Investigation of the benefits of the oxazolidinium cation for plastic crystal and ionic liquid electrolytes

Abstract

Organic ionic plastic crystals (OIPCs) are promising for developing safer energy storage electrolytes. However, there remains a significant knowledge gap regarding how different cation-anion combinations influence their core properties, and cyclic ether-based cations have received limited attention. This study reports the synthesis and characterization of OIPCs based on the N-ethyl-N-methyl-oxazolidinium cation [C2moxa]+ and demonstrates the first instance of oxazolidinium OIPCs being combined with lithium salts to create electrolytes. The [C2moxa]+ cation was paired with [FSI]-, [TFSI]-, [BF4]-, [PF6]- and [FTFSI]- anions. A study of the thermal, transport and electrochemical properties was performed. Among the new salts developed, [C2moxa][BF4] exhibited the most promising characteristics, including the lowest entropy of melting (ΔS = 7 J mol−1 K−1), an extended phase I range (10°C–130°C), the highest conductivity of 8 x 10−6 S cm−1 at 30°C, and an electrochemical stability window of 4.8 V. When the [C2moxa][BF4] and [C2moxa][FSI] were mixed with lithium salts (10, 20 and 50 mol% Li+) of the same anion, the highest conductivity of 2 x 10−3 S cm−1 at 30°C was found for the 20 mol% LiFSI/[C2moxa][FSI] electrolyte. Finally, preliminary lithium plating/stripping experiments and coulombic efficiency (CE) measurements demonstrate stability for lithium cycling for all four [C2moxa]+ electrolytes.