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C as a new anode material for lithium-ion battery
Journal of Alloys and Compounds 622 (2015) 250–253
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Synthesis and performance of LiVTiO4/C as a new anode material for lithium-ion battery Jie Liu a,⇑, Chenqiang Du a, Yaqing Lin a, Zhiyuan Tang a,⇑, Xinhe Zhang b a b
Department of Applied Chemistry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China McNair Technology Company Limited, Dongguan City, Guangdong 523700, China
a r t i c l e
i n f o
Article history: Received 26 June 2014 Accepted 10 October 2014 Available online 17 October 2014 Keywords: Lithium vanadium titanate Anode Spinel Carbon coating Lithium-ion battery
a b s t r a c t Spinel LiVTiO4/C compound is prepared via a facile sol–gel method. The structure and morphology of LiVTiO4/C are investigated by X-ray diffraction (XRD), scanning electronmicroscopy (SEM), and transmission electron microscopy (TEM). The results indicate that the LiVTiO4/C has a carbon coating layer ranging between 2 and 3 nm. Besides, the existence of VO2 improves the conductivity to some extent. The electrochemical properties of LiVTiO4/C as an anode material for lithium-ion battery are investigated for the first time. The synthesized LiVTiO4/C material exhibits a high charge capacity of 231.6 mA h g1 after 50 cycles at 0.02 A g1, with a capacity retention ratio of 96.5%. The excellent reversible capacity and excellent cycling stability indicate that the LiVTiO4/C composite has a great potential application in lithium-ion battery. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries have been widely used in various electronic consumer products due to their high capacity and power density [1–3]. In recent years, the demand for high performance in lithium-ion batteries has stimulated researchers to explore different types of anode materials. Graphite and Li-ion intercalated compound Li4Ti5O12 (LTO) have been investigated extensively. Graphite is a conventional anode material and has a high capacity of 372 mA h g1 [4]. However, due to several critical disadvantages, such as low power density, and safety hazards [5], its application in lithium-ion batteries is limited. Li4Ti5O12 has been demonstrated to be one of the most promising electrode materials, because it has a flat voltage range, high reversible capacity, and especially the long cycling performance due to no structural change (zerostrain insertion material) during charge–discharge cycling. Moreover, Li4Ti5O12 possesses slightly lower conductivity and poor lithium-ion diffusion properties that also limit its application for high energy density devices [6–9]. Thus, extensive efforts have been put on the development of new electrode materials for lithium-ion batteries with higher capacity, excellent rate capability and reliable safety. Recently, a new family of electrode materials with a general formula of LiMTiO4 (M is transition metal) have been ⇑ Corresponding authors. Tel.: +86 769 83017180; fax: +86 769 83195372 (J. Liu). E-mail addresses: [email protected] (J. Liu), [email protected] (Z. Tang). http://dx.doi.org/10.1016/j.jallcom.2014.10.059 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
investigated widely [4,10–12]. The unit cell of the normal spinel structure comprises 32 cubic close packed O atoms with the Liions located in the tetrahedral sites (8a) and the Ti-ions are positioned in the octahedral sites (16d) while the M-ions (M = V, Mn, Fe) distribute over both the tetrahedral (8a) and octahedral sites (16d) [11,13,14]. Among them, LiVTiO4 is especially attractive. Barker et al. first synthesized LiVTiO4 and investigated it as a cathode material [13,15]. With the potential range of 2.0–4.0 V (vs. Li/Li+), the insertion/extraction reaction of LiVTiO4 is located at 2.85/3.16 V, respectively, which corresponds to the V4+/V3+ redox couple. Unfortunately, the relatively low capacity of about 92 mA h g1 limits its potential application as a cathode material [13]. On the other hand, vanadium-based anode materials are very well known to possess a high capacity, such as LiVPO4F [16,17] and Li3VO4 [18,19]. Thus, introducing vanadium as the transition metal M into the spinel LiMTiO4 family could be a reasonable attempt in the search for a new anode material with high reversible capacity. However, there is little study that has been carried out for LiVTiO4 as an anode material for lithiumion battery. In the present study, we make an attempt to synthesize spinel LiVTiO4/C composite via a facile sol–gel method and investigated it as an anode material for the first time. The results of electrochemical measurements indicate that the novel anode material LVTiO4/C has a great potential application in lithiumion batteries.
