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Synthesis parameter dependence of the electrochemical performance of solvothermally synthesized Li4Ti5O12

Synthesis parameter dependence of the electrochemical performance of solvothermally synthesized... Mater Renew Sustain Energy (2014) 3:24 DOI 10.1007/s40243-014-0024-7 OR IGINAL PAPER Synthesis parameter dependence of the electrochemical performance of solvothermally synthesized Li Ti O 4 5 12 • • • Qian Yang Hailei Zhao Jie Wang • • Jing Wang Chunmei Wang Xinmei Hou Received: 6 September 2013 / Accepted: 3 February 2014 / Published online: 22 February 2014 The Author(s) 2014. This article is published with open access at Springerlink.com Abstract Pure Li Ti O with high crystallinity was Introduction 4 5 12 successfully synthesized by a solvothermal process. The effects of initial Li/Ti ratio and post-heating temperature on Lithium-ion batteries have attracted much attention as the phase evolution, particle morphology and electro- important energy supply in portable electronic devices, chemical properties were systematically investigated. hybrid electrical vehicles and electrical vehicles because of Excess lithium, compared to the theoretical value in their high power and energy density [1–3]. At present, new Li Ti O , was required to get pure Li Ti O due to the electrode materials exhibiting excellent rate capability and 4 5 12 4 5 12 condensation reaction. Low Li/Ti ratio led to the appear- high safety performance are urgently demanded to meet the ance of secondary phase rutile TiO , while high heat- requirement of electrical vehicles. The spinel lithium tita- treatment temperature easily resulted in particle agglom- nate Li Ti O is being considered as an ideal anode 4 5 12 eration of Li Ti O powder. The existence of rutile TiO material in lithium-ion batteries due to its unique charac- 4 5 12 2 decreased the specific capacity, and the particle agglom- teristics, including very flat charge/discharge voltage pla- erate had a strong negative effect on the rate capability of teaus and a small structural change during charge/discharge electrode. The sample synthesized at the optimized con- processes. The zero-strain insertion characteristic provides dition exhibited a stable specific capacity of 150 mAh/g material with an excellent cycling performance [4, 5], and a good rate performance. while that of the flat operating voltage at 1.55 V (versus Li /Li) can avoid the deposition of dendritic metallic Keywords Li Ti O  Solvothermal synthesis  Heat lithium, therefore a high operational safety can be expected 4 5 12 treatment  Electrochemical performance [6, 7]. Despite the high Li deintercalation/intercalation potential, it can, in principle, be coupled with high-voltage cathodes such as LiNi Mn O to provide a cell with an 0.4 1.6 4 operating voltage of approximately 3 V [8]. However, Li Ti O is an insulator, its rate capability is 4 5 12 greatly limited by its inherently low lithium-ion diffusivity and electronic conductivity. Typical approaches to resolve this problem include employing nanoparticles to reduce the Q. Yang  H. Zhao (&)  J. Wang  J. Wang  C. Wang School of Materials Science and Engineering, University diffusion length of lithium ions, and increase the contact of Science and Technology Beijing, Beijing 100083, China area between the electrode and the electrolyte [9–11], e-mail: hlzhao@ustb.edu.cn 3? 3? doping Li Ti O with aliovalent cation (Al ,Ga , 4 5 12 3? 2? 5? Co ,Mg ,Ta )[12–14] in Li and Ti sites to produce H. Zhao 3? 4? Beijing Key Lab of Advanced Energy Materials, mixed valence of Ti /Ti , and thus increase the elec- Beijing 100083, China tronic conductivity, and incorporating directly the con- ductive second phase (carbon, Ag and so on) [7, 15, 16]. X. Hou Actually, the particle size and the crystalline ordering School of Metallurgical and Ecological Engineering, University degree have strong impacts on the electrochemical of Science and Technology Beijing, Beijing 100083, China 123 Page 2 of 9 Mater Renew Sustain Energy (2014) 3:24 properties of electrode. Small-sized active material can not the partial organic compounds. The precipitate was dried at only reduce the lithium-ion diffusion distance, but also 80 C in air for 3 h. To obtain well-crystallized Li Ti O , 4 5 12 increases the contact area with conductive reagent and the precursor was calcined at 800 C for 2 h in air with a electrolyte solution, thus can decrease the local current heating rate of 5 C/min. At last, the effect of heat-treat- density and mitigate the electrode polarization. The high ment temperature on the particle morphology and electro- crystallinity is believed to be beneficial to the good cycling chemical properties was investigated. The precursor with stability of electrode [9]. Compared to the doped materials, the optimal Li/Ti ratio based on the above results was the pure material is easier to be synthesized and handled in subjected to calcination at temperatures of 400, 600, and practical operations. Many methods, including conven- 800 C, respectively. tional solid-state reaction [12–14], sol–gel method [6, 17, 18], solvothermal technique [19–22], combustion synthesis Characterization [23], rheological phase reaction [11] and other synthesis routes, have been exploited to prepare Li Ti O materials. Phase purity and crystallinity of the synthesized samples 4 5 12 Among them, solvothermal technique with simple and were identified by means of powder X-ray diffraction flexible controls has spurred considerable interests. (XRD) performed on a Rigaku D/MAX-A diffractometer Although Li Ti O powders prepared by solvothermal with Cu Ka radiation source (k = 1.54056 A) in the range 4 5 12 method have been investigated extensively [19–22], the of 10 B 2h B 90, while the morphology and size distri- work concerning the effect of the synthesis parameters on bution of precursors and post-treated powders were the electrochemical properties is very limited. Considering observed on a LEO-1450 scanning electron microscope that the practical composition of the synthesized material (SEM). The actual molar ratio of Li/Ti in the precursor was via solvothermal route is usually different from the nomi- determined by inductively coupled plasma atomic emission nal composition, in this work, the effect of initial Li/Ti spectrometer (ICP-AES) (IRIS Intrepid II XSP). The ratio in starting solution on the phase purity and the elec- thermal behavior of the precursor powders was examined trochemical properties was investigated. The influence of by a thermogravimetry–differential thermal