Electrochemical performance of amorphous and crystalline Sn 2P 2O 7 anodes in secondary lithium batteries

Abstract

Amorphous and crystalline Sn 2P 2O 7 can be considered as intrinsic P-doped glasses, similar to the products from the binary solid state reaction between SnO and P 2O 5. Amorphous Sn 2P 2O 7 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 Sn 2P 2O 7 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 Sn 2P 2O 7. A higher potential is required for the complete release of Li + in crystalline Sn 2P 2O 7 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 P 2O 7 4- into PO 4 3- and PO 3 - 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.