Electrochem_insitu XRD LiSns

Electrochemical and In Situ X-Ray Diffraction Studies of the Reaction of Uthium with Tin Oxide Composites Ian A. Courtney and J. R. Dahn* Department of Physics, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5 ABSTRACT We report our electrochemical and in situ x-ray diffraction experiments on a variety of tin oxide based compounds; SnO, Sn02, Li2SnO3, and SnSiO3 glass, as cathodes opposite lithium metal in a rechargeable Li-ion coin cell. These materials demonstrate discharge capacities on the order of 1000 mAh/(g Sn), which is consistent with the alloying capacity limit of 4.4 Li atoms per Sn atom, or 991 mAh/(g Sn). These materials also demonstrate significant irreversible capacities ranging from 200 mAh/(g active) to 700 mAh/(g active). In situ x-ray diffraction experiments on these materials show that by introducing lithium, lithium oxide and tin form first, which is then followed by the formation of the various Li-Sn alloy phases. When lithium is removed the original material does not reform. The ending composition is metallic tin, pre- sumably mixed with amorphous lithium oxide. The oxygen from the tin oxide in the starting material bonds irreversibly with lithium to form an amorphous Li20 matrix. The Li-Sn alloying process is quite reversible; perhaps due to the formation of this lithia "matrix" which helps to keep the electrode particles mechanically connected together. Introduction Lithium-ion batteries rely on intercalation, which is the "reversible insertion of guest atoms into host solids such that the structure of the host is not significantly altered."1 If lithium can be intercalated into a host that is not altered by many successive lithium insertions and removals, then this host will probably have good cycle life in electrochemical cells. The amount of lithium that can be intercalated into a host, or its capacity, is as important as the cycle life for practical applications. Much of the present materials research done on Li-ion batteries focuses on finding electrode materials with improved cycleability and increased capacity for lithium intercalation. A conventional anode material is graphite2 which can intercalate one Li per six C under ambient conditions. The maximum theoretical capacity of graphite is 372 mAh/g and its volumetric capacity is 800 mAh/ml. Much research has been done on Li-metal alloys as anode materials for lithium and Li-ion cells, see, for examples, Ref. 3-5. Besenhard et al.6 show that some Li- metal alloys have similar or, in the case of silicon, higher lithium atom packing densities than lithium metal itself. Therefore alloy anodes are attractive both on a volumetric and gravimetric capacity basis. The difficulty with metal alloys is that there is a two and sometimes threefold volume change associated with the alloying of lithium. This can cause the "cracking" and "crumbling" of the alloy anode,6 and subsequently the conductivity of the electrode is reduced and the internal resistance of the cell is increased, resulting in poor cycleability. Recent developments suggest that alloy systems can be made to work well as anodes for Li-ion cells. For example, Fujifilm Celltec Co., Ltd., has announced its plans to man- ufacture and sell a new battery (trademark STALION), commencing early 1997, whose anode is an amorphous tin- based composite oxide and not carbon. This new anode is claimed to have theoretical volumetric and gravimetric capacity advantages over carbon of four and two times, respectively. Idota et al.,7 in one of approximately 200 patent applications surrounding the Fuji cell, describe the preparation and electrochemical behavior of hundreds of tin, as well as other group IV, oxides and oxide composite glasses. They claim that the mechanism involved in the reversible reaction of lithium with the anode electrode is one of intercalation, and that the process does not involve the formation of tin. The compounds described in Ref. 7 react with about 4 to 7 Li atoms per tin atom, which is much larger than that normally observed for intercalation systems, where a reversible range of one or two lithium atoms per metal atom is more typical. Therefore, interca- * Electrochemical Society Active Member. 372 mAh/g X 2.2 g/ml (density of graphite) = 800 mAh/mL lation as the mechanism seemed unlikely to us, so we decided to investigate four compounds representative of those studied by Idota et al., that is, SnO, Sn02, Li2SnO3, and SnSiO3 glass. Experimental Materials preparation—The SnO and Sn02 used throughout were commercially available (Aesar, 99.9%). To confirm the crystal structures of the samples they were analyzed using a Siemens D5000 diffractometer with a Cu K x-ray tube and diffracted beam monochrometer operat- ing in the Bragg-Brentano (flat plate sample) geometry. To prepare Li2SnO3, equimolar amounts of Li2CO3 (FMC) and Sn02 were dry blended, heated in an alumina heating boat at 10°C/mm to 1000°C in air in a Lindberg tube furnace, soaked for 7 h, and cooled slowly to room temperature. The resultant white powder was ground in an auto- grinder for 20 mm, and was then analyzed by x-ray diffraction (XRD). To prepare SiSnO3, equimolar amounts of SnO and Si02 (Aesar, 99.6%) were dry-blended and heated in a graphite heating boat. The sample was heated at 10°C/mm to 1000°C in a constant flow of Argon (Linde, Ultra High Pure 99.999%) inside a quartz reaction tube 4 ft long and 2 in. diam inside a Lindberg horizontal tube furnace. The material was soaked at 1000°C for 12 h and then quenched to room temperature by taking the tube out of the furnace. The product was yellow glassy SnSiO3. This was then ground in an auto-grinder for 20 mm and the powder was analyzed using XRD. We also prepared a second sample of this type, but where the melt was allowed to cool slowly by turning the furnace off and letting the sample cool inside the furnace. The resultant sample was also analyzed using XRD. Electrochemical testing—Electrodes of the four materials and also of tin metal powder (Aldrich, 99.5%) were prepared by coating slurries of the respective powders (85% by weight), Super S carbon black (10% by weight), and polyvinylidene fluoride (PVDF) (5% by weight) dissolved in N-methyl pyrrolidmnone (NMP) on a copper foil substrate. The thickness of the coated film was about 300 p.m. After coating, the electrodes were dried for 4 h at 106°C and pressed between plates at 2.0 X l0 Pa. The electrodes were cut into 1.44 cm2 squares and weighed. Coin-type test cells were constructed using these electrodes in 2320 (23 mm diam and 2.0 mm thick) coin cell hardware. The cells used a polypropylene microporous separator, an electrolyte (1 M LiPF6 dissolved in a 30:70 volume percent (v/o) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC)), and a 125 p.m thick, 1.44 cm2 lithium foil for the negative electrode. Cells were assembled and crimped closed in an argon-filled glove box. Details of cell assembly can be found in Ref. 8. All cells were tested with a constant current of 37.2 mA/g and J Electrochem. Soc., Vol. 144, No. 6, June 1997 The Electrochemical Society, Inc. 2045 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.146.237.55 Downloaded on 2015-07-28 to IP

Electrochem_insitu XRD LiSns

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