Enhanced lithium ionic conductivity and study of the relaxation and giant dielectric properties of spark plasma sintered Li5La3Nb2O12 nanomaterials

Nanocrystalline Li5La3Nb2O12 (LLN) lithium ion conductor with garnet-like structure is synthesized by mechanical milling and solid state reaction routes. The nanopowder has an average grain size of 26 nm. The product nanopowder is sintered by conventional (at 900 °C) and spark plasma sintering (SPS) at different temperatures of 800 °C, 850 °C and 900 °C. The conventionally sintered ceramics have coarse grained structure with grain size of 1–2 μm, whereas the SPS ceramics have nano-sized grains with grain size as small as 50–100 nm at low SPS temperatures. The electrical properties have been studied by impedance spectroscopy measurements. The SPS-850 ceramics exhibit ionic conductivity value of 3.7×10−5 S/cm at 27 °C, which is one order of magnitude larger than the conventionally sintered sample. For the first time we explore two regions of the temperature dependent conductivity below and above room temperature with different activation energies, most probably due to two Li+ ions migration pathways. The relaxation properties of the studied materials have been analyzed in the conductivity and electric modulus formalisms. Different factors are suggested to contribute to the enhanced conductivity including the hopping frequency and the concentration of mobile Li+ ions.