Structural study of monoclinic Li2FeSiO4 by X-ray diffraction and Mössbauer spectroscopy

•Monoclinic Li2FeSiO4 polymorph that crystallizes in P21/n space group is analyzed.•A combined X-ray diffraction and Mössbauer spectroscopy study reveal the antisite defect.•Small part of iron ion occupies exclusively Li(2) crystallographic position.•The bond-valence energy landscape suggests that Li conductivity is two-dimensional in the (101) layer.

A composite powder Li2FeSiO4/C is synthesized through a solid state reaction at 750 °C. The Rietveld crystal structure refinement is done in the monoclinic P21/n space group. It is found that the crystal structure is prone to “antisite” defect where small part of iron ion occupies exclusively Li(2) crystallographic position, of two different lithium tetrahedral positions (Li(1) and Li(2)). This finding is also confirmed by Mössbauer spectroscopy study: the sextet evidenced in the Mössbauer spectrum is assigned to the iron ions positioned at the Li(2) sites. A bond-valence energy landscape calculation is used to predict the conduction pathways of lithium ions. The calculations suggest that Li conductivity is two-dimensional in the (101) plane. Upon galvanostatic cyclings the structure starts to rearrange to inverse βII polymorph.