J. Phys. Chem. C 2009, 113, 20733–20737
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Structural and Electrochemical Characterization of Li2MnSiO4 Cathode Material Ilias Belharouak,* A. Abouimrane, and K. Amine Chemical Sciences and Engineering DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439 ReceiVed: June 15, 2009; ReVised Manuscript ReceiVed: October 15, 2009
The candidate cathode material Li2MnSiO4 for lithium-ion cells was synthesized by an all-acetate precursor sol/gel method under a reducing atmosphere at 600, 700, and 800 °C. The material prepared at 700 °C was a pure phase and had the structural order of Li3PO4 orthorhombic (S.G. Pmn21) phase. The temperature dependence of the molar magnetic susceptibility of Li2MnSiO4 was found to be consistent with an antiferromagnetic material with a Ne´el temperature of 12 K. The calculated effective moment confirmed that the observed magnetic behavior involves Mn2+ ions in a high spin configuration in tetrahedral sites. Scanning electron microscopy of Li2MnSiO4 showed large aggregates (10 to 50 µm) composed of nanosized particles (100-200 nm). The as-prepared material was almost electrochemically inactive despite the presence of 15 wt % carbon additive. The material was treated by carbon coating using cellulose carbon source precursor and particle size reduction using high-energy ball milling. In coin-cell tests, the carbon-coated and ballmilled materials yielded charge capacities of 190 and 172 mAh/g, respectively, under a current density of 10 mA/g. At present, the cationic mixing between Li+ and Mn2+ ions in their mutual crystallographic sites is the main impediment to the achievement of the full theoretical capacity of Li2MnSiO4 (333 mAh/g). Introduction After two decades of extensive R&D, lithium-ion battery technology has appeared to be approaching optimal power and energy density with cost and safety remaining as major concerns. With the increasing demand for ever more fuel efficient cars, Li-ion batteries are regarded nowadays as the immediate technology that can power plug-in hybrid electric vehicles. A typical Li-ion battery can store 150 Wh of electricity in 1 kg. By comparison, it takes 6 kg of lead-acid battery to store the same amount of energy. Despite this advancement in Li-ion batteries, no further dramatic improvement in energy density is anticipated with existing materials and cell designs because of strict weight and volume constraints.1 Since the strategy of reducing the inactive materials at the battery pack level has been exhausted over the past decade, the only route to surmount the energy density shortfall is the development of higher specific capacity materials and/or higher cell voltage. Recently, a silicate family (Li2MSiO4, where M ) Fe, Mn, and Co) has been introduced because its high theoretical capacity may be exploited if the transition metal ions can be oxidized and reduced reversibly from their lowest oxidation state (2+) to a higher oxidation state (4+).2-4 This condition requires the extraction/ insertion of two lithium ions from the host structure, with the generation of 333 mAh/g theoretical capacity according to the following scheme: Li2Mn2+SiO4 / Mn4+SiO4 + 2Li+ + 2ein the case of Li2MnSiO4. The manganese (Mn2+/4+) redox couple is of particular interest because it exhibits a high potential (vs Lio), and resources to prepare the material are plentiful and clean. The preparation of Li2MnSiO4 material is, however, not trivial due to the possible presence of mixed phases and/or impurities such as MnO, MnSiO3, and Li2SiO3.3,5 Dominko et al.3 found that 0.9 Li+ ions are extracted but only 0.6 Li+ can be inserted at room temperature, and that complex structural * To whom correspondence should be addressed. E-mail: belharouak@ anl.gov.
behavior occurs during the charge, in which the structure becomes amorphous.6 The aim of the present paper is to report on a new synthesis route by an all-acetate sol/gel method that was found to be successful in preparing high-purity Li2MnSiO4. Carbon coating and particle size reduction by high-energy ball milling were found to be effective ways to enhance the electrochemical activity of this material. This improved electrochemical performance is discussed in light of structural investigations carried out by X-ray diffraction and magnetic and electron paramagnetic resonance measurements. Experimental Section Li2MnSiO4 was synthesized by a sol-gel method from lithium acetate, manganese acetate, and silicon acetate precursors. Stoichiometric amounts of CH3COOLi · 2H2O, (CH3COO)2Mn · 4H2O and Si(OCOCH3)4 were dissolved in acetic acid and stirred until the formation of a gel. Thereafter, the gel product was dried at 100 °C and then slowly heated to 600, 700, and 800 °C for 24 h under He/H2 (3.5%) gas flow to prevent the oxidation of Mn2+ ions. To prepare carbon-coated Li2MnSiO4, cellulose was added to the initial mix. The amount of carbon that had coated the material was determined by thermal gravimetric analysis of 10 mg Li2MnSiO4/C powder placed inside a platinum pan. Typically, around 7 wt % of carbon was found in the mix. The phases in the samples were identified by powder X-ray diffraction (Siemens D5000 diffractometer); Cu KR was used as the radiation source. The samples were scanned from 2θ ) 10 to 80° at a scan rate of 5 s/0.02°. The scanning electron microscope (SEM, Hitachi S-4700-II) at the Electron Microscopy Center of Argonne National Laboratory was used to analyze the sample morphology. The magnetization of Li2MnSiO4 was measured with a superconducting quantum interference device (SQUID, MPMS-9 Quantum Design) in the temperature range of 5-300 K with a magnetic field of 10 000 Gauss. Electron paramagnetic reso-
10.1021/jp905611s CCC: $40.75 2009 American Chemical Society Published on Web 11/09/2009
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Figure 1. X-ray patterns of Li2MnSiO4 prepared at 600, 700, and 800 °C under reducing atmosphere. Calculated pattern also shown at top.
