The effect of cation chemistry on physicochemical behaviour of superconcentrated NaFSI based ionic liquid electrolytes and the implications for Na battery performance

Abstract There is growing interest in ionic liquid based electrolytes for Na metal and Na-ion batteries. Here we compare three quite distinct bis(fluorosulfonyl)imide (FSI) anion based ionic liquids with small alkyl phosphonium (trimethyl isobutyl phosphonium, methyl tri-isobutyl: P111i4, P1i4i4i4) or alkoxy ammonium counter cations (N-ethyl-2-(2-methoxyethoxy)-N,N-bis(2-(2-methoxyethoxy)ethyl)ethan-1-ammonium bis(fluorosulfonyl)imide: N2(2O2O1)3) mixed at near 1:1 mol ratio with NaFSI. The conductivities of these electrolytes range from 4.4 mScm−1 for the smallest P111i4FSI:NaFSI system to 0.3 mScm−1 for the N2(2O2O1)3FSI:NaFSI mixture at 50 °C. This difference in conductivity is interestingly not reflected in the cyclic voltammetry for Na/Na+ where the maximum peak current density of 10 mAcm−2 is surprisingly high for the poorly conductive N2(20201)3FSI:NaFSI solution (e.g. 17 mAcm−2 for P111i4FSI:NaFSI). The overpotentials observed for Na symmetric cell cycling show very little differences after initial stabilising/conditioning for the three electrolytes being 50 mV for P111i4FSI:NaFSI and 100 mV for the others (at 0.1 mA cm−2). Also the Na+ transport number is similar for the three electrolytes ranging from 0.33 to 0.37. Full cells were prepared with layered transition metal oxide cathodes: O3-Na2/3(Fe2/3Mn1/3)O2), P2-Na2/3(Fe2/3Mn1/3)O2 and P2-Na2/3(Mn0.8Fe0.1Ti0.1)O2. While for the O3/P2-Na2/3(Fe2/3Mn1/3)O2 structures the device performance is consistent with the electrolyte properties, with the P2-Na2/3(Mn0.8Fe0.1Ti0.1)O2 cathode the N2(2O2O1)3FSI:NaFSI electrolyte cycling extremely well. The P111i4FSI and N2(2O2O1)3FSI yield almost equivalent specific capacities of approximately 180 and 160 mAhg−1 respectively at C/10 rate.