J. Chil. Chem. Soc, 55, N° 3 (2010) 343-346.

 

RHEOLOGICAL PHASE SYNTHESIS AND CHARACTERIZATION OF MICRO-SIZED Li₄Ti₅O₁₂

LINGLING XIE¹, XIAOYU CAO¹*, CHANGWEI LIU¹, CHIWEI WANG²

¹School of Chemistry and Chemical Engineering, Henan University of Technology,Zhengzhou, 450001 People's Republic of China e-mail: caoxy@haut.edu.cn

²Tianjin EV Energies Co., Ltd, Tianjin, 300112 People's Republic of China

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ABSTRACT

Zero-strain anode material of Li₄Ti₅O₁₂ for lithium ion battery was successfully synthesized via the rheological phase reaction (RPR) method. The as-prepared powders were characterized by means of powder X-ray diffraction (XRD), scanning electron microscope (SEM) and particle size distribution analysis (PSD), and the electrochemical properties of the powders were evaluated by the galvanostatic discharge test and cyclic voltammetry (CV). The results revealed that well-crystallized uniform micro-sized Li₄Ti₅O₁₂ powders were obtained at 800°C for different calcination times via the simple template-free rheological phase route. Among these RPR-derived Li₄Ti₅O₁₂ powders, one synthesized at 800°C for 22 h displays the initial discharge capacity of 184.3 mAh/g and excellent characteristic of cyclic voltammetry.

Keywords: rheological phase synthesis; characterization; Li₄Ti₅O₁₂; anode material; lithium ion batteries

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INTRODUCTION                                                                             

Spinel Li₄Ti₅O₁₂ as an anode material has received increasing interest over past years due to its potential applications in lithium ion batteries^1,2,3,4,5,6. Both the excellent Li intercalation/de-intercalation kinetics and structural stability make it achieve the good high-rate capability and reversibility^6,7. Apart from the cheapness and non-toxicity, Li₄Ti₅O₁₂ also displays a flat voltage plateau located at about 1.5 V vs. Li/Li⁺, which is higher than the reduction potential of the conventional electrolyte solvents^8,9. This feature avoids the reduction of solvent and the formation of a solid electrolyte interface, which accordingly alleviate the potential problem of lithium deposition and render the material significantly safer than graphite. Thus, Li₄Ti₅O₁₂ possesses a continuous interest to the lithium battery industry though the energy density of cell is slightly sacrificed when it is coupled with a 4 V cathode material.

At present, Li₄Ti₅O₁₂ has been widely prepared by various synthetic methods. Among these preparation methods, the conventional solid state reaction is usually used to synthesize Li₄Ti₅O₁₂ powder at high temperature starting from lithium salts and titanium oxides^{1,2,3,4,5,6,7}. These methods need large thermal energy and long reactive time, which may results in the undesirable large particle size and particle heterogeneity and accordingly incur an adverse effect on its electrochemical performances such as discharge capacity and rate capability. In particular, many high temperature calcination processes are made in a nitrogen or oxygen gas stream^1,2,3,4,6,7, which leads to the cost increase. As far as the soft chemistry synthesis methods such as the low temperature chemical lithiation method¹⁰, the sol-gel synthesis method¹¹, the macroemulsion synthesis method¹², the hydrothermal synthesis method¹³, the ball milling-assisted sol-gel method¹⁴ and spray pyrolysis method¹⁵ are concerned, the particle size of thus prepared Li₄Ti₅O₁₂ powders with various morphologies is much finer and more uniform compared with the conventional solid state synthesis powder due to preparing the precursors constituted of ultra-fine particles. As a consequence, the soft-chemical synthesis of Li₄Ti₅O₁₂ powders display the high discharge capacity and excellent rate capability. However, many soft chemistry synthetic methods often suffer from the complicated operation and thereby are not suitable for the mass production in despite of a relative low calcination temperature.

