Understanding the Formation of the Truncated Morphology of High-Voltage Spinel LiNi0.5Mn1.5O4 via Direct Atomic-Level Structural Observations

High-voltage spinel LiNi0.5Mn1.5O4 cathode materials typically exhibit a perfect octahedral morphology; i.e., only the {111} planes are observed. However, a truncated octahedral morphology is sometimes observed with the appearance of both the {100} planes and the {111} planes. The underlying mechanism of this morphological transformation is unclear. CS corrected scanning transmission electron microscopy (STEM) techniques were used to study LiNi0.5Mn1.5O4 samples lifted by a focused ion beam (FIB) to determine the atomic-level crystal and electronic structures of the octahedral and truncated octahedral morphologies. STEM images directly show that the appearance of the {100} planes in the truncated octahedral particles of LiNi0.5Mn1.5O4 is closely associated with the atomic-level migration of Ni and Mn ions in the surface region. The STEM electron energy loss spectroscopy (EELS) confirms the presence of oxygen-deficient and Ni-rich areas, particularly in the region close to the newly formed {100} planes. The formation of the {100} planes is sensitive to residual SO42- ions on the surface originating from the sulfates used to prepare LiNi0.5Mn1.5O4. The presence of a small amount of SO42- inhibits the formation of {100} planes. First-principles computer simulations reveal that the adsorption of SO42- on the LiNi0.5Mn1.5O4 surface results in a reduction in the energy required for the formation of the {111} planes. Furthermore, the two O atoms of SO42- can form bonds, improving the stability of the low-coordinated Ni/Mn ions on the {111} planes.