Study on surface structural control for performance improvement in energy harvesting and storage applications

Abstract

Recently, as demand for wearable devices and Internet of Things(IoTs) systems requiring high energy density has increased, demand for eco-friendly energy harvesting using energy existing around them such as solar, heat, and wind as well as research on performance improvement of lithium(Li)-ion batteries that supply power to each device individually is increasing. According to such social demand, Li-ion batteries using materials with high energy density and devices for harvesting various types of eco-friendly energy have been researched and developed. However, a Li-ion battery has low cycle stability and Coulombic Effiency(CE) because of the volume expansion occurring charging and discharging, and a harvesting device has low power conversion efficiency, making it difficult to apply it to commercial products. Therefore, there is a need for a new research method to improve the output of electronic devices to fill energy demand. This thesis suggests novel methods to improve the performance of eco-friendly harvesting devices for use as auxiliary power sources as well as energy storage devices through various surface structure control. This thesis begins with an overview on representative energy harvesting and storage applications including triboelectric nanogenerators(TENGs) and Li-ion batteries in Chapter 1, in which an overall system, working mechanisms, and their limitations are described. Chapter 2 and 3 are described a case of manufacturing various structures to improve electrical performance and applying them to Li-ion batteries, which are representative energy storage devices. The first topic of this thesis (Chapter 2) is silcon(Si) anodes of Li-ion batteries with Si structures that can be repeatedly produced via using laser interference lithography(LIL) in combination with the metal assisted chemical etching(MACE) process. This facile process enabled us to produce a large number of Si rods from one patterned Si, thereby saving time and cost. In particular, to investigate the effect of boron doping and its concentration on the electrochemical battery performance, various Si rods anode were prepared. As a result, the lightly doped Si rods (1015 atoms cm-3), anode exhibited the highest capacity (3377 mAh g-1) at the first discharge at 0.05 C, high initial coulombic efficiency of 98.1% and excellent cycling stability (1137 mAh g-1 after 500 cycles). In the following chapter (Chapter 3), a lithium metal anode battery with a structure that uses two layer of different lithiophility is described. In order to suppress the formation of Li dendrites generated by non-unifomr charge distribution, which causes low cycle stablitiy, a hybrid current collector combined with lithiophobic and lithophilic layers is manufactured. The current collector applied with the structure suppresses the vertical growth of Li dendrite and causes horizontal growth, so that Li is electrodeposited uniformly, thereby improving high CE and electrical performances. Chapter 4 and 5 are described TENGs to which new structures for high electrical power conversion efficiency through surface control is applied. In Chapter 4, new TENGs that exhibits high electrical performance by intercalating an Al layer into dielectric films to enable bidirectional friction are described. Electrostatic charges are readily induced between the dielectric surface and the intercalated Al layer at both sides. To investigate the effect of the double-contact and intercalated Al layer, this study compared the electrical performance characteristics by fabricating a single-contact TENG without the intercalated Al, a single-contact TENG with the intercalated Al, and a double-contact TENG with the intercalated Al. It is found that the double-contact TENG exhibits the highest open-circuit voltage (Voc) of 232 V, a short-circuit current (Isc) of 0.256 mA at a high frequency of 412 Hz, and a maximum output power density of 32.8 W m-2 at a wind velocity of 15.1 m s-1. Moreover, up to 36 LED bulbs are successfully lighted stably. In last Chapter 5 of this thesis, the wind-driven TENGs with cross-shaped dielectric film, which can generate energy in various directions. we describe a new type of wind-driven triboelectric nanogenerator (TENG) with a bent cross-shaped dielectric film in four directions along with an intercalated Al layer within it. The dielectric film can be fluttered by arbitrarily blown wind, thereby collecting wind energy regardless of wind direction.