Expanded graphite/copper oxide composite electrodes for cell kinetic balancing of lithium-ion capacitor

Lithium-ion batteries (LIBs), favored for their high energy density, have slow charging issues due to redox reactions in both electrodes. To overcome these issues, we present a hybrid energy storage system that delivers high energy density without sacrificing power density. This hybrid system is made up of an expanded graphite/copper oxide composite as the anode material and a porous carbon as the positive electrode material. The proposed composite anode is fabricated in one step; it shows a high specific capacity and a high specific power density due to a lowered lithium -ion insertion barrier of expanded graphite derived from copper oxide synthesis and pre-lithiation. The high theoretical capacity of copper oxide is effective in yielding high energy density. Furthermore, the reaction kinetic balancing needed for a lithium-ion capacitor is determined by the mass ratio of its electrodes. As a result, it is confirmed that internal resistance is more important than specific capacity for a lithium-ion capacitor, unlike a Li-ion battery. The optimized hybrid cell shows a high specific energy density of 212.3 Wh kg⁻¹ at a specific power density of 1.3 kW kg⁻¹ and maintains 85% of its initial energy density after 500 cycles.