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Electrochemical performance of nanocrystalline Li2CoTiO4 cathode materials for lithium ion batteries
Journal of Alloys and Compounds 618 (2015) 210–216
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Electrochemical performance of nanocrystalline Li2CoTiO4 cathode materials for lithium ion batteries Meng Yang a, Xiangyu Zhao a,⇑, Liqun Ma a,⇑, Hui Yang a, Xiaodong Shen a,⇑, Yajuan Bian a,b a b
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China DLG Power Battery (Zhangjiagang) Co., Ltd, Zhangjiagang 215600, PR China
a r t i c l e
i n f o
Article history: Received 29 April 2014 Received in revised form 19 August 2014 Accepted 19 August 2014 Available online 27 August 2014 Keywords: Titanate materials Cation disordered Nanocrystalline Electrochemical performance
a b s t r a c t Cation disordered Li2CoTiO4 titanate with 3D lithium ion channels could be a promising new cathode material for lithium ion batteries due to its high theoretical capacity. Herein the Li2CoTiO4 materials with tunable nanostructures were synthesized by a sol–gel method and subsequent heat treatment at different temperatures. The microstructure and electrochemical properties of the nanocrystalline Li2CoTiO4 materials have been systematically investigated. The Li2CoTiO4 material synthesized at lower temperature possessed smaller particle size and grain size, and allowed a higher reversible extraction of lithium ions per formula unit. Furthermore, the small particle size enabled insertion of lithium along short diffusion paths, and thus an increase of the lithium ion diffusion coefficient. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The research on lithium ion batteries with high performance is very important to develop electric vehicles and store sustainable energy such as wind energy and solar energy [1]. The development of the lithium ion battery with high capacity and rate capability was largely influenced by the performance of the electrode materials. Many efforts have been devoted to the research of the cathode materials such as the mature lithium-insertion materials including the layered LiMO2 (e.g., LiCoO2, LiNiO2, LiMnO2 or their solid solution) [2,3] and the spinel LiMn2O4 [4,5]. In order to achieve a wide application, some polyanionic materials including LiFePO4 [6–8], LiVPO4F [9], Li2MSiO4 [10–14] and Li2MTiO4 (M = Mn, Fe, Co, Ni) [15] have been attracting much attention in recent years. The Li2MTiO4 material shows a cubic cation disordered rock salt structure with the Fm3m space group. The metal atoms are disordered at the octahedral sites of cubic closet packing (CCP) array of anions (Fig. 1). The Ti4+ in the structure has a strong bond with O2, enabling the transition metals to be easily oxidized to higher oxidation state. Cation disorder rock salt phase Li2MTiO4 (M = Mn, Fe, Co, Ni) could allow reversible insertion and extraction of two lithium ions (approx. 300 mA h/g capacity) in principle, which may satisfy the high capacity characteristic. The research on Li2MTiO4 material was firstly reported in 2003 [16]. Subsequently, ⇑ Corresponding authors. Tel.: +86 25 83587234; fax: +86 25 83240205. E-mail addresses: [email protected] (X. Zhao), [email protected] (L. Ma), [email protected] (X. Shen). http://dx.doi.org/10.1016/j.jallcom.2014.08.163 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
this series materials including Li2FeTiO4, Li2MnTiO4, Li2NiTiO4 and Li2xVTiO4 were investigated by several groups [17–22]. The investigation on the Li2CoTiO4 was firstly reported by our group in 2012 [23]. The carbon-coated Li2CoTiO4 delivered a reversible capacity of 144.3 mA h g1 in the first cycle, and showed an capacity retention rate of 75.1% after 70 cycles. However, the cation disorder structure means that lithium ion migration may be impeded by other cations (M2+, Ti4+) distributed in the crystal lattice, and therefore the lithium ion transfer was limited. Assuming that the grain and particle size of the material could be infinitely reduced to the unit cell (Fig. 1), the diffusion of lithium ions would not be blocked by other cations and could be realized in any direction. This means that reducing the grain and particle size could be an effective way to improve the lithium ion diffusion. In this work, nanocrystalline Li2CoTiO4 materials were synthesized. The structure and electrochemical properties of Li2CoTiO4 materials with different particle scales were investigated. In order to get a more clear investigation, the pure Li2CoTiO4 materials were synthesized and used.
2. Experimental 2.1. Preparation of Li2CoTiO4 materials Stoichiometric amounts of lithium acetate dihydrate, cobalt acetate tetrahydrate and tetra-n-butyl titanate were dispersed in ethanol, and were stirred and refluxed at 80 °C for 20 h to enable formation of sol. Then the sol was evaporated at 120 °C for 12 h. The obtained mixture was heat-treated in a gas-tight furnace at different temperatures (550, 600, 650 and 700 °C) for 10 h under nitrogen flow.
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Fig. 1. Schematic diagram of the crystal structure for Li2CoTiO4.
3. Results and discussion
Fig. 2. Refined XRD pattern of the Li2CoTiO4 material synthesized at 650 °C.
