<p>Two-dimensional (2D) materials with tunable electronic transitions are vital for next-generation electronics and energy storage technologies. Here, we demonstrate a novel, thermally-driven, reversible modulation of electronic conductivity in the quaternary heteroanionic 2D van der Waals superlattice Bi₄O₄SeCl₂. Systematic thermal annealing of bulk Bi₄O₄SeCl₂ induces a striking reversible transition from insulating to semiconducting behavior, accompanied by a 10⁶ times magnitude enhancement in electrical conductivity. This dramatic transformation originates from thermally mediated defect annihilation and grain boundary elimination, substantially reducing carrier scattering barriers and enhancing intrinsic carrier concentration. Moreover, leveraging its intrinsic layered structure, we successfully exfoliated Bi₄O₄SeCl₂ into nanosheets (~ 1.5&#xa0;nm in thickness) and utilized these exfoliated layers as interfacial coatings on Zn electrodes, markedly improving dendrite suppression and extending the cycling stability of Zn-ion hybrid capacitors and Zn/MnO₂ batteries. Our work elucidates crucial defect-driven microstructural mechanisms responsible for thermally-induced reversible electronic transitions, establishing heteroanionic Bi₄O₄SeCl₂ as a promising platform for advanced electronic and energy-storage materials engineering.</p> Graphical abstract <p></p>