b Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 239026, China
Lithium ion battery as an energy storage device is attracting more and more attentions due to its high energy and power density. The spinel Li4Ti5O12 has a good reversibility and exhibits a very volume change during charge-discharge process, the Li insertion potential of Li4Ti5O12 is relatively high (~1.5 V) in comparison with that of ~0.1 V for graphite, which can effectively prevent safety problems associated with carbon-based anode materials for the operating voltage is higher than the reduction potential of most organic electrolyte solvents [1-3]. On the other hand, Li4Ti5O12 has low electrochemical conductivity with 10-13 S/cm  at room temperature, which makes it suffer from the problem of poor rate capability. Thus, some strategies have been proposed to overcome the significant drawbacks, by reducing the particle size [5-8], doping [9-12] and surface modification [13, 14].
Herein, reducing the particle size and surface modification were carried out to solve the low electrochemical conductivity by a simple sol-gel method. First of all, stoichiometric amounts of TiO2 (0.005 mol), and LiNO3 (0.004 mol) were added in 80 mL distilled water under magnetic stirring at 60 ℃ for 0.5 h. Secondly, citric acid (0.009 mol) with titanium in equal molar ratio was added to the obtained yellow solution. Then the solution was heated at 80 ℃ under constant stirring to remove the excess water till the solution became a gel. Then the gel was placed in an oven and kept at 80 ℃ for 48 h. Thus, the dry gel was calcined at 750, 800, 850 and 900 ℃, respectively, in air for 12 h at N2 atmosphere to obtain four powders.
The XRD patterns of carbon-coated Li4Ti5O12 obtained at various temperatures is shown in Fig. 1, which is characterized by X-ray diffraction (XRD) using a diffract meter (BRUKER D8 ADVANCE, Cu Kα radiation). For the sample sintered at 750 ℃, a small amount of TiO2 phase is detected in addition to the dominated a cubic spinel-type Li4Ti5O12 with a space of Fd3m, while for the samples sintered at 800 ℃, 850 ℃ and 900 ℃, all the diffraction peaks can be indexed as a single phase. The diffraction intensity becomes stronger along with the increasing temperature, indicating a better crystalline of the sample. The contents of carbon measured by TG (Q600) are 16.1%, 13.5%, 4.7% and 6.5% in the final product of the samples sintered at 750 ℃, 800 ℃, 850 ℃ and 900 ℃, respectively. And the carbon should be amorphous since no peaks attributed to crystalline carbon can be detected in the XRD patterns.
|Fig. 1. XRD patterns of Li4Ti5O12 sample calcined at different temperatures.|
Representative microstructure of the carbon-coated Li4Ti5O12 sample sintered at different temperatures is compared in Figs. 2(a-d), which was examined by a scanning electron microscopy (SEM, JSM-6510LV). From Figs. 2a-c, it can be observed that all the samples have better dispersion and narrower particles size distribution of 200-300 nm, especially the sample sintered at 850 ℃, which agree well with the TEM (EVO-18) image shown in Fig. 2e. On the contrary, the sample sintered at 900 ℃ (shown in Fig. 2d) non-uniform particles can be seen, which are agglomerated together. Fig. 2f reveals that a thin carbon shell with thickness of about 6.5 nm has coated onto Li4Ti5O12 to form a carbon-coated Li4Ti5O12 core-shell nanostructures, which in accordance with the XRD and TG analysis results. It is believed that this carbon layer increasing electrochemical performance of the electrode.
|Fig. 2. SEM micrographs of the powders synthesized at various temperatures (a) 750 ℃, (b) 800 ℃, (c) 850 ℃ and (d) 900 ℃). (e, f) TEM images of sample sintered at 850 ℃.|
The electrochemical properties of the synthesized Li4Ti5O12 sample was analyzed with Li4Ti5O12/Li half cells. The working electrode was prepared by mixing the as-synthesis ample Li4Ti5O12 with acetylene black and polyvinylidiene fluoride (PVDF) binder in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP). The counter electrode was metallic lithium and the electrolyte was 1 mol/L LiPF6 in EC/DEC (1:1). Coin cells (CR2032) were assembled in an argon-filled dry-box. Fig. 3 shows the CV curves in the first cycle of the Li4Ti5O12 powders calcined at various temperatures at a scan rate of 0.2 mV/s between 1.0 V and 3.0 V. It is well known that the cathodic and anodic peaks of the Li4Ti5O12 about 1.75 V and 1.45 V are attributed to the redox of Ti4+/Ti3+ and the reduction, respectively. The strength of cathodic peaks for Li4Ti5O12 powders calcined at 850 ℃ is weaker than other Li4Ti5O12 powders at different temperature, but the strength of anodic peaks for Li4Ti5O12 powders calcined at 850 ℃ is stronger than other Li4Ti5O12, indicating that the Li4Ti5O12 powders calcined at 850 ℃ exhibits a higher coulomb efficiency than other Li4Ti5O12. The results of CV curves can be in good agreement with those of galvanostatic charge and discharge.
|Fig. 3. Cyclic voltammetry curve of the Li4Ti5O12 powders calcined at various temperatures at a scan rate of 0.2 mA/s.|
For comparison, cycling stabilities of sample sintered at different temperatures are presented in Fig. 4. The electrochemical performance of these cells was cycled with a Newware multichannel battery cycler between 1.0 V and 3.0 V at room temperature. The sample sintered at 850 ℃ shows both higher discharge specific capacity and better cycling stabilities than others. After 100 cycles at 1C, the discharge specific capacity can retain 163.5 mAh/g, which is 175 mAh/g of the value obtained in the first cycle. All columbic efficiencies can retain almost 100% except for the first cycle. The capacities of the carbon-coated Li4Ti5O12 decrease along with the increasing of current rates, and even at high current rate of 10C, the capacities can still retain about 80 mAh/g.
