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Electrochemical Intercalation of Mg2+ in Magnesium Manganese Silicate and Its Application as High-Energy Rechargeable Magnesium Battery Cathode Yanna NuLi, Jun Yang,* Jiulin Wang, and Yun Li Department of Chemical Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China ReceiVed: April 7, 2009; ReVised Manuscript ReceiVed: May 18, 2009
A study on electrochemical intercalation of bivalent cation Mg2+ in magnesium manganese silicate is reported. Reversible Mg2+ intercalation can be demonstrated and the process is enhanced by use of nanoscopic particles, resulted from a lower interfacial charge transfer resistance and a shorter solid-state diffusion distance of Mg2+ in the host. As a step toward novel rechargeable magnesium battery system, we have demonstrated the feasibility of the electrochemical oxidation, reduction, and cycling of magnesium manganese silicate. The results show that the silicate compound could be a good host for Mg2+ intercalation and a potential cathode material for high-energy rechargeable magnesium batteries. Introduction So far, study of the electrochemical intercalation is mainly devoted to graphite intercalation compounds and the reactions of univalent cations such as H+ and Li+. However, the insertion of Mg2+ is of particular theoretical and practical interest due to an analogous insertion electrochemistry of Mg2+ and Li+, and a prospective application in ion-transfer battery systems. The relatively low price and higher expected safety than lithium make magnesium a natural choice for use as an anode material in ion-transfer batteries.1 However, it is not easy to realize reversible Mg2+ insertion and extraction in a host, owing to the strong polarization of small and divalent Mg2+ compared to Li+.2,3 For the application as a host for electrochemically reversible intercalation of magnesium, vacant sites in the host plays an important role. So far, either a small capacity or a low voltage has been reported for magnesium insertion electrode. Therefore, it is necessary to search for new types of competitive insertion materials for the development of high-energy rechargeable magnesium batteries. Because of the large enough interstitial voids to uptake guest species and high structural stability based on three-dimensional framework, polyanionic compounds (XO4n-; X ) P, Si, Ge, etc.) have been successfully used as hosts of Li+ insertion/extraction and as promising cathodes for lithium-ion batteries.4 However, there are few reports on reversible Mg2+ insertion in polyanionic compounds. Recently, our research group first reported the electrochemical intercalation and deintercaltion of Mg2+ in magnesium manganese silicate synthesized by a high-temperature solid-state reaction and sol-gel routes, although the kinetic and cycling performance remains unsatisfactory.5,6 It is well known that the electrochemical characteristics of intercalation compounds strongly depend on the morphology of the materials and the structural perfection.7 Herein, we report Mg2+ intercalation in magnesium manganese silicate synthesized by a molten salt approach. The molten salt synthesis (MSS) method has been reported to be one of the simplest means to prepare pure and stoichiometric multicomponent oxide powders, * To whom correspondence should be addressed. Tel: (+86) 21-54747667. Fax: (+86) 21-5474-7667. E-mail:
[email protected]. Address: 800 Dongchuan Road, Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China.
in which the molten salts are utilized as solvent or reacting species, or sometimes both.8,9 The well-crystallized materials can be synthesized at rather low temperatures and in a short time because the salt melt exhibits higher ion diffusion rate and strong dissolving capability.10-14 The as-prepared materials show fast kinetics and excellent cycling stability for the intercalation and deintercaltion of Mg2+ as contrasted to that reported previously by us. Experimental Section Synthesis and Characterization. All the chemical reagents were analytically pure and used without further purification. KCl (melting point is 780 °C) was used as flux after drying for 3 h at 150 °C under vacuum. The starting materials were magnesium oxide (MgO), manganese(II) carbonate (MnCO3) and silicon dioxide (SiO2, 15-20 nm) powders. The stoichiometric amounts for the precursors were accurately controlled with the molar ratio of 1.03:0.97:1 for Mg:Mn:Si. The mixture (flux/reactants ) 4, by molar ratio) was hand-ground in a mortar by pestle for a few minutes, and was poured into a corundum crucible. Then, the powder mixture was dried at 120 °C for 5 h under vacuum to minimize the water content in the mixture. After that, the mixture was immediately transferred to a tube furnace and heated in a reductive (Ar + 5 wt % H2) atmosphere at 350 °C for 2 h to remove carbonate groups, followed by final firing at various temperatures at a rate of 2 °C /min for 6 h, then cooling to room temperature naturally. Finally, the product was washed three times with deionized water to dissolve any remaining salt, separated by centrifugation, and dried under vacuum at 100 °C for 2 h. The preparation procedure for electrolyte solution of 0.25 mol L-1 Mg(AlCl2EtBu)2/THF has already been reported in detail and the experimental method is described briefly as follows:15 proper amounts of MgBu2 solution (1 M in hexane, Aldrich) and AlEtCl2 solution (1 M in heptane, Aldrich) in the ratio of 1:2 were mixed at room temperature, and a white solid precipitation formed immediately. After stirring for 48 h, the hexane and heptane were completely evaporated, and a proper amount of high purity tetrahydrofuran (THF, distilled with benzophenone containing sodium chips under argon protection) was added to form the desired 0.25 mol L-1 solution. All
10.1021/jp903188b CCC: $40.75 2009 American Chemical Society Published on Web 06/22/2009
Electrochemical Intercalation of Mg2+ in Mg1.03Mn0.97SiO4
Figure 1. X-ray powder diffraction patterns of the samples that were synthesized at different temperatures and the standard pattern of JCPDSICDD 83-1546.
