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Template-engaged synthesis of spinel-layered Li1.5MnTiO4+δ nanorods as a cathode material for Li-ion batteries
Journal of Power Sources 355 (2017) 134e139
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Template-engaged synthesis of spinel-layered Li1.5MnTiO4þd nanorods as a cathode material for Li-ion batteries Ngoc Hung Vu a, Sanjith Unithrattil a, Van Hien Hoang a, Sangeun Chun b, **, Won Bin Im a, * a School of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea b School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea
h i g h l i g h t s A promising Li1.5MnTiO4þd cathode: spinel-layered structure with high capacity. A Li1.5MnTiO4þd nanorod was synthesized by using a b-MnO2 nanorod template. The Li1.5MnTiO4þd nanorods exhibit improved rate performance at high C-rate. The structure and electrochemical performances of Li1.5MnTiO4þd were studied.
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Article history: Received 4 February 2017 Received in revised form 4 April 2017 Accepted 14 April 2017
Spinel-layered composites of Li1.5MnTiO4þd were studied for their use as high-energy, low-cost, and environmentally benign cathode materials. The bulk particles showed an attractive specific capacity of up to 250 mAh g1 at C/10. To improve the performance of this cathode at a high C-rate, a spinel-layered Li1.5MnTiO4þd nanorod was successfully synthesized using a b-MnO2 nanorod template. The nanorod, which had an average diameter of 200 nm and a length of 1 mm, showed specific capacity as high as the bulk particle at C/10. However, owing to a one-dimensional nanostructure with a large effective contact area for Liþ diffusion, the nanorod sample exhibited enhanced capacities 11% (170 mAh g1) and 167% higher (80 mAh g1) at 1C and 10C rates, respectively, compared to the bulk particles. Moreover, both samples showed good cycle stability and capacity retention of over 85% after 100 cycles at 1C. © 2017 Elsevier B.V. All rights reserved.
Keywords: LiMnTiO4 Nanorod Template Spinel framework Li-ion battery
1. Introduction Among commercial cathode materials for Li-ion batteries (LIBs) such as LiCoO2 and LiFePO4, spinel LiMn2O4 with a threedimensional tunnel structure for the migration of Liþ has been considered one of the best candidates because of its low cost, nontoxicity, abundance, and easy preparation [1]. However, this material often displays a limited capacity and working voltage range and is thus used at high voltages (>3 V) due to the Jahn-Teller distortion that occurs below 3 V. In order to solve this problem, earlier studies have focused on the chemical modification of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Chun), [email protected] (W.B. Im). http://dx.doi.org/10.1016/j.jpowsour.2017.04.055 0378-7753/© 2017 Elsevier B.V. All rights reserved.
LiMn2O4 by partial substitution of Mn4þ by Ti4þ in LiMn2O4, which contributes to the formation of another attractive spinel cathode material, LiMnTiO4. In this compound, Ti forms stronger bonds with O (662 kJ mol1) in comparison with Mn (402 kJ mol1). Therefore, a more stable [Mn2-xTix]O4 framework could be obtained without sacrificing the advantages of LiMn2O4 [2e6]. However, the partial substitution with Ti decreases the capacity of LiMn2O4 (<200 mAh g1 in the 2e4.8 V) [6,7]. To address this problem, we previously reported a new material, Li1þzMnTiO4þd (where z ¼ 0, 0.5 and 1.0, and 0 d < 0.67), which has an improved capacity and cycling stability compared to current materials for cathode applications [7]. This material, Li1.5MnTiO4þd, showed the highest capacity among these cathodes. However, the material was synthesized by a solidstate reaction, which resulted in non-uniform, large particles. In addition, its morphology limited the contact area it had with the
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2. Experimental
Fig. 1. (a) XRD pattern and (b, c) SEM images of b-MnO2 as obtained from the hydrothermal reaction.
electrolyte, leading to a low capacity and poor C-rate performance, which are both drawbacks for battery applications [8e10]. An effective approach to improve the capacity, as well as the rate performance of the cathode material, is to exploit nanomaterials that have high surface areas, and in particular, one-dimensional (1D) nanomaterials. Moreover, cation disordered materials recently were studied and it might be a potential cathode with high capacity for LIBs [11]. The synthesis conditions were found to be a key which not only influence the crystallinity, but also cation disordering [12]. In this study, we successfully synthesized a cation disordered Li1.5MnTiO4þd nanorod by using a b-MnO2 nanorod template. The spinel-layered heterostructure of the nanorod enabled the facile electron transport along the 1-D geometry, and reduced the Li-ion diffusion length, resulting in an enhanced discharge capacity and Crate performance.
