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TiNb6O17: a new electrode material for lithium-ion batteries† Chunfu Lin,*a Guizhen Wang,a Shiwei Lin,*a Jianbao Lia and Li Lu*b
Received 17th February 2015, Accepted 20th April 2015 DOI: 10.1039/c5cc01494h www.rsc.org/chemcomm
TiNb6O17 shows a similar crystal structure to Ti2Nb10O29 (Wadsley– Roth shear structure) but has larger lattice parameters and 0.49% cation vacancies, leading to its large Li+ ion diffusion coefficients. It exhibits a large initial discharge capacity of 383 mA h g1 at 0.1 C,
Based on the two-electron transfer between Nb5+ and Nb3+ ions and the one-electron transfer between Ti4+ and Ti3+ ions, Ti–Nb–O compounds with a general chemical formula of TiNbxO2+2.5x have large theoretical capacities shown in eqn (1):
high rate performance and good cyclability.
C = 403 5441/(133x + 80) (mA h g1) The great success that lithium-ion batteries (LIBs) have experienced in portable electronic devices is now being extended to electric vehicles (EVs), which require both high power density and high energy density.1 For commercial LIBs, graphite is usually selected as the anode material due to its large theoretical capacity of 372 mA h g1 and good reversible discharge– charge for Li+ ions. However, it suffers severe safety hazards rooted in the formation and growth of lithium dendrites when operated at low potentials and high currents, and thus fails to be used in EVs.2 A fundamental solution is to replace graphite by other anode materials with high safety, among which intercalationtype Li4Ti5O12 may be the most promising one.3 Li4Ti5O12 has a spinel crystal structure, in which cations occupy both octahedral and tetrahedral sites.4 After being modified by doping with alien ions, compositing with a second conductive phase and/or reducing the particle size, this material exhibits not only high safety and good cyclability but also high rate performance.5 However, its intrinsically low theoretical capacity (175 mA h g1 between 3.0 and 1.0 V vs. Li/Li+) cannot be effectively increased. Consequently, although Li4Ti5O12 can fulfil the requirement of high power density, it cannot fulfil that of high energy density, significantly limiting its practical applications in EVs. Therefore, exploring new anode materials with the same advantages of Li4Ti5O12 but larger capacities is an urgent task in the field of EVs.
(1)
For instance, TiNb2O7 (x = 2) and Ti2Nb10O29 (x = 5), respectively, possess the theoretical capacities of 388 and 396 mA h g1,6–8 which are B1.2 times larger than that of Li4Ti5O12 and even exceed that of graphite. They show Wadsley–Roth shear structures built by m n N (m = n = 3 for TiNb2O7; m = 4 and n = 3 for Ti2Nb10O29, Fig. 1a and Fig. S1, ESI†) ReO3-type blocks, where m and n are, respectively, the length and width of the blocks in numbers of octahedral.9 All cations (Nb5+ and Ti4+ ions with molar ratios of 2 : 1 for TiNb2O7 and 5 : 1 for Ti2Nb10O29) are disordered in octahedral sites sharing corners and edges. Since all cations occupy the octahedral sites, the Wadsley–Roth shear structure is more open than the spinel structure, inferring a larger Li+ ion diffusion coefficient of TiNbxO2+2.5x than that of Li4Ti5O12. Despite the large theoretical capacities of TiNb2O7 and Ti2Nb10O29, they suffer low intrinsic conductivity, resulting in poor rate performances. Thus, they can fulfil the requirement of high energy density but cannot fulfil that of high power density. To our best knowledge, TiNb2O7, Ti2Nb10O29 and Ti2Nb2O9 are the only three known TiNbxO2+2.5x anode materials.6–8,10 The following studies are very limited.11–19 Here, we report a new TiNbxO2+2.5x compound with a chemical formula of TiNb6O17 (x = 6), which is fabricated using a facile solid-state reaction method. According to eqn (1), the theoretical capacity of TiNbxO2+2.5x increases with x. Thus, TiNb6O17
a
Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, College of Materials and Chemical Engineering, Hainan University, Haikou 570228, Hainan, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-898-66290185; Tel: +86-898-66290185 b Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore. E-mail: [email protected]; Fax: +65-67791459; Tel: +65-65162236 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cc01494h
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Fig. 1
Crystal structures of (a) Ti2Nb10O29 and (b) TiNb6O17.
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Fig. 2 XRD patterns and Rietveld refinement results of (a) TiNb6O17 and (b) Ti2Nb10O29.
