Chem. Mater. 2001, 13, 2397-2402
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Nanocomposite Li-Ion Battery Anodes Produced by the Partial Reduction of Mixed Oxides Pimpa Limthongkul, Haifeng Wang, and Yet-Ming Chiang* Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received December 29, 2000. Revised Manuscript Received April 17, 2001
A method based on the thermochemical reduction of mixed oxides is demonstrated for creating ultrafine metal-ceramic composites for lithium storage. Mixed oxides containing a more noble metal, selected to be capable of alloying with lithium at potentials useful as a Li-ion battery anode, are partially reduced to form electrochemically active composites of the dispersed metal in a conductive oxide matrix. The starting oxides SbVO4 rutile and Sb2Mn2O7 distorted fluorite are discussed as examples. Materials are characterized using XRD, SEM, and TEM, and electrochemical tests are presented. Reversible charge capacities of 300-400 mAh/g (1800-2500 mAh/cm3) are shown to be possible in these systems.
Introduction Increasing demand for higher energy density battery systems has motivated an ongoing search for new storage electrodes with high gravimetric and volumetric charge capacity and excellent cycleability. Carbon is presently used as the negative electrode in Li-ion rechargeable batteries due to its high capacity (372 mAh/g and 837 mAh/cm3), good cycleability, and low cost.1 Due to their high theoretical energy densities, a number of Li-metal alloy systems have also been investigated as candidates for negative electrodes.2-4 However, because of the large volume changes and mechanical failure that accompanies Li cycling, capacity retention upon cycling is usually poor. Research in tin oxide containing anodes5,6 shows that improved cycleability is possible if the metal phase is electrochemically reduced in-situ to ultrafine particulates. These oxides especially offer higher volumetric capacity than carbon (>1800 mAh/cm3) and can exhibit good cycleability, but a major limiting factor is the large irreversibility loss during the first cycle, due to irreversible lithium consumption accompanying the initial electrochemical reduction of tin oxide to tin metal.6 Reduction of first cycle irreversibility while retaining good cycleability has been described7,8 for some composites containing ultrafine dispersions of lithium-active metals produced by mechanical milling. These studies suggest that fine dispersions of lithium-active metals can be robust anodes.3-4,8,9 (1) Megahed, S.; Scrosati, B. J. Power Sources 1994, 51, 79. (2) Fauteux, D.; Koksbang, P. J. Appl. Electrochem. 1993, 23, 1. (3) Huggins, R. A. J. Power Sources 1989, 26, 109. (4) Yang, J.; Winter, M.; Besenhard, J. O. Solid State Ionics 1996, 90, 281. (5) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395. (6) Courtney, I. A.; Dahn, J. J. Electrochem. Soc. 1997, 144, 2045. (7) Mao, O.; Turner, R. L.; Courtney, I. A.; Fredericsen, B. D.; Buckett, M. I.; Krause, L. J.; Dahn, J. R. Electrochem. Solid State Lett. 1999, 2 (1), 3. (8) Ehrlich, G. M.; Durand, C.; Chen, X.; Hugener, T. A.; Spiess, F.; Suib, S. L. J. Electrochem. Soc. 2000, 147 (3), 886.
In the present research, the partial reduction of mixed oxides using simple thermochemical processes has been explored as an alternative approach to fabricating electrochemically active metal-ceramic nanocomposites with reduced first-cycle irreversibility loss and in which phase and volume changes during lithium insertion and removal can be accommodated. Experiments have been conducted in several systems, as discussed more completely elsewhere.10 Here we focus on oxides in the SbV-O and Sb-Mn-O family to illustrate of this general approach. Partial Reduction In partial reduction reactions a ternary or higher order oxide (or nitride, sulfide, etc.) is subjected to thermochemical conditions that reduce the most noble metal(s) but leaves the less noble metal(s) in an oxidized form:11 partially
MeIaMeIIbOc 9 8 aMeI + MeIIbOy + (c - y)/2O2 reduced (1) Here MeI is more noble than MeII. Depending on the starting composition and the relative diffusion rates of oxygen and the cations, either internal reduction, wherein the reduced species precipitates inside an oxide matrix, or external reduction, wherein the reduced species forms at the outer surface, can occur.11 Examples of previously studied systems include Mg-Me-O (Me ) Ni, Co, Fe, Cu), Al-Me-O (Me ) Cr, Fe, Ni), FeMn-O, and Fe-Cr-O.13-18 While partial reduction (9) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144 (9), 2942. (10) Limthongkul, P.; Wang, H.; Chiang, Y.-M. Electrochemical Society Proceedings, Symposium on Rechargeable Lithium Batteries; 198th Meeting of the Electrochemical Society, Phoenix, AZ, Oct 2227, 2000; Electrochemical Society: Pennington, NJ, in press. (11) Schmalzried H.; Backhaus-Ricoult, M. Prog. Solid State Chem. 1993, 22, 1. (12) Narayan, J.; Chen, Y. Philos. Mag. 1984, A49, 475.
