High-resolution X-ray mapping of fluorinated binders in lithium-ion battery electrodes

This study looks at energy-dispersive X-ray spectroscopy (EDX) maps of fluorine in NMC 622 cathodes and the efforts made to improve the spatial resolution of fluorine mapping. The transition to electric vehicles demands faster and efficient production of next-generation lithium-ion batteries. To achieve this goal, the industry needs to take advantage of state-of-the-art battery characterization techniques in the pursuit of gaining a greater understanding of electrode structure and production. The binder location within an electrode is critical to electrochemical performance and contains fluorine, yielding an opportunity to use it as a marker and a way to visualize binder distribution and therefore, better understand process-structure relations. However, fluorine is difficult to differentiate from cobalt and manganese in an EDX spectrum due to similar Kα energy. Fluorine also interacts with the electron beam, potentially leading to poor spatial resolution. This paper examines different EDX parameters and compares the spatial resolution of fluorine in the maps of lithium-ion cathode cross sections. Analysis of the EDX maps showed that reducing the accelerating voltage from 20 to 5 kV improved the spatial resolution of fluorine 10-fold, from 2553 to 238 nm, supported by CASINO simulations. The EDX maps also indicated that imaging for one long scan at a 2500 μs dwell time produced a higher spatial resolution than imaging for 10 scans at 250 μs. Repeated line scans of the sample showed the extent of fluorine mobility; fluorine-rich zones emit less, while fluorine-free zones begin to emit more fluorine X-rays. This work shows that the spatial resolution of fluorine maps can be increased by imaging at 5 kV and scanning for one pass at 2500 μs. This methodology can be used to create more representative EDX maps of the binder in the cathodes. Visual analysis or further processing with an image analysis can reveal binder distributions and potential binder gradients. This technique is useful in understanding how changes to electrode manufacturing can change the electrode structure and binder distribution.