temperature solid-state reaction [15]. Recently, Gratzel et al. [16,17] has reported that electrode from nanocrystalline spinel lithium titanate exhibit excellent activity towards Li insertion very quickly, even at charging rates as high as 250 °C. So this nanocrystalline spinel Li4Ti5O12 may become promising anode for lithium ion battery or cathode for supercapacitor [18,19]. Titanate nanotubes or nanowires prepared by hydrothermal reaction in concentrated NaOH solution is ionexchangeable by transition metal ions [20,21]. Anatase TiO2 nanotubes can be transformed into perovskite BaTiO3 or SrTiO3 nanotubes by hydrothermal processing [22]. Here, we report the topochemical preparation of nanostructured spinel lithium titanate from hydrogen titanate nanotubes/nanowires precursors by hydrothermal lithium ion exchange processing. Because the shape and dimensional size of hydrogen titanate precursors (nanotube, nanowire, nanorod or nanobelt) are controllable by changing the hydrothermal conditions, hence the spinel Li4Ti5O12 products. The formation mechanism and the electrochemical lithium insertion properties of this nanostructured spinel Li4Ti5O12 are discussed and investigated. The well reversible cyclic voltammetric and lithium insertion cycling performance of both electrodes indicate the nanostructured spinel Li4Ti5O12 may find promising applications in lithium ion batteries and electrochemical cells.
2. Experimental section Nanostructured spinel lithium titanate was prepared from hydrogen titanate nanotubes/nanowires by a simple low temperature hydrothermal ion exchange processing and subsequent heat treatment. Hydrogen titanate nanotubes/nanowires was prepared using a slightly modified previously published processing [23– 25]. Hydrogen titanate nanotubes were prepared from industrial TiO2 powders via sonochemical–hydrothermal reaction in concentrated NaOH solution (10 M) at 120–140 °C for 24–48 h [26], higher alkaline concentration (15 M) and temperature (150–170 °C) and longer reaction time (>72 h) would result in nanowires counterpart. The white product was neutralized by diluted HNO3, including aging in acid condition for at least 8 h to ensure fully acidified product, then washed by
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de-ionized water for several times. Finally, the product was dried in a vacuum drier at 80 °C for 3 h. In a typical procedure for preparing spinel lithium titanate nanotubes/nanowires, 2.0 g hydrogen titanate nanotubes or nanowires powder was mixed with a 40 mL, 0.2 M LiOH aqueous solution in a cone flask, (all the de-ionized water used here had been freshly degassed with ultrasonic irradiation for at least 5 min for removing dissolved CO2), stirred for 30 min, the resulting suspension was transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 120 °C for 24–36 h. The resulting white precipitate was filtered, washed with de-ionized water and ethanol for several times, respectively. Then the powder was separated from the washing solution by suction filter, dried in a vacuum at 80 °C for at least 3 h, finally calcined at 300–500 °C for 2–6 h. The morphology and the structure of the as prepared spinel lithium titanate nanotubes/nanowires was characterized by transmission electron microscopy (TEM, Hitachi H-800 equipped with Energy Dispersive X-ray Detector), field emitting gun scanning electron microscopy (FEG-SEM, JEOL JSM-6301 F) and X-ray diffraction measurement (XRD, Rigaku, D/max-RB ˚ ), respectively. using Cu Ka radiation with k = 1.5418 A High resolution TEM was performed on Philips TECNAI F30 with acceleration voltage of 300 kV. The BET specific surface area was measured by N2 absorption–desorption method at liquid nitrogen temperature on Quantachrome NOVA 4000. Electrochemical properties were measured on electrodes prepared using mixtures comprising 80 wt% active material, 10–12 wt% acetylene black, and 10–8 wt% polyvinylidene fluoride (PVDF) binder. The electrode films were fabricated by doctor blade technique on aluminum foil. The cells consisted of the electrode, a lithium metal counter electrode and the electrolyte of a 1 M solution of LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC 1:1 v/v; Merck). The cells were constructed and handled in an Ar-filled glove box and were evaluated using coin-type cells (CR2032). The galvanostatic discharge/charge tests were carried out using a LAND Celltest-2001A (Wuhan, China) system between 2.5 and 1 V vs. the Li counter electrode. Cyclic voltammogram was recorded from 3.0 to 1.0 V at a scan rate of 0.05 mV/s, using IM6e electrochemical workstation (Germany), with electrolyte of 1 M solution of LiPF6 in EC/DMC (1:1 v/v; Merck) and counter and reference electrode of Li metal disk.
