High-Ionicity Electrolytes Based on Bulky Fluoroborate Anions for Stable Na-Metal Cycling

Abstract

Currently available electrolyte materials for sodium battery technology remain unoptimized, despite the success of their lithium counterparts. Herein, highly fluorinated borate ester anions are synthesized and characterized in terms of their physical and electrochemical properties as sodium salts in 1,2dimethoxyethane (DME) solvent. Walden analysis and nuclear magnetic resonance pulsed field gradient diffusion measurements are utilized to probe the ionicity of the electrolytes, demonstrating that sodium 1,1,1,3,3,3-(tetrakis)hexafluoroisopropoxy borate (NaB(hfip) 4 •3DME) possesses notably high ionicity when compared to NaPF 6 and NaFSI at an equivalent Na + concentration. NaB(hfip) 4 /DME is explored electrochemically, with the anion showing excellent oxidative stability and the desired passivation behavior toward Al surfaces, akin to NaPF 6 . Investigating the electrolyte in Na-metal symmetrical cells shows very impressive cycling behavior with very low (<2 mV) overpotentials observed at 0.1 mA cm −2 for over 100 cycles at 0.1 mAh cm −2 . Cycling is investigated via postmortem analysis of the electrode surfaces, confirming Na deposition and revealing substantially reduced electrolyte decomposition when compared to NaPF 6 . The results herein demonstrate that NaB(hfip) 4 •3DME has the physical and electrochemical properties required for a next-generation Na-electrolyte material.

Fluoroborate ionic liquids as sodium battery electrolytes

“…8c, with the 1b at RT sample producing AlF 3 (687.7 eV) whereas for the Na[FSI]: 1b sample, NaF and [FSI] − were the major species present. 14,32,34 The –CF 3 moiety detected for 1b at 50 °C is attributed to residual electrolyte in the porous Al surface which can be seen in the SEM micrograph in Fig. S35 (ESI†).…”

“…Na[B(OCH 2 CF 3 ) 4 ] (Na[B(tfe) 4 ]), Li[B(OCH(CF 3 ) 2 ) 4 ]· x DME (Li[B(hfip) 4 ]·3DME) and N 2(2O2O1)3 Br were synthesised according to published methods. 29,32,34…”

“…Moreover, the anion shows superior cathodic stability against aluminium, evenly passivating the surface with AlF 3 and NaF. 34…”

“…29 The fluoroborate salts, sodium (tetrakis)hexafluoroisopropoxy borate (Na[B(hfip) 4 ]ÁxDME, where DME = 1,2-dimethoxyethane) and sodium (tetrakis)trifluoroethoxy borate (Na[B(tfe) 4 ]), were first reported in 2011 and 2014 respectively, 30,31 however their use as battery electrolytes in organic solvents was only recently realised. [32][33][34] From our recent publication, an electrolyte comprised of 0.5 M Na[B(hfip) 4 ] in DME demonstrated stable electrochemical cycling over 300 cycles at 0.1 mA cm À2 in a Na8Na symmetrical cell with very low overpotentials, below 2 mV. 34 This impressive electrochemical performance was attributed to the weakly coordinating nature of [B(hfip)] À which results in high ion-dissociation of Na + .…”

Electrocatalysis of Fe‐N‐C Bonds Driving Reliable Interphase and Fast Kinetics for Phosphorus Anode in Sodium‐Ion Batteries

“…Fluorinated electrolyte components such as sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(fluorosulfonyl)imide (NaFSI), sodium trifluoromethanesulfonate (NaOTf), and vinyl fluorocarbonate (FEC) have been widely reported to regulate the content of inorganic component NaF in SEI, which possesses high chemical stability, low Na + diffusion barrier and strong mechanical toughness. [17][18][19] It is generally believed that NaF decomposed by fluorine-containing electrolytes helps to promote the cycle life of batteries. [20,21] Therefore, it is theoretically feasible to precisely control the C-F separation chemistry in fluorinated electrolyte to regulate the NaF content, but the practical operation remains very challenging.…”