Solid State Ionics 322 (2018) 39–43
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Outstanding lithium-storage performance of carbon-free Li4Ti5O12 anode material for rechargeable lithium-ion batteries
Yonghui Shang School of Chemistry & Chemical Engineering, Xianyang Normal University, Xianyang 712000, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Li4Ti5O12 anode Carbon-free Rate capability Lithium-ion batteries
In this work, carbon-free Li4Ti5O12 material has been successfully prepared by using a conventional sol-gel method. X-ray diﬀraction (XRD) result reveals that the as-prepared pristine Li4Ti5O12 sample has the spinel-type structure with a cubic lattice. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images indicate that the morphology of Li4Ti5O12 material is well-crystallized with a uniform size distribution in the range of 300–700 nm. Moreover, high-resolution TEM image demonstrates that the surface of Li4Ti5O12 particle is very smooth without any carbon coating. When used as anode material for lithium-ion batteries, the pristine Li4Ti5O12 exhibits outstanding lithium-storage performances such as high reversible capacity, excellent rate capability and stable cycling property. Therefore, we can speculate that the as-prepared Li4Ti5O12 sample without any carbon materials is a promising candidate for the application of high-performance anode for lithium-ion batteries.
delivered high discharge capacities of 153.9 and 147.9 mAh g−1 at 10 and 20 C respectively. Doping with other metal ions [13–15] is another strategy to improve the intrinsic electronic conductivity of Li4Ti5O12. What's more, synthesis of nanosized Li4Ti5O12 particles [16,17] is a promising approach to reduce the Li+-ion diﬀusion path and thus improve the Li+-ion diﬀusion eﬃciency. However, it remains a great challenge to prepare nanostructured Li4Ti5O12 materials with desirable architectures for fulﬁlling the requirements of excellent rate capability and stable long-cycle life. In this current work, we designed and prepared the carbon-free Li4Ti5O12 material used as anode for rechargeable lithium-ion batteries by using a simple sol-gel approach. The results showed that the wellcrystallized Li4Ti5O12 sample had the spinel-type structure and the surface of Li4Ti5O12 particle was smooth without any carbon coating. The electrochemical measurements revealed that the pristine Li4Ti5O12 anode exhibited outstanding lithium-storage performances including rate capability and long-cycle life. Thus, the as-prepared carbon-free Li4Ti5O12 material can be considered as a promising anode for highperformance lithium-ion batteries.
Nowadays, rechargeable lithium-ion batteries with high energy/ power density and good safety have been regarded as one of the most promising energy devices for electric vehicles and hybrid electric vehicles [1,2]. Nevertheless, the search for advanced anode materials to meet the requirements of good rate capability and long-cycle life is still critical for high-performance lithium-ion batteries. Compared with the commercial graphite-based anodes, spinel Li4Ti5O12 material has attracted great attention for lithium-ion batteries anode due to its intrinsic characteristics such as high thermal stability, good safety, excellent rate capability and long-cycle life [3–5]. Moreover, it shows a ﬂat Li+ insertion/extraction potential plateau at around 1.55 V (vs. Li/ Li+) and a high theoretical capacity of 175 mAh g−1 [6,7]. Despite of these advantages, the Li4Ti5O12 material suﬀers from the inherent poor electronic conductivity (ca. 10−13 S cm−1) and moderate Li+-ion diffusion coeﬃcient (10−8 cm2 s−1) [8,9], which greatly inhibit its practical application for energy storage. Up to the present, many strategies have been devoted to solve these issues. Carbon coating [5,7,9–12] has been regarded as the most common approach to improve the lithium-storage performance of Li4Ti5O12. Owing to this carbon layer, which could be able to prevent the growth of Li4Ti5O12 particles during the sintering process and enhance its apparent electronic conductivity, the high-rate capability and long-cycle stability of Li4Ti5O12 will be improved. For instance, Lin's group  reported that the carbon-coated Li4Ti5O12 was prepared by using a facile one-pot sol-gel route and the obtained composite
E-mail address: [email protected]
https://doi.org/10.1016/j.ssi.2018.04.020 Received 13 March 2018; Received in revised form 17 April 2018; Accepted 26 April 2018 0167-2738/ © 2018 Elsevier B.V. All rights reserved.
