Novel Type of Li-Ion Capacitor with Improved Energy/Power Performance

Abstract

Constantly, the world is looking for the perfect energy storage and conversion system. Two energy storage technologies are currently the most popular - electric double-layer capacitors (EDLCs) and lithium-ion batteries (LIBs). On the one hand, EDLCs cannot (and is not obliged to) compete with Li-based technology in terms of energy density; this feature is quite often sought as the reason for the limited commercialization of EDLCs. On the other hand, LIBs have shorter lifespan and lower power values comparing to EDLCs. Therefore, it seems reasonable to merge both technologies into one compact device, called a hybrid lithium-ion capacitor (LIC). The hybridization of energy storage mechanisms allows for acquiring higher energy density values than EDLCs while maintaining high specific power and a great number of charge/discharge cycles. The major problem occurring during LICs assembling is the pre-lithiation stage. Auxiliary metallic lithium electrode is one of the solutions – in fact, this approach was encouraging; however, after the pre-lithiation process, the metallic lithium electrode should be entirely utilized or removed and become replaced by a carbon electrode. This raises technical and safety issues because the exchange requires an inert environment. Another approach uses composite materials as positive electrodes, allowing full intercalation of lithium into graphite structure.Nonetheless, after the pre-lithiation process, the dead mass of the composite stays in the system and deteriorates the efficiency [1]. A different method of pre-lithiation using lithium from electrolyte was presented by Beguin et al. [2]. In this approach, the main problem identified was the consumption of lithium ions from the electrolyte and the diminished electrolytic conductivity of the solution. In our work, we propose another concept that allows for full intercalation of the negative electrode without an auxiliary electrode or less-conductive composite on the positive side. In principle, the redox-active lithium salt dissolved in 1 mol∙L-1 LiPF6 in EC:DMC electrolyte served as a charge-balancer. Thus, additional Li+ in the redox agent structure made the Li+ concentration in the electrolyte non-affected. Full electrochemical tests, including cyclic voltammetry, galvanostatic charge/discharge and impedance spectroscopy, constant power discharge, were carried out. In further studies, optimal LIC operating conditions, i.e., voltage (2.2 – 4.2 V) and current density (0.2 A⸱g-1), were determined. The system has also been compared with commercial products. Both, redox and non-redox processes occurring on both electrodes were identified. The capacitor's cycle life and the effect of the addition of inorganic salt on the conductivity were also investigated. Furthermore, detailed analysis has been conducted using operando mass spectrometry coupled with gas chromatography, SEM/EDS, XRD, Raman and FTIR spectroscopy.