Thermal plasma synthesis and electrochemical properties of high-voltage LiNi0.5Mn1.5O4 nanoparticles

Abstract

The synthesis of LiNi0.5Mn1.5O4 has been reported to change the crystal structure with the oxygen partial pressure and affect the battery characteristics. LiNi0.5Mn1.5O4 involves the formation of impurities, such as LixNi1−xO, LixMn3−xO4, and Li2CO3, at a high temperature range exceeding 700 °C because oxygen loss occurs during synthesis. LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4 (space group Fd3m) of Mn4+. The crystal structure of the cubic-spinel nanoparticles approached a LiNi0.5Mn1.5O4 single phase as the flow rate of O2 increased from 2.5 to 5 l min−1. The formation of LiNi0.5Mn1.5O4 was shown to be accelerated by increasing the O2 gas flow rate. The measured current–voltage characteristics of LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4 nanoparticles formed in thermal plasma.