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J. Liu et al. / Journal of Alloys and Compounds 622 (2015) 250–253 2. Experimental 2.1. Materials preparation All of the reactants and solvents were analytical grade and used without further purification. LiVTiO4/C nanoparticles were synthesized via a facile sol–gel method. In a typical process, a stoichiometric amount of LiCH3COO2H2O and V2O5 were dissolved into deionized water and the mixture was ultrasonically treated for 0.5 h to form solution A. Then, a stoichiometric amount of tetrabutyl titanate (TBT) was dissolved in anhydrous ethanol to form solution B. Afterwards, the as-prepared solution A was added gradually into solution B with vigorous stirring at room temperature. The resultant solution was heated in a water bath at 70 °C to vaporize the contained water and ethanol completely, which was followed by a heat treatment at 350 °C for 4 h in N2 atmosphere to allow a pyrolysis process, thus yielding the precursor. The precursor was ground and calcined at 800 °C for 8 h in N2 atmosphere to obtain the carbon coated LiVTiO4 composite.
2.2. Materials characterization X-ray diffraction patterns (XRD) were obtained using an X-ray diffractometer (Rigaku RINT2000) with Cu Ka radiation and recorded between 10° and 80° with a step count of 0.02° and scanning speed of 3° min1. The morphology of the samples was observed using a scanning electron microscope (Hitachi S4800) (SEM). Transmission Electron Microscopy (TEM) studies of the samples were conducted using a JEM-2100F Transmission Electron Microscope operated at 200 kV.
2.3. Electrochemical measurements Electrochemical performances tests were studied by employing 2032 coin type cells. 80 wt.% active materials were mixed and grounded with 10 wt.% polyvinylidene fluoride (PVDF) powders as binder and 10 wt.% super-P as the conductive assistant materials. The mixture was added into N-methyl-2-pyrrolidinone (NMP) solvent to form a homogeneous slurry. The slurry was coated on a copper foil and dried at 120 °C in a vacuum oven for 24 h. Metallic lithium foils were used as the counter electrodes. 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 in volume) served as electrolyte, and polypropylene (Celgard Ò 2300, Celgard Inc. USA) was used as the separator. The cells were assembled in a glove box filled with high purity argon gas (O2 and H2O levels <5 ppm). The cyclic voltammetry (CV) tests were conducted on an electrochemical workstation (CHI1040B, ChenHua, China) at a scanning rate of 0.1 mV s1 in the voltage range of 1.0–3.0 V and 0.02–3.0 V (vs. Li/ Li+). And the charge/discharge tests were performed in the voltage range of 1.0– 3.0 V and 0.02–3 V (vs. Li/Li+) at different current densities on an automatic batteries tester (Land CT 2001A, Wuhan, China).
3. Results and discussion 3.1. Structure and morphology The XRD pattern of the LiVTiO4/C and the results of Rietveld refinement are shown in Fig. 1a and b. The main peaks are indexed in the space group Fd-3m, which are consistent with the spinel Li8Cr5Ti11O32 (JCPDS No. 47-0138). A minor impurity phase of VO2 (space group pbnm, JCPDS No. 73-0514) is also identified, and the content is detected to be 2.2(1) wt.% by the Rietveld refinement method. It is worth mentioning that the paramontroseite VO2 has a very attractive room-temperature conductivity of 1.02 S cm1, higher than many other inorganic conducting materials [20], thus, the minor VO2 impurity can improve the conductivity to some extent and the existence of VO2 is acceptable. The lattice parameter value of the synthesized spinel phase is found to be a = 8.337(2) Å, which is a little bigger than the previous report [13]. SEM image of the LiVTiO4/C is shown in Fig. 1c. It can be found that the LiVTiO4/C is composed of many irregular particles with rough surfaces. HRTEM is used to further study the microstructure of the as-prepared composite. As shown in Fig. 1d and e, the existence of VO2 can be clearly confirmed. Additionally, the LiVTiO4 particles have been coated a carbon layer with a thickness of 2–3 nm. The conductive carbon can not only improve the electronic conductivity but also inhibit the growth of the LiVTiO4 particles. The residual carbon content in LiVTiO4/C is 5.6%, as confirmed by a CHN analyzer. The selected area electron diffraction (SAED) (the inset of Fig. 1e) pattern appears as individual spots
