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A symmetrical and co-operating effect of Mg-Zr codoping on Li4Ti5O12 anode materials
Solid State Ionics 326 (2018) 63–68
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
A symmetrical and co-operating effect of Mg-Zr codoping on Li4Ti5O12 anode materials
T
⁎
Qinglin Li, Bing Xue, Yi Tan , Kai Wang, Jianming Sun School of Materials Science and Engineering, Dalian University of Technology, Liaoning, Dalian 116024, China Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Liaoning, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Li4-xMgxTi5-yZryO12 Mg/Zr Symmetrical Co-operating
Uniform nanoparticles of Li4Ti5O12 were prepared using solid-state method coupled with Mg and Zr ions codoped which defined as LTO-R1, LTO-R2, LTO-R3 (R = Mg/Zr). Results show that Mg-Zr doped Li4Ti5O12 have the finest particles with diameters distributed in the range of 10 to 100 nm featured relatively high specific capacity and good rate capacity. The first discharge capacity of LTO-R2 is 186.6 mAh g−1 at 0.57C (1.0–2.6 V), and rate performance improve 56.7% at 10C compared pure LTO. The symmetrical and co-operating effect of codoped Mg-Zr ions on the morphology and the electrochemical property of LTO were intensively discussed in this paper.
1. Introduction Lithium-ion batteries (LIBs) are first choice as energy source for these applications in virtue of their high gravimetric and volumetric energy density. Conventionally, anode materials of LIBs are graphite negative electrode. Graphite negative electrodes inevitably react with electrolyte components due to their low working potential [1–3]. So, it cannot meet the demand of high safety. Spinel LTO has been widely investigated as a promising anode candidate for high electrochemical performance of LIBs. Due to the discharge plateau at 1.55 V (vs. Li/ Li+), it can avoid the reduction of the electrolyte on electrode surface and the formation of a solid-electrolyte interface (SEI) layer. Then will enhance the safety performance [4–8]. LTO is also called “Zero Strain” materials when Li-ion insertion/extraction with miniscule volume change which benefits from the 3D structure as showed in Fig. 1 [9]. The remarkable structure stability results in excellent cycle performance and capacity retention in long cycle life [10–12]. However, pure LTO shows poor lithium storage properties at high rates and low specific capacity (175 mAh g−1), which have huge negative effect on its commercial applications. Besides low electronic conductivity (10−13 S cm−1) and low Li+ diffusion coefficient −9 −13 2 −1 (10 –10 cm S ) also limit its performance [13–15]. Various approaches have been used to solve the above-mentioned problems, such as lattice ion doping, nanostructure-Li4Ti5O12 and surface modification with conductive materials [16,17]. However, many of the
technologies developed are too complicated to be cost-effective so hinder its applications. Solid-state method has been realized in industry at present for its simple operation and low production cost. But the conventional solid-state method would easily cause aggregation of the LTO particles at the expense of losing material's nanostructure. To tackle the problems, rational productive technology need to be developed. Doped electrode materials which synthesized by solid-state method are been considered promising to realize its application. Peng Lu, Libin Gao both adopted solid-state method to synthesize Li4-xNaxTi5xZrxO12(0 ≤ x ≤ 0.2),Li1−xZrx/4Fe0.99Co0.01PO4 (0 ≤ x ≤ 0.02) respectively [18,19]. Moreover, Caixia Qiu et al. and Shenouda and Murali codoped ions by a hydrothermal method which showed excellent rate performance at high current [20,21]. Codoping must be an effective way to improve the performance of LTO. However, all of them do not take discussion on the interaction of codoped ions and neglect the influence of the codoped ratio. This study is the first to composite Li4-xMgxTi5-yZryO12 by solid state method which is a simple and mass-producible way. The paper infers the effect of Mg and Zr ions on the morphology and electrochemical performance, also exams the different ratio of Mg and Zr ions. These issues have not been reported until now. Li4-xMgxTi5-yZryO12 expresses high specific capacity and long cycle life than pure LTO when the doped ratio is optimized. The capacity and rate performance of the batteries are both improved. Thus, this work provides a low cost way to solve the problem of LTO.
⁎ Corresponding author at: School of Materials Science and Engineering, Dalian University of Technology, No.2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province 116023, China. E-mail address: [email protected] (Y. Tan).
https://doi.org/10.1016/j.ssi.2018.09.015 Received 16 July 2018; Received in revised form 19 September 2018; Accepted 25 September 2018 0167-2738/ © 2018 Elsevier B.V. All rights reserved.
Solid State Ionics 326 (2018) 63–68
Q. Li et al.
Fig. 1. (a) Spinel- and (b) rock-salt-LTO structures. Yellow tetrahedral represents lithium, and green octahedral represents disordered lithium and titanium. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) XRD patterns of LTO, LTO-R1, LTO-R2, LTO-R3 (b) the partial enlarged view(17° ≤ 2θ ≤ 19°).