analysis (TG– the post-heat-treatment temperature on the electrochemical DTA) instrument (Netzsch STA 449 C) with a heating rate performance of Li Ti O electrode was also addressed. of 10 C/min from room temperature to 900 C under air. 4 5 12 The synthesized Li Ti O exhibited excellent rate capa- 4 5 12 bility and cycling performance, showing the solvothermal Electrochemical measurement synthesis is a promising method to obtain high-perfor- mance Li Ti O anode material. Half-cells were used to evaluate the electrochemical per- 4 5 12 formance. Celgard 2400 microporous membrane was used as separator, a lithium foil as negative electrode, and 1 M Experimental LiPF dissolved in a mixture of ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and ethylene carbonate Materials synthesis (EC) with a volume ratio of 1:1:1 as the electrolyte. The working electrode was made from the mixture of active The spherical precursors of Li Ti O powders were syn- material Li Ti O , acetylene black (AB) and polyvinyli- 4 5 12 4 5 12 thesized by solvothermal method using lithium acetate dene fluoride (PVdF) in a weight ratio of 85:10:5. The (LiAc, AR C99.0 %, Beijing Yili Fine Chemicals Co., slurry of the mixture was uniformly pasted on aluminum Ltd.) and tetrabutyl titanate [Ti(O(CH ) CH ) , denoted as foil, dried and cut into disks. Then the working electrode 2 3 3 4 Ti(OR) ,AR C99.0 %, Beijing Jinlong Chemical Reagent was dried under vacuum at 120 C for 24 h before elec- Co., Ltd.) as Li and Ti cation sources, respectively. The trochemical evaluation. Cell assembly was carried out in a molar ratios of the mixtures were fixed at different pro- glove box filled with high-purity argon where the oxygen portions (Li/Ti ratio = 0.8–1.4). Ti(OR) was dissolved in and water vapor contents were each \1 ppm. ethanol under magnetic stirring, and then LiAc was added The galvanostatic charging–discharging test was into the mixtures with further stirring to obtain a homo- employed to evaluate the cycle stability and electrochem- geneous dispersion system. The concentration of Ti(OR) ical capacity of the samples by a computer-controlled Land -4 in ethanol was 1.4 9 10 mol/ml. The transparent solu- CT 2001A battery test system. The cell was cycled at tion was then transferred into a 100 ml teflon-lined stain- different current densities, and the cut-off voltage for less steel autoclave and kept at 180 C for 24 h. After charging and discharging processes was 1.0–2.5 V, cooling down to room temperature, a milky white precursor respectively. The specific capacity was calculated based on was prepared. The produced powder was washed and fil- the whole weight of the synthesized samples, including tered with ethanol to eliminate the unreacted reagents and Li Ti O and possible impurity TiO . AC electrochemical 4 5 12 2 123 Mater Renew Sustain Energy (2014) 3:24 Page 3 of 9 impedance spectroscopy was measured by a Solartron remarkably and finally tends to disappear. When the molar 1260/1287 (UK) impedance analyzer in the frequency ratio of Li/Ti reaches 1.4, the single-phase spinel type range from 1 MHz to 0.01 Hz at the state of fully lithiated. Li Ti O without any impurity is obtained. 4 5 12 Experiments were carried out at room temperature. Compared to the theoretical ratio, the excess lithium required to obtain pure single-phase Li Ti O in the 4 5 12 solvothermal synthesis route is related to the reaction Results and discussion mechanism of Ti(OR) and LiAc. Under solvothermal condition, part of tetrabutyl titanate Ti(OR) may first Effect of Li/Ti molar ratio resolve in ethanol and take alcoholysis reaction to form Ti(OR) (OH) , as expressed by reaction (1). The 4 - x x Due to the complicated coordination process of tetrabutyl Ti(OR) (OH) monomers then condense with LiAc to 4 - x x titanate [Ti(OR) ] with LiAc in the solvothermal condition, produce Ti(OR) (OH) (OLi) via reaction (2). Dif- 4 - x x - y y the chemical composition of the resultant is usually dif- ferent condensation reactions occur among the clusters of ferent from the nominal composition. To prepare high- Ti(OR) OH) (OLi) . The condensation may occur 4 - x( x - y y purity Li Ti O , it is essential to control the starting molar 4 5 12 between Ti–(OH) groups, producing H O as by-product, ratio of Li/Ti in the reaction mixtures. Different solutions while the reaction may also occur between Ti–OH and Ti– with Li/Ti ratio = 0.8, 1.0, 1.2 and 1.4 were solvother- OLi producing LiOH, corresponding to reaction (3)and (4), mally treated in an autoclave, and the precipitates after heat respectively. The product of the solvothermal reaction was treatment were subjected to the phase identification by composed of mainly Li–Ti–O amorphous material with XRD. The results are shown in Fig. 1. The samples with some remained organic radicals that already show the basic Li/Ti molar ratios of 0.8, 1.0, 1.2, 1.4 are named as lattice structure of Li Ti O , as shown in Fig. 6 (sample 4 5 12 LTO0.8, LTO1.0, LTO1.2 and LTO1.4, respectively. For LTO), and small amount of LiOH as by-product. Therefore, all samples, diffraction peaks indexed on the cubic spinel part of the lithium remains in the solution. If Li source LiAc phase Li Ti O with Fd3m space group (JCPDS No. 4 5 12 is not excess in the starting materials than the theoretically 49-0207) are observed. However, some additional diffrac- required amount, then the attached lithium in the solid tion peaks corresponding to rutile TiO with P42/mnm particles after condensation reactions (3)and (4)will be space group (JCPDS No. 21-1276) are also detected at the inadequate to form Li Ti O , leading to the generation of 4 5 12 same time except for sample LTO1.4. Although the theo- trace of TiO after heat treatment, as evidenced in Fig. 1. retical Li/Ti ratio of Li Ti O is 0.8, sample LTO0.8 4 5 12 shows strong TiO peaks, demonstrating that a significant Ti(OR) þ xC H OH ! Ti(OR) ðOH) þ xC H OR 2 5 2 5 4 4x x amount of lithium remained in the solution. With increas- ð0 \x\ 4Þð1Þ ing Li/Ti ratio, the relative peak intensity of Li Ti O 4 5 12 increases gradually, while that of TiO decreases Ti(OR) ðOH) þ yCH COOLi 2 4x x 3 ! Ti(OR) ðOH) ðOLi) þ yCH COOH ð2Þ 4x xy y 3 ð0 \y\xÞ 2Ti(OR) ðOH) ðOLi) 4x xy y !ðOLi) ðOH) ðRO) TiOTi(OR) y xy1 4x 4x ðOH) ðOLi) þ H O ð3Þ xy1 y 2Ti(OR) ðOH) ðOLi) 4x xy y !