nance (EPR) experiments were conducted on a Bruker Elexys E580 spectrometer equipped with a helium cryostat. The g tensor values were calibrated for homogeneity and accuracy by comparison to a coal standard (g ) 2.00285 ( 0.00005). Both SQUID and EPR measurement were conducted at the Center for Nanoscale Materials of Argonne National Laboratory. Electrochemical measurements were carried out on CR2032type coin cells. The positive electrodes were typically made of 75 wt % active material, 15 wt % acetylene black as the conductive agent, and 10 wt % polyvinylidene difluoride binder. The electrolyte was 1.2 M LiPF6 dissolved in a (3:7 v/v) mixture of ethylene carbonate and ethyl methyl carbonate. The cells were assembled with lithium metal as the negative electrode and were tested in the voltage range of 1.5-4.8 V under a current density of 10 mA/g.
Figure 2. Representation of the structure of Li2MnSiO4 with a Li3PO4 (Pmn21) structural order.
Results and Discussion Figure 1 shows the X-ray patterns of Li2MnSiO4 samples prepared at 600, 700, and 800 °C. The figure also shows the agreement between these patterns and the calculated X-ray diagram of Li2MnSiO4 (uppermost spectrum), whose structural model is a derivative of an orthorhombic form of Li3PO4 (S.G. Pmn21). As suggested by Tarte and Cahay,7 (Li2Mn)(SiO4) can be isostructural to (Li2(4b)Li(2a))(PO4) trilithium phosphate, where Mn2+ ions are located in the 2a tetrahedral sites within the [SiO4]4- anionic silicate framework that replaces the [PO4]3anionic phosphate framework. The remaining lithium ions occupy the 4b tetrahedral sites in between silicon and manganese tetrahedra (Figure 2). Under our experimental conditions, the Li2MnSiO4 samples (Figure 1) contained impurities such as MnO8 at 600 °C and Mn2SiO49 and Li2SiO310 at 800 °C, in agreement with other reports.11,12 The sample prepared at 700 °C seemed to be the most pure phase. Note that trials to use other space groups as suggested by others5,12 to fit the structure of Li2MnSiO4 were not successful. We speculate that an overlap exists between the stability domains of the main and impurity phases, so that it is very difficult to isolate an impurity-free Li2MnSiO4 phase. The Li2MnSiO4 would then partially decompose according to the following possible schemes: Li2MnSiO4 f Li2SiO3 + MnO and/or Li2MnSiO4 f 1/2 Li4SiO4 + 1/2 Mn2SiO4. The samples prepared at 700 °C showed the lowest level of impurities and thus were selected for further structural and electrochemical characterization. The orthorhombic lattice parameters [a ) 6.308(9) Å, b ) 5.385(9) Å, and c ) 4.999(6) Å] were refined by Rietveld profile matching13 and are consistent with values published by others.3,14 The observed and calculated patterns and the Bragg positions of
Figure 3. Observed and calculated X-ray diffraction patterns and the Bragg positions of Li2MnSiO4.
Li2MnSiO4 are shown in Figure 3. The observed diffraction lines can be indexed according to the space group Pmn21 of Li3PO4;15 however, weak peaks were observed around 2θ ) 35, 31, and 19°, which were attributed to Mn2SiO4 and Li2SiO3 impurity phases. Nonetheless, the Li2MnSiO4 prepared by the all-acetate sol/gel method has resulted in the highest material purity in comparison with previous results.3,16 Figure 4a,b shows the temperature dependence of the molar magnetic susceptibility, χm(T), and the inverse magnetic susceptibility, 1/χm(T), respectively, for Li2MnSiO4 prepared at 700 °C. The susceptibility χm is defined as the ratio between the magnetization (M) and the field (H). The magnetization was measured under an applied field of 10 000 gauss between 5 and 300 K with field-cooled and zero-fieldcooled conditions. The temperature dependence of χm for Li2MnSiO4 is typical for an antiferromagnetic material. The magnetic susceptibility shows a maximum at the Ne´el temperature (TN ) 12 K), below which χm increased with temperature, and above which the material is typically paramagnetic, following the Curie-Weiss law. In the paramagnetic part, the magnetic susceptibility can be expressed as follows:
χm )
C T-θ
(1)
Li2MnSiO4 Cathode Material
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Figure 5. Electron paramagnetic resonance spectrum of Li2MnSiO4 at 5 K.
