Effect of Microstructure on Ionic Transport in Silica-Based Sodium Containing Nanoconfined Systems and Their Electrochemical Performance as Electrodes

Glasses having compositions xNa(2)O center dot 10P(2)O(5)center dot(100-(10+x))SiO2, with x having values of 20 and 35, were synthesized by a facile sol-gel technique within a mesoporous silica (SBA-15) template. Nanocrystals of cristobalite with a diameter of around 4 nm were grown within the amorphous phase by subjecting the composite to a suitable heat treatment. Details of the internal microstructural features were revealed through X-ray diffraction and transmission electron microscopy studies. High-resolution X-ray photoelectron spectroscopy studies affirmed the existence of Si4+/Si2+ and P5+/P3+ species in both glass compositions. In addition, the presence of both bridging and non-bridging oxygen ions caused the generation of defects at the interfacial regime, which resulted in the lowering of activation energy for sodium-ion transport within the nanocomposites. DC conductivity of all the nanocomposite samples was evaluated using complex impedance spectroscopy data. The AC conductivity behaviors of both systems have been analyzed considering both power law and stretched exponential relaxation functions. The influence of calcination temperature on the microstructural framework and its effect on sodium ion conductivity were investigated. The nanoglass-ceramic system exhibited higher room-temperature ionic conductivity (similar to 10(-4.5) S center dot cm(-1)) and delivered a maximum capacity of 628.6 F/g at a scan rate of 100 mV/s. This was ascribed to the interfaces provided by the nanoglass and SBA-15 as well as the nanoglass and nanocrystals, which furnish a faster avenue for ion movement. The activation energy was found to be similar to 0.053 eV. The nanoglass-ceramic system is capable of withstanding over 200 successive cycles with a >80% retention of its initial capacitance. Such intriguing features of the nanoglass-ceramic system make it a potential candidate for electrode materials in sodium-ion solid-state batteries.