The b-MnO2 nanorod was synthesized according to a previous work [13]. In a typical synthesis, Mn(CH3COO)2$4H2O (2.45 g) and Na2S2O8 (1.19 g) (Sigma-Aldrich) were dissolved in 70 mL of distilled water, followed by stirring to form a homogeneous clear solution. The mixed solution was transferred to a 100 mL Teflonlined stainless steel autoclave, and heated at 120 C for 12 h for the hydrothermal reaction to occur. After the reaction, the final precipitated products were washed sequentially with deionized water and ethanol to remove the sulfate ions and other remnants by filtration. The obtained powder was subsequently dried at 100 C for 12 h in air. To prepare the nanorod sample, b-MnO2 nanorods (0.435 g) were added to 10 mL of anhydrous ethanol, where titanium isopropoxide (Ti(OiPr)4, 1.48 mL) was dissolved with 3 mL acetic acid. Thereafter, LiOH (0.315 g) were added to the above dispersion, followed by triethanolamine (2 mL), which was added as a chelating agent. The mixture was sonicated and stirred for 12 h, and subsequently dried at 100 C overnight. Then, the mixture was calcined at 600 or 700 C for 12 h in air at a heating rate of 1 C min1. The Li:Mn:Ti ratio in the obtained compounds was 1.5:1:1. The Li1.5MnTiO4þd nanorod was denoted as LMTO-NR. The bulk particles were prepared by a solid-state reaction method. First, stoichiometric amounts of Li2CO3 (99.9%, SigmaAldrich), MnCO3 (99.9%, Kojundo Chemicals), and TiO2 (99.5%, Sigma-Aldrich) were mixed using a mortar. The resultant mixture was heated in an alumina crucible at 800 C for 12 h, at a heating rate of 2 C min1. The Li1.5MnTiO4þd bulk particles were denoted as LMTO-BP. The structure of the samples was characterized using X-ray diffraction (XRD) (Cu Ka radiation, Philips X'Pert) in the angle range of 10 2q 120 , with a step size of 0.026 . The Rietveld refinement was made with the General Structure Analysis System (GSAS) program [14]. Field emission scanning electron microscopy (FESEM) (S-4700, Hitachi) and high-resolution transmission electron microscopy (HRTEM) (FEI Tecnai F20) at 200 kV were used to determine the sizes and morphologies of the particles. The specific surface area of particles was evaluated by the BET method (BELSORP-mini II, BEL Japan, Inc.). All measurements were carried out in the Korea Basic Science Institute (KBSI), Gwangju, Republic of Korea. The cathode was prepared with a mass ratio of 75:10:15 of the active material, conductive carbon (KETJEN black), and teflonized acetylene black (TAB) binder, respectively. The mass of active material was 7.5 mg. This mixture was thoroughly mixed and pressed onto a stainless steel mesh, and dried under vacuum at 120 C for 12 h. A 2032 coin-type cell, which was fabricated in an Ar-filled glove box, consisted of the cathode and Li metal anode separated by a polymer membrane, together with glass fiber. The electrolyte was made from a 1:1 mixture of ethylene carbonate and dimethyl carbonate containing 1 M LiPF6. The cells were aged for 12 h before the electrochemical measurements. The charge-discharge measurements were carried out using a NAGANO BTS-2004H battery charger between 1.5 and 4.8 V vs. Liþ/Li. 3. Result and discussion Fig. 1 shows the XRD patterns and SEM images of the hydrothermally synthesized b-MnO2. All peaks were matched with those of the reference, revealing a tetragonal symmetry with a P42/mnm space group, in accordance with the previous report [13]. No additional impurity peaks were detected (Fig. 1a). The SEM images (Fig. 1b and c) showed that the particles consisted of nanorods with an average diameter of 120 nm
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Fig. 2. XRD of LMTO-NR at 600 C, 700 C, and LMTO-BP. Enlarged XRD patterns in the Bragg's angle range of 17.5e19.5 and 19e35 are shown to depict the (111) plane and the presence of a layered superlattice peak denoted by the (020)M plane, respectively. The open triangles represent impurity peaks of Li1þxMn2-xO4. (b) XRD Rietveld refinement pattern for LMTO-NR at 700 C, and LMTO-BP. The black dots represent the observed profile, the blue and red lines represent the fitted profile and difference profile (at the bottom), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Rietveld refinement and crystal data from the XRD data. The numbers in parentheses are the estimated standard deviations of the last significant figure. M and Li1 represent to transition metal and Li, respectively. formula
LMTO-BP
LMTO-NR
space group phase fraction a ¼ b ¼ c (Å) 8a site occupancy 16d site occupancy
Fd3 m 70% 8.2794 (5) 92.5% Li 7.5% Mn 2.5% Li 52.5% Mn 45% Ti
Fd3 m 70% 8.2773 (7) 92.5% Li 7.5% Mn 10% Li 45% Mn 45% Ti
space group phase fraction a (Å) b (Å) c (Å) b ( ) M site occupancy
C2/c 30% 5.0491 (24) 8.7153 (41) 9.7815 (20) 100.1744 (33) 32.5% Mn 67.5% Ti
Li1 site
100%
Rp (%) Rwp (%) c2 (%)
2.59 3.66 2.95
C2/c 30% 5.0262 (11) 8.7202 (8) 9.6696 (10) 100.1597 (69) 32.5% Mn 60% Ti 7.5% Li 33.33% Mn 33.33% Ti 33.33% Li 1.67 2.51 2.939
and an average length of 2 mm. Fig. 2a shows the XRD patterns of the as-prepared materials. To optimize the synthesis conditions, the mixture was annealed at two different temperatures. As can be seen from Fig. 2a, LMTO-NR at 600 C had impurity peaks belonging to Li1þxMn2-xO4. According to reference [15], at elevated temperatures from 300 to 500 C, Liþ gradually incorporates into the b-MnO2 phase, and the spinel composition of Li1þxMn2-xO4 is formed. The decomposition of Li1þxMn2-xO4 occurred with the formation of Li2MnO3 and LiMn2O4 at temperatures over 600 C. At high temperatures (over 600 C), the diffusion of Mn and Ti was favorable, leading to the formation of
LiMn2exTixO4, and Li2MnyTi1eyO3 [16]. LMTO-NR at 700 C and LMTO-BP had the same peaks position, implying no change in the structure of these materials. All the peaks can be indexed to the cubic spinel structure in the Fd3 m space group, except for the (020)M superlattice peak at 20.5 (where M represents monoclinic). This superlattice peak can be a unique peak of the layered phase, distinguishing it from the spinel phase, as these phases have very similar XRD patterns. Moreover, the diffraction peaks shifted toward lower diffraction angles due to the larger ionic radii of the Ti4þ ions that displaced the Mn4þ ions in the spinel sub-lattice [6,7,17]. A (220) diffraction peak was observed for all samples, suggesting that the Ti substitution induced heavy atoms, especially Mn, to occupy the Li-occupying 8a sites [18e22]. Importantly, the intensity of the (220) peak in the diffraction pattern of the Li1.5MnTiO4þd was lower than that of LiMnTiO4 because of the excess Li. This characteristic favored the diffusion of Liþ through the 8a e 16c e 8a path [7]. The full width at half-maximum peak decreased with increasing reaction temperature, suggesting that LMTO-BP had the highest crystallinity. The reaction temperature not only affects to the crystallinity of a material, but also the cation disorder [23]. This phenomenon impacts the electrochemical performance of two samples, as it will be discussed later. The LMTO-NR at 700 C was further investigated and compared with LMTO-BP. The crystal structures of LMTO-BP and LMTO-NR at 700 C were refined using the Rietveld method to explore the effect of the reaction temperature on the structure of these samples. The Rietveld refinement of the two samples was performed using a spinel structure with space group Fd3 m, and a monoclinic layered structure with space group C2/c (Table 1). The fraction of the layered phase and spinel phase in both samples was 30% and 70%, respectively, and these values were consistent with a previous report [7]. For the LMTO-NR, which was synthesized at low temperatures, the Rietveld results showed cation disorder in both phases where Li could occupy a transition metal site, and vice versa. The spinel phase was rich in Li, while the layered phase was poor in Li, and the lattice parameters in both phases decreased in comparison with
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Fig. 3. (a, b) SEM images of LMTO-NR; (c) HRTEM image of LMTO-NR; (d, e) SEM images of LMTO-BP; (f) HRTEM image of LMTO-BP. The S and L subscripts represent the spinel and layered structures, respectively.