2.2. Structural characterization Structural and crystallographic analyses of as prepared material were determined by X-ray diffraction (XRD) with a Cu Ka radiation. Transmission electron microscopy (TEM) analysis was performed with JEOL JEM-2010 operated at an accelerating voltage of 200 kV. The morphology of the sample was characterized by a HITACHI S-4800 field-emission scanning electron microscopy (FE-SEM) at a high tension of 5 kV. 2.3. Electrochemical measurement Electrochemical experiments were performed using coin cell (CR2032) assembled in glove box. The Li2CoTiO4 electrode was made by dispersing 80 wt.% active material, 10 wt.% carbon black and 10 wt.% polyvinylidene fluoride (PVDF) binder in N-Methyl-2-pyrrolidinone (NMP) solvent to form a slurry, which was then spread on to an aluminum foil. Li foil was used as the anode. The electrolyte was 1 M LiPF6 dissolved in the mixture of ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1, v/v). Charge and discharge testing was performed at a constant current (10, 20, 40, 80, 160 or 320 mA g1) between 1.5 and 4.8 V at 298 K by using a BT2000 battery testing system (Arbin, USA). Electrochemical impedance spectroscopy (EIS) of the electrode was obtained at a frequency range from 10 KHz to 0.01 Hz with an amplitude of 5 mV.
Fig. 2 shows the XRD pattern of Li2CoTiO4 synthesized at 650 °C. The refined XRD spectrum indicates that the as-prepared Li2CoTiO4 has a single rock salt phase structure with the Fm3m space group. The refined cell parameter is a = 4.168 (9) Å. The agreement factors are Rp = 8.66% and Rwp = 11.01%, respectively. To confirm the structure of the prepared sample, TEM analysis was carried out. Fig. 3 displays the high resolution transmission electron microscopy (HRTEM) of the Li2CoTiO4 sample synthesized at 650 °C. The HRTEM image in Fig. 3(A) shows clear lattice fringes with d-spacing of 0.241 and 0.208 nm, corresponding to the (1 1 1) and (2 0 0) planes of rock salt phase Li2CoTiO4, respectively. The inset in Fig. 3(A) is the fast Fourier transform (FFT) pattern of the square area, which also points out that the Li2CoTiO4 possesses the rock salt phase structure. In order to provide more detailed information of the microstructure, the observed HRTEM image was locally enlarged and exhibited in Fig. 3(B). The periodic dots arranging at (1 1 1) and (2 0 0) planes (white lines) possess different brightness and scales, as indicated by the arrows. These different dots are distributed at random in the crystal planes, which could cause obvious distortion of structural unit (denoted with the dotted pane) in a short-range. This lattice arrangement may be attributed to the difference in the cationic radius (Li+: 0.68 Å; Co2+: 0.72 Å; Ti4+: 0.68 Å). The results mentioned above confirmed that the single rock salt Li2CoTiO4 phase can be obtained at the calcining temperature of 650 °C. In order to know whether the calcining temperature has a significant effect on the microstructure and morphology of Li2CoTiO4 materials, different calcining temperatures were used. Fig. 4 shows the FE-SEM and TEM images of the Li2CoTiO4 materials calcined at different temperatures. As shown in the SEM images, all of the samples presented an aggregation feature. From the TEM images, it can be found that the particle size of the Li2CoTiO4 material increases from 30–40 nm at 550 °C to 150– 200 nm at 700 °C. The calcination at lower temperature contributes to the formation of small particles. Fig. 5 shows the XRD
direction (the inset is the corresponding FFT image for the quadrate area) and (B) typical two-dimensional Fig. 3. (A) HRTEM image of the Li2CoTiO4 material along ½0 1 1 lattice image of Li2CoTiO4.
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Fig. 4. FE-SEM images of the Li2CoTiO4 materials calcined at different temperatures.
patterns of the Li2CoTiO4 materials calcined at different temperatures. All the diffraction peaks are assigned to the rock salt Li2CoTiO4 phase without any impurities. However, a discrepancy in the intensity and FWHM (full width at half maximum) of the diffraction peaks for the materials calcined at different temperatures was observed. This indicates that the calcining temperature affects the crystallinity of the Li2CoTiO4 materials. The grain size of the Li2CoTiO4 sample calcined at various temperatures was determined by Scherrer’s formula. The (2 0 0) peak was selected for calculation of the grain size, which has values of 29.1, 45.8, 68.0 and 116.1 nm for the materials calcined at 550 °C, 600 °C, 650 °C and 700 °C, respectively. The calcination at higher temperature caused the
grain growth. According to the results of the XRD and SEM for the Li2CoTiO4 materials, we found that the particle size and crystalline grain size of the sample increased by increasing the calcination temperature. Fig. 6 shows the charge and discharge curves of the Li2CoTiO4 electrodes at a current density of 10 mA g1 during the first three cycles. The samples except for that synthesized at 700 °C display similar charge and discharge profiles during the first three cycles. An obvious irreversible capacity and a large polarization between the charge and discharge processes during the first cycle were observed. The obvious irreversible capacity could be explained by the irreversibility of structure change of the Li2CoTiO4 during the
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Fig. 5. XRD patterns of the Li2CoTiO4 materials calcined at different temperatures.
first oxidation process, which has been confirmed by Küezma et al. [17]. The large polarization was mainly caused by the cation disordered structure. Disordered distribution of transition metal cations could act as a migration barrier for the lithium ion diffusion. In such cases, the lithium ion diffusion would require a large activation energy of diffusion, resulting in a large polarization during charge and discharge process. Moreover, the Li2CoTiO4 material calcined at 550 °C, 600 °C or 650 °C shows evident charge and discharge plateaus. The sample synthesized at 550 °C shows the best electrochemical performance, and has a maximum charge capacity of 247.7 mA h g1 (equivalent to 1.68 lithium per formula unit) and a discharge capacity of 150.8 mA h g1 (about 1.02 lithium per formula unit) at the first cycle. This indicates that the Li2CoTiO4 material calcined at a lower temperature could allow a reversible insertion/extraction of more than one lithium ion per formula unit.
