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Structure and Electrochemical Properties of Li4Ti5O12 Prepared via Low-Temperature Precipitation
This work aimed to prepare the spinel phase Li4Ti5O12 by a combination of the low-temperature precipitation technique and assisted calcination step. X-ray diffraction (XRD) revealed that the intermediated phase was Li2TiO3, and the spinel phase could be evidently formed at 700°C for 12 to 20 hours. The morphology of spinel powder, determined by SEM and TEM, exhibited a good distribution at the submicrometric scale that promoted a fast kinetic of Li migration and an excellent performance at the high-rate cycling test. The stable performances were achieved in the charge-discharge test at different current densities: 80 mA/g (165 mAh/g), 320 mA (160 mAh/g), and 1600 mA (145 mAh/g) upon 100 cycles. Moreover, we observe a capacity retention of 48% (corresponding 80 mA/g) at a high rate of 5000 mAh/g. The cyclic voltammetry measurement displayed a reversible system and revealed the lithium diffusion coefficient of 1.15 × 10−11 cm2/s.
Spinel Li4Ti5O12 has become an attractive anode material for high-power and fast-charging Li-ion battery because of its excellent cycle performance, structural stability, and little change in the unit cell volume during lithium intercalation/deintercalation. The spinel structure of Li4Ti5O12 provides a three-dimensional network for Li migration [1, 2]. The process of lithium insertion/extraction occurs as the two-phase mechanism at 1.55 V (vs. Li+/Li), characterized by a long flat plateau in the voltage profile. The voltage plateau happens to be much above the reduction potentials of most common electrolyte solvents mitigating the formation of the solid electrolyte interface (SEI). In spite of these beneficial properties, Li4Ti5O12 does not yet meet all the requirements for successful use in fast-charging applications caused by its poor electrical conductivity and sluggish diffusion of lithium ions. [3, 4].
In the literature, many efforts have been aimed to overcome these disadvantages by doping or carbon coating of particles [5, 6] to enhance the conductivity of LTO particles; or alternative synthesis methods fabricate the nano-size LTO with controlled and regular morphologies aiming to shorten the lithium diffusion pathway. It has been reported that nanocrystalline Li5Ti5O12 electrode materials have excellent performance even at high charging rate and high temperature; therefore, they are useful when applied in high-rate charge/discharge Li-ion batteries. Different synthesis methods have been investigated such as sol-gel processes, high-temperature annealing treatment, solution-combustion, and hydrothermal to improve the phase purity and electrochemical performance. Hao et al.  synthesized Li4Ti5O12 particles by a sol-gel method with citric acid as a chelating agent and Li2CO3 and tetrabutyl titanate as starting materials. Mixtures of Li4Ti5O12 and rutile-TiO2 were observed after heating at 800°C for 20 hours in air, while the pure Li4Ti5O12 phase with an average particle size of 500 nm was obtained at 850°C for 24 hours. Li et al.  prepared Li4Ti5O12 with nanotubes and nanowires morphology by a hydrothermal lithium exchange processing. XRD analysis showed a trace of anatase-TiO2 in spinel Li4Ti5O12 nanotubes calcinated at 350°C for 2 hours. The Li4Ti5O12 spinel was synthesized by a sol-gel method using titanium isopropoxide, LiOH, and 2-methoxy ethanol and subsequent treatment with high ball milling by Kim et al.  It was also notified that a trace of rutile-TiO2 was observed for the powder annealed at 850°C for 5 hours. According to previous studies, the heat treatment at high temperature in synthesizing the Li4Ti5O12 spinel obviously led to the formation of rutile or anatase-TiO2, and these impurities are detrimental to the electrochemical performance of the spinel phase. Consequently, low-temperature synthesis or minimum heat treatment by employing a suitable aqueous particulate sol-gel route or intermediate precipitate phase has been recently suggested. Chui and coworker [10, 11] utilized the extreme excess ratio of Ti versus Li (Ti : Li = 6 : 1) to prepare the spinel phase Li4Ti5O12 through the intermediated phase lithium titanate hydrate Li1.81H0.19Ti2O52H2O (LTH).
Our work aimed to prepare the nanosized spinel Li4Ti5O12 powder from the intermediated cubic phase Li2TiO3 which was obtained from a low-temperature precipitation. The effect of temperature and duration of calcination on the formation of the spinel phase was regarded. The structural and morphological properties were characterized by powder X-ray diffraction, Raman spectroscopy, and scanning/transmission electronic microscopy (SEM/TEM). The high-rate performance through galvanostatic charge-discharge and the diffusion coefficient of lithium ions (DLi) in the host Li4Ti5O12 were measured and compared with the bulk material.
