Controlling Particle Size and Phase Purity of “Single-Crystal” LiNi0.5Mn1.5O4 in Molten-Salt-Assisted Synthesis

Spinel-structured lithium nickel manganese oxide (LiNi0.5Mn1.5O4, LNMO) is a low-cost battery cathode material with a high operating voltage (∼4.7 V vs Li+/Li), relatively high capacity (∼147 mA h g–1), and good rate performance. Previous studies have suggested that large particle size, single-crystal-like morphology, and high phase purity are favorable for achieving good electrochemical performance. However, simultaneous control over these properties is difficult for conventional solid-state synthesis at high temperatures. Here, we report a molten-salt-assisted method to prepare large-size (median grain size D50 = 16.8 μm) single-crystal LNMO with molten lithium molybdate (Li2MoO4) serving as the medium of ion diffusion and crystal growth. In situ X-ray diffraction (XRD) studies of the material preparation process reveal structural disorder and formation of the rock-salt impurity phase at high temperatures, which are reverted upon cooling. The correlations between the cooling rate and structural ordering/phase purity at the single-particle level are further studied using correlative scanning electron microscopy and Raman spectroscopy (SEM-Raman) techniques. SEM-Raman studies for the first time reveal that the Ni-rich rock-salt impurity phase actually exists at the interior of the large-size LNMO particles, which is difficult to be detected by conventional XRD or Raman spectroscopy because of limitations in probing depth. We further confirm that slow cooling is the key to increasing structural ordering and enhancing electrochemical performance of LNMO. The optimized LNMO sample shows excellent cycle performance by retaining ∼85% initial capacity after 300 charge–discharge cycles and acceptable level of rate capability despite its large particle size. Our results highlight the importance of mechanistic studies into material synthesis, which provide the basis for designing better materials and more efficient preparation methods.