ðOLi) ðOH) ðRO) TiOTi(OR) y1 xy 4x 4x ðOH) ðOLi) þ LiOH ð4Þ xy1 y Analyzed by means of ICP-AES, the actual Li/Ti molar ratio in the precursor of sample LTO1.4 is 1.0, indicating that some lithium remained in solution, which is in good agreement with reaction (4). Considering that the Li/Ti is 0.8 in Li Ti O , the high Li/Ti (1.0) in the precursor of 4 5 12 LTO1.4 implies that some lithium is lost during calcination [17, 24, 25]. To investigate the thermal decomposition behavior of Fig. 1 XRD patterns of Li Ti O samples after heat treated at 4 5 12 the precursor, TG–DTA examination was performed on the 800 C in air 123 Page 4 of 9 Mater Renew Sustain Energy (2014) 3:24 The particle morphologies of Li Ti O precursors with 4 5 12 different molar ratios of Li/Ti are shown in Fig. 3. The samples are all composed of well-dispersed spherical par- ticles with some small particles adhering to their surface. With increasing Li/Ti ratio, the number of small particles decreases remarkably, the average particle size reduces from 3 to 2 lm and the particle distribution tends to be more uniform. After heat treatment at 800 C in air, the powders exhibit even smaller particle size of about 1–1.5 lm and smoother particle surface, as presented in Fig. 4. This is considered to be mainly resulted from the decomposition of organic groups on the particle surface of Li Ti O precursor. 4 5 12 With Li Ti O /Li half-cell, the cycling performances of 4 5 12 samples LTO0.8, LTO1.0, LTO1.2 and LTO1.4 at 0.2 C Fig. 2 TG–DTA curves of precursor with initial Li/Ti = 1.4 were examined, and the results are shown in Fig. 5a. All the synthesized active materials of LTO0.8, LTO1.0, LTO1.2 and LTO1.4 display a stable cycling performance. The precursor of sample LTO1.4. The result is shown in Fig. 2. specific capacity of these samples increases with increasing In the TG curve, the total mass loss obtained in the tem- Li/Ti ratio, and sample LTO1.4 exhibits the highest specific perature range from room temperature to 900 Cis capacity among these samples. The existence of rutile TiO approximately 25.64 %. The first step of mass loss about is apparently detrimental to lithium storage capacity of the 19.06 % occurred between room temperature and 180 C, samples. This is in good agreement with the literature results corresponding to the endothermic peak at 86.7 C in the that only small amounts of lithium ions can be intercalated DTA curve, is due to the removal of adsorbed ethanol and in rutile TiO at room temperature [26, 27]. water molecules. The second step of mass loss occurred in The initial discharge–charge potential curves of samples 180–600 C, associating with the exothermic peak at LTO0.8, LTO1.0, LTO1.2 and LTO1.4 are shown in Fig. 5b. 271.9 C, is attributed to the loss of the organics and the All the samples exhibit only one typical discharge/charge formation of Li Ti O phase. When the temperature is 4 5 12 potential plateau of Li Ti O representing a two-phase reac- 4 5 12 above 600 C, no major weight loss was examined, indi- tion between Li Ti O and Li Ti O [28], no plateau corre- 4 5 12 7 5 12 cating that the decomposition of organic groups was sponds to the lithiation/delithiation process of rutile TiO [29]. completed. Fig. 3 SEM images of Li Ti O precursors obtained 4 5 12 by solvothermal reaction with a Li/Ti = 0.8, b Li/Ti = 1.0, c Li/Ti = 1.2, d Li/Ti = 1.4 123 Mater Renew Sustain Energy (2014) 3:24 Page 5 of 9 Fig. 4 SEM images of Li Ti O samples after heat 4 5 12 treated at 800 C in air a LTO0.8, b LTO1.0, c LTO1.2, d LTO1.4 heat-treatment investigation was conducted. Precursor of sample LTO1.4, showing pure phase and high specific capacity after 800 C-treatment, was selected to subject the heat-treatment test to clarify the effect of treating temper- ature on the structure and electrochemical performance of synthesized Li Ti O . The precursor and the samples heat 4 5 12 treated at 400, 600 and 800 C are denoted as LTO, LTO4, LTO6 and LTO8, accordingly. Figure 6 shows the XRD patterns of these samples. They are all identified with a pure cubic phase Li Ti O . The precursor features amor- 4 5 12 phous structure. The appearance of messy background and the broad peaks with weak intensities indicate the poor crystallinity of the formed precursor. The peak intensities of the spinel phase significantly enhance when the heat- treatment temperature increases, exhibiting the improve- ment of crystallinity. When the heat-treating temperature is up to 600 C, good crystallinity and pure spinel phase Li Ti O have been identified from the XRD data. The 4 5 12 lattice parameters calculated according to the XRD data are 8.3395 (3), 8.3661 (9), 8.3695 (9) A for LTO4, LTO6 and LTO8, respectively, which are in good agreement with Fig. 5 a Discharge specific capacity of Li Ti O prepared from 4 5 12 previous reported values [6, 30, 31]. The average crystallite starting materials with different Li/Ti ratios; b initial discharge– sizes calculated from the full width at half maximum charge curves of the samples LTO0.8, LTO1.0, LTO1.2 and LTO1.4 (FWHM) of peak (111) are 11.47, 44.70 and 50.04 nm for The results indicate that the impurity rutile TiO in the samples LTO4, LTO6 and LTO8, respectively. The crystallite size does not have electrochemical activity in this condition. increases with the increase of heat-treatment temperature. The results demonstrate that pure Li Ti O powders with 4 5 12 high crystallinity, small crystallite size can be successfully Effect of heat-treatment temperature synthesized by solvothermal method. Additionally, the synthesized Li Ti O powders display high dispersity and Considering that the electrochemical performances of 4 5 12 Li Ti O are closely related with its crystallinity, a further good sphericity without particle agglomeration, which are 4 5 12 123 Page 6 of 9 Mater Renew Sustain Energy (2014) 3:24 Fig. 6 XRD patterns of the precursor and the final powders calcined at different temperatures beneficial for both of electrochemical performance and practical electrode preparation. In most previous studies, spinel Li Ti O was synthesized by solid-state reaction at 4 5 12 800–1000 C for 5–24 h [12–14]. Compared with the conventional solid-state technique, the calcination tem- perature of the solvothermally synthesized products was greatly decreased and the dwelling time was significantly shortened. The morphologies of the Li Ti O powders after heat Fig. 7 SEM images of samples a LTO4, b LTO6, c LTO8 4 5 12 treatment are shown in Fig. 7. With increasing tempera- ture, the particle size decreases slightly and the particle aspects should be taken into account. The first is the surface becomes much smoother. However, the 800 C- crystallinity of powders. High crystallinity is usually ben- treated sample LTO8 shows an obvious particle agglom- eficial to the good electrochemical performance of eration, several