Design of High Energy Density Anode Materials for Next-generation Energy Storage Systems

As environmental regulation on conventional fossil fuel energy would be tightened, interest in renewable energy and energy storage devices have been gradually increased. Especially, the electric vehicle (EV) field which mainly includes a great portion of the battery market, would be estimated to grow more than 100 times bigger than the current size in 20 years. However, the insufficient energy density of the current lithium ion battery (LIB) system hinders the popularization of the EVs. Therefore, the demands for high energy density energy storage systems have been increased sharply owing to the development of long-range EVs. The potential solutions for the enhancing energy density are the introduction of silicon (Si), germanium (Ge) anode based LIB and lithium metal battery (LMB) systems. The main difference between LIB and LMB is the number of host material. LIB consist of two host materials of anode/cathode, electrolyte, and separator. In LIB, lithium (Li) is stored as an ion state, thus it shows good electrolyte compatibility, stable battery performance, but relatively low energy density. On the other hands, LMB has one host material. Instead of anode host materials, metallic Li is occupied. We can expect higher gravimetric and volumetric energy density in LMB owing to the extra elimination of host material in the anode side. However, deposited Li metal is thermodynamically unstable and causes Li dendrite growth, resulting in low Coulombic efficiency and safety issue. Herein, we introduce diverse strategies of Si/Ge based anodes (LIB) and gel polymer electrolyte (GPE)/electrolyte additives (LMB) for high energy density battery systems. In Si/Ge LIB, we focus on solving the original nature of large volume expansion (>300%) and low conductivity by the combination of volume expansion compensation and conductive layer. On the other hands, various attempts for Li dendrite suppression, for example, uniform Li deposition or mechanical strength are carried out in the LMB system. In chapter II, we design a 3D porous structure with aluminum (Al) core and aluminum oxide (Al2O3) layer through selective etching and wet oxidation process. The volume expansion during electrochemical lithiation is compensated owing to the porous structure. Al core inside supports the overall particle structure and outer Al2O3 thin layer enhances ionic conductivity and structural robustness, resulting in stable long-term electrochemical performance. In chapter III, a nitrogen-doped Ge/C composite secondary particle is reported via gelation and carbothermal reduction process using natural carbon source and germanium oxide nanoparticles. This secondary particle strategy is effective for preventing volume expansion in both particle level and electrode level owing to the existence of enough free volume inside. In addition, nitrogen dopant successfully increases the electric conductivity, leading to higher Coulombic efficiency during cycling. In chapter IV, we report a functionalized carbon nanodot (CD) additive for LMB which is well dispersible in commercial carbonate electrolyte. In addition, CDs include surface negative charges and various functional groups, which are easily controllable through modulating the amount and types of precursors. The synergistic interplay between high negative surface charges and the configuration of nitrogen in CD plays a critical role in strong Li-ion affinity, as determined by various characterization and computational analysis. Therefore, CD draws Li ions through the strong binding energy under electric field and moves together. This CD-based shuttling not only provides the nucleation sites for Li-ions, but also induces uniform transportation of Li ions to the current collector, resulting in homogeneous Li plating. In chapter V, we report a mechanically strengthened GPE for LMB. The GPE includes vinyl functionalized silica nanoparticles which forms direct bonds between vinyl end group and polymer chains with the thermally induced in-situ crosslinking process. The inorganic particle included GPE exhibits enough strength which suppresses the growth of Li dendrite mechanically. As a result, we secure the improved electrochemical property, prevention of electrolyte depletion, and minimized degradation layer of Li metal.