Fast and High Capacitive Electrode Materials for Asymmetric Supercapacitors

Lithium ion capacitors (LICs) are unique energy storage devices that combine the high power technology of conventional EDLCs with the high energy density characteristics of lithium ion batteries. An important technology in LICs is lithium pre-dope to a carbon negative electrode (NE), but how the depth of charge by pre-dope affects the charging and discharging performance of LICs is not well understood so far. In this study, pitch-coated graphite (PCG), which suppresses self-discharge, was used as NE. Noting that PCG has several stage structures, the objective of this study is to investigate the effects of NE utilization conditions (utilization stages and cycle utilization capacity) on the cycling behavior of LICs. In other words, we will report the effect of different utilizing stages with controlled depth of charge by pre-doping on the cycle behavior.<br/>A four-terminal aluminum laminate cell was used with activated carbon as the positive electrode material, PCG as NE, Li metal foil as the counter electrode and reference electrode for pre-doping, and 1.2 M LiPF₆ /EC : PC : DEC (3 : 1 : 4 v/v/v) as the electrolyte. Pre-doping and cycling tests were performed according to planned negative electrode utilization conditions. Pre-doping was performed up to a certain utilizing stage in a high-temperature environment, and the cycle test was conducted at room temperature for 500 cycles under constant current conditions at 5C.<br/>First, we found that the smaller the cycle capacity, the more stable the cycle behavior is, and that there is obvious Li loss in the early cycles, which does not affect the cycle stability regardless of the NE utilization conditions. We observed the change in electrode potential (potential swing) for three cycle test conditions with a cycle capacity range of 150 mAh g^-1 at different utilizing stages. When the cycling is centered on “dilute stage II”, the utilization stage shifts to a shallower direction with cycling, and the NE potential increases and degrades. It is considered that Li is continuously lost from the NE during cycling. When we utilize “stage II” as the utilizing stage, compared to the previous result, the NE potential does not change with cycling and shows stable cycling behavior. When we utilize “stage I”, although there is a concern about an increase in the resistance of the NE because the electrode potential falls below 0 V vs. Li / Li⁺ during charging, the cycle behavior looks stable. From these results, we considere that the utilizing stage has a significant impact on the cycle characteristics. Based on the above, we will extensively discuss the influence of the utilizing stage on the cycle behavior.