The sample preparation protocol employed in this study follows the methodology outlined in our previous work , with a brief summary provided below for completeness. Calibrating UV-Vis spectroscopic methods requires the precise preparation of single-species reference electrolytes and their mixtures. This was achieved by carefully pipetting, mixing, and diluting reference solutions of V II , V III , V IV , and V V , while adjusting the vanadium concentration and the state of charge (SOC) as needed. Using deionized water and a high-precision scale, we ensured a pipetting accuracy of 0.4%. For calibration, we prepared 66 samples across three vanadium concentrations (1.82 M, 1.52 M, and 1.21 M), with SOCs ranging from 0% to 100%. Concentrations below 1.21 M were excluded due to their limited industrial relevance.