We report the formation of crystalline dispersible Li x Co1–x O y (with y⩽1) nanoparticles with an unusual rock-salt phase containing ∼15 at. % Li in the crystalline structure. This is the first time that this composition was formed at temperatures as low as 150 °C under conditions of a solvothermal process, although it is referred to as a high-temperature metastable phase in a very limited number of known publications. The Li0.15Co0.85O y nanoparticles of 2–3 nm size completely transform to high-temperature LiCoO2 (HT-LCO) nanoparticles at 560 °C in the presence of slightly overstoichiometric amounts of Li source. The presence of lithium in the CoO lattice slows down the kinetics of its phase transformation, enabling to obtain very small HT-LCO nanocrystals during the subsequent calcination. The HT-LCO particles formed after this transformation have an elongated shape with a mean size of about 17 × 60 nm, which is targeted as an optimum size for battery applications. An attractive feature of the Li0.15Co0.85O y nanoparticles is their high dispersibility enabling their assembly into different nanostructures with optimized morphology. Open porous HT-LCO electrodes prepared via self-assembly of Li0.15Co0.85O y nanoparticles and Pluronic F127 as a structure-directing agent demonstrate very good performances at high current densities representing short charge/discharge times below 10 min. Even at a charge/discharge time of 72 s (50C), 50% of the theoretical capacity has been preserved. After 250 cycles at a charge/discharge time of 6 min (10C), over 60% of the initial discharge capacity was retained.