Solid state NMR studies of phosphate/tin matrix formed on electrochemical insertion into SnP2O7

Tin pyrophosphate was synthesised and tested as an anode in lithium ion batteries. This material has a very good reversible capacity of 360 mA h g−1 with good capacity retention. The source of this good cycling behaviour is thought to be related to the phosphate matrix, which we have investigated in this study. The pyrophosphate is irreversibly reduced during the lithium insertion in the first cycle, converting the phosphorus from PV to PIV as confirmed by XPS, coulometry and IR. 31P MAS NMR indicates a single species near in position to that expected for Li3PO4 but this species would be inconsistent with the observed phosphate reduction. 6Li MAS NMR shows no presence of Li2O. IR exhibits similar bands to pyrophosphate but these are shifted to lower frequency, suggesting a decrease in bond order. Hence, it seems the pyrophosphate linkage retained but the phosphate is reduced, indicating the dominant phosphate species in the matrix is a (P2IVO7)6− species analogous to disilicate.