Highly concentrated solutions of sodium bis(fluorosulfonyl)imide (NaFSI) with a series of bis(fluorosulfonyl)imide-based ionic liquids incorporating either alkylphosphonium or alkoxy ammonium cations (P111i4FSI:NaFSI, P-1i4i4i4 FRNaFSI), N2(20201)3FSI:NaFSI) are combined with NaFePO4 cathodes to demonstrate excellent cycling performance with respect to potential range, rate capability, cycle life and elevated temperature stability. With a capacity of 85 mAhg(-1) and a capacity retention of 95% over 100 cycles (50 degrees C, C/2) P-111i4 FSI:NaFSI matches or outperforms conventional organic solvent based electrolytes as well as other ionic liquid electrolytes in terms of capacity, elevated temperature performance and cycle stability. The electrolyte conductivity does not correlate with the capacity, suggesting that this is not the primary factor determining the cell performance. The solid electrolyte interphase determines the cycle stability with SEM and XPS techniques suggesting the presence of NaOH, Na2S and NaF on the Na metal surface post cycling.