2. Materials and methods
2.3. Electrochemical characterization
2.1. Synthesis of Li4-xMgxTi5-yZryO12
Electrochemical measurements were performed using CR2025 coin half-cell. The LTO electrodes were composed of Li4-xMgxTi5-yZryO12 nanoparticles, Super-P and polyvinylidene fluoride (PVDF) with a weight ratio of 80:10:10 and added N-methyl-2-pyrrolidone (NMP) to make slurry. Then its coated on copper foil and followed drying at 120 °C for 12 h in vacuum oven. After this, cut into discs for assembling cells. The metallic lithium was used as anode. The 1 M LiPF6 solution in ethylene carbonate (EC)/diethyl carbonate (DEC)/ethyl methyl carbonate (EMC) (volume ratio: 1:1:1) were used as the electrolyte. Microporous polyethylene (Celgard 2500) served as the separator. The operation of assembling half-cell was carried out in an Argon filled glove box. The galvanostatic charge-discharge cycle test at different current rates was performed in the potential range of 1.0–2.6 V on a Land CT2001A battery test system. Here 1C is corresponding to the current density of 175 mAh g−1. Cyclic voltammetry (CV) was conducted on a CHI660E electrochemical workstation over the potential range of 1.0–2.6 V at a scanning rate of 0.1 mV s−1. All the above electrochemical tests were carried out at ambient temperature.
Li4-xMgxTi5-yZryO12 was prepared using solid-state reaction as following. The precursor materials (Li2CO3, TiO2, Mg(OH)2, ZrO2) were added into a mill pot with the atom ratio besides the amount of Li excess 6 wt%. Then 60 mL ethanol alcohol was added into the above mixture as solvent, followed by ball milling 3 h at 400 r/min. Subsequently, the precursor slurry was dried at 100 °C for 8 h. Finally, the white powder was ground in the agate mortar and further annealed at 750 °C for 10 h in air to form the Li4-xMgxTi5-yZryO12 powder. As comparison, LTO materials were prepared at the same processes.
2.2. Materials characterization The phase compositions of all samples were measured by X-ray diffraction (XRD, SHIMANZU XRD-6000 using Cu Kα radiation with λ = 1.5418 Å). The morphology was observed by scanning electron microscopy (SEM, ZEISS SUPRA 55) and transmission electron microscope (TEM, Tecnai F30). The elemental compositions were determined by energy disperse spectroscopy (EDS, OXFORD INCA).
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3. Results and discussion
the advantage of small size for agglomeration. Thus, we infer that Mg ions can effectively reduce the size of LTO. Moreover, Zr ions act as the interstitial phases to disperse and block the aggregation of particles in the sintering process, which obviously improve the homogeneity. Herein, we infer that Mg and Zr ions have the symmetric and complementary interactions on the morphology. The result suggests that it is essential to control the ratio of Mg and Zr, then choose a best R. The reduction of particle size will shorten the diffusion path of Li ions. The improvement of uniformity will lighten the polarization during the discharge and charge process, and above all, will improve the electrochemical performance of LTO. As for the LRO-R2 showed the best morphology of above results, so the TEM analysis and selected area electron diffraction (SAED) testing of LTO-R2 was characterized in Fig. 4. It has smaller grain size around 100 nm and distributed uniformly which is agreed with the result of SEM. The regular distribution of sharp spots reveals that the samples are monocrystal materials with a good crystal structure in Fig. 4(c). The HRTEM pattern Fig. 4(b) of LTO-R2 shows that the structure of sample is well crystallized. As shown in Fig. 4(b), the interplanar spacing of LTO-R2 is 0.48 nm which is well consistent with the lattice space (111) facets of spinel LTO. In order to check Mg and Zr ions embedded, EDS was taken for LTOR2. As showed in Fig. 5. It was clearly seen that Mg and Zr ions distribute uniformly and the amount of Mg ions was more than Zr ions accorded with the experiments.
3.1. Symmetrical effect on the morphology of LTO In order to examine the mechanism of codoped Mg and Zr ions, different ratio of Mg and Zr doped was checked. Fig. 2a shows that LTOR1, LTO-R2 and LTO-R3 samples are typical spinel structure just as pure LTO and no impurity. It explains that Mg and Zr ions are perfectly embedded in Li, Ti sites and do not change the structure of pure LTO. Fig. 2b indicates that when R = 0, 1, 2and3, the 2 theta of LTO, LTOR1, LTO-R2, LTO-R3 are 18.331°, 18.264°, 18.335° and 18.392° respectively. According to the Formulas (1)–(3), where the λ = 1.5418 Å, the lattice parameters were calculated as 8.3763, 8.4063, 8.3741and 8.3483 Å respectively.
