Lithium Insertion Behaviour of Li 1+x V 3 O 8 Prepared by Sol-Gel Method in Methanol
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SOL-GEL
J. KAWAKITA, Y KATAYAMA, T. MTURA, T. KISHI Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kouhoku-ku, Yokohama 223, JAPAN, [email protected] ABSTRACT Lil+xV3 0 8 (LT-M form) was obtained by sol-gel method in CH 3OH. This form, prepared at 350'C, possessed smaller grain size and better electrochemical performance than the HT form prepared by conventional high temperature synthesis. High discharge capacity (372 mAh-g': x = 4.0) and reversible discharge and charge cycle were attained. When heated at 200'C, CH 3OH molecules remained in the compound and crystallinity became lower by lithium insertion over x = 2.0. The lithium de-intercalation was irreversible. INTRODUCTION The layered lithium trivanadate, Lil+xV308, has been investigated as a cathode material for lithium secondary batteries [1,2,3]. The main advantage as an active material is its ability to accommodate up to three moles of Li+ ions per formula unit. Vanadium exists in pentavalent state though Li(I) is contained in the host structure before discharge. A good cycling performance is derived from the fact that the layers composed of V3 08 are linked strongly by an ionic bond between Li(I) ions and octahedrally coordinated oxygen atoms faced to the layers and the host structure changes little on lithium insertion and extraction[4]. Recently, it was reported that amorphous and low crystalline Lil+xV308 (LT form) prepared by sol-gel method in an aqueous solution was superior to one (HT form) obtained by conventional high temperature synthesis in terms of electrochemical property such as the discharge capacity [5,6]. So far, LT form has been synthesized only in an aqueous solution. However, water is one of the hindrances that have to be excluded from the non-aqueous lithium cell. In this work Lil+,V 3 0 8 (LT-M form) is prepared by sol-gel method using CH 3 OH as a solvent. Moreover, the structural characteristics and the electrochemical properties have been investigated and compared with those of HT form. EXPERIMENT Lil+xV 30 8 (LT-M form) was prepared by sol-gel method, referring the report by Pistoia et al. [5]. LiOCH 3 and CH 3 OH were substituted for LiOH and H2 0, respectively. V 20 5 was slowly added to 0.6 M LiOCH 3 / CH 3 OH solution and the concentration of it was 0.75 M. This operation was carried out in a glove box filled with argon gas. Then the mixture was stirred at 50'C for two days under argon atmosphere. Greenish brown precipitate was formed, and was heated and evacuated at 90'C for 12 hours to evaporate solvent CH 3OH. A fine powder was obtained by mild grinding of the sample. While this powder was highly hygroscopic, it became non-hygroscopic after the heat treatment at 200 and 350'C in dry air.
391 Mat. Res. Soc. Symp. Proc. Vol. 496 01998 Materials Research Society
Thermogravimetry (TG) was carried out on the samples heated at 200 and 350'C. As the reference sample, Lil. 2 V3 0 8 (HT form) was prepared by melting the mixture of Li 2 CO 3 and V20 5 in an appropri
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