small particles stick together to form a Li Ti O negative electrode [9]. On the other hand, par- 4 5 12 large one, showing a poor dispersity. ticle agglomeration is unfavorable for the diffusion of To investigate the influence of heat-treatment tempera- lithium ions due to the elongated diffusion distance. The ture on the electrochemical performance, the cycling per- characteristics of good crystallinity and small particle size formances of the samples LTO4, LTO6 and LTO8 were of sample LTO6 guarantee its high specific capacity. Due examined. As shown in Fig. 8, sample LTO4 displays to the obvious particle agglomeration, sample LTO8 extremely high irreversible capacity compared to other exhibits the lowest specific capacity, even worse than samples, mainly resulting from the remained organic sample LTO4. Besides the long diffusion path of lithium groups on the particle surface due to its lower heating ions of the agglomerated particles that impedes the kinetics temperature. All samples show good cycling stability since of lithium intercalation into the LTO8 host structure, the the second cycle, while sample LTO6 exhibits the highest lowered specific surface area should be taken into consid- electrochemical capacity of 150 mAh/g among all the eration because it can reduce the contact area between the samples. To understand the dependence of electrochemical electrode and the AB, and therefore deteriorate the elec- capacity of LTO on the heating temperature, several tronic conduction. Furthermore, the lowered specific 123 Mater Renew Sustain Energy (2014) 3:24 Page 7 of 9 Fig. 8 Discharge capacities of samples LTO4, LTO6 and LTO8 Fig. 10 AC impedance spectra of Li/Li Ti O cells with LTO6 and 4 5 12 LTO8 as active material AC impedance measurement was performed on LTO6 and LTO8 electrodes, the results are shown in Fig. 10. Each curve is comprised of a depressed semi-circle in the high-medium frequency region and an oblique straight line in the low frequency region. The semi-circle in the high- medium frequency region is mainly related to the charge- transfer process, while the inclined line in the low fre- quency region is attributed to the Warburg impedance that reflects lithium-ion diffusion behavior in the Li Ti O 4 5 12 electrode [34]. Sample LTO6 displays a significantly lower resistance of charge transfer than that of sample LTO8, which is certainly associated with the larger specific sur- Fig. 9 Specific capacities of samples LTO6 and LTO8 at different rates between 1.0 and 2.5 V face area of active material LTO6. These results can interpret well the experimental results in Figs. 8 and 9. surface area can result in the increase in actual current density on particle surface, and thus cause a large polari- zation of the electrode, which is another reason for the low Conclusion specific capacity of sample LTO8 [6, 31–33]. Figure 9 illustrates the rate capabilities of LTO6 and Pure and well-crystallized Li Ti O powders were syn- 4 5 12 LTO8 electrodes. The discharge capacity of both samples thesized from Ti(O(CH ) CH ) and LiAc by solvothermal 2 3 3 4 decreases gradually with increasing current density. Nev- route involving a further heat treatment at relatively low ertheless, the sample LTO6 maintains a higher reversible temperature with short dwelling time. The Li/Ti molar ratio capacity at the same current density as compared to the in starting materials and post-heat-treatment temperature sample LTO8, indicating the good rate capability of sample have strong impacts on the electrochemical performance of LTO6. At lower current density of 0.2 C, both the samples Li Ti O anode material. Excess lithium, compared to the 4 5 12 reveal relatively good capacity characteristics due to the theoretical value in Li Ti O , is required for the synthesis 4 5 12 sufficient insertion/extraction of lithium ions. High current of pure-phase Li Ti O in this preparation condition. Low 4 5 12 density makes their specific capacity different. The Li/Ti ratio (\1.4 in atom) easily results in the coexistence capacity ratio of C /C is 0.65 and 0.44 for samples of the secondary phase TiO , while high treatment tem- 2C 0.2 C 2 LTO6 and LTO8, respectively. The good rate capability as perature leads to particle agglomeration of Li Ti O 4 5 12 well as the high specific capacity of sample LTO6 makes it powders. The former causes the decrease in specific a promising material for a better commercial application. capacity, but the latter deteriorates the rate capability of 123 Page 8 of 9 Mater Renew Sustain Energy (2014) 3:24 12. Huang, S., Wen, Z., Zhu, X., Lin, Z.: Effects of dopant on the electrode. The 600 C-treated sample LTO6 with starting electrochemical performance of Li Ti O as electrode material 4 5 12 LiAc/Ti(OR) = 1.4 exhibits the highest specific capacity for lithium ion batteries. J. Power Sources 165, 408–412 (2007) and best rate performance due to its high purity, good 13. Chen, C.H., Vaughey, J.T., Jansen, A.N., Dees, D.W., Kahaian, crystallinity and excellent dispersity. At 0.5 C, LTO6 dis- A.J., Goacher, T., Thackeray, M.M.: Studies of Mg-substituted Li Mg Ti O spinel electrodes (0 B x B 1) for lithium plays a stable reversible capacity of ca. 150 mAh/g. EIS 4 - x x 5 12 batteries. J. Electrochem. Soc. 148, A102–A104 (2001) measurement reveals that LTO6 has lower charge-transfer 14. Wolfenstine, J., Allen, J.L.: Electrical conductivity and charge resistance compared to the 800 C-treated LTO8, which is compensation in Ta doped Li Ti O . J. Power Sources 180, 4 5 12 mainly attributable to the high specific surface area and 582–585 (2008) 15. Cheng, L., Yan, J., Zhu, G.N., Luo, J.Y., Wang, C.X., Xia, Y.Y.: small particle size of sample LTO6. General synthesis of carbon-coated nanostructure Li Ti O as a 4 5 12 high rate electrode material for Li-ion intercalation. J. Mater. 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Colin, J.F., Godbole, V., Nova ´k, P.: In situ neutron diffraction 4 5 12 modified rheological phase reaction. Electrochim. Acta 54, study of Li insertion in Li Ti O . Electrochem. Commun. 12, 4 5 12 5629–5633 (2009) 804–807 (2010) 123 Mater Renew Sustain Energy (2014) 3:24 Page 9 of 9 31. Jung, H.G., Kim, J., Scrosati, B., Sun, Y.K.: Micron-sized, car- 34. Yoshikawa, D., Kadoma, Y., Kim, J.-M., Ui, K., Kumagai, N., bon-coated Li Ti O as high power anode material for advanced Kitamura, N., Idemoto, Y.: Spray-drying synthesized lithium- 4 5 12 lithium batteries. J. Power Sources 196, 7763–7766 (2011) excess Li Ti O and its electrochemical property as 4 ? x 5 - x 12 - d 32. Zhang, N., Liu, Z., Yang, T., Liao, C., Wang, Z., Sun, K.: Facile negative electrode material for Li-ion batteries. Electrochim. preparation of nanocrystalline Li Ti O and its high electro- Acta 55, 1872–1879 (2010) 4 5 12 chemical performance as anode material for lithium-ion batteries. Electrochem. Commun. 13, 654–656 (2011) 33. Arico ` , A.S., Bruce, P., Scrosati, B., Tarascon, J.-M., van Schalkwijk, W.: Nanostructured materials for advanced energy conversion and storage devices. Nat. Mater. 4, 366–377 (2005) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals

Synthesis parameter dependence of the electrochemical performance of solvothermally synthesized Li4Ti5O12

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Material Science; Materials Science, general; Renewable and Green Energy; Renewable and Green Energy
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Abstract

Mater Renew Sustain Energy (2014) 3:24 DOI 10.1007/s40243-014-0024-7 OR IGINAL PAPER Synthesis parameter dependence of the electrochemical performance of solvothermally synthesized Li Ti O 4 5 12 • • • Qian Yang Hailei Zhao Jie Wang • • Jing Wang Chunmei Wang Xinmei Hou Received: 6 September 2013 / Accepted: 3 February 2014 / Published online: 22 February 2014 The Author(s) 2014. This article is published with open access at Springerlink.com Abstract Pure Li Ti O with high crystallinity was Introduction 4 5 12 successfully synthesized by a solvothermal process. The effects of initial Li/Ti ratio and post-heating temperature on Lithium-ion batteries have attracted much attention as the phase evolution, particle morphology and electro- important energy supply in portable electronic devices, chemical properties were systematically investigated. hybrid electrical vehicles and electrical vehicles because of Excess lithium, compared to the theoretical value in their high power and energy density [1–3]. At present, new Li Ti O , was required to get pure Li Ti O due to the electrode materials exhibiting excellent rate capability and 4 5 12 4 5 12 condensation reaction. Low Li/Ti ratio led to the appear- high safety performance are urgently demanded to meet the ance of secondary phase rutile TiO , while high heat- requirement of electrical vehicles. The spinel lithium tita- treatment temperature easily resulted in particle agglom- nate Li Ti O is being considered as an ideal anode 4 5 12 eration of Li Ti O powder. The existence of rutile TiO material in lithium-ion batteries due to its unique charac- 4 5 12 2 decreased the specific capacity, and the particle agglom- teristics, including very flat charge/discharge voltage pla- erate had a strong negative effect on the rate capability of teaus and a small structural change during charge/discharge electrode. The sample synthesized at the optimized con- processes. The zero-strain insertion characteristic provides dition exhibited a stable specific capacity of 150 mAh/g material with an excellent cycling performance [4, 5], and a good rate performance. while that of the flat operating voltage at 1.55 V (versus Li /Li) can avoid the deposition of dendritic metallic Keywords Li Ti O  Solvothermal synthesis  Heat lithium, therefore a high operational safety can be expected 4 5 12 treatment  Electrochemical performance [6, 7]. Despite the high Li deintercalation/intercalation potential, it can, in principle, be coupled with high-voltage cathodes such as LiNi Mn O to provide a cell with an 0.4 1.6 4 operating voltage of approximately 3 V [8]. However, Li Ti O is an insulator, its rate capability is 4 5 12 greatly limited by its inherently low lithium-ion diffusivity and electronic conductivity. Typical approaches to resolve this problem include employing nanoparticles to reduce the Q. Yang  H. Zhao (&)  J. Wang  J. Wang  C. Wang School of Materials Science and Engineering, University diffusion length of lithium ions, and increase the contact of Science and Technology Beijing, Beijing 100083, China area between the electrode and the electrolyte [9–11], e-mail: hlzhao@ustb.edu.cn 3? 3? doping Li Ti O with aliovalent cation (Al ,Ga , 4 5 12 3? 2? 5? Co ,Mg ,Ta )[12–14] in Li and Ti sites to produce H. Zhao 3? 4? Beijing Key Lab of Advanced Energy Materials, mixed valence of Ti /Ti , and thus increase the elec- Beijing 100083, China tronic conductivity, and incorporating directly the con- ductive second phase (carbon, Ag and so on) [7, 15, 16]. X. Hou Actually, the particle size and the crystalline ordering School of Metallurgical and Ecological Engineering, University degree have strong impacts on the electrochemical of Science and Technology Beijing, Beijing 100083, China 123 Page 2 of 9 Mater Renew Sustain Energy (2014) 3:24 properties of electrode. Small-sized active material can not the partial organic compounds. The precipitate was dried at only reduce the lithium-ion diffusion distance, but also 80 C in air for 3 h. To obtain well-crystallized Li Ti O , 4 5 12 increases the contact area with conductive reagent and the precursor was calcined at 800 C for 2 h in air with a electrolyte solution, thus can decrease the local current heating rate of 5 C/min. At last, the effect of heat-treat- density and mitigate the electrode polarization. The high ment temperature on the particle morphology and electro- crystallinity is believed to be beneficial to the good cycling chemical properties was investigated. The precursor with stability of electrode [9]. Compared to the doped materials, the optimal Li/Ti ratio based on the above results was the pure material is easier to be synthesized and handled in subjected to calcination at temperatures of 400, 600, and practical operations. Many methods, including conven- 800 C, respectively. tional solid-state reaction [12–14], sol–gel method [6, 17, 18], solvothermal technique [19–22], combustion synthesis Characterization [23], rheological phase reaction [11] and other synthesis routes, have been exploited to prepare Li Ti O materials. Phase purity and crystallinity of the synthesized samples 4 5 12 Among them, solvothermal technique with simple and were identified by means of powder X-ray diffraction flexible controls has spurred considerable interests. (XRD) performed on a Rigaku D/MAX-A diffractometer Although Li Ti O powders prepared by solvothermal with Cu Ka radiation source (k = 1.54056 A) in the range 4 5 12 method have been investigated extensively [19–22], the of 10 B 2h B 90, while the morphology and size distri- work concerning the effect of the synthesis parameters on bution of precursors and post-treated powders were the electrochemical properties is very limited. Considering observed on