3. Results and discussion Representative microstructures of prepared lithium titanate nanotubes and nanowires are presented in Fig. 1. The TEM micrograph in Fig. 1(a) shows the
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Fig. 1. (a) TEM micrograph of hydrothermal (120 °C for 24 h) lithium ion exchanged lithium titanate nanotubes before heat treatment. (b) Sample (a) after heat treatment at 350 °C for 2 h in air atmosphere. (c) FEG-SEM micrograph of hydrothermal (120 °C for 24 h) lithium ion exchanged lithium titanate nanowires before heat treatment. (d) A single nanowire TEM micrograph of sample (c) after heat treatment at 500 °C for 2 h in air atmosphere. (e) HRTEM microstructure of sample (d), up-right insert is electron diffraction pattern.
typical hydrothermal lithium ion exchanged nanotubes product before heat treatment, while Fig. 1(b) reveals lithium titanate nanotubes after heat treatment at 350 °C for 2 h in air atmosphere. Examinations of those images show that the lithium ion exchanged products keep almost the same morphology with raw hydrogen titanate nanotubes precursors [26]; furthermore, the
calcined product can keep nanotube like framework well. The outer diameter of lithium titanate nanotubes is about 10 nm, and their lengths varies from hundreds nm to over several lm. Fig. 1(c) is FEG-SEM microgram of lithium titanate nanowires before heat treatment. It showed that the lithium titanate nanowires also keep the similar morphology to hydrogen titanate nanowires precursor [27], which indicates that they can be successfully transformed from their hydrogen titanate nanowires precursor by topochemical hydrothermal ion exchange process. Figs. 1(d) and (e) are TEM and HRTEM microstructures of a single spinel lithium titanate nanowire after heat treatment at 500 °C for 2 h, respectively. The spinel lithium titanate nanowires seem the accumulation of nanocrystalline fine particles along the axes direction of nanowires. The high-resolution electron microscopy results show that the d1 1 1 spacing of 0.48 nm is well in accordance with the bulk spinel Li4Ti5O12, which indicate the well-crystallized spinel phase in these one dimensional nanostructured materials prepared from low temperature heat treatment. Considering the poor heat resistance of hydrogen titanate nanotubes [21], this result, on the other hand, implies that the lithium ion exchange product shows improved heat resistance stability than their hydrogen titanate nanotubes precursors. The plausible reason may lie in the lithium atom substituting the hydrogen atom sites can avoid the hydrogen titanate decomposed into anatase TiO2 and H2O [21], thus avoid collapsing of nanotube framework. The EDX and XRF measurement results both show that there is no residual Na+ detected both in lithium titanate nanotubes and their hydrogen titanate nanotubes precursor, this indicates that the acid treatment of raw sodium titanate nanotubes could effectively substitute the sodium by hydrogen. This acid treated hydrogen titanate nanotubes precursor ensures the preparation of phase pure lithium titanate nanotubes by hydrothermal lithium ion exchange processing. The BET surface test results show that the specific surface area of this lithium titanate nanotubes and nanowires are 237.6 and 126.7 m2g 1, respectively, which is comparable with annealed products of hydrogen titanate nanotubes and nanowires at corresponding temperature. Fig. 2 is a typical X-ray diffraction patterns of hydrothermal lithium ion exchanged hydrogen titanate nanotubes (b) and nanowires (c) after heat treatment with the comparison of standard XRD pattern of spinel Li4Ti5O12 (a). All diffraction peaks of both samples can be indexed as spinel lithium titanate(cubic phase, space group Fd 3m) in accordance with spinel Li4Ti5O12 (JCPDS Card No. 49-0207). The unit cell parameters for spinel lithium titanate nanotubes have been deter˚ , in agreement with bulk cubic matemined to be 8.378 A rials. The broadening of the diffraction pecks is ascribed to nanoscale structure of the material. There is trace
J. Li et al. / Electrochemistry Communications 7 (2005) 894–899
Fig. 2. XRD patterns of: (a) standard spinel Li4Ti5O12 PDF card #490207; (b) Li4Ti5O12 nanotubes by lithium ion exchanging at 120 °C for 24 h and calcined at 350 °C for 2 h; (c) Li4Ti5O12 nanowires by lithium ion exchanging at 120 °C for 24 h and calcined at 500 °C for 2 h.
anatase TiO2 detected in spinel Li4Ti5O12 nanotubes marked by asterisk, this trace impurity may origin from its hydrogen titanate nanotubes samples. Comparing the monoclinic hydrogen titanate nanotubes/nanowires, the spinel lithium titanate is cubic lattice, which indicate that there are some structural transformation between the hydrogen titanate precursors and lithium ion exchanged samples. The lithium ion exchange processing was performed at different conditions to further investigate this transformation. Fig. 3 is a typical X-ray diffraction pattern of lithium ion exchanged hydrogen titanate nanotubes at different temperature and different pH conditions after heat treatment. The XRD results show that only hydrothermal (120 °C) lithium ion exchanging in basic conditions can result in cubic spinel lithium titanate products (as
Fig. 3. XRD pattern of calcined samples at 350 °C for 2 h with different lithium ion exchange conditions: (a) lithium ion exchange was performed at 60 °C with pH value of 14; (b) lithium ion exchanged was performed at 120 °C with pH value of 14; (c) lithium ion exchange was performed at 120 °C with pH value of 6.
