High-voltage lithium-ion capacitors enabled by a multifunctional phosphite electrolyte additive

Combining the high energy of lithium-ion batteries with the high power and excellent cycling stability of supercapacitors, lithium-ion capacitors (LICs) stand for an emerging hybrid technology for next-generation electrochemical energy storage. Among them, the configuration composed of a battery-type cathode and a capacitor-type anode is advantageous owing to its unnecessity for electrode prelithiation but disadvantageous because of its limited operating voltage. Herein, using triphenyl phosphite (TPPi) as a powerful multifunctional electrolyte additive to not only scavenge electrolyte impurities but also form robust, uniform, and thin cathode/electrolyte interface film on cathode and solid-electrolyte interphase film on anode, we develop high-voltage LICs from a high-potential spinel LiNi0.5Mn1.5O4 cathode and an activated carbon anode. In addition to the important multifunctionalities of TPPi, we modulate the capacity ratio between the cathode and anode and composite the electrodes with carbon nanotubes (CNTs) to enhance the overall performance for our LICs. Therefore, having the synergy from the multifunctionalities of TPPi, capacity modulation between the cathode and anode, and electrode compositing with CNTs, the optimized LIC shows a high operating voltage of 3.45 V, high energy density of 61.6 Wh kg−1, high power density of 52.5 kW kg−1, and long cycle life with a capacity retention of 91.8% after 6000 cycles, outperforming all of its previously reported counterparts having a similar configuration incorporating a battery cathode and a capacitor anode. The rational strategy developed in this work is versatile and should be applicable to other electrode/electrolyte systems for advanced hybrid electrochemical energy storage.