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Cite this: Chem. Commun., 2015, 51, 2641 Received 4th November 2014, Accepted 2nd January 2015
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A novel rechargeable battery with a magnesium anode, a titanium dioxide cathode, and a magnesium borohydride/tetraglyme electrolyte† Shuojian Su,ab Zhenguo Huang,c Yanna NuLi,*ab Felure Tuerxun,ab Jun Yangab and Jiulin Wangab
DOI: 10.1039/c4cc08774g www.rsc.org/chemcomm
The strong push for electric vehicles and large-scale power storage systems has generated intense interest in rechargeable magnesium batteries due to the innate merits associated with the magnesium metal anode in terms of volumetric capacity, abundance, and operational safety. Herein, we report a novel pathway toward the development of an advanced battery containing a magnesium anode, a titanium dioxide cathode, and a magnesium borohydride/tetraglyme electrolyte, which delivers high specific capacity, as well as exceptional cycle life and rate capability. This work demonstrates the importance of compatibility of the electrochemical activities of the cathode materials and electrolytes in rechargeable Mg batteries.
As some of the most successful means of electrochemical energy storage, Li-ion batteries are powering most of the electronic devices required by today’s information-rich mobile society. Recently, they have also been tested in hybrids, plug-in hybrids, and electric vehicles (EVs). In order for EVs to be competitive in the driving range and cost with those powered by combustion engines, batteries with a higher capacity will be required.1 Using the Li metal as the anode can improve both the gravimetric and the volumetric capacity of Li-ion batteries. The formation of Li dendrites during cycling, however, will cause a fatal short circuit.2,3 Unlike the Li metal, magnesium can be electrodeposited rather smoothly without the dendritic growth.4 Furthermore, Mg can provide a higher theoretical volumetric capacity than Li (3832 and 2062 mA h cm 3, respectively). This makes Mg batteries more competitive when only limited space is available to mount the battery packs.5 In addition, Mg is more abundant a
School of Chemistry and Chemical Engineering, Shanghai Electrochemical Energy Devices Research Center, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China. E-mail: [email protected]; Fax: +86-21-5474-1297; Tel: +86-21-5474-5887 b Hirano Institute for Materials Innovation, Shanghai Jiao Tong University, Shanghai 200240, P. R. China c Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia † Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c4cc08774g
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and more widely distributed than Li in the Earth’s crust. It is also possible to prepare the electrode in both humid and oxygen-rich environments. These merits make the rechargeable magnesium batteries great candidates for next-generation energy storage. So far, studies of Mg rechargeable batteries have been limited to only a few choices of cathode material and electrolyte. The early rechargeable Mg batteries, consisting of a Mg metal anode, a Chevrel phase Mo6S8 cathode, and an organic magnesium organohaloaluminate Mg(AlCl2BuEt)2/tetrahydrofuran (THF) electrolyte, showed impressive cycle life and low capacity fading over prolonged cycling.6 These batteries, however, suffer from kinetically torpid Mg2+ ion insertion/extraction and diffusion at the cathode due to the strong polarization effect of the divalent Mg2+. Moreover, the batteries show quite low overall energy densities because of the high formula weight and low voltage of the Mo6S8 insertion cathodes.6 They are thus only considered as replacements for nickel-cadmium or lead-acid batteries. The (PhMgCl)2-AlCl3/ THF electrolyte prepared from readily available precursors has also been studied.7 Muldoon proposed magnesium/sulfur batteries with non-nucleophilic electrolytes containing active species formed from the reaction between hexamethyldisilazide magnesium chloride and aluminium trichloride.1,5 Further work is needed to effectively prevent the dissolution of sulphur and intermediate polysulfides in the THF, which is a crucial component of the electrochemically active species [Mg2(m-Cl)36THF]+.5 Recently, a class of inorganic magnesium salt solutions consisting of nonnucleophilic MgCl2 and Lewis acidic compounds such as AlCl3 in ethereal solvents were studied.8,9 The electrolytes demonstrate high anodic stability and exceptional coulombic efficiency. Herein, we report the development of a novel rechargeable battery, consisting of a Mg anode, a cheap and non-toxic commercial TiO2 cathode, and a 0.5 mol L 1 Mg(BH4)2/LiBH4/tetraglyme (TG) ([LiBH4] = 1.5 mol L 1) electrolyte. The battery delivers high specific capacity, excellent cycle life, and impressive rate capability. As a slightly ionic and halide-free inorganic salt, Mg(BH4)2 in THF, dimethoxyethane (DME), or diglyme (DGM) can potentially provide high stability on the non-inert current collectors used in a practical rechargeable Mg battery system.10,11 LiBH4 was employed