a LEO-1450 scanning electron microscope that the practical composition of the synthesized material (SEM). The actual molar ratio of Li/Ti in the precursor was via solvothermal route is usually different from the nomi- determined by inductively coupled plasma atomic emission nal composition, in this work, the effect of initial Li/Ti spectrometer (ICP-AES) (IRIS Intrepid II XSP). The ratio in starting solution on the phase purity and the elec- thermal behavior of the precursor powders was examined trochemical properties was investigated. The influence of by a thermogravimetry–differential thermal analysis (TG– the post-heat-treatment temperature on the electrochemical DTA) instrument (Netzsch STA 449 C) with a heating rate performance of Li Ti O electrode was also addressed. of 10 C/min from room temperature to 900 C under air. 4 5 12 The synthesized Li Ti O exhibited excellent rate capa- 4 5 12 bility and cycling performance, showing the solvothermal Electrochemical measurement synthesis is a promising method to obtain high-perfor- mance Li Ti O anode material. Half-cells were used to evaluate the electrochemical per- 4 5 12 formance. Celgard 2400 microporous membrane was used as separator, a lithium foil as negative electrode, and 1 M Experimental LiPF dissolved in a mixture of ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and ethylene carbonate Materials synthesis (EC) with a volume ratio of 1:1:1 as the electrolyte. The working electrode was made from the mixture of active The spherical precursors of Li Ti O powders were syn- material Li Ti O , acetylene black (AB) and polyvinyli- 4 5 12 4 5 12 thesized by solvothermal method using lithium acetate dene fluoride (PVdF) in a weight ratio of 85:10:5. The (LiAc, AR C99.0 %, Beijing Yili Fine Chemicals Co., slurry of the mixture was uniformly pasted on aluminum Ltd.) and tetrabutyl titanate [Ti(O(CH ) CH ) , denoted as foil, dried and cut into disks. Then the working electrode 2 3 3 4 Ti(OR) ,AR C99.0 %, Beijing Jinlong Chemical Reagent was dried under vacuum at 120 C for 24 h before elec- Co., Ltd.) as Li and Ti cation sources, respectively. The trochemical evaluation. Cell assembly was carried out in a molar ratios of the mixtures were fixed at different pro- glove box filled with high-purity argon where the oxygen portions (Li/Ti ratio = 0.8–1.4). Ti(OR) was dissolved in and water vapor contents were each \1 ppm. ethanol under magnetic stirring, and then LiAc was added The galvanostatic charging–discharging test was into the mixtures with further stirring to obtain a homo- employed to evaluate the cycle stability and electrochem- geneous dispersion system. The concentration of Ti(OR) ical capacity of the samples by a computer-controlled Land -4 in ethanol was 1.4 9 10 mol/ml. The transparent solu- CT 2001A battery test system. The cell was cycled at tion was then transferred into a 100 ml teflon-lined stain- different current densities, and the cut-off voltage for less steel autoclave and kept at 180 C for 24 h. After charging and discharging processes was 1.0–2.5 V, cooling down to room temperature, a milky white precursor respectively. The specific capacity was calculated based on was prepared. The produced powder was washed and fil- the whole weight of the synthesized samples, including tered with ethanol to eliminate the unreacted reagents and Li Ti O and possible impurity TiO . AC electrochemical 4 5 12 2 123 Mater Renew Sustain Energy (2014) 3:24 Page 3 of 9 impedance spectroscopy was measured by a Solartron remarkably and finally tends to disappear. When the molar 1260/1287 (UK) impedance analyzer in the frequency ratio of Li/Ti reaches 1.4, the single-phase spinel type range from 1 MHz to 0.01 Hz at the state of fully lithiated. Li Ti O without any impurity is obtained. 4 5 12 Experiments were carried out at room temperature. Compared to the theoretical ratio, the excess lithium required to obtain pure single-phase Li Ti O in the 4 5 12 solvothermal synthesis route is related to the reaction Results and discussion mechanism of Ti(OR) and LiAc. Under solvothermal condition, part of tetrabutyl titanate Ti(OR) may first Effect of Li/Ti molar ratio resolve in ethanol and take alcoholysis reaction to form Ti(OR) (OH) , as expressed by reaction (1). The 4 - x x Due to the complicated coordination process of tetrabutyl Ti(OR) (OH) monomers then condense with LiAc to 4 - x x titanate [Ti(OR) ] with LiAc in the solvothermal condition, produce Ti(OR) (OH) (OLi) via reaction (2). Dif- 4 - x x - y y the chemical composition of the resultant is usually dif- ferent condensation reactions occur among the clusters of ferent from the nominal composition. To prepare high- Ti(OR) OH) (OLi) . The condensation may occur 4 - x( x - y y purity Li Ti O , it is essential to control the starting molar 4 5 12 between Ti–(OH) groups, producing H O as by-product, ratio of Li/Ti in the reaction mixtures. Different solutions while the reaction may also occur between Ti–OH and Ti– with Li/Ti ratio = 0.8, 1.0, 1.2 and 1.4 were solvother- OLi producing LiOH, corresponding to reaction (3)and (4), mally treated in an autoclave, and the precipitates after heat respectively. The product of the solvothermal reaction was treatment were subjected to the phase identification by composed of mainly Li–Ti–O amorphous material with XRD. The results are shown in Fig. 1. The samples with some remained organic radicals that already show the basic Li/Ti molar ratios of 0.8, 1.0, 1.2, 1.4 are named as lattice structure of Li Ti O , as shown in Fig. 6 (sample 4 5 12 LTO0.8, LTO1.0, LTO1.2 and LTO1.4, respectively. For LTO), and small amount of LiOH as by-product. Therefore, all samples, diffraction peaks indexed on the cubic spinel part of the lithium remains in the solution. If Li source LiAc phase Li Ti O with Fd3m space group (JCPDS No. 4 5 12 is not excess in the starting materials than the theoretically 49-0207) are observed. However, some additional diffrac- required amount, then the attached lithium in the solid tion peaks corresponding to rutile TiO with P42/mnm particles after condensation reactions (3)and (4)will be space group (JCPDS No. 21-1276) are also detected at the inadequate to form Li Ti O , leading to the generation of 4 5 12 same time except for sample LTO1.4. Although the theo- trace of TiO after heat treatment, as evidenced in Fig. 1. retical Li/Ti ratio of Li Ti O is 0.8, sample LTO0.8 4 5 12 shows strong TiO peaks, demonstrating that a significant Ti(OR) þ xC H OH ! Ti(OR) ðOH) þ xC H OR 2 5 2 5 4 4x x amount of lithium remained in the solution. With increas- ð0 \x\ 4Þð1Þ ing Li/Ti ratio, the relative peak intensity of Li Ti O 4 5 12 increases gradually, while that of TiO decreases Ti(OR) ðOH) þ yCH COOLi 2 4x x 3 ! Ti(OR) ðOH) ðOLi) þ yCH COOH ð2Þ 4x xy y 3 ð0 \y\xÞ 2Ti(OR) ðOH) ðOLi) 4x xy y !