2d sin θ = λ
(1)
a = d∗ N
(2) (3)
N = h2 + k 2 + l 2 2+
Li et al. has indicated the ionic radius of Mg was nearly the same as that of Li+, and Mg ions were easily incorporated into the Li sites [22]. Chen et al. also prepared spinel Li4-xMgxTi5O12 (0 ≤ x ≤ 1) by solid-state reaction [23]. Furthermore, Caixia Qiu et al. got Li4-xMgxTi5xZrxO12 [20]. Through the above literature, it can be proved that Mg ions will take place of Li ions, Zr ions will replace the position of Ti. The size of Zr4+ (0.8 Å) is larger than that of Ti4+ (0.68 Å), since the substitution of Zr for Ti sites, the lattice parameter will increase. In addition, the ionic radius of Mg2+(0.66 Å) was slightly smaller than that of Li+(0.68 Å) which will reduce the lattice parameter. When R = 1, the effect of Zr ions on lattice parameters is greater than that of Mg ions, so that the lattice parameters increase. With the increasing of R, the lattice parameters decrease. When R = 2, the influence of Mg and Zr ions on the lattice parameters is counteracted, thus the lattice parameters are closed to pure LTO. When the R is further increased, the lattice parameters are reduced. Next, the SEM analysis was carried out to probe the effect of Mg and Zr ions on the morphology of LTO. As evidenced in Fig. 3, with the increasing of R, the particles size decrease greatly and the homogeneity of LTO have been improved obviously (Fig. 3a-d). The results indicated that Mg and Zr ions have the significant influence on the morphology of LTO. Although the particles size decrease with the R increasing, it lost
3.2. Cooperating effect on the electrochemical of LTO The galvanostatic charge/discharge was test at various current. Fig. 6 presents the initial charge/discharge curves of LTO, LTO-R1, LTO-R2, LTO-R3 at various rate from 0.57C to 10C over the potential range of 1.0–2.6 V. It can be observed that four samples form flat charge/discharge plateaus, which can be attributed to the redox reaction of Ti4+/Ti3+ in LTO. The potential plateau becomes much shorter and bends down more seriously, and the voltage difference becomes larger with increasing the current densities, indicating the increased polarization for the samples. Comparing with other samples, we note that the LTO-R2 has longer platform at high current densities 5C and 10C. The results show remarkably enhanced rate capability and the improved performance at high current densities in particular. The
Fig. 3. SEM of LTO, LTO-R1, LTO-R2, LTO-R3. 65
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Fig. 4. TEM and SAED image of LTO-R2.
charge specific capacity of LTO-R2 is 90.8 mAh g−1 at 10C while that of pure LTO is 39.3 mAh g−1, which improved 56.7%. Besides, the results of charge capacity are shown in Fig. 6. The differences of platform become more and more obvious with increasing the current densities. There are the reasons. It was published that Zr doped won't obviously change the Op-bands and Tid-bands in the LTO but can increase the lattice parameter. The lattice expansion of LTO through the substitution of Ti4+ with Zr4+, an ion with a relatively large ionic radius, might have benefits for facile and fast intercalation/deintercalation of lithium ions during lithium ion diffusion [24]. As previous report [25], the rate capability of electrodes is influenced by the lithium ion insertion/extraction kinetics corresponding to the charge transfer reaction and lithium ion diffusion in the bulk of the material, so Zr ions doping will perfect the rate performance. (See Fig. 7.) According to the discharge curve of samples (Fig. 8) which was tested at 0.57C, the LTO-R2 presented the highest charge capacity (186.6 mAh g−1) and the best cycle stability. The coulombic efficiency of each sample reached 100% after 100 cycles. It has been reported that the electrochemical performances are strongly related to the existence of defects in the material: the octahedral defects (16d) reduce the capacity and the tetrahedral defects (8a) create an irreversible insertion mechanism [26]. Ti3+ will increase tetrahedral defects so can improve the capacity. Because one Mg atom has one more valence electron than one Li atom does, when the Li atoms are substituted with Mg atoms,
there will be more valence electrons in the system. The shifting of the Fermi level to the Ti 3d bands also indicates that there are some electrons in the Ti 3d bands in the Mg substituted Li4Ti5O12. Therefore, Ti4+ ions are partially reduced to Ti3+ to increase the specific capacity [26]. However, the charge capacity retention of samples is respectively 70.4%, 90.1%, 90.8%, 89.9%. (LTO, LTO-R1, LTO-R2, LTO-R3) which are low. We infer that the uneven grain size and the uncertainty substitution of Mg caused the low retention of capacity. The dopants might hinder the Li diffusion if they share the 8a sites with Li. Then will degrade the electrochemical performance of LTO [27]. Fig. 9 is the typical cyclic voltammetry (CVs) of LTO, LTO-R1, LTOR2 and LTO-R3 electrodes between 1.0 and 2.6 V were tested at the second cycle as given. The potential difference between reduction and oxidation peaks can reflect the polarization degree of the electrode. The potential difference of LTO-R1, LTO-R2, LTO-R3 is lower than that of pure LTO. This observation suggests that codoping reduces the electrode polarization and improves the reversibility of LTO. In particular, the LTO-R2 electrode has the smallest potential difference among these samples which reveals the lowest polarization degree corresponded to the results. What's more, the samples show a pair of sharp and reversible redox peaks at round 1.55 V in all curves, indicating the good activity of these anodes. The reduction peaks located at round 1.5 V corresponds to the discharge process of Li+ intercalation and the oxidation peaks appear at around 1.7 V corresponds to the charge process
Fig. 5. EDS of LTO-R2. 66
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Fig. 6. The galvanostatic charge/discharge test of LTO, LTO-R1, LTO-R2, LTO-R3 at various current.
Fig. 7. The rate performance of samples.
of Li+ deintercalation. Thus those results suggest that LTO by doping Mg and Zr elements have the co-operating effect on improving the electrochemical of LTO. Mg ions can obviously improve the capacity of LTO and Zr ions perfect the rate performance. Proper proportion of Mg and Zr ions are beneficial to the improvement of material properties. In addition, the consequences of electrochemical test were agreed with the results of morphology analysis.