2.1. Synthesis of Nanostructured Li4Ti5O12
Spinel-type Li4Ti5O12 was prepared via a low-temperature precipitation method combining with the calcining process from 600 to 800°C. Titanium butoxide Ti(OBu)4 with a normal purity of 97% (Sigma Aldrich, d = 1.491 g/mL at 20°C, M = 340.39 g/mole) and lithium hydroxide monohydrate LiOH·H2O with a normal purity of 99.95% (Sigma Aldrich) were used as titanium and lithium precursors. 10.50 mg Ti(OBu)4 was dropped into 25 mL solution of 1 M LiOH solution under vigorous stirring at 5°C with the molar ratio Li : Ti of 1 : 1.33. Then, the produced powder was collected by centrifugation and washed several times with deionized water to neutral pH to eliminate the excess of LiOH and dried at 120°C for overnight. The as-prepared powder was annealed at 600–800°C for 12 hours.
2.2. Structural Characterizations
The structure of samples was identified by X-ray diffraction (XRD) in D8-Advance (Bruker) diffractometer using CuKα radiation (λ = 1.5406 Å). XRD patterns were collected in the range 10°–70° with a step of 0.029° per second. Lattice parameters were calculated by software CelRef. Raman spectra were measured with a XploRA Raman microspectrometer (Jobin-Yvon-Horiba), using a He : Ne laser (632.8 nm) as the excitation source. The morphologies and the distribution of grain size were determined by using scanning electron microscope (FE-SEM S4800 Hitachi, Japan) and transmission electronic microscopy (TEM, JEOL 1400, Japan).
2.3. Electrochemical Characterizations
The electrochemical performance, including rate capability and charge/discharge capacity of spinel Li4Ti5O12, was evaluated at room temperature with the coin-cell type CR-2025 in which a metallic lithium foil was used as the counterelectrode. The composite electrodes were made of the active material (80 wt.%), conducting acetylene black (7.5 wt.%), carbon graphite (7.5 wt.%), and polyvinylidene fluoride (PVdF) binder (5 wt.%) homogeneously dispersed in N-methyl-2-pyrrolidone (NMP) solvent. The slurry was coated on an aluminum foil and cut to pieces with diameter of 10 mm (an area surface of 0.785 cm2) and then dried at 120°C in vacuum for 12 hours. The coin cell was assembled in an argon-filled glove box (M. Braun Co., [O2] < 1 ppm and [H2O] < 1 ppm). The employed organic electrolyte was a mixture of 1 M LiPF6 with ethylene carbonate (EC) and dimethyl carbonate (DMC) at volumetric ratio 2 : 1.
A constant current protocol in the range of 20 mA/g to 5000 mA/g was used for formation cycles in a potential range of 1–2.5 V (vs. Li+/Li). The cyclic voltammetry (CV) was performed at different scan rates between the voltage of 10 and 120 μV/s to evaluate the lithium coefficient diffusion related to the charge/discharge process.
3. Results and Discussion
3.1. Structure and Morphology
Considering the structure of Li4Ti5O12, the previous studies [1, 2, 4] demonstrated a cubic spinel (unit cell a = 8.3557 Å) with a space group of F3dm. The three ions Li+ are situated at the tetrahedral 8a site, the other ions Li+ and ions Ti+ randomly locate in the octahedral 16d sites with a ratio of 1 : 5, and the oxygen atoms totally are located at the 32e site; this suggests that [Li3]8a[Li1Ti5]16d[O12]32e is the probable structure. Figure 1 compares the XRD patterns of the intermediated phase and final samples calcinated at different temperatures from 500°C to 800°C for 12 hours. The intermediate phase can be indexed in the orthorhombic phase Li2TiO3 (JSPDS: 00-003-1024). At 500°C, the spinel Li4Ti5O12 phase was apparently formed, but the transformation was uncompleted; the remaining of the intermediate phase was still observed in the XRD diagram. At 600°C, XRD patterns provided the main peaks of the Li4Ti5O12 phase (JSPDS: 000-049-0207) , indicating the complete transformation from the intermediated phase to Li4Ti5O12 phase. However, it was noticed in the samples thermally treated, and Li4Ti5O12 presents wide peaks, which indicated the formation of the small crystalline material. The pure spinel phase Li4Ti5O12 was obtained at 700°C and 800°C, and it could be obviously observed that the crystallinity degree of Li4Ti5O12 improved with increase in calcination temperature. In contrast, the calcination at 800°C led a trace of rutile-TiO2. When the calcinating temperature was raised from 600°C to 700°C, the lattice parameter a slightly increased: 8.3477 Å (600°C) to 8.3599 Å (700°C). Colbow et al.  reported a = 8.357 Å, and Harisson et al.  also reported a = 8.358 Å which is in good agreement with our value at 700°C. The sample calcinated at 800°C exhibited unexpectedly the high value of the lattice parameter and 8.3716 Å due to the emergence of the TiO2 rutile phase, respectively.
