Effect of MgO addition on the ionic conductivity of LiGe₂(PO₄)₃ ceramics

Abstract

Effect of the addition of MgO, along with Li₂O, as sintering aid on the densification and ionic conductivity of Li-analogues of Nasicon has been studied. For this purpose, a series of compounds, LiGe₂(PO₄)₃ (LGP)+x(MgO)+y(Li₂O), x=y=0-0.7; x=0.2, 0.5 and y=0; x=0 and y=0.2, 0.5 and Li_1.4[Ge_1.6Al_0.4](PO₄)₃ (LAGP)+x(MgO)+x(Li₂O), x=0-0.3, have been synthesized and characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy (XPS), density, porosity and microstructure (by scanning electron microscope, SEM) of the sintered pellets. Results show that the additives do not get incorporated in to the crystal lattice of the Nasicon but act as a binder and drastically reduce the porosity of the pellets and improve connectivity between the grains. Binder addition enhances the ionic conductivity (σ_ionic: bulk, σ_B and total, σ_T) at any given temperature in the LGP series and the σ_T reaches 1.2×10⁻⁴ S cm⁻¹ at 100 °C in LGP 5, which is three orders of magnitude higher than that shown by LGP 0 at the same temperature. Similar enhancement of σ_B is also seen. However, the respective energies of activation, E_a(T) and E_a(B) behave differently with the binder addition. The LAGP series also show an enhancement of σ_T with the binder addition, by a factor of two and the E_a(T) remains almost unchanged. Present study shows that MgO acts as a binder and helps in reducing the porosity and improving the σ_ionic of the Li-Nasicon phases.