LMTO-BP. For LMTO-BP, only a small portion of the transition metal occupied Li sites, and vice versa, in the spinel phase. In the layered phase, the transition metal had an ordered atomic arrangement with Li. The FESEM images shown in Fig. 3(a e d) display the morphology and particle sizes of the materials. LMTO-BP showed octahedral particles, with a broad particle size distribution in the range of 350e500 nm, and a shape typical of spinel materials synthesized via solid-state reactions [7]. The nanorods were uniform with a diameter of ~200 nm and length ~1 mm. After firing, the nanorods became bigger and shorter than the pristine b-MnO2 nanorods. Fig. 3e and f shows the HRTEM images, which further revealed the distribution of the spinel and layered domains of the two samples. The fringe spacing of 0.478 nm corresponds to the (002) plane of Li2MnyTi1eyO3, or the (111) plane of LiMn2exTixO4, in both samples. The composite structure of the LiMn2exTixO4 and Li2MnyTi1eyO3 phases showed higher Liþ diffusivities for both charge and discharge processes than LiMnTiO4 and the original LiMn2O4, since the layered structure improved the diffusivity of the Liþ ions [7,24]. For the LMTO-NR, the HRTEM images showed crystalline growth along the 〈110〉 crystallographic direction. Fig. 4a and c shows the charge-discharge profiles of the two samples after the first three cycles. In the first charging cycle, all samples had similar profiles, and a plateau at around 4.2 V was observed, characteristic of Liþ ion extraction from the tetrahedral 8a sites in spinel cathodes.
This was followed by the oxidation of Mn3þ to Mn4þ (which was clearly observed in the dQ/dV plot) [4]. The small plateau at 4.5 V was assigned to the activation of the layered phase. During the discharge process, a voltage plateau appear at around 4.1 and 2.8 V, corresponding to the two-phase reaction that formed the Li2MnTiO4 phase, where the extra Liþ ions were inserted into the 16c sites [4]. The discharge capacities of the two samples were similar at low C rates, which suggest a similarity in the crystal structure of both samples. To further understand the electrochemical performance of these samples, the dQ/dV plots of both samples at the 1st, 2nd, and 3rd cycle are shown in Fig. 4b,d. For the 1st charge, these samples showed peaks at ~4 V and ~4.5 V. These peaks correspond to the extraction of Liþ from the 8a sites of the pristine samples, the oxidation of Mn3þ to Mn4þ, and the activation of the layered phase. These peaks, which were observed for both samples, matched with the voltage plateau (Fig. 4a,c). The peak above 4.4 V predominantly corresponds to the highly irreversible activation process of stripping Li2O off the Li2MnyTi1eyO3 component, and the formation of an active [Mn, Ti]O2-like component [25]. Note that because of the high bonding energy of Ti e O (662 kJ mol1), the plateau was small. Because of the irreversibility of the activation reaction, there was not a corresponding reduction peak observed in the initial discharge process; however, a new peak was induced at ~3 V, which stemmed from the reduction reaction of Mn4þ to Mn3þ [25]. The small peak that appeared below 2.5 V for both samples might be
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Fig. 4. (a, b) Voltage profiles and dQ/dV plot of LMTO-NR at C/10 for the first three cycles; (c, d) voltage profiles and dQ/dV plot of LMTO-BP at C/10 for the first three cycles; (e) cycling stability curves of LMTO-NR, and LMTO-BP at different C-rates; (f) cycling stability curves of LMTO-NR, and LMTO-BP at 1C rate.