|Fig. 4. The cycling performance of the Li4Ti5O12 powders calcined at various temperatures.|
As shown in Fig. 5, the carbon-coated Li4Ti5O12 sample has a reversible discharge specific capacity of 163.5 mAh/g at 1C in the 1st-5th cycle. Furthermore, it can export more than 80 mAh/g at 10C rate. It should be emphasized that discharge capacities of 163 mAh/g and at 1C can be recovered even after discharge at 10C.
|Fig. 5. Shows the rate abilities of carbon-coated Li4Ti5O12 powders calcined at 850 ℃ with different discharge rates, and finally in 1C succession.|
The electrochemical impedance spectrum (EIS) of the electrode after five cycles has been provided in Fig. 6. The impedance of these cells after different cycles was also measured with an electrochemical workstation (CHI604a) in the frequency range from 0.01 Hz to 100 KHz at a scanning rate of 0.1 mV/s. The EIS spectrum shows the compressed semicircle from the high to medium frequency range of each spectrum, which describes the charge transfer resistance (Rct) and constant-phase element (CPE, representing the double-layer capacitance, taking into account the roughness of the particle surface) for these electrodes, the straight line in a low frequency range is the Li+ ion Warburg diffusion resistance in the solid electrode material .
|Fig. 6. Nyqusit plots of Li4Ti5O12 powders calcined at different temperatures.|
Carbon-coated Li4Ti5O12 sample has been synthesized by sol-gel method. Our experimental results showed that the carbon-coated Li4Ti5O12 sample has excellent structure stability. Although the valence of titanium is +4, it can be easily synthesized even in the presence of carbon and reducing atmosphere. The final product of sample sintered at 850 ℃ demonstrates a favorable electronic conductivity with 4.7% residual carbon. The carbon-coated Li4Ti5O12 presents a discharge specific capacity of 175 mAh/g at 1C in the first cycle and a reversible capacity of 163.5 mAh/g can be obtained after 100 cycles. The sample sintered at 850 ℃ shows both higher capacity and better cycling stabilities than others.Itsdue to the samples sintered at 850 ℃ have better dispersion and narrower particles size distribution of 200-300 nm, and the carbon layer makes great contribution to the restriction of grain growth and increasing electronic conductivity, which is significant for improving the electrochemical performance of the electrode [16, 17].Acknowledgement
This work was supported by the Natural Science Foundation of Anhui Province Education Department (No. 2014kjA167).
G.Q. Liu, L. Wen, G.Y. Liu, J. Alloys Compd. 509(2011) 6427-6432. DOI:10.1016/j.jallcom.2011.03.078
T. Yuan, R. Cai, R. Ran, J. Alloys Compd. 505(2010) 367-373. DOI:10.1016/j.jallcom.2010.04.253
T.F. Yi, B. Chen, H.Y. Shen, J. Alloys Compd. 558(2013) 11-17. DOI:10.1016/j.jallcom.2013.01.018
C.H. Chen, J.T. Vaughey, A.N. Jansen, J. Electrochem. Soc. 148(2001) A102-A104. DOI:10.1149/1.1344523
A.S. Prakash, P. Manikandan, K. Ramesha, J. Chem. Mater. 22(2010) 2857-2863. DOI:10.1021/cm100071z
C.C. Li, Q.H. Li, L.B. Chen, et al., Appl. Mater. Interfaces 4(2012) 1233-1238. DOI:10.1021/am2018145
S.H. Yu, A. Pucci, T. Herntrich, J. Mater. Chem. 26(2010) 806-810.
J. Lim, E. Choi, V. Mathew, J. Electrochem. Soc. 158(2011) 275-280. DOI:10.1149/1.3527983
Y.J. Bai, C. Gong, Y.X. Qi, J. Mater. Chem. 22(2012) 19054-19060. DOI:10.1039/c2jm34523d
T.F. Yi, H.P. Liu, Y.R. Zhu, J. Power Sources 215(2012) 258-265. DOI:10.1016/j.jpowsour.2012.04.080
Y.H. Yin, S.Y. Li, Z.J. Fan, J. Mater. Chem. Phys. 130(2011) 186-190. DOI:10.1016/j.matchemphys.2011.06.062
B.B. Tia, H.F. Xiang, L. Zhang, J. Solid State Electrochem. 16(2012) 205-211. DOI:10.1007/s10008-011-1305-z
J.P. Zhu, W. Zu, J.J. Yang, et al., Nanotechnology 12(2012) 2539-2542.
J. Liu, X.F. Li, M. Cai, Electrochim. Acta 93(2013) 195-201. DOI:10.1016/j.electacta.2012.12.141
M.V. Reddy, S. Madhavi, Subba Rao G.V., B.V.R. Chowdari, J. Power Source 162(2006) 1312-1321. DOI:10.1016/j.jpowsour.2006.08.020
J. Peng, Y.T. Zuo, G. Li, et al., Chin. Chem. Lett. 27(2016) 1559-1562. DOI:10.1016/j.cclet.2016.02.028
L. Deng, W.H. Yang, S.X. Zhou, Chin. Chem. Lett. 26(2015) 1529-1534. DOI:10.1016/j.cclet.2015.06.009