• Hollenkamp, Anthony
• Glenn, Oldham
• Forsyth, Craig
• Barghamadi, Marzieh
• Mccallum, Rory
• Mahon, Peter

Abstract

Ionic liquid type electrolytes (ILELs) based on boronium cations, (trimethylamine) (dimethylethylamine)dihydroborate [N111N112BH2]+ are revisited as they have barely been studied for Li-battery applications as alternatives to the ubiquitous [Cxmpyr]+ and [R4P]+ cations. We demonstrate the potential of the binary and ternary ILELs with bis(trifluoromethanesulfonyl)imide [TFSI]- and bis(fluorosulfonyl)imide [FSI]- anions with comparison to N-butyl-N-methylpyrrolidinium [C4mpyr][TFSI] as a reference. The conductivity of the neat ILs and their 0.5 molkg-1 Li-salt containing mixtures (25 °C = 0.5-1.68 mS cm-1), differential scanning calorimetry (DSC) (Tg -83 to -77 °C, Tm 14 to 28 °C), solid-solid state transitions for [N111N112BH2][FSI]), cyclic voltammetry (CV) (ESW 5.6 V), Li|LFP and Li|Li cells, and a crystal structure of the Li environment have been investigated. The binary mixture of [N111N112BH2][TFSI] + 0.8 molkg-1 Li[TFSI] yielded crystalline material for which the X-ray structure showed a four coordinated square planar [Li(TFSI)2]- environment with OO chelation, while two coordination environments were reported for Li+ centres in [Cxpyr][Li2(TFSI)3] with higher anion numbers, both tetrahedral mono-dentate coordination by four TFSI, and five-coordinate, square-based pyramidal coordination by three TFSI anions. Charge-discharge cycling studies were conducted on lithium metal electrodes in a symmetrical Li|Li coin cell configuration for 1500 charge–discharge cycles at 50 °C and a current density of 0.2 mA cm−2. When used as the electrolyte in Li|LFP half cells, several hundred charge-discharge cycles with high initial discharge capacity ( 155 mAh g-1 at50 °C) and good retention (0.03 to 0.05% capacity loss per cycle) are possible, at cycle rates of 0.1-0.25 C (0.07-0.2 mA cm−2). Good discharge capacity ( 135 mAh g-1) was also achieved at 20 °C/0.1C with the ternary electrolyte system.

Topics

• density
• mass spectrometry
• thermogravimetry
• differential scanning calorimetry
• Lithium
• current density
• cyclic voltammetry