caused by the reduction of Mn, and it disappeared in the next cycle. The reduction peak at ~1.8 V originated mainly from the redox reaction of Ti in LiMn2exTixO4. Such Ti3þ/Ti4þ redox reactions were found to be useful for increasing the intrinsic electronic conductivity of the host materials [4]. The difference in the intensity of these peaks at 3 and 1.8 V, in the two samples, might be due to different crystallinity and cation disorder in each phase of both samples. The higher the crystallinity, the more difficult the activation [12]. Therefore, the peak at ~3 V was clear for LMTO-NR, but it was overlapped for LMTO-BP. Moreover, the cation disorder in the layered phase of LMTO-NR favored the transport of Liþ through percolating zero-transition-metal pathways, therefore the contribution of the layered phase was clearly observed [11]. However, in the spinel phase, the effects of cation disorder were opposite. The LMTO-BP sample, which had less cation disorder, favored the diffusion of Li through the 8a e 16c e 8a path, so the peak at 1.8 V for this sample was stronger than the one in LMTO-NR [6]. The performance of these materials at different C-rates is shown in Fig. 4e. The rate capabilities of the LMTO-NR were significantly higher than those of LMTO-BP at high C-rates. At a C/10 rate, both samples showed similar discharge capacities in the first few cycles. However, at high C-rates, the improvement was apparent. The LMTO-NR sample delivered stable capacities of 240 mAh g1 at C/ 10, 220 mAh g1 at C/5, 190 mAh g1 at C/2, 170 mAh g1 at 1C, 150 mAh g1 at 2C, 105 mAh g1 at 5C, and 80 mAh g1 at 10C. The LMTO-NR sample delivered 167% greater capacity than the bulk particle sample at 10C. As shown by the results of the C-rate experiments, the 1-D morphology and high surface area (6.3735 m2 g1) of the LMTO-NR enabled high capacities at rates two times higher than those of LMTO-BP (which has a surface area
of 1.4332 m2 g1). More importantly, the reversible capacity of the samples was 100% at C/10 after a full sequence measurement with high currents of up to 10C. To investigate the cycling stability of these samples, galvanostatic charge-discharge measurements were carried out in the voltage range of 1.5e4.8 V at room temperature. The cycling performance of the cells at a 1C rate (with a current density of 154 mA g1) is shown in Fig. 4f. The capacity of the LMTO-NR and LMTO-BP were as high as 185 mAh g1 and 167 mAh g1, respectively, at the first cycle. For the subsequent cycles, the capacity was stable at 170 mAh g1 and 160 mAh g1 for LMTO-NR and LMTO-BP, respectively. The special 1-D morphology favored the diffusion of Li, especially at high C-rates, giving an enhanced capacity of LMTONR compared to LMTO-BP. After 100 cycles, the capacity retention of LMTO-NR and LMTO-BP was 85 and 88%, respectively. The Coulombic efficiency of both samples increased during the first few cycles then was stabilized at ~99% because of the activation of layered [26]. 4. Conclusions We developed a promising Li1.5MnTiO4þd cathode material based on a cheap and non-toxic transition metal. The XRD analysis and electrochemical behavior of the samples confirmed the existence of a spinel-layered composite. The electrochemical performance of this cathode was investigated on the bulk particles, and the nanorod sample. The nanorod sample, which was synthesized using a b-MnO2 nanorod template, and at optimized synthesis conditions (700 C for 12 h), showed a performance rate two times higher than the particles, at high C-rates. The difference in the
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electrochemical behavior of these materials was not only due to their morphology, but also the cation disorder in each phase. Finally, both samples exhibited a high capacity of 250 mAh g1 at C/ 10, and good cycle stability with capacity retention of over 85% after 100 cycles at 1C. By possessing these attractive properties, we believe that this material is a potential cathode for the development of rechargeable Li-ion batteries. Conflict of interest There are no conflicts of interest to declare. Acknowledgements This research was supported by the Strategic Key-Material Development, and the Materials and Components Research and Development, both of which are funded by the Ministry of Knowledge Economy (MKE, Korea, 10044203). This work was also financially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (NRF2014R1A1A1002909, 2016R1E1A2020571). References [1] J.M. Tarascon, E. Wang, F.K. Shokoohi, W.R. McKinnon, S. Colson, The spinel phase of LiMn2O4 as a cathode in secondary lithium cells, J. Electrochem. Soc. 138 (1991) 2859e2864. [2] X. Zhang, M. Yang, X. Zhao, Y. Wang, M. Wang, L. Ma, The spinel phase LiMnTiO4 as a potential cathode for rechargeable lithium ion batteries, J. Mater. Sci. Mater. Electron. 26 (2015) 6366e6372. [3] N. Krins, F. Hatert, K. Traina, L. Dusoulier, I. Molenberg, J.F. Fagnard, P. Vanderbemden, A. Rulmont, R. Cloots, B. Vertruyen, LiMn2xTixO4 spineltype compounds (x 1): structural, electrical and magnetic properties, Solid State Ion. 177 (2006) 1033e1040. [4] R. Chen, M. Knapp, M. Yavuz, R. Heinzmann, D. Wang, S. Ren, V. Trouillet, S. Lebedkin, S. Doyle, H. Hahn, H. Ehrenberg, S. Indris, Reversible Liþ storage in a LiMnTiO4 spinel and its structural transition mechanisms, J. Phys. Chem. C 118 (2014) 12608e12616. Arillo, M.L. Lo pez, C. Pico, M.L. Veiga, Structural, thermal and magnetic [5] M.A. properties of LiMnTiO4 spinel in different atmospheres, Solid State Sci. 10 (2008) 1612e1619. [6] S. Wang, J. Yang, X. Wu, Y. Li, Z. Gong, W. Wen, M. Lin, J. Yang, Y. Yang, Toward high capacity and stable manganese-spinel electrode materials: a case study of Ti-substituted system, J. Power Sources 245 (2014) 570e578. [7] N.H. Vu, P. Arunkumar, S. Won, H.J. Kim, S. Unithrattil, Y. Oh, J.