Novel binary deep eutectic electrolytes for rechargeable Li-ion batteries based on mixtures of alkyl sulfonamides and lithium perfluoroalkylsulfonimide salts

Ionic liquids (IL's) were proposed for use in Li-ion batteries (LIBs), in order to mitigate some of the well-known drawbacks of LiPF₆/mixed organic carbonates solutions. However, their large cations seriously decrease lithium transference numbers and block lithium insertion sites at electrode-electrolyte interfaces, leading to poor LIB rate performance. Deep eutectic electrolytes (DEEs) (which share some of the advantages of ILs but possess only one cation, Li⁺), were then proposed, in order to overcome the difficulties associated with ILs. We report herein on the preparation, thermal properties (melting, crystallization, and glass transition temperatures), transport properties (specific conductivity and viscosity) and thermal stability of binary DEEs based on mixtures of lithium bis(trifluoromethane)sulfonimide or lithium bis(fluoro)sulfonimide salts with an alkyl sulfonamide solvent. Promise for LIB applications is demonstrated by chronoamperometry on Al current collectors, and cycling behavior of negative and positive electrodes. Residual current densities of 12 and 45 nA cm^-2 were observed at 5 V vs. Li/Li⁺ on aluminum, 1.5 and 16 nA cm^-2 at 4.5 V vs. Li/Li⁺, respectively for LiFSI and LiTFSI based DEEs. Capacities of 220, 130, and 175 mAh· g^-1 were observed at low (C/13 or C/10) rates, respectively for petroleum coke, LiMn_1/3Ni_1/3Co_1/3O₂ (a.k.a. NMC 111) and LiAl_0.05Co_0.15Ni_0.8O₂ (a.k.a. NCA).