Nanoscale Mapping of Transition Metal Ordering in Individual LiNi_0.5Mn_1.5O₄ Particles Using 4D-STEM

The electrochemical performance of the spinel LiNi_0.5Mn_1.5O₄, a high-voltage positive electrode material for Li-ion batteries (LIBs), is influenced by the transition metal arrangement in the octahedral network, leading to disordered ( $Fd\underline{3}m$ S.G.) and ordered (P4₃32 S.G.) structures. However, widely used techniques lack the spatial resolution necessary to elucidate the ordering phenomenon at the particle scale. Using the 4D-STEM technique, we present the first direct observation of ordering distribution in individual LiNi_0.5Mn_1.5O₄ particles with nanometric spatial resolution. We propose a quantification method for the local degree of ordering based on the ratio of ordered to disordered spinel lattices along the particle thickness extracted from electron diffraction spot intensities. In an ordered spinel LiNi_0.5Mn_1.5O₄, the transition metal ordering is consistently observed throughout the primary particle. However, the extent of ordering in the spinel phase depends on its distribution at the particle scale, a factor influenced by the annealing conditions. The 4D-STEM analysis elucidates the boundary between highly-ordered and low-ordered LiNi_0.5Mn_1.5O₄ particles.