with the coefficients F i,k,l,m being listed in Table . Note that the summations extend over all integral values of i, k, l, and m such that i + k + l + m < 4, hence the third-order multivariate polynomial fit. The regression for ν + has a root mean squared error (RMSE) of 1.48 • 10 -2 mm 2 /s while that for ν -has a RMSE of 3.16 • 10 -2 mm 2 /s. Figure shows contour maps of the fitted viscosity function for the posolyte (top) and the negolyte (bottom), for SoC = 0 (left), 0.5 (center), and 1 (right) at 20 • C. In all cases, the viscosity is seen to increase with the total vanadium concentration, and, more weakly, with the sulfates concentration. The Supplementary Information includes two additional figures showing viscosity contours for SoC = (0, 0.5, 1) at 10 • C and 30 • C (Figures and). Just as in the previous section, the region corresponding to values of vanadium and sulfate concentrations outside the parametric range of this study is labeled as unexplored. Across all cases, but particularly noticeable in the negolyte, we observe a shift in trend near the boundary of this unexplored region, suggesting a reduced influence of sulfates concentration on viscosity. This trend shift is stronger c S and as concentrations depart from the studied range. In fact, viscosity isocontours eventually reverse their direction deeper into the unexplored domain. This could simply be an artifact, as the empirical regression of ν -is not expected to accurately capture viscosity variations in this range due to the lack of experimental data. The posolyte viscosity ν + shows a similar behavior, but to a lesser extend and farther away from the studied parametric range.