associated with concentric rings, indicating the polycrystalline nature of the LiVTiO4/C composite. 3.2. Electrochemical performance Fig. 2 shows the electrochemical properties of as-prepared LiVTiO4/C. Fig. 2a and b present the cyclic voltammetry (CV) curves and galvanostatic charge/discharge curves cycled at 0.02 A g1 in 1.0–3.0 V. It can be found that the LiVTiO4/C composite exhibits an intense anodic peak at 1.79 V and a weak cathodic peak at 1.65 V, which indicates the reduction of LiVTiO4 to Li2VTiO4 via a two-phase process [21]. Thus, the lithium insertion/extraction reaction for LiVTiO4/C can be summarized as 3þ
þ
LiV TiO4 þ Li þ e $ Li2 V2þ TiO4
ð1Þ 1
As depicted in Eq. (1), a theoretical capacity of 158 mA h g can be delivered corresponding to 1.0 Li uptake per formula. As shown in Fig. 2b, the initial discharge capacity of LiVTiO4/C is 155.3 mA h g1, which is very close to the theoretical capacity. This suggests that almost all of the LiVTiO4 transformed into Li2VTiO4 after discharge to 1.0 V. The initial charge capacity of LiVTiO4/C is only 110.9 mA h g1, which can be attributed to the local structural changes and/or to polarization of the cell [22]. Besides, a couple of peaks located at 2.15/2.10 V are also detected, which could be ascribed to the crystal chemical changes which are inherently linked to cation distributions within the as-prepared material and further studies are required [23]. Additionally, with the charge/discharge cycle going on, another new couple of plateaus located at 1.53/1.48 V appear gradually, as shown in Fig. 2b. This should be ascribed to partial Ti4+/Ti3+ redox couple participating in the charge/discharge process. Fig. 2c and d shows the CV curves and galvanostatic charge/ discharge curves cycled at 0.02 A g1 in a more wide potential window (0.02–3.0 V). Comparing with Fig. 2a, besides two pairs of redox peaks which located at 2.15/2.10 V and 1.79/1.65 V, it also presents an irreversible cathodic peak at 0.85 V, which correspond to the electrolyte irreversible reduction decomposition to form SEI film [21]. Additionally, a couple of repeatable redox peaks below 0.5 V are also detected, where this process is analogous to that observed for the Li1.33Ti1.67O4 composite and is speculated to be the reversible formation of anion, deficient quasi-rocksalt species [23]. According to the large capacity we obtained with a depth discharge process to 0.02 V, as shown in Fig. 2d, it is thought that another 1.0 mol lithium-ions per formula could be intercalated into LiVTiO4/C. The lithium insertion/extraction reaction for LiVTiO4/C in 0.02–1.0 V can be summarized as þ
Li2 V2þ TiO4 þ Li þ e $ Li3 Vþ TiO4
ð2Þ
Thus, the whole lithium insertion/extraction reaction for LiVTiO4/C in 0.02–3.0 V can be summarized as 3þ
þ
LiV TiO4 þ 2Li þ 2e $ Li3 Vþ TiO4
ð3Þ 1
Based on Eq. (3), a theoretical capacity of 316 mA h g can be delivered, corresponding to 2.0 Li uptake per formula. As shown in Fig. 2d, the initial discharge capacity of LiVTiO4 is up to 329.5 mA h g1 and the charge capacity is only 239.8 mA h g1. The irreversible capacity should be due to the formation of SEI film and dead lithium in the structure. Furthermore, the disappearance of the slope at 0.85 V in the following cycles demonstrates the formation of SEI film in the initial discharge process, which is in good agreement with the results of CV curves. Additionally, a couple of plateaus located at 1.5 V appear in the charge/discharge profiles after 20 cycles, which should be ascribed to the Ti4+/Ti3+ redox couple.
J. Liu et al. / Journal of Alloys and Compounds 622 (2015) 250–253
×
(533)
(444)
(622)
(440) (531)
(511) (151)
(221)
(131) (331)
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(130)
(222)
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Instensity (a.u.)
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Li Cr Ti O : 47-0138 8
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Fig. 1. (a) XRD pattern and (b) Rietveld refinement results for LiVTiO4/C; (c) SEM and (d and e) HRTEM images for LiVTiO4/C composite.
3.0
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(b) First cycle Subsequent cycles
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Potential (V vs.Li/Li + ) Fig. 2. (a and c) Cyclic voltammetry profiles of LiVTiO4/C at a scan rate of 0.1 mV s
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and (b and d) charge-discharge curves of LiVTiO4/C composite at the current of 0.02 A g1.
J. Liu et al. / Journal of Alloys and Compounds 622 (2015) 250–253
be associated with the electrode-activation process during lithium insertion/extraction into/out from LiVTiO4/C [24].
