Stable Cycling of Lithium Batteries Using Novel Boronium-Cation-Based Ionic Liquid Electrolytes

Boronium-cation-based room-temperature ionic liquids (RTILs) were applied for the first time as novel supporting electrolytes in rechargeable Li|LiFePO 4 batteries. The physicochemical properties of three different materials (L 1 L 2 )BH 2 −NTf 2, 3a (L 1, L 2 = 1-methyl imidazole (mim)), 3b (L 1, L 2 = 1-butylimidazole (bim), and 3c (L 1 = trimethylamine (N 111 ), L 2 = dimethylethylamine (N 112 )), which are readily synthesized from inexpensive and commercially available starting materials, were established by DSC, TGA, conductivity, and cyclic voltammetry. These RTILs are stable up to between 238 and 335 °C and display sufficient conductivities and electrochemical windows (4.3−5.8 V) to be compatible with the Li anode of a battery. Stable battery cycling with good capacity retention was possible for >300 cycles with (N 111 )(N 112 )BH 2 −NTf 2 + LiNTf 2 solutions at charge − discharge rates C /10 and C /5 between 50 and 30 °C. By contrast, a C4mpyr−NTf 2 + LiNTf 2 electrolyte system performed less well under the same conditions despite the higher conductivity of C4mpyr−NTf 2 compared to the boronium RTIL 3c . Li battery cycling was also possible with the imidazole units containing material (bim) 2 BH 2 −NTf 2 for 140 cycles at 80 °C. These new materials could emerge as important electrolytes for various electrochemical applications.