In 1994, Fuji Photo Film Co., Ltd., Japan, filed a patent for nonaqueous Li-ion batteries in which tin composite oxides (TCOs) were used as the active anode material. 1 Since then, much attention has been given to the potential of substituting this class of compounds for carbon negative electrodes in Li-ion batteries. Tin oxide composites are attractive because of their high gravimetric and volumetric specific capacities relative to carbon, which enable cells of higher energy densities to be built. 2 It is reported 2 that most tin composite oxides are amorphous and the Sn(II)-O sites are active for lithium insertion. Although some early Li 7 NMR measurements have been rationalised in terms of an intercalation mechanism for charge and discharge reactions, more recent experimental results based on X-ray diffraction (XRD) analyses of the active materials at different stages of charge and discharge are more satisfactorily explained by the alloying mechanism made famous by Dahn and associates. 3,4 The alloying mechanism assumes the formation of a lithia (Li 2 O) matrix mostly during the first charging cycle. This matrix binds the lithium-tin regions together and smoothens the large volume changes associated with alloying and dealloying in reversible charge and discharge reactions.Both pristine and doped tin oxides 2-6 have been investigated as anode materials. All of these materials show reversible capacities higher than 500 mAh/g but large capacity losses in their first cycles. They were synthesized by classical solid state reactions involving two or more components. In this study, we consider amorphous and crystalline Sn 2 P 2 O 7 as intrinsic P-doped tin oxide composites, and investigated their performance as anode materials for Li-ion batteries. We have chosen Sn 2 P 2 O 7 to circumvent the mixing problem inherent in most solid state reactions. We are particularly interested in the effects of crystallinity on the electrochemical performance of Sn 2 P 2 O 7 . ExperimentalMaterials preparation.-Crystalline Sn 2 P 2 O 7 (98%, Aldrich) was used as received. For the preparation of amorphous Sn 2 P 2 O 7 , crystalline Sn 2 P 2 O 7 was pelletized at 2 ϫ 10 6 Pa and fired in flowing nitrogen in a Lindberg/Blue M tube furnace. After 2 h at 900ЊC, the melt was removed from the furnace and immediately quenched in air between two stainless plates at room temperature. The solid thus formed was carefully ground to powder and the above procedure repeated once more using the ground Sn 2 P 2 O 7 powder as the starting material, and eight more hours of heating at 900ЊC.Composition and structure determinations.-Elemental composition was determined by inductively coupled plasma (ICP) spectroscopy on a Perkin Elmer Optima 3000DV, using digestions of the melt-quenched samples in 1:1 (by volume) mixture of HCl/HNO 3 at 60ЊC. The crystal structure of Sn 2 P 2 O 7 was confirmed by XRD, using a Philips PW1710 diffractometer and Cu K␣ X-ray source ( ϭ 1.54 Å). The 2 range was initially set at 10-70Њ, but was subsequently narrowed to 10-42Њ ...
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