Unveiling ion-coordination in super-concentrated borate-based ionic liquid electrolyte

The fluoroborate anions have emerged as promising candidates to complement the existing suite of ionic liquid electrolyte anions, particularly with the recent preparation of the ionic liquid N-ethyl-N,N,N-tris(2-(2-methoxyethoxy)ethyl)ammonium tetrakis(1,1,1- 3,3,3-hexafluoroisopropoxy)borate ([N_2(2O2O1)3][B(hfip)₄]) which shows excellent properties for sodium battery technologies. In this work, we report physicochemical and electrochemical characterisation of [N_2(2O2O1)3][B(hfip)₄] electrolytes with varying Na[FSI] concentrations which demonstrate the equimolar Na[FSI]:[N_2(2O2O1)3][B(hfip)₄] mixture to offer the best ionic behaviour. Electrochemical performance of the optimised electrolyte was evaluated through sodium cycling in sodium symmetrical cells which demonstrate electrochemical stability towards Na at 0.5 mA cm⁻² over 100 cycles. The cycling stability is attributed to a low impedance interface derived predominately from [FSI]⁻ and its decomposition products. Spectroscopic studies discern the coordination environment of Na⁺ where [N_2(2O2O1)3]⁺ actively coordinates to Na⁺ via the ethereal alkyl chains at all concentrations studied. Nuclear magnetic resonance indicates that both [B(hfip)₄]⁻ and [FSI]⁻ participate in Na⁺ coordination at the equimolar concentration, likely existing as a labile ion-cluster with Na⁺. This study shows equimolar Na[FSI]:[N_2(2O2O1)3][B(hfip)₄] to be a suitable electrolyte for sodium electrochemistry, offering control over interface formation via implementation of the more robust [B(hfip)₄]⁻ anion.