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Optimization of Fe-Air Battery Cycling Processes with SEM EDS

Iron-air batteries (Fe-air batteries) are a sustainable and cost-effective energy storage solution. These batteries, made using abudant iron, excel in environmental friendliness, are produced using non-toxic materials, and have a high energy density ideal for both electric vehicles and grid storage.

Fe-air batteries have a long lifetime, with a high number of charge/discharge cycles, and posess inherent safety advantages over lithium-ion batteries. As a result, Fe-air batteries show promise in helping stabilize grids and to reduce reliance on rare, geopolitically sensitive materials. This makes them well suited for renewable energy integration and the development of a more sustainable energy future.

Figure 1a: SE image of a processed battery electrode sample. The sample is highly topographic on the submicron scale.

The research and optimization of complete iron-air battery material systems involves studying different deposition and cycling processes. One such process is the redistribution of elements, such as sulfur, during battery cycling. 

Here we present an example of material optimization, where the sulfur redistribution due to the cycling process could be detected via element mapping using a Bruker XFlash® 760 detector with a conventional EDS detection geometry.

In the non-cycled battery material sulfur is distributed homogenously (not shown here). This is in constrast to battery materials that have undergone cycling which, as demonstrated in these EDS maps, show significant redistribution and localized enrichment of sulfur. 

Figure 1b: EDS element maps with SE image overlay acquired using a Bruker XFlash® 760 EDS detector. Redistribution and enrichment of sulfur on the sample surface can be detected.

Seeing within Intragranular Spaces using the Annular XFlash® FlatQUAD Detector

EDS analysis of samples with a grain structure on the micrometer scale without any preceding sample preparation (mechanical or FIB polishing) will always be affected by shadowing, especially when performing a low-kV analysis or detecting light elements.

Annular EDS detectors such as Bruker's XFlash® FlatQUAD, eliminate this issue, as the significantly higher take off angle allows visualization of signals from inbetween grains. This, combined with the detector's high sensitivity, enables analysts to detect and visualize element inhomogeneities in a much more efficient way.  

Below a direct comparison of EDS maps acquired using a conventional detector (Bruker XFlash® 760) and annular detector (Bruker XFlash® FlatQUAD) can be seen.

These maps demonstrate the unmatched sensitivity of the FlatQUAD detector when mapping complex sample structures as well as the ability for the system to see in between grains with no need for sample preparation. In the elemental maps acquired using FlatQUAD the local enrichment of sulfur can be seen within intragranular spaces. 

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Figure 2a: EDS maps without SE image overlay acquired using a Bruker XFlash® 760 EDS detector. Sulfur and iron can be detected, however the sulfur distribution maps are affected by shadowing of the grainy sample structure.
Figure 2b: EDS maps without SE image overlay acquired using a Bruker XFlash® FlatQUAD detector. Sulfur and iron can be detected and mapped clearly. The maps show the presence of sulfur in intragranular spaces, proving the occurrence of chemical redistribution on the microscale during cycling.