Interphase design from ionic liquid cation mixtures and multi-mode surface analysis for safe and stable Na metal batteries

Abstract

Enhancing the cycling performance of sodium metal batteries requires deliberate electrolyte design to optimize interfacial structuring and sodium deposition. This study explores novel ionic liquid (IL) electrolyte mixtures containing 20 mol% phosphonium cations ((tributylmethyl phosphonium) P₁₄₄₄⁺ and (trimethyl isobutyl phosphonium) P_111i4⁺) as co-solvent additives in a base (N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide) C₃mpyrFSI electrolyte. The effects of these mixtures on electrochemical performance, physicochemical properties, interfacial nano-structuring, and sodium deposition morphology are investigated using a multifaceted approach, including cell cycling, cyclic voltammetry (CV), differential scanning calorimetry (DSC), ionic conductivity measurements, and in situ techniques such as neutron reflectometry (NR), atomic force microscopy (AFM), and optical microscopy (OM). The addition of the larger P₁₄₄₄⁺ cation, while decreasing ionic conductivity, surprisingly exhibits reduced polarisation overpotential and significantly extended lifespan and improved solid electrolyte interphase (SEI) formation kinetics. Our results, supported by NR, AFM, and in situ optical studies, reveal that incorporating P₁₄₄₄⁺ as a co-solvent disrupts interfacial nano-structuring. Under applied negative potentials, this disruption increases the presence of Na⁺ cations at the interface and their coordinated FSI⁻ anions, leading to enhanced SEI formation kinetics as evidenced by CV results. This facilitates improved cycling stability and more uniform sodium deposition morphology. This work highlights the potential of mixed-cation ILs in achieving long-term performance and stability in sodium metal batteries. By shedding light on the poorly understood mechanisms underlying SEI-related performance improvements, it provides new strategies for optimizing interfacial structuring and electrolyte design.