Fast Li-ion conduction at grain boundaries in (La,Li)NbO3 polycrystals

Abstract

We have investigated optimum synthesis condition and the lithium-ion conductivity of lithium lanthanum niobate (LLNbO) polycrystals which is a candidate for solid-state electrolyte in all solid-state Li-ion secondary batteries. The LLNbO polycrystals sintered at 1470 K for 12 h show the highest sintered density, the largest average grain size and the largest ionic conductivity in both the bulk and at the grain boundary. Unlike the other oxide electrolytes, the Li-ion conductivity at the grain boundary is three-times larger than that in the bulk, which is statistically confirmed by using random walk Metropolis Hastings algorithm. We also evaluated the activation energies for Li-ion conduction in LLNbO polycrystals and we found the activation energy at the grain boundary is also 1.5 times lower than that in the bulk. Atomic resolution electron microscopy elucidates that the conventional (La,Li)TiO3 has a strong La-enrichment at the grain boundary, while in LLNbO no significant La-enrichment was observed at the grain boundaries. The present results suggest that the local atomic configurations at the grain boundary strongly affects the Li-ion conductivity and it is therefore important to control the local atomic structures in polycrystalline electrolytes.