Reactive co-sputtered Li–Nb–O thin films with tunable ionic conductivity and dielectric properties for energy storage applications

Lithium niobate (LiNbO₃) has emerged as a promising solid-state electrolyte for energy storage systems owing to its favorable ionic conductivity and distinctive physicochemical properties. However, conventional solution-based synthesis routes are limited by Li loss, compromising performance. This study reports for the first time the development of Li-Nb-O thin films using reactive magnetron co-sputtering of Nb and Li targets at various DC powers and constant RF power. This approach yields the growth of amorphous films at room temperature, as confirmed by X-ray diffraction. Morphological studies revealed that increasing DC power promotes grain growth. EDX analysis indicated a rise in Nb/O atomic ratio from 0.28 to 0.44 with increasing DC power (20-80 W). XPS experiments provided evidence supporting the dominant presence of Nb-oxygen bonds and a growing contribution from weakly bound Li species at higher Nb concentrations. Moreover, the Li-to-Nb atomic ratio was found to range from 2.31 to 1.36 with increasing DC power (20-80 W), corroborating the formation of a Li-rich compositional region. Temperature-dependent impedance spectroscopy demonstrated enhanced ionic diffusion in Li-Nb-O films with higher Nb content, attributed to a reduction in activation energy. The highest room-temperature ionic conductivity of co-sputtered films reached 1.33 × 10^-7 S cm^-1. Metal-insulator-metal capacitors based on the optimized Li-Nb-O thin films exhibited a high capacitance (~ 624.86 nF cm^-2), a large dielectric constant (~ 141.21), and a low loss tangent. In this way, a electric double layer was formed at the electrolyte/electrode interfaces. These findings underscore the potential of reactive co-sputtered Li-Nb-O films as solid-state electrolytes for energy/charge storage applications.