2. Experimental 2.1. Material preparation The pristine Li4Ti5O12 sample (P-Li4Ti5O12) was prepared by using a conventional sol-gel method with lithium acetate dehydrate (CH3COOLi·2H2O), tetrabutyl titanate (Ti(OC4H9)4) and citric acid
Solid State Ionics 322 (2018) 39–43
Fig. 1. (a) XRD patterns and (b) Raman spectrum of the as-prepared P-Li4Ti5O12 sample.
3. Results and discussion
(C6H8O7·H2O) as chelating agent. In a typical synthesis, the CH3COOLi·2H2O and Ti(OC4H9)4 were separately dissolved in ethanol solution with Li:Ti molar ratio of 4.5:5. Afterwards, the CH3COOLi solution was slowly added to the Ti(OC4H9)4 solution under continuous stirring at 25 °C for 4 h. Then, the mixed solution was further added to the ethanol solution containing C6H8O7·H2O. The mixture was stirred at 25 °C for 2 h and gently heated at 70 °C until a gel precursor was formed. Finally, the powder was calcinated at 350 °C for 6 h and then calcinated at 850 °C for 12 h in a ﬂowing air atmosphere to get the PLi4Ti5O12 sample.
The XRD patterns of the as-prepared P-Li4Ti5O12 sample are shown in Fig. 1a. All of the diﬀraction peaks of P-Li4Ti5O12 can be indexed to a cubic spinel structure with Fd-3m space group, which are in good
2.2. Material characterization The phase and crystal structure of the as-prepared material were investigated by X-ray powder diﬀraction (XRD) analysis system (Xpert MPD DY1219) equipped with a Cu Kα radiation source (λ = 0.15406 nm). Raman spectroscopy was carried out on a laser Raman spectrometer (Dongwoo DM500i) with the 532 nm line of an Ar ion laser as the excitation source. The morphology and microstructure of P-Li4Ti5O12 sample were recorded by scanning electron microscope (SEM, JEOL-7401) and transmission electron microscopy (TEM, JEOL, JEM-2100F). To further characterize the surface morphology of PLi4Ti5O12 particles, the high-resolution transmission electron microscopy (HRTEM) was also performed. The electronic conductivity of the as-prepared P-Li4Ti5O12 material was measured with an RTS-8 linear four-point probe measurement system. 2.3. Electrochemical measurements CR2032 coin-type cells were used to evaluate the lithium-storage performance of the as-prepared P-Li4Ti5O12 anode material. The working electrodes were prepared by spreading the slurry of the active material (85 wt%), super P (10 wt%) and poly(vinylidene ﬂuoride) binder (5 wt%) dissolved in N-methylpyrrolidone solvent, which was uniformly spread on a Cu foil and dried at 110 °C overnight to remove the residual solvent. The coin cells were assembled in an Ar-ﬁlled glove box using pure lithium foil as the counter electrode, the microporous polypropylene membrane (Celgard 2400) as the separator. The electrolyte was 1 M LiPF6 solution in ethylene carbonate (EC): diethyl carbonate (DEC): ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1. The mass loading of the working electrode was about 2.8 mg cm−2. The charge/discharge tests were carried out over potential ranging from 1.0 V to 2.5 V (vs. Li+/Li) at diﬀerent current rates based on the theoretical capacity of 175 mAh g−1. The electrochemical impedance spectroscopy (EIS) measurement was carried out on an electrochemical workstation (PARSTAT) in the frequency range of 0.01 Hz to 100 kHz by applying an AC signal of 5 mV. All the electrochemical measurements were performed at room temperature (25 °C).
