Ionic Competition between Na⁺ and H⁺ in Aqueous Sodium-Ion Battery Electrolytes

Aqueous electrolytes have become a research hotspot because of their high safety and low cost, while the inevitable ionization phenomenon of water in aqueous solution leads to the existence of competitive ions (H⁺) except the active ions. In this article, we take aqueous Na base electrolyte as an example to clear the ion competition behavior by modeling, simulating together with experimental verification. First, the reaction tendency of the two ions (Na⁺ and H⁺) is obtained by calculating the Gibbs energy change of the reaction. Furthermore, the properties of electrolytes with different concentrations including transportation are obtained by modeling. After that, relevant experiments are also proceeded to verify the simulation results. Then, the ion competition behavior is analyzed by in situ observation by controlling the constant concentration of Na⁺: the high concentration of Na⁺ can reduce the proportion of H⁺ and reduce the competitiveness of H⁺; a high concentration of Na⁺ causes the increased viscosity and reduces the ion diffusion. Based on this, the correlation between ion competitiveness and ion ratio is also confirmed by keeping the concentration of Na⁺ unchanged and adjusting the concentration of H⁺ (adjusting pH). The influence of the ion competition phenomenon (Na⁺ and H⁺) is the reaction characteristics of the substance itself and the ratio of ion concentration. Finally, the electrochemical performance is further verified in 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDI) symmetric cells and in full-cells with vanadium phosphate sodium (NVP) as the cathode and PTCDI as the anode.