-W. Lee, W.B. Im, Effects of excess Li on the structure and electrochemical performance of Li1þzMnTiO4þd cathode for Li-ion batteries, Electrochim. Acta 225 (2017) 458e466. ment, P. Hovington, A. Mauger, [8] D. Liu, C. Gagnon, J. Trottier, A. Guerfi, D. Cle C.M. Julien, K. Zaghib, Improvement of the rate property of LiMn1.45Ni0.45Cr0.1O4 cathode for Li-ion batteries, Electrochem. Commun. 41 (2014) 64e67.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Template-engaged synthesis of spinel-layered Li1.5MnTiO4þd nanorods as a cathode material for Li-ion batteries Ngoc Hung Vu a, Sanjith Unithrattil a, Van Hien Hoang a, Sangeun Chun b, **, Won Bin Im a, * a School of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea b School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea
h i g h l i g h t s A promising Li1.5MnTiO4þd cathode: spinel-layered structure with high capacity. A Li1.5MnTiO4þd nanorod was synthesized by using a b-MnO2 nanorod template. The Li1.5MnTiO4þd nanorods exhibit improved rate performance at high C-rate. The structure and electrochemical performances of Li1.5MnTiO4þd were studied.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2017 Received in revised form 4 April 2017 Accepted 14 April 2017
Spinel-layered composites of Li1.5MnTiO4þd were studied for their use as high-energy, low-cost, and environmentally benign cathode materials. The bulk particles showed an attractive specific capacity of up to 250 mAh g1 at C/10. To improve the performance of this cathode at a high C-rate, a spinel-layered Li1.5MnTiO4þd nanorod was successfully synthesized using a b-MnO2 nanorod template. The nanorod, which had an average diameter of 200 nm and a length of 1 mm, showed specific capacity as high as the bulk particle at C/10. However, owing to a one-dimensional nanostructure with a large effective contact area for Liþ diffusion, the nanorod sample exhibited enhanced capacities 11% (170 mAh g1) and 167% higher (80 mAh g1) at 1C and 10C rates, respectively, compared to the bulk particles. Moreover, both samples showed good cycle stability and capacity retention of over 85% after 100 cycles at 1C. © 2017 Elsevier B.V. All rights reserved.
Keywords: LiMnTiO4 Nanorod Template Spinel framework Li-ion battery
1. Introduction Among commercial cathode materials for Li-ion batteries (LIBs) such as LiCoO2 and LiFePO4, spinel LiMn2O4 with a threedimensional tunnel structure for the migration of Liþ has been considered one of the best candidates because of its low cost, nontoxicity, abundance, and easy preparation [1]. However, this material often displays a limited capacity and working voltage range and is thus used at high voltages (>3 V) due to the Jahn-Teller distortion that occurs below 3 V. In order to solve this problem, earlier studies have focused on the chemical modification of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Chun), [email protected] (W.B. Im). http://dx.doi.org/10.1016/j.jpowsour.2017.04.055 0378-7753/© 2017 Elsevier B.V. All rights reserved.
LiMn2O4 by partial substitution of Mn4þ by Ti4þ in LiMn2O4, which contributes to the formation of another attractive spinel cathode material, LiMnTiO4. In this compound, Ti forms stronger bonds with O (662 kJ mol1) in comparison with Mn (402 kJ mol1). Therefore, a more stable [Mn2-xTix]O4 framework could be obtained without sacrificing the advantages of LiMn2O4 [2e6]. However, the partial substitution with Ti decreases the capacity of LiMn2O4 (<200 mAh g1 in the 2e4.8 V) [6,7]. To address this problem, we previously reported a new material, Li1þzMnTiO4þd (where z ¼ 0, 0.5 and 1.0, and 0 d < 0.67), which has an improved capacity and cycling stability compared to current materials for cathode applications [7]. This material, Li1.5MnTiO4þd, showed the highest capacity among these cathodes. However, the material was synthesized by a solidstate reaction, which resulted in non-uniform, large particles. In addition, its morphology limited the contact area it had with the
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2. Experimental
Fig. 1. (a) XRD pattern and (b, c) SEM images of b-MnO2 as obtained from the hydrothermal reaction.
electrolyte, leading to a low capacity and poor C-rate performance, which are both drawbacks for battery applications [8e10]. An effective approach to improve the capacity, as well as the rate performance of the cathode material, is to exploit nanomaterials that have high surface areas, and in particular, one-dimensional (1D) nanomaterials. Moreover, cation disordered materials recently were studied and it might be a potential cathode with high capacity for LIBs [11]. The synthesis conditions were found to be a key which not only influence the crystallinity, but also cation disordering [12]. In this study, we successfully synthesized a cation disordered Li1.5MnTiO4þd nanorod by using a b-MnO2 nanorod template. The spinel-layered heterostructure of the nanorod enabled the facile electron transport along the 1-D geometry, and reduced the Li-ion diffusion length, resulting in an enhanced discharge capacity and Crate performance.