ðOLi) ðOH) ðRO) TiOTi(OR) y xy1 4x 4x ðOH) ðOLi) þ H O ð3Þ xy1 y 2Ti(OR) ðOH) ðOLi) 4x xy y !ðOLi) ðOH) ðRO) TiOTi(OR) y1 xy 4x 4x ðOH) ðOLi) þ LiOH ð4Þ xy1 y Analyzed by means of ICP-AES, the actual Li/Ti molar ratio in the precursor of sample LTO1.4 is 1.0, indicating that some lithium remained in solution, which is in good agreement with reaction (4). Considering that the Li/Ti is 0.8 in Li Ti O , the high Li/Ti (1.0) in the precursor of 4 5 12 LTO1.4 implies that some lithium is lost during calcination [17, 24, 25]. To investigate the thermal decomposition behavior of Fig. 1 XRD patterns of Li Ti O samples after heat treated at 4 5 12 the precursor, TG–DTA examination was performed on the 800 C in air 123 Page 4 of 9 Mater Renew Sustain Energy (2014) 3:24 The particle morphologies of Li Ti O precursors with 4 5 12 different molar ratios of Li/Ti are shown in Fig. 3. The samples are all composed of well-dispersed spherical par- ticles with some small particles adhering to their surface. With increasing Li/Ti ratio, the number of small particles decreases remarkably, the average particle size reduces from 3 to 2 lm and the particle distribution tends to be more uniform. After heat treatment at 800 C in air, the powders exhibit even smaller particle size of about 1–1.5 lm and smoother particle surface, as presented in Fig. 4. This is considered to be mainly resulted from the decomposition of organic groups on the particle surface of Li Ti O precursor. 4 5 12 With Li Ti O /Li half-cell, the cycling performances of 4 5 12 samples LTO0.8, LTO1.0, LTO1.2 and LTO1.4 at 0.2 C Fig. 2 TG–DTA curves of precursor with initial Li/Ti = 1.4 were examined, and the results are shown in Fig. 5a. All the synthesized active materials of LTO0.8, LTO1.0, LTO1.2 and LTO1.4 display a stable cycling performance. The precursor of sample LTO1.4. The result is shown in Fig. 2. specific capacity of these samples increases with increasing In the TG curve, the total mass loss obtained in the tem- Li/Ti ratio, and sample LTO1.4 exhibits the highest specific perature range from room temperature to 900 Cis capacity among these samples. The existence of rutile TiO approximately 25.64 %. The first step of mass loss about is apparently detrimental to lithium storage capacity of the 19.06 % occurred between room temperature and 180 C, samples. This is in good agreement with the literature results corresponding to the endothermic peak at 86.7 C in the that only small amounts of lithium ions can be intercalated DTA curve, is due to the removal of adsorbed ethanol and in rutile TiO at room temperature [26, 27]. water molecules. The second step of mass loss occurred in The initial discharge–charge potential curves of samples 180–600 C, associating with the exothermic peak at LTO0.8, LTO1.0, LTO1.2 and LTO1.4 are shown in Fig. 5b. 271.9 C, is attributed to the loss of the organics and the All the samples exhibit only one typical discharge/charge formation of Li Ti O phase. When the temperature is 4 5 12 potential plateau of Li Ti O representing a two-phase reac- 4 5 12 above 600 C, no major weight loss was examined, indi- tion between Li Ti O and Li Ti O [28], no plateau corre- 4 5 12 7 5 12 cating that the decomposition of organic groups was sponds to the lithiation/delithiation process of rutile TiO [29]. completed. Fig. 3 SEM images of Li Ti O precursors obtained 4 5 12 by solvothermal reaction with a Li/Ti = 0.8, b Li/Ti = 1.0, c Li/Ti = 1.2, d Li/Ti = 1.4 123 Mater Renew Sustain Energy (2014) 3:24 Page 5 of 9 Fig. 4 SEM images of Li Ti O samples after heat 4 5 12 treated at 800 C in air a LTO0.8, b LTO1.0, c LTO1.2, d LTO1.4 heat-treatment investigation was conducted. Precursor of sample LTO1.4, showing pure phase and high specific capacity after 800 C-treatment, was selected to subject the heat-treatment test to clarify the effect of treating temper- ature on the structure and electrochemical performance of synthesized Li Ti O . The precursor and the samples heat 4 5 12 treated at 400, 600 and 800 C are denoted as LTO, LTO4, LTO6 and LTO8, accordingly. Figure 6 shows the XRD patterns of these samples. They are all identified with a pure cubic phase Li Ti O . The precursor features amor- 4 5 12 phous structure. The appearance of messy background and the broad peaks with weak intensities indicate the poor crystallinity of the formed precursor. The peak intensities of the spinel phase significantly enhance when the heat- treatment temperature increases, exhibiting the improve- ment of crystallinity. When the heat-treating temperature is up to 600 C, good crystallinity and pure spinel phase Li Ti O have been identified from the XRD data. The 4 5 12 lattice parameters calculated according to the XRD data are 8.3395 (3), 8.3661 (9), 8.3695 (9) A for LTO4, LTO6 and LTO8, respectively, which are in good agreement with Fig. 5 a Discharge specific capacity of Li Ti O prepared from 4 5 12 previous reported values [6, 30, 31]. The average crystallite starting materials with different Li/Ti ratios; b initial discharge– sizes calculated from the full width at half maximum charge curves of the samples LTO0.8, LTO1.0, LTO1.2 and LTO1.4 (FWHM) of peak (111) are 11.47, 44.70 and 50.04 nm for The results indicate that the impurity rutile TiO in the samples LTO4, LTO6 and LTO8, respectively. The crystallite size does not have electrochemical activity in this condition. increases with the increase of heat-treatment temperature. The results demonstrate that pure Li Ti O powders with 4 5 12 high crystallinity, small crystallite size can be successfully Effect of heat-treatment temperature synthesized by solvothermal method. Additionally, the synthesized Li Ti O powders display high dispersity and Considering that the electrochemical performances of 4 5 12 Li Ti O are closely related with its crystallinity, a further good sphericity without particle agglomeration, which are 4 5 12 123 Page 6 of 9 Mater Renew Sustain Energy (2014) 3:24 Fig. 6 XRD patterns of the precursor and the final powders calcined at different temperatures beneficial for both of electrochemical performance and practical electrode preparation. In most previous studies, spinel Li Ti O was synthesized by solid-state reaction at 4 5 12 800–1000 C for 5–24 h [12–14]. Compared with the conventional solid-state technique, the calcination tem- perature of the solvothermally synthesized