Outstanding compatibility of hard-carbon anodes for sodium-ion batteries in ionic liquid electrolytes

: Hard carbons (HC) from natural bio-waste have been investigated as anodes for sodium-ion batteries in electrolytes based on the 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIFSI) and N-trimethyl-N-butylammonium bis(fluorosulfonyl)imide (N1114FSI) ionic liquids. The Na+ intercalation process has been analyzed by cyclic voltammetry tests, performed at different scan rates for hundreds of cycles, in combination with impedance spectroscopy measurements to decouple bulk and interfacial resistances of the cells. The Na+ diffusion coefficient in the HC host has been also evaluated via the Randles-Sevcik equation. Battery performance of HC anodes in the ionic liquid electrolytes has been evaluated in galvanostatic charge/discharge cycles at room temperature. The evolution of the SEI (Solid Electrochemical Interface) layer grown on the HC surface has been carried out by Raman spectroscopy. Overall the sodiation process of the HC host is highly reversible and reproducible. In particular, a capacity retention exceeding 98 % of the initial value has been recorded in N1114FSI electrolytes after more than 1500 cycles with a coulombic efficiency above 99 %, largely beyond standard carbonate-based electrolytes. Raman, transport properties and impedance confirms that ILs disclose the formation of SEI layers with superior ability to support the reversible Na+ intercalation with the possible minor contributions from the EMI+ cation.