300
Charge specific capacity (mAh g-1)
(a) 250
0.02V-3.0V 1.0V-3.0V
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4. Conclusions
219.31 224.42
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The spinel LiVTiO4/C composite is synthesized successfully and investigated as an anode material for the first time. The results of SEM and TEM reveal that the as-prepared LiVTiO4 has a carbon coating layer with the thickness of 2–3 nm. In the whole insertion/extraction process, LiVTiO4 experiences through a reversible two-phase reaction between LiVTiO4 and Li2VTiO4 in 1.0–3.0 V, and Li2VTiO4 transforms to Li3VTiO4 with a depth discharge process to 0.02 V. The LiVTiO4/C shows excellent cycling stability and considerable charge specific capacity. Especially in a wide potential window of 0.02–3.0 V, the LiVTiO4/C composite delivers high capacities of 239.92 mA h g1, 224.42 mA h g1, 219.31 mA h g1, 193.43 mA h g1 and 137.92 mA h g1 at the currents of 0.02 A g1, 0.1 A g1, 0.3 A g1, 0.5 A g1 and 1.0 A g1, respectively. The excellent electrochemical performance suggests that the synthesized LiVTiO4/C material is a promising anode candidate for lithium-ion battery.
193.43 143.11
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Acknowledgments
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We acknowledge financial support from the Cooperation Project in Industry, Education and Research of Guangdong Province and Ministry of Education of PR China (No. 2010A090200002).
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References
100 Cut-off: 0.02-3.0 V
75 50
0
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Clycle number Fig. 3. (a) Rate capability of LiVTiO4/C with the cut-off voltage of 1.0–3.0 and 0.02– 3.0 V. (b) Cycling performance of LiVTiO4/C composite.
The rate capability of LiVTiO4/C in different potential windows had also been investigated. As shown in Fig. 3a, when cycled in 0.02–3.0 V, the LiVTiO4/C delivers high charge capacities of 239.92 mA h g1, 224.42 mA h g1, 219.31 mA h g1 and 193.43 mA h g1 at the currents of 0.02 A g1, 0.1 A g1, 0.3 A g1 and 0.5 A g1, respectively. Even the current is as high as 1.0 A g1, a charge capacity of 137.92 mA h g1 still can be obtained. For comparison, when discharged to 1.0 V, LiVTiO4/C can only present 143.11 mA h g1, 88.63 mA h g1, 56.47 mA h g1 and 38.12 mA h g1 at the currents of 0.02 A g1, 0.1 A g1, 0.3 A g1 and 0.5 A g1, respectively. The large capacity differences should be ascribed to the further intercalation into the structure of LiVTiO4/C below 1.0 V. Fig. 3b shows the cycling performance of LiVTiO4/C at 0.02 and 0.3 A g1 in 0.02–3.0 V. The synthesized LiVTiO4/C material exhibits a high charge capacity of 231.6 mA h g1 after 50 cycles at the current of 0.02 A g1, with a capacity retention ratio of 96.5%. Besides, a capacity of 214.9 mA h g1 can be achieved after 150 cycles at 0.3 A g1. Interestingly, the charge capacity increases gradually during the first 50 cycles, which might
[1] B. Kang, G. Ceder, Nature 458 (2009) 190. [2] R. Koksbang, J. Barker, H. Shi, M.Y. Saidi, Solid State Ionics 84 (1996) 1. [3] J. Kim, H. Kim, K.Y. Park, Y.U. Park, S. Lee, H.S. Kwon, H.I. Yoo, K. Kang, J. Mater. Chem. A 2 (2014) 8632. [4] X.Y. Feng, C. Shen, N. Ding, C.H. Chen, J. Mater. Chem. 22 (2012) 20861. [5] S.L. Chou, J.Z. Wang, H.K. Liu, S.X. Dou, J. Phys. Chem. C 115 (2011) 16220. [6] K. Naoi, S. Ishimoto, Y. Isobe, S. Aoyagi, J. Power Sources 195 (2010) 6250. [7] G.N. Zhu, Y.G. Wang, Y.Y. Xia, Energy Environ. Sci. 5 (2012) 6652. [8] T.F. Yi, L.J. Jiang, J. Shu, C.B. Yue, R.S. Zhu, H.B. Qiao, J. Phys. Chem. Solids 71 (2010) 1236. [9] M. Nakayama, Y. Ishida, H. Ikuta, M. Wakihara, Solid State Ionics 117 (1999) 265. [10] V. Aravindan, W. Chuiling, S. Madhavi, J. Mater. Chem. 22 (2012) 16026. [11] M.A. Arillo, M.L. Lopez, E. Perez-Cappe, C. Pico, M.L. Veiga, Solid State Ionics 107 (1998) 307. [12] L. Wang, Q.Z. Xiao, L.J. Wu, G.T. Lei, Z.H. Li, Solid State Ionics 236 (2013) 43. [13] J. Barker, M.Y. Saidi, J.L. Swoyer, Solid State Ionics 167 (2004) 413. [14] M.A. Arillo, G. Cuello, M.L. López, P. Martín, C. Pico, M.L. Veiga, Solid State Sci. 7 (2005) 25. [15] J. Barker, US patent No. 2003/0064288. [16] J.A. Mba, L. Croguennec, N.I. Basir, J. Barker, C. Masquelier, J. Electrochem. Soc. 159 (2012) A1171. [17] J.X. Wang, X.H. Li, Z.X. Wang, B. Huang, Z.G. Wang, H.J. Guo, J. Power Sources 251 (2014) 325. [18] W. Kim, B. Min, H.C. Choi, Y.J. Lee, Y.U. Jeong, J. Electrochem. Soc. 161 (2014) A1302. [19] W. Kim, Y.U. Jeong, Y.J. Lee, Y.J. Kim, J.H. Song, J. Power Sources 244 (2013) 557. [20] C.Z. Wu, Z.P. Hu, W. Wang, M. Zhang, J.L. Yang, Y. Xie, Chem. Commun. 33 (2008) 3891. [21] R. Ma, L.Y. Shao, K.Q. Wu, M. Shui, D.J. Wang, J.G. Pan, N.B. Long, Y.L. Ren, J. Shu, Appl. Mater. Interf. 5 (2013) 8615. [22] D. Dambournet, I. Belharouak, K. Amine, Inorg. Chem. 49 (2010) 2822. [23] A.D. Robertson, H. Tukamoto, J.T.S. Irvine, J. Electrochem. Soc. 146 (1999) 3958. [24] R. Yi, J. Feng, D. Lv, M.L. Gordin, S. Chen, D. Choi, D. Wang, Nano Energy 2 (2013) 498.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Synthesis and performance of LiVTiO4/C as a new anode material for lithium-ion battery Jie Liu a,⇑, Chenqiang Du a, Yaqing Lin a, Zhiyuan Tang a,⇑, Xinhe Zhang b a b
