Role of Fe3+ doping vis-à-vis secondary phases on the electrical transport of LiTi2(PO4)3 solid electrolyte

Fast ion conducting solid-electrolytes, with diverse technological applications, have been studied critically in recent years. Among various prototype structures, NASICON structured materials are known for their comparatively high bulk conductivities, which could be further improved by selective substitution at cationic sites. Present work reports the effect of Fe3+ doping at the Ti4+ sites vis-à-vis secondary phases on the ionic conductivity of NASICON structured lithium titanium phosphate (LiTi2(PO4)3 or LTP) solid electrolyte. Li1+xTi2−xFex(PO4)3 (x = 0.0, 0.1 and 0.2) was synthesized using the solid-state reaction method. Crystal structure, morphology, chemical composition, and ionic conductivity were studied using room-temperature powder X-ray diffraction (p-XRD), field emission scanning (FESEM) and high-resolution transmission (HRTEM) electron microscopy, and temperature-dependent impedance spectroscopy. Very low bulk activation energies were found for all the samples, attributed to interstitial diffusion via a concerted migration. The room-temperature ionic conductivity initially increased upon Fe3+ doping (x = 0.1) and dropped subsequently (x = 0.2). The aberrant growth of electrolyte grains, associated gas pores, and cracks formed during sintering were successfully reduced by the LiTiOPO4 phase formation upon Fe doping, initially raising the grain boundary conductivity. However, doped samples also showed segregation of another secondary phase, Li2FeTi(PO4)3, whose larger weight fraction at x = 0.2 severely restricted the Li-ion migration resulting in sudden conductivity loss. These results suggest the need to optimise the microstructure, especially the amount of secondary phases, which contribute to the grain boundary resistance, affecting the ionic conductivity of the samples.