Combination of neutron diffraction, impedance spectroscopy and DFT analysis of lithium diffusion pathways provides an understanding of lithium-ion mobility in layered oxides through a case study of Li₂La(TaTi)O₇. This new material is a Li-conducting oxide, which contains stacks of (Ta/Ti)O₆ octahedra separated by a layer of lithium ions, forming a Ruddlesden–Popper type structure. Experiments show that two strategies, i.e., shortening the Li hopping distance and inducing defects in the Li-layer, can successfully improve the ionic conductivity. The DFT analyses reveal the orientation of lithium diffusion pathways and the energy barriers in these pathways, which are directly correlated with the atomic arrangement of this material. These results have broad implications with regard to the design of a new class of Li-conducting oxides based on Ruddlesden–Popper oxides.