Oxalate

In the Laboratory

Advanced Chemistry Classroom and Laboratory

edited by

Joseph J. BelBruno Dartmouth College Hanover, NH 03755

Preparation of Spinel-Type Cathode Materials from Carbonate/Oxalate Powder Mixtures A Solid-State Experiment for Advanced Undergraduate Inorganic Chemistry Laboratory Birgül Zümreo˘ glu-Karan* and Elif Yılmazer Department of Chemistry, Hacettepe University-Beytepe, 06532 Ankara, Turkey; *[email protected]

The most suitable cathode materials for rechargeable lithium-ion batteries are the so-called “insertion” or “intercalation” compounds capable of accepting and releasing lithium ions. Among these, the layered oxides LiCoO2 and LiNiO2 and the cubic spinel LiMn2O4 in particular have been used as high-voltage (around 4 V) active substances (1, 2). The LiMn2O4 spinel has considerable advantages in starting materials, low cost, and nonpolluting properties. The conventional procedure for preparing LiMn2O4 is to react stoichiometric amounts of Li2CO3 and MnO2 in air at 800 °C (1). The synthesis can also be accomplished by the simultaneous decomposition of Li2CO3 and MnCO3 in air Li2CO3 + 4MnCO3+ 2O2 → Li2Mn4O9 + 5CO2 Li2Mn4O9 → 2LiMn2O4 +

1⁄

2 O2

(1) (2)

or under nitrogen at lower temperatures (below 600 °C) (3, 4 ). We present an experiment to introduce the topic of solidstate chemical synthesis to students and to illustrate the applications of thermoanalytical (5, 6 ) and X-ray diffraction (7 ) techniques for the characterization of solid materials. The method involves the study of the reactions between alternating combinations of powder Li and Mn carbonates and oxalates. The most favorable combination and the reaction conditions for the formation and composition of the Li-Mn-oxide spinel were investigated by evaluating the thermogravimetric decomposition data and powder X-ray diffraction patterns. Experimental Procedure Simple lithium and manganese salts that are expected to give oxide structures upon thermal decomposition are chosen. We have performed four sets of experiments using combinations of Li2CO3 (Merck) and Li2C2O4 (BDH) with MnCO3 (Aldrich) and MnC2O4⭈4H2O (synthesized by the student from aqueous solutions of Mn(NO3)2⭈4H2O and oxalic acid; the number of water molecules was confirmed by thermogravimetric analysis [TG]). The starting Li and Mn compounds are intimately mixed in a 1:4 molar ratio (providing the Li:Mn stoichiometric ratio of 1:2) in an agate mortar to prepare the following mixtures: Li2CO3 + MnCO3, Li2C2O4 + MnCO3, Li2CO3 + MnC2O4⭈4H2O, and Li2C2O4 + MnC2O4⭈4H2O. Each mixture is heated and analyzed in an instrument such as a DuPont 951 TG thermal analyzer, in platinum

crucibles (conditions: nitrogen atmosphere, 10 mL min᎑1; heating rate: 20 °C min᎑1, up to 600 °C). The temperature range throughout which the decomposition takes place is noted for each set of combinations of reactants. TG data are recorded again and runs are terminated at 500 and 600 °C. IR spectra of the samples are taken by the KBr disk method to see if decarbonation is complete (following the peaks in the 1430–1500 cm᎑1 region corresponding to the ν3 mode of the carbonate group). Powder X-ray patterns are also obtained to compare with the literature about stoichiometric and defect spinels. The most suitable salt combination that decarbonates at the lowest temperature and gives the best X-ray pattern is determined. To demonstrate the effects of temperature and annealing on the composition and structure of the spinel, this salt mixture is prepared in stoichiometric but larger amounts and subjected to the following treatments in a tubular furnace: heating to 500 °C, heating to 600 °C, and heating to 500 °C and 600 °C and annealing at these temperatures for one hour (heating rate: 12 °C min᎑1; nitrogen flow: 10 mL min᎑1). The contents of lithium in the products are analyzed by flame photometry (FP) and of manganese by atomic absorption spectroscopy (AAS). Powder X-ray patterns are recorded to make correlations with the known spinel phases. Hazards There are no significant hazards related to substances or procedures. Results Figure 1 displays the thermal decomposition curves of the following mixtures. Li2CO3 + MnCO3 Mixture. It is clearly seen that decomposition is almost complete at around 450 °C. The IR spectrum of the sample heated to 500 °C shows no carbonate peaks. The observed mass loss between 200 and 450 °C (29.5%) agrees well with the expected value (29%) for the formation of the defect spinel (eq 1). There is a subsequent smooth decrease of ca. 6% up to 850 °C; this is possibly due to the transformation of the defect spinel into the stoichiometric spinel (expected 3%, eq 2). The 6% (5% for curve b) mass loss below 200 °C indicates the presence of some water trapped in the MnCO3 structure (also confirmed by the TG