4. Conclusion LTO-R1, LTO-R2 and LTO-R3 were successfully synthesized by a simple solid-state method which is the nano size particles. The structure and morphology studies show that Mg-Zr doped can distinctly modify the particle size and uniformity of LTO. The two doping elements cooperate to improve the electrochemical property of LTO by this mechanism: the larger Zr4+ into Ti4+ exaggerate the lattice parameters of LTO, faster Li+ diffusion coefficient, change the rate performance. Then Mg2+ into Li+ raise the electron concentration to increase specific capacity. On the other hand, Zr ions doping can reduce the particles 67
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ions have the symmetrical and complementary effect on morphology. So, the ratio of Mg and Zr ions is a vital factor. Thus, an appropriate amount of Mg and Zr codoping can be an effective way to enhance the electrochemical performance of LTO as anode materials for rechargeable LIBs. References [1] D. Brandell, F. Jeschull, T. Nordh, R. Younesi, K. Edström, C. Ten Gstedt, ChemElectroChem 4 (2017) 2683–2692. [2] E.J. Berg, C. Villevieille, D. Streich, S. Trabesinger, P. Novk, J. Electrochem. Soc. 162 (2015) A2468–A2475. [3] J.B. Goodenough, K.S. Park, J. Am. Chem. Soc. 135 (2013) 1167–1176. [4] T. Meng, R. Zeng, Z. Sun, F. Yi, D. Shu, J. Electrochem. Soc. 165 (2018) A1046–A1053. [5] Y. Wang, J. Zhao, J. Qu, F. Wei, W. Song, Y.-G. Guo, B. Xu, ACS Appl. Mater. Interfaces 8 (2016) 26008–26012. [6] C. Chen, Y. Huang, H. Zhang, X. Wang, G. Li, Y. Wang, L. Jiao, H. Yuan, J. Power Sources 278 (2015) 693–702. [7] L. Yu, H.B. Wu, X.W. Lou, Adv. Mater. 25 (2013) 2296–2300. [8] K.-S. Park, A. Benayad, D.-J. Kang, S.-G. Doo, J. Am, Chem. Soc. 130 (2008) 14930–14931. [9] X. Sun, P.V. Radovanovic, B. Cui, New J. Chem. 39 (2015) B641–B644. [10] B. Zhao, X. Deng, R. Ran, M. Liu, Z. Shao, Adv. Energy Mater. 6 (2016) 1500924. [11] T.-F. Yi, S.-Y. Yang, Y. Xie, J. Mater. Chem. A 3 (2015) 5750–5777. [12] C. Kim, N.S. Norberg, C.T. Alexander, R. Kostecki, J. Cabana, Adv. Funct. Mater. 23 (2013) 1214–1222. [13] K. Wu, X. Lin, L. Shao, M. Shui, N. Long, Y. Ren, J. Power Sources 259 (2014) 177–182. [14] J.S. Chen, Y.L. Tan, C.M. Li, Y.L. Cheah, D. Luan, S. Madhavi, F.Y. Boey, L.A. Archer, X.W. Lou, J. Am. Chem. Soc. 132 (2010) 6124–6130. [15] C.M. Doherty, R.A. Caruso, B.M. Smarsly, C.J. Drummond, Chem. Mater. 21 (2009) 2895–2903. [16] H. Liu, C. Su, X. Li, Y. Guo, Solid State Ionics 320 (2018) 113–117. [17] Z. Huang, P.F. Luo, Solid State Ionics 311 (2017) 52–57. [18] P. Lu, X. Huang, Y. Ren, J. Ding, H.Y. Wang, S. Zhou, RSC Adv. 6 (2016) 90455–90461. [19] L. Gao, Z. Xu, S. Zhang, L. Xu, K. Tang, Solid State Ionics 305 (2017) 52–56. [20] C. Qiu, Z. Yuan, L. Liu, S. Cheng, J. Liu, Chin. J. Chem. 31 (2013) 819–825. [21] A.Y. Shenouda, K.R. Murali, J. Power Sources 176 (2008) 332–339. [22] F.Y. Li, M. Zeng, J. Li, H. Xu, Int. J. Electrochem. 10 (2015) 10445–10453. [23] C.H. Chen, J.T. Vaughey, A.N. Jansen, D.W. Dees, A.J. Kahaian, J. Electrochem. Soc. 148 (2001) A102. [24] J.G. Kim, M.S. Park, S.M. Hwang, Y.U. Heo, T. Liao, Z. Sun, et al., ChemSusChem 7 (2014) 1451–1457. [25] J. Liu, A. Manthiram, J. Phys. Chem. C 113 (2009) 15073–15079. [26] P. Kubiak, A. Garcia, M. Womes, L. Aldon, L. Olivier-Fourcade, P.E. Lippens, J. Power Sources 119 (2003) 626–630. [27] D. Liu, C. Uyang, J. Shu, J. Jiang, Z. Wang, L. Chen, Phys. Status Solidi 243 (2006) 1835–1841.
Fig. 8. The cycle performance of samples.
