Lithium ion conducting solid electrolytes
Lithium ion conducting solids have potential application as the solid electrolytes in new battery systems. A systematic study has been made of solid solution formation, phase diagrams and conductivity in four lithium containing systems. Two of these: LiI-Aℓ₂O₃ and Li₄SiO₄-Li₃PO₄ have been studied previously and one-LiI/Aℓ₂O₃ is used in commercial batteries. The other two systems: Li₄Si₄O-Li₃AsO₄ and Li₄SiO₄-Li₃VO₄ are new. The phase diagrams of the systems Li₄SiO₄/Li₃PO₄, Li₄Si₄O/Li₃AsO₄ and Li₄SiO₄/Li₃VO₄ were determined. They are of binary eutectic character and each contains limited regions of solid solutions of Li₄SiO₄ (silicate structure) and the Li₃XO₄ end member (γ structure). The a.c. conductivity of polycrystalline samples was measured over the range of 0 to ~ 300°C for about ten compositions, in each system. Both solid solutions series in all three systems have much higher conductivities than the pure end members. Maximum conductivities were observed in compositions ~35 to 75% Li₃XO₄, with values of 2 x 10⁻⁶ ohm⁻¹cm⁻¹ at 25°C, rising to 2 x 10⁻² ohm⁻¹cm⁻¹ at 300°C for Li₄SiO₄/Li₃PO₄ system. These values increase by factors ~2 and 4 in the systems Li₄Si₄O/Li₃AsO₄ and Li₄SiO₄/Li₃VO₄ respectively. Arrhenius prefactors and activation energies are discussed in terms of the mechanism of conduction. The effect of preparation conditions and starting materials on the conductivity of LiI/Aℓ₂O₃ electrolyte was studied systematically. It was found that use of high surface area α-alumina is necessary in order to achieve highly conducting LiI/Aℓ₂O₃ electrolyte. It is better to prepare samples by heating in a static gas atmosphere rather than in flowing gas, in order to avoid loss of iodine. Experiments should be carried out in dry box conditions in order to avoid moisture attack. The highest conductivity found for LiI/Aℓ₂O₃ electrolyte in this work is ~4 x 10⁻⁵ ohm⁻¹cm⁻¹ at 25°C and rising to 1 x 10⁻³ ohm⁻¹cm⁻¹ at 70°C.