The b-MnO2 nanorod was synthesized according to a previous work [13]. In a typical synthesis, Mn(CH3COO)2$4H2O (2.45 g) and Na2S2O8 (1.19 g) (Sigma-Aldrich) were dissolved in 70 mL of distilled water, followed by stirring to form a homogeneous clear solution. The mixed solution was transferred to a 100 mL Teflonlined stainless steel autoclave, and heated at 120 C for 12 h for the hydrothermal reaction to occur. After the reaction, the final precipitated products were washed sequentially with deionized water and ethanol to remove the sulfate ions and other remnants by filtration. The obtained powder was subsequently dried at 100 C for 12 h in air. To prepare the nanorod sample, b-MnO2 nanorods (0.435 g) were added to 10 mL of anhydrous ethanol, where titanium isopropoxide (Ti(OiPr)4, 1.48 mL) was dissolved with 3 mL acetic acid. Thereafter, LiOH (0.315 g) were added to the above dispersion, followed by triethanolamine (2 mL), which was added as a chelating agent. The mixture was sonicated and stirred for 12 h, and subsequently dried at 100 C overnight. Then, the mixture was calcined at 600 or 700 C for 12 h in air at a heating rate of 1 C min1. The Li:Mn:Ti ratio in the obtained compounds was 1.5:1:1. The Li1.5MnTiO4þd nanorod was denoted as LMTO-NR. The bulk particles were prepared by a solid-state reaction method. First, stoichiometric amounts of Li2CO3 (99.9%, SigmaAldrich), MnCO3 (99.9%, Kojundo Chemicals), and TiO2 (99.5%, Sigma-Aldrich) were mixed using a mortar. The resultant mixture was heated in an alumina crucible at 800 C for 12 h, at a heating rate of 2 C min1. The Li1.5MnTiO4þd bulk particles were denoted as LMTO-BP. The structure of the samples was characterized using X-ray diffraction (XRD) (Cu Ka radiation, Philips X'Pert) in the angle range of 10 2q 120 , with a step size of 0.026 . The Rietveld refinement was made with the General Structure Analysis System (GSAS) program [14]. Field emission scanning electron microscopy (FESEM) (S-4700, Hitachi) and high-resolution transmission electron microscopy (HRTEM) (FEI Tecnai F20) at 200 kV were used to determine the sizes and morphologies of the particles. The specific surface area of particles was evaluated by the BET method (BELSORP-mini II, BEL Japan, Inc.). All measurements were carried out in the Korea Basic Science Institute (KBSI), Gwangju, Republic of Korea. The cathode was prepared with a mass ratio of 75:10:15 of the active material, conductive carbon (KETJEN black), and teflonized acetylene black (TAB) binder, respectively. The mass of active material was 7.5 mg. This mixture was thoroughly mixed and pressed onto a stainless steel mesh, and dried under vacuum at 120 C for 12 h. A 2032 coin-type cell, which was fabricated in an Ar-filled glove box, consisted of the cathode and Li metal anode separated by a polymer membrane, together with glass fiber. The electrolyte was made from a 1:1 mixture of ethylene carbonate and dimethyl carbonate containing 1 M LiPF6. The cells were aged for 12 h before the electrochemical measurements. The charge-discharge measurements were carried out using a NAGANO BTS-2004H battery charger between 1.5 and 4.8 V vs. Liþ/Li. 3. Result and discussion Fig. 1 shows the XRD patterns and SEM images of the hydrothermally synthesized b-MnO2. All peaks were matched with those of the reference, revealing a tetragonal symmetry with a P42/mnm space group, in accordance with the previous report [13]. No additional impurity peaks were detected (Fig. 1a). The SEM images (Fig. 1b and c) showed that the particles consisted of nanorods with an average diameter of 120 nm
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Fig. 2. XRD of LMTO-NR at 600 C, 700 C, and LMTO-BP. Enlarged XRD patterns in the Bragg's angle range of 17.5e19.5 and 19e35 are shown to depict the (111) plane and the presence of a layered superlattice peak denoted by the (020)M plane, respectively. The open triangles represent impurity peaks of Li1þxMn2-xO4. (b) XRD Rietveld refinement pattern for LMTO-NR at 700 C, and LMTO-BP. The black dots represent the observed profile, the blue and red lines represent the fitted profile and difference profile (at the bottom), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Rietveld refinement and crystal data from the XRD data. The numbers in parentheses are the estimated standard deviations of the last significant figure. M and Li1 represent to transition metal and Li, respectively. formula
LMTO-BP
LMTO-NR
space group phase fraction a ¼ b ¼ c (Å) 8a site occupancy 16d site occupancy
Fd3 m 70% 8.2794 (5) 92.5% Li 7.5% Mn 2.5% Li 52.5% Mn 45% Ti
Fd3 m 70% 8.2773 (7) 92.5% Li 7.5% Mn 10% Li 45% Mn 45% Ti
space group phase fraction a (Å) b (Å) c (Å) b ( ) M site occupancy
C2/c 30% 5.0491 (24) 8.7153 (41) 9.7815 (20) 100.1744 (33) 32.5% Mn 67.5% Ti
Li1 site
100%
Rp (%) Rwp (%) c2 (%)
2.59 3.66 2.95
C2/c 30% 5.0262 (11) 8.7202 (8) 9.6696 (10) 100.1597 (69) 32.5% Mn 60% Ti 7.5% Li 33.33% Mn 33.33% Ti 33.33% Li 1.67 2.51 2.939
and an average length of 2 mm. Fig. 2a shows the XRD patterns of the as-prepared materials. To optimize the synthesis conditions, the mixture was annealed at two different temperatures. As can be seen from Fig. 2a, LMTO-NR at 600 C had impurity peaks belonging to Li1þxMn2-xO4. According to reference [15], at elevated temperatures from 300 to 500 C, Liþ gradually incorporates into the b-MnO2 phase, and the spinel composition of Li1þxMn2-xO4 is formed. The decomposition of Li1þxMn2-xO4 occurred with the formation of Li2MnO3 and LiMn2O4 at temperatures over 600 C. At high temperatures (over 600 C), the diffusion of Mn and Ti was favorable, leading to the formation of
LiMn2exTixO4, and Li2MnyTi1eyO3 [16]. LMTO-NR at 700 C and LMTO-BP had the same peaks position, implying no change in the structure of these materials. All the peaks can be indexed to the cubic spinel structure in the Fd3 m space group, except for the (020)M superlattice peak at 20.5 (where M represents monoclinic). This superlattice peak can be a unique peak of the layered phase, distinguishing it from the spinel phase, as these phases have very similar XRD patterns. Moreover, the diffraction peaks shifted toward lower diffraction angles due to the larger ionic radii of the Ti4þ ions that displaced the Mn4þ ions in the spinel sub-lattice [6,7,17]. A (220) diffraction peak was observed for all samples, suggesting that the Ti substitution induced heavy atoms, especially Mn, to occupy the Li-occupying 8a sites [18e22]. Importantly, the intensity of the (220) peak in the diffraction pattern of the Li1.5MnTiO4þd was lower than that of LiMnTiO4 because of the excess Li. This characteristic favored the diffusion of Liþ through the 8a e 16c e 8a path [7]. The full width at half-maximum peak decreased with increasing reaction temperature, suggesting that LMTO-BP had the highest crystallinity. The reaction temperature not only affects to the crystallinity of a material, but also the cation disorder [23]. This phenomenon impacts the electrochemical performance of two samples, as it will be discussed later. The LMTO-NR at 700 C was further investigated and compared with LMTO-BP. The crystal structures of LMTO-BP and LMTO-NR at 700 C were refined using the Rietveld method to explore the effect of the reaction temperature on the structure of these samples. The Rietveld refinement of the two samples was performed using a spinel structure with space group Fd3 m, and a monoclinic layered structure with space group C2/c (Table 1). The fraction of the layered phase and spinel phase in both samples was 30% and 70%, respectively, and these values were consistent with a previous report [7]. For the LMTO-NR, which was synthesized at low temperatures, the Rietveld results showed cation disorder in both phases where Li could occupy a transition metal site, and vice versa. The spinel phase was rich in Li, while the layered phase was poor in Li, and the lattice parameters in both phases decreased in comparison with
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Fig. 3. (a, b) SEM images of LMTO-NR; (c) HRTEM image of LMTO-NR; (d, e) SEM images of LMTO-BP; (f) HRTEM image of LMTO-BP. The S and L subscripts represent the spinel and layered structures, respectively.
