Ceramic/polymer nanocomposites are promising materials for energy storage applications. The most common approach to optimize energy storage properties relies on combining a ceramic and a polymer having the highest possible dielectric permittivity and breakdown strength, respectively. It is also known that another significant factor is the aspect ratio of nanofillers where a higher aspect ratio gives rise to a higher effective dielectric permittivity of the composite. There is thus a duality between the dielectric permittivity of the ceramic and its aspect ratio that we propose to study in this paper. To achieve this, high-k nanocomposites were designed based on biodegradable polymer matrix polylactic acid (PLA) and different inorganic nanofillers having different shapes (spherical, rod and wire) and dielectric properties. The effects of the aspect ratio, core–shell structure, dielectric permittivity and volume fraction of the nanofillers on the dielectric and energy storage performances of PLA-based nanocomposites were assessed. An enhanced energy storage density and recovered energy density of 3.63 and 1.80 J cm−3, respectively, were obtained in nanocomposites based on rod-like fillers. We discussed, from a theoretical model, that, below the percolation threshold, the obtained properties resulted from a compromise between both the dielectric permittivity and the aspect ratio of the ceramic. This investigation provides an exciting opportunity to advance our knowledge of ceramic/polymer nanocomposites for energy storage applications.< Réduire