Correlation of Electrical Conductivity and Microstructure with the Band Gap of Oxysulfide Glass-Ceramics for Na-Ion Battery

Abstract

In the present investigation, a glass-ceramics with the composition of xNa2S + (100–x)P2S5, where x = 40, 45, 50 and 55, were successfully synthesized by employing the melt-quenching method. Comprehensive characterization of the glass-ceramic samples was conducted using X-ray diffraction (XRD), UV-visible spectroscopy, impedance spectroscopy (IS) and field emission scanning electron microscopy (FE-SEM) techniques. The XRD analysis revealed the presence of three distinct phases, namely NaPO3, Na2S2O3 and Na3PS4, in all samples. Significantly, NaPO3 and Na2S2O3 exhibited an orthorhombic crystal structure, while Na3PS4 displayed a tetragonal crystal structure. The densities of the synthesized samples fell within the range of 2.24 to 2.35 g/cc, surpassing those of Li2S-based solid electrolytes commonly used in portable devices. The band gap of the materials varied from 2.99 to 3.60 eV. Significantly, an inverse relationship between Na2S content (modifier) and band gap was observed, indicating a decrease in band gaps with increasing Na2S content. This phenomenon is beneficial for enhancing ionic conductivity. At ambient temperature, samples with x values of 50 and 55 demonstrated remarkable conductivity on the order of 10-4 S cm-1. Overall, the synthesized glass ceramics exhibit promising features, such as higher density compared to conventional Li2S-based solid electrolytes and favourable band gap values, suggesting their potential application in enhancing ionic conductivity for various electronic devices.