Tuning of Na⁺ Concentration in an Ionic Liquid Electrolyte to Optimize Solid-Electrolyte Interphase at Microplasma-Synthesized Graphene Anode for Na-Ion Batteries

Two-dimensional (2D) materials are promising anodes for Na-ion batteries owing to their unique architectures and tunable physiochemical properties. However, their high surface area requires sophisticated electrolyte/electrode interface control to improve the charge-discharge efficiency and reversibility. This study uses microplasma-synthesized graphene nanosheets (MPGNSs) as a model 2D material. The effects of NaFSI concentration in an N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid (IL) electrolyte are systematically investigated. It is found that the chemical composition of the solid-electrolyte interphase depends on the electrolyte formulation, leading to distinct Coulombic efficiency, discharge capacity, rate capability, and cyclability of the MPGNS electrodes. The thermal reactivity of the sodiated MPGNSs (in terms of exothermic onset temperature and total heat released) upon heating is studied using differential scanning calorimetry. The IL electrolyte with a proper Na⁺ fraction is superior to a conventional organic carbonate electrolyte for practical NIB applications.