A Hexafluorophosphate‐Based Ionic Liquid as Multifunctional Interfacial Layer between High Voltage Positive Electrode and Solid‐State Electrolyte for Sodium Secondary Batteries

Abstract Sodium secondary batteries have gained accolades as future energy storage devices due to their low costs and environmental benignity, but are heavily impeded by the poor anodic stabilities of most electrolytes, including solid‐state electrolytes (SSE), that render them incompatible with high‐voltage positive materials. This study reports, for the first time, a new synthesis technique using a fluorohydrogenate ionic liquid (IL)precursor to prepare a [DEME][PF 6 ] ([DEME] + : N , N ‐diethyl‐ N ‐methyl‐ N ‐(2‐methoxyethyl) ammonium) with high yield and high purity. Herein, a Na[PF 6 ]‐[DEME][PF 6 ] IL is formulated and subjected to a series of electrochemical tests to validate its performance in battery applications. The present IL harbors a strong oxidative stability (up to 5.2 V on Pt and >4.5 V on conductive carbon electrodes) that aids in the suppression of oxidative decompositions of one typical SSE, for example, beta alumina solid electrolyte (BASE), thereby extending their electrochemical window in hybrid SSE systems. A hybrid solid‐state Na secondary battery employing a high voltage positive electrode, Na 3 V 2 (PO 4 ) 2 F 3 , is assembled using the BASE/IL configuration, and features energy density and superior cycling performance. This work demonstrates that sandwiching an SSE between the oxidatively stable [PF 6 ] − IL can be an effective design for high voltage operation Na secondary batteries.