A Study on Modification of Components to Enhance Performance of High-Energy-Density Lithium-Ion Batteries

Abstract

With significant advances in the performance of battery-powered application devices, such as portable electronic devices and electric vehicles, the demand for high-energy-density batteries has remarkably increased. In particular, lithium-ion batteries (LIBs) have garnered increasing interest owing to their relatively high energy densities, eco-friendliness, and efficient operation. Secondary battery technologies that guarantee high-energy-density and long-term operation are crucial for the development and distribution of electric vehicles capable of long-distance driving. However, since existing commercialized LIBs are limited in capacity and energy density, efforts are being made to achieve higher energy densities within each major component of such batteries. Although individual research pertaining to each major component can improve certain effects, it is difficult to ensure synergy between those components in the integrated battery. In order to realize high overall energy density, a high-capacity electrode material and operation under high-voltage must be enabled simultaneously. In this study, all major components were modified to produce a high-energy-density battery, thereby ensuring sufficiently high-voltage and capacity to allow for a long battery life and enhanced performance. Chapter 2 presents an enhancement in battery capacity by introducing a novel sulfide-based electrolyte additive, developed by controlling the unstable surface of each electrode during the high-voltage operation of a high-capacity battery composed of high-nickel-content NMC cathode material. In addition, it was confirmed that the introduction of Li metal mitigates the unstable dendrites that form on the surface, thereby guaranteeing longer duration and improving the battery's electrochemical performance. Consequently, when a small amount of a single additive is used, the cathode and anode react simultaneously, exhibiting excellent cycle performance in the operation of a high-energy-density battery. Chapter 3 describes the preparation of a Si-based anode via vapor deposition; this electrode was then used to overcome the capacity limit of anode that are primarily graphite-based. The electrical conductivity of the Si-based anode was improved via phosphorus (P) doping. Furthermore, the introduction of silicon oxides (SiOx), which have good physical properties and conductivity, onto the Si film surface yielded a higher capacity, and also improved the capability of battery to respond to changes in volume. Capacity was improved by preparing a composite electrode of graphite and Si veneer-type particles (SVPs) expanded to the microscale, wherein several layers of nano-sized Si anode were alternately coated via vapor deposition. The SVPs within the composite electrode improved the rate performance owing to their high conductivity and increased the Li+ diffusion coefficient. Alternatively, an electrode was directly prepared on a Cu current collector by thin film deposition. By introducing an oxide layer to the top and bottom of the Si deposition layer to change in volume and buffer for the high conductivity, electrochemical properties and surface stability were confirmed to improve. The correlation between the long-term operation performance of the battery and the reduction of irreversible capacity through conductivity was also analyzed. Chapter 4 evaluates the improvement in battery performance obtained by reducing the negative factors. These unfortunate factors are incurred by adhesion between the electrode and separator, caused by the process of coating an adhesive layer on the separator to minimize the effect of gas formed by the side reactions of electrolytes and additives during high-voltage operation. Enhancements in heat resistance and electrolyte affinity were achieved by introducing a ceramic coating layer. Consequently, with the electrode and separator adjoined, high-energy-density battery operation performance was confirmed to improve. High-capacity electrode and high-voltage operation are requirements that must be fulfilled simultaneously in the operation of high-energy-density LIBs. In this study, beyond modifying just individual components to satisfy these requirements, mutual simultaneous applications of all interrelated components were examined. Thus, synergistic relationships between major components were examined in depth in order to resolve the issues that may occur in high-energy-density batteries from a macroscopic perspective.