Overcoming the probing-depth dilemma in spectroscopic analyses of batteries with muon-induced X-ray emission (MIXE)

Abstract

Battery research often encounters the challenge of determining chemical information, such as composition and elemental oxidation states, of a layer buried within a cell stack in a non-destructive manner. Spectroscopic techniques based on X-ray emission or absorption are well-suited and commonly employed to reveal this information. However, the attenuation of X-rays as they travel through matter creates a challenge when trying to analyze layers buried at depths exceeding hundred micrometers from the sample's surface. In the context of battery research, the limited escape depth of X-rays often necessitates the design of experiment-specific cells with thinner inner layers, despite the risk that these tailored cells may not exactly replicate the cycling behavior of larger commercial cells. Muon-induced X-ray emission (MIXE) is a non-destructive spectroscopic technique that involves implanting negative muons into a sample and detecting the highly energetic muonic X-rays generated when these muons are captured by the sample's atoms. By virtue of the high energy of muonic X-rays, the depth of analysis of MIXE greatly exceeds that of other X-ray based techniques. In this article, we introduce the technique and lay the groundwork for employing MIXE in future in situ/operando analyses of batteries. We demonstrate that MIXE can detect nearly all elements, including low atomic number ones such as Li. Additionally, we establish the quantitative nature of MIXE through the precise determination of LiNi_xMn_yCo_1−x−yO₂ (NMC) electrode stoichiometries. Finally, we demonstrate that MIXE enables the acquisition of depth-resolved chemical information from a 700 μm thick cell, in good agreement with simulation results.