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According to the Dollo’s law, biological evolution that authorize a species new functions, simultaneously create a mechanical or physiological limitations. This perspective has been also applicable to technological innovation, and energy storage systems have seemed to strictly obey the rules. Rigid external passive structures have been thought to be essential for the conventional energy storage systems, because packaging with robust wearing maintain the structure of a device and minimize damages that are caused by impacts from external forces and the environment. However, these advantageous characteristics of conventional device packaging results in critical limitations for further innovation of energy storage systems, such as (1) low degree of deformation, (2) scalability of device unit and (3) thermal and pressure controllability. As the vertebrates blazed a trail in the biological evolution with endoskeleton frame, we propose a novel construction of energy storage device with an analogy to bio-systems. Reverse stacking to fabricate endoskeletal frame would allow energy storage devices deformable and consequently applicable to diverse electronics with generosity.

For a brand-new electrode material with transition metal-free and high electrochemical performance for Li-ion battery (LIB), graphene and disulfide-based framework was designed and synthesized by hybridizing graphene and organic compounds. For the designing of the material, we focused on the four important electrochemical properties: specific capacity, potential sensitiveness, cyclic stability and high Li+/e- mobility. As a result, graphene framework with a layered structure constructed by the formation of disulfide bonds between terminal dithiol organic linker molecules and thiol-functionalized graphene sheets. The organic linker molecules play a role as pillars which maintains the framework structure and prevents restacking of graphene sheets. Various types of dithiol organics were investigated as linker components in GSF for evaluating the effect of its length or structure on electrochemical properties. As a result, distinctive electrochemical performance among the synthesized materials was observed depending on current density, implies the involvement of multiple Li-incorporation mechanisms. Based on the differential analyses and electrochemical impedance spectroscopy (EIS), the linker-dependent Li storage mechanism in GSFs was suggested. Also, reversible breakage-formation of disulfide bond within GSF was proved by ex-situ XPS analysis with varied state of charge (SOC), which represents self-healing functionality of the material. The GSF proposed in this study could be a promising electrode material for transition metal-free, low cost and high performance energy storage system.

A demand for lithium-oxygen (Li-O­2) batteries with high storage capacity is increasing due to its application for the energy storage systems and electric vehicles. However, there are still critical bottlenecks for fabrication of a commercial Li-O2 battery. The cyclability of Li-O2 battery is very poor compared to lithium-ion battery. The main reasons for this problem are a variety of side reactions during the battery operation, which inactivate the lithium anode and cathode catalyst. To solve this problem, reversible charge/discharge reactions should be presented. During discharge reaction of Li-O2 battery, various superoxide and peroxide species, such as O2 -, LiO2 and Li2O2 appear. These nucleophilic molecules could react with electrolyte and carbon electrode. A dominant portions of side-reactions originate from this process. Therefore, we adopted a catalyst support with surface oxygen-defects to stabilize the nucleophilic molecules. In this study, we used zirconia as a support and observed that electrocatalyst with zirconia support exhibits better cyclability compared to that without support. Experimental and computational approaches are exploited to understand this phenomenon and we found that the reactivity of superoxide and peroxide species decrease with the oxygen-defective support. Our concept proposed in this study could be the clue for the design of cathode catalyst for reversible charge/discharge reactions.

Supplying precise power is a significant factor in many electric devices to secure optimized performance, energy efficiency and device protection. For example, in OLEDs, the energy efficiency is highly dependent on the applied potential. However, most of electrode materials for batteries have its unique thermodynamic value, which is almost impossible to be adjusted. Therefore, tunability of the electrochemical property of materials is highly desirable for improving the performance of electric devices. MLi2Ti6O14 (MLTO, M = 2 Na, Sr, Pb, Ca, Ba etc.) have been paid attention for the next-generation anode material for lithium-ion batteries (LIB) because of its lower working voltage than that of Li4Ti5O12 (LTO). Also, they shows flat potential plateau as well as LTO. Their operating potential and equivalent amount of intercalatible lithium ions varies depend on the M species. However, all of their crystal structures are analogous, except minor differences. Based on these observations, we achieved precise tuning of the electrochemical properties of MLTO with two coexisting M species by modulating stoichiometry of M species. In this research, we used Na, Sr-coexisting MLTO (Na2-2xSrxLi2Ti6O14, NaSrLTO). Na2Li2Ti6O14 (NaLTO) has the lowest working potential (1.25 V) as well as the lowest reversibly intercalatible Li+ (2.0 equiv.-1) among the MLTOs, and SrLi2Ti6O14 (SrLTO) is diametrical of NaLTO (1.4 V and 2.5 equiv.-1). In NaSrLTO, operating potential gradually rises and equivalent inserted Li+ increases as higher ratio of Sr exists. Ab initio calculations revealed that the gradