Influence of structure and composition upon performance of tin phosphate based negative electrodes for lithium batteries

Tin oxide and amorphous tin borophosphates have recently received significant attention as possible new negative electrode materials for lithium batteries. In this study, we have carefully investigated a number of different well-characterised tin phosphates as electrodes in Li-ion cells, in order to better understand the mode of operation of these materials and how their performance is related to structure and composition. The materials that were investigated were crystalline cubic and layered SnP₂O₇, LiSn₂(PO₄)₃, Sn₂P₂O₇, and Sn₃(PO₄)_2, and amorphous Sn₂BPO₆. Cubic SnP₂O₇ showed the best performance with a reversible specific charge capacity of >360 mA h g⁻¹ and a capacity retention of 96% over 50 cycles when cycled between 0.02 and 1.2 V versus Li_m. The three Sn(IV) materials showed lower initial reversible capacity but better capacity retention than the three Sn(II) materials in the study. Their higher proportion of inert matrix material can partly explain this. However, cubic SnP₂O₇ cycled significantly better than its layered polymorph, which shows that the structure of the starting material is also of great importance. Another important conclusion drawn from the results is that it is not necessary for the starting material to be amorphous, or if crystalline, to have small grain size, to cycle well. The three pyrophosphates all show an initial reduction capacity that corresponds to around 2 Li per P₂O₇⁴⁻ unit more than is predicted by theory. This might be explained by reductive break-up of the POP bond.