has a theoretical capacity of 397 mA h g1, larger than those of TiNb2O7 and Ti2Nb10O29. The influences of crystal structures on material properties and electrochemical performances are investigated. The results show that TiNb6O17 has a large Li+ ion diffusion coefficient, large capacity, high rate performance and good cyclability, thus fulfilling the requirements of both high power density and high energy density for EVs. The X-ray diffraction (XRD) spectrum of TiNb6O17 is plotted in Fig. 2a. Its sharp diffraction peaks indicate its good crystallinity arising from the high-temperature calcination. To our best knowledge, no studies on TiNb6O17 have been reported so far. As a result, no existing crystal data for TiNb6O17 can be found. However, it was found that the XRD spectrum of TiNb6O17 is similar to that of Ti2Nb10O29 (Fig. 2b). Therefore, it can be reasonably deduced that TiNb6O17 has a similar crystal structure to Ti2Nb10O29, i.e., a Wadsley–Roth shear structure with an A2/m space group. The structural similarity was confirmed by the high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses (Fig. S2, ESI†). Using this crystal data as the initial crystal data for TiNb6O17, the spectrum of TiNb6O17 was successfully refined by the Rietveld method, and the results are tabulated in Table 1. As a comparison, the refinement for Ti2Nb10O29 was also conducted. Based on the comparison between the observed and calculated patterns of TiNb6O17, it can be concluded that all diffraction peaks of TiNb6O17 conform to the Ti2Nb10O29 structure without any impurity phases (such as TiO2, Nb2O5 or TiNb2O7). Compared with Ti2Nb10O29, TiNb6O17 shows three different features in the crystal structure. Firstly, the Nb5+ : Ti4+ ratio of TiNb6O17 is 6 : 1, larger than that of Ti2Nb10O29 (5 : 1); the larger ratio indicates more Nb5+ ions participating in the twoelectron reaction between Nb5+ and Nb3+ ions, leading to the larger theoretical capacity of TiNb6O17 (eqn (1)). Secondly, TiNb6O17 has a considerably large amount of cation vacancies (0.49% vs. all cations sites). Hence, TiNb6O17 possesses a defective Ti2Nb10O29-type structure (Fig. 1b) in which 14.7% of Ti4+ ions in Ti2Nb10O29 are replaced by Nb5+ ions and cation vacancies, following eqn (2).
0000
2Nb2O5 = 4NbTi + VTi + 10O o Table 1
(2)
Finally, TiNb6O17 shows larger lattice parameters (a, b and c) and unit cell volume. These increased values can be due to the larger amount of Nb5+ ions with larger size (0.64 Å) than that of Ti4+ ions (0.605 Å) and the existence of the cation vacancies.20 Fig. S3a and b (ESI†) display the particle morphologies and sizes of TiNb6O17 and Ti2Nb10O29, respectively. Both samples exhibit similar morphologies with wide particle size distributions ranging from less than 100 nm to more than 1 mm, but TiNb6O17 has a larger average particle size than Ti2Nb10O29. The difference in the particle size is further confirmed by their specific surface area tests. The BET specific surface area of TiNb6O17 is 1.06 m2 g1, which is 19% smaller than that of Ti2Nb10O29 (1.26 m2 g1). This comparison suggests that the average particle size of TiNb6O17 is 19% larger than that of Ti2Nb10O29. The cyclic voltammetry (CV) measurements of the TiNb6O17/Li and Ti2Nb10O29/Li cells were carried out at a scanning rate of 0.1 mV s1 for four cycles and then successively at 0.3, 0.5 and 0.7 mV s1 for one cycle each between 3.0 and 0.8 V vs. Li/Li+, as shown in Fig. 3a and b. For each cell, the intensive cathodic peak shifts to a larger potential after the first cycle. This shift is probably due to the change in the electronic structure of TiNb6O17/Ti2Nb10O29 originating from the irreversible lithiation process in the first cycle (note that the initial coulombic efficiency for each cell is not 100% as presented below).17 There are two cathodic peaks centered at B1.88 and B1.61 V vs. Li/Li+ as well as three anodic peaks at B1.92, B1.73 and B1.25 V vs. Li/Li+ in the second cycle of the Ti2Nb10O29/Li cell, which are consistent with the previous report from Wu et al.8 Compared with the Ti2Nb10O29/Li cell, the TiNb6O17/Li cell also exhibits two cathodic and three anode peaks at the similar positions but its relative peak intensities are slightly different. Such differences may be attributed to the different Nb5+ : Ti4+ ratios between TiNb6O17 and Ti2Nb10O29. These peaks can be related to the Ti3+/Ti4+, Nb4+/Nb5+ and Nb3+/Nb4+ redox couples. Based on the X-ray absorption near edge spectroscopy of a TiNb2O7/Li cell, Guo et al. showed that during the lithiation process the reductions of the Ti4+ and Nb5+ ions start simultaneously; Ti4+ ions are continuously reduced to Ti3+ ions; and Nb5+ ions are continuously reduced to Nb4+ ions and then to Nb3+ ions.15 Therefore, it can be reasonably deduced that the respective cathodic/anodic peak of the TiNb6O17/Li and Ti2Nb10O29/Li cells cannot be assigned to a certain redox couple. The average working potentials of both cells are B1.70 V vs. Li/Li+, taking the middle points between the intensive cathodic and anodic peaks. This value is similar to those of the TiNb2O7/Li cell (B1.64 V vs. Li/Li+) and the Li4Ti5O12/Li cell (B1.57 V vs. Li/Li+).5,17 Such high working potential avoids the deposition of lithium dendrites on TiNb6O17 and Ti2Nb10O29 particle
Results of crystal analysis by Rietveld refinements in TiNb6O17 and Ti2Nb10O29
Sample
a (Å)
b (Å)
c (Å)
a, g (1)
b (1)
V (Å3)
fTia
fNbb
Rwpc
Rpd
w2 e
TiNb6O17 Ti2Nb10O29
15.54285(63) 15.52368(58)
3.81695(12) 3.81104(11)
20.56735(76) 20.54769(67)
90(—) 90(—)
113.077(3) 113.058(3)
1122.541(87) 1118.512(78)
0.1659(—) 0.1667(—)
0.8292(—) 0.8333(—)
0.1099 0.0998
0.0861 0.0789
7.961 6.593
a
Occupancy of Ti4+ ions in 4i sites.