Fig. 9. CV curves of LTO, LTO-R1, LTO-R2, LTO-R3.
gathering accounted for Mg ions doping, Mg ions doping will refine the grain size which will be large result in Zr doped. Therefore, Mg and Zr
68
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
A symmetrical and co-operating effect of Mg-Zr codoping on Li4Ti5O12 anode materials
T
⁎
Qinglin Li, Bing Xue, Yi Tan , Kai Wang, Jianming Sun School of Materials Science and Engineering, Dalian University of Technology, Liaoning, Dalian 116024, China Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Liaoning, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Li4-xMgxTi5-yZryO12 Mg/Zr Symmetrical Co-operating
Uniform nanoparticles of Li4Ti5O12 were prepared using solid-state method coupled with Mg and Zr ions codoped which defined as LTO-R1, LTO-R2, LTO-R3 (R = Mg/Zr). Results show that Mg-Zr doped Li4Ti5O12 have the finest particles with diameters distributed in the range of 10 to 100 nm featured relatively high specific capacity and good rate capacity. The first discharge capacity of LTO-R2 is 186.6 mAh g−1 at 0.57C (1.0–2.6 V), and rate performance improve 56.7% at 10C compared pure LTO. The symmetrical and co-operating effect of codoped Mg-Zr ions on the morphology and the electrochemical property of LTO were intensively discussed in this paper.
1. Introduction Lithium-ion batteries (LIBs) are first choice as energy source for these applications in virtue of their high gravimetric and volumetric energy density. Conventionally, anode materials of LIBs are graphite negative electrode. Graphite negative electrodes inevitably react with electrolyte components due to their low working potential [1–3]. So, it cannot meet the demand of high safety. Spinel LTO has been widely investigated as a promising anode candidate for high electrochemical performance of LIBs. Due to the discharge plateau at 1.55 V (vs. Li/ Li+), it can avoid the reduction of the electrolyte on electrode surface and the formation of a solid-electrolyte interface (SEI) layer. Then will enhance the safety performance [4–8]. LTO is also called “Zero Strain” materials when Li-ion insertion/extraction with miniscule volume change which benefits from the 3D structure as showed in Fig. 1 [9]. The remarkable structure stability results in excellent cycle performance and capacity retention in long cycle life [10–12]. However, pure LTO shows poor lithium storage properties at high rates and low specific capacity (175 mAh g−1), which have huge negative effect on its commercial applications. Besides low electronic conductivity (10−13 S cm−1) and low Li+ diffusion coefficient −9 −13 2 −1 (10 –10 cm S ) also limit its performance [13–15]. Various approaches have been used to solve the above-mentioned problems, such as lattice ion doping, nanostructure-Li4Ti5O12 and surface modification with conductive materials [16,17]. However, many of the
technologies developed are too complicated to be cost-effective so hinder its applications. Solid-state method has been realized in industry at present for its simple operation and low production cost. But the conventional solid-state method would easily cause aggregation of the LTO particles at the expense of losing material's nanostructure. To tackle the problems, rational productive technology need to be developed. Doped electrode materials which synthesized by solid-state method are been considered promising to realize its application. Peng Lu, Libin Gao both adopted solid-state method to synthesize Li4-xNaxTi5xZrxO12(0 ≤ x ≤ 0.2),Li1−xZrx/4Fe0.99Co0.01PO4 (0 ≤ x ≤ 0.02) respectively [18,19]. Moreover, Caixia Qiu et al. and Shenouda and Murali codoped ions by a hydrothermal method which showed excellent rate performance at high current [20,21]. Codoping must be an effective way to improve the performance of LTO. However, all of them do not take discussion on the interaction of codoped ions and neglect the influence of the codoped ratio. This study is the first to composite Li4-xMgxTi5-yZryO12 by solid state method which is a simple and mass-producible way. The paper infers the effect of Mg and Zr ions on the morphology and electrochemical performance, also exams the different ratio of Mg and Zr ions. These issues have not been reported until now. Li4-xMgxTi5-yZryO12 expresses high specific capacity and long cycle life than pure LTO when the doped ratio is optimized. The capacity and rate performance of the batteries are both improved. Thus, this work provides a low cost way to solve the problem of LTO.
⁎ Corresponding author at: School of Materials Science and Engineering, Dalian University of Technology, No.2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province 116023, China. E-mail address: [email protected] (Y. Tan).
https://doi.org/10.1016/j.ssi.2018.09.015 Received 16 July 2018; Received in revised form 19 September 2018; Accepted 25 September 2018 0167-2738/ © 2018 Elsevier B.V. All rights reserved.
Solid State Ionics 326 (2018) 63–68
Q. Li et al.
Fig. 1. (a) Spinel- and (b) rock-salt-LTO structures. Yellow tetrahedral represents lithium, and green octahedral represents disordered lithium and titanium. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) XRD patterns of LTO, LTO-R1, LTO-R2, LTO-R3 (b) the partial enlarged view(17° ≤ 2θ ≤ 19°).