Fig. 2. SEM images of the obtained P-Li4Ti5O12 particles. 40
Solid State Ionics 322 (2018) 39–43
Fig. 3. (a–c) TEM and (d) HRTEM images of P-Li4Ti5O12 particles.
can be seen that the P-Li4Ti5O12 sample has a regular shape and the particle sizes are about 0.3–0.7 μm. To further investigate the microstructure of P-Li4Ti5O12 particles, TEM and HRTEM were performed as presented in Fig. 3. It is found from Fig. 3a,b that the primary particles of P-Li4Ti5O12 sample are about 500 nm in size. The small particles with large surface area are expected to increase the electrode/electrolyte contact area and reduce the Li+-ion diﬀusion pathway during the electrochemical reaction . As illustrated in Fig. 3c,d, the surface of P-Li4Ti5O12 is smooth without any carbon coating layer. Besides, the PLi4Ti5O12 particles are also well crystallized with the d-spacing value of about 0.47 nm, suggesting the P-Li4Ti5O12 sample with great phase purity. Fig. 4 shows the initial charge/discharge curves and cycling performance of P-Li4Ti5O12 anode material at 0.1C in the potential range of 1.0 and 2.5 V (vs. Li+/Li). As shown in Fig. 4a, the P-Li4Ti5O12 electrode delivers the high charge and discharge capacities of 165.2 and 170.6 mAh g−1 with a Coulombic eﬃciency of about 96.8%. The results
agreement with these in JCPDS card No. 26-1198. The results are also consistent well with the previously reported literatures [12,16,17]. Besides, no other impurity phases can be detected in the XRD patterns, suggesting that the conventional sol-gel route is a good method to fabricate the well crystallized Li4Ti5O12 material. The P-Li4Ti5O12 sample was further investigated by Raman spectrum in the wavenumbers of 0–2000 cm−1 as illustrated in Fig. 1b. Obviously, ﬁve bands at around 237, 355, 425, 676 and 750 cm−1 can be found in the Raman spectrum of P-Li4Ti5O12. The low Raman band at 237 cm−1 is assigned to the bending vibrations of OeTieO bonds while the frequency bands at 355 and 425 cm−1 are ascribed to the stretching vibrations of the LieO bonds in LiO6 and LiO4 polyhedra respectively . Moreover, the high frequency bands at 676 and 750 cm−1 can be attributed to the vibrations of TieO bonds in TiO6 octahedra . No D-band and Gband are detected in the wavenumbers of 1000–2000 cm−1, revealing that there is no carbon material in the P-Li4Ti5O12 sample. The morphology of P-Li4Ti5O12 particles is illustrated in Fig. 2. It
Fig. 4. (a) Initial charge/discharge curves and (b) cycling performance of P-Li4Ti5O12 anode material at 0.1 C in the potential range of 1.0 and 2.5 V (vs. Li+/Li). 41
Solid State Ionics 322 (2018) 39–43
Fig. 6. (a) Equivalent circuit model, corresponding to the impedance diagrams; (b) Nyquist plots of P-Li4Ti5O12 electrode.