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b
Occupancy of Nb5+ ions in 4i sites. c Weighted profile residual.
d
Profile residual. e Goodness of fit.
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Fig. 3 CV curves of (a) TiNb6O17 and (b) Ti2Nb10O29, and (c) relationship between the peak current density of cathodic/anodic reaction ip and square root of sweep rate v0.5.
surfaces and suppresses the reduction of an electrolyte, guaranteeing their high safety. The polarization of the TiNb6O17/Li cell between the intensive cathodic and anodic peaks in the second cycle is 0.104 V, which is 0.019 V lower than that of the Ti2Nb10O29/Li cell (0.123 V). The Li+ ion diffusion coefficients of TiNb6O17 and Ti2Nb10O29 were determined from the CV results. As can be seen from Fig. 3c, the peak current density of the intensive cathodic/anodic reaction ip is proportional to the square root of the sweep rate v0.5, which shows the linear semi-infinite diffusion in cathodic and anodic processes. Consequently, the Randles–Sevcik equation (eqn (3)) can be applied,21 based on which the Li+ ion diffusion coefficient D can be calculated. ip = 0.4463n1.5F1.5CSR0.5T0.5D0.5v0.5
(3)
where n, F, C, S, R and T are the charge transfer number, Faraday’s constant, molar concentration of Li+ ions in solid, surface area per unit weight of active materials, molar gas constant and absolute temperature, respectively. The Li+ ion diffusion coefficients of TiNb6O17 (4.28 1014 cm2 s1 for lithiation and 5.48 1014 cm2 s1 for delithiation) are B7 times larger than those of Ti2Nb10O29 (5.43 1015 cm2 s1 for lithiation and 6.52 1015 cm2 s1 for delithiation). The improvements may be rooted in the crystalline characteristics of TiNb6O17. Compared with Ti2Nb10O29, TiNb6O17 exhibits larger unit cell volume and 0.49% cation vacancies, which may respectively lead to the larger size and number of Li+ ion transport paths in the crystal structure, facilitating the Li+ ion transport during the discharge (lithiation) and charge (delithiation) processes. Therefore, the significantly larger Li+ ion diffusion coefficients in TiNb6O17 were achieved. Furthermore, it is worth noting that the average Li+ ion diffusion coefficient of TiNb6O17 is B45 times larger than that of TiNb2O7 (1.05 1015 cm2 s1) and B269 times larger than that of Li4Ti5O12 (1.81 1016 cm2 s1),18 indicating the advanced crystal structure of TiNb6O17. In addition, since the Li+ ion diffusion
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Fig. 4 (i) Initial discharge–charge profiles at 0.1 C, second discharge– charge profiles at (ii) 0.1 C, (iii) 0.5 C, (iv) 1 C, (v) 2 C and (vi) 5 C of (a) TiNb6O17 and (b) Ti2Nb10O29, and (c) cyclability of TiNb6O17/Li and Ti2Nb10O29/Li cells at 5 C after aging at 0.1 C for 5 cycles along with Coulombic efficiency of the TiNb6O17/Li cell. Identical discharge–charge rates were used.
coefficients for TiNb6O17 and Ti2Nb10O29 during the delithiation process are larger than those during the lithiation process, it can be confirmed that the lithiation process is the ratelimiting step for both samples. Fig. 4a and b respectively, illustrate the galvanostatic discharge– charge profiles of the TiNb6O17/Li and Ti2Nb10O29/Li cells at different rates (0.1, 0.5, 1, 2 and 5 C) from 3.0 to 0.8 V vs. Li/Li+. The discharge profile of each cell at 0.1 C reveals three different regions: a sloping region down to B1.65 V vs. Li/Li+, attributed to a solid-solution reaction; a plateau at B1.65 V vs. Li/Li+, reflecting a two-phase reaction; and a further sloping region until reaching the discharge limit, assigned to another solid-solution reaction.8 Similarly, there are three corresponding regions during the charge process. The discharge plateau at B1.65 V vs. Li/Li+ and the charge plateau at B1.68 V vs. Li/Li+, respectively, match with the intensive cathodic peak at B1.61 V vs. Li/Li+ and the anodic peak at B1.73 V vs. Li/Li+ in the CV curves (Fig. 3). The initial discharge capacity of the Ti2Nb10O29/Li cell is 355 mA h g1. In contrast, the TiNb6O17/Li cell delivers a larger value of 383 mA h g1, which corresponds to 12.5 Li per formula unit and is as high as 96.5% of its theoretical capacity (397 mA h g1). Similarly, at the same rate, the Ti2Nb10O29/Li cell shows an initial Coulombic efficiency of 82.8% and a charge capacity of 294 mA h g1, while those for the TiNb6O17/Li cell (85.4% and 328 mA h g1) are larger. These capacities of the TiNb6O17/Li cell are larger than those of the previously reported TiNb2O7/Li and Ti2Nb10O29/Li cells.6–14 The large capacities of the TiNb6O17/Li cell can be due to its good electrochemical kinetics and large theoretical capacity. When the current rate increases, the capacities decrease; the plateaus become inconspicuous; the discharge profiles monotonically drop; and the charge profiles monotonically rise. These phenomena can be ascribed to the increasing electrode polarization. In comparison with the Ti2Nb10O29/Li cell, the TiNb6O17/Li
