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

• Co-crystalline Li(GLN) 2 BF 4 displays successive Li + -transport channels. • Weak Li···N bonding and abundant H-bonding confer a low melting point of 60 °C. • Grain-boundary fluidity enables intimate interfacial contact without external pressure. • Liquid-like Li + conduction for high-performance solid-state batteries. • Superior structural stability is evidenced by in-situ wide-angle X-ray scattering. 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) 2 BF 4 (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||LiFePO 4 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. Li(GLN) 2 BF 4 , a top-performing solid electrolyte, is constructed from ordered Li···C N coordination bonds, which grant it a unique set of properties distinct from conventional ceramics. Notably, its low melting point of 60 °C endows grain-boundary fluidity, facilitating intimate interface contact and eliminating the need for pressure treatments. Collectively, these outstanding features enable liquid-like Li-ion conduction for high-performance solid-state batteries.