Synthesis and characterisation of some novel lithium-containing solid electrolytes
A range of γ-LISICON type solid solutions of general formula Li_4-3xGa_xGeO_4 have been prepared which extend from compositions close to, but not including, Li_4GeO_4, up to Li_2.5Ga_0.5GeO_4 at high temperatures. Phase diagram studies, which included comprehensive studies of the join Li_4GeO_4-Li_2.5GeO_4 and the binary join Li_2GeO_3-LiGaO_2, were carried out. Lattice parameter data for the solid γ-solid solution are presented along with data for a new phase β-Li_2.5Ga_0.5GeO_4 and an ordered γ-phase, C-Li2.8Ga0.4GeO4. The structures of γ and β-Li2.5Ga0.5GeO4 are stoichiometric derivatives of γ and β-Li3PO4, as shown by using Rietveld refinement of X-ray powder diffraction data. Both phases show some Li / Ga disorder, which is unusual for stoichiometric compounds and the γ-phase may be regarded as 'entropy-stabilised'. A discussion of possible mechanisms for theβ↔γ transition is also given. These γ-LISICON solid solutions exhibit high lithium ion conductivity; the maximum conductivity was observed for the composition, Li_3.7Ga_0.1GeO_4. Conductivity Arrhenius plots are markedly non-linear and conductivity data depend on sample pre-treatment below ~260°C. The sub-solidus phase relations in the ternary system Li2O-TiO2-P2O5 were investigated to study the stoichiometry of LiTi2(PO4)3, which is a controversial lithium ion conducting material. One new ternary phase, LiTiPO5 and its arsenate analogue, LiTiAsO5 were synthesised and their structures determined by X-ray Rietveld refinements. These phases are isostructural with α-Fe_2PO_5. Conductivity studies showed these new materials to be insulators. Finally, a range of solid solutions between LiTi_2(PO_4)_3 and TiNb(PO_4)_3 have been prepared, which have the general formula: Li_1-xTi_2-xNb_x(POP_4)_3. A partial phase diagram study in the ternary system TiO_2-Nb_2O_5-P_2O_5 was carried out. Lattice parameter data for the solid solution Li_1-xTi_2-xNb_x(PO_4)_3 is presented. Conductivity results indicate that the materials are modest lithium ion conductors; maximum conductivity is observed for the composition x = 0.8.