Sonochemically synthesized vanadyl phosphate dihydrate cathodes

Sonochemically Synthesized Vanadyl Phosphate
change in capacity under high discharge rate of 0.3 Dihydrate Cathodes
C is observed. Sonochemically prepared sample exhibits also good reversibility for the Telecommunication Basic Research Lab., electrochemical lithium insertion/extraction. Electronics and Telecommunications Research Preliminary results on the electrochemical performance indicate that sonochemically synthesized VOPO  4 2H2O can be a promising cathode material for rechargeable lithium battery. Introduction
Recently, chemical bonding nature-potential level correlation has been investigated for Nasicon-type electrode materials LixM2(XO4)3 (M = transition metal; X = S, P, As) [1,2], where the higher ionicity of the M-O bond was found to give rise to the higher potential of the Mn+/M(n+1)+ redox couple. Electrochemical lithium insertion in the rhombohedral Li3V2(PO4)3 [3] and the β-form of VOPO4 [4] was investigated. The potentials in batteries based on these materials showed 3.77 and 3.98 V vs. Li+/Li, respectively. However, less attention has been paid to vanadyl phosphate dihydrate VOPO  4 2H2O because of being more interested in its catalytic property. VOPO  4 2H2O has been usually prepared by long-term boiling of a V suspension in aqueous phosphoric acid according to Ladwig’s method [5]. We have developed a method for rapid synthesis of VOPO  4 2H2O. In this paper, we report on sonochemical Figure 1. First discharge curve for the
Experimental
References
follows. The aqueous phosphoric solution contained Goodenough, S. Okada, H. Ohtsuka, H. Arai, and J. Yamaki, Solid State Ionics, 92, 1 (1996) 2O5 powder was exposed to high-intensity ultrasound radiation for 15 min by employing a direct 2. C. Masquelier, A.K. Padhi, K.S. Nanjundaswamy, immersion titanium alloy horn operating at 20 kHz, and J.B. Goodenough, J. Solid State Chem., 135, with intensity of ~ 70 W/cm2. The temperature of the reaction solution rose to 70 oC during sonication. The resulting yellow solid was filtered, washed several Masquelier, and L. Nazar, Chem. Mater., 12, 3240 times with acetone and dried at ambient atmosphere. The synthesized material was characterized 4. J. Gaubicher, T. Le Mercier, Y. Chabre, J. using X-ray diffraction (XRD), Fourier transformed Angenault, and M. Quarton, J. Electrochem. Soc., infrared (FTIR), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). 5. G. Ladwig, Anorg. Allg. Chem., 338, 266 (1965) Electrochemical studies were carried out using a Macfile-II galvanostat system. A Swagelok-type cell
consisted of the mixture of the synthesized
VOPO  4 2H2O, the Super P black (MMM Carbon Co.)
and the polytetrafluoroethylene (70:20:10 wt%) as a
cathode, Li foil as an anode and 0.95 M solution of
LiPF6 in ethylene carbonate/ dimethyl carbonate
(50:50 vol%) as an electrolyte, which was
discharged and charged under the constant current
ranging from 4.16 to 39.4 mA/g.
Results and Discussion
XRD confirms the phase purity of tetragonal VOPO  4 2H2O with a = 6.2 Å and c = 7.4 Å. The particle size of the sonochemically synthesized material is determined to be 1 – 3 µm by TEM, which is smaller than the size (about 10 µm) of the sample prepared by the long-term refluxing method. Electrochemical reduction of VOPO  4 2H2O under a constant current of 4.16 mA/g (corresponding to 0.03 C) shows that almost 1 Li can be inserted, which results in the capacity of 135 mAh/g (Fig. 1). The cell potential shows a plateau at 3.6 V. Little

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