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

In oxide electrolytes, crystalline phosphate with NASICON (NaSuper Ionic Conductor)-type structures such as Li_1+xAl_xTi_2−x(PO₄)₃ (LATP) and Li_1+xAl_xGe_2−x(PO₄)₃ (LAGP) are known as excellent Li ion conductors. Perovskite Li_0.5−3xLa_0.5+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.