In this work, we introduce a simple synthetic route for preparing the homogenized micro-sized Li₄Ti₅O₁₂ powders by a rheological phase reaction method. It is a process of preparing compounds or materials from a solid-liquid rheological mixture. Firstly, the solid reactants are fully mixed in a proper molar ratio. Then a proper amount of water or other solvent is added to a solid-liquid rheological body where the solid particles and liquid substance are uniformly distributed. After further treatment, the final product is obtained. The structure, morphology and electrochemical properties of the as-synthesized Li₄Ti₅O₁₂ powders have been investigated.

EXPERIMENTAL

Li₄Ti₅O₁₂ powder was prepared through a rheological phase reaction process. LiOHH₂O and anatase TiO₂ were used as starting materials and fully mixed by grinding in a molar ratio of 4:5. A proper amount of redistilled water was added to obtain a rheological body and the mixtures were heated at 90 °C for 5 h in a cylindrical Teflon-lined stainless autoclave. After being dried at 120 °C, the precursor was moved into a furnace at 800 °C for 8 h, 16 h and 22 h in air, respectively and then cooled to room temperature to obtain final Li₄Ti₅O₁₂ powders. All the reagents were analytical reagent grade and as received.

X-ray diffraction (XRD) analysis was carried out on an XRD-Pert Pro diffractometer (XPERT PRO MPD, Netherlands) with Co Kα radiation source (λ = 1.78901 Å). The morphological feature of the as-prepared powders was observed on the SEM- JSM6380LA(JEOL, Japan). The analysis of particle size for the powders was carried out with Mastersizer-2000 (Malvern instruments, England) using a laser granulometer.

The working electrodes were composed of Li₄Ti₅O₁₂ powders, acetylene black and polytetrafluoethylene emulsion binder (60 wt.%) at a weight ratio of 8:1:1. The stainless-steel meshes were used as the current collectors. The CR2016 coin cells were assembled with pure lithium foil as a counter electrode, Celgard-2400 as the separator and 1 mol/L LiClO₄ dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) solution (v/v, 1:1) as the electrolyte. The discharge tests were performed at constant specific current of 30 mA/g with a cut-off voltage of 1.0 V in a multi-channel battery tester (Neware, Shenzhen) at 25 °C. The cells were assembled in an Argon-filled dry glove box. Cyclic voltammetry (CV) test was carried out on a CHI660A electrochemical workstation (Chenhua, Shanghai) at a scan rate of 0.05mV/s using lithium foils as the reference and the counter electrodes.

RESULTS AND DISCUSSION

Fig. 1 shows the XRD patterns of PRP-derived Li₄Ti₅O₁₂ powders heat-treated at 800 °C for different calcination times. It can be obviously seen from the XRD that all the powders display sharp diffraction peaks, indicating an enhanced degree of crystallinity. For the powder obtained at 800 °C for 8 h in Fig. l(a), the main Bragg diffraction peaks of Li₄Ti₅O₁₂ phase have emerged apart from an impurity peak marked with asterisk (); at about 20-21.99°. With prolonging the calcinations time from 8 h to 16 h and 22 h, all the peaks of as-prepared powders exhibit the characteristic diffraction lines of Li₄Ti₅O₁₂ phase without any miscellaneous phase, which are identified as Li^(8a)[Li_1/3Ti_5/3]^(16d) O₁₂^(32e) and indexed to the cubic system with space group Fd¯3m.