Fig. 7. (A) Impedance spectra of Li2CoTiO4 electrodes synthesized at different temperatures. Blue solid lines are the fitting curves by the insert equivalent circuit. (B) The relationship between Zre and x1/2 at low-frequency region of the asprepared Li2CoTiO4 electrodes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Charge and discharge curves of the Li2CoTiO4 electrodes at the first three cycles.
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Table 1 Electrochemical kinetic parameters of Li2CoTiO4 electrodes synthesized at different temperatures. Calcination temperature (°C)
R1 (ohm)
R2 (ohm)
r
D (cm2 s1)
550 600 650 700
124.6 130.3 115.1 79.21
207.8 259.5 526.5 1050.1
24.50 29.78 45.12 93.68
9.26 1016 6.26 1016 4.63 1016 6.33 1017
Whereas the material synthesized at a high temperature of 700 °C only shows sloping charge and discharge profiles, and a charge and discharge capacity of merely 40–50 mA h g1. The lower calcination temperature contributed to the formation of small particles and grains, which not only enabled insertion of lithium along very short diffusion paths, but also reduced lithium ion transfer barrier. Moreover, small grain size of the material could provide a large amount of grain boundaries for lithium ion diffusion. Therefore, it is important to decrease the particle size of this material. To get further understanding of the electrochemical kinetic performance of this new cathode material, we have carried out the electrochemical impedance measurement. Fig. 7 shows the electrochemical impedance spectra of the Li2CoTiO4 electrodes. There are two obvious capacitive loops at high and medium frequencies, and a long straight line at low frequency in Fig. 7(A). An equivalent circuit used to explain the spectra is shown in the insert. The parameters R1 and CPE1 are responsible for the surface layer resistance and capacitance, whereas R2 and CPE2 correspond to the lithium ion intercalation/deintercalation reaction resistance and interfacial capacitance, respectively [24]. The long straight lines in the low frequency range are caused by Warburg impedance (Wo) associated with lithium ion diffusion. The values of the parameters for
the Li2CoTiO4 electrodes can be calculated and listed in Table 1. From the long straight line in the low frequency range, the lithium ion diffusion coefficient is estimated according to the following equation [25],
D ¼ R2 T 2 Þ=ð2A2 n4 F 4 C 2 r2
ð1Þ
where D is the diffusion coefficient of lithium ion; R is the gas constant (8.31 J K1 mol1); T is the absolute temperature (300 K); A is the surface area of the electrode; n is the number of electrons per molecule; F is the Faraday constant (96,500 C mol1); C is the concentration of lithium ion (mol cm3) and r is the Warburg factor (ohm s1/2) which has relationship with
Z re / rx1=2
ð2Þ
The calcination at higher temperature led to the grain and particle growth, which decreased the surface layer resistance of the electrode. On the contrary, it decreased the surface area of electrode material, which increased the lithium ion intercalation/deintercalation reaction resistance. For example, the R1 increases from 79.21 (700 °C) to 124.6 (550 °C) ohm, and the corresponding R2 decreases from 1050.1 to 207.8 ohm. Furthermore, the lithium ion diffusion coefficient of Li2CoTiO4 electrode increases from 6.33 1017 (700 °C) to 9.26 1016 (550 °C) cm2 S1, which is comparable to the reported values of Li2NiTiO4 [20] (e.g. 6.33 1017 cm2 S1) and lower than that of LiCoO2 [26] (e.g. 1013–107 cm2 S1). An increased lithium ion diffusion coefficient for the sample with small grain and particle sizes could be explained by two aspects. The small grain or particle can provide more unhindered ion channel for the lithium ion diffusion, which means that the obstacle from transition metal cations could be
Fig. 8. Cycling stability curves (green line for coulombic efficiency) of the Li2CoTiO4 electrodes synthesized at different temperatures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. EIS patterns of the Li2CoTiO4 electrodes for different cycles: (A) Nyquist plots and (B) Bode plots.
the appearance of another time constant, corresponding to another capacitive loop in the Nyquist plot. According to the above observations, we infer that the SEI film could form on the electrode in the initial several cycles, and led to a change of the impedance spectrum. The equivalent circuits for the EIS spectra of the electrodes are shown in Fig. 10. The parameters R1 and CPE1 mean the surface layer resistance and capacitance, respectively. R2 and CPE2 correspond to the lithium ion intercalation/deintercalation reaction resistance and interfacial capacitance, respectively. The parameters R3 and CPE3 appearing in Fig. 10b are attributed to SEI film resistance and capacitance, respectively. Fig. 10. Equivalent circuits for the Li2CoTiO4 electrodes at different cycles: (a) 0th and (b) 5th cycle.
