E-Mobility Engineering | September/October 2023 35 Battery surface analysis | Focus This multi-technique analysis is used for the analysis of cathodes, anodes and separator materials, such as looking at the surface chemistry of pristine electrodes and comparing that to the surface chemistry of used electrodes. Cathode materials such as lithium, nickel, manganese and cobalt (NMC) change in stoichiometry depending on whether the cell is charged or discharged. To demonstrate that, one experiment profiled two NMC cathode samples on an XPS system. One sample was taken from a pristine, unused cell, while the other was from a cell that had been through several charge-discharge cycles and was in a charged state when the cell was disassembled. Because lithium is highly sensitive to air and moisture, the electrode materials were loaded into a vacuum transfer module in a glove box for transport to the XPS system. The survey spectra in the figure above shows peaks for the cathode materials with lithium, nickel, manganese, cobalt and oxygen, and the binder medium and oxygencontaining polymers that hold the materials together. In the pristine sample (green), the binder is evident as a significant amount of residue on the surface. This could be important when first using the cathode, if the binder residue is mobile within the electrolyte or reacts to begin the formation of a surface layer that impedes ion transport. The cycled cathode (red) still shows the presence of the binder as well as residue from the electrolyte at the surface. Comparing the NMC components (excluding oxygen) between the two samples (right figure), the relative intensities of lithium, nickel, manganese and cobalt were similar. However, the amount of lithium detected in the cycled cathode was around 40% of that observed in the pristine cathode. This was to be expected in a sample from a charged cell, where the lithium ions have been transported towards the anode, depleting the lithium level in the cathode. Depth profiling analysis of the lithium phosphorus oxynitride solid electrolyte can help determine the structure of the film as a function of depth. A 1 keV monatomic argon ion beam induces lithium ‘pile-up’ at the interface because the monatomic ion beam induces lithium mobility in the sample. Using a cluster ion source prevents this from happening, resulting in an accurate measurement of the composition of the electrolyte film. Depth distribution of elemental and chemical states can be determined by combining argon ion sputtering to break up the surface. A spectrometer can be configured with a monatomic argon ion source or a more advanced ion source with a cluster of argon ions (Arn+) , which has proved important in the correct determination of lithium concentration through solid electrolyte materials. A conventional monatomic argon ion source can induce lithium ion migration through the electrolyte material X-ray photoelectron spectroscopy (XPS) is a key surface analysis technique that provides quantitative elemental and chemical state information about the top layers of a material. XPS is essential for understanding the interface between electrolytes and electrodes. The cathode and anode materials of lithium-ion cells can be studied to confirm post-cycling changes in composition, to understand changes in the chemistry of the electrode components, and to determine how the solid electrolyte interface layer varies in depth as it develops. This technique has proved useful in studying the surface pre-treatment of graphite electrode materials to slow the irreversible consumption of material during battery charging. Surface analysis tools include vacuum transfer modules for safe transport of sensitive battery samples from the glove box, where the samples are prepared for the instrument where the properties are measured without exposure to ambient atmosphere. Surface profile of an NMC cathode before and after cycling The shape of lithium metal crystals determined using cryoEM (Courtesy of UCLA)
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