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

Abstract

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.

Manipulating Electrode/Electrolyte Interphases of Sodium-Ion Batteries: Strategies and Perspectives

“…116 It was also pointed out that the concentration of 50 mol % NaFSI in C 3 mpyr-FSI results in the most favorable interfacial layer, in which the concentration factor is more important than the ionic conductivity of the electrolyte itself. 118 By putting the concentration strategy into experimental practice, Chang et al 119 tuned the SEI chemistry by matching electrolytes consisting of NaFSI in C 3 mpyr-FSI (0.5, 1, 2, and 3 M, respectively) with a two-dimensional (2D) graphene anode material. The obtained SEI layer from 2 M NaFSI showed the best protective function via balanced organic-inorganic constituents (NaF, Na 2 CO 3 , and Na 2 CO 2 R species), while an organic species-rich SEI was produced from 0.5 M, and the inorganic species-rich SEI from 3 M NaFSI could not sufficiently stabilize the electrode-electrolyte interface.…”

Electrolytes toward High‐Voltage Na3V2(PO4)2F3 Positive Electrode Durable against Temperature Variation

“…Note here that half‐cell configuration using sodium metal as a counter electrode, exhibits large impedance, making it unsuitable for evaluation of the impedance information in sodium secondary battery target electrodes. [ 47,60 ] Figure 6g shows the impedance Nyquist plots and fitted curves from the NVPF‐C/NVPF‐C symmetric cell (SOC = 50%). The equivalent circuit used to fit the plots is shown as the inset in Figure 6h, and the EIS parameters are provided in Table S7, Supporting Information.…”

“…However, sodium cations have milder acidity compared to lithium cations and thus tend to form inhomogeneous and fragile SEI layers with higher solubility. [ 7,38,43–49 ] This allows further surface reactions between the electrode and electrolyte that induce further degradation and extra gas evolution that may create safety problems. [ 40,50 ] For this reason, optimization of sodium electrolytes is a vital step towards the realization of high performance, safe and durable sodium secondary batteries.…”

Benefits of the Mixtures of Ionic Liquid and Organic Electrolytes for Sodium-ion Batteries

“…These observations are consistent with previously reported IL electrolytes for Na batteries which demonstrated lower irreversible capacities than their OL counterparts. 15,21,27,[49][50][51][52] Across the various current densities, the ILOL systems demonstrated improved rate capability, as illustrated in Figure 7b. Although the ILOL 100 achieves the highest reversible capacity at 100 mA g -1 , the ILOL 20 and ILOL 50 attain higher capacity retention at the higher current densities.…”