A low-melting solid electrolyte with grain-boundary fluidity for solid-state batteries

Solid electrolytes (SEs) are urgently needed as key components of solid-state batteries (SSBs). However, the limited physical contact between the SE and electrode gives rise to interfacial issues, causing interrupted charge transport and significant resistance at the interface. In this study, we propose a co-crystalline SE, Li(GLN)₂BF₄ (GLN, glutaronitrile), exhibiting a combination of properties not found in conventional ceramics, notably a low melting point of 60 °C and its grain-boundary fluidity. These features facilitate intimate interfacial contact without external pressure, thereby enabling liquid-like Li⁺ conduction for high-performance SSBs. Consequently, this SE exhibits an ionic conductivity of 1.43 × 10⁻⁴ S cm⁻¹ at 30 °C and a lithium-ion transference number of 0.74. Importantly, it exhibits superior structural stability during electrochemical cycling as evidenced by in-situ wide-angle X-ray scattering. Benefitting from these properties, Li||Li symmetric cells exhibit stable operation for 600 h, while Li||LiFePO₄ cells retain 92.3 % of its initial capacity after 400 cycles, all operating at room temperature and under zero externally applied pressure. This work paves new avenues for exploring co-crystalline substances that can concurrently achieve interfacial compatibility and chemical stability, in contrast to ceramic electrolytes.