A novel Cu(II)-azolate metal-organic framework (MOF) with tubular pores undergoes a reversible single crystal to single crystal transition between neutral and anionic phases upon reaction with stoichiometric amounts of halide or pseudohalide salts. The stoichiometric transformation between the two phases allows loading of record amounts of charge-balancing Li+, Na+, and Mg2+ ions for MOFs. Whereas the halide/pseudohalide anions are bound to the metal centers and thus stationary, the cations move freely within the one-dimensional pores, giving rise to single-ion solid electrolytes. The respective Li+, Na+-, and Mg2+-loaded materials exhibit high ionic conductivity values of 4.4 x 10(-5), 1.8 x 10(-5), and 8.8 x 10(-7) S/cm. With addition of LiBF4, the Li+ conductivity improves to 4.8 x 10(-4) S/cm. These are the highest values yet observed for MOF solid electrolytes.