In order to examine the impact of calcination time, the sample at 700°C was treated at variable scheduling (Figure 2). The remaining intermediate phase was still observed for 4 hours and 8 hours, while the pure phase Li4Ti5O12 only appeared at the duration from 12 hours to 20 hours. However, with 24 hours of calcination, the TiO2 anatase phase was merged in the LTO phase. XRD patterns of samples at 12, 16, and 20 hours are identical and indexed to the pure spinel phase. The lattice parameter a can be determined to be 8.3595 Å which is in good agreement with the reported standard values even though the wide XRD peaks indicated the small crystallite of LTO. The crystallite size of the LTO phase was, respectively, calculated from their reflections using the Scherrer equation based on the full width at half maximum (FWHM) of diffraction peaks:where dhkl is the average crystallite size, k is the constant depending on the crystalline shape (0.9), λ is the radiation X-ray wavelength of Cu (1.5406 Å), β is the FWHM of the most intense peak, and 2θ is the diffraction angle. The rough estimation of crystallite size is around 40 nm for all three samples. The submicrometric LTO could suggest a fast kinetic of lithium insertion due to shortening of the lithium pathway diffusion.
Raman active modes of spinel Li4Ti5O12 involve F2g, Eg, and A1g following the calculation of symmetry group , which represented the stretching and bending vibration of Ti-O bonds in [TiO6] octahedra and Li-O bonds in [LiO4] and [LiO6] polyhedra. [14, 15] At high frequency, two vibration modes A1g at 671 cm−1 and 740 cm−1 were assigned to the vibrations of Ti-O bonds, while the stretching vibrations of the Li-O bonds were attributed by two modes in region medium frequency 430 cm−1 and 374 cm−1. The last modes in low frequency (235 cm−1) were attributed to the bending vibrations of O-Ti-O bonds. In brief, we succeeded to produce the pure phase Li4Ti5O12 treated at 700°C for three schedules (12, 16, and 20 hours) without structural difference.
Figure 3 shows the SEM and TEM morphology of the sample Li4Ti5O12 treated at 700°C for 16 hours. It can be seen that the sample consists of submicro-scaled primary particles with a homogeneous size distribution, which are in turn composed of larger particles of 100 μm. From the TEM image, the primary particles consist of nanorods of 10 nm in length (Figure 4).
3.2. Electrochemical Performance
The electrochemical performance of the spinel nanosized Li4Ti5O12 as anode materials for lithium ion batteries has been investigated. Figure 5(a) illustrates the voltage profiles at low current density 20 mA/g of three samples Li4Ti5O12 calcinated at 700°C for 12, 16, and 20 hours. The samples delivered a first discharge capacity of 178 mAh/g with a cutoff value of 1.0 (vs. Li+/Li). This is in line with the theoretical capacity of 175 mAh·g−1 meaning that the excess lithium ions were intercalated into LTO, and no SEI film formed during the first charge which is usually formed in the range 0.2–0.8 V vs. Li+/Li . In the subsequent cycles, the capacity discharge was stabilized at 167 mAh/g without obvious capacity fading. A flat plateau at 1.65 V vs. Li+/Li precisely corresponding to the Ti3+/Ti4+ redox reaction means that is the insertion/extraction of lithium ions during the charge/discharge process. In addition, a small electrode polarization of about 0.02 V (below 20 mV) is observed between charge and discharge curves indicating the existence of good interparticle electrical contacts and better ion transport owing to the Li4Ti5O12 nanoparticles and good stability of the spinel structure during Li migration. Furthermore, all three spinel samples exhibited excellent cycling stability with a capacity retention of 100% upon 100 cycles (Figure 5(b)).