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cell exhibits smaller polarization and larger discharge/charge capacities at all the ranges of the current rates. For instance, at 5 C (B2 A g1), the TiNb6O17/Li cell is able to deliver a charge capacity of 178 mA h g1, which is almost twice that of the Ti2Nb10O29/Li cell (90 mA h g1) and even surpasses the theoretical capacity of the popular Li4Ti5O12/Li cell (175 mA h g1). Note that the average particle size of TiNb6O17 is 19% larger than that of Ti2Nb10O29, which results in a longer distance for the Li+ ion transport in the particles and thus has a negative effect on the rate performance. However, the larger Li+ ion diffusion coefficient of TiNb6O17 overwhelms the former negative effect, and leads to its overall better conduction. Consequently, a better rate performance of the TiNb6O17/Li cell was achieved. All these discharge– charge results are in good agreement of the CV analysis. Furthermore, the TiNb6O17/Li cell also shows good cyclability at 5 C (Fig. 4c). The initial charge capacity is up to 178 mA h g1 and the reversible charge capacity stabilizes at 171 mA h g1 even after 100 cycles, which presents an outstanding capacity retention of 95.6%. Comparatively, the capacity retention of the Ti2Nb10O29/ Li cell is only 74.7%. Meanwhile, the Coulombic efficiency of the TiNb6O17/Li cell always maintains at B100% during the cycling (Fig. 4c), suggesting its highly reversible characteristic in the successive runs. In addition, the similarity of the ex situ XRD patterns of the TiNb6O17 electrodes at different states of charge (Fig. S4a, ESI†) demonstrates the intercalation/deintercalation characteristic and good structural reversibility of TiNb6O17, which also contribute to their good cyclability. As a new intercalation-type anode material, TiNb6O17 has been prepared using a facile solid-state reaction method. It exhibits a similar crystal structure with Ti2Nb10O29 (a Wadsley– Roth shear structure with an A2/m space group) but has a larger unit cell volume and 0.49% cation vacancies (vs. all cations sites). Due to this advanced crystal structure, TiNb6O17 shows large Li+ ion diffusion coefficients of 4.28 1014 cm2 s1 (lithiation) and 5.48 1014 cm2 s1 (delithiation), which are B7 times larger than those of Ti2Nb10O29. As a result, TiNb6O17 exhibits outstanding electrochemical performances in terms of the capacity, rate performance and cyclability. At 0.1 C, it has a large initial discharge capacity of 383 mA h g1 and charge capacity of 328 mA h g1. At 5 C, it still retains a large charge capacity of 178 mA h g1 with only 4.6% loss after 100 cycles. Therefore, TiNb6O17, with the same advantages of Li4Ti5O12 but
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significantly larger capacities, is able to fulfil the requirements of both high power density and high energy density, and thus possesses great potential for the applications in the LIBs of EVs. This research is supported by the National Natural Science Foundation of China (Grant No. 51202050 and 51162007) and the Scientific Research Setup Fund of Hainan University. The support from the Scientific Research Innovation Team of the Characteristic Resources Development and Utilization in Hainan Island with the Special Fund for the Midwest Universities Comprehensive Strength Promotion is also acknowledged.
Notes and references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
M. Armand and J. M. Tarascon, Nature, 2008, 451, 652. S. S. Zheng, J. Power Sources, 2006, 161, 1385. J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587. T. Ohzuku, A. Ueda and N. Yamamoto, J. Electrochem. Soc., 1995, 142, 1431. C. F. Lin, X. Y. Fan, Y. L. Xin, F. Q. Cheng, M. O. Lai, H. H. Zhou and L. Lu, J. Mater. Chem. A, 2014, 2, 9982. J. T. Han, Y. H. Huang and J. B. Goodenough, Chem. Mater., 2011, 23, 2027. J. T. Han and J. B. Goodenough, Chem. Mater., 2011, 23, 3404. X. Y. Wu, J. Miao, W. Z. Han, Y. S. Hu, D. F. Chen, J. S. Lee, J. Kim and L. Q. Chen, Electrochem. Commun., 2012, 25, 39. J. F. Colin, V. Pralong, M. Hervieu, V. Caignaert and B. Raveau, Chem. Mater., 2008, 20, 1534. R. J. Cava, D. W. Murphy and S. M. Zahurak, J. Electrochem. Soc., 1983, 130, 2345. L. Fei, Y. Xu, X. F. Wu, Y. L. Li, P. Xie, S. G. Deng, S. Smirnov and H. M. Luo, Nanoscale, 2013, 5, 11102. S. Jayaraman, V. Aravindan, P. S. Kumar, W. C. Ling, S. Ramakrishna and S. Madhavi, ACS Appl. Mater. Interfaces, 2014, 6, 8660. K. Tang, X. K. Mu, P. A. van Aken, Y. Yu and J. Maier, Adv. Energy Mater., 2013, 3, 49. C. Jo, Y. Kim, J. Hwang, J. Shim, J. Chun and J. Lee, Chem. Mater., 2014, 26, 3508. B. K. Guo, X. Q. Yu, X. G. Sun, M. F. Chi, Z. A. Qiao, J. Liu, Y. S. Hu, X. Q. Yang, J. B. Goodenough and S. Dai, Energy Environ. Sci., 2014, 7, 2220. Q. S. Cheng, J. W. Liang, Y. C. Zhu, L. L. Si, C. Guo and Y. T. Qian, J. Mater. Chem. A, 2014, 2, 17258. X. Lu, Z. L. Jian, Z. Fang, L. Gu, Y. S. Hu, W. Chen, Z. X. Wang and L. Q. Chen, Energy Environ. Sci., 2011, 4, 2638. C. F. Lin, S. Yu, S. Q. Wu, S. W. Lin, Z. Z. Zhu, J. B. Li and L. Lu, J. Mater. Chem. A, 2015, 3, 8627. T. Takashima, T. Tojo, R. Inada and Y. Sakurai, J. Power Sources, 2015, 276, 113. R. D. Shannon, Acta Crystallogr., Sect. A: Found. Crystallogr., 1976, 32, 751. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York, 2nd edn, 2001.