LMTO-BP. For LMTO-BP, only a small portion of the transition metal occupied Li sites, and vice versa, in the spinel phase. In the layered phase, the transition metal had an ordered atomic arrangement with Li. The FESEM images shown in Fig. 3(a e d) display the morphology and particle sizes of the materials. LMTO-BP showed octahedral particles, with a broad particle size distribution in the range of 350e500 nm, and a shape typical of spinel materials synthesized via solid-state reactions [7]. The nanorods were uniform with a diameter of ~200 nm and length ~1 mm. After firing, the nanorods became bigger and shorter than the pristine b-MnO2 nanorods. Fig. 3e and f shows the HRTEM images, which further revealed the distribution of the spinel and layered domains of the two samples. The fringe spacing of 0.478 nm corresponds to the (002) plane of Li2MnyTi1eyO3, or the (111) plane of LiMn2exTixO4, in both samples. The composite structure of the LiMn2exTixO4 and Li2MnyTi1eyO3 phases showed higher Liþ diffusivities for both charge and discharge processes than LiMnTiO4 and the original LiMn2O4, since the layered structure improved the diffusivity of the Liþ ions [7,24]. For the LMTO-NR, the HRTEM images showed crystalline growth along the 〈110〉 crystallographic direction. Fig. 4a and c shows the charge-discharge profiles of the two samples after the first three cycles. In the first charging cycle, all samples had similar profiles, and a plateau at around 4.2 V was observed, characteristic of Liþ ion extraction from the tetrahedral 8a sites in spinel cathodes.
This was followed by the oxidation of Mn3þ to Mn4þ (which was clearly observed in the dQ/dV plot) [4]. The small plateau at 4.5 V was assigned to the activation of the layered phase. During the discharge process, a voltage plateau appear at around 4.1 and 2.8 V, corresponding to the two-phase reaction that formed the Li2MnTiO4 phase, where the extra Liþ ions were inserted into the 16c sites [4]. The discharge capacities of the two samples were similar at low C rates, which suggest a similarity in the crystal structure of both samples. To further understand the electrochemical performance of these samples, the dQ/dV plots of both samples at the 1st, 2nd, and 3rd cycle are shown in Fig. 4b,d. For the 1st charge, these samples showed peaks at ~4 V and ~4.5 V. These peaks correspond to the extraction of Liþ from the 8a sites of the pristine samples, the oxidation of Mn3þ to Mn4þ, and the activation of the layered phase. These peaks, which were observed for both samples, matched with the voltage plateau (Fig. 4a,c). The peak above 4.4 V predominantly corresponds to the highly irreversible activation process of stripping Li2O off the Li2MnyTi1eyO3 component, and the formation of an active [Mn, Ti]O2-like component [25]. Note that because of the high bonding energy of Ti e O (662 kJ mol1), the plateau was small. Because of the irreversibility of the activation reaction, there was not a corresponding reduction peak observed in the initial discharge process; however, a new peak was induced at ~3 V, which stemmed from the reduction reaction of Mn4þ to Mn3þ [25]. The small peak that appeared below 2.5 V for both samples might be
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Fig. 4. (a, b) Voltage profiles and dQ/dV plot of LMTO-NR at C/10 for the first three cycles; (c, d) voltage profiles and dQ/dV plot of LMTO-BP at C/10 for the first three cycles; (e) cycling stability curves of LMTO-NR, and LMTO-BP at different C-rates; (f) cycling stability curves of LMTO-NR, and LMTO-BP at 1C rate.
