Quantitative Synchrotron XRD and Cryogenic EXAFS Reveal the Structural Role of Antimony Ions in Lead–Acid Batteries

The incorporation of trace amounts of Sb₂O₃ into the positive electrode plate of lead-acid batteries effectively suppresses the softening and shedding of the active material, thereby extending the cycle life. Although this beneficial effect has been recognized for decades, its mechanistic origin has remained elusive owing to the ultratrace concentration of antimony (Sb), which prevents detection by conventional techniques. Here, we combine quantitative multicomponent analysis by synchrotron X-ray diffraction (XRD) with cryogenic (10 K) extended X-ray absorption fine structure (EXAFS) spectroscopy to directly probe the structural role of Sb ions during charge-discharge cycling. Quantitative XRD analysis reveals that during formation, Sb ions are substitutionally incorporated into Pb sites within the PbO₂ lattice. Cryogenic EXAFS measurements further demonstrate that during discharge, as PbO₂ converts into PbSO₄, a fraction of the Sb ions appear to migrate from the PbO₂ lattice toward particle surfaces, where they reinforce interparticle bonding. This reversible incorporation and surface relocation of Sb ions account for the observed stabilization of the active material and suppression of softening and shedding. Our findings highlight how advanced synchrotron-based techniques enable direct mechanistic insight at the atomic scale, providing critical guidance for the rational design of next-generation lead-acid batteries with enhanced performance and reduced environmental impact.