Department of Applied Chemistry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China McNair Technology Company Limited, Dongguan City, Guangdong 523700, China
a r t i c l e
i n f o
Article history: Received 26 June 2014 Accepted 10 October 2014 Available online 17 October 2014 Keywords: Lithium vanadium titanate Anode Spinel Carbon coating Lithium-ion battery
a b s t r a c t Spinel LiVTiO4/C compound is prepared via a facile sol–gel method. The structure and morphology of LiVTiO4/C are investigated by X-ray diffraction (XRD), scanning electronmicroscopy (SEM), and transmission electron microscopy (TEM). The results indicate that the LiVTiO4/C has a carbon coating layer ranging between 2 and 3 nm. Besides, the existence of VO2 improves the conductivity to some extent. The electrochemical properties of LiVTiO4/C as an anode material for lithium-ion battery are investigated for the first time. The synthesized LiVTiO4/C material exhibits a high charge capacity of 231.6 mA h g1 after 50 cycles at 0.02 A g1, with a capacity retention ratio of 96.5%. The excellent reversible capacity and excellent cycling stability indicate that the LiVTiO4/C composite has a great potential application in lithium-ion battery. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries have been widely used in various electronic consumer products due to their high capacity and power density [1–3]. In recent years, the demand for high performance in lithium-ion batteries has stimulated researchers to explore different types of anode materials. Graphite and Li-ion intercalated compound Li4Ti5O12 (LTO) have been investigated extensively. Graphite is a conventional anode material and has a high capacity of 372 mA h g1 [4]. However, due to several critical disadvantages, such as low power density, and safety hazards [5], its application in lithium-ion batteries is limited. Li4Ti5O12 has been demonstrated to be one of the most promising electrode materials, because it has a flat voltage range, high reversible capacity, and especially the long cycling performance due to no structural change (zerostrain insertion material) during charge–discharge cycling. Moreover, Li4Ti5O12 possesses slightly lower conductivity and poor lithium-ion diffusion properties that also limit its application for high energy density devices [6–9]. Thus, extensive efforts have been put on the development of new electrode materials for lithium-ion batteries with higher capacity, excellent rate capability and reliable safety. Recently, a new family of electrode materials with a general formula of LiMTiO4 (M is transition metal) have been ⇑ Corresponding authors. Tel.: +86 769 83017180; fax: +86 769 83195372 (J. Liu). E-mail addresses: [email protected] (J. Liu), [email protected] (Z. Tang). http://dx.doi.org/10.1016/j.jallcom.2014.10.059 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
investigated widely [4,10–12]. The unit cell of the normal spinel structure comprises 32 cubic close packed O atoms with the Liions located in the tetrahedral sites (8a) and the Ti-ions are positioned in the octahedral sites (16d) while the M-ions (M = V, Mn, Fe) distribute over both the tetrahedral (8a) and octahedral sites (16d) [11,13,14]. Among them, LiVTiO4 is especially attractive. Barker et al. first synthesized LiVTiO4 and investigated it as a cathode material [13,15]. With the potential range of 2.0–4.0 V (vs. Li/Li+), the insertion/extraction reaction of LiVTiO4 is located at 2.85/3.16 V, respectively, which corresponds to the V4+/V3+ redox couple. Unfortunately, the relatively low capacity of about 92 mA h g1 limits its potential application as a cathode material [13]. On the other hand, vanadium-based anode materials are very well known to possess a high capacity, such as LiVPO4F [16,17] and Li3VO4 [18,19]. Thus, introducing vanadium as the transition metal M into the spinel LiMTiO4 family could be a reasonable attempt in the search for a new anode material with high reversible capacity. However, there is little study that has been carried out for LiVTiO4 as an anode material for lithiumion battery. In the present study, we make an attempt to synthesize spinel LiVTiO4/C composite via a facile sol–gel method and investigated it as an anode material for the first time. The results of electrochemical measurements indicate that the novel anode material LVTiO4/C has a great potential application in lithiumion batteries.