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TiNb6O17: a new electrode material for lithium-ion batteries† Chunfu Lin,*a Guizhen Wang,a Shiwei Lin,*a Jianbao Lia and Li Lu*b
Received 17th February 2015, Accepted 20th April 2015 DOI: 10.1039/c5cc01494h www.rsc.org/chemcomm
TiNb6O17 shows a similar crystal structure to Ti2Nb10O29 (Wadsley– Roth shear structure) but has larger lattice parameters and 0.49% cation vacancies, leading to its large Li+ ion diffusion coefficients. It exhibits a large initial discharge capacity of 383 mA h g1 at 0.1 C,
Based on the two-electron transfer between Nb5+ and Nb3+ ions and the one-electron transfer between Ti4+ and Ti3+ ions, Ti–Nb–O compounds with a general chemical formula of TiNbxO2+2.5x have large theoretical capacities shown in eqn (1):
high rate performance and good cyclability.
C = 403 5441/(133x + 80) (mA h g1) The great success that lithium-ion batteries (LIBs) have experienced in portable electronic devices is now being extended to electric vehicles (EVs), which require both high power density and high energy density.1 For commercial LIBs, graphite is usually selected as the anode material due to its large theoretical capacity of 372 mA h g1 and good reversible discharge– charge for Li+ ions. However, it suffers severe safety hazards rooted in the formation and growth of lithium dendrites when operated at low potentials and high currents, and thus fails to be used in EVs.2 A fundamental solution is to replace graphite by other anode materials with high safety, among which intercalationtype Li4Ti5O12 may be the most promising one.3 Li4Ti5O12 has a spinel crystal structure, in which cations occupy both octahedral and tetrahedral sites.4 After being modified by doping with alien ions, compositing with a second conductive phase and/or reducing the particle size, this material exhibits not only high safety and good cyclability but also high rate performance.5 However, its intrinsically low theoretical capacity (175 mA h g1 between 3.0 and 1.0 V vs. Li/Li+) cannot be effectively increased. Consequently, although Li4Ti5O12 can fulfil the requirement of high power density, it cannot fulfil that of high energy density, significantly limiting its practical applications in EVs. Therefore, exploring new anode materials with the same advantages of Li4Ti5O12 but larger capacities is an urgent task in the field of EVs.
(1)
For instance, TiNb2O7 (x = 2) and Ti2Nb10O29 (x = 5), respectively, possess the theoretical capacities of 388 and 396 mA h g1,6–8 which are B1.2 times larger than that of Li4Ti5O12 and even exceed that of graphite. They show Wadsley–Roth shear structures built by m n N (m = n = 3 for TiNb2O7; m = 4 and n = 3 for Ti2Nb10O29, Fig. 1a and Fig. S1, ESI†) ReO3-type blocks, where m and n are, respectively, the length and width of the blocks in numbers of octahedral.9 All cations (Nb5+ and Ti4+ ions with molar ratios of 2 : 1 for TiNb2O7 and 5 : 1 for Ti2Nb10O29) are disordered in octahedral sites sharing corners and edges. Since all cations occupy the octahedral sites, the Wadsley–Roth shear structure is more open than the spinel structure, inferring a larger Li+ ion diffusion coefficient of TiNbxO2+2.5x than that of Li4Ti5O12. Despite the large theoretical capacities of TiNb2O7 and Ti2Nb10O29, they suffer low intrinsic conductivity, resulting in poor rate performances. Thus, they can fulfil the requirement of high energy density but cannot fulfil that of high power density. To our best knowledge, TiNb2O7, Ti2Nb10O29 and Ti2Nb2O9 are the only three known TiNbxO2+2.5x anode materials.6–8,10 The following studies are very limited.11–19 Here, we report a new TiNbxO2+2.5x compound with a chemical formula of TiNb6O17 (x = 6), which is fabricated using a facile solid-state reaction method. According to eqn (1), the theoretical capacity of TiNbxO2+2.5x increases with x. Thus, TiNb6O17
a
Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, College of Materials and Chemical Engineering, Hainan University, Haikou 570228, Hainan, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-898-66290185; Tel: +86-898-66290185 b Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore. E-mail: [email protected]; Fax: +65-67791459; Tel: +65-65162236 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cc01494h
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Fig. 1
Crystal structures of (a) Ti2Nb10O29 and (b) TiNb6O17.