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to increase the electrochemical performance. In this work, the quinquedentate ligand TG, featuring higher boiling/flash points compared with DGM, DME, and THF, and thus improved safety, is employed as the solvent for Mg(BH4)2.12 As shown in Fig. S1 in the ESI†, the electrolyte shows anodic stability up to 2.4 V vs. Mg RE on stainless steel and good compatibility with the Mo6S8 cathode. TiO2 has been reported as a promising anode material for Li-ion batteries owing to its good capacity retention, low selfdischarge rate, and low volume expansion (3–4%) during lithium insertion.13 There are only a few reports on TiO2 as a cathode material for rechargeable Mg batteries.14 We find that the electrolyte is the key factor in exploring the electrochemical activity of TiO2. As shown in Fig. S2 (ESI†), TiO2 exhibits poor Mg2+ insertion/extraction performance in the organic magnesium organohaloaluminate electrolyte, i.e., 0.4 mol L 1 (PhMgCl)2-AlCl3/ THF. When the TiO2 cathode is coupled with a compatible electrolyte, the batteries show much enhanced capacity and rate capability. Commercially available TiO2 (Aladdin Industrial Corporation) was used without further treatment. The corresponding X-ray diffraction (XRD) pattern, and scanning and transmission electron microscopy (SEM and TEM) images are presented in Fig. 1. The diffraction peaks can be indexed to tetragonal anatase TiO2 with JCPDS No. 21-1272 (space group: I41/amd). The primary spherical crystals are found to be around 25–30 nm in size, and tend to aggregate into larger particles. The Brunauer–Emmett–Teller (BET) surface area of commercial TiO2 is 84.542 m2 g 1. Fig. 2a shows the voltage profiles of the TiO2/Mg coin cell with the 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1) electrolyte between 0.5 and 1.7 V at a rate of 0.2 C (1 C = 168 mA h g 1). During the first discharge, TiO2 exhibits a well-defined plateau at approximately 0.87 V. The first charge plateau is observed at a slightly higher potential of 1.16 V. In the subsequent two cycles, the discharge platform moves up slightly to about 0.9 V, and the charge platform remains almost the same at 1.15 V, so TiO2 features low polarization (Vch Vdis). The first discharge
Fig. 1 X-ray diffraction pattern of commercial TiO2 with the standard pattern of JCPDS-ICDD 21-1272 (a). SEM (b), and low- (c) and high-resolution (d) TEM images of commercial TiO2.
2642 | Chem. Commun., 2015, 51, 2641--2644
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Fig. 2 (a) Discharge–charge curves for the first 3 cycles of the TiO2/Mg coin cell with the 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1) electrolyte at 0.2 C. (b) Discharge and charge capacities versus cycle number at 0.2 C.
capacity is 155.8 mA h g 1, and the corresponding charge capacity is 147.3 mA h g 1. The irreversible capacity in the first cycle may arise from side reactions involving trace water absorbed on the TiO2 surface and/or ions intercalated into irreversible sites. There is a little decrease in the second and third cycles, with the discharge and charge capacities being 148.9 mA h g 1 and 147.2 mA h g 1, and 146.8 mA h g 1 and 145.9 mA h g 1, respectively. Fig. 2b displays the discharge and charge capacities with respect to cycle number at 0.2 C over 90 cycles. The close match between the discharge and charge capacities manifests excellent reversibility. The reversible specific capacity of the electrode remains at the quite considerable value of B140 mA h g 1 for all of the cycles, demonstrating excellent cycling stability. Mg electrochemistry in Mg(BH4)2/LiBH4/DGM electrolytes with various LiBH4 concentrations (0–2.0 mol L 1) has been systematically studied.11 The current density and efficiency for Mg deposition–dissolution are the highest for the electrolyte containing 1.5 mol L 1 LiBH4, probably resulting from a higher conductivity. Fig. S3 (ESI†) shows the cycling performance of the TiO2/Mg coin cells with Mg(BH4)2/LiBH4/TG electrolytes of different LiBH4 concentrations (1.0, 1.5 and 2.0 mol L 1). The cell with 1.5 mol L 1 LiBH4 shows slight higher capacities and better cycling performance. In order to investigate the phase transitions during discharge and charge, ex situ XRD patterns (Fig. 3b) were collected at the points A to F in Fig. 3a. The pristine electrode at point A shows typical tetragonal anatase TiO2 with lower peak intensity due to
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Fig. 4 (a) Ex situ Raman spectra from points A–F indicated in Fig. 3a. (b) and (c) are enlarged regions of (a) at the ranges of interest. The numbers indicate the band wavenumber (cm 1) maximum.