251
J. Liu et al. / Journal of Alloys and Compounds 622 (2015) 250–253 2. Experimental 2.1. Materials preparation All of the reactants and solvents were analytical grade and used without further purification. LiVTiO4/C nanoparticles were synthesized via a facile sol–gel method. In a typical process, a stoichiometric amount of LiCH3COO2H2O and V2O5 were dissolved into deionized water and the mixture was ultrasonically treated for 0.5 h to form solution A. Then, a stoichiometric amount of tetrabutyl titanate (TBT) was dissolved in anhydrous ethanol to form solution B. Afterwards, the as-prepared solution A was added gradually into solution B with vigorous stirring at room temperature. The resultant solution was heated in a water bath at 70 °C to vaporize the contained water and ethanol completely, which was followed by a heat treatment at 350 °C for 4 h in N2 atmosphere to allow a pyrolysis process, thus yielding the precursor. The precursor was ground and calcined at 800 °C for 8 h in N2 atmosphere to obtain the carbon coated LiVTiO4 composite.
2.2. Materials characterization X-ray diffraction patterns (XRD) were obtained using an X-ray diffractometer (Rigaku RINT2000) with Cu Ka radiation and recorded between 10° and 80° with a step count of 0.02° and scanning speed of 3° min1. The morphology of the samples was observed using a scanning electron microscope (Hitachi S4800) (SEM). Transmission Electron Microscopy (TEM) studies of the samples were conducted using a JEM-2100F Transmission Electron Microscope operated at 200 kV.
2.3. Electrochemical measurements Electrochemical performances tests were studied by employing 2032 coin type cells. 80 wt.% active materials were mixed and grounded with 10 wt.% polyvinylidene fluoride (PVDF) powders as binder and 10 wt.% super-P as the conductive assistant materials. The mixture was added into N-methyl-2-pyrrolidinone (NMP) solvent to form a homogeneous slurry. The slurry was coated on a copper foil and dried at 120 °C in a vacuum oven for 24 h. Metallic lithium foils were used as the counter electrodes. 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 in volume) served as electrolyte, and polypropylene (Celgard Ò 2300, Celgard Inc. USA) was used as the separator. The cells were assembled in a glove box filled with high purity argon gas (O2 and H2O levels <5 ppm). The cyclic voltammetry (CV) tests were conducted on an electrochemical workstation (CHI1040B, ChenHua, China) at a scanning rate of 0.1 mV s1 in the voltage range of 1.0–3.0 V and 0.02–3.0 V (vs. Li/ Li+). And the charge/discharge tests were performed in the voltage range of 1.0– 3.0 V and 0.02–3 V (vs. Li/Li+) at different current densities on an automatic batteries tester (Land CT 2001A, Wuhan, China).
