Structural and electric properties of Na super ionic conductor solid electrolyte MIIx/2Li1−xTi2(PO4)3 (MII = Mg, Zn, and Cd)

Abstract

All-solid-state lithium-ion batteries, employing solid electrolytes, offer a promising solution to address safety concerns inherent in conventional lithium-ion batteries. Among the various types of Li-ion solid electrolytes, LiTi₂(PO₄)₃ (LTP) with the Na Super Ionic CONductor (NASICON) structure stands out as a particularly attractive material, despite its relatively low ionic conductivity at room temperature. One approach to enhance the performance of LTP solid electrolytes involves modifying the network size or redistributing Li cations and vacancies within the adjacent sites of the NASICON structure. Therefore, this study seeks to replace lithium ions with divalent cations, thereby increasing the concentration of vacancies, and facilitates the migration of Li⁺ ions between adjacent partially populated M1 sites. Introducing divalent elements not only augments vacancies in the lithium sites but also induces variations in local disorder within NASICON structures. Consequently, NASICON compounds, M^II_x/2Li_1−xTi₂(PO₄)₃ (M^II = Mg, Zn, and Cd), were synthesized via the sol–gel method, and their structural, microstructural, and electrical properties were thoroughly analyzed using a variety of techniques. The presence of divalent cations in the M1 site results in a reduction of symmetry and an enhancement of local disorder. A correlation between ionic conductivity and structure was established, which was linked to the disorder of lithium atoms within the structure. Electric modulus formalism was employed to explore electric relaxation, revealing that the diffusion and relaxation processes are thermally activated.