Investigation of Hybrid Battery/Ultracapacitor Electrode Customization for Energy Storage Applications with Different Energy and Power Requirements Using HPPC Cycling

This article explores hybrid energy storage devices in which an individual electrode is composed of a mixture of the active materials used in lithium-ion batteries and ultracapacitors, allowing them to exhibit characteristics of both device types. In order to explore the breadth of options between a pure battery electrode and a pure ultracapacitor electrode, seven different electrode compositions containing mixtures of lithium iron phosphate (LiFePO4) and activated carbon have been investigated. The hybrid electrodes have been implemented in an aqueous lithium sulfate electrolyte with single-electrode measurements using a reference electrode. The hybrid pulse power characterization (HPPC) test profile has been employed to characterize the energy and power capability of each electrode composition. An approach for converting single-electrode cycling results to full cell predictions suitable for the HPPC analysis procedure is presented. By executing this procedure, battery scale factors representing estimates of the quantity of active material required to achieve a set of energy storage target requirements have been calculated. Models of full 18650-format hybrid cells are developed to extend results to optimize a tailored energy storage system on the basis of total cell mass and volume. These results show that the required amount of active materials, total mass, or total volume of an energy storage system can be minimized for a given application by using a hybrid electrode and adjusting the constituent fraction of battery and ultracapacitor materials.