Development of Quantitative Techniques with Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) for Li Characterization in High Energy Batteries.

Abstract

Energy dispersive x-ray spectroscopy (EDS) is the standard technique when it comes to material characterization with scanning electron microscopy (SEM). However, for lithium related materials, even a windowless EDS detector with optimized electronics will have limitations in the detection and quantification of Li content due to low sensitivity (>20 wt%) and lack of understanding of Li x-ray emission processes [1]. TOF-SIMS in a focused ion beam scanning electron microscope (FIB-SEM) allows imaging with a resolution greater than 50 nm and high sensitivity for Li (>2 ppm [2]). However, quantification using TOF-SIMS is difficult, mainly due to matrix effects and the change of sputtering yield with primary beam angle incidence (edge effects). Quantification models already exist for TOF-SIMS using secondary implanted standards [3], however those are developed for trace elemental analysis of implants in homogenous, simple structures. Battery related materials have high elemental concentrations with complex stoichiometry. Since Li is the key element in battery materials and the sputtering yield is very high for Li (very high sensitivity), a quantitative model must be developed with TOF-SIMS that can be applied to electrode materials, as well as lithium metal and solid electrolyte interface (SEI). In this work, experiments will be made first on standard lithium nickel cobalt manganese oxide (NCM) cathodes using a FIB-SEM TESCAN Lyra3 microscope equipped with a TOF-SIMS analyzer from the collaborative work of TESCAN and TOFWERK AG. A primary Ga + beam with a landing energy of 30 kV was used for all the experiments. Samples were prepared using an IM4000PLUS ion milling system from Hitachi High Technologies. Figure 1 shows an example of a TOF-SIMS acquisition on standard NCM particle for Li 7. Signal variations are seen on the particle and at interfaces with the porous medium. For quantification purposes, true signal variations must be distinguished from variations due to edges effect. In order to minimize those effects, samples have been prepared with the ion milling system to ensure a flat surface for analysis. Figure 2 shows the TOF-SIMS recorded intensities for active battery materials on standard NCM cathodes with different degree of lithium insertion (x). According to the results, Li intensity decreases with decreasing x values. However, the decrease in Li intensity from x=1 to x=0.25, about 30%, does not correlate with the theoretical decrease of Li content…