Nanoscale Phase Evolution in Lithium-air Batteries as Revealed by In-situ Small Angle X-ray Scattering
Rechargeable Lithium-air batteries promise extraordinary energy densities with sustainable materials. So far, practical application is hindered due to fast degradation and a limited fundamental understanding of structure-property relationships. In-situ SAXS/WAXS with a new electrochemical scattering cell allows quantifying the lithium peroxide formation and dissolution in the nanoporous carbon cathode during charging and discharging.
Many of today’s obstacles and chances of next generation electrochemical energy storage lie in the structural complexity of such multiphase material systems. Function and properties are not only rooted in their chemistry but at least as much in their structure, all the way from atomic to nanometer (and sub-µm) length scales.
An intriguing example for a complex structural transition is the reversible electrodeposition of electronically and ionically insulating active materials in conversion-type battery electrodes (e.g. metal-air or metal-sulfur battery cathodes). Currently, the lack of knowledge of suitable structure-sensitive in-situ techniques impedes the progress of such battery systems.
Rechargeable lithium-air (Li-O2) batteries can in theory achieve extraordinary energy densities and, at the same time use sustainable cathode materials. However, fast degradation due to side reactions and limited cyclability hinders their practical application. During discharge, Li+ ions migrate from the Li metal anode to the nanoporous carbon cathode where dissolved O2 is reduced to form solid Li2O2. Current understanding states that in the final reduction step Li2O2 is either formed via solution-mediated disproportionation of dissolved LiO2 (solution mechanism) or by direct reduction of LiO2 at the carbon-electrolyte interface (surface mechanism). The solution mechanism leads to generally larger, toroidal Li2O2 particles (> 50 nm) growing towards the pore center, the surface mechanism leads to a thin conformal coating with a thickness limited to around 7 nm (tunneling thickness). The fraction of Li2O2 formed via above mentioned mechanisms is believed to control the degree of pore filling and hence the discharge capacities. However, most mechanistic descriptions of Li2O2 nucleation and growth are vague and a direct evidence for a thin Li2O2 surface film is missing so far. (1)
Here, we expand the possibilities of in-situ small- and wide-angle X-ray scattering (SAXS/WAXS) to study the reversible deposition/dissolution of Li2O2 solids during discharging/charging a Li-O2 battery using a unique customized electrochemical SAXS cell. Combined with stochastic modelling of the nanoporous carbon cathode structure and numerical Fourier transformation we quantify nucleation and growth of thin disc-shaped Li2O2 crystallites. Hypothetical, modelled SAXS intensities of a conformal Li2O2 surface coating of a few nm proof the absence of such film even under nominally prototypical conditions.
1. Freunberger, S. A. et al. Chemical Science 8, 6716-6729 (2017).
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