3. Results and discussion 3.1. Structure and morphology The XRD pattern of the LiVTiO4/C and the results of Rietveld refinement are shown in Fig. 1a and b. The main peaks are indexed in the space group Fd-3m, which are consistent with the spinel Li8Cr5Ti11O32 (JCPDS No. 47-0138). A minor impurity phase of VO2 (space group pbnm, JCPDS No. 73-0514) is also identified, and the content is detected to be 2.2(1) wt.% by the Rietveld refinement method. It is worth mentioning that the paramontroseite VO2 has a very attractive room-temperature conductivity of 1.02 S cm1, higher than many other inorganic conducting materials [20], thus, the minor VO2 impurity can improve the conductivity to some extent and the existence of VO2 is acceptable. The lattice parameter value of the synthesized spinel phase is found to be a = 8.337(2) Å, which is a little bigger than the previous report [13]. SEM image of the LiVTiO4/C is shown in Fig. 1c. It can be found that the LiVTiO4/C is composed of many irregular particles with rough surfaces. HRTEM is used to further study the microstructure of the as-prepared composite. As shown in Fig. 1d and e, the existence of VO2 can be clearly confirmed. Additionally, the LiVTiO4 particles have been coated a carbon layer with a thickness of 2–3 nm. The conductive carbon can not only improve the electronic conductivity but also inhibit the growth of the LiVTiO4 particles. The residual carbon content in LiVTiO4/C is 5.6%, as confirmed by a CHN analyzer. The selected area electron diffraction (SAED) (the inset of Fig. 1e) pattern appears as individual spots
associated with concentric rings, indicating the polycrystalline nature of the LiVTiO4/C composite. 3.2. Electrochemical performance Fig. 2 shows the electrochemical properties of as-prepared LiVTiO4/C. Fig. 2a and b present the cyclic voltammetry (CV) curves and galvanostatic charge/discharge curves cycled at 0.02 A g1 in 1.0–3.0 V. It can be found that the LiVTiO4/C composite exhibits an intense anodic peak at 1.79 V and a weak cathodic peak at 1.65 V, which indicates the reduction of LiVTiO4 to Li2VTiO4 via a two-phase process [21]. Thus, the lithium insertion/extraction reaction for LiVTiO4/C can be summarized as 3þ
þ
LiV TiO4 þ Li þ e $ Li2 V2þ TiO4
ð1Þ 1
As depicted in Eq. (1), a theoretical capacity of 158 mA h g can be delivered corresponding to 1.0 Li uptake per formula. As shown in Fig. 2b, the initial discharge capacity of LiVTiO4/C is 155.3 mA h g1, which is very close to the theoretical capacity. This suggests that almost all of the LiVTiO4 transformed into Li2VTiO4 after discharge to 1.0 V. The initial charge capacity of LiVTiO4/C is only 110.9 mA h g1, which can be attributed to the local structural changes and/or to polarization of the cell [22]. Besides, a couple of peaks located at 2.15/2.10 V are also detected, which could be ascribed to the crystal chemical changes which are inherently linked to cation distributions within the as-prepared material and further studies are required [23]. Additionally, with the charge/discharge cycle going on, another new couple of plateaus located at 1.53/1.48 V appear gradually, as shown in Fig. 2b. This should be ascribed to partial Ti4+/Ti3+ redox couple participating in the charge/discharge process. Fig. 2c and d shows the CV curves and galvanostatic charge/ discharge curves cycled at 0.02 A g1 in a more wide potential window (0.02–3.0 V). Comparing with Fig. 2a, besides two pairs of redox peaks which located at 2.15/2.10 V and 1.79/1.65 V, it also presents an irreversible cathodic peak at 0.85 V, which correspond to the electrolyte irreversible reduction decomposition to form SEI film [21]. Additionally, a couple of repeatable redox peaks below 0.5 V are also detected, where this process is analogous to that observed for the Li1.33Ti1.67O4 composite and is speculated to be the reversible formation of anion, deficient quasi-rocksalt species [23]. According to the large capacity we obtained with a depth discharge process to 0.02 V, as shown in Fig. 2d, it is thought that another 1.0 mol lithium-ions per formula could be intercalated into LiVTiO4/C. The lithium insertion/extraction reaction for LiVTiO4/C in 0.02–1.0 V can be summarized as þ
Li2 V2þ TiO4 þ Li þ e $ Li3 Vþ TiO4
ð2Þ
Thus, the whole lithium insertion/extraction reaction for LiVTiO4/C in 0.02–3.0 V can be summarized as 3þ
þ
LiV TiO4 þ 2Li þ 2e $ Li3 Vþ TiO4
ð3Þ 1
Based on Eq. (3), a theoretical capacity of 316 mA h g can be delivered, corresponding to 2.0 Li uptake per formula. As shown in Fig. 2d, the initial discharge capacity of LiVTiO4 is up to 329.5 mA h g1 and the charge capacity is only 239.8 mA h g1. The irreversible capacity should be due to the formation of SEI film and dead lithium in the structure. Furthermore, the disappearance of the slope at 0.85 V in the following cycles demonstrates the formation of SEI film in the initial discharge process, which is in good agreement with the results of CV curves. Additionally, a couple of plateaus located at 1.5 V appear in the charge/discharge profiles after 20 cycles, which should be ascribed to the Ti4+/Ti3+ redox couple.