chemical preparations were carried out in an argon-filled glovebox (Mbraun, Unilab, Germany). X-ray powder diffraction (XRD) analysis was conducted on a Rigaku diffractometer D/MAX-2200/PC equipped with Cu KR radiation (λ ) 0.15418 nm) with 2θ ranging from 15 to 75° at a rate of 4° min-1 to analyze the structure of the expected products. Ex situ XRD measurements were conducted on electrodes to detect the phase evolution resulting from electrochemical reactions. The electrochemical processes were performed at a rate of C/20 (1C means the rate charged or discharged completely in 1 hour) and stopped at characteristic potentials (according to the first charge-discharge curves). The electrodes were removed from cell and washed by THF, then dried and contained in airtight holders. All the preparation was done in the argon-filled glovebox. The particle morphology was observed using scanning electron microscopy (SEM) on a JEOL field-emission microscope (JSM-7401F) and transmission electron microscopy (TEM) on a JEOL high-resolution electron microscope (JEM-2010). The Mg/Mn molar ratio (Si cannot be tested) was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using Iris Advangtage 1000 spectroscope (Thermo Electron). Electrochemical Measurements. Electrode slurry was prepared by mixing 78 wt % active material, 12 wt % super-P carbon powder (Timcal), and 10 wt % poly(vinylidene fluoride) (PVDF) dissolved in N-methyl-2-pyrrolidinone. The electrodes in diameter of Φ12.5 mm were formed by coating the slurry onto copper foil current collectors, drying at 80 °C for 1 h, pressing at 2 MPa, and drying at 100 °C for 4 h under vacuum. The active material loading was about 0.3-0.4 mg and the typical thickness of the active layer was 50 µm. Electrochemical behavior of the test materials was examined via CR2016 coin cells with magnesium ribbon counter electrode, Enter PE membrane separator, and 0.25 mol L-1 Mg(AlCl2EtBu)2/THF electrolyte. The cells were assembled in the argon-filled glovebox. Cyclic voltammetry measurements were performed on a CHI650C Electrochemical Workstation (Shanghai, China). Galvanostatic charge-discharge measurements were conducted at ambient temperature on a Land battery measurement system (Wuhan, China) with the cutoff voltage of 2.1/1.5 V versus Mg/ Mg2+. Results and Discussion Structure and Morphology. Figure 1 shows the XRD patterns of powder samples that were prepared at different temperatures. Mn2O3 and Mg6MnO8 phases can be detected in the product treated at 700 °C, and crystalline phase
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12595 Mg1.03Mn0.97SiO4 (JCPDS-ICDD database, 83-1546) are successfully obtained up to 800 °C. The Mg/Mn molar ratio is further confirmed by ICP analysis measurements.The literature reveals mixed site occupation of the same octahedral sites by Mg and Mn in the crystal structure.16 That is, 8% of the M1 (4a) sites is occupied by Mn2+ and 92% by Mg2+, and 89% of M2 (4c) sites is occupied by Mn2+ and 11% by Mg2+. The material prepared at 800 °C shows broader peaks, which indicates its smaller crystallite size. The peaks sharpen and the product becomes highly crystalline with increasing the temperature. The result shows that magnesium manganese silicate phase can rapidly be obtained at a moderate temperature by the MSS method, compared with the high-temperature solidstate reaction (at 1250 °C for 10 h) and sol-gel routes (at 700 °C for 24 h using tetraethyl silicate and at 900 °C for 24 h using silicon dioxide as silicon source).5,6 Figure 2a-c compares SEM images of Mg1.03Mn0.97SiO4 particles synthesized at different temperatures. The sample synthesized at 1000 °C is composed of crystalline well-shaped microparticles (1-2 µm), apparently different from the coarse particles obtained by the solid state reaction.5 The particle size decreases with decreasing the temperatures and the nanosized particles were obtained at 800 °C. TEM analysis in Figure 2d further shows that the particle distribution is narrow and most particles are averaged at about 80-90 nm. Compared with the samples obtained by the sol-gel routes,5,6 the degree of agglomeration is lower and the uniformity of these powders is