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Fig. 2 XRD patterns and Rietveld refinement results of (a) TiNb6O17 and (b) Ti2Nb10O29.
has a theoretical capacity of 397 mA h g1, larger than those of TiNb2O7 and Ti2Nb10O29. The influences of crystal structures on material properties and electrochemical performances are investigated. The results show that TiNb6O17 has a large Li+ ion diffusion coefficient, large capacity, high rate performance and good cyclability, thus fulfilling the requirements of both high power density and high energy density for EVs. The X-ray diffraction (XRD) spectrum of TiNb6O17 is plotted in Fig. 2a. Its sharp diffraction peaks indicate its good crystallinity arising from the high-temperature calcination. To our best knowledge, no studies on TiNb6O17 have been reported so far. As a result, no existing crystal data for TiNb6O17 can be found. However, it was found that the XRD spectrum of TiNb6O17 is similar to that of Ti2Nb10O29 (Fig. 2b). Therefore, it can be reasonably deduced that TiNb6O17 has a similar crystal structure to Ti2Nb10O29, i.e., a Wadsley–Roth shear structure with an A2/m space group. The structural similarity was confirmed by the high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses (Fig. S2, ESI†). Using this crystal data as the initial crystal data for TiNb6O17, the spectrum of TiNb6O17 was successfully refined by the Rietveld method, and the results are tabulated in Table 1. As a comparison, the refinement for Ti2Nb10O29 was also conducted. Based on the comparison between the observed and calculated patterns of TiNb6O17, it can be concluded that all diffraction peaks of TiNb6O17 conform to the Ti2Nb10O29 structure without any impurity phases (such as TiO2, Nb2O5 or TiNb2O7). Compared with Ti2Nb10O29, TiNb6O17 shows three different features in the crystal structure. Firstly, the Nb5+ : Ti4+ ratio of TiNb6O17 is 6 : 1, larger than that of Ti2Nb10O29 (5 : 1); the larger ratio indicates more Nb5+ ions participating in the twoelectron reaction between Nb5+ and Nb3+ ions, leading to the larger theoretical capacity of TiNb6O17 (eqn (1)). Secondly, TiNb6O17 has a considerably large amount of cation vacancies (0.49% vs. all cations sites). Hence, TiNb6O17 possesses a defective Ti2Nb10O29-type structure (Fig. 1b) in which 14.7% of Ti4+ ions in Ti2Nb10O29 are replaced by Nb5+ ions and cation vacancies, following eqn (2).
0000
2Nb2O5 = 4NbTi + VTi + 10O o Table 1
(2)
Finally, TiNb6O17 shows larger lattice parameters (a, b and c) and unit cell volume. These increased values can be due to the larger amount of Nb5+ ions with larger size (0.64 Å) than that of Ti4+ ions (0.605 Å) and the existence of the cation vacancies.20 Fig. S3a and b (ESI†) display the particle morphologies and sizes of TiNb6O17 and Ti2Nb10O29, respectively. Both samples exhibit similar morphologies with wide particle size distributions ranging from less than 100 nm to more than 1 mm, but TiNb6O17 has a larger average particle size than Ti2Nb10O29. The difference in the particle size is further confirmed by their specific surface area tests. The BET specific surface area of TiNb6O17 is 1.06 m2 g1, which is 19% smaller than that of Ti2Nb10O29 (1.26 m2 g1). This comparison suggests that the average particle size of TiNb6O17 is 19% larger than that of Ti2Nb10O29. The cyclic voltammetry (CV) measurements of the TiNb6O17/Li and Ti2Nb10O29/Li cells were carried out at a scanning rate of 0.1 mV s1 for four cycles and then successively at 0.3, 0.5 and 0.7 mV s1 for one cycle each between 3.0 and 0.8 V vs. Li/Li+, as shown in Fig. 3a and b. For each cell, the intensive cathodic peak shifts to a larger potential after the first cycle. This shift is probably due to the change in the electronic structure of TiNb6O17/Ti2Nb10O29 originating from the irreversible lithiation process in the first cycle (note that the initial coulombic efficiency for each cell is not 100% as presented below).17 There are two cathodic peaks centered at B1.88 and B1.61 V vs. Li/Li+ as well as three anodic peaks at B1.92, B1.73 and B1.25 V vs. Li/Li+ in the second cycle of the Ti2Nb10O29/Li cell, which are consistent with the previous report from Wu et al.8 Compared with the Ti2Nb10O29/Li cell, the TiNb6O17/Li cell also exhibits two cathodic and three anode peaks at the similar positions but its relative peak intensities are