Fig. 1: XRD patterns of Li4Ti5O12 powders calcined at 800 °C for different calcination times(a) 8 h, (b) 16 h, (c) 22 h

Fig. 2 presents a set of the different magnified SEM micrographs of RPR-derived Li₄Ti₅O₁₂ powders calcined at 800 °C for 8, 16 and 22 h, respectively. It is easily found that homogenized, uniform and smooth Li₄Ti₅O₁₂ particles are obtained in all situations. In the case of the Li₄Ti₅O₁₂ powder calcined at 800 °C for 8 h in Fig. 2(a-b), the particle sizes are about 1.3-3.9 um in diameter. After heating at 800 °C for 16 and 22 h, the particles tend to become relative smaller and more dispersed. Moreover, Fig. 2(e-f) also displays that a lot of particles are nearly spherical in shape and dense in density, whereas no visible particle aggregation can be found. It is widely acknowledged that uniform micro-sized electrode-active powder with high tap density is very desirable to the practical application in lithium-ion batteries due to avoiding the side reactions at the interface between the electrolyte and the electrode caused by nano-sized particles. As a result, a high discharge capacity for the Li₄Ti₅O₁₂ powder calcined at 800 °C for 22 h is to follow on.

Fig. 2: Different magnified SEM images of Li4Ti5O12 powders obtained at 800 °C for different calcinations times (a-b) 8 h, (c-d) 16 h, (e-f) 22 h; (a, c and e) x 1000, (b,d and f)x2000.

It should be pointed that particle size analysis can demonstrate the entire picture of the pattern where the particles are distributed in the bulk but SEM analysis only gives the magnification of the selected portion of the powder and represents the averaged out size of the particles. In order to understand the PSD of PRP-derived Li₄Ti₅O₁₂ powders, the PSD experiment is conducted and the result is shown in Fig. 3. It gives the particle size distribution in terms of percentage of total particle volume vs. particle diameter of Li₄Ti₅O₁₂ powders. As seen in Fig. 3(a-b), the distribution curves all presents one large population and two small shoulder peaks. However, the Li₄Ti₅O₁₂ powder calcined at 800 °C for 22 h has one large population and one small shoulder peak with a mean size of about 3.2 μm. Table 1 summarizes the selected particle size distribution dada of Li₄Ti₅O₁₂ powders. D_10% denotes the maximal particle size when the volume percentage of the cumulate volume to the total volume amounts to 10%. The meaning of D_50% and D_90% can be analogical. It is noticed that the Li₄Ti₅O₁₂powder synthesized at 800 °C for 22 h is superior to the other powders in the small and uniform-distributed particle size, and its mean particle size is only 3.215 μm.

Fig. 3: Particle size distribution curves of Li4Ti5O12 powders obtained at 800 °C for different calcination times (a) 8 h, (b) 16 h, (c) 22 h

In general, templates or surfactants are essential in the reaction system for preparing ultra-fine powders with homogenized particles. However, neither templates ñor surfactants are used in this RPR process. This phenomenon should be related to the synthetic method and the reaction condition. During the synthesis of the Li₄Ti₅O₁₂ precursor, the surface area of the solid particles can be utilized efficiently while the contact between solid particle and fluid is close and uniform. Meanwhile, the heat exchange between them can be carried out easily and quickly. Therefore, the growth rates of Li₄Ti₅O₁₂nuclei are likely to be identical and controllable. In the subsequent calcination process, the well-crystallized uniform micro-sized Li₄Ti₅O₁₂ powders were obtained. On the basis of the above-mentioned analysis, we tentatively deduce that the RPR process for the synthesis of Li₄Ti₅O₁₂ powders should be divided into three steps, as described in Fig. 4. Firstly, dissolved LiOHH₂0 can be immersed the particle gap of Ti0₂ and coated on its particle surface to attain homogenized mixing of the raw materials. Next, LiOH·H₂0 gradually loses crystal water and becomes LiOH under the heat-treatment condition. Lastly, with the calcination temperature heightening, the resultant LiOH and TiO₂ initiate chemical reactions on the solid-solid interface by losing hydroxyl and convert to Li₄Ti₅O₁₂.

Fig. 4: Schematic diagram of RPR process for forming micro-sized LÍ4TÍ5012 particles.