reduced during the lithium insertion/extraction process. On the other hand, the small grain size delivers a large amount of grain boundary, which favors lithium ion diffusion. Fig. 8 shows the discharge capacity of the electrodes as a function of the cycling number. All samples exhibited capacity decay and low coulombic efficiency in initial several cycles, and showed a decline of the discharge capacity when increasing the current density. For example, the electrode only shows a discharge capacity of less than 10 mA h g1 or could not discharge at a high current density of 320 mA g1. The capacity retention rates after 80 cycles are 68.8% (for 550 °C sample), 63.9% (for 600 °C sample) and 91.1% (for 650 °C sample). However, small particles and grains obtained at lower calcination temperature are beneficial to improve the rate capability. The sample calcined at 550 °C can deliver a discharge capacity of 15 mA h g1 at 160 mA g1, while the sample calcined at 700 °C almost could not charge and discharge. The synthesis of the Li2CoTiO4 material with ultrafine particle size may favor its rate capability. The capacity decay and low coulombic efficiencies in the initial several cycles might be attributed to irreversibility of structure change [23] and the formation of solid electrolyte interface (SEI) on the surface of Li2CoTiO4 electrodes. After several cycles, the structure and SEI tended to be stable according to the EIS results in Figs. 9 and 10. Fig. 9 shows the Nyquist and Bode plots of the Li2CoTiO4 electrodes synthesized at 650 °C. The electrode showed two obvious capacitive loops at high and medium frequencies and a long straight line at low frequency in the Nyquist plot (Fig. 9A), which was in agreement with the result from the Bode-phase plots, as shown in Fig. 9B. After 5 cycles, the impedance spectrum of the electrode changed and showed three consecutive capacitive loops, which could be better recognized from the Bode presentation in Fig. 9B. In the Bode phase plot, an obvious peak can be observed in the 2539–81.38 Hz frequency range. This indicates
4. Conclusions In summary, the different scale particles of Li2CoTiO4 with 3D lithium ion channels were successfully prepared and then used as the cathode materials in lithium ion batteries. The detailed information of cation disorder structure for Li2CoTiO4 was confirmed by TEM technology. The results of XRD and SEM indicated that controllable nanostructure materials can be obtained by decreasing the calcination temperature. The Li2CoTiO4 material synthesized at 550 °C showed a maximum discharge capacity of 150.8 mA h g1 (about 1.02 lithium per formula unit). Moreover, the electrochemical kinetic performance of the Li2CoTiO4 electrode can be improved obviously by decreasing the calcination temperature. For example, the lithium ion intercalation/deintercalation reaction resistance decreased from 1050.1 (700 °C) to 207.8 ohm (550 °C), and the lithium ion diffusion coefficient increased from 6.33 1017 (700 °C) to 9.26 1016 cm2 S1 (550 °C). Acknowledgements This work was supported by the China Postdoctoral Science Foundation (2012M521064). The support from the National Natural Science Foundation of China (51201089) and the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) were also acknowledged. References [1] A. Manthiram, J. Phys. Chem. Lett. 2 (2011) 176–184. [2] F. Cheng, J. Liang, Z. Tao, J. Chen, Adv. Mater. 23 (2011) 1695–1715. [3] Y.K. Sun, Z. Chen, H.J. Noh, D.J. Lee, H.G. Jung, Y. Ren, S. Wang, C.S. Yoon, S.T. Myung, K. Amine, Nat. Mater. 11 (2012) 942–947. [4] F.X. Wang, S.Y. Xiao, Y. Shi, L.L. Liu, Y.S. Zhu, Y.P. Wu, J.Z. Wang, R. Holze, Electrochim. Acta 93 (2013) 301–306. [5] Q. Zhang, J. Mei, X. Wang, W. Fan, F. Wang, W. Lu, F. Tang, J. Alloys Comp. 606 (2014) 249–253. [6] X.M. Liu, P. Yan, Y.Y. Xie, H. Yang, X.D. Shen, Z.F. Ma, Chem. Commun. 49 (2013) 5396–5398.
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[7] L.H. Hu, F.Y. Wu, C.T. Lin, A.N. Khlobystov, L.J. Li, Nat. Commun. 4 (2013) 1687– 1692. [8] R. Rosaiah, O.M. Hussain, J. Alloys Comp. 614 (2014) 13–19. [9] J. Wang, X. Li, Z. Wang, H. Guo, Y. Li, Z. He, B. Huang, J. Alloys Comp. 581 (2013) 836–842. [10] J. Yang, X. Kang, L. Hu, X. Gong, D. He, T. Peng, S. Mu, J. Alloys Comp. 572 (2013) 158–162. [11] Y.C. Wang, S.X. Zhao, P.Y. Zhai, F. Li, C.W. Nan, J. Alloys Comp. 614 (2014) 271– 276. [12] M.K. Devaraju, T. Tomai, A. Unemoto, I. Honma, RSC Adv. 3 (2013) 608–615. [13] M.M. Kalantarian, S. Asgari, P. Mustarelli, J. Mater. Chem. A 1 (2013) 2847– 2855. [14] M.S. Islam, R. Dominko, C. Masquelier, C. Sirisopanaporn, A.R. Armstrong, P.G. Bruce, J. Mater. Chem. 21 (2011) 9811–9818. [15] S.R.S. Prabaharan, M.S. Michael, H. Ikuta, Y. Uchimoto, M. Wakihara, Solid State Ionics 172 (2004) 39–45. [16] L. Sebastian, J. Gopalakrishnan, J. Solid State Chem. 172 (2003) 171–177.
