March 2020
We here report for the first time the characterization of Li−O2 battery cathodes by 3D tomography, allowing us to distinguish and localize Li2O2 from carbon material and pores. Such characterization, which coupled X-ray nano-CT with in- line Zernike phase contrast, was carried out on a few cathodes discharged independently until there are different depths of discharge. First and foremost, we demonstrated the feasibility of this technique theoretically onto the cathode of the Li−O2 system. In practice, at 8 keV this technique provides a high resolution (nm), contrast, and a large field of view (50 μm). Throughout the present paper, the X-ray Zernike phase contrast nanotomography for Li−O2 battery provides intuitive visualizations and valuable quantitative information. We successfully retrieved the pore size distribution and inter- connectivity in different cathode electrodes. The surface passivation has also been analyzed. In the studied material, the analysis shows a heterogeneous spatial distribution and inefficient occupation of Li2O2, probably due to the highly tortuous electrode and low diffusivity of oxygen. In addition, future work with this technique should be extended to other parts of the battery, such as a lithium anode, or a separator for investigating transport properties. Last but not least, the nondestructive property and nonvacuum requirement of this technique turn out to be robust for the development of the operando experiment. The investigation of the Li−O2 cathode kinetics should be carried out in future work.