2. Materials and methods
2.3. Electrochemical characterization
2.1. Synthesis of Li4-xMgxTi5-yZryO12
Electrochemical measurements were performed using CR2025 coin half-cell. The LTO electrodes were composed of Li4-xMgxTi5-yZryO12 nanoparticles, Super-P and polyvinylidene fluoride (PVDF) with a weight ratio of 80:10:10 and added N-methyl-2-pyrrolidone (NMP) to make slurry. Then its coated on copper foil and followed drying at 120 °C for 12 h in vacuum oven. After this, cut into discs for assembling cells. The metallic lithium was used as anode. The 1 M LiPF6 solution in ethylene carbonate (EC)/diethyl carbonate (DEC)/ethyl methyl carbonate (EMC) (volume ratio: 1:1:1) were used as the electrolyte. Microporous polyethylene (Celgard 2500) served as the separator. The operation of assembling half-cell was carried out in an Argon filled glove box. The galvanostatic charge-discharge cycle test at different current rates was performed in the potential range of 1.0–2.6 V on a Land CT2001A battery test system. Here 1C is corresponding to the current density of 175 mAh g−1. Cyclic voltammetry (CV) was conducted on a CHI660E electrochemical workstation over the potential range of 1.0–2.6 V at a scanning rate of 0.1 mV s−1. All the above electrochemical tests were carried out at ambient temperature.
Li4-xMgxTi5-yZryO12 was prepared using solid-state reaction as following. The precursor materials (Li2CO3, TiO2, Mg(OH)2, ZrO2) were added into a mill pot with the atom ratio besides the amount of Li excess 6 wt%. Then 60 mL ethanol alcohol was added into the above mixture as solvent, followed by ball milling 3 h at 400 r/min. Subsequently, the precursor slurry was dried at 100 °C for 8 h. Finally, the white powder was ground in the agate mortar and further annealed at 750 °C for 10 h in air to form the Li4-xMgxTi5-yZryO12 powder. As comparison, LTO materials were prepared at the same processes.
2.2. Materials characterization The phase compositions of all samples were measured by X-ray diffraction (XRD, SHIMANZU XRD-6000 using Cu Kα radiation with λ = 1.5418 Å). The morphology was observed by scanning electron microscopy (SEM, ZEISS SUPRA 55) and transmission electron microscope (TEM, Tecnai F30). The elemental compositions were determined by energy disperse spectroscopy (EDS, OXFORD INCA).
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3. Results and discussion
the advantage of small size for agglomeration. Thus, we infer that Mg ions can effectively reduce the size of LTO. Moreover, Zr ions act as the interstitial phases to disperse and block the aggregation of particles in the sintering process, which obviously improve the homogeneity. Herein, we infer that Mg and Zr ions have the symmetric and complementary interactions on the morphology. The result suggests that it is essential to control the ratio of Mg and Zr, then choose a best R. The reduction of particle size will shorten the diffusion path of Li ions. The improvement of uniformity will lighten the polarization during the discharge and charge process, and above all, will improve the electrochemical performance of LTO. As for the LRO-R2 showed the best morphology of above results, so the TEM analysis and selected area electron diffraction (SAED) testing of LTO-R2 was characterized in Fig. 4. It has smaller grain size around 100 nm and distributed uniformly which is agreed with the result of SEM. The regular distribution of sharp spots reveals that the samples are monocrystal materials with a good crystal structure in Fig. 4(c). The HRTEM pattern Fig. 4(b) of LTO-R2 shows that the structure of sample is well crystallized. As shown in Fig. 4(b), the interplanar spacing of LTO-R2 is 0.48 nm which is well consistent with the lattice space (111) facets of spinel LTO. In order to check Mg and Zr ions embedded, EDS was taken for LTOR2. As showed in Fig. 5. It was clearly seen that Mg and Zr ions distribute uniformly and the amount of Mg ions was more than Zr ions accorded with the experiments.
3.1. Symmetrical effect on the morphology of LTO In order to examine the mechanism of codoped Mg and Zr ions, different ratio of Mg and Zr doped was checked. Fig. 2a shows that LTOR1, LTO-R2 and LTO-R3 samples are typical spinel structure just as pure LTO and no impurity. It explains that Mg and Zr ions are perfectly embedded in Li, Ti sites and do not change the structure of pure LTO. Fig. 2b indicates that when R = 0, 1, 2and3, the 2 theta of LTO, LTOR1, LTO-R2, LTO-R3 are 18.331°, 18.264°, 18.335° and 18.392° respectively. According to the Formulas (1)–(3), where the λ = 1.5418 Å, the lattice parameters were calculated as 8.3763, 8.4063, 8.3741and 8.3483 Å respectively.
