Phase equilibrium during the synthesis of LiNi0.46Mn1.54O4: comprehensive X-ray & neutron powder diffraction study

Abstract

A combination of synchrotron X-ray powder diffraction (SXRPD) and neutron powder diffraction (NPD) is used to investigate phase equilibrium during the synthesis of LiNi_0.46Mn_1.54O₄ (LNMO) powders from a reagent mixture. A Li-deficient disordered LNMO begins to form at T ≈ 460 °C and as the temperature increases, oxygen release triggers the formation of impurity phases. Advanced structural characterization of quenched LNMO samples, along with in situ SXRPD experiments, reveals that a layered oxide impurity crystallizes between 700 °C and 900 °C. At temperatures of 900 °C and above, this impurity phase transforms into a rock-salt type one, while a Li-rich layered oxide impurity also emerges. This leads to the coexistence of three phases at T ≥ 900 °C: LNMO spinel, rock salt, and Li-rich layered oxide. These transformations affect significantly the composition of the targeted LNMO spinel phase, which highlights the challenges in achieving phase purity with the desired stoichiometry in this complex system. The findings provide valuable insights for optimizing the LNMO synthesis so as to prepare high-performance positive electrode materials.