Lithium-based oxide nanoparticles have recently shown significant advantages as cathode materials for lithium ion batteries, showing higher ion exchanging rates related to the high surface area. Among them, LiNiO2 has been considered an attractive candidate due to its relatively low cost, high discharge capacity, reversibility, and low toxicity. However, the synthesis of nanosized LiNiO2 typically favors the formation of Li2CO3 and NiO phases, which critically affect the performance of the cathode nanoparticles. In this work, we describe the synthesis of lithium nickel oxide nanoparticles using a modified polymeric precursor method. As the formation of NiO and Li2CO3 was unavoidable, high temperatures would be required to obtain a carbonate-free LiNiO2. In order to avoid large coarsening of the particles associated with those treatments, samples were treated at lower temperatures and cleaned from surface Li2CO3 contaminants using acidic washing. The procedure successfully removed the carbonate, and also resulted in crystallite size reduction (28.1–15.2 nm) and controlled delithiation, simulating the lithium deficient conditions during electrochemical lithium displacement.