Figure 4. Temperature dependence of (a) the magnetic susceptibility and (b) the inverse of magnetic susceptibility of Li2MnSiO4.
where θ is the Weiss constant (negative number for antiferromagnetic materials), and C is the Curie constant, expressed as: 2 2 NAPeff µB C) 3KB
(2)
In this equation, NA is the Avogadro number, Peff is the effective magnetic moment, µB is the Bohr magneton, and KB is the Boltzmann constant. The Curie and the Weiss constants, after deduction from the linear fitting of the 1/χm versus T data, were found to equal 4.54 emu · K · mol-1 and -29 K, respectively. Therefore, the experimental paramagnetic effective moment was µeff ) 6.0 µB, which is consistent with the spin-only calculated paramagnetic moment of Mn2+ ions (µeff ) 5.92 µB). This result clearly confirms that the observed magnetic behavior is that of Mn2+ (d5) ions located in tetrahedral sites. Indeed, according to crystal field theory, a high spin configuration is always expected in tetrahedral sites due to the weakness of the surrounding field. Figure 5 shows the X-band EPR spectrum of Li2MnSiO4 measured at 5 K with a modulation amplitude of 1 G and power of 2.09 mW. The EPR spectrum consists of a main Lorentzian line with g ) 2.003, close to the calculated g ) 2.0 for Mn2+ ions in a paramagnetic state. The large value of the line width (40 mT) for Mn2+ ions is believed to indicate magnetic interaction or coupling at low temperature, which is consistent with the observation of antiferromagnetic order at 5 K. Figure 6 shows the SEM photographs of the Li2MnSiO4 materials prepared at 700 °C. Large aggregates (10 to 50
µm) are observed for the as-prepared material (Figure 6a). These aggregates are made of nanosized particles (100-200 nm) that were cemented together during the calcination of the material (Figure 6b). The observed morphology may not be suitable to achieve the full theoretical capacity. On the one hand, Li2MnSiO4 itself is an insulating material; on the other hand, despite the presence of carbon black additive only the small particles at the surface of the large agglomerates can be electrochemically active. The core of the large aggregates, however, is insufficiently conductive to allow for rapid and full lithium extraction and insertion. Therefore, methods such as carbon coating and high-energy ball milling were considered to enhance the electronic conductivity and shorten the pathways for lithium mobility. By use of a cellulose precursor, Li2MnSiO4 was coated with carbon on the surface of the large particles (Figure 6c). Energy dispersive X-ray analysis was used to check the homogeneity of the coating and also for a qualitative elemental analysis of the chemical species that compose the Li2MnSiO4/C composite material (Figure 7). The exact amount of carbon that had coated the material, however, was determined by thermal gravimetric analysis and was found to be around 7 wt %. After 4 h of ball milling, the large agglomerates had broken into smaller particles without inducing a noticeable structural change (Figure 6d). Figure 8 shows the charge/discharge profiles of the asprepared (a), carbon-coated (b), ball-milled (c), and Li2MnSiO4 cathode materials in coin cells. All cells were assembled with lithium metal as the negative electrode and were tested in the voltage range of 1.5-4.8 V under a current density of 10 mA/ g. Because of its large aggregates and insulating characteristics, the as-prepared Li2MnSiO4 was almost electrochemically inactive despite the presence of 15 wt % carbon black, where specific capacities of only 10 and 4 mAh/g were achieved in the first charge and discharge, respectively. When coated with 7 wt % carbon (but 15 wt % total carbon in the electrode), the charge and discharge capacities were much improved, 190 and 135 mAh/g, with a coloumbic efficiency of 71%. The capacity and electrochemical activity also improved when the material was subjected to ball milling in the presence of 15 wt % carbon black. However, only 172 and 115 mAh/g capacities were obtained in this case. The capacity retention of the carbon-coated and ball-milled Li2MnSiO4 materialsisshowninFigure9over15charge-discharge cycles. In both cases, the low coloumbic efficiencies (71 and 67%) observed in the first cycle improved with cycle number and reached 93 and 91% after 15 cycles, respectively. However,
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Figure 6. Scanning electron microscopy images of (a,b) as-prepared Li2MnSiO4, (c) carbon-coated Li2MnSiO4, and (d) high-energy ball-milled Li2MnSiO4.
Figure 7. Energy dispersive X-ray spectrum of carbon-coated Li2MnSiO4.