Fig. 3 (a) Voltage profiles of TiO2 during the first discharge and charge. (b) Ex situ XRD patterns from points A–F indicated in (a). The diffraction peak of the stainless-steel substrate is marked as SS.
the interference from the conductive carbon and binder. The peaks start to dwindle at point B with no other appreciable change, showing the retention of the tetragonal anatase phase. At point C, a fairly weak peak (marked as # in the pattern) appears at about 221, indicating the formation of a new compound/phase. It is typically difficult to elucidate a local structural change based upon an XRD pattern, because it only provides overall structural information on a material. The new peak remains visible at points D and E, and disappears at point F (1.7 V), where only the tetragonal phase remains, indicative of good reversibility. The detailed local structural change in TiO2 during the discharge and charge process was further investigated by ex situ Raman spectroscopy (Fig. 4). At the initial state (point A), the spectrum is characterized by three bands at 145, 198, and 396 cm 1 corresponding to O–Ti–O bending (Eg and B1g modes) and two bands at 515 and 636 cm 1 corresponding to Ti–O stretching (A1g, B1g and Eg modes), which agree well with those of typical anatase TiO2.15 During the discharge and charge, the main bands of pure anatase are well preserved, with only slight changes in the position and intensity (Fig. 4a). The spectroscopic changes can be better observed in the enlarged patterns (Fig. 4b and c). At point B, the spectrum is very close to that of pure TiO2. The intensity of the bands decreases during ion intercalation at points C and D. Meanwhile, the bands at 396 and 515 cm 1 become wider and several new bands appear (Fig. 4c), which are associated with the change in the TiO2 framework due to ion insertion. Upon oxidation, i.e. ion extraction, the new peaks become less visible and disappear eventually at point F,
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where a full charge is reached. Upon full charge, only the main bands associated with TiO2 are observed, and the slight shifts in positions are likely caused by the residual strain in the lattice after ion intercalation/extraction. This result demonstrates the structural integrity of TiO2 during discharge and charge, which is critical for long cycle life. Mg intercalation into TiO2 was confirmed by energy-dispersive X-ray spectroscopy (EDS) (see Fig. S4, ESI†) and inductively coupled plasma (ICP) spectroscopy measurements for the discharged electrodes at points B and D (Fig. 3a). Prior to characterization, the TiO2 cathode was soaked and washed several times to remove the Mg and Li borohydrides. Both EDS and ICP results indicate that the amount of Mg increases from B to D. Co-intercalation of Li+ ions seems to have taken place since the Li : Mg molar ratio increases from 1.406 for B to 2.395 for point D. The XRD patterns of TiO2 discharged at points C and D have a peak at 221 (Fig. 3b), which is not related to Li insertion into TiO2.16 This system seems to be different from the hybrid battery where only Li insertion into Mo6S8 cathode material occurred when the Mg–Li dual salt was used.17 Further research on the reaction mechanism is now in progress. Cyclic voltammetry was used to further investigate the electrochemical behavior of TiO2 in the 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1) electrolyte at different scan rates. As shown in Fig. 5, only a pair of prominent cathodic/anodic peaks can be observed. The actual peak potentials depend on the scan rate, and the reversibility increases with decreasing scan rate. The cathodic (insertion) and anodic (extraction) peaks are in accordance with the plateaus of the discharging/charging curves. The main redox reaction responsible for the electrochemical activity is the Ti4+/Ti3+ conversion during the discharge process and vice versa during the charge process. The transition from Ti4+ to Ti3+ in TiO2 leads to an increase in the electronic conductivity18 and thus decreases polarization, as shown in Fig. 2a. To evaluate the rate performance of TiO2, the capacities at various current rates from 0.1 to 2 C were measured, and the results are displayed in Fig. 6. The discharge and charge capacities
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delivers excellent electrochemical performance, such as high specific capacity, excellent cycling stability, and rate capability. The co-intercalation of Li+ and Mg2+ seems to contribute to the exceptional performance of the batteries. This finding opens up a new alternative for the development of rechargeable magnesium batteries. Financial support from the National Natural Science Foundation of China (No. 21273147), Shanghai Municipal Science and Technology Commission (Project No. 11JC1405700), and Science and Technology Commission of Shanghai Municipality (14DZ2250800) is gratefully acknowledged. Fig. 5 Typical cyclic voltammograms of anatase TiO2 in the 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1) electrolyte at different scan rates.