caused by the reduction of Mn, and it disappeared in the next cycle. The reduction peak at ~1.8 V originated mainly from the redox reaction of Ti in LiMn2exTixO4. Such Ti3þ/Ti4þ redox reactions were found to be useful for increasing the intrinsic electronic conductivity of the host materials [4]. The difference in the intensity of these peaks at 3 and 1.8 V, in the two samples, might be due to different crystallinity and cation disorder in each phase of both samples. The higher the crystallinity, the more difficult the activation [12]. Therefore, the peak at ~3 V was clear for LMTO-NR, but it was overlapped for LMTO-BP. Moreover, the cation disorder in the layered phase of LMTO-NR favored the transport of Liþ through percolating zero-transition-metal pathways, therefore the contribution of the layered phase was clearly observed [11]. However, in the spinel phase, the effects of cation disorder were opposite. The LMTO-BP sample, which had less cation disorder, favored the diffusion of Li through the 8a e 16c e 8a path, so the peak at 1.8 V for this sample was stronger than the one in LMTO-NR [6]. The performance of these materials at different C-rates is shown in Fig. 4e. The rate capabilities of the LMTO-NR were significantly higher than those of LMTO-BP at high C-rates. At a C/10 rate, both samples showed similar discharge capacities in the first few cycles. However, at high C-rates, the improvement was apparent. The LMTO-NR sample delivered stable capacities of 240 mAh g1 at C/ 10, 220 mAh g1 at C/5, 190 mAh g1 at C/2, 170 mAh g1 at 1C, 150 mAh g1 at 2C, 105 mAh g1 at 5C, and 80 mAh g1 at 10C. The LMTO-NR sample delivered 167% greater capacity than the bulk particle sample at 10C. As shown by the results of the C-rate experiments, the 1-D morphology and high surface area (6.3735 m2 g1) of the LMTO-NR enabled high capacities at rates two times higher than those of LMTO-BP (which has a surface area
of 1.4332 m2 g1). More importantly, the reversible capacity of the samples was 100% at C/10 after a full sequence measurement with high currents of up to 10C. To investigate the cycling stability of these samples, galvanostatic charge-discharge measurements were carried out in the voltage range of 1.5e4.8 V at room temperature. The cycling performance of the cells at a 1C rate (with a current density of 154 mA g1) is shown in Fig. 4f. The capacity of the LMTO-NR and LMTO-BP were as high as 185 mAh g1 and 167 mAh g1, respectively, at the first cycle. For the subsequent cycles, the capacity was stable at 170 mAh g1 and 160 mAh g1 for LMTO-NR and LMTO-BP, respectively. The special 1-D morphology favored the diffusion of Li, especially at high C-rates, giving an enhanced capacity of LMTONR compared to LMTO-BP. After 100 cycles, the capacity retention of LMTO-NR and LMTO-BP was 85 and 88%, respectively. The Coulombic efficiency of both samples increased during the first few cycles then was stabilized at ~99% because of the activation of layered [26]. 4. Conclusions We developed a promising Li1.5MnTiO4þd cathode material based on a cheap and non-toxic transition metal. The XRD analysis and electrochemical behavior of the samples confirmed the existence of a spinel-layered composite. The electrochemical performance of this cathode was investigated on the bulk particles, and the nanorod sample. The nanorod sample, which was synthesized using a b-MnO2 nanorod template, and at optimized synthesis conditions (700 C for 12 h), showed a performance rate two times higher than the particles, at high C-rates. The difference in the
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electrochemical behavior of these materials was not only due to their morphology, but also the cation disorder in each phase. Finally, both samples exhibited a high capacity of 250 mAh g1 at C/ 10, and good cycle stability with capacity retention of over 85% after 100 cycles at 1C. By possessing these attractive properties, we believe that this material is a potential cathode for the development of rechargeable Li-ion batteries. Conflict of interest There are no conflicts of interest to declare. Acknowledgements This research was supported by the Strategic Key-Material Development, and the Materials and Components Research and Development, both of which are funded by the Ministry of Knowledge Economy (MKE, Korea, 10044203). This work was also financially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (NRF2014R1A1A1002909, 2016R1E1A2020571). References [1] J.M. Tarascon, E. Wang, F.K. Shokoohi, W.R. McKinnon, S. Colson, The spinel phase of LiMn2O4 as a cathode in secondary lithium cells, J. Electrochem. Soc. 138 (1991) 2859e2864. [2] X. Zhang, M. Yang, X. Zhao, Y. Wang, M. Wang, L. Ma, The spinel phase LiMnTiO4 as a potential cathode for rechargeable lithium ion batteries, J. Mater. Sci. Mater. Electron. 26 (2015) 6366e6372. [3] N. Krins, F. Hatert, K. Traina, L. Dusoulier, I. Molenberg, J.F. Fagnard, P. Vanderbemden, A. Rulmont, R. Cloots, B. Vertruyen, LiMn2xTixO4 spineltype compounds (x 1): structural, electrical and magnetic properties, Solid State Ion. 177 (2006) 1033e1040. [4] R. Chen, M. Knapp, M. Yavuz, R. Heinzmann, D. Wang, S. Ren, V. Trouillet, S. Lebedkin, S. Doyle, H. Hahn, H. Ehrenberg, S. Indris, Reversible Liþ storage in a LiMnTiO4 spinel and its structural transition mechanisms, J. Phys. Chem. C 118 (2014) 12608e12616. Arillo, M.L. Lo pez, C. Pico, M.L. Veiga, Structural, thermal and magnetic [5] M.A. properties of LiMnTiO4 spinel in different atmospheres, Solid State Sci. 10 (2008) 1612e1619. [6] S. Wang, J. Yang, X. Wu, Y. Li, Z. Gong, W. Wen, M. Lin, J. Yang, Y. Yang, Toward high capacity and stable manganese-spinel electrode materials: a case study of Ti-substituted system, J. Power Sources 245 (2014) 570e578. [7] N.H. Vu, P. Arunkumar, S. Won, H.J. Kim, S. Unithrattil, Y. Oh, J.-W. Lee, W.B. Im, Effects of excess Li on the structure and electrochemical performance of Li1þzMnTiO4þd cathode for Li-ion batteries, Electrochim. Acta 225 (2017) 458e466. ment, P. Hovington, A. Mauger, [8] D. Liu, C. Gagnon, J. Trottier, A. Guerfi, D. Cle C.M. Julien, K. Zaghib, Improvement of the rate property of LiMn1.45Ni0.45Cr0.1O4 cathode for Li-ion batteries, Electrochem. Commun. 41 (2014) 64e67.
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