Growth Manner of Octahedral-Shaped Li(Ni1/3Co1/3Mn1/3)O2 Single Crystals in Molten Na2SO4

The synthesis of shape-controlled crystals has been a highly attractive research topic in modern materials chemistry. In this work, the growth of Li(Ni1/3Co1/3Mn1/3)O2 (NCM) crystals in molten sulfate or carbonate salts (flux) at 1000 °C was systematically studied under various conditions. In situ X-ray diffraction during the growth and thermogravimetry-differential thermal analysis revealed that the growth of NCM crystals in the flux was controlled by liquid-phase sintering according to the Ostwald ripening principle. We studied the effect of Na+ in the flux on the crystal shapes and found that Na+ was critical in forming octahedral crystals with well-developed facets. Single crystals with well-developed facets were obtained homogeneously from Na2SO4, while truncated polyhedral crystals of smaller size were obtained from Li2SO4. The shape-controlled NCM crystals showed discharge capacities approaching 160 mAh g–1 in the operating voltage range of 2.8–4.4 V vs Li/Li+ under a low current density of 0.1 C, independent of flux composition. This suggests that the Li+ and transition-metal ions in the individual NCM crystals were highly ordered into hexagonal arrangements belonging to the R3̅m space group, without cation mixing.