2, University of the Basque Country, Leioa, Bizkaia, Spain
In this work we report lithium ion capacitors (LIC) with extraordinary energy-to-power ratios based on olive-pit recycled carbons, supported with graphene as a conducting agent. LICs typically present limited energy at high power densities due to the sluggish kinetics of the battery-type electrode. In order to circumvent this limitation, the olive-pit derived hard carbon (HC) was embedded in reduced graphene oxide (rGO). The addition of rGO into the negative electrode not only wraps HC particles but forms a 3D interpenetrating carbon network, facilitating Li-ion diffusion and enhancing the electronic conductivity -which becomes critical- at high power densities. Impedance analysis reveals that charge transfer and contact resistance within the electrode is considerably inferior in the presence of rGO. Furthermore, it remains constant upon cycling, independent to the applied current density, while in the absence of rGO resistance is highly depending on the applied current density. Resistance reduction within the battery-type electrode is reflected in capacity gain at 10C, which is over 100%, rising from 75 mAh g-1 to more than 150 mAh g-1. This fact triggers the overall LIC performance, allowing assembling an ultrafast LIC delivering 80 Wh kg-1 at a power ratio of 10000 W kg-1, to our knowledge one of the best reported energy densities at such a high delivery of power.