Determination of battery separator permeability by scanning electrochemical microscopy

Separators are critical components in batteries. In closed systems, like lithium-ion batteries, they should ensure a good ion transport between the two electrodes. In open systems, such as flow batteries, separators should also avoid the crossover of the redox species in the flowing electrolytes. We focused our studies on separators for redox flow batteries (RFBs), which are widely applied as electrochemical energy storage systems, specifically in combination with renewable energy systems like photovoltaic or wind turbines. We pioneered the use of scanning electrochemical microscopy to characterize the efficiency of different commercial membranes, specifically for all-copper RFB, by evaluating the cupric ion permeation through these membranes. In fact, one drawback of this system is represented by the permeation of cupric ions in the negative half-cell that leads to the dissolution of the copper deposit and results in the battery's self-discharge. Several types of membranes have been tested as separators to limit this process. Finite elements simulations were also performed to quantitatively interpret the electrochemical measurements and to correlate separator permeabilities to membrane molecular characteristics such as hydrophobicity and ion exchange capability. Estimates of copper ion diffusion coefficients and permeabilities were provided for a subset of the separators investigated.