The initial discharge curves of the Li₄Ti₅O₁₂ powders obtained at 800 °C for different calcination times are shown in Fig. 5. According to Fig. 5, the powders heated at 800 °C for 8 h, 16h and 22h deliver the initial discharge capacity of 160.7 mAh/g, 178.8 mAh/g and 184.3 mAh/g, respectively. Meanwhile, there exists one typical flat discharge plateau located at about 1.46 V in the voltage curves for the lithium intercalation of three Li₄Ti₅O₁₂ powders. Among them, the plateau of the Li₄Ti₅O₁₂ powder obtained at 800°C for 22 h is obviously elongated compared to the Li₄Ti₅O₁₂ powders obtained at 800°C for 8 h and 16 h. The above phenomenon can be ascribed to the differences in the particle size of these powders. As was reported^16,17,18, the smaller grain size will result in a shorter diffusion path of Li⁺ ions and effectively restrains the concentration polarization inside a porous electrode. At the same time, the small particle size means a large active area, which can dramatically lower the effective current density and reduce the electrochemical polarization^16,18. Consequently, the relative large particle size and presence of impurity decrease the discharge capacity of the powder synthesized at 800 °C for 8 h. In contrast, RPR-derived powder caclined at 800 °C for 22 h exhibits highest discharge capacity because of its more uniform and smaller particle size. In addition, the Li₄Ti₅O₁₂ powders obtained at 800 °C for 22 h display a lower discharge plateau than that of the Li₄Ti₅O₁₂ powders obtained at 800°C for 16 h. In the case of Li₄Ti₅O₁₂ anode, reduced discharge plateau will favor to achieve a high operation voltage when Li₄Ti₅O₁₂ and other cathodes such as LiCoO₂, LiMn₂O₄, LiNi_1-x-y, Co_x Mn_y O₂, and LiFePO₄, are assembled into lithium ion batteries.

Fig. 5: The initial discharge curves of Li4Ti5O12 powders obtained at 800 °C for different calcination times (a) 8 h, (b) 16 h, (c) 22 h.

Fig. 6 shows the CV curves of Li₄Ti₅O₁₂ powder prepared at 800 °C for 22 h. It can be seen that the there is one pair of redox peaks in the voltage range of 1.0-3.0 V, which should be attributed to the redox of Ti⁴⁺/Ti³⁺ couple. The well defined cathode and anode peak centered at 1.43 V and 1.75 V indicates a high crystallinity, as reflected in XRD patterns. Also, the profiles of CV curves for the initial two cycles are almost overlapped, suggesting a stable crystal structure of zero-strain Li₄Ti₅O₁₂ anode¹.

Fig. 6: Cyclic voltammograms of Li4Ti5O12 powder obtained at 800 °C for 22 h.

In sum, RPR route processes great advantages for preparing Li₄Ti₅O₁₂ powder with the homogenized particle. This green soft chemical synthesis method does not need any templates or surfactants. Especially, this method is suitable for producing Li₄Ti₅O₁₂ anode materials in battery industry due to its simplicity and feasibility.

CONCLUSIONS

Rheological phase reaction derived Li₄Ti₅O₁₂ powders have been synthesized in this work. Well-crystallized uniform micro-sized Li₄Ti₅O₁₂ powders with narrow particle distribution range are obtained at 800 °C for different calcination times. RPR-derived powder made at 800 °C for 22 h yields the initial discharge capacity of as high as 184.3 mAh/g with a cut-off voltage of 1.0 V at a current rate of 30 mA/g owing to its reduced particle size. Besides, it also exhibits an excellent characteristic of cyclic voltammetry. Therefore, this RPR process demonstrates to be an effective and promising method for preparing a practical electro-active Li₄Ti₅O₁₂ powders with uniform micro-sized particles when the large-scale production is concerned.

ACKNOWLEDGEMENT

This work was supported by the Key Science and Technology Project of Henan Province of China under the grant No. 092102210153.

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(Received: November 26, 2009 - Accepted: June 15, 2010)