Highly conductive LiH2PO4-based solid electrolyte at intermediate temperatures through a polymerization-hydrolysis treatment

Abstract

Establishing expandable solid electrolytes with high ionic conductivity at approximately 200 °C remains a significant challenge for various electrocatalytic applications. In this study, we developed a self-standing electrolyte membrane composed of H₃PO₄-containing LiH₂PO₄ and a quartz fiber (QF) matrix using a unique polymerization-hydrolysis (PH) synthesis method. We used a simple mixture of LiH₂PO₄ and H₃PO₄ as the precursor, and heating-induced phosphate polymerization resulted in a unique glassy sol formation that could be reshaped easily. Upon hydrolysis pretreatment with polyphosphate, LiH₂PO₄ was formed, and the electrolyte membrane exhibited high ionic conductivity. We optimized the infiltrating amount of H₃PO₄ to vary the maximum conductivity of the electrolyte. We conducted conductivity measurements under diverse temperatures and water humidities to determine the operating conditions for the electrolyte membrane. The LiH₂PO₄-based membrane maintained high conductivity (21–28 mS cm⁻¹) over a broad and comparatively low temperature range (100–200 °C) when supplied with water vapor. It is hypothesized that a phase containing H₂O and H₃PO₄ is formed at the interface between the LiH₂PO₄ particles, which contributes to the high conductivity. The scalability of the present fabrication method was confirmed by successfully forming a 100 mm Φ QF sheet displaying uniform conductivities through the sheet.