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showed in Fig. 3(b)). The lower lithium ion exchanging temperature (such as 60 °C) cannot prepare phase pure spinel lithium titanate nanotubes, but a mixture of anatase TiO2 and spinel lithium titanate as showed in Fig. 3(a). Furthermore, the prolonging of reaction time cannot reach phase pure spinel lithium titanate yet. The lower pH conditions, however, result in phase pure anatase TiO2, as showed in Fig. 3(c), which is coincident with the results of Li et al. [21]. These results indicate that the temperature and pH value of solution during lithium ion exchange processing are key factors for converting hydrogen titanate nanotubes/nanowires into cubic spinel lithium titanate nanotubes/nanowires. The spinel compounds have the generic formula AB2X4, in the spinel structure the X atoms are close packed and the A atoms occupy 1/8 of the tetrahedral interstices and the B atoms occupy 1/2 of the octahedral voids. For spinel Li4Ti5O12 (also be formulated as Li[Li1/3Ti5/3]O4), one Li atoms occupy the tetrahedral interstices, while other 1/3 Li atoms and 5/3 Ti atoms occupy the octahedral voids together. So the hydrothermal lithium ion exchange processing may help migration of lithium atom from tetrahedral interstices into octahedral voids, thus the hydrothermal lithium ion exchange processing leads to a phase change as showed in Fig. 2(c). So the hydrothermal condition is necessary to prepare spinel Li4Ti5O12 nanotubes/nanowires by ion exchange processing from hydrogen titanate nanotubes/nanowires precursors. In the mean time, Li+ has the equivalent ionic radius with Ti4+, ˚ , which makes the structure stable after about 0.68 A Li+ ion diffusion into Ti4+ sites (octahedral voids) in lithium titanate nanotubes/nanowires. On the contrary, ˚ , which is much the ionic radius of Na+ is about 0.97 A larger than the radius of Ti4+, so the sodium titanate nanotubes are difficult to be directly transformed into spinel structure analogue like Li4Ti5O12 [21]. Anyway, future work will involve the detailed mechanistic studies of the formation and growth of cubic spinel lithium titanate from monoclinic hydrogen titanate precursors, which may help to understand the precise composition and structure of hydrogen titanate nanotubes/nanowires derived from alkali-hydrothermal approaches that difficult to analysis directly by chemical and physical methods [28,29]. Fig. 4 is typical cyclic voltammogram of electrode made from spinel Li4Ti5O12 nanotubes and nanowires, respectively. The lithium insertion into and extraction from these electrodes are both reversible. The absolute values of specific peak current of anodic branch (lithium ion extraction) and cathodic branch (lithium ion insertion) are comparable, which is different from that of bulk spinel Li4Ti5O12 electrode, where the specific peak currents are usually asymmetric [30]. The difference in peak currents between anodic and cathodic branches is mainly attribute to the slow lithium ion diffusivity in
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Fig. 5. Voltage profile of spinel Li4Ti5O12 nanowires at different discharge/charge rates.
Fig. 4. Cyclic voltammogram of spinel Li4Ti5O12 nanotubes (a) and nanowires (b) in 1 M LiPF6 + EC/DMC (1/1, v/v) at scan rate of 0.05 mV/s. The geometric surface area of electrode is 0.83 cm2.