Spinel Li4Ti5O12 is subjected to cycling at different charge-discharge rates such as 80 mA/g, 320 mA/g, and 1600 mA/g to evaluate the capacity-rate relationship (Figure 6(a)). The charge-discharge curve at current 320 mA/g seems to quasi-superimpose on the one at current 80 mA/g without potential difference. The sample exhibited a capacity value of 165 mAh/g (80 mAh/g) and 160 mAh/g (320 mAh/g), a value close to the theoretical value of 175 mAh/g. At extremely high current of 1600 mAh/g, there is a slight drop in capacity values (12.5% decrease) together with a drop in the discharge voltage plateau at 1.5 V (vs. Li+/Li) which causes high electrode polarization (140 mV). However, the cycling performance upon 100 cycles at all three constant current show an excellent Coulombic efficiency; typically, the capacity retention remained 100% after 100 cycles with the capacity of 165 mAh/g, 160 mAh/g, and 135 mAh/g, respectively.
The rate capability is also important parameter for evaluating an electrode material addressed to the battery as well as supercapacitor application. A constant current discharge test was performed on Li4Ti5O12 electrodes varying the rate from 20 mA/g to 5000 mA/g during discharge in the voltage range of 1.0–2.5 V, with the voltage profiles visible in Figures 7(a)–7(c). The charge-discharge profiles at rates below 1000 mA/g (Figure 7(a)) show a low polarization from 20 mV (40 mA/g) to 90 mV (960 mA/g) indicating the electron transfer efficiency and lithium diffusion pathway. Our spinels exhibit the better rate capability than those reported in the literature . A capacity of 145 mAh/g was achieved at current 400 mA/g, and this capacity amounts to 85% of capacity at the low current of 20 mA/g. From the current density above 1200 mA/g, we observe the critical impact of rates on the voltage profiles and the specific capacity. The more the current density applied, the more the discharge plateau shortened. Traditionally, the lithium insertion/extraction into host Li4Ti5O12 occurred via a two-phase mechanism; herein, it showed a sloping voltage curve similar to that of a one-phase reaction due to the large electrode polarization.
Figure 7(d) illustrates the electrode polarization and specific capacity as function of the C-rate, and the highest current 5000 mA/g corresponds to rate 30°C. In the high current density performance, the electrochemical profiles significantly depended on the electronic conduction, which served as the electrode polarization. The drastic increase of polarization voltage from 100 mV to 350 mV was observed when the currents applied varied from 1200 mA/g to 5000 mA/g. Our spinel Li4Ti5O12 still shows an impressive performance at such high current densities. At three highest currents 4500 mA/g (26°C), 4750 mA/g (28°C), and 5000 mA/g (30°C), the capacities 90 mAh/g, 85 mAh/g, and 80 mAh/g were achieved, respectively. It is noteworthy that the capacity (80 mAh/g) obtained with nanosized LTO at 30°C rate is much higher than that obtained at the 5°C rate (95 mAh/g) with bulk LTO. Indeed, the morphology of mesoporous and nanoparticle LTO powders provide a short diffusion path for Li and pore fill of electrolytes ensuring a high flux of Li.
Cyclic voltammetry analysis has been carried out to further investigate the electrochemical properties of spinel Li4Ti5O12. Figure 8(a) shows the cyclic voltammetry curve at different scan rates to determine the kinetic of Li-migration process. The pair of anodic/cathodic peaks of spinel Li4Ti5O12, located at 1.62 V/1.55 V at the scan rate of 10 μV/s, demonstrate reversible lithium intercalation and deintercalation of host Li4Ti5O12, respectively. The peak separation increases with the increasing scan rate that indicated the effect of electrode polarization owing to the limited diffusion process. The plot of cathodic peak current density (ipc) as a function of the square root of the scan rate () in Figure 8(b) shows a good linear relation between ipc and (R = 0.9999). Herein, the equation Randles–Sevcik  can be applied to determine the diffusion coefficient of lithium ions (DLi), characterized as the kinetics of the lithium insertion process. The diffusion coefficient of lithium ions was calculated to be 1.15 × 10−11 cm2/s.
In summary, spinel Li4Ti5O12 has been successfully prepared through a low temperature precipitation process to obtain a particle-size distribution at the submicrometric scale. The electrochemical measurements show that spinel LTO exhibited the discharge capacity of 178 mAh/g at 0.1°C in the first discharge and favorable cyclic reversibility at different current rates of 20, 80, 320, and 1600 mA/g with a capacity of 167, 165, 160, and 135 mAh/g upon 100 cycles. Furthermore, the spinel samples show an excellent performance at extremely high rate, which is capable of delivering a capacity of 80 mAh/g at current 5000 mA/g.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This research was funded by the Department of Science and Technology in Ho Chi Minh City (DOST) under contract no. 135/2017/HĐ-SKHCN.
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