Recently high capacity energy storage devices are highlighted due to rapid growth of electric vehicles(EVs) and large-scale grid energy storage system. Especially Li ion batteries (LIBs) is the most suitable energy storage devices for EVs performing with high energy density. However, LIBs have some problems. The first is slow charging time issues owing to intrinsic reaction mechanism. The second is operating voltage range limit which is directly connected to energy density derived from revisable capacity of cathodic material. The last is cost of cathode materials because of their scarcity. To overcome these issues, we introduce hybrid energy storage system that delivers high energy density without sacrificing power density. This hybrid system is made up of expanded graphite/copper oxide composite (GCuO) as the negative electrode and activated carbon (AC) as the positive electrode. . The GCuO is high capacity materials by faradaic reaction. The AC provide large capacitance by non-faradaic reaction. Additionally, both the active materials are abundant and low price. So, this system shows the three types of energy storage mechanism: a Intercalation reaction derived from the expanded graphite, a conversion reaction from the copper oxide, and a physical adsorption/ desorption of ion the interface between the AC electrode and the electrolyte

The GCuO is synthesized by a simple process, through heat treatment of the Cu ion complex and pristine graphite. While the Cu ion complex is thermal decomposed, H₂ and CO₂ gases are generated. These gases induce the graphite planes expanded, the gap between the planes is increased from 0.34nm to 0.40nm. Also, copper oxide is in-situ formed on the graphite plane. The expanded graphite has a lower Li⁺ ion intercalation barrier than pristine graphite. So the synthesized material leads to a fast diffusion of lithium ions on the anode. As a result, a high power density can be obtained by enhanced charge transfer between the electrode and the electrolyte. The copper oxide provides additional capacity to the anode. Structure of GCuO were confirmed with HR-TEM (High Resolution Transmission Electron Microscopy), EDS (Energy Dispersive Spectrometer), HR-XRD (High Resolution X-ray Diffraction) and Raman Spectroscopy. And the specific surface area of AC was characterized by BET(Brunauer-Emmett-Teller) analysis and the value was 2131m² g^-1. The large surface area provided large capacitance by the ion adsorption/desorption.

The half-cell test was performed with Li metal electrode. The GCuO half-cell exhibited good specific capacity (~500 mAh g^-1 at 0.2C) and life characteristics (83% capacity retention up to 250 cycles ). The AC half-cell showed specific capacity (~60 mAh g^-1 at 1A g^-1) and excelent cycle ability (98% capacity retention up to 500cycle). Before the full cell assembled, GCuO electrode needs to be pre-lithiation step under proper conditions since the positive electrode is not a lithium-ion source unlike the LIBs. The pre-lithiation is very effective for lowering the potential of the negative electrode and forming a stable SEI (Solid Electrolyte Interface) layer. Due to the lowered anode potential, a full cell had a wide operating potential window, which can obtain a higher energy density. In addition, the cycle characteristics of the cell were also enhanced by preventing additional loss of lithium-ion in the electrolyte . The volume change of copper oxide occurs while the copper oxide lithiation / delithiation. The lithiated CuO (enlarged CuO) makes graphite planes gap expanded. Accordingly, repetitive charging/discharging of GCuO can improve the hybrid system’s performance. The full cell with repetitive pre-lithiation has a higher energy density (~1.5 times) than the one pre-lithiated. Finally, optimized full cell shows high specific energy of about 210 Wh kg^-1 at specific power of 270W kg^-1 , which is equivalent to LIB’s energy density performance and the specific energy remain 70Wh kg^-1 when the specific power of 10kW kg^-1 within the voltage range of 1.0~4.0V. In addition, the optimized full cell showed excellent energy efficiency over 90%, which is better than that of LIBs with 80%. In conclusion, GCuO is outstanding and promising anode active material for hybrid energy storage system.