better. The formation of uniform and less-aggregated crystalline powder can be attributed to the medium action of KCl flux during calcination process. Electrochemical Properties. Figure 3 shows typical steady state (after 5-6 preliminary cycles) voltammetric curves of the electrode comprising Mg1.03Mn0.97SiO4 synthesized at 1000 °C at 10 and 50 µV s-1. The voltammograms show a typical and expected feature of the electrochemical magnesium deintercalation and intercalation reactions. Two steps of Mg2+ deintercalation/intercalation can be distinguished, corresponding to two pairs of anodic/cathodic peaks marked as A/A′ and B/B′, respectively. In the Mg1.03Mn0.97SiO4 structure, there are two kinds of occupation sites for Mg atoms, M1 and M2. Because the M1-O bonds can be formed more easily,16 Mg atoms prefer to occupy M1 sites at first in thermodynamics. Peaks A/A′ and B/B′ can be attributed to the deintercalation/intercalation of Mg2+ from M1 and M2, respectively. The decrease in the scan rate results in the approach between A/A′ and B/B′ redox potential peaks. This implies that Mg2+ deintercalation and intercalation reaction processes are limited by the solid-state diffusion. Furthermore, the capacity fraction of peaks B/B′ decreases with decreasing the scan rate. It means that Mg2+ intercalation in M2 sites is kinetically superior to that in M1 sites, although the occupation of Mg2+ in M1 sites is thermodynamically superior to that in M2 sits. Figure 4 exhibits typical steady state charge-discharge curves (the 15th cycle) of the electrode comprising Mg1.03Mn0.97SiO4 synthesized at 1000 °C at C/20 (about 15.7 mA g-1). A capacity of 253.8 mAh g-1 (0.81 Mg2+ per unit formula), that is, 80.7% of the theoretical value 314.6 mAh g-1, is extracted from the pristine sample during charging, and 241.2 mAh g-1 (0.77 Mg2+ per unit formula) is intercalated back over the following discharge. The charge-discharge curves clearly show two main plateaus related to the redox peaks of A/A′ and B/B′ in Figure 3. The discharge voltage plateaus can be distinguished at 1.6 and 1.1 V, corresponding to the intercalation of Mg2+ into M1 and M2, respectively. The distribution of the total capacity for
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Figure 2. SEM images of Mg1.03Mn0.97SiO4 particles prepared at (a) 1000, (b) 900, and (c) 800 °C. (d) TEM imaging of Mg1.03Mn0.97SiO4 particles prepared at 800 °C.
Figure 3. Typical steady state (after 5-6 preliminary cycles) cyclic voltammograms of the electrode comprising Mg1.03Mn0.97SiO4 synthesized at 1000 °C at 10 and 50 µV S1-, respectively. The active electrode mass was about 0.4 mg.
Figure 5. Comparison of the charge-discharge curves of Mg1.03Mn0.97SiO4 electrodes comprising (a) microparticles (synthesized at 1000 °C) and (b) nanoparticles (synthesized at 800 °C) at C/5.
Figure 4. Typical steady state charge-discharge curves of the electrode comprising Mg1.03Mn0.97SiO4 synthesized at 1000 °C at C/20.
the length of two discharge potential plateaus is about 8:1, close to the magnesium occupation fraction at M1 and M2.
Figure 5 presents chronopotentiometric responses of magnesium deintercalation-intercalation of the electrodes based on Mg1.03Mn0.97SiO4 microparticles (synthesized at 1000 °C) and nanoparticles (synthesized at 800 °C) at C/5 (62.9 mA g-1). The initial cycle curves are different from the following ones. The initial charging process shows two voltage plateaus but only one flat and stable plateau for the following charge. It appears
Electrochemical Intercalation of Mg2+ in Mg1.03Mn0.97SiO4
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12597 appear mostly before 2θ ) 40° as shown in Figure 1, the scanning range is controlled here from 15 to 40° to remove the influence of copper current collector (a strong peak appeared at 43.3°) on the electrode active material. Although the data with weak peak intensities are insufficient to facilitate accurate Rietveld refinement to indicate the occurrence of an ion rearrangement during the first cycle, the XRD patterns of Mg1.03Mn0.97SiO4 change little upon charge and discharge and the structure stability is proved. This ensures its good rechargeability.
