Role of crystallographic orientation in atom probe analysis of Li-ion battery cathode materials

Abstract

Determining composition of Li-ion battery (LIB) cathodes at the nanoscale is important to understanding cathode performance. However, in the widely adopted layered transition metal oxide cathode materials, the high crystallographic anisotropy of lithium transport makes characterization particularly challenging due to the potential mobility of Li during characterization. Atom probe tomography (APT) holds promise to provide sub-nanometer, three-dimensional chemical information of such cathode materials, and in particular the lithium distribution within these materials. However, such analysis assumes Li does not migrate under the intense electric field required for APT. Using lithium cobalt oxide (LiCoO₂) as a model system, we evaluate the role of crystallographic orientation in APT analysis of anisotropic battery materials. When the crystal orientation favors Li transport, the measured Li/Co ratio is highly dependent upon applied laser pulse energy and ranges from near stoichiometric for a 1 pJ pulse, to as high as 6.4 for a 10 pJ pulse. In contrast, when the orientation impedes ionic transport, Li migration is largely suppressed, and the Li/Co ratio reaches only 1.8 using a 10 pJ laser pulse. Using an extrinsically deposited metallic capping layer, localized Li migration is largely stabilized, the Pearson coefficient is reduced for all evaluated orientations. The results presented here shed light on the impact of and emphasize the necessity to report crystallographic orientation on APT analysis results for materials with fast transport characteristics.