Structural and electrical studies of B3+-and-In3+-ion co-doped Li1.3Al0.3Ti1.7(PO4)3 solid electrolytes

In this study, B3+ and In3+ ions were co-doped at the Al3+ sites of Li1.3Al0.3Ti1.7(PO4)3 electrolyte using a solid-state reaction with the goal of obtaining electrolytes with remarkable Li-ion conductivities. B3+-ion doping enhanced the densification of Li1.3Al0.3-yByTi1.7(PO4)3 electrolytes, and Li1.3Al0.22B0.08Ti1.7(PO4)3 electrolyte delivered the highest relative density of 97.6%. Subsequent In3+-ion doping further enhanced densification, and Li1.3Al0.21B0.08In0.01Ti1.7(PO4)3 electrolyte delivered the highest relative density of 98.2%. However, the elec-trolyte densification diminished rapidly upon further increasing the In3+-ion content. The peaks in the X-ray diffraction patterns of the Li1.3Al0.3-yByTi1.7(PO4)3 and Li1.3Al0.22-xB0.08InxTi1.7(PO4)3 electrolytes were indexed to rhombohedral NASICON-type LiTi2(PO4)3 with the space group R3c with trace amounts of LiTiPO5 secondary phase. Upon incorporating In3+ ions into the lattice of Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte, the cell volume and the transport channels for Li+ ions were expanded, while B3+-ion incorporation slightly contracted the cell. The scanning electron microscopy and Raman spectroscopy results indicated that B3+-ion substitution into the Li1.3Al0.3Ti1.7(PO4)3 lattice to obtain Li1.3Al0.3-yByTi1.7(PO4)3 electrolytes, which led to the increase in the structural order of the lattices and promoted the formation of microstructures with superior crystallinity and smaller and more uniform grain sizes. Further, B3+ and In3+-ions co-doped Li1.3Al0.22-xB0.08InxTi1.7(PO4)3 elec-trolytes showed lower structural order, larger grain size, lower grain-size uniformity, and more pore contents than those of undoped Li1.3Al0.3Ti1.7(PO4). The ionic conductivities of the Li1.3Al0.3-yByTi1.7(PO4)3 electrolytes increased with increasing concentration of B3+-ion dopant and reached a maximum of 8.35x 10-4 S/cm for Li1.3Al0.22B0.08Ti1.7(PO4)3. Among the Li1.3Al0.22-xB0.08InxTi1.7(PO4)3 electrolytes, Li1.3Al0.21B0.08In0.01Ti1.7(PO4)3 presented the highest ionic conductivity of 1.08 x 10-3 S/cm, which was closely correlated to its high relative density. The contribution of electronic conductivity to total conductivity was found to be trivial.