Thermal plasma synthesis and electrochemical properties of high-voltage LiNi_0.5Mn_1.5O₄ nanoparticles

The synthesis of LiNi_0.5Mn_1.5O₄ has been reported to change the crystal structure with the oxygen partial pressure and affect the battery characteristics. LiNi_0.5Mn_1.5O₄ involves the formation of impurities, such as Li_xNi_1-xO, Li_xMn_3-xO₄, and Li₂CO₃, at a high temperature range exceeding 700 °C because oxygen loss occurs during synthesis. LiNi_0.5Mn_1.5O₄ electrochemically contains Mn⁴⁺, however, Mn³⁺ is formed because of oxygen deficiency. The Li-Ni-Mn-oxide causes a disproportionation of Mn³⁺ in an oxygen-deficient state. The synthesized Li-Ni-Mn-oxide nanoparticles at 10,000 K by induction thermal plasma formed spinel-type LiNi_0.5Mn_1.5O₄ (space group Fd3m) of Mn⁴⁺. The crystal structure of the cubic-spinel nanoparticles approached a LiNi_0.5Mn_1.5O₄ single phase as the flow rate of O₂ increased from 2.5 to 5 l min^-1. The formation of LiNi_0.5Mn_1.5O₄ was shown to be accelerated by increasing the O₂ gas flow rate. The measured current-voltage characteristics of LiNi_0.5Mn_1.5O₄ nanoparticles appeared at around 4.7-4.8 V as the reaction peak of Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺. In contrast, the Mn of the Li-Ni-Mn-oxide nanoparticles synthesized in the oxygen-deficient state was less than trivalent, which caused disproportionation of Mn. The measured current-voltage characteristics showed peak of an oxygen desorption at near 4.6 V. This study investigated the factors affecting the crystal structure formation and electrochemical properties of high-voltage LiNi_0.5Mn_1.5O₄ nanoparticles formed in thermal plasma.