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

journal contribution
posted on 01.04.2018, 00:00 authored by Matthias HilderMatthias Hilder, Patrick HowlettPatrick Howlett, D Saurel, E Gonzalo, Andrew Basile, M Armand, T Rojo, Mega KarMega Kar, D R MacFarlane, Maria ForsythMaria Forsyth
© 2018 Elsevier Ltd 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−1for the smallest P111i4FSI:NaFSI system to 0.3 mScm−1for 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−2is surprisingly high for the poorly conductive N2(20201)3FSI:NaFSI solution (e.g. 17 mAcm−2for 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)O2and P2-Na2/3(Mn0.8Fe0.1Ti0.1)O2. While for the O3/P2-Na2/3(Fe2/3Mn1/3)O2structures the device performance is consistent with the electrolyte properties, with the P2-Na2/3(Mn0.8Fe0.1Ti0.1)O2cathode 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−1respectively at C/10 rate.

History

Journal

Electrochimica acta

Volume

268

Pagination

94 - 100

Publisher

Elsevier

Location

Amsterdam, The Netherlands

ISSN

0013-4686

Language

eng

Publication classification

C1 Refereed article in a scholarly journal

Copyright notice

2018, Elsevier