[17] M. Kuezma, R. Dominko, D. Hanzˇel, A. Kodre, I. Arcˇon, A. Meden, M. Gaberšcˇek, J. Electrochem. Soc. 156 (2009) A809–A816. [18] M. Kuezma, R. Dominko, A. Meden, D. Makovec, M. Bele, J. Jamnik, M. Gaberšcˇek, J. Power Sources 189 (2009) 81–88. [19] R. Dominko, C.V.A. Garrido, M. Bele, M. Kuezma, I. Arcon, M. Gaberscek, J. Power Sources 196 (2011) 6856–6862. [20] R. Trócoli, M. Cruz-Yusta, J. Morales, J. Santos-Peña, Electrochim. Acta 100 (2013) 93–100. [21] Y. Kawano, A. Kitajou, S. Okada, J. Power Sources 242 (2013) 768–774. [22] Y. Wang, Y. Wang, F. Wang, Nanoscale Res. Lett. 9 (2014) 197–200. [23] M. Yang, X.Y. Zhao, Y. Bian, L.M. Ma, Y. Ding, X.D. Shen, J. Mater. Chem. 22 (2012) 6200–6205. [24] Y. Zhou, J. Wang, Y. Hu, R. O’Hayer, Z. Shao, Chem. Commun. 46 (2010) 7151– 7153. [25] G. Peng, L.L. Zhang, X.L. Yang, S. Duan, G. Liang, Y.H. Huang, J. Alloys Comp. 570 (2013) 1–6. [26] Y.H. Rho, K. Kanamura, J. Electrochem. Soc. 151 (2004) A1406–A1411.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Electrochemical performance of nanocrystalline Li2CoTiO4 cathode materials for lithium ion batteries Meng Yang a, Xiangyu Zhao a,⇑, Liqun Ma a,⇑, Hui Yang a, Xiaodong Shen a,⇑, Yajuan Bian a,b a b
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China DLG Power Battery (Zhangjiagang) Co., Ltd, Zhangjiagang 215600, PR China
a r t i c l e
i n f o
Article history: Received 29 April 2014 Received in revised form 19 August 2014 Accepted 19 August 2014 Available online 27 August 2014 Keywords: Titanate materials Cation disordered Nanocrystalline Electrochemical performance
a b s t r a c t Cation disordered Li2CoTiO4 titanate with 3D lithium ion channels could be a promising new cathode material for lithium ion batteries due to its high theoretical capacity. Herein the Li2CoTiO4 materials with tunable nanostructures were synthesized by a sol–gel method and subsequent heat treatment at different temperatures. The microstructure and electrochemical properties of the nanocrystalline Li2CoTiO4 materials have been systematically investigated. The Li2CoTiO4 material synthesized at lower temperature possessed smaller particle size and grain size, and allowed a higher reversible extraction of lithium ions per formula unit. Furthermore, the small particle size enabled insertion of lithium along short diffusion paths, and thus an increase of the lithium ion diffusion coefficient. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The research on lithium ion batteries with high performance is very important to develop electric vehicles and store sustainable energy such as wind energy and solar energy [1]. The development of the lithium ion battery with high capacity and rate capability was largely influenced by the performance of the electrode materials. Many efforts have been devoted to the research of the cathode materials such as the mature lithium-insertion materials including the layered LiMO2 (e.g., LiCoO2, LiNiO2, LiMnO2 or their solid solution) [2,3] and the spinel LiMn2O4 [4,5]. In order to achieve a wide application, some polyanionic materials including LiFePO4 [6–8], LiVPO4F [9], Li2MSiO4 [10–14] and Li2MTiO4 (M = Mn, Fe, Co, Ni) [15] have been attracting much attention in recent years. The Li2MTiO4 material shows a cubic cation disordered rock salt structure with the Fm3m space group. The metal atoms are disordered at the octahedral sites of cubic closet packing (CCP) array of anions (Fig. 1). The Ti4+ in the structure has a strong bond with O2, enabling the transition metals to be easily oxidized to higher oxidation state. Cation disorder rock salt phase Li2MTiO4 (M = Mn, Fe, Co, Ni) could allow reversible insertion and extraction of two lithium ions (approx. 300 mA h/g capacity) in principle, which may satisfy the high capacity characteristic. The research on Li2MTiO4 material was firstly reported in 2003 [16]. Subsequently, ⇑ Corresponding authors. Tel.: +86 25 83587234; fax: +86 25 83240205. E-mail addresses: [email protected] (X. Zhao), [email protected] (L. Ma), [email protected] (X. Shen). http://dx.doi.org/10.1016/j.jallcom.2014.08.163 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
this series materials including Li2FeTiO4, Li2MnTiO4, Li2NiTiO4 and Li2xVTiO4 were investigated by several groups [17–22]. The investigation on the Li2CoTiO4 was firstly reported by our group in 2012 [23]. The carbon-coated Li2CoTiO4 delivered a reversible capacity of 144.3 mA h g1 in the first cycle, and showed an capacity retention rate of 75.1% after 70 cycles. However, the cation disorder structure means that lithium ion migration may be impeded by other cations (M2+, Ti4+) distributed in the crystal lattice, and therefore the lithium ion transfer was limited. Assuming that the grain and particle size of the material could be infinitely reduced to the unit cell (Fig. 1), the diffusion of lithium ions would not be blocked by other cations and could be realized in any direction. This means that reducing the grain and particle size could be an effective way to improve the lithium ion diffusion. In this work, nanocrystalline Li2CoTiO4 materials were synthesized. The structure and electrochemical properties of Li2CoTiO4 materials with different particle scales were investigated. In order to get a more clear investigation, the pure Li2CoTiO4 materials were synthesized and used.
