Electrochemical performance of amorphous and crystalline Sn2P2O7 anodes in secondary lithium batteries

Amorphous and crystalline Sn2P2O7 can be considered as intrinsic P-doped glasses, similar to the products from the binary solid stare reaction between SnO and P2O5. Amorphous Sn2P2O7 WaS prepared by melt-quenching the crystalline form. IR spectroscopy and inductively coupled plasma measurements indicated that both forms are phase pure homogeneous materials. In the potential range 0-1.2 V (vs. Li+/Li), amorphous and crystalline Sn2P2O7 can deliver reversible specific capacities of 520 and 400 mAh/g, respectively. However, both display nearly the same capacity and fade characteristics when the upper cutoff limit is increased to 1.4 V. Cyclic voltammetry and differential capacity plots indicated the presence of two energetically different Li+ sites in crystalline Sn2P2O7 A higher potential is required for the complete release of Li+ in crystalline Sn2P2O7 and this explains the lower capacity of this polymorph. X-ray diffraction detected the presence of tin and tin alloy phases in charged and discharged samples, and an alloying mechanism is proposed to explain the reversibility in charge and discharge reactions. Dissociation of P2O74- into PO43- and PO3- after repeated cycling was evidenced by IR spectroscopy. The large initial capacity loss could be explained by the requisite initial irreversible reaction to form metallic tin. (C) 1999 The Electrochemical Society. S0013-4651(99)02-014-5. All rights reserved.