The high theoretical energy density of Li-O2 batteries as required for electrification of transport has pushed Li-O2 research to the forefront. The poor cyclability of this system due to incomplete Li2O2 oxidation is one of the major hurdles to be crossed if it is ever to deliver a high reversible energy density. Here we present the use of nano seed crystallites to control the size and morphology of the Li2O2 crystals. The evolution of the Li2O2 lattice parameters during operando X-ray diffraction demonstrates that the hexagonal NiO nanoparticles added to the activated carbon electrode act as seed crystals for equiaxed growth of Li2O2, which is confirmed by scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX) elemental maps also showing preferential formation of Li2O2 on the NiO surface. Even small amounts of NiO (∼5 wt %) particles act as preferential sites for Li2O2 nucleation, effectively reducing the average size of the primary Li2O2 crystallites and promoting crystalline growth. This is supported by first principle calculations, which predict a low interfacial energy for the formation of NiO-Li2O2 interfaces. The eventual cell failure appears to be the consequence of electrolyte side reactions, indicating the necessity of more stable electrolytes. The demonstrated control of the Li2O2 crystallite growth by the rational selection of appropriate nano seed crystals appears to be a promising strategy to improve the reversibility of Li-air electrodes.