The ion conduction mechanism of isovalent cation-doped Li2SO4

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SO₄:(x)Me₂ SO₄ (where Me = Na, K, Rb, and Ag; x=0.0025to0.09) is systematically investigated by using complex impedance spectroscopy. The solid solubility limit up to 2 mole% of Me₂SO₄ in β-Li₂SO₄ is determined by X-ray powder diffraction, scanning electron microscopy, and differential scanning calorimetry techniques. The partial replacement of Li⁺ with Me⁺ of bigger ionic size increases the conductivity due to lattice opening. The maximum conductivity enhancement is achieved by Ag⁺ substitution. A simple model, based on change in entropy and Coulomb interaction, is applied to understand the increase in conductivity. A semi-quantitative treatment provides some useful conclusions such as the decrease in activation energy of ion conduction and increase in ion jump probability after doping. It is found that the normal expansion of the lattice by substituting larger isovalent ions alone is not sufficient, but requires a proper ionic size for conductivity enhancement in β-Li₂SO₄.