We generated the samples of desired composition via pipetting previously prepared reference solutions of vanadium electrolytes, sulfuric acid and water with phosphoric acid. For the negolyte, we mixed V 2+ and V 3+ (V II /V III ) and, for the posolyte, VO 2+ and VO + 2 (V IV /V V ). The pipettes were calibrated using deionized water and a scale, yielding 0.3% repeatability and 0.45% accuracy. We prepared the reference solutions via electrochemical charge/discharge from an initial equimolar V III /V IV commercial solution (Oxkem Limited, UK). The electrolyte data sheet was provided with, a vanadium, sulfuric acid and phosphoric acid concentration of, respectively, 1.8 M, 4.6 M, and 0.05 M. However, we deemed these values unreliable for our study since the commercial datasheet indicated a 10% error in the vanadium and sulfate concentrations. In particular, the density of 98% sulfuric acid is 1.83 g/cm 3 . Thus, we expect the concentration (ca. 22% in weight) of sulfuric acid to have a significant impact on the density measurements. For these reasons, we determined the sulfate, phosphate and vanadium concentration of our starting electrolyte, whose exact composition is listed in Table . We quantified the sulfate concentration using the procedure outlined in Oreiro et al. , based on the precipitation of sulfate ions and barium ions as well as precise density measurement. We measured the total vanadium concentration via spectrophotometric titration of VO 2+ 2 , oxidized by potassium permanganate (KMnO 4 ). The phosphoric acid concentration was measured by induction coupled plasma-optical emission spectroscopy (ICP-OES). Details regarding these procedures are given in the Supplementary Information. Additional information regarding the electrochemical preparation is described in our recent paper by Maurice et al. . It is worth noting that the reference V II solutions were degassed before pipetting, as during the charge of the negolyte some diluted hydrogen appears because of the electrolysis of water.