solid-state body of bulk spinel Li4Ti5O12. Almost the same peak currents in electrode made from spinel Li4Ti5O12 nanotubes or nanowires, however, may indicate the improved kinetics performance of these nanostructured electrode materials. The shoulder peaks in the spinel Li4Ti5O12 nanotubes may be ascribed to the trace anatase TiO2 impurity as showed in the XRD pattern, however, this trace impurity is not so obvious in nanowires counterpart. The trace anatase TiO2 impurity may exist its hydrogen titanate nanotubes precursors when the raw anatase TiO2 has not been fully transformed into hydrogen titanate nanotubes during the alkaline-hydrothermal process. Because of the higher hydrothermal temperature and longer reaction time when preparing hydrogen titanate nanowires than that of nanotubes, there is no obvious anatase detected in hydrogen titanate nanowires precursor and spinel Li4Ti5O12 nanowires product. So it is necessary to optimize the reaction conditions when preparing phase pure hydrogen titanate nanotubes precursor to ensure phase pure spinel Li4Ti5O12 nanotubes. Fig. 5 shows the typical voltage profile of nanostructured electrode made from spinel Li4Ti5O12 nanowires at different discharge/charge rates. This voltage profile is similar to that of normal spinel Li4Ti5O12 powders,
however, the superior rate capability in spinel Li4Ti5O12 nanowires is observed. This improved rate capability of lithium insertion may be ascribed to the reduced dimensional size mitigated the influence of slow lithium ion diffusivity in solid body, hence enhanced electrochemical kinetics. Fig. 6 shows the cycling performance of lithium insertion into and extraction from spinel Li4Ti5O12 nanowires. It clearly indicates the excellent cycling stability of as prepared spinel Li4Ti5O12 nanowires, which may imply that the well-crystallized spinel phase can be achieved at relative low temperature. Owing to the shape and morphology are controllable of one dimensional hydrogen titanate precursors (such as nanotube, nanowire, nanorod and nanobelt) by varying the reaction conditions, so the shape and morphology of spinel Li4Ti5O12 are also controllable by this topochemical transformation of hydrothermal lithium ion exchanging approaches. This spinel Li4Ti5O12 with different shape and morphology may become promising lithium insertion materials and may find versatile applications in high performance hybrid electrochemical cells and lithium ion batteries.
Fig. 6. Galvanostatic cycling performance of spinel Li4Ti5O12 nanowires at different discharge/charge rates.
J. Li et al. / Electrochemistry Communications 7 (2005) 894–899
4. Conclusions In summary, we have succeeded in controllable synthesizing of spinel lithium titanate nanotubes/nanowires with uniform morphology and high specific surface area by means of topochemical hydrothermal ion exchange reaction from hydrogen titanate nanotubes/nanowires precursors. The formation temperature of this cubic spinel Li4Ti5O12 is greatly lower than that of solid-state reaction and sol–gel approaches. Because of the versatile shape and morphology of alkali-hydrothermal derived hydrogen titanate nano-materials, this ion-exchange methodology opens a new controllable way to prepare nanostructured spinel Li4Ti5O12 materials with different shape and morphology. The cyclic voltammetric results of both electrodes indicate enhanced electrochemical kinetics for lithium insertion by reducing the distance of lithium ion diffusion in solid-state body. The electrochemical lithium insertion cycling tests show excellent cycling stability and improved rate capability, which make these novel one-dimensional nanostructured materials become promising anode materials for lithium ion batteries and cathode materials for electrochemical supercapacitors.
Acknowledgements This work was supported by National High Tech Research & Development Programme (Grant No. 2003AA302320) and National Natural Science Funds of China (Grant Nos. 50372033 and 50472005), as well as the Basic Research Funds of Tsinghua University (Grant No. JC2003040). We also thank Dr. Wang, R. M. (Electron Microscopy Laboratory and Department of Physics, Peking University) for his help with the HRTEM characterization.
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Controllable formation and electrochemical properties of one-dimensional nanostructured spinel Li4Ti5O12 Junrong Li a
a,b
, Zilong Tang a, Zhongtai Zhang
a,*
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Beijing Institute of Space Medico-engineering, Beijing 100094, PR China Received 3 May 2005; accepted 15 June 2005 Available online 5 August 2005
Abstract Novel spinel Li4Ti5O12 with nanotubes/nanowires morphology and high surface area has been prepared by a low temperature hydrothermal lithium ion exchange processing from hydrogen titanate nanotubes/nanowires precursors. The shape and morphology of spinel Li4Ti5O12 are controllable by varying the hydrogen titanate precursors (nanotube, nanowire, nanorod and nanobelt) from alkaline-hydrothermal approach. The crystal structure and morphology of the as-prepared lithium titanate nanotubes/nanowires have been investigated by TEM, HRTEM and XRD, respectively. The formation temperature of spinel Li4Ti5O12 nanotubes/nanowires is lower than that of bulk materials counterpart prepared by solid-state reaction or by sol–gel processing. The well reversible cyclic voltammetric results of both electrodes indicate enhanced electrochemical kinetics for lithium insertion. These novel onedimensional nanostructured materials may find promising applications in lithium ion batteries and electrochemical cells. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Spinel Li4Ti5O12; Nanotubes; Nanowires; Controllable formation; Hydrothermal ion exchange; Electrochemical Li-ion insertion
1. Introduction Cubic spinel oxide of lithium titanate has attracted attention as a superior electrode material for energy storage cells as a zero-strain lithium insertion hosts [1,2], because of the extremely small expansion and contraction during the charge and discharge processes (lithium ion insertion and extraction), respectively [3]. Recently, this cubic spinel Li4Ti5O12 also finds versatile applications in asymmetric hybrid electrochemical supercapacitors [4,5], electrochemical generators [6], all-solid-state lithium ion batteries [7,8] and room temperature Li metal batteries [9]. * Corresponding author. Tel.: +8610 6277 2623; fax: +8610 6278 3046. E-mail addresses: [email protected] (J. Li), zzt@tsinghua. edu.cn (Z. Zhang).