2d sin θ = λ
(1)
a = d∗ N
(2) (3)
N = h2 + k 2 + l 2 2+
Li et al. has indicated the ionic radius of Mg was nearly the same as that of Li+, and Mg ions were easily incorporated into the Li sites [22]. Chen et al. also prepared spinel Li4-xMgxTi5O12 (0 ≤ x ≤ 1) by solid-state reaction [23]. Furthermore, Caixia Qiu et al. got Li4-xMgxTi5xZrxO12 [20]. Through the above literature, it can be proved that Mg ions will take place of Li ions, Zr ions will replace the position of Ti. The size of Zr4+ (0.8 Å) is larger than that of Ti4+ (0.68 Å), since the substitution of Zr for Ti sites, the lattice parameter will increase. In addition, the ionic radius of Mg2+(0.66 Å) was slightly smaller than that of Li+(0.68 Å) which will reduce the lattice parameter. When R = 1, the effect of Zr ions on lattice parameters is greater than that of Mg ions, so that the lattice parameters increase. With the increasing of R, the lattice parameters decrease. When R = 2, the influence of Mg and Zr ions on the lattice parameters is counteracted, thus the lattice parameters are closed to pure LTO. When the R is further increased, the lattice parameters are reduced. Next, the SEM analysis was carried out to probe the effect of Mg and Zr ions on the morphology of LTO. As evidenced in Fig. 3, with the increasing of R, the particles size decrease greatly and the homogeneity of LTO have been improved obviously (Fig. 3a-d). The results indicated that Mg and Zr ions have the significant influence on the morphology of LTO. Although the particles size decrease with the R increasing, it lost
3.2. Cooperating effect on the electrochemical of LTO The galvanostatic charge/discharge was test at various current. Fig. 6 presents the initial charge/discharge curves of LTO, LTO-R1, LTO-R2, LTO-R3 at various rate from 0.57C to 10C over the potential range of 1.0–2.6 V. It can be observed that four samples form flat charge/discharge plateaus, which can be attributed to the redox reaction of Ti4+/Ti3+ in LTO. The potential plateau becomes much shorter and bends down more seriously, and the voltage difference becomes larger with increasing the current densities, indicating the increased polarization for the samples. Comparing with other samples, we note that the LTO-R2 has longer platform at high current densities 5C and 10C. The results show remarkably enhanced rate capability and the improved performance at high current densities in particular. The
Fig. 3. SEM of LTO, LTO-R1, LTO-R2, LTO-R3. 65
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Fig. 4. TEM and SAED image of LTO-R2.
charge specific capacity of LTO-R2 is 90.8 mAh g−1 at 10C while that of pure LTO is 39.3 mAh g−1, which improved 56.7%. Besides, the results of charge capacity are shown in Fig. 6. The differences of platform become more and more obvious with increasing the current densities. There are the reasons. It was published that Zr doped won't obviously change the Op-bands and Tid-bands in the LTO but can increase the lattice parameter. The lattice expansion of LTO through the substitution of Ti4+ with Zr4+, an ion with a relatively large ionic radius, might have benefits for facile and fast intercalation/deintercalation of lithium ions during lithium ion diffusion [24]. As previous report [25], the rate capability of electrodes is influenced by the lithium ion insertion/extraction kinetics corresponding to the charge transfer reaction and lithium ion diffusion in the bulk of the material, so Zr ions doping will perfect the rate performance. (See Fig. 7.) According to the discharge curve of samples (Fig. 8) which was tested at 0.57C, the LTO-R2 presented the highest charge capacity (186.6 mAh g−1) and the best cycle stability. The coulombic efficiency of each sample reached 100% after 100 cycles. It has been reported that the electrochemical performances are strongly related to the existence of defects in the material: the octahedral defects (16d) reduce the capacity and the tetrahedral defects (8a) create an irreversible insertion mechanism [26]. Ti3+ will increase tetrahedral defects so can improve the capacity. Because one Mg atom has one more valence electron than one Li atom does, when the Li atoms are substituted with Mg atoms,
there will be more valence electrons in the system. The shifting of the Fermi level to the Ti 3d bands also indicates that there are some electrons in the Ti 3d bands in the Mg substituted Li4Ti5O12. Therefore, Ti4+ ions are partially reduced to Ti3+ to increase the specific capacity [26]. However, the charge capacity retention of samples is respectively 70.4%, 90.1%, 90.8%, 89.9%. (LTO, LTO-R1, LTO-R2, LTO-R3) which are low. We infer that the uneven grain size and the uncertainty substitution of Mg caused the low retention of capacity. The dopants might hinder the Li diffusion if they share the 8a sites with Li. Then will degrade the electrochemical performance of LTO [27]. Fig. 9 is the typical cyclic voltammetry (CVs) of LTO, LTO-R1, LTOR2 and LTO-R3 electrodes between 1.0 and 2.6 V were tested at the second cycle as given. The potential difference between reduction and oxidation peaks can reflect the polarization degree of the electrode. The potential difference of LTO-R1, LTO-R2, LTO-R3 is lower than that of pure LTO. This observation suggests that codoping reduces the electrode polarization and improves the reversibility of LTO. In particular, the LTO-R2 electrode has the smallest potential difference among these samples which reveals the lowest polarization degree corresponded to the results. What's more, the samples show a pair of sharp and reversible redox peaks at round 1.55 V in all curves, indicating the good activity of these anodes. The reduction peaks located at round 1.5 V corresponds to the discharge process of Li+ intercalation and the oxidation peaks appear at around 1.7 V corresponds to the charge process
Fig. 5. EDS of LTO-R2. 66
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Fig. 6. The galvanostatic charge/discharge test of LTO, LTO-R1, LTO-R2, LTO-R3 at various current.
Fig. 7. The rate performance of samples.
of Li+ deintercalation. Thus those results suggest that LTO by doping Mg and Zr elements have the co-operating effect on improving the electrochemical of LTO. Mg ions can obviously improve the capacity of LTO and Zr ions perfect the rate performance. Proper proportion of Mg and Zr ions are beneficial to the improvement of material properties. In addition, the consequences of electrochemical test were agreed with the results of morphology analysis.
