In this work, we revisit the rudimentary approach of using a thin-film electrode system to study solid-state diffusion impedance of the state-ofthe-art LiNi 0.5 Mn 1.5 O 2 (LNMO) electrode materials. Thanks to the welldefined diffusion length and geometry of the thin-film configuration coupled with the advancements of modern characterization techniques, the anomalous diffusion impedance is associated with inhomogeneous solid-state diffusion coefficients originating from different diffusion paths. Atomistic simulation results further suggest the presence of fast grain-boundaries diffusion and slow lattice diffusion within LNMO cathode materials. Moreover, a parallel-diffusion Warburg (PDW) model is proposed to quantify the different solid-state diffusion coefficients. The applicability the PDW model is validated on both thin-film LNMO electrodes and thin-coated commercial LiNi 0.80 Co 0.15 Al 0.05 O 2 (NCA) electrodes.