1388-2481/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2005.06.012
Regarding previous related work, the spinel oxide Li4Ti5O12 has been synthesized by a high-temperature solid-state reaction of TiO2 and Li2CO3 or LiOH at the temperature of 800–1000 °C [3]. Apparently, this synthetic route will give microcrystalline products. Nanocrystalline Li4Ti5O12 has been prepared by highenergy ball milling of the conventional microcrystalline spinel Li4Ti5O12 prepared by high temperature solidstate reaction [10,11]. The electrochemical performance of the milled product with particles around 600 nm in size, however, is not significantly different from that of the non-milled starting material. Amatucci et al. [12] reported on nanocrystalline Li4Ti5O12 exhibiting a very promising charging rate and cycling stability in a hybrid cell with a supercapacitor-like counter electrode. Sol–gel processing seems to be a viable way to prepare nanocrystalline spinel Li4Ti5O12; however, the expensive organo-metal precursors will prevent its massive
J. Li et al. / Electrochemistry Communications 7 (2005) 894–899
application in energy storage devices [13,14]. Nanocrystalline cubic spinel Li–Ti–O phases were prepared by low temperature hydrothermal reaction of TiO2 and LiOH in water at temperatures between 130 and 200 °C for 20 h, and this low temperature hydrothermal prepared spinel phases show enhanced charge transfer kinetics, in both heterogeneous charge transfer constants and lithium ion diffusion coefficients than corresponding values of conventional spinel Li4Ti5O12 derived from high temperature solid-state reaction [15]. Recently, Gratzel et al. [16,17] has reported that electrode from nanocrystalline spinel lithium titanate exhibit excellent activity towards Li insertion very quickly, even at charging rates as high as 250 °C. So this nanocrystalline spinel Li4Ti5O12 may become promising anode for lithium ion battery or cathode for supercapacitor [18,19]. Titanate nanotubes or nanowires prepared by hydrothermal reaction in concentrated NaOH solution is ionexchangeable by transition metal ions [20,21]. Anatase TiO2 nanotubes can be transformed into perovskite BaTiO3 or SrTiO3 nanotubes by hydrothermal processing [22]. Here, we report the topochemical preparation of nanostructured spinel lithium titanate from hydrogen titanate nanotubes/nanowires precursors by hydrothermal lithium ion exchange processing. Because the shape and dimensional size of hydrogen titanate precursors (nanotube, nanowire, nanorod or nanobelt) are controllable by changing the hydrothermal conditions, hence the spinel Li4Ti5O12 products. The formation mechanism and the electrochemical lithium insertion properties of this nanostructured spinel Li4Ti5O12 are discussed and investigated. The well reversible cyclic voltammetric and lithium insertion cycling performance of both electrodes indicate the nanostructured spinel Li4Ti5O12 may find promising applications in lithium ion batteries and electrochemical cells.
2. Experimental section Nanostructured spinel lithium titanate was prepared from hydrogen titanate nanotubes/nanowires by a simple low temperature hydrothermal ion exchange processing and subsequent heat treatment. Hydrogen titanate nanotubes/nanowires was prepared using a slightly modified previously published processing [23– 25]. Hydrogen titanate nanotubes were prepared from industrial TiO2 powders via sonochemical–hydrothermal reaction in concentrated NaOH solution (10 M) at 120–140 °C for 24–48 h [26], higher alkaline concentration (15 M) and temperature (150–170 °C) and longer reaction time (>72 h) would result in nanowires counterpart. The white product was neutralized by diluted HNO3, including aging in acid condition for at least 8 h to ensure fully acidified product, then washed by
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de-ionized water for several times. Finally, the product was dried in a vacuum drier at 80 °C for 3 h. In a typical procedure for preparing spinel lithium titanate nanotubes/nanowires, 2.0 g hydrogen titanate nanotubes or nanowires powder was mixed with a 40 mL, 0.2 M LiOH aqueous solution in a cone flask, (all the de-ionized water used here had been freshly degassed with ultrasonic irradiation for at least 5 min for removing dissolved CO2), stirred for 30 min, the resulting suspension was transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 120 °C for 24–36 h. The resulting white precipitate was filtered, washed with de-ionized water and ethanol for several times, respectively. Then the powder was separated from the washing solution by suction filter, dried in a vacuum at 80 °C for at least 3 h, finally calcined at 300–500 °C for 2–6 h. The morphology and the structure of the as prepared spinel lithium titanate nanotubes/nanowires was characterized by transmission electron microscopy (TEM, Hitachi H-800 equipped with Energy Dispersive X-ray Detector), field emitting gun scanning electron microscopy (FEG-SEM, JEOL JSM-6301 F) and X-ray diffraction measurement (XRD, Rigaku, D/max-RB ˚ ), respectively. using Cu Ka radiation with k = 1.5418 A High resolution TEM was performed on Philips TECNAI F30 with acceleration voltage of 300 kV. The BET specific surface area was measured by N2 absorption–desorption method at liquid nitrogen temperature on Quantachrome NOVA 4000. Electrochemical