Fig. 6 Rate capability of anatase TiO2 in the 0.5 mol L 1 Mg(BH4)2/LiBH4/ TG ([LiBH4] = 1.5 mol L 1) electrolyte at varied current rates from 0.1 to 2 C.
match each other well during cycling. At a relatively low rate of 0.1 C, TiO2 exhibits an initial discharge capacity of 168.8 mA h g 1 and a charge capacity of 161.9 mA h g 1, very close to the theoretical capacity (168 mA h g 1). The discharge capacity at 0.2 C and 1 C is 148.9 mA h g 1 and 123.7 mA h g 1, respectively. It is worth noting that at the higher rate of 2 C, the capacity still reaches 85 mA h g 1. In addition, a capacity of B143 mA h g 1 and 148 mA h g 1 at 0.2 and 0.1 C, respectively, is retained after many discharge/charge cycles at various high rates. The excellent rate capability of the commercial TiO2 cathode is clearly comparable to or even better than some reported Mg-insertion materials prepared by complicated methods.19–22 In conclusion, commercial TiO2 in a highly compatible electrolyte of 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1)
2644 | Chem. Commun., 2015, 51, 2641--2644
Notes and references 1 J. Muldoon, C. B. Bucur, A. G. Oliver, T. Sugimoto, M. Matsui, H. S. Kim, G. D. Allred, J. Zajicek and Y. Kotani, Energy Environ. Sci., 2012, 5, 5941. 2 J. M. Tarascon and M. Armand, Nature, 2001, 414, 359. 3 O. Crowther and A. C. West, J. Electrochem. Soc., 2008, 155, A806. 4 M. Matsui, J. Power Sources, 2011, 196, 7048. 5 H. S. Kim, T. S. Arthur, G. D. Allred, J. Zajicek, J. G. Newman, A. E. Rodnyansky, A. G. Oliver, W. C. Boggess and J. Muldoon, Nat. Commun., 2011, 2, 427. 6 D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich and E. Levi, Nature, 2000, 407, 724. 7 D. Aurbach, G. S. Suresh, E. Levi, A. Mitelman, O. Mizrahi, O. Chusid and M. Brunelli, Adv. Mater., 2007, 19, 4260. 8 R. E. Doe, R. Han, J. Hwang, A. J. Gmitter, I. Shterenberg, H. D. Yoo, N. Pour and D. Aurbach, Chem. Commun., 2014, 50, 243. 9 T. B. Liu, Y. Y. Shao, G. S. Li, M. Gu, J. Z. Hu, S. C. Xu, Z. M. Nie, X. L. Chen, C. M. Wang and J. Liu, J. Mater. Chem. A, 2014, 2, 3430. 10 R. Mohtadi, M. Matsui, T. S. Arthur and S. J. Hwang, Angew. Chem., Int. Ed., 2012, 51, 9780. 11 Y. Y. Shao, T. B. Liu, G. S. Li, M. Gu, Z. M. Nie, M. Engelhard, J. Xiao, D. P. Lv, C. M. Wang, J. G. Zhang and J. Liu, Sci. Rep., 2013, 3, 3130. 12 F. Tuerxun, Y. S. Abulizi, Y. N. NuLi, S. J. Su, J. Yang and J. L. Wang, J. Power Sources, 2015, 276, 255. 13 Z. H. Chen, I. Belharouak, Y.-K. Sun and K. Amine, Adv. Funct. Mater., 2013, 23, 959. 14 Y. C. Liu, L. F. Jiao, J. Chen, H. Q. Liu, K. Z. Cao, Y. J. Wang, Proceedings of the 2014 Electrochemical Conference on Energy & the Environment, Abstract #200. ´k, L. Dupont and E. Baudrin, 15 L. J. Hardwick, M. Holzapfel, P. Nova Electrochim. Acta, 2007, 52, 5357. 16 G. Sudant, E. Baudrin, D. Larcher and J. M. Tarascon, J. Mater. Chem., 2005, 15, 1263. 17 Y. W. Cheng, Y. Y. Shao, J. G. Zhang, V. L. Sprenkle, J. Liu and G. S. Li, Chem. Commun., 2014, 50, 9644. 18 T. V. Thi, A. K. Rai, J. Gim, S. Kim and J. Kim, J. Alloys Compd., 2014, 598, 16. 19 A. Mitelman, M. D. Levi, E. Lancry, E. Levi and D. Aurbach, Chem. Commun., 2007, 4212. 20 Y. NuLi, J. Yang, Y. S. Li and J. L. Wang, Chem. Commun., 2010, 46, 3794. 21 Y. L. Liang, R. J. Feng, S. Q. Yang, H. Ma, J. Liang and J. Chen, Adv. Mater., 2011, 23, 640. 22 Y. C. Liu, L. F. Jiao, Q. Wu, J. Du, Y. P. Zhao, Y. C. Si, Y. J. Wang and H. T. Yuan, J. Mater. Chem. A, 2013, 1, 5822.
