Polyoxyanion compounds, particularly the olivine-phosphate LiFePO 4 , are receiving considerable attention as alternative cathodes for rechargeable lithium batteries. More recently, an entirely new class of polyoxyanion cathodes based on the orthosilicates, Li 2 MSiO 4 (where M ¼ Mn, Fe, and Co), has been attracting growing interest. In the case of Li 2 FeSiO 4 , iron and silicon are among the most abundant and lowest cost elements, and hence offer the tantalising prospect of preparing cheap and safe cathodes from rust and sand! This Highlight presents an overview of recent developments and future challenges of silicate cathode materials focusing on their structural polymorphs, electrochemical behaviour and nanomaterials chemistry.
Small differences in the FeO(4) arrangements (orientation, size, and distortion) do influence the equilibrium potential measured during the first oxidation of Fe(2+) to Fe(3+) in all polymorphs of Li(2)FeSiO(4).
Li(2)FeSiO(4), 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(+), Fe(2+), Si(4+)) 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 Li(2)FeSiO(4) (obtained from ceramic synthesis at 700 degrees C, 800 degrees C and 900 degrees C) which may contain significant amounts of structural defects. Two polymorphs of Li(2)FeSiO(4) were isolated and investigated through X-ray diffraction and electron microscopy. A new form of Li(2)FeSiO(4) (space group Pmnb with a = 6.2853(5), b = 10.6592(8) A and c = 5.0367(4) A or alternatively P2(1)/n with a = 6.2819(1) A, b = 10.6575(2) A, c = 5.0371(1) A, beta = 90.032(7) degrees ) prepared at 900 degrees 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 P2(1)/n, a = 8.2253(5) A, b = 5.0220(1) A, c = 8.2381(4) A, beta = 99.230(2) degrees ) previously reported by Nishimura et al., lead to crystals exempt of structural defects (at 700 degrees C) or built up by an intergrowth between the low temperature polymorph and a residue of the high temperature one.
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