INVESTIGATION ON INTERNAL HYBRID SYSTEM OF LITHIUM ION BATTERY AND SUPERCAPACITOR FOR ENERGY STORAGE APPLICATIONS

Abstract

Currently, the demand of electrochemical energy storage devices for energy storage applications such as portable electronic devices, power tools and hybrid electric vehicles (HEVs) is increasing day by day. Although, achieving high energy density as well as high power density within a single device is not that easy. But meeting both the characteristics within a single device is under vigorous investigation. A hybrid system of Li-ion battery and supercapacitor is one of the possible solutions to this problem because Li-ion battery is known for high energy density and supercapacitor is known for high power density and therefore, their hybrid system will have both the characteristics. Present dissertation work deals with the electrochemical characteristics of bi-material electrodes of lithium manganese oxide (LiMn2O4 abbreviated as LMO) and activated carbon (AC) and these bi-material electrodes are acting as cathodes in hybrid half cells and lithium metal is acting as anode (counter electrode) for all cells. Nanocrystalline lithium manganese oxide has been prepared by sol-gel method. Performance and life of the Li-ion batteries depends on the purity of materials of electrodes because the impurities may react with the anode and cathode materials or electrolytes and thus decreases the life of battery. Phase verification of the calcined LMO powders have been done by X-Ray diffraction. Morphology and particles size of LMO and AC powders have been determined by field emission scanning electron microscopy (FE-SEM). Electrochemical performance of all four bi-material electrodes (LMO/AC r0.5, LMO/AC r0.25, LMO/AC r0.65 and LMO/AC r0.75) has been tested by charging-discharging behaviors, capacity retention and CV behaviors. The bi-material electrodes were positively polarized between 3.0V and 4.3V vs. Li/Li+ and their specific charge, energy density, power density were measured. Specific charge and energy density were measured at 0.1C, 1C and 10C rates whereas power density was calculated at 10C rate only. It has been found that the LMO/AC r0.25 bielectrode is having lowest specific capacity whereas LMO/AC r0.75 bi-material electrode has highest specific capacity. The specific capacity and the energy density are higher at 0.1C rates than at 1C rates for all electrodes. Power density of LMO/AC r0.5 bi-material electrode is coming out to be slightly higher than the LMO/AC r0.25 bi-material electrode, also highest among all hybrid systems.