Structural dependence of effective mass transport properties in lithium battery electrodes

Scanning electrochemical microscopy (SECM) has recently been reported as a convenient technique to characterize the apparent diffusion coefficients in porous media (D*) such as lithium-ion battery electrodes. The technique relies on hemispherical diffusion at the probe in the presence and absence of porous media. However, transition towards cylindrical diffusion follows as the diffusion layer becomes comparable to the media thickness, gradually limiting direct correlation between D* and the SECM probe steady state current. In this work, we examine the experimental parameters that define this transitional frontier using simulations. A best practice thickness criterium is derived that ensures that the measured steady state current is A 90% of that found in the ideal hemispherical case. Below this thickness limit the response was found to follow an empirical formulation with only two variables. Fundamentally, this quantifies diffusion in the transition regime between hemispherical and cylindrical transport. Practically, this correction formulation may be included into standard steady-state current expression to account for the deviation from hemispherical transport in thinelectrodes. The analysis consolidates SECM as a reliable tool to characterize liquid phase mass transport within porous films over wide range of thicknesses, including films supported on blocking substrates, like lithium-ion battery electrodes.

Automated summary

Scanning electrochemical microscopy (SECM) is a useful technique for characterizing the diffusion coefficients in porous media, with a transitional frontier from hemispherical to cylindrical diffusion. Experimental parameters were examined through simulations to derive a best practice thickness criterium and found an empirical formulation in the response below a certain thickness limit. This analysis solidifies SECM as a reliable tool for characterizing mass transport in porous films over a wide range of thicknesses.