Abstract <p>Sodium metal has demonstrated significant potential for various applications due to its exceptional performance characteristics. The electrochemical behavior of Na₂B₄O₇ and the underpotential deposition mechanism of Ca(II) and B on the electrode surface were investigated in a BaCl₂-NaCl-CaCl₂ molten salt system at 873 K using cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry (CA), and open-circuit potentiometry (OCP). B₄O₇²⁻ undergoes a quasi-reversible, one-step, three-electron reduction to B on a tungsten electrode in the molten salt system. The diffusion coefficient of B₄O₇²⁻ in the BaCl₂-NaCl-CaCl₂ molten salt system was determined to be 3.24 × 10^-4 cm² s⁻¹, and the nucleation mechanism was identified as instantaneous nucleation. Under the interaction between B and Ca(II), part of the Ca(II) undergoes underpotential deposition on the B surface to form the intermediate phase CaB₆, which is a diffusion-controlled, quasi-reversible process. X-ray diffraction (XRD) analysis of the electrolysis products confirmed that the reduction product of B₄O₇²⁻ is B, and the intermediate phase is CaB₆. This study provides valuable insights into impurity separation in the electrolytic production of sodium metal and establishes a theoretical foundation for reducing impurity accumulation and improving current efficiency.</p> Graphical abstract <p></p>