slightly different. Such differences may be attributed to the different Nb5+ : Ti4+ ratios between TiNb6O17 and Ti2Nb10O29. These peaks can be related to the Ti3+/Ti4+, Nb4+/Nb5+ and Nb3+/Nb4+ redox couples. Based on the X-ray absorption near edge spectroscopy of a TiNb2O7/Li cell, Guo et al. showed that during the lithiation process the reductions of the Ti4+ and Nb5+ ions start simultaneously; Ti4+ ions are continuously reduced to Ti3+ ions; and Nb5+ ions are continuously reduced to Nb4+ ions and then to Nb3+ ions.15 Therefore, it can be reasonably deduced that the respective cathodic/anodic peak of the TiNb6O17/Li and Ti2Nb10O29/Li cells cannot be assigned to a certain redox couple. The average working potentials of both cells are B1.70 V vs. Li/Li+, taking the middle points between the intensive cathodic and anodic peaks. This value is similar to those of the TiNb2O7/Li cell (B1.64 V vs. Li/Li+) and the Li4Ti5O12/Li cell (B1.57 V vs. Li/Li+).5,17 Such high working potential avoids the deposition of lithium dendrites on TiNb6O17 and Ti2Nb10O29 particle
Results of crystal analysis by Rietveld refinements in TiNb6O17 and Ti2Nb10O29
Sample
a (Å)
b (Å)
c (Å)
a, g (1)
b (1)
V (Å3)
fTia
fNbb
Rwpc
Rpd
w2 e
TiNb6O17 Ti2Nb10O29
15.54285(63) 15.52368(58)
3.81695(12) 3.81104(11)
20.56735(76) 20.54769(67)
90(—) 90(—)
113.077(3) 113.058(3)
1122.541(87) 1118.512(78)
0.1659(—) 0.1667(—)
0.8292(—) 0.8333(—)
0.1099 0.0998
0.0861 0.0789
7.961 6.593
a
Occupancy of Ti4+ ions in 4i sites.
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b
Occupancy of Nb5+ ions in 4i sites. c Weighted profile residual.
d
Profile residual. e Goodness of fit.
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Fig. 3 CV curves of (a) TiNb6O17 and (b) Ti2Nb10O29, and (c) relationship between the peak current density of cathodic/anodic reaction ip and square root of sweep rate v0.5.
surfaces and suppresses the reduction of an electrolyte, guaranteeing their high safety. The polarization of the TiNb6O17/Li cell between the intensive cathodic and anodic peaks in the second cycle is 0.104 V, which is 0.019 V lower than that of the Ti2Nb10O29/Li cell (0.123 V). The Li+ ion diffusion coefficients of TiNb6O17 and Ti2Nb10O29 were determined from the CV results. As can be seen from Fig. 3c, the peak current density of the intensive cathodic/anodic reaction ip is proportional to the square root of the sweep rate v0.5, which shows the linear semi-infinite diffusion in cathodic and anodic processes. Consequently, the Randles–Sevcik equation (eqn (3)) can be applied,21 based on which the Li+ ion diffusion coefficient D can be calculated. ip = 0.4463n1.5F1.5CSR0.5T0.5D0.5v0.5
(3)
where n, F, C, S, R and T are the charge transfer number, Faraday’s constant, molar concentration of Li+ ions in solid, surface area per unit weight of active materials, molar gas constant and absolute temperature, respectively. The Li+ ion diffusion coefficients of TiNb6O17 (4.28 1014 cm2 s1 for lithiation and 5.48 1014 cm2 s1 for delithiation) are B7 times larger than those of Ti2Nb10O29 (5.43 1015 cm2 s1 for lithiation and 6.52 1015 cm2 s1 for delithiation). The improvements may be rooted in the crystalline characteristics of TiNb6O17. Compared with Ti2Nb10O29, TiNb6O17 exhibits larger unit cell volume and 0.49% cation vacancies, which may respectively lead to the larger size and number of Li+ ion transport paths in the crystal structure, facilitating the Li+ ion transport during the discharge (lithiation) and charge (delithiation) processes. Therefore, the significantly larger Li+ ion diffusion coefficients in TiNb6O17 were achieved. Furthermore, it is worth noting that the average Li+ ion diffusion coefficient of TiNb6O17 is B45 times larger than that of TiNb2O7 (1.05 1015 cm2 s1) and B269 times larger than that of Li4Ti5O12 (1.81 1016 cm2 s1),18 indicating the advanced crystal structure of TiNb6O17. In addition, since the Li+ ion diffusion
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Fig. 4 (i) Initial discharge–charge profiles at 0.1 C, second discharge– charge profiles at (ii) 0.1 C, (iii) 0.5 C, (iv) 1 C, (v) 2 C and (vi) 5 C of (a) TiNb6O17 and (b) Ti2Nb10O29, and (c) cyclability of TiNb6O17/Li and Ti2Nb10O29/Li cells at 5 C after aging at 0.1 C for 5 cycles along with Coulombic efficiency of the TiNb6O17/Li cell. Identical discharge–charge rates were used.