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Cite this: Chem. Commun., 2015, 51, 2641 Received 4th November 2014, Accepted 2nd January 2015
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A novel rechargeable battery with a magnesium anode, a titanium dioxide cathode, and a magnesium borohydride/tetraglyme electrolyte† Shuojian Su,ab Zhenguo Huang,c Yanna NuLi,*ab Felure Tuerxun,ab Jun Yangab and Jiulin Wangab
DOI: 10.1039/c4cc08774g www.rsc.org/chemcomm
The strong push for electric vehicles and large-scale power storage systems has generated intense interest in rechargeable magnesium batteries due to the innate merits associated with the magnesium metal anode in terms of volumetric capacity, abundance, and operational safety. Herein, we report a novel pathway toward the development of an advanced battery containing a magnesium anode, a titanium dioxide cathode, and a magnesium borohydride/tetraglyme electrolyte, which delivers high specific capacity, as well as exceptional cycle life and rate capability. This work demonstrates the importance of compatibility of the electrochemical activities of the cathode materials and electrolytes in rechargeable Mg batteries.
As some of the most successful means of electrochemical energy storage, Li-ion batteries are powering most of the electronic devices required by today’s information-rich mobile society. Recently, they have also been tested in hybrids, plug-in hybrids, and electric vehicles (EVs). In order for EVs to be competitive in the driving range and cost with those powered by combustion engines, batteries with a higher capacity will be required.1 Using the Li metal as the anode can improve both the gravimetric and the volumetric capacity of Li-ion batteries. The formation of Li dendrites during cycling, however, will cause a fatal short circuit.2,3 Unlike the Li metal, magnesium can be electrodeposited rather smoothly without the dendritic growth.4 Furthermore, Mg can provide a higher theoretical volumetric capacity than Li (3832 and 2062 mA h cm 3, respectively). This makes Mg batteries more competitive when only limited space is available to mount the battery packs.5 In addition, Mg is more abundant a
School of Chemistry and Chemical Engineering, Shanghai Electrochemical Energy Devices Research Center, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China. E-mail: [email protected]; Fax: +86-21-5474-1297; Tel: +86-21-5474-5887 b Hirano Institute for Materials Innovation, Shanghai Jiao Tong University, Shanghai 200240, P. R. China c Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia † Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c4cc08774g
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and more widely distributed than Li in the Earth’s crust. It is also possible to prepare the electrode in both humid and oxygen-rich environments. These merits make the rechargeable magnesium batteries great candidates for next-generation energy storage. So far, studies of Mg rechargeable batteries have been limited to only a few choices of cathode material and electrolyte. The early rechargeable Mg batteries, consisting of a Mg metal anode, a Chevrel phase Mo6S8 cathode, and an organic magnesium organohaloaluminate Mg(AlCl2BuEt)2/tetrahydrofuran (THF) electrolyte, showed impressive cycle life and low capacity fading over prolonged cycling.6 These batteries, however, suffer from kinetically torpid Mg2+ ion insertion/extraction and diffusion at the cathode due to the strong polarization effect of the divalent Mg2+. Moreover, the batteries show quite low overall energy densities because of the high formula weight and low voltage of the Mo6S8 insertion cathodes.6 They are thus only considered as replacements for nickel-cadmium or lead-acid batteries. The (PhMgCl)2-AlCl3/ THF electrolyte prepared from readily available precursors has also been studied.7 Muldoon proposed magnesium/sulfur batteries with non-nucleophilic electrolytes containing active species formed from the reaction between hexamethyldisilazide magnesium chloride and aluminium trichloride.1,5 Further work is needed to effectively prevent the dissolution of sulphur and intermediate polysulfides in the THF, which is a crucial component of the electrochemically active species [Mg2(m-Cl)36THF]+.5 Recently, a class of inorganic magnesium salt solutions consisting of nonnucleophilic MgCl2 and Lewis acidic compounds such as AlCl3 in ethereal solvents were studied.8,9 The electrolytes demonstrate high anodic stability and exceptional coulombic efficiency. Herein, we report the development of a novel rechargeable battery, consisting of a Mg anode, a cheap and non-toxic commercial TiO2 cathode, and a 0.5 mol L 1 Mg(BH4)2/LiBH4/tetraglyme (TG) ([LiBH4] = 1.5 mol L 1) electrolyte. The battery delivers high specific capacity, excellent cycle life, and impressive rate capability. As a slightly ionic and halide-free inorganic salt, Mg(BH4)2 in THF, dimethoxyethane (DME), or diglyme (DGM) can potentially provide high stability on the non-inert current collectors used in a practical rechargeable Mg battery system.10,11 LiBH4 was employed
