XAFS studies on battery materials: data analysis supported bya chemometric approach

The development of low-cost, efficient storage of off-peak electric power and of electrical energy generated by energy sources other than fossil fuels is a global priority. Rechargeable batteries, which are based on high rate intercalation reaction of lithium, and more recently, sodium ions into nanostructured and microstructured porous materials, offer efficient electrochemical energy storage [1]. The rate and efficiency of such intercalation reactions are of fundamental importance to the chemists because they permit to design and tailor new host structures for intercalation batteries. Because of the amorphous, ill-crystalline state led by these materials during the intercalation/release processes, X-ray Absorption Spectroscopy (XAS) is the technique of choice for retrieving structural and electronic information. The use of EXAFS spectroscopy in the field of battery materials has been recently reviewed [2].XAFS experiments have been conducted at ELETTRA [3] on cathode materials using the operandomode. This allowsto check the structural and electronic reversibility of thebattery system while at least one full galvanostatic cycle is performed. XAS data have been recordedon vanadiumoxide materials, lithium rich materials and Prussian Blue-like cathodes for Li batteries. Typically, alarge amount of data is recorded duringtheseexperiments. For instance, the study of the charge or the discharge process of a battery produces something like 100–300 spectra, depending on the experimental conditions (data acquisition protocol and battery discharge rate).A chemometric approachcan be used to analyze such an amount of data and to obtain acomplete understanding of the cell dynamic during the electrochemical process. Among some different chemometric techniques, the application of the Multicurve Resolution Analysis -Alternate Least Squares (MCR-ALS) has been demonstrated to be very useful in the study of XASspectra on battery systems [4]. Following this approach not only the number of species but also the existence range of the various species involved during the electrochemicalapproach can berevealed without needing any preexisting model or a prioriinformation about the system.Once the species have been hypothesized,a common EXAFS analysis can be performed for the structural identification of the selected species in a givenrange of existence. The potentiality (and weaknesses) of the MCR-ALS analysis ofXAS spectra will be presented in this contribution.XAFS measurements at ELETTRA have been done through the 20110095, 20130225, and20145337projects. MG acknowledges theRFO funding of the University of Bologna.