Deciphering Potential Shifts in Highly Concentrated LiTFSI Aqueous Electrolytes

Uncertainties persist in determining the electrochemical stability window of concentrated electrolytes due to the Nernst shift (ΔEN) and the liquid junction potential (ΔEj). This study investigates potential shifts in LiTFSI aqueous solutions across a broad concentration range. By characterizing Li-ion transference numbers and thermodynamic factors, we identify concentration- and activity coefficient-dependent ΔEN and ΔEj in LiTFSI electrolytes, observed in concentration cells with LiFePO4 electrodes. Our findings reveal that a sharp increase in the mean ionic activity coefficient at high salt concentrations drives a substantial upward shift in the Li-ion (de)-intercalation potential, which contributes 65% of the total potential shift between 1 and 5 M solutions. In contrast, ΔEj has a less significant impact. Correcting for ΔEj then allowed an accurate analysis of how LiTFSI concentration affects the redox characteristics of the Fe(CN)63–/4– couple. These findings help deepen our understanding of electrochemical dynamics in highly concentrated solutions.