properties were measured on electrodes prepared using mixtures comprising 80 wt% active material, 10–12 wt% acetylene black, and 10–8 wt% polyvinylidene fluoride (PVDF) binder. The electrode films were fabricated by doctor blade technique on aluminum foil. The cells consisted of the electrode, a lithium metal counter electrode and the electrolyte of a 1 M solution of LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC 1:1 v/v; Merck). The cells were constructed and handled in an Ar-filled glove box and were evaluated using coin-type cells (CR2032). The galvanostatic discharge/charge tests were carried out using a LAND Celltest-2001A (Wuhan, China) system between 2.5 and 1 V vs. the Li counter electrode. Cyclic voltammogram was recorded from 3.0 to 1.0 V at a scan rate of 0.05 mV/s, using IM6e electrochemical workstation (Germany), with electrolyte of 1 M solution of LiPF6 in EC/DMC (1:1 v/v; Merck) and counter and reference electrode of Li metal disk.
3. Results and discussion Representative microstructures of prepared lithium titanate nanotubes and nanowires are presented in Fig. 1. The TEM micrograph in Fig. 1(a) shows the
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Fig. 1. (a) TEM micrograph of hydrothermal (120 °C for 24 h) lithium ion exchanged lithium titanate nanotubes before heat treatment. (b) Sample (a) after heat treatment at 350 °C for 2 h in air atmosphere. (c) FEG-SEM micrograph of hydrothermal (120 °C for 24 h) lithium ion exchanged lithium titanate nanowires before heat treatment. (d) A single nanowire TEM micrograph of sample (c) after heat treatment at 500 °C for 2 h in air atmosphere. (e) HRTEM microstructure of sample (d), up-right insert is electron diffraction pattern.
typical hydrothermal lithium ion exchanged nanotubes product before heat treatment, while Fig. 1(b) reveals lithium titanate nanotubes after heat treatment at 350 °C for 2 h in air atmosphere. Examinations of those images show that the lithium ion exchanged products keep almost the same morphology with raw hydrogen titanate nanotubes precursors [26]; furthermore, the
calcined product can keep nanotube like framework well. The outer diameter of lithium titanate nanotubes is about 10 nm, and their lengths varies from hundreds nm to over several lm. Fig. 1(c) is FEG-SEM microgram of lithium titanate nanowires before heat treatment. It showed that the lithium titanate nanowires also keep the similar morphology to hydrogen titanate nanowires precursor [27], which indicates that they can be successfully transformed from their hydrogen titanate nanowires precursor by topochemical hydrothermal ion exchange process. Figs. 1(d) and (e) are TEM and HRTEM microstructures of a single spinel lithium titanate nanowire after heat treatment at 500 °C for 2 h, respectively. The spinel lithium titanate nanowires seem the accumulation of nanocrystalline fine particles along the axes direction of nanowires. The high-resolution electron microscopy results show that the d1 1 1 spacing of 0.48 nm is well in accordance with the bulk spinel Li4Ti5O12, which indicate the well-crystallized spinel phase in these one dimensional nanostructured materials prepared from low temperature heat treatment. Considering the poor heat resistance of hydrogen titanate nanotubes [21], this result, on the other hand, implies that the lithium ion exchange product shows improved heat resistance stability than their hydrogen titanate nanotubes precursors. The plausible reason may lie in the lithium atom substituting the hydrogen atom sites can avoid the hydrogen titanate decomposed into anatase TiO2 and H2O [21], thus avoid collapsing of nanotube framework. The EDX and XRF measurement results both show that there is no residual Na+ detected both in lithium titanate nanotubes and their hydrogen titanate nanotubes precursor, this indicates that the acid treatment of raw sodium titanate nanotubes could effectively substitute the sodium by hydrogen. This acid treated hydrogen titanate nanotubes precursor ensures the preparation of phase pure lithium titanate nanotubes by hydrothermal lithium ion exchange processing. The BET surface test results show that the specific surface area of this lithium titanate nanotubes and nanowires are 237.6 and 126.7 m2g 1, respectively, which is comparable with annealed products of hydrogen titanate nanotubes and nanowires at corresponding temperature. Fig. 2 is a typical X-ray diffraction patterns of hydrothermal lithium ion exchanged hydrogen titanate nanotubes (b) and nanowires (c) after heat treatment with the comparison of standard XRD pattern of spinel Li4Ti5O12 (a). All diffraction peaks of both samples can be indexed as spinel lithium titanate(cubic phase, space group Fd 3m) in accordance with spinel Li4Ti5O12 (JCPDS Card No. 49-0207). The unit cell parameters for spinel lithium titanate nanotubes have been deter˚ , in agreement with bulk cubic matemined to be 8.378 A rials. The broadening of the diffraction pecks is ascribed to nanoscale structure of the material. There is trace
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Fig. 2. XRD patterns of: (a) standard spinel Li4Ti5O12 PDF card #490207; (b) Li4Ti5O12 nanotubes by lithium ion exchanging at 120 °C for 24 h and calcined at 350 °C for 2 h; (c) Li4Ti5O12 nanowires by lithium ion exchanging at 120 °C for 24 h and calcined at 500 °C for 2 h.