2. Experimental 2.1. Preparation of Li2CoTiO4 materials Stoichiometric amounts of lithium acetate dihydrate, cobalt acetate tetrahydrate and tetra-n-butyl titanate were dispersed in ethanol, and were stirred and refluxed at 80 °C for 20 h to enable formation of sol. Then the sol was evaporated at 120 °C for 12 h. The obtained mixture was heat-treated in a gas-tight furnace at different temperatures (550, 600, 650 and 700 °C) for 10 h under nitrogen flow.
M. Yang et al. / Journal of Alloys and Compounds 618 (2015) 210–216
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Fig. 1. Schematic diagram of the crystal structure for Li2CoTiO4.
3. Results and discussion
Fig. 2. Refined XRD pattern of the Li2CoTiO4 material synthesized at 650 °C.
2.2. Structural characterization Structural and crystallographic analyses of as prepared material were determined by X-ray diffraction (XRD) with a Cu Ka radiation. Transmission electron microscopy (TEM) analysis was performed with JEOL JEM-2010 operated at an accelerating voltage of 200 kV. The morphology of the sample was characterized by a HITACHI S-4800 field-emission scanning electron microscopy (FE-SEM) at a high tension of 5 kV. 2.3. Electrochemical measurement Electrochemical experiments were performed using coin cell (CR2032) assembled in glove box. The Li2CoTiO4 electrode was made by dispersing 80 wt.% active material, 10 wt.% carbon black and 10 wt.% polyvinylidene fluoride (PVDF) binder in N-Methyl-2-pyrrolidinone (NMP) solvent to form a slurry, which was then spread on to an aluminum foil. Li foil was used as the anode. The electrolyte was 1 M LiPF6 dissolved in the mixture of ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1, v/v). Charge and discharge testing was performed at a constant current (10, 20, 40, 80, 160 or 320 mA g1) between 1.5 and 4.8 V at 298 K by using a BT2000 battery testing system (Arbin, USA). Electrochemical impedance spectroscopy (EIS) of the electrode was obtained at a frequency range from 10 KHz to 0.01 Hz with an amplitude of 5 mV.
Fig. 2 shows the XRD pattern of Li2CoTiO4 synthesized at 650 °C. The refined XRD spectrum indicates that the as-prepared Li2CoTiO4 has a single rock salt phase structure with the Fm3m space group. The refined cell parameter is a = 4.168 (9) Å. The agreement factors are Rp = 8.66% and Rwp = 11.01%, respectively. To confirm the structure of the prepared sample, TEM analysis was carried out. Fig. 3 displays the high resolution transmission electron microscopy (HRTEM) of the Li2CoTiO4 sample synthesized at 650 °C. The HRTEM image in Fig. 3(A) shows clear lattice fringes with d-spacing of 0.241 and 0.208 nm, corresponding to the (1 1 1) and (2 0 0) planes of rock salt phase Li2CoTiO4, respectively. The inset in Fig. 3(A) is the fast Fourier transform (FFT) pattern of the square area, which also points out that the Li2CoTiO4 possesses the rock salt phase structure. In order to provide more detailed information of the microstructure, the observed HRTEM image was locally enlarged and exhibited in Fig. 3(B). The periodic dots arranging at (1 1 1) and (2 0 0) planes (white lines) possess different brightness and scales, as indicated by the arrows. These different dots are distributed at random in the crystal planes, which could cause obvious distortion of structural unit (denoted with the dotted pane) in a short-range. This lattice arrangement may be attributed to the difference in the cationic radius (Li+: 0.68 Å; Co2+: 0.72 Å; Ti4+: 0.68 Å). The results mentioned above confirmed that the single rock salt Li2CoTiO4 phase can be obtained at the calcining temperature of 650 °C. In order to know whether the calcining temperature has a significant effect on the microstructure and morphology of Li2CoTiO4 materials, different calcining temperatures were used. Fig. 4 shows the FE-SEM and TEM images of the Li2CoTiO4 materials calcined at different temperatures. As shown in the SEM images, all of the samples presented an aggregation feature. From the TEM images, it can be found that the particle size of the Li2CoTiO4 material increases from 30–40 nm at 550 °C to 150– 200 nm at 700 °C. The calcination at lower temperature contributes to the formation of small particles. Fig. 5 shows the XRD
direction (the inset is the corresponding FFT image for the quadrate area) and (B) typical two-dimensional Fig. 3. (A) HRTEM image of the Li2CoTiO4 material along ½0 1 1 lattice image of Li2CoTiO4.
