Amorphous and Nanocrystalline Oxide Electrodes for Rechargeable Lithium Batteries
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at. Res. Soc. Symp. Proc. Vol. 496 © 1998 Materials Research Society
studied with coin-type cells employing metallic lithium anodes and 1M LiC10 4 in propylene carbonate / 1,2-dimethoxyethane (1:1 ratio) electrolyte. The electrodes were fabricated by mixing the vacuum- or air-heated samples with 25 wt% fine carbon and 5 wt% polytetrafluoroethylene, rolling the mixture into thin sheets, and punching out circular electrodes of about 2 cm 2 area. RESULTS AND DISCUSSION Reduction of the oxo ions with borohydrides give both the binary oxides MO, and the ternary oxides AxMYOz depending upon the reaction pH and the concentration and volume of borohydride [3-6]. The chemical reaction for the formation of, for example, the binary oxide V0 can be given as 2K 3 VO 4 + 2KBH 4 + 6H 20
-
2KBO 2 + 2VO 2 + 6KOH + 71H2
2
(1)
Although the reducing power of borohydride increases with decreasing pH [2], the increasing degree of condensation of the oxo ions with decreasing pH to give poly anions [8] leads to a complex dependence of the reduction products on pH as the condensed poly anions are more difficult to reduce than the monomeric (M0 4 )n- ions. The reaction conditions to obtain the binary oxides that were used as electrodes in this study are given in Table 1. The chemical compositions given in Table 1 refer to that of the samples used for making the electrodes after heating as described in the experimental section. Table 1 Synthesis conditions and product compositions Reactants
pH
Product
0.25 M K3 VO 4 (50 ml) + 0.25 M KBH 4 (300 ml)
4
V0 2
0.25 M K2CrO4 (50 ml) + 0.1 M KBH 4 (10 ml)
4
CrO1.99
0.25 M K2 MoO 4 (50 ml) + 2.5 M KBH4 (50 ml)
I
MOO 2 .3
0.25 M KMnO4 (100 ml) + 2.5 M KBH 4 (5 ml)
1
MnO 2
The V0 2 sample after heating in vacuum at 240 'C was found to be nanocrystalline as revealed by TEM and X-ray diffraction. It belong to the metastable form called V0 2 (B) and transforms irreversibly to the more stable rutile form above 300 'C as indicated by X-ray diffraction and DSC [9]. The first discharge-charge curves and cyclability of nanocrystalline V0 2 (B) after heating in vacuum at 240 °C are shown in Fig. 1. It shows superior performance compared to that obtained in the literature by a reduction of NH 4VO 3 [10]. While the V0 2 (B) prepared by the literature methods insert about 0.5 lithium per vanadium, the V0 2(B) prepared by our solution-based approach inserts reversibly about 0.9 Li per V with a larger capacity of about 290 mAh/g and excellent cyclability. We believe, the smaller particle size achieved by the solutionbased approach and the accompanying microstructure play a critical role in giving better electrode properties. In addition, V0 2 (B) is tedious to synthesize as a single phase material by the literature procedures [11]. Our method is simple and gives readily single phase material. 422
The Cr0 2 sample after annealing in vacuum at 250 'C was found to be amorphous as indicated by the absence of diffraction peaks in the X-ray pattern. The amorphous nature was also confirmed by the abse
