Effects of Si4+ and Co3+ co-doping on the structural and electrical conductivity of Li1.3Al0.3Ti1.7(PO4)3 ceramic electrolytes

Abstract

In this study, we investigated the co-doping of Si⁴⁺ and Co³⁺ ions at the P⁵⁺ and Al³⁺ sites of the Li_1.3Al_0.3Ti_1.7(PO₄)₃ electrolyte to enhance Li-ion conductivities. Si⁴⁺ doping increased the maximum relative density of the sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ electrolyte, although it slightly raised the densification temperature. The Li_1.35Al_0.3Ti_1.7P_2.95Si_0.05O₁₂ electrolyte achieved 96.1% theoretical density at 925 °C. Subsequent doping with Co³⁺ ions further improved densification, with the Li_1.35Al_0.29Co_0.01Ti_1.7P_2.95Si_0.05O₁₂ electrolyte reaching a peak relative density of 97.5% at 850 °C. However, higher levels of Si⁴⁺ or Co³⁺ ions significantly degraded densification. X-ray diffraction results of the Li_1.3+xAl_0.3Ti_1.7P_3−xSi_xO₁₂ and Li_1.35Al_0.3−yCo_yTi_1.7P_2.95Si_0.05O₁₂ electrolytes identified a LiTi₂(PO₄)₃ phase with rhombohedral NASICON-type structure belonging to the R\(\:\overline{3}\)c space group, along with a small quantity of the LiTiOPO₄ phase. Substitution of Si⁴⁺ ions into the Li_1.3Al_0.3Ti_1.7(PO₄)₃ lattice increased Li⁺ ion transport channels due to the expansion of the cell volume. Raman spectroscopy and scanning electron microscopy revealed that the addition of both dopants improved the microstructure by enhancing the lattice structural order and leading to a higher degree of uniformity and smaller grains and higher crystallinity. X-ray photoelectron spectroscopy demonstrated that the incorporation of Si⁴⁺ and Co³⁺ ions into LATP lattice inhibited the reduction of Ti⁴⁺ ions to Ti³⁺ ions. The ionic conductivities of Li_1.3+xAl_0.3Ti_1.7P_3−xSi_xO₁₂ electrolytes increased with higher Si⁴⁺-ion concentrations, reaching of 5.1 × 10^− 4 S/cm at 25 °C. In the case of co-doping of Si⁴⁺ and Co³⁺ ions, Li_1.35Al_0.29Co_0.01Ti_1.7P_2.95Si_0.05O₁₂ composition exhibited the highest ionic conductivity of 1.0 × 10^− 3 S/cm at 25 °C. The enhanced electrical conductivities were closely associated with high relative densities, excellent grain size uniformity, improved grain connectivity, and suppression of lower Ti⁴⁺ ion reduction.