Comparing the Physicochemical, Electrochemical, and Structural Properties of Boronium versus Pyrrolidinium Cation-Based Ionic Liquids and Their Performance as Li-Ion Battery Electrolytes

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 [C x mpyr]+ and [R4P]+ cations. We demonstrate the potential of binary and ternary ILELs with bis­(trifluoromethanesulfonyl)­imide [TFSI]− and bis­(fluorosulfonyl)­imide [FSI]− anions in comparison with N-butyl-N-methylpyrrolidinium [C4mpyr]­[TFSI] as a reference. The conductivity of neat ILs and their 0.5 mol kg–1 Li-salt-containing mixtures (σ25°C = 0.5–1.68 mS cm–1), differential scanning calorimetry (T g = −83 to −77 °C, T m = 14 to 28 °C), solid–solid-state transitions for [N111N112BH2]­[FSI], cyclic voltammetry (electrochemical stability window 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 mol kg–1 of Li­[TFSI] yielded a crystalline material for which the X-ray structure showed a four-coordinate square-planar [Li­(TFSI)2]− environment with O ∩ O chelation, while two-coordinate environments were reported for Li+ centers in [C x pyr]­[Li2(TFSI)3] with higher anion numbers, both tetrahedral monodentate coordination by four TFSI anions 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 an electrolyte in Li|LFP half cells, several hundred charge–discharge cycles with high initial discharge capacity (∼155 mA h g–1 at 50 °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). A good discharge capacity (∼135 mA h g–1) was also achieved at 20 °C/0.1 C with the ternary electrolyte system.