coefficients for TiNb6O17 and Ti2Nb10O29 during the delithiation process are larger than those during the lithiation process, it can be confirmed that the lithiation process is the ratelimiting step for both samples. Fig. 4a and b respectively, illustrate the galvanostatic discharge– charge profiles of the TiNb6O17/Li and Ti2Nb10O29/Li cells at different rates (0.1, 0.5, 1, 2 and 5 C) from 3.0 to 0.8 V vs. Li/Li+. The discharge profile of each cell at 0.1 C reveals three different regions: a sloping region down to B1.65 V vs. Li/Li+, attributed to a solid-solution reaction; a plateau at B1.65 V vs. Li/Li+, reflecting a two-phase reaction; and a further sloping region until reaching the discharge limit, assigned to another solid-solution reaction.8 Similarly, there are three corresponding regions during the charge process. The discharge plateau at B1.65 V vs. Li/Li+ and the charge plateau at B1.68 V vs. Li/Li+, respectively, match with the intensive cathodic peak at B1.61 V vs. Li/Li+ and the anodic peak at B1.73 V vs. Li/Li+ in the CV curves (Fig. 3). The initial discharge capacity of the Ti2Nb10O29/Li cell is 355 mA h g1. In contrast, the TiNb6O17/Li cell delivers a larger value of 383 mA h g1, which corresponds to 12.5 Li per formula unit and is as high as 96.5% of its theoretical capacity (397 mA h g1). Similarly, at the same rate, the Ti2Nb10O29/Li cell shows an initial Coulombic efficiency of 82.8% and a charge capacity of 294 mA h g1, while those for the TiNb6O17/Li cell (85.4% and 328 mA h g1) are larger. These capacities of the TiNb6O17/Li cell are larger than those of the previously reported TiNb2O7/Li and Ti2Nb10O29/Li cells.6–14 The large capacities of the TiNb6O17/Li cell can be due to its good electrochemical kinetics and large theoretical capacity. When the current rate increases, the capacities decrease; the plateaus become inconspicuous; the discharge profiles monotonically drop; and the charge profiles monotonically rise. These phenomena can be ascribed to the increasing electrode polarization. In comparison with the Ti2Nb10O29/Li cell, the TiNb6O17/Li
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cell exhibits smaller polarization and larger discharge/charge capacities at all the ranges of the current rates. For instance, at 5 C (B2 A g1), the TiNb6O17/Li cell is able to deliver a charge capacity of 178 mA h g1, which is almost twice that of the Ti2Nb10O29/Li cell (90 mA h g1) and even surpasses the theoretical capacity of the popular Li4Ti5O12/Li cell (175 mA h g1). Note that the average particle size of TiNb6O17 is 19% larger than that of Ti2Nb10O29, which results in a longer distance for the Li+ ion transport in the particles and thus has a negative effect on the rate performance. However, the larger Li+ ion diffusion coefficient of TiNb6O17 overwhelms the former negative effect, and leads to its overall better conduction. Consequently, a better rate performance of the TiNb6O17/Li cell was achieved. All these discharge– charge results are in good agreement of the CV analysis. Furthermore, the TiNb6O17/Li cell also shows good cyclability at 5 C (Fig. 4c). The initial charge capacity is up to 178 mA h g1 and the reversible charge capacity stabilizes at 171 mA h g1 even after 100 cycles, which presents an outstanding capacity retention of 95.6%. Comparatively, the capacity retention of the Ti2Nb10O29/ Li cell is only 74.7%. Meanwhile, the Coulombic efficiency of the TiNb6O17/Li cell always maintains at B100% during the cycling (Fig. 4c), suggesting its highly reversible characteristic in the successive runs. In addition, the similarity of the ex situ XRD patterns of the TiNb6O17 electrodes at different states of charge (Fig. S4a, ESI†) demonstrates the intercalation/deintercalation characteristic and good structural reversibility of TiNb6O17, which also contribute to their good cyclability. As a new intercalation-type anode material, TiNb6O17 has been prepared using a facile solid-state reaction method. It exhibits a similar crystal structure with Ti2Nb10O29 (a Wadsley– Roth shear structure with an A2/m space group) but has a larger unit cell volume and 0.49% cation vacancies (vs. all cations sites). Due to this advanced crystal structure, TiNb6O17 shows large Li+ ion diffusion coefficients of 4.28 1014 cm2 s1 (lithiation) and 5.48 1014 cm2 s1 (delithiation), which are B7 times larger than those of Ti2Nb10O29. As a result, TiNb6O17 exhibits outstanding electrochemical performances in terms of the capacity, rate performance and cyclability. At 0.1 C, it has a large initial discharge capacity of 383 mA h g1 and charge capacity of 328 mA h g1. At 5 C, it still retains a large charge capacity of 178 mA h g1 with only 4.6% loss after 100 cycles. Therefore, TiNb6O17, with the same advantages of Li4Ti5O12 but
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significantly larger capacities, is able to fulfil the requirements of both high power density and high energy density, and thus possesses great potential for the applications in the LIBs of EVs. This research is supported by the National Natural Science Foundation of China (Grant No. 51202050 and 51162007) and the Scientific Research Setup Fund of Hainan University. The support from the Scientific Research Innovation Team of the Characteristic Resources Development and Utilization in Hainan Island with the Special Fund for the Midwest Universities Comprehensive Strength Promotion is also acknowledged.
Notes and references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
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