Figure 6. Ex situ XRD patterns for nanoparticle Mg1.03Mn0.97SiO4 electrodes during oxidation and consequent reduction at C/20 (a) uncharged, (b) charged to 1.85 V, (c) charged to 2.1 V, (d) discharged to 1.6 V, (e) discharged to 1.0 V, and (f) discharged to 0.5 V.
that the intersite exchange occurs between magnesium ions and manganese ions only during the first cycle. Moreover, after the first cycle, the voltage difference between charge and discharge becomes smaller. Especially, it is noted that the nanoparticle electrode has lower charging voltage than the microparticle one, indicating its smaller electrochemical polarization. On the other hand, although the microparticle electrode presents larger discharge capacity in the first cycle, its initial coulombic efficiency is lower (81% versus 85%) and the capacity retention in the subsequent cycles is worse. The initial capacity loss may partly arise from the electrolyte decomposition and surface filming on the electrodes. Furthermore, the slow ion diffusion rate and low electronic conductivity in the original silicate are also responsible for the initial irreversibility.17 The advantage of the Mg1.03Mn0.97SiO4 nanoparticles can be explained in terms of faster kinetics due to the larger surface area and shorter solid-state diffusion length of ions in the host, once this diffusion is the rate determining step.18 In addition, the finer host particles correspond to smaller absolute volume changes during Mg deintercalation and reintercalation, which is favorable for the electrode stability. The cycling result in Figure 5 further evidences the pronounced improvement in the electrochemical response of the systems by the use of nanoparticles as the active mass. The reversible capacity of the nanoparticle electrode can maintain 120.1 mAh g-1 upon the 80th cycle, against 96.0 mAh g-1 for the microparticle electrode. As is well known, the particle size is critical in determining useful capacity and charge/ discharge rates for low-conductive polyanion compounds. Herein, it is clearly shown that the electrochemical performance of the materials synthesized by molten-salt process is superior to that of high-temperature solid state reaction and sol-gel routes, especially at a higher charge-discharge rate.5,6 The uniform and highly dispersive particle structure is an important factor for magnesium manganese silicate to achieve good electrochemical performance. Figure 6 shows the evolution of ex situ XRD patterns for nanoparticle Mg1.03Mn0.97SiO4 electrodes during electrochemical reaction in the voltage range from 2.1 to 0.5 V versus magnesium at C/20. Since the main peaks for Mg1.03Mn0.97SiO4
Conclusions The electrochemical intercalation of divalent magnesium cations into Mg1.03Mn0.97SiO4 was studied with regard to its use as electroactive species in ion-transfer battery systems. As a novel cathode for rechargeable magnesium batteries, the material in uniform and nanodispersed granularity shows high voltage, large capacity, and good capacity retention. Its use makes a new approach to develop advanced rechargeable magnesium batteries. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (No. 20603022) and National 973 Program (No. 2007CB209700). References and Notes (1) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Nature 2000, 274, 724. (2) Gregory, T. D.; Hoffman, R. J.; Winterton, R. C. J. Electrochem. Soc. 1990, 137, 775. (3) Nova´k, P.; Imhof, R.; Haas, O. Electrochim. Acta 1999, 45, 351. (4) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188. (5) Feng, Z. Z.; Yang, J.; NuLi, Y. N.; Wang, J. L.; Wang, X. J. Electrochem. Commun. 2008, 10, 1291. (6) Feng, Z. Z.; Yang, J.; NuLi, Y. N.; Wang, J. L. J. Power Sources 2008, 184, 604. (7) Levi, E.; Gofer, Y.; Vestfreed, Y.; Lancry, E.; Aurbach, D. Chem. Mater. 2002, 14, 2767. (8) Chiu, C. C.; Li, C. C.; Desu, S. B. J. Am. Ceram. Soc. 1991, 74, 38. (9) Yoon, K. H.; Cho, Y. S.; Lee, D. H.; Kang, D. H. J. Am. Ceram. Soc. 1991, 76, 1373. (10) Tang, W.; Kanoh, H.; Ooi, K. Electrochem. Solid-State Lett. 1998, 145-146, 1. (11) Yang, X.; Tang, W.; Kanoha, H.; Ooi, K. J. Mater. Chem. 1999, 9, 2683. (12) Han, C.; Hong, Y.; Park, C. M.; Kim, K. J. Power Sources 2001, 92, 95. (13) Liang, H. Y.; Qiu, X. P.; Zhang, S. C.; He, Z. Q.; Zhu, W. T.; Chen, L. Q. Electrochem. Commun. 2004, 6, 505. (14) Bai, Y.; Shi, H. J.; Wang, Zh.X.; Chen, L. Q. J. Power Sources 2007, 167, 504. (15) Aurbach, D.; Schechter, A.; Moshkovich, M.; Cohen, Y. J. Electrochem. Soc. 2001, 148, A1004. (16) Francis, C. A.; Ribbe, P. H. Am. Mineral. 1980, 65, 1263. (17) Andersson, A. S.; Thomas, J. O. J. Power Sources 2001, 97/98, 498. (18) Suresh, G. S.; Levi, M. D.; Aurbach, D. Electrochim. Acta 2008, 53, 3889.
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