Joaquin Rodriguez-Lopez1,Abhiroop Mishra1,Dipobrato Sarbapalli1
University of Illinois at Urbana Champaign1
Joaquin Rodriguez-Lopez1,Abhiroop Mishra1,Dipobrato Sarbapalli1
University of Illinois at Urbana Champaign1
Lattice oxygen loss during cathode charging is one of the main limitations for high-voltage lithium ion batteries and a critical player in various surface cathode phenomena including densification cathode electrolyte interphase (CEI) formation, and electrolyte degradation processes. Thus, developing techniques capable of evaluating the degradative O<sub>2</sub> evolution phenomena with high versatility, e.g. with (electro)chemical and spatial resolution and the capability to correlate to other surface phenomena, is pressing towards understanding and designing better cathodes. In this presentation we will discuss a novel in situ oxygen detection strategy using scanning electrochemical microscopy (SECM) in the substrate generation/tip collection mode. By approaching the tip to the substrate within a few microns, we are able to collect the generated O<sub>2</sub> within few milliseconds after its generation at the cathode with unprecedented sensitivity and signal to noise ratio. This allowed us to generate plots that quantified the rate of oxygen evolution vs. electrode potential. Our first results on LiCoO<sub>2</sub>, LiNi<sub>0.33</sub>Mn<sub>0.33</sub>Co<sub>0.33</sub>O<sub>2</sub> and LiNi<sub>0.8</sub>Mn<sub>0.1</sub>Co<sub>0.1</sub>O<sub>2 </sub> revealed an unprecedented two-stage oxygen evolution behavior from commercial cathodes with an unreported feature at ~3.3 V vs Li<sup>+</sup>/Li during the first charge cycle. At the main oxygen evolution features above ~3.3 V vs Li<sup>+</sup>/Li, SECM mapping highlighted spatial and temporal heterogeneities as the tip was positioned on different evolution sites and at different times. Further exploration using the feedback mode of SECM enables us to correlate the state of the surface to its charge transfer properties. Since such an interface could also be the source of reactive oxygen species (ROS), we have deployed generation-collection techniques and surface-interrogation SECM methods to quantify and evaluate the properties of ROS and radical intermediates formed at the interface, thus highlighting the chemical versatility of our approach. Our SECM toolbox is a unique complement to other methods used to characterize interfacial processes at battery materials, and requires very little sample preparation, being capable of addressing large format electrodes down to individual particles. This tool will help create new prospects for quantitative and spatially resolved investigations of degradation processes in operating lithium ion battery cathodes.