Li9Mg3[PO4]4F3 and Li2Mg[PO4]F: Crystal Structures and Ionic Conductivities

Abstract

In oxide electrolytes, crystalline phosphate with NASICON (NaSuper Ionic Conductor)-type structures such as Li1+xAlxTi2 −x(PO4)3 (LATP) and Li1+xAlxGe2 −x(PO4)3 (LAGP) are known as excellent Li ion conductors. Perovskite Li0.5 −3xLa0.5+xTiO3 (LLT) exhibits the high level of bulk conductivity of 10−3 S cm−1, but LATP and LLT electrolytes have Ti element in crystalline structure, and thus transition metal such as Ti is easily reduced by lithium metal negative electrode; it is thus difficult for these electrolytes to use in lithium metal batteries.Recently, we started the systematic study of crystal structure and the electrochemical properties of fluorophosphate phases which do not contain transition metal elements. Indeed, in the ternary phase diagram LiMgPO4-LiF-Li3PO4, two new lithium rich phases have been discovered (Li2Mg[PO4]F and Li9Mg3[PO4]4F3). Li2Mg[PO4]F crystallizes with the well-known Li2Ni[PO4]F-type structure, whereas the Li9Mg3[PO4]4F3 crystallizes with a new-type of structure. Li9Mg3[PO4]4F3 exhibits an ionic conductivity of 10-4 S cm-1 at 300 °C, which is much higher than that of Li3VO4 (σ 300 °C =1.6x10-6 S cm-1), γ-Li3PO4 (σ 300 °C =2.2 ´ 10-8 S cm-1) or Li2SO4 (σ 300 °C =2 ´ 10-7 S cm-1). This new phase would be also of great interest as positive electrode for Li-ion batteries if Mg2+ could be replaced by Fe2+ or Mn2+cations [1].[1] Crystal structures of the new fluorophosphates Li9Mg3[PO4]4F3 and Li2Mg[PO4]F and ionic conductivities of selected compositions, H. Ben Yahia, M. Shikano, T. Takeuchi, H. Kobayashi, M. Itoh, J. Mater. Chem. A (2014)DOI:10.1039/C3TA15264B.