XRF-Microanalysis of Electrodes for Li-Ion Batteries

Spatial correlation between morphological properties and chemical phase visualizes transport mechanism and phase transformations, e.g. in electrodes for Li-ion batteries. This can help to find origin of aging and failure mechanism and finally, to build better batteries. Quasi nondestructive micro X-ray fluorescence analysis allows for in operandoand intermediate stages investigations such as elemental migration and can reveal structure-properties relationships while keeping the sample in a well-protected environment. Making use of the fairly large penetration length of the X-rays, representative volumes of the electrodes can be studied. The method is extremely suitable to correlate the spatial distribution of two or more elements.

Cutting edge instrumentation in this field is the MAIA fluorescence detector which allows for ultra fast mapping of areas of multiple mm² area with sub-micron resolution within minutes. This allows examination of full electrodes. 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.

For the present study, samples of LiNi_0.5Mn_1.5O₄ electrodes cycled vs. Li were investigated at beamline P06 at DESY, Hamburg Germany. 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. This method is by no means limited to the investigated material, but is easily applicable to other transition metals.