4. Conclusion LTO-R1, LTO-R2 and LTO-R3 were successfully synthesized by a simple solid-state method which is the nano size particles. The structure and morphology studies show that Mg-Zr doped can distinctly modify the particle size and uniformity of LTO. The two doping elements cooperate to improve the electrochemical property of LTO by this mechanism: the larger Zr4+ into Ti4+ exaggerate the lattice parameters of LTO, faster Li+ diffusion coefficient, change the rate performance. Then Mg2+ into Li+ raise the electron concentration to increase specific capacity. On the other hand, Zr ions doping can reduce the particles 67
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ions have the symmetrical and complementary effect on morphology. So, the ratio of Mg and Zr ions is a vital factor. Thus, an appropriate amount of Mg and Zr codoping can be an effective way to enhance the electrochemical performance of LTO as anode materials for rechargeable LIBs. References [1] D. Brandell, F. Jeschull, T. Nordh, R. Younesi, K. Edström, C. Ten Gstedt, ChemElectroChem 4 (2017) 2683–2692. [2] E.J. Berg, C. Villevieille, D. Streich, S. Trabesinger, P. Novk, J. Electrochem. Soc. 162 (2015) A2468–A2475. [3] J.B. Goodenough, K.S. Park, J. Am. Chem. Soc. 135 (2013) 1167–1176. [4] T. Meng, R. Zeng, Z. Sun, F. Yi, D. Shu, J. Electrochem. Soc. 165 (2018) A1046–A1053. [5] Y. Wang, J. Zhao, J. Qu, F. Wei, W. Song, Y.-G. Guo, B. Xu, ACS Appl. Mater. Interfaces 8 (2016) 26008–26012. [6] C. Chen, Y. Huang, H. Zhang, X. Wang, G. Li, Y. Wang, L. Jiao, H. Yuan, J. Power Sources 278 (2015) 693–702. [7] L. Yu, H.B. Wu, X.W. Lou, Adv. Mater. 25 (2013) 2296–2300. [8] K.-S. Park, A. Benayad, D.-J. Kang, S.-G. Doo, J. Am, Chem. Soc. 130 (2008) 14930–14931. [9] X. Sun, P.V. Radovanovic, B. Cui, New J. Chem. 39 (2015) B641–B644. [10] B. Zhao, X. Deng, R. Ran, M. Liu, Z. Shao, Adv. Energy Mater. 6 (2016) 1500924. [11] T.-F. Yi, S.-Y. Yang, Y. Xie, J. Mater. Chem. A 3 (2015) 5750–5777. [12] C. Kim, N.S. Norberg, C.T. Alexander, R. Kostecki, J. Cabana, Adv. Funct. Mater. 23 (2013) 1214–1222. [13] K. Wu, X. Lin, L. Shao, M. Shui, N. Long, Y. Ren, J. Power Sources 259 (2014) 177–182. [14] J.S. Chen, Y.L. Tan, C.M. Li, Y.L. Cheah, D. Luan, S. Madhavi, F.Y. Boey, L.A. Archer, X.W. Lou, J. Am. Chem. Soc. 132 (2010) 6124–6130. [15] C.M. Doherty, R.A. Caruso, B.M. Smarsly, C.J. Drummond, Chem. Mater. 21 (2009) 2895–2903. [16] H. Liu, C. Su, X. Li, Y. Guo, Solid State Ionics 320 (2018) 113–117. [17] Z. Huang, P.F. Luo, Solid State Ionics 311 (2017) 52–57. [18] P. Lu, X. Huang, Y. Ren, J. Ding, H.Y. Wang, S. Zhou, RSC Adv. 6 (2016) 90455–90461. [19] L. Gao, Z. Xu, S. Zhang, L. Xu, K. Tang, Solid State Ionics 305 (2017) 52–56. [20] C. Qiu, Z. Yuan, L. Liu, S. Cheng, J. Liu, Chin. J. Chem. 31 (2013) 819–825. [21] A.Y. Shenouda, K.R. Murali, J. Power Sources 176 (2008) 332–339. [22] F.Y. Li, M. Zeng, J. Li, H. Xu, Int. J. Electrochem. 10 (2015) 10445–10453. [23] C.H. Chen, J.T. Vaughey, A.N. Jansen, D.W. Dees, A.J. Kahaian, J. Electrochem. Soc. 148 (2001) A102. [24] J.G. Kim, M.S. Park, S.M. Hwang, Y.U. Heo, T. Liao, Z. Sun, et al., ChemSusChem 7 (2014) 1451–1457. [25] J. Liu, A. Manthiram, J. Phys. Chem. C 113 (2009) 15073–15079. [26] P. Kubiak, A. Garcia, M. Womes, L. Aldon, L. Olivier-Fourcade, P.E. Lippens, J. Power Sources 119 (2003) 626–630. [27] D. Liu, C. Uyang, J. Shu, J. Jiang, Z. Wang, L. Chen, Phys. Status Solidi 243 (2006) 1835–1841.
Fig. 8. The cycle performance of samples.
Fig. 9. CV curves of LTO, LTO-R1, LTO-R2, LTO-R3.
gathering accounted for Mg ions doping, Mg ions doping will refine the grain size which will be large result in Zr doped. Therefore, Mg and Zr
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