Abstract
Lithium ion conductivity in layered oxides Li 2 Ca 1.5 Nb 3 O 10 , Li 2 Ca 1.5 TaNb 2 O 10 and Li 2 Ca 1.5 Ta 2 NbO 10 is improved over their Sr-analogue due to the formation of a supercell. The greater mobility of lithium is confirmed by dielectric and complex modulus analyses. The formation of a supercell and its impact on lithium-ion conductivity have been studied through synthesis of three layered oxides, Li 2 Ca 1.5 Nb 3 O 10 , Li 2 Ca 1.5 TaNb 2 O 10 and Li 2 Ca 1.5 Ta 2 NbO 10 , related to Ruddlesden-Popper structure-type. Neutron diffraction experiments show that these materials feature a supercell, which is significantly larger (~√2 a × ~√2 b × ~ 1 c ) than that of a typical Ruddlesden-Popper oxide. Electrochemical impedance spectroscopy shows that the formation of the new supercell is associated with enhanced lithium-ion conductivity of these materials as compared with the Sr-analogue, Li 2 Sr 1.5 Nb 3 O 10 , which lacks the supercell. In addition, a systematic trend is observed in the ionic conductivity: Li 2 Ca 1.5 Ta 2 NbO 10 < Li 2 Ca 1.5 TaNb 2 O 10 < Li 2 Ca 1.5 Nb 3 O 10 . The Arrhenius analysis in the temperature range 25–400 °C shows that activation energy for the temperature-dependent rise in conductivity follows a similar trend. Detailed analyses of real and imaginary components of impedance, dielectric properties, tangent loss, and complex modulus show the systematic increase in lithium-ion mobility. The dielectric values mirror the same trend as ionic conductivity, where the most conductive material shows the highest dielectric properties. In addition, the same trend is observed in the peak and dispersion of dielectric loss and complex modulus as a function of angular frequency, indicating a systematic rise in lithium-ion mobility. This fundamental study is aimed at exploring the impact of structural modifications on ionic conductivity in solids.
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