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analysis of MnCO3 used). Li2C2O4 + MnCO3 Mixture. The decomposition extends to 850 °C with a mass loss of ca. 37% (expected 35.5%). At 600 °C, the system still contains nondecomposed carbonate material, as indicated by the presence of a very strong peak around 1500 cm᎑1 in the IR spectrum. Li2CO3 + MnC2O4⭈4H2O Mixture. Although the TG curve for this mixture reaches a plateau at around 500 °C, decarbonation is not complete at 600 °C. The mass loss of 58% at 800 °C is close to the theoretical value of 61%. Li2C2O4 + MnC2O4⭈4H 2O Mixture. Decomposition continues up to high temperatures. Mass loss at 800 °C is 58.1% (62.4%, theoretically) and carbonate decomposition is incomplete at 600 °C. For the mixtures containing MnC2O4⭈4H2O, the thermogravimetric mass loss values corresponding to the dehydration step are not consistent with the calculated values. This is not surprising, considering the high hydration tendency of the lithium ions present in these mixtures, which hold a fraction of the water molecules up to high temperatures, follow a different decomposition pathway, and give X-ray patterns not exactly agreeing with either the defect or stoichiometric spinels. On the other hand, the decomposition of mixtures containing Li2C2O4 exhibits completely different X-ray patterns when annealed at 600 °C for 1 h, indicating the formation of different products. The results show that the most suitable salt combination is the 1:4 mixture of Li2CO3 and MnCO3. Figure 2 shows XRD diagrams of student samples prepared from the carbonate mixture and Figure 3 gives those for the defect Li2Mn4O9 and stoichiometric LiMn2O4 from the literature, for comparison of the important projection reflections. The effects of temperature and annealing procedure on the composition and the structure of the spinels can be summarized as follows. The [111] reflection, which is characteristic of the cubic spinel lattice, is not observed at 500 °C (Fig. 2 [i]). The AAS and FP analyses of this product reveal the Li0.89Mn1.43O4 composition. The considerable degree of nonstoichiometry indicates a non-spinel phase. The [111] peak appears upon annealing for 1 h at 500 °C (Fig. 2 [ii]). At 600 °C, the X-ray pattern resembles that of the defect spinel (Fig. 2 [iii]). When the Li2CO3 + MnCO3 mixture is heated to 600 °C and annealed for 1 h at this temperature the pattern is similar to that of stoichiometric spinel (Fig. 2 [iv]). The composition of this sample is Li1.16Mn2.07O4, which is close to the stoichiometric LiMn2O4, within the limits of experimental error. Additional small peaks and intensity variations point to a multi-phase composition, since only the lithium-rich spinels, Li1+x Mn2-xO (0.1 ≤ x < 0.33), have been known to yield single-phase materials with the typical diffraction pattern of a cubic spinel (8). However, the nonstoichiometry in the products is difficult to estimate using our methods and apparatus. Powder neutron diffraction experiments should be undertaken to determine the cationic and anionic vacancies. Discussion The experiment described here is designed for final-year undergraduates. The method uses simple reagents available 1208

Figure 1. Thermogravimetric mass loss curves for the 1:4 mixtures of (a) Li2CO3 + MnCO3, (b) Li2C2O4 + MnCO3, (c) Li2CO3 + MnC2O4⭈4H2O, (d) Li2C2O4 + MnC2O4⭈4H2O.

Figure 2. Powder XRD patterns of the products obtained from the Li2CO3 + MnCO3 mixture after (i) heating to 500 °C, (ii) heating to 500 °C and annealing at 500 °C for 1 h, (iii) heating to 600 °C, (iv) heating to 600 °C and annealing at 600 °C for 1 h. (Philips PW 1140/00 Dy 687 Diffractometer, CuKa, Ni filter.)

in inorganic chemistry laboratories, and TG and XRD techniques that are available in most research laboratories. If departmental facilities are lacking, a diffractogram can be provided by another department or division as we have been serviced by our geology department. The lithium and manganese contents of the samples can be determined by AAS if

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In the Laboratory

techniques that have widespread use in solid state chemistry, and (iii) apply various methods of chemical analysis to determine the composition of the products. The electrochemical performance of the spinels prepared by the students can be tested in collaboration with the physical chemistry laboratory to satisfy the students that the synthesis actually yields a cathode material. Acknowledgments

Figure 3. Powder XRD patterns of the (a) stoichiometric and (b) defect spinels (from Thackeray et al. 1994, ref 4 ).

T. Fırat and S¸ . Özcan, Department of Engineering Physics, Hacettepe University, are gratefully acknowledged for the design, construction, and calibration of the tubular furnace used in this paper. Literature Cited

the instrument furnishes the necessary lamps, or by means of other common analysis methods. FP is a practical technique for students’ use, particularly to analyze the Li+ ions. The synthetic procedure involves the grinding of the starting metal compounds in proper stoichiometry followed by heating in a furnace. XRD evaluation here is restricted to demonstrating the diffraction phenomenon for a cubic crystal using the powder method. Actual solving of structure would be an excellent exercise for perhaps a graduate project. The experiment enables students to (i) examine various combinations of lithium and manganese salts to give the desired products, (ii) become familiar with the TG and XRD

1. Megahed, S.; Scrosati, B. J. Power Sources 1994, 51, 79. 2. Fey, G. T. K.; Wang, K. S.; Yang, S. M. J. Power Sources 1997, 68, 159. 3. de Kock, A.; Rossouw, M. H.; de Picciotto, L. A.; Thackeray, M. M.; Goodenough, J. B. Mater. Res. Bull. 1990, 25, 657. 4. Thackeray, M. M.; Rossouw, M. H. J. Solid State Chem. 1994, 113, 441. 5. Hill, J. O.; Magee, R. J. J. Chem. Educ. 1988, 65, 1025. 6. Wendlandt, W. W. J. Chem. Educ. 1972, 49, A623. 7. Azaroff, L. V.; Donahue, R. J. Laboratory Experiments in X-Ray Crystallography; McGraw-Hill: New York, 1969. 8. Endres, P.; Ott, A.; Kemmler-Sack, S.; Jager, A.; Mayer, H. A.; Praas, H.-W.; Brandt, K. J. Power Sources 1997, 69, 145.

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