Operando Chemical Analysis of Batteries by Quantitative X-Ray Spectrometry

Quantitative characterization methods allow for the reliable correlation of the functionality of energy materials with the underlaying chemical or physical properties. These correlations are required for the directed development of advanced materials to reach target functionalities such as specific battery capacities. The traceability of analytical methods revealing quantitative information on chemical properties often relies on calibration samples, the spatial elemental distributions of which must be very similar to the sample of interest. To establish traceability to the SI, an alternative approach lays in the physical calibration of the analytical instrument’s response behavior and efficiency as well as in the use of good atomic fundamental data. This approach has been established by Germany’s metrology institute PTB for x-ray spectrometry (XRS). In different operational configurations the information depth, discrimination capability and sensitivity of XRS can be tuned, especially when using synchrotron radiation. Time-resolved and hybrid, i.e. multimodal approaches, provide access to complementary analytical information of different kind of batteries (NMC, LiS and SIB) under ex-situ to operando conditions while using calibrated instrumentation. The latter is a prerequisite to real quantitative conversion and transport rates.<br/><br/>For ex-situ hybrid depth profiling, angular-resolved X-Ray Fluorescence (XRF) analysis and Near-Edge X-ray Absorption Spectroscopy (NEXAFS) measurements were used to investigate the depth-dependent Ni2+/Ni3+ ratio and Ni/O ratio in NCM111, 622 and 811 cathode materials. The results show a gradient of the Ni/O and Ni2+/Ni3+ ratio from the surface to the bulk of the NMC particles, with higher oxygen and Ni2+ content on the surface.<br/><br/>The improved understanding of degradation mechanisms is essential to developing next generation batteries. Quantitative operando NEXAFS in fluorescence detection mode has been used during multiple charge–discharge cycles on both electrodes of lithium–sulfur (Li/S) cells. This enables the absolute quantification of dissolved polysulfides (PS) with respect to both the local polysulfides concentration and the average chain length. Using novel self-standing 80% C/S composite electrodes long-term hybrid operando XRS investigations (XRF and NEXAFS) were performed allowing to determine quantitative and time-resolved information on relevant sulfur species over 90 cycles of this LiS battery.<br/> <br/>[1] B. Beckhoff, Nanomaterials, 2022, <b>12</b>, 2255<br/>[2] C. Zech et al., J. Mater. Chem. A, 2021,<b> 9</b>, 10231<br/>[3] C. Zech et al., J. Anal. At. Spectrom., 2021, <b>36</b>, 2056<br/>[4] European Partnership on Metrology, "Operando metrology for energy storage materials" OpMetBat project,<br/> https://opmetbat.inrim.it/