Elucidating the Role of Crystallographic Orientation in Atom Probe Analysis for Anisotropic Metal-Ion Battery Cathode Materials

Li-ion batteries (LIBs) have played a pivotal role in electrochemical energy storage and have garnered extensive research interest since their emergence. Layered transition metal oxides (LTMOs), widely adopted Li-ion battery cathode materials, are crucial for enhancing overall energy density. To gain deeper insights into nanoscale material changes during cycling which could lead to capacity loss, atom probe tomography (APT) offers promise due to its sub-nanometer, three-dimensional (3-D) resolution, and high chemical sensitivity. However, challenges related to Li migration under the required intense operational electric fields of APT have limited APT’s applications. Here we utilize air-stable lithium cobalt oxide (LCO) as a model system to elucidate the role of crystallographic orientation in APT analysis for the typically anisotropic battery materials. Our findings reveal that the Li/Co ratio detected by APT is highly dependent on applied laser pulse energy, ranging from stoichiometric (1pJ pulses) to 6.4 (10pJ pulses) when the orientation favors Li transport (Li-ion fast diffusing direction is parallel to the applied electric field). In contrast, even under 10pJ pulses, when the LCO is orientated such Li ion transport is impeded, the Li/Co ratio is 1.8 (near stoichiometric). Additionally, we discuss the effectiveness of an extrinsically deposited metallic capping layer in stabilizing localized Li migration for air-stable LTMO materials, reducing the Pearson coefficient from 0.98 to 0.43. The observed effects of sample orientation on APT could explain the inconsistent stoichiometry reported by APT for structurally similar LTMO materials. Our results emphasize the necessity of reporting crystallographic orientation in APT analyses, not only for battery materials but also for a broader class of materials with anisotropic atomic and ionic transport characteristics.