For several different applications ranging from automobiles to portable electronics, lithium-ion (Li-ion) battery assembles have become an ideal energy storage option, as these high energy density battery cells are lightweight and cost effective options as compared to other battery technology and are therefore appropriate for use in transport applications. Research developments on Li-ion batteries have involved increases in the energy density of the batteries while simultaneously reducing the weight and costs of these storage systems.
While operating Li-ion batteries, the lithium that stored in the anode is oxidized, followed by a transport of the Li+ ions through the electrolyte and separator lm to the cathode. In the cathode, the anion is then oxidized to create a compound that can store the arriving lithium ions. While the cell is recharged following its use, the flow of ions is in the opposite direction and thereby reduced back to its lithium metal form to be stored for future use in the anode, which is typically composed of graphite and lithium that is intercalated into the graphite structure. The cathode is comprised of a lithium metal oxide, however its exact composition varies depending upon the required characteristics of the cell.
The most commonly used cathode materials include:
- Lithium Cobalt (LiCoO2)
- Lithium Manganese (LiMn2O4)
- Lithium Phosphate (LiFePO4)
- Lithium Nickel Manganese Cobalt (NMC; Li(NiMnCo)O2)
These oxides can change in their stoichiometry, depending on whether the cell is charged or discharged.
Figure 1. Li-ion cell in operation
A byproduct of the charge and discharge process involves the formation of the solid-electrolyte interphase (SEI) layer on the anode, which competes with the reversible lithium intercalation process. Over the lifetime of the battery, the SEI layer will contribute to the lowering of capacity and the ultimate failure of the cell, therefore understanding the SEI layer is an area of significant interest to improve overall performance of the cell. Depth profiling by the Nexsa XPS System offers an accurate method to characterize and identify the complexity of chemistries that comprise both the SEI and interphase layers.
Lithium is very sensitive to air and moisture, therefore introducing Li-ion electrode materials into the XPS system must be achieved without any possible exposure to air. To achieve this, the samples are loaded into the Vacuum Transfer Module (VTM) in a glove box, which is compatible with the K-Alpha and Nexsa instruments, followed by evacuation into the glove box antechamber and a final transportation to the XPS system. As the VTM is held together by air pressure, it automatically opens during the pump-down cycle in the system load-lock and integrated into the standard and automated sample transfer process.
Figure 2. The vacuum transfer module allows samples that have been prepared in an inert environment to be transferred into the spectrometer chamber without exposure to air.
In the following experiments, two cathode samples, including one pristine and unused sample and the other, which was from a cell that had been through several charge and discharge cycles, were investigated.
The cathode material, Li(NixMnyCoz)O2, was prepared using a binder medium, which is a mixture of uorine and oxygen containing polymers, to hold the material together. The pristine sample showed a significant amount of binder residue on the surface, which could affect its mobility in the electrolyte, or allow it to react to begin the formation of a surface layer and ultimately impede ion transport.
Figure 3. Survey spectra from pristine cathode (blue) and cycled cathode (red) samples
The relative intensities of the Ni, Mn, and Co components were determined to be very similar between the two samples, however the amount of Li detected was approximately 40% of that which was seen in the pristine cathode. This is as expected in a sample from a charged cell, in which the Li ion transport has been towards the anode and away from the cathode, thereby resulting in a depleted level of lithium in the cathode.
Figure 4. Composition variation for the NMC components
By using the vacuum transfer module and the Nexsa XPS System it is possible to analyze Li-ion battery components including both unused and cycled cathode samples to determine the expected variation in lithium content.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.
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