High-Performance Lithium-Ion Hybrid Supercapacitors Based on Lithium Salt/Imidazolium Ionic Liquid Electrolytes and Ni-Doped LiMn₂O₄ Cathode Materials

In this work, we evaluate the effect of Ni-doping LiMn2O4 spinels, LiNixMn2–xO4 (0.01 ≤ x ≤ 0.10) on the performance of lithium-ion hybrid supercapacitors (Li-HSCs) based on a mixture of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and a 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) ionic liquid (IL) as the electrolyte and mesoporous carbon as the anode. Although the positive contribution of metal doping of lithium-ion insertion materials in the electrochemical properties is known for Li-ion batteries, this effect is little studied in battery/supercapacitor hybrid full cells based on these compounds, especially with the use of electrolytes based on ionic liquids at room temperature. This issue is addressed in the present work. Among all studied devices, the Li-HSC based on LiNi0.01Mn1.99O4 was able to deliver the highest energy density values, in the range of 21.7–40.9 W h kg–1. Also, the devices based on low nickel-content spinels (x ≤ 0.03) showed high performance with excellent cycling stability, being able to retain around 79–85% of its initial energy density after 1500 cycles of charge/discharge at 400 mA g–1.