Lithiation of amorphous carbon negative electrode for Li ion capacitor

Electrochemical double-layer capacitors (EDLCs) are well suited to time-dependent electrical power output, but in terms of their overall energy density, these devices are much more limited in comparison to Li ion batteries. However, asymmetric supercapacitors that utilize a lithiated negative electrode and a high surface area positive electrode can result in significant improvements in the overall energy density of the device. Key to the development of a LixC6 asymmetric supercapacitor, or Li ion capacitor, is the preparation of the lithiated negative electrode. A common method of accomplishing this is to allow Li ions to intercalate into the carbon material under a spontaneous potential gradient. However, the insertion process can be very slow and may take several hours to reach equilibrium. However, an electrochemically driven lithiation process could significantly decrease the electrode preparation time, which would have great benefits for the development of these devices. Three methods for the insertion of Li into activated carbon electrodes were evaluated in this work that examined the examined the effect of electrochemically-driven lithiation of an activated carbon powder electrode as it may compare to unaided lithiation. In addition, in one of the configurations, the sacrificial Li metal electrode faced the ‘back’ side of the carbon electrode, which provides the advantage in that the cell would not have to be disassembled and re-assembled to evaluate the completed Li capacitor cell, since the positive electrode could readily be added to the cell during the initial assembly. During the lithiation process, the current flow and total charge passed were monitored and an electrochemical impedance analysis was performed to gain insight into the formation of the SEI layer on the carbon surface during the Li insertion process.

► Detail mechanisms for the Li-ion asymmetric supercapacitors were explained.
► Electrochemically driven lithiation shorten the time, as compared to unaided lithiation.
► The current density for pre-lithiation has significant impact on the SEI layer formation.
► The Li ion capacitor could fill the energy density space between EDLC and Li ion batteries.