anatase TiO2 detected in spinel Li4Ti5O12 nanotubes marked by asterisk, this trace impurity may origin from its hydrogen titanate nanotubes samples. Comparing the monoclinic hydrogen titanate nanotubes/nanowires, the spinel lithium titanate is cubic lattice, which indicate that there are some structural transformation between the hydrogen titanate precursors and lithium ion exchanged samples. The lithium ion exchange processing was performed at different conditions to further investigate this transformation. Fig. 3 is a typical X-ray diffraction pattern of lithium ion exchanged hydrogen titanate nanotubes at different temperature and different pH conditions after heat treatment. The XRD results show that only hydrothermal (120 °C) lithium ion exchanging in basic conditions can result in cubic spinel lithium titanate products (as
Fig. 3. XRD pattern of calcined samples at 350 °C for 2 h with different lithium ion exchange conditions: (a) lithium ion exchange was performed at 60 °C with pH value of 14; (b) lithium ion exchanged was performed at 120 °C with pH value of 14; (c) lithium ion exchange was performed at 120 °C with pH value of 6.
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showed in Fig. 3(b)). The lower lithium ion exchanging temperature (such as 60 °C) cannot prepare phase pure spinel lithium titanate nanotubes, but a mixture of anatase TiO2 and spinel lithium titanate as showed in Fig. 3(a). Furthermore, the prolonging of reaction time cannot reach phase pure spinel lithium titanate yet. The lower pH conditions, however, result in phase pure anatase TiO2, as showed in Fig. 3(c), which is coincident with the results of Li et al. [21]. These results indicate that the temperature and pH value of solution during lithium ion exchange processing are key factors for converting hydrogen titanate nanotubes/nanowires into cubic spinel lithium titanate nanotubes/nanowires. The spinel compounds have the generic formula AB2X4, in the spinel structure the X atoms are close packed and the A atoms occupy 1/8 of the tetrahedral interstices and the B atoms occupy 1/2 of the octahedral voids. For spinel Li4Ti5O12 (also be formulated as Li[Li1/3Ti5/3]O4), one Li atoms occupy the tetrahedral interstices, while other 1/3 Li atoms and 5/3 Ti atoms occupy the octahedral voids together. So the hydrothermal lithium ion exchange processing may help migration of lithium atom from tetrahedral interstices into octahedral voids, thus the hydrothermal lithium ion exchange processing leads to a phase change as showed in Fig. 2(c). So the hydrothermal condition is necessary to prepare spinel Li4Ti5O12 nanotubes/nanowires by ion exchange processing from hydrogen titanate nanotubes/nanowires precursors. In the mean time, Li+ has the equivalent ionic radius with Ti4+, ˚ , which makes the structure stable after about 0.68 A Li+ ion diffusion into Ti4+ sites (octahedral voids) in lithium titanate nanotubes/nanowires. On the contrary, ˚ , which is much the ionic radius of Na+ is about 0.97 A larger than the radius of Ti4+, so the sodium titanate nanotubes are difficult to be directly transformed into spinel structure analogue like Li4Ti5O12 [21]. Anyway, future work will involve the detailed mechanistic studies of the formation and growth of cubic spinel lithium titanate from monoclinic hydrogen titanate precursors, which may help to understand the precise composition and structure of hydrogen titanate nanotubes/nanowires derived from alkali-hydrothermal approaches that difficult to analysis directly by chemical and physical methods [28,29]. Fig. 4 is typical cyclic voltammogram of electrode made from spinel Li4Ti5O12 nanotubes and nanowires, respectively. The lithium insertion into and extraction from these electrodes are both reversible. The absolute values of specific peak current of anodic branch (lithium ion extraction) and cathodic branch (lithium ion insertion) are comparable, which is different from that of bulk spinel Li4Ti5O12 electrode, where the specific peak currents are usually asymmetric [30]. The difference in peak currents between anodic and cathodic branches is mainly attribute to the slow lithium ion diffusivity in
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Fig. 5. Voltage profile of spinel Li4Ti5O12 nanowires at different discharge/charge rates.