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Fig. 4. FE-SEM images of the Li2CoTiO4 materials calcined at different temperatures.
patterns of the Li2CoTiO4 materials calcined at different temperatures. All the diffraction peaks are assigned to the rock salt Li2CoTiO4 phase without any impurities. However, a discrepancy in the intensity and FWHM (full width at half maximum) of the diffraction peaks for the materials calcined at different temperatures was observed. This indicates that the calcining temperature affects the crystallinity of the Li2CoTiO4 materials. The grain size of the Li2CoTiO4 sample calcined at various temperatures was determined by Scherrer’s formula. The (2 0 0) peak was selected for calculation of the grain size, which has values of 29.1, 45.8, 68.0 and 116.1 nm for the materials calcined at 550 °C, 600 °C, 650 °C and 700 °C, respectively. The calcination at higher temperature caused the
grain growth. According to the results of the XRD and SEM for the Li2CoTiO4 materials, we found that the particle size and crystalline grain size of the sample increased by increasing the calcination temperature. Fig. 6 shows the charge and discharge curves of the Li2CoTiO4 electrodes at a current density of 10 mA g1 during the first three cycles. The samples except for that synthesized at 700 °C display similar charge and discharge profiles during the first three cycles. An obvious irreversible capacity and a large polarization between the charge and discharge processes during the first cycle were observed. The obvious irreversible capacity could be explained by the irreversibility of structure change of the Li2CoTiO4 during the
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Fig. 5. XRD patterns of the Li2CoTiO4 materials calcined at different temperatures.
first oxidation process, which has been confirmed by Küezma et al. [17]. The large polarization was mainly caused by the cation disordered structure. Disordered distribution of transition metal cations could act as a migration barrier for the lithium ion diffusion. In such cases, the lithium ion diffusion would require a large activation energy of diffusion, resulting in a large polarization during charge and discharge process. Moreover, the Li2CoTiO4 material calcined at 550 °C, 600 °C or 650 °C shows evident charge and discharge plateaus. The sample synthesized at 550 °C shows the best electrochemical performance, and has a maximum charge capacity of 247.7 mA h g1 (equivalent to 1.68 lithium per formula unit) and a discharge capacity of 150.8 mA h g1 (about 1.02 lithium per formula unit) at the first cycle. This indicates that the Li2CoTiO4 material calcined at a lower temperature could allow a reversible insertion/extraction of more than one lithium ion per formula unit.
Fig. 7. (A) Impedance spectra of Li2CoTiO4 electrodes synthesized at different temperatures. Blue solid lines are the fitting curves by the insert equivalent circuit. (B) The relationship between Zre and x1/2 at low-frequency region of the asprepared Li2CoTiO4 electrodes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Charge and discharge curves of the Li2CoTiO4 electrodes at the first three cycles.
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Table 1 Electrochemical kinetic parameters of Li2CoTiO4 electrodes synthesized at different temperatures. Calcination temperature (°C)
R1 (ohm)
R2 (ohm)
r
D (cm2 s1)
550 600 650 700
124.6 130.3 115.1 79.21
207.8 259.5 526.5 1050.1
24.50 29.78 45.12 93.68
9.26 1016 6.26 1016 4.63 1016 6.33 1017
Whereas the material synthesized at a high temperature of 700 °C only shows sloping charge and discharge profiles, and a charge and discharge capacity of merely 40–50 mA h g1. The lower calcination temperature contributed to the formation of small particles and grains, which not only enabled insertion of lithium along very short diffusion paths, but also reduced lithium ion transfer barrier. Moreover, small grain size of the material could provide a large amount of grain boundaries for lithium ion diffusion. Therefore, it is important to decrease the particle size of this material. To get further understanding of the electrochemical kinetic performance of this new cathode material, we have carried out the electrochemical impedance measurement. Fig. 7 shows the electrochemical impedance spectra of the Li2CoTiO4 electrodes. There are two obvious capacitive loops at high and medium frequencies, and a long straight line at low frequency in Fig. 7(A). An equivalent circuit used to explain the spectra is shown in the insert. The parameters R1 and CPE1 are responsible for the surface layer resistance and capacitance, whereas R2 and CPE2 correspond to the lithium ion intercalation/deintercalation reaction resistance and interfacial capacitance, respectively [24]. The long straight lines in the low frequency range are caused by Warburg impedance (Wo) associated with lithium ion diffusion. The values of the parameters for
the Li2CoTiO4 electrodes can be calculated and listed in Table 1. From the long straight line in the low frequency range, the lithium ion diffusion coefficient is estimated according to the following equation [25],
D ¼ R2 T 2 Þ=ð2A2 n4 F 4 C 2 r2
ð1Þ
where D is the diffusion coefficient of lithium ion; R is the gas constant (8.31 J K1 mol1); T is the absolute temperature (300 K); A is the surface area of the electrode; n is the number of electrons per molecule; F is the Faraday constant (96,500 C mol1); C is the concentration of lithium ion (mol cm3) and r is the Warburg factor (ohm s1/2) which has relationship with
Z re / rx1=2
ð2Þ
The calcination at higher temperature led to the grain and particle growth, which decreased the surface layer resistance of the electrode. On the contrary, it decreased the surface area of electrode material, which increased the lithium ion intercalation/deintercalation reaction resistance. For example, the R1 increases from 79.21 (700 °C) to 124.6 (550 °C) ohm, and the corresponding R2 decreases from 1050.1 to 207.8 ohm. Furthermore, the lithium ion diffusion coefficient of Li2CoTiO4 electrode increases from 6.33 1017 (700 °C) to 9.26 1016 (550 °C) cm2 S1, which is comparable to the reported values of Li2NiTiO4 [20] (e.g. 6.33 1017 cm2 S1) and lower than that of LiCoO2 [26] (e.g. 1013–107 cm2 S1). An increased lithium ion diffusion coefficient for the sample with small grain and particle sizes could be explained by two aspects. The small grain or particle can provide more unhindered ion channel for the lithium ion diffusion, which means that the obstacle from transition metal cations could be
Fig. 8. Cycling stability curves (green line for coulombic efficiency) of the Li2CoTiO4 electrodes synthesized at different temperatures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. EIS patterns of the Li2CoTiO4 electrodes for different cycles: (A) Nyquist plots and (B) Bode plots.