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to increase the electrochemical performance. In this work, the quinquedentate ligand TG, featuring higher boiling/flash points compared with DGM, DME, and THF, and thus improved safety, is employed as the solvent for Mg(BH4)2.12 As shown in Fig. S1 in the ESI†, the electrolyte shows anodic stability up to 2.4 V vs. Mg RE on stainless steel and good compatibility with the Mo6S8 cathode. TiO2 has been reported as a promising anode material for Li-ion batteries owing to its good capacity retention, low selfdischarge rate, and low volume expansion (3–4%) during lithium insertion.13 There are only a few reports on TiO2 as a cathode material for rechargeable Mg batteries.14 We find that the electrolyte is the key factor in exploring the electrochemical activity of TiO2. As shown in Fig. S2 (ESI†), TiO2 exhibits poor Mg2+ insertion/extraction performance in the organic magnesium organohaloaluminate electrolyte, i.e., 0.4 mol L 1 (PhMgCl)2-AlCl3/ THF. When the TiO2 cathode is coupled with a compatible electrolyte, the batteries show much enhanced capacity and rate capability. Commercially available TiO2 (Aladdin Industrial Corporation) was used without further treatment. The corresponding X-ray diffraction (XRD) pattern, and scanning and transmission electron microscopy (SEM and TEM) images are presented in Fig. 1. The diffraction peaks can be indexed to tetragonal anatase TiO2 with JCPDS No. 21-1272 (space group: I41/amd). The primary spherical crystals are found to be around 25–30 nm in size, and tend to aggregate into larger particles. The Brunauer–Emmett–Teller (BET) surface area of commercial TiO2 is 84.542 m2 g 1. Fig. 2a shows the voltage profiles of the TiO2/Mg coin cell with the 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1) electrolyte between 0.5 and 1.7 V at a rate of 0.2 C (1 C = 168 mA h g 1). During the first discharge, TiO2 exhibits a well-defined plateau at approximately 0.87 V. The first charge plateau is observed at a slightly higher potential of 1.16 V. In the subsequent two cycles, the discharge platform moves up slightly to about 0.9 V, and the charge platform remains almost the same at 1.15 V, so TiO2 features low polarization (Vch Vdis). The first discharge
Fig. 1 X-ray diffraction pattern of commercial TiO2 with the standard pattern of JCPDS-ICDD 21-1272 (a). SEM (b), and low- (c) and high-resolution (d) TEM images of commercial TiO2.
2642 | Chem. Commun., 2015, 51, 2641--2644
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Fig. 2 (a) Discharge–charge curves for the first 3 cycles of the TiO2/Mg coin cell with the 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1) electrolyte at 0.2 C. (b) Discharge and charge capacities versus cycle number at 0.2 C.
capacity is 155.8 mA h g 1, and the corresponding charge capacity is 147.3 mA h g 1. The irreversible capacity in the first cycle may arise from side reactions involving trace water absorbed on the TiO2 surface and/or ions intercalated into irreversible sites. There is a little decrease in the second and third cycles, with the discharge and charge capacities being 148.9 mA h g 1 and 147.2 mA h g 1, and 146.8 mA h g 1 and 145.9 mA h g 1, respectively. Fig. 2b displays the discharge and charge capacities with respect to cycle number at 0.2 C over 90 cycles. The close match between the discharge and charge capacities manifests excellent reversibility. The reversible specific capacity of the electrode remains at the quite considerable value of B140 mA h g 1 for all of the cycles, demonstrating excellent cycling stability. Mg electrochemistry in Mg(BH4)2/LiBH4/DGM electrolytes with various LiBH4 concentrations (0–2.0 mol L 1) has been systematically studied.11 The current density and efficiency for Mg deposition–dissolution are the highest for the electrolyte containing 1.5 mol L 1 LiBH4, probably resulting from a higher conductivity. Fig. S3 (ESI†) shows the cycling performance of the TiO2/Mg coin cells with Mg(BH4)2/LiBH4/TG electrolytes of different LiBH4 concentrations (1.0, 1.5 and 2.0 mol L 1). The cell with 1.5 mol L 1 LiBH4 shows slight higher capacities and better cycling performance. In order to investigate the phase transitions during discharge and charge, ex situ XRD patterns (Fig. 3b) were collected at the points A to F in Fig. 3a. The pristine electrode at point A shows typical tetragonal anatase TiO2 with lower peak intensity due to
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Fig. 4 (a) Ex situ Raman spectra from points A–F indicated in Fig. 3a. (b) and (c) are enlarged regions of (a) at the ranges of interest. The numbers indicate the band wavenumber (cm 1) maximum.
