The underestimated charge storage capability of carbon cathodes for advanced alkali metal-ion capacitors

Li-ion capacitors (LICs) are emerging as complementary energy storage devices to Li-ion batteries to satisfy some specific applications where high power density and long cycle life are required. Due to the wide usage of LICs, LICs with promising energy density are urgently needed; however, at this stage, the achievement of this type of LICs is the main challenge. In this study, we increased the energy density of LICs via both material optimization and charge storage mechanism exploration. Moreover, porous carbon with a high surface area of over 2800 m² g^-1 was fabricated from alkali lignin via a traditional KOH activation method assisted by self-activation. A wide voltage window of 1.0-4.8 V was applied where the synergistic storage of anions and cations was achieved. This shows that a deep discharge down to 1.0 V is necessary for the complete desorption of anions, which also triggers the adsorption of cations (Li⁺), resulting in increased capacity. However, a compromise must be made in the energy efficiency due to intensified battery polarization upon deep discharging. Furthermore, considering the natural abundance of sodium and potassium over lithium, Na- and K-ion capacitors have been investigated for sustainable development using the as-prepared carbon materials.