Fig. 4. Cyclic voltammogram of spinel Li4Ti5O12 nanotubes (a) and nanowires (b) in 1 M LiPF6 + EC/DMC (1/1, v/v) at scan rate of 0.05 mV/s. The geometric surface area of electrode is 0.83 cm2.
solid-state body of bulk spinel Li4Ti5O12. Almost the same peak currents in electrode made from spinel Li4Ti5O12 nanotubes or nanowires, however, may indicate the improved kinetics performance of these nanostructured electrode materials. The shoulder peaks in the spinel Li4Ti5O12 nanotubes may be ascribed to the trace anatase TiO2 impurity as showed in the XRD pattern, however, this trace impurity is not so obvious in nanowires counterpart. The trace anatase TiO2 impurity may exist its hydrogen titanate nanotubes precursors when the raw anatase TiO2 has not been fully transformed into hydrogen titanate nanotubes during the alkaline-hydrothermal process. Because of the higher hydrothermal temperature and longer reaction time when preparing hydrogen titanate nanowires than that of nanotubes, there is no obvious anatase detected in hydrogen titanate nanowires precursor and spinel Li4Ti5O12 nanowires product. So it is necessary to optimize the reaction conditions when preparing phase pure hydrogen titanate nanotubes precursor to ensure phase pure spinel Li4Ti5O12 nanotubes. Fig. 5 shows the typical voltage profile of nanostructured electrode made from spinel Li4Ti5O12 nanowires at different discharge/charge rates. This voltage profile is similar to that of normal spinel Li4Ti5O12 powders,
however, the superior rate capability in spinel Li4Ti5O12 nanowires is observed. This improved rate capability of lithium insertion may be ascribed to the reduced dimensional size mitigated the influence of slow lithium ion diffusivity in solid body, hence enhanced electrochemical kinetics. Fig. 6 shows the cycling performance of lithium insertion into and extraction from spinel Li4Ti5O12 nanowires. It clearly indicates the excellent cycling stability of as prepared spinel Li4Ti5O12 nanowires, which may imply that the well-crystallized spinel phase can be achieved at relative low temperature. Owing to the shape and morphology are controllable of one dimensional hydrogen titanate precursors (such as nanotube, nanowire, nanorod and nanobelt) by varying the reaction conditions, so the shape and morphology of spinel Li4Ti5O12 are also controllable by this topochemical transformation of hydrothermal lithium ion exchanging approaches. This spinel Li4Ti5O12 with different shape and morphology may become promising lithium insertion materials and may find versatile applications in high performance hybrid electrochemical cells and lithium ion batteries.
Fig. 6. Galvanostatic cycling performance of spinel Li4Ti5O12 nanowires at different discharge/charge rates.
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4. Conclusions In summary, we have succeeded in controllable synthesizing of spinel lithium titanate nanotubes/nanowires with uniform morphology and high specific surface area by means of topochemical hydrothermal ion exchange reaction from hydrogen titanate nanotubes/nanowires precursors. The formation temperature of this cubic spinel Li4Ti5O12 is greatly lower than that of solid-state reaction and sol–gel approaches. Because of the versatile shape and morphology of alkali-hydrothermal derived hydrogen titanate nano-materials, this ion-exchange methodology opens a new controllable way to prepare nanostructured spinel Li4Ti5O12 materials with different shape and morphology. The cyclic voltammetric results of both electrodes indicate enhanced electrochemical kinetics for lithium insertion by reducing the distance of lithium ion diffusion in solid-state body. The electrochemical lithium insertion cycling tests show excellent cycling stability and improved rate capability, which make these novel one-dimensional nanostructured materials become promising anode materials for lithium ion batteries and cathode materials for electrochemical supercapacitors.
Acknowledgements This work was supported by National High Tech Research & Development Programme (Grant No. 2003AA302320) and National Natural Science Funds of China (Grant Nos. 50372033 and 50472005), as well as the Basic Research Funds of Tsinghua University (Grant No. JC2003040). We also thank Dr. Wang, R. M. (Electron Microscopy Laboratory and Department of Physics, Peking University) for his help with the HRTEM characterization.
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