TIN OXIDE BASED COMPOSITE ANODES FOR LITHIUM ION BATTERIES

Abstract

Simple melt-quench procedures were used to prepare three tin based composite oxides (TCOs), namely amorphous Sn2P2O7, amorphous Sn2BPO6 and amorphous Sn2B2O5, which were subsequently investigated for their suitabilities as anode materials in rechargeable lithium ion batteries. Amorphous Sn2P2O7 and crystalline Sn2P2O7 can be considered as intrinsic P doped glasses, similar to the product from the binary solid state reaction between SnO and P2O5. Amorphous Sn2P2O7 was prepared by melt - quenching the crystalline form. IR spectroscopy and inductively coupled plasma (ICP) measurements indicated that both forms are homogeneous materials without phase separation. In the potential range of 0 to 1.2V (vs. Li/Li+), amorphous Sn2P2O7 delivers a reversible capacity of 520mAh/g which is higher than that of crystalline Sn2P2O7 (400mAh/g). However, both display nearly the same capacity and fade characteristics with cycling when the upper cut-off limit is increased to 1.4 V. Cyclic voltammetry and differential capacity plots indicated the presence of two energetically different sites for Li+ in crystalline Sn2P2O7. A higher potential is required for the complete release of Li+ in the crystalline form and this may explain the lower capacity of the latter relative to amorphous Sn2P2O7. X-ray diffraction (XRD) measurements 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 also indicated by IR spectroscopy. BPO4 was used as an intrinsic mixture of B2O3 and P2O5 to react with SnO to form amorphous Sn2BPO6, and to alleviate the latter formation via the ternary reaction between SnO and the volatile oxides of P2O5 and B2O3. Amorphous Sn2B2O5 was also synthesized by melt-quenching, using a mixture of SnO and B2O3. At low currents, Sn2P2O7 and Sn2BPO6 show better capacity retention than Sn2B2O5. The propensity of Sn2B2O5 for phase transition has led to the aggregation of tin and tin-lithium alloys into large particles. Experimentally, this was supported by evidence from XRD and differential capacity plots. At high currents and high potentials, Sn2BPO6 has the best capacity and cycle life among the three tested TCOs. This is perhaps due to the structural integrity of the BPO64- framework in repeated cycling (unlike P2O74- and B2O54-, IR spectroscopy did not identify any BPO64- dissociation). Again, the reversibility in charge and discharge reactions from the second cycle onwards is best explained by the alloying mechanism. For all of the TCOs investigated, a large initial capacity loss (ICL) still exists. This is attributed to the requisite initial irreversible reaction between lithium and TCOs to disperse tin in a Li2O or lithium oxyanion environment. Other capacity loss mechanisms such as passive layer formation may also be present to account for the large experimental ICL values.