A series of trifluoromethanesulfonamide solvents were synthesized with systematically controlled ring size (4–6) at the N-terminal to tune their steric and electronic properties to realize enhanced contact ion pairs for the formation of an anion-derived solid-electrolyte interface (SEI) and compatibility with the NMC811 cathode. Comparative analyses of electrolytes revealed that the 1.6 M LiFSI 1-azetidine trifluoromethanesulfonamide (AzTFSA) electrolyte presents the ideal combination of steric and electronic effects along with high oxidation stability up to 5 V and a Coulombic efficiency of 99.2% in Cu–Li half-cells at 1 mA cm–2 and 1 mAh cm–2. The corresponding full cells using 20 μm of Li foil paired with the NCM811 cathode by a negative and positive capacity ratio (N/P) of 2.5, achieve 80% capacity retention after 150 cycles at 0.5C. Even at a high charge cutoff voltage of 4.6 V, the Li|NCM811 full cell still realizes 92% retention at 0.5C after 100 cycles.