Fig. 3 (a) Voltage profiles of TiO2 during the first discharge and charge. (b) Ex situ XRD patterns from points A–F indicated in (a). The diffraction peak of the stainless-steel substrate is marked as SS.
the interference from the conductive carbon and binder. The peaks start to dwindle at point B with no other appreciable change, showing the retention of the tetragonal anatase phase. At point C, a fairly weak peak (marked as # in the pattern) appears at about 221, indicating the formation of a new compound/phase. It is typically difficult to elucidate a local structural change based upon an XRD pattern, because it only provides overall structural information on a material. The new peak remains visible at points D and E, and disappears at point F (1.7 V), where only the tetragonal phase remains, indicative of good reversibility. The detailed local structural change in TiO2 during the discharge and charge process was further investigated by ex situ Raman spectroscopy (Fig. 4). At the initial state (point A), the spectrum is characterized by three bands at 145, 198, and 396 cm 1 corresponding to O–Ti–O bending (Eg and B1g modes) and two bands at 515 and 636 cm 1 corresponding to Ti–O stretching (A1g, B1g and Eg modes), which agree well with those of typical anatase TiO2.15 During the discharge and charge, the main bands of pure anatase are well preserved, with only slight changes in the position and intensity (Fig. 4a). The spectroscopic changes can be better observed in the enlarged patterns (Fig. 4b and c). At point B, the spectrum is very close to that of pure TiO2. The intensity of the bands decreases during ion intercalation at points C and D. Meanwhile, the bands at 396 and 515 cm 1 become wider and several new bands appear (Fig. 4c), which are associated with the change in the TiO2 framework due to ion insertion. Upon oxidation, i.e. ion extraction, the new peaks become less visible and disappear eventually at point F,
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where a full charge is reached. Upon full charge, only the main bands associated with TiO2 are observed, and the slight shifts in positions are likely caused by the residual strain in the lattice after ion intercalation/extraction. This result demonstrates the structural integrity of TiO2 during discharge and charge, which is critical for long cycle life. Mg intercalation into TiO2 was confirmed by energy-dispersive X-ray spectroscopy (EDS) (see Fig. S4, ESI†) and inductively coupled plasma (ICP) spectroscopy measurements for the discharged electrodes at points B and D (Fig. 3a). Prior to characterization, the TiO2 cathode was soaked and washed several times to remove the Mg and Li borohydrides. Both EDS and ICP results indicate that the amount of Mg increases from B to D. Co-intercalation of Li+ ions seems to have taken place since the Li : Mg molar ratio increases from 1.406 for B to 2.395 for point D. The XRD patterns of TiO2 discharged at points C and D have a peak at 221 (Fig. 3b), which is not related to Li insertion into TiO2.16 This system seems to be different from the hybrid battery where only Li insertion into Mo6S8 cathode material occurred when the Mg–Li dual salt was used.17 Further research on the reaction mechanism is now in progress. Cyclic voltammetry was used to further investigate the electrochemical behavior of TiO2 in the 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1) electrolyte at different scan rates. As shown in Fig. 5, only a pair of prominent cathodic/anodic peaks can be observed. The actual peak potentials depend on the scan rate, and the reversibility increases with decreasing scan rate. The cathodic (insertion) and anodic (extraction) peaks are in accordance with the plateaus of the discharging/charging curves. The main redox reaction responsible for the electrochemical activity is the Ti4+/Ti3+ conversion during the discharge process and vice versa during the charge process. The transition from Ti4+ to Ti3+ in TiO2 leads to an increase in the electronic conductivity18 and thus decreases polarization, as shown in Fig. 2a. To evaluate the rate performance of TiO2, the capacities at various current rates from 0.1 to 2 C were measured, and the results are displayed in Fig. 6. The discharge and charge capacities
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delivers excellent electrochemical performance, such as high specific capacity, excellent cycling stability, and rate capability. The co-intercalation of Li+ and Mg2+ seems to contribute to the exceptional performance of the batteries. This finding opens up a new alternative for the development of rechargeable magnesium batteries. Financial support from the National Natural Science Foundation of China (No. 21273147), Shanghai Municipal Science and Technology Commission (Project No. 11JC1405700), and Science and Technology Commission of Shanghai Municipality (14DZ2250800) is gratefully acknowledged. Fig. 5 Typical cyclic voltammograms of anatase TiO2 in the 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1) electrolyte at different scan rates.