products was greatly decreased and the dwelling time was significantly shortened. The morphologies of the Li Ti O powders after heat Fig. 7 SEM images of samples a LTO4, b LTO6, c LTO8 4 5 12 treatment are shown in Fig. 7. With increasing tempera- ture, the particle size decreases slightly and the particle aspects should be taken into account. The first is the surface becomes much smoother. However, the 800 C- crystallinity of powders. High crystallinity is usually ben- treated sample LTO8 shows an obvious particle agglom- eficial to the good electrochemical performance of eration, several small particles stick together to form a Li Ti O negative electrode [9]. On the other hand, par- 4 5 12 large one, showing a poor dispersity. ticle agglomeration is unfavorable for the diffusion of To investigate the influence of heat-treatment tempera- lithium ions due to the elongated diffusion distance. The ture on the electrochemical performance, the cycling per- characteristics of good crystallinity and small particle size formances of the samples LTO4, LTO6 and LTO8 were of sample LTO6 guarantee its high specific capacity. Due examined. As shown in Fig. 8, sample LTO4 displays to the obvious particle agglomeration, sample LTO8 extremely high irreversible capacity compared to other exhibits the lowest specific capacity, even worse than samples, mainly resulting from the remained organic sample LTO4. Besides the long diffusion path of lithium groups on the particle surface due to its lower heating ions of the agglomerated particles that impedes the kinetics temperature. All samples show good cycling stability since of lithium intercalation into the LTO8 host structure, the the second cycle, while sample LTO6 exhibits the highest lowered specific surface area should be taken into consid- electrochemical capacity of 150 mAh/g among all the eration because it can reduce the contact area between the samples. To understand the dependence of electrochemical electrode and the AB, and therefore deteriorate the elec- capacity of LTO on the heating temperature, several tronic conduction. Furthermore, the lowered specific 123 Mater Renew Sustain Energy (2014) 3:24 Page 7 of 9 Fig. 8 Discharge capacities of samples LTO4, LTO6 and LTO8 Fig. 10 AC impedance spectra of Li/Li Ti O cells with LTO6 and 4 5 12 LTO8 as active material AC impedance measurement was performed on LTO6 and LTO8 electrodes, the results are shown in Fig. 10. Each curve is comprised of a depressed semi-circle in the high-medium frequency region and an oblique straight line in the low frequency region. The semi-circle in the high- medium frequency region is mainly related to the charge- transfer process, while the inclined line in the low fre- quency region is attributed to the Warburg impedance that reflects lithium-ion diffusion behavior in the Li Ti O 4 5 12 electrode [34]. Sample LTO6 displays a significantly lower resistance of charge transfer than that of sample LTO8, which is certainly associated with the larger specific sur- Fig. 9 Specific capacities of samples LTO6 and LTO8 at different rates between 1.0 and 2.5 V face area of active material LTO6. These results can interpret well the experimental results in Figs. 8 and 9. surface area can result in the increase in actual current density on particle surface, and thus cause a large polari- zation of the electrode, which is another reason for the low Conclusion specific capacity of sample LTO8 [6, 31–33]. Figure 9 illustrates the rate capabilities of LTO6 and Pure and well-crystallized Li Ti O powders were syn- 4 5 12 LTO8 electrodes. The discharge capacity of both samples thesized from Ti(O(CH ) CH ) and LiAc by solvothermal 2 3 3 4 decreases gradually with increasing current density. Nev- route involving a further heat treatment at relatively low ertheless, the sample LTO6 maintains a higher reversible temperature with short dwelling time. The Li/Ti molar ratio capacity at the same current density as compared to the in starting materials and post-heat-treatment temperature sample LTO8, indicating the good rate capability of sample have strong impacts on the electrochemical performance of LTO6. At lower current density of 0.2 C, both the samples Li Ti O anode material. Excess lithium, compared to the 4 5 12 reveal relatively good capacity characteristics due to the theoretical value in Li Ti O , is required for the synthesis 4 5 12 sufficient insertion/extraction of lithium ions. High current of pure-phase Li Ti O in this preparation condition. Low 4 5 12 density makes their specific capacity different. The Li/Ti ratio (\1.4 in atom) easily results in the coexistence capacity ratio of C /C is 0.65 and 0.44 for samples of the secondary phase TiO , while high treatment tem- 2C 0.2 C 2 LTO6 and LTO8, respectively. The good rate capability as perature leads to particle agglomeration of Li Ti O 4 5 12 well as the high specific capacity of sample LTO6 makes it powders. The former causes the decrease in specific a promising material for a better commercial application. capacity, but the latter deteriorates the rate capability of 123 Page 8 of 9 Mater Renew Sustain Energy (2014) 3:24 12. Huang, S., Wen, Z., Zhu, X., Lin, Z.: Effects of dopant on the electrode. The 600 C-treated sample LTO6 with starting electrochemical performance of Li Ti O as electrode material 4 5 12 LiAc/Ti(OR) = 1.4 exhibits the highest specific capacity for lithium ion batteries. J. Power Sources 165, 408–412 (2007) and best rate performance due to its high purity, good 13. Chen, C.H., Vaughey, J.T., Jansen, A.N., Dees, D.W., Kahaian, crystallinity and excellent dispersity. At 0.5 C, LTO6 dis- A.J., Goacher, T., Thackeray, M.M.: Studies of Mg-substituted Li Mg Ti O spinel electrodes (0 B x B 1) for lithium plays a stable reversible capacity of ca. 150 mAh/g. EIS 4 - x x 5 12 batteries. J. Electrochem. Soc. 148, A102–A104 (2001) measurement reveals that LTO6 has lower charge-transfer 14. Wolfenstine, J., Allen, J.L.: Electrical conductivity and charge resistance compared to the 800 C-treated LTO8, which is compensation in Ta doped Li Ti O . J. Power Sources 180, 4 5 12 mainly attributable to the high specific surface area and 582–585 (2008) 15. Cheng, L., Yan, J., Zhu, G.N., Luo, J.Y., Wang, C.X., Xia, Y.Y.: small particle size of sample LTO6. General synthesis of carbon-coated nanostructure Li Ti O as a 4 5 12 high rate electrode material for Li-ion intercalation. J. Mater. 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Materials for Renewable and Sustainable EnergySpringer Journals

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