Aging in Li-ion batteries is accompanied by structural and compositional changes of the active and passive material of the electrodes. Visualization of spatial correlation between morphological properties and chemical phase can help to find origins of aging and failure mechanisms and to understand the ongoing processes. Micro X-ray fluorescence (XRF) analysis allows for chemical imaging of elemental distributions of multiple 3d elements simultaneously, or distribution of the state of charge even at low concentrations or under operando conditions. Utilizing focused synchrotron radiation, representative volumes of the electrodes can be studied with sub-micrometer resolution on acceptable time scales.

Cutting edge instrumentation in this field is the Maia fluorescence detector. It allows for ultra-fast mapping of areas of multiple mm² with high spatial resolution within minutes. This allows e.g. examination of large electrode areas. The backscattering geometry of the detector enables easy mounting of flat samples such as electrodes or pouch-cells. By variation of the energy in the vicinity of an absorption edge, conclusions on local variations of the state of charge can be drawn, probing the oxidation state of the redox-active element.  Furthermore, confocal XRF enables spatially resolved measurements of the deposition of transition metals on the graphitic anode without destruction. This will help to understand the impact on capacity fade and degradation.

For the present study, samples of LiNi_0.5Mn_1.5O₄ electrodes cycled vs. Li or graphite were investigated at beamline P06 at DESY, Hamburg Germany.¹ The experiment revealed significant inhomogeneities in the local Ni/Mn ratio upon cycling which is correlated to thinning of the electrode. Higher cycling rates showed an increased impact and higher material losses. Inhomogeneities in the phase transformation were also found during the charge and discharge process. The graphitic anodes showed significant amounts of deposited transition metals, few wt%, both Ni and Mn, deeply penetrating into the pores of the electrode. On the other hand, a study on commercial LiFePO₄ cathodes with outstanding rate capabilities showed an extremely high degree of homogeneity throughout the investigated areas. These methods are by no means limited to the investigated materials, but are easily transferred to other transition metal containing electrode materials.

1. U. Boesenberg et al., Chem.Mater., 27, 2525–2531 (2015)