Fig. 6 Rate capability of anatase TiO2 in the 0.5 mol L 1 Mg(BH4)2/LiBH4/ TG ([LiBH4] = 1.5 mol L 1) electrolyte at varied current rates from 0.1 to 2 C.
match each other well during cycling. At a relatively low rate of 0.1 C, TiO2 exhibits an initial discharge capacity of 168.8 mA h g 1 and a charge capacity of 161.9 mA h g 1, very close to the theoretical capacity (168 mA h g 1). The discharge capacity at 0.2 C and 1 C is 148.9 mA h g 1 and 123.7 mA h g 1, respectively. It is worth noting that at the higher rate of 2 C, the capacity still reaches 85 mA h g 1. In addition, a capacity of B143 mA h g 1 and 148 mA h g 1 at 0.2 and 0.1 C, respectively, is retained after many discharge/charge cycles at various high rates. The excellent rate capability of the commercial TiO2 cathode is clearly comparable to or even better than some reported Mg-insertion materials prepared by complicated methods.19–22 In conclusion, commercial TiO2 in a highly compatible electrolyte of 0.5 mol L 1 Mg(BH4)2/LiBH4/TG ([LiBH4] = 1.5 mol L 1)
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Notes and references 1 J. Muldoon, C. B. Bucur, A. G. Oliver, T. Sugimoto, M. Matsui, H. S. Kim, G. D. Allred, J. Zajicek and Y. Kotani, Energy Environ. Sci., 2012, 5, 5941. 2 J. M. Tarascon and M. Armand, Nature, 2001, 414, 359. 3 O. Crowther and A. C. West, J. Electrochem. Soc., 2008, 155, A806. 4 M. Matsui, J. Power Sources, 2011, 196, 7048. 5 H. S. Kim, T. S. Arthur, G. D. Allred, J. Zajicek, J. G. Newman, A. E. Rodnyansky, A. G. Oliver, W. C. Boggess and J. Muldoon, Nat. Commun., 2011, 2, 427. 6 D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich and E. Levi, Nature, 2000, 407, 724. 7 D. Aurbach, G. S. Suresh, E. Levi, A. Mitelman, O. Mizrahi, O. Chusid and M. Brunelli, Adv. Mater., 2007, 19, 4260. 8 R. E. Doe, R. Han, J. Hwang, A. J. Gmitter, I. Shterenberg, H. D. Yoo, N. Pour and D. Aurbach, Chem. Commun., 2014, 50, 243. 9 T. B. Liu, Y. Y. Shao, G. S. Li, M. Gu, J. Z. Hu, S. C. Xu, Z. M. Nie, X. L. Chen, C. M. Wang and J. Liu, J. Mater. Chem. A, 2014, 2, 3430. 10 R. Mohtadi, M. Matsui, T. S. Arthur and S. J. Hwang, Angew. Chem., Int. Ed., 2012, 51, 9780. 11 Y. Y. Shao, T. B. Liu, G. S. Li, M. Gu, Z. M. Nie, M. Engelhard, J. Xiao, D. P. Lv, C. M. Wang, J. G. Zhang and J. Liu, Sci. Rep., 2013, 3, 3130. 12 F. Tuerxun, Y. S. Abulizi, Y. N. NuLi, S. J. Su, J. Yang and J. L. Wang, J. Power Sources, 2015, 276, 255. 13 Z. H. Chen, I. Belharouak, Y.-K. Sun and K. Amine, Adv. Funct. Mater., 2013, 23, 959. 14 Y. C. Liu, L. F. Jiao, J. Chen, H. Q. Liu, K. Z. Cao, Y. J. Wang, Proceedings of the 2014 Electrochemical Conference on Energy & the Environment, Abstract #200. ´k, L. Dupont and E. Baudrin, 15 L. J. Hardwick, M. Holzapfel, P. Nova Electrochim. Acta, 2007, 52, 5357. 16 G. Sudant, E. Baudrin, D. Larcher and J. M. Tarascon, J. Mater. Chem., 2005, 15, 1263. 17 Y. W. Cheng, Y. Y. Shao, J. G. Zhang, V. L. Sprenkle, J. Liu and G. S. Li, Chem. Commun., 2014, 50, 9644. 18 T. V. Thi, A. K. Rai, J. Gim, S. Kim and J. Kim, J. Alloys Compd., 2014, 598, 16. 19 A. Mitelman, M. D. Levi, E. Lancry, E. Levi and D. Aurbach, Chem. Commun., 2007, 4212. 20 Y. NuLi, J. Yang, Y. S. Li and J. L. Wang, Chem. Commun., 2010, 46, 3794. 21 Y. L. Liang, R. J. Feng, S. Q. Yang, H. Ma, J. Liang and J. Chen, Adv. Mater., 2011, 23, 640. 22 Y. C. Liu, L. F. Jiao, Q. Wu, J. Du, Y. P. Zhao, Y. C. Si, Y. J. Wang and H. T. Yuan, J. Mater. Chem. A, 2013, 1, 5822.
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