The combination of lone-pair effects on Pb2+ cations and the smaller electronegativity of I- anions into the pentaborate framework generates a phase-matchable material, Pb2B5O9I, with the largest powder SHG response among borates, about 13.5 times that of KDP (KH2PO4), and transparency over the near-UV to middle-IR region. DFT calculations on electronic structure and cutoff-energy-dependent SHG coefficients confirm these origins.