DecipheringPotential Shifts in Highly ConcentratedLiTFSI Aqueous Electrolytes

Uncertainties persist in determining the electrochemical stability window of concentrated electrolytes due to the Nernst shift (Δ<i>E</i>_N) and the liquid junction potential (Δ<i>E</i>_j). 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 Δ<i>E</i>_N and Δ<i>E</i>_j 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, Δ<i>E</i>_j has a less significant impact. Correcting for Δ<i>E</i>_j then allowed an accurate analysis of how LiTFSI concentration affects the redox characteristics of the Fe(CN)₆^3–/4– couple. These findings help deepen our understanding of electrochemical dynamics in highly concentrated solutions.