Unravelling the electrical and thermal behaviour of as-synthesised oxynitride glass-ceramic/ LLZTO composite through spark plasma sintering

The advancement of lithium metal-based batteries strongly depends on the successful commercialisation of solid-state electrolytes (SSE). In the absence of binder materials, pure SSE pellets are prone to structural degradation. This study investigates the microstructure, morphology, and thermal properties of Ca_7.5Al_15.1Si_17.5O_56.5N_5.5-based glass-ceramics doped with varying amounts (15-75 wt%) of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) via the spark plasma sintering (SPS) technique. X-ray diffraction (XRD) analysis confirms that the undoped glass remains amorphous, while doped samples exhibit crystalline phases such as La₂Zr₂O₇, LaTaON₂, LiAlO₂, ZrSiO₂, and LiAlSi₃O₈. The degree of crystallinity increases with rising LLZTO content. Scanning electron microscopy (SEM) studies reveal progressive alterations in microstructural features with increasing LLZTO concentration. Fourier transform infrared (FTIR) spectroscopy reveals characteristic band shifts, indicating enhanced crystallisation at higher LLZTO contents. The density increases from 2.45 g/cm³(undoped) to 3.92 g/cm³at the highest doping level. Thermal conductivity peaks at 1.69 W/m·K for the L1 sample but declines with further doping. Thermal expansion increases from 4.3 ppm/°C (undoped) to 5.4 ppm/°C (L2 sample), and then slightly decreases with higher doping levels. The conductivity spectra show semiconductor-like behaviour, characterised by a distinct direct current (DC) region at low frequencies transitioning to an alternating current (AC) region at higher frequencies, along with a general increase in conductivity with increasing LLZTO content. The DC conductivity rises with temperature in an Arrhenius-type behaviour, with activation energies varying from 0.78 eV to 0.99 eV, indicating thermally activated ionic transport that is primarily influenced by lithium-ion conduction in lower LLZTO-doped samples and oxygen-ion conduction at higher concentrations. Impedance analysis further corroborates the existence of two relaxation processes corresponding to different conduction mechanisms: one associated with lithium-ion conduction and the other with oxygen-ion conduction.