Composition and Oxidation State Changes of NCM Materials over Cycling via Simultaneous EDS-EELS 3D Tomography

Battery life and capacity are paramount factors in determining the efficacy of a battery [1]. Lithium nickel cobalt manganese oxide (NCM) has emerged as a highly promising cathode material, renowned for its extended battery life and improved capacity [2-3]. Despite the significance of structural degradation as a governing factor of battery life and capacity, the precise mechanism remains largely elusive, although several degradation mechanisms have been suggested [4-5]. In this research, we simultaneously measured energy dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) data during a single tomography tilt series acquisition utilizing scanning transmission electron microscopy (STEM) (see Fig. 1 for more details). The simultaneous EDS-EELS tomography allowed us to obtain multi-modal structural information regarding the 3D composition distribution (see Fig. 2a-d) and 3D oxidation states (see Fig. 2e-f) of elements without the need for repeated electron exposure. The multi-modal 3D structural information obtained from varying cycles holds immense potential for providing a clearer understanding of the degradation mechanism of NCM materials [6]. A schematic representation of simultaneous EDS-EELS 3D tomography. HAADF tilt series, EDS tilt series of each element, and EEL spectra tilt series are simultaneously acquired during a single tilt series measurement. After the 3D reconstruction, the 3D HAADF density map, 3D elemental composition maps, and 3D oxidation state maps for each element are obtained. The 3D rendering of elemental composition and oxidation state maps of NCM111 particles is depicted. The panel (a) to (d) show the elemental composition maps of a selected particle in pristine state, and map after 50, 100, and 200 cycles, respectively. The 3D elemental composition changes over the cycles are depicted for (a) O, (b) Mn, (c) Co, and (d) Ni, with a 50 nm scale bar in panel (a). The 3D oxidation state maps of (e) Mn and (f) Co from the selected particle after 200 cycles are also displayed. In panels (e) and (f), the images represent the 3D oxidation state maps of Mn2+, Mn3+, Mn4+, and the overlapped Mn oxidation states, as well as Co2+, Co3+, and the overlapped Co oxidation states, respectively.