the appearance of another time constant, corresponding to another capacitive loop in the Nyquist plot. According to the above observations, we infer that the SEI film could form on the electrode in the initial several cycles, and led to a change of the impedance spectrum. The equivalent circuits for the EIS spectra of the electrodes are shown in Fig. 10. The parameters R1 and CPE1 mean the surface layer resistance and capacitance, respectively. R2 and CPE2 correspond to the lithium ion intercalation/deintercalation reaction resistance and interfacial capacitance, respectively. The parameters R3 and CPE3 appearing in Fig. 10b are attributed to SEI film resistance and capacitance, respectively. Fig. 10. Equivalent circuits for the Li2CoTiO4 electrodes at different cycles: (a) 0th and (b) 5th cycle.
reduced during the lithium insertion/extraction process. On the other hand, the small grain size delivers a large amount of grain boundary, which favors lithium ion diffusion. Fig. 8 shows the discharge capacity of the electrodes as a function of the cycling number. All samples exhibited capacity decay and low coulombic efficiency in initial several cycles, and showed a decline of the discharge capacity when increasing the current density. For example, the electrode only shows a discharge capacity of less than 10 mA h g1 or could not discharge at a high current density of 320 mA g1. The capacity retention rates after 80 cycles are 68.8% (for 550 °C sample), 63.9% (for 600 °C sample) and 91.1% (for 650 °C sample). However, small particles and grains obtained at lower calcination temperature are beneficial to improve the rate capability. The sample calcined at 550 °C can deliver a discharge capacity of 15 mA h g1 at 160 mA g1, while the sample calcined at 700 °C almost could not charge and discharge. The synthesis of the Li2CoTiO4 material with ultrafine particle size may favor its rate capability. The capacity decay and low coulombic efficiencies in the initial several cycles might be attributed to irreversibility of structure change [23] and the formation of solid electrolyte interface (SEI) on the surface of Li2CoTiO4 electrodes. After several cycles, the structure and SEI tended to be stable according to the EIS results in Figs. 9 and 10. Fig. 9 shows the Nyquist and Bode plots of the Li2CoTiO4 electrodes synthesized at 650 °C. The electrode showed two obvious capacitive loops at high and medium frequencies and a long straight line at low frequency in the Nyquist plot (Fig. 9A), which was in agreement with the result from the Bode-phase plots, as shown in Fig. 9B. After 5 cycles, the impedance spectrum of the electrode changed and showed three consecutive capacitive loops, which could be better recognized from the Bode presentation in Fig. 9B. In the Bode phase plot, an obvious peak can be observed in the 2539–81.38 Hz frequency range. This indicates
4. Conclusions In summary, the different scale particles of Li2CoTiO4 with 3D lithium ion channels were successfully prepared and then used as the cathode materials in lithium ion batteries. The detailed information of cation disorder structure for Li2CoTiO4 was confirmed by TEM technology. The results of XRD and SEM indicated that controllable nanostructure materials can be obtained by decreasing the calcination temperature. The Li2CoTiO4 material synthesized at 550 °C showed a maximum discharge capacity of 150.8 mA h g1 (about 1.02 lithium per formula unit). Moreover, the electrochemical kinetic performance of the Li2CoTiO4 electrode can be improved obviously by decreasing the calcination temperature. For example, the lithium ion intercalation/deintercalation reaction resistance decreased from 1050.1 (700 °C) to 207.8 ohm (550 °C), and the lithium ion diffusion coefficient increased from 6.33 1017 (700 °C) to 9.26 1016 cm2 S1 (550 °C). Acknowledgements This work was supported by the China Postdoctoral Science Foundation (2012M521064). The support from the National Natural Science Foundation of China (51201089) and the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) were also acknowledged. References [1] A. Manthiram, J. Phys. Chem. Lett. 2 (2011) 176–184. [2] F. Cheng, J. Liang, Z. Tao, J. Chen, Adv. Mater. 23 (2011) 1695–1715. [3] Y.K. Sun, Z. Chen, H.J. Noh, D.J. Lee, H.G. Jung, Y. Ren, S. Wang, C.S. Yoon, S.T. Myung, K. Amine, Nat. Mater. 11 (2012) 942–947. [4] F.X. Wang, S.Y. Xiao, Y. Shi, L.L. Liu, Y.S. Zhu, Y.P. Wu, J.Z. Wang, R. Holze, Electrochim. Acta 93 (2013) 301–306. [5] Q. Zhang, J. Mei, X. Wang, W. Fan, F. Wang, W. Lu, F. Tang, J. Alloys Comp. 606 (2014) 249–253. [6] X.M. Liu, P. Yan, Y.Y. Xie, H. Yang, X.D. Shen, Z.F. Ma, Chem. Commun. 49 (2013) 5396–5398.
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