10.1021/cm0014280 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/12/2001
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tion metal oxides are among the most attractive candidates for the host metal oxide MeIIyOz for this reason. Selection Criteria for Partially Reduced Anodes
Figure 1. Schematic illustrating partial reduction of a mixed oxide producing an internal metal precipitate MeI of reduced volume in an oxide MeIIzO, followed by lithiation of the metal to an alloy LixMeI with accompanying volume expansion.
reactions have been studied as a basic phenomenon13,18 and used for fabricating materials for potential applications as structural materials14-15,19 and optical storage media,16,20 to our knowledge this is the first instance where it has been used to synthesize electrochemically active materials. For use as a lithium storage electrode, our objective is to produce fine particles of a lithium-active metal, MeI, enclosed within or dispersed among particles of a metal-oxide matrix MeIIbOy. The matrix itself may or may not be lithium active. Materials made by this process can have a number of advantages: (1) Firstcycle irreversibility can be reduced since the electrochemically driven displacement reactions that Li2O forms as a byproduct upon first lithium insertion6 can be avoided. These displacement reactions are typically irreversible or poorly reversible. (2) Ultrafine metal particles, which appear to better tolerate cyclic volume changes than coarser particles or bulk metal,4,9,21 can be produced. Partial reduction can produce metal particles as small in diameter as a few nanometers. (3) Volume shrinkage of the metal oxide upon reduction can provide room for the subsequent expansion of the metal upon lithiation (Figure 1). Note that the total volume change from starting oxide to lithiated compound can be almost zero for some systems. (4) Chemical compatibility between cell components may be improved. Passivating layers are known to exist between pure elemental negative electrodes and liquid electrolytes. Where internal reduction of a partially reduced mixed oxide is achieved, the active metal particles can be isolated from the electrolyte by an oxide that is more chemically compatible. (5) High electronic and ionic conductivity of the composite can be achieved. Transi(13) Ostyn, K. M.; Carter, C. B.; Koehne, M., Falke, H.; Schmalzried, H. J. Am. Ceram. Soc. 1984, 67, 679. (14) Subramanian, R.; U ¨ stu¨ndag, E.; Sass, S. L.; Dieckmann, R. Mater. Sci. Eng. 1995, A195, 51. (15) U ¨ stu¨ndag, E.; Subramanian, R.; Dieckmann, R.; Sass, S. L. Acta Metall. Mater. 1995, 43, 383. (16) Smith, J. A.; Limthongkul, P.; Hartsuyker, L.; Kim, S. Y.; Sass, S. L. J. Appl. Phys. 1998, 83, 2719. (17) Ricoult D. L.; Schmalzried, H. J. Mater. Sci. 1987, 22, 2257. (18) Schmalzried, H. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 1186. (19) Smith, J. A.; Limthongkul, P.; Sass, S. L. Processing and Fabrication of Advanced Materials IV; Srivatsan, T. S., Moore, J. J., Eds.; The Minerals, Metals and Materials Society: Warrendale, PA, 1996; p 457. (20) U ¨ stu¨ndag, E.; Subramanian, R.; Dieckmann, R.; Sass, S. L. In Situ Comps. Proc. Symposium; Singh, M., Lewis, D., Eds.; Met. Mater. Soc.: Warrendale, PA, 1994; p 97. (21) Huggins, R. A.; Nix, W. D. Ionics 2000, 6, 57.
Since most metals studied to date alloy at voltages
