Al–Sc dual-doped LiGe2(PO4)3 – a NASICON-type solid electrolyte with improved ionic conductivity

Abstract

LiGe₂(PO₄)₃ (LGP), a NASICON-type solid electrolyte, has many advantages such as its superior electrochemical and thermal stability for use in all solid-state lithium batteries. However, its low ionic conductivity is one of the challenges that can hinder its practical application commercially. In this work, the influence of adding different amounts of scandium and aluminum on the Li⁺ conductivity of LGP was investigated computationally and experimentally. Substituting 25% of Ge⁴⁺ ions in the LGP structure with Al³⁺ and/or Sc³⁺ ions to obtain doped LGP in the form of Li_1+x+yAl_xSc_yGe_2−x−y(PO₄)₃, where x + y = 0.5, led to more Li⁺ ions in the 36f vacant sites (M₂) and resulted in enhanced ionic conductivity of the material. In both approaches, the highest bulk Li⁺ conductivity of 5.826 mS cm⁻¹ was obtained for Li_1.5Al_0.33Sc_0.17Ge_1.5(PO₄)₃ from the experimental measurement. The activation energy was also investigated theoretically using the nudged elastic band method, and the lowest value (0.279 eV) was obtained for this composition. Furthermore, the Li_1+x+yAl_xSc_yGe_2−x−y(PO₄)₃ electrolytes were synthesized using a melt-quenching method and subsequently transformed into a glass–ceramic material through heat treatment. X-ray diffraction, electrochemical impedance spectroscopy and cyclic voltammetry were used to characterize the structure, measure the Li⁺ conductivity and determine the electrochemical window of the synthesized glass–ceramic material, respectively. There was a remarkable agreement between the computationally calculated and experimentally measured values of ionic conductivity, activation energy and electrochemical window. Finally, its applicability in a solid-state battery was tested, and it showed good electrochemical performance.