from 0.2 C to 5 C. It can deliver the reversible capacities of 168.9 mAh g−1 at 0.2 C, 165.2 mAh g−1 at 0.5 C, 160.3 mAh g−1 at 1 C and 153.7 mAh g−1 at 2 C, respectively. Even at a high rate of 5 C, the initial discharge capacity can still approach 141.2 mAh g−1. Moreover, it can be noted that the P-Li4Ti5O12 also exhibits stable cycling properties at each current rates. The long-cycle performance of P-Li4Ti5O12 at 2 C is shown in Fig. 5b. Obviously, it can deliver a reversible capacity of 142.9 mAh g−1 after 300 cycles with a capacity retention ratio of 92.7%. The rate performances of P-Li4Ti5O12 sample in this work and that in other reports are illustrated in Table 1. The excellent rate capability of P-Li4Ti5O12 anode can be attributed to the nanosized PLi4Ti5O12 particles which can reduce the Li+-ion diﬀusion path and improve the Li+-ion diﬀusion eﬃciency . To understand the electrochemical behavior of P-Li4Ti5O12 anode, EIS test was performed before cycling and after 50 cycles at 0.1 C. Fig. 6a gives the equivalent circuit model used to simulate the EIS data. As shown in Fig. 6b, it can be seen that the Nyquist plots of P-Li4Ti5O12 are composed of a semicircle in the high frequency region and a straight line in the low frequency region. Herein, the small intercept is related to the solution resistance (Rs). The depressed semicircle represents the charge-transfer resistance (Rct) and the double-layer capacitance (CPE), whereas the straight line is attributed to the Warburg impedance (Zw) associated with the diﬀusion behavior of Li+-ion within the electrode particles [24,25]. The values of Rct for P-Li4Ti5O12 are 103 and 129 Ω before cycling and after 50 cycles, respectively. The small Rct is expected to overcome the restriction of kinetics during the charge/discharge procedure and enlarge the depth of Li+-ion insertion/extraction . Note that the electronic conductivity of the as-prepared PLi4Ti5O12 material was measured by the four-electrode method and the conductivity of the sample reaches 5.7 × 10−9 S cm−1. Thus, the asprepared P-Li4Ti5O12 anode material exhibits superior lithium-storage performance.
Fig. 5. High-rate performances of P-Li4Ti5O12 anode at various current rates between 1.0 and 2.5 V (vs. Li+/Li). Table 1 Comparison on the rate performances of P-Li4Ti5O12 sample in this work and that in other reports. Samples
P-Li4Ti5O12 Li4Ti5O12 Li4Ti5O12 Li4Ti5O12
Rate capacity (mAh g−1)
168.9 – 169/0.1C 167.9
165.2 163.6 – 166.7
160.3 160.1 158 165.3
153.7 154.3 – 161.9
141.2 142.1 139 156.7
This work   
are also better than that of other reported literature . Besides, the PLi4Ti5O12 presents ﬂat charge/discharge plateaus at about 1.55 V, revealing the characteristic of two-phase reaction based on the Ti4+/Ti3+ redox couple via the following reaction: Li4Ti5O12 + 3Li+ + 3e− ↔ Li7Ti5O12 [22,23]. Meanwhile, it can be seen from Fig. 4b that the PLi4Ti5O12 anode exhibits a good cycling performance, delivering a discharge capacity of 166.5 mAh g−1 over 50 cycles with a capacity retention ratio of 97.6%. The high-rate performances of P-Li4Ti5O12 anode are also performed at various current rates between 1.0 and 2.5 V (vs. Li+/Li). As illustrated in Fig. 5a, the speciﬁc capacities of P-Li4Ti5O12 decrease slowly
4. Conclusions In summary, the carbon-free Li4Ti5O12 material used as anode for rechargeable lithium-ion batteries has been successfully designed and prepared via the sol-gel route. The obtained P-Li4Ti5O12 sample is systematically investigated by XRD, Raman spectrum, SEM, TEM, HRTEM. The electrochemical tests reveal that the P-Li4Ti5O12 anode 42
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exhibits superior battery properties including rate capability and longcycle life. It delivers high reversible capacities of 170.6, 168.9, 165.2, 160.3, 153.7 and 141.2 mAh g−1 at 0.1, 0.2, 0.5, 1, 2 and 5 C, respectively, and still retains 92.7% of the initial capacity at 2 C after 300 cycles. The excellent electrochemical properties can be attributed to the high structure stability and nanosized particles of P-Li4Ti5O12 material. The present work demonstrates that the carbon-free Li4Ti5O12 is a promising anode for high-rate and long-life lithium-ion batteries.
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