Conductivity of cold sintered diphasic composites containing a ceramic active material and a solid-state electrolyte or carbon for all solid-state batteries

For solid-state batteries based on ceramic materials, the means of effective materials processing and co-processing presents a significant challenge. The high sintering temperature required for many solid electrolytes induces alkali volatilization, in addition to preventing the co-processing with other battery materials, such as carbon or active materials. In this paper, we demonstrate how cold sintering, a low temperature sintering process driven by chemomechanical pressure-solution creep in the presence of a transient solvent and uniaxial pressure, can fabricate dense ceramic matrix composites containing an electroactive material with either carbon or a ceramic solid electrolyte with minimal loss of phase/function. The model system is composed of the Na₃V₂(PO₄)₃ (NVP) active material, the Na₃Zr₂Si₂PO₁₂ (NZSP) solid electrolyte, and carbon black/carbon nanofibers. The conductivity of the carbon-NVP composites is fitted to a classical percolation model, demonstrating low percolation thresholds (2-4 vol%). By measuring the electrical properties of a range of composite compositions, a more complete view of mixed conduction phenomena through the diphasic composites is obtained. In contrast, impedance spectroscopy measurements of diphasic composites containing active material and solid electrolyte (NVP and NZSP, respectively) reveals a gradual transition to fast ionic conduction as the composition ranges from pure active material to primarily solid electrolyte dominated (vol% NZSP > 90%). These results may also be useful in future studies which require a detailed understanding of conductivity as a function of composition in order to optimize for electrochemical properties such as volumetric energy density or rate performance.