Unveiling the dynamic change in the ionic conductivity of a solid-state binary mixture comprising an organic ionic plastic crystal and LiBF4

The binary composites of organic ionic plastic crystals (OIPCs) and lithium salts are a promising new class of solid electrolytes for all-solid-state batteries (ASSBs). One of the primary areas of research to enable their application into battery devices is to understand their complex physicochemical features, in particular their phase behaviors in response to changing compositions and the ensuing correlation with ionic conductivities. Herein, the phase behaviors and transport properties of binary OIPC–Li + salt composites are reported over the full compositional window using N-ethyl-N-methyl pyrrolidinium tetrafluoroborate ([C2mpyr][BF4]) and lithium tetrafluoroborate (LiBF4). The phase diagram of the Lix[C2mpyr]1–x[BF4] electrolyte was constructed and used to not only provide valuable insights into the key composition–property relationships but also guide the selection of salt concentration in OIPC composites for Li+-based ASSBs. Differential scanning calorimetry and 19F nuclear magnetic resonance (NMR) spectroscopy revealed the emergence of intermediate phases as the composition was varied. Significant and sudden variations in ionic conductivity at or near the phase transitions were attributed to the presence of these intermediate, secondary phases which ultimately provide the dominant contribution to the ionic conductivity of the Lix[C2mpyr]1–x[BF4] electrolyte.