Figure 9. Capacity vs cycle number of (a) carbon-coated Li2MnSiO4 and (b) high-energy ball-milled Li2MnSiO4.
Conclusions
Figure 8. Voltage profiles of (a) as-prepared Li2MnSiO4, (b) carboncoated Li2MnSiO4, and (c) high-energy ball-milled Li2MnSiO4.
this improvement was accompanied by a decrease of the capacity for both materials. Indeed, only 98 and 79 mAh/g discharge capacities were retained after 15 cycles.
An all-acetate precursor sol/gel method has been used to produce Li2MnSiO4 with a minimal level of impurities and with submicrometer particle sizes. Despite the agglomeration of the nanosize particles during calcination, carbon coating and highenergy ball milling enhanced the electronic conductivity and reduced the lithium diffusion pathways of the material, resulting in relatively high capacities. From this study we infer two needs before Li2MnSiO4 material can achieve practical high-energy battery utilization. First is overcoming the insulating properties of this silicate material. By applying the same strategies as used in the case of LiFePO4 olivine, carbon coating and particle size reduction, we have been able to partially activate the material electrochemically. However, extraction/insertion of only one lithium ion appears to be at work, given the partial achievement
Li2MnSiO4 Cathode Material of the 333 mAh/g theoretical capacity for Li2MnSiO4. Second is a need to better understand the structure of Li2MnSiO4, even though numerous researchers have already studied this material.3,6,14 A close examination of this structure suggests the possibility of a cationic mix between Mn2+ and Li+ ions, since both ions are located in tetrahedral sites and possess close ionic radii (0.59 Å for Li+ and 0.66 Å for Mn2+, compared with 0.26 Å for Si4+).17 This cationic mixing would likely cause the blocking of the pathways available for lithium ion diffusion in an ideal Li3PO4 (Pmn21) type structure and thus reduce the capacity. Acknowledgment. The authors would like to thank Gary L. Henriksen for his support throughout the accomplishment of this work and Nada Dimitrijevic for EPR measurements. This research was funded by the U.S. Department of Energy, FreedomCAR and Vehicle Technologies Office, through contract DE-AC02-06CH11357. References and Notes (1) Battery Test Manual for Plug-in Hybrid Electric Vehicles; Prepared for the U.S. Department of Energy: Idaho National Laboratory, March 2008. (2) Nyten, A.; Aboumrane, A.; Armand, M.; Gustafson, T.; Thomas, J. O. Electrochem. Commun. 2005, 7, 156.
J. Phys. Chem. C, Vol. 113, No. 48, 2009 20737 (3) Dominko, R.; Bele, M.; Gaberscek, M.; Meden, A.; Remskar, M.; Jamnik, J. Electrochem. Commun. 2006, 8, 217. (4) Lyness, C.; Delobel, B.; Armstrong, A. R.; Bruce, P. G. Chem. Commun., 2007, 4890. (5) Politaev, V. V.; Petrenko, A. A.; Nalbandyan, V. B.; Medvedev, B. S.; Shvetsova, E. S. J. Solid State Chem. 2007, 180, 1045. (6) Dominko, R.; Bele, M.; Kokalj, A.; Gaberscek, M.; Jamnik, J. J. Power Sources 2007, 174, 457. (7) Tarte, P.; Cahay, R. C. R. Acad. Sci. Paris 1970, C271, 777. (8) Barrett, C. A.; B Evans, E. J. Am. Ceram. Soc. 1964, 47, 533. (9) Fujino, K.; Sasaki, S.; Takeuchi, Y.; Sadanaga, R. Acta Crystallogr., Sect. B 1981, 37, 513. (10) Hesse, K. F. Acta Crystallogr., Sect. B 1977, 33, 901. (11) Arroyo de Dompablo, M. E.; Amador, U.; Gallardo-Amores, J. M.; Mora´na, E.; Ehrenberg, H.; Dupont, L.; Dominko, R. J. Power Sources 2009, 189, 638. (12) Arroyo de Dompablo, M. E.; Dominko, R.; Gallardo-Amores, J. M.; Dupont, L.; Mali, G.; Ehrenberg, H.; Jamnik, J.; Moran, E. Chem. Mater. 2008, 20, 5574. (13) Rodriguez-Carvajal, J. Powder Diffraction Satellite Meeting of the XVth Congress of IUCr, Toulouse, France, 1990; p 127. (14) Li, Y. X.; Gong, Z. L.; Yang, Y. J. Power Sources 2007, 174, 528. (15) Keffer, C.; Mighell, A. D.; Mauer, F.; Swanson, H.; Block, S. Inorg. Chem. 1967, 6, 119. (16) Ghosh, P.; Mahanty, S.; Basu, R. N. J. Electrochem. Soc. 2009, 156 (8), A677. (17) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
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