Polymorphism and structural defects in Li2FeSiO4

Abstract

Li₂FeSiO₄, an interesting material with potential applications as the positive electrode in lithium batteries, shows complex crystal chemistry due to the versatility of cation ordering (Li⁺, Fe²⁺, Si⁴⁺) within tetrahedral sites of buckled hexagonal close packed layers of oxygen atoms. This study, conducted through X-ray and electron diffraction experiments, focuses on three samples of Li₂FeSiO₄ (obtained from ceramic synthesis at 700 °C, 800 °C and 900 °C) which may contain significant amounts of structural defects. Two polymorphs of Li₂FeSiO₄ were isolated and investigated through X-ray diffraction and electron microscopy. A new form of Li₂FeSiO₄ (space group Pmnb with a = 6.2853(5), b = 10.6592(8) Å and c = 5.0367(4) Å or alternatively P2₁/n with a = 6.2819(1) Å, b = 10.6575(2) Å, c = 5.0371(1) Å, β = 90.032(7)°) prepared at 900 °C, shows cooperative small displacements of lithium cations from one tetrahedral site (up) to another (down). Attempts to prepare the second, low-temperature, polymorph (space group P2₁/n, a = 8.2253(5) Å, b = 5.0220(1) Å, c = 8.2381(4) Å, β = 99.230(2)°) previously reported by Nishimura et al., lead to crystals exempt of structural defects (at 700 °C) or built up by an intergrowth between the low temperature polymorph and a residue of the high temperature one.