Synergistic effects of lithium doping and organic phases on the electrical and dielectric properties of sodium titanate solid-state electrolytes

Abstract

Energy storage technologies have attracted widespread attention due to the growing global demand. In this context, we present a detailed study of the structural, electrical, and dielectric properties of sodium titanate samples, with Na₂Ti₃O₇ as the predominant crystalline phase and Na₂Ti₆O₁₃ as the secondary phase, following lithium addition. Additionally, the synthesis route also produced samples capable of retaining organic phases, derived from the solvents used, even after thermal treatment at 900 °C. The above materials were characterized using X-ray diffraction (XRD) with Rietveld refinement, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), complex impedance spectroscopy, distribution of relaxation times analysis (DRT), and density-functional calculations. In samples with retained organic phases, the presence of C₂₀H₁₀O₂ was observed in the sample without lithium addition, whereas, lithium hydrogen maleate (C₄H₇LiO₆) and C₂₀H₁₀O₂ were detected in the sample on lithium addition. Density functional analysis was performed to obtain detailed information on the various phases. Moreover, the gap values of the phases demonstrated an increase in conductivity due to the presence of the organic phases. Scanning electron microscopy revealed a morphology already known for sodium titanate. It is characterized by uniform rod-like structures, with particle sizes ranging between 1 μm and 2 μm in samples without organic phases. In contrast, samples with organic phases exhibited a distinct morphology, with low uniformity and significant variations in particle sizes. FTIR spectroscopy revealed the predominance of bands below 920 cm⁻¹, characteristic of Ti – O, Ti – O – Ti, and Na – O bonds. Furthermore, complex impedance spectroscopy revealed a two- order-of-magnitude increase in conductivity for the 1 % lithium sample containing organic phases, compared to the pure ceramic sodium titanate sample. Dielectric analysis reveals a synergistic effect between lithium incorporation and organic phase retention, leading to enhanced energy storage capacity. Distribution of relaxation times (DRT) analysis shows strong agreement with the equivalent circuit models obtained via ZView software from the complex impedance data. The above results confirm the feasibility of the use of sodium titanate in solid-state electrolytes.