J. Liu et al. / Journal of Alloys and Compounds 622 (2015) 250–253
×
(533)
(444)
(622)
(440) (531)
(511) (151)
(221)
(131) (331)
(121)
(130)
(222)
(120)
(220)
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— — II
Obs Cal Obs-Cal Bragg
Instensity (a.u.)
(400)
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(311)
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Li Cr Ti O : 47-0138 8
5 11 32
VO : PDF# 73-0514 2
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2Theta (degree)
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Fig. 1. (a) XRD pattern and (b) Rietveld refinement results for LiVTiO4/C; (c) SEM and (d and e) HRTEM images for LiVTiO4/C composite.
3.0
(a)
(b) First cycle Subsequent cycles
0.10 0.05 0.00 -0.05
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Potential (V vs.Li/Li + ) Fig. 2. (a and c) Cyclic voltammetry profiles of LiVTiO4/C at a scan rate of 0.1 mV s
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and (b and d) charge-discharge curves of LiVTiO4/C composite at the current of 0.02 A g1.
J. Liu et al. / Journal of Alloys and Compounds 622 (2015) 250–253
be associated with the electrode-activation process during lithium insertion/extraction into/out from LiVTiO4/C [24].
300
Charge specific capacity (mAh g-1)
(a) 250
0.02V-3.0V 1.0V-3.0V
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4. Conclusions
219.31 224.42
200
The spinel LiVTiO4/C composite is synthesized successfully and investigated as an anode material for the first time. The results of SEM and TEM reveal that the as-prepared LiVTiO4 has a carbon coating layer with the thickness of 2–3 nm. In the whole insertion/extraction process, LiVTiO4 experiences through a reversible two-phase reaction between LiVTiO4 and Li2VTiO4 in 1.0–3.0 V, and Li2VTiO4 transforms to Li3VTiO4 with a depth discharge process to 0.02 V. The LiVTiO4/C shows excellent cycling stability and considerable charge specific capacity. Especially in a wide potential window of 0.02–3.0 V, the LiVTiO4/C composite delivers high capacities of 239.92 mA h g1, 224.42 mA h g1, 219.31 mA h g1, 193.43 mA h g1 and 137.92 mA h g1 at the currents of 0.02 A g1, 0.1 A g1, 0.3 A g1, 0.5 A g1 and 1.0 A g1, respectively. The excellent electrochemical performance suggests that the synthesized LiVTiO4/C material is a promising anode candidate for lithium-ion battery.
193.43 143.11
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137.92
100 88.63
56.47
50 38.12 0 0.0
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Acknowledgments
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We acknowledge financial support from the Cooperation Project in Industry, Education and Research of Guangdong Province and Ministry of Education of PR China (No. 2010A090200002).
175 150 125
References
100 Cut-off: 0.02-3.0 V
75 50
0
50
100
150
Clycle number Fig. 3. (a) Rate capability of LiVTiO4/C with the cut-off voltage of 1.0–3.0 and 0.02– 3.0 V. (b) Cycling performance of LiVTiO4/C composite.
The rate capability of LiVTiO4/C in different potential windows had also been investigated. As shown in Fig. 3a, when cycled in 0.02–3.0 V, the LiVTiO4/C delivers high charge capacities of 239.92 mA h g1, 224.42 mA h g1, 219.31 mA h g1 and 193.43 mA h g1 at the currents of 0.02 A g1, 0.1 A g1, 0.3 A g1 and 0.5 A g1, respectively. Even the current is as high as 1.0 A g1, a charge capacity of 137.92 mA h g1 still can be obtained. For comparison, when discharged to 1.0 V, LiVTiO4/C can only present 143.11 mA h g1, 88.63 mA h g1, 56.47 mA h g1 and 38.12 mA h g1 at the currents of 0.02 A g1, 0.1 A g1, 0.3 A g1 and 0.5 A g1, respectively. The large capacity differences should be ascribed to the further intercalation into the structure of LiVTiO4/C below 1.0 V. Fig. 3b shows the cycling performance of LiVTiO4/C at 0.02 and 0.3 A g1 in 0.02–3.0 V. The synthesized LiVTiO4/C material exhibits a high charge capacity of 231.6 mA h g1 after 50 cycles at the current of 0.02 A g1, with a capacity retention ratio of 96.5%. Besides, a capacity of 214.9 mA h g1 can be achieved after 150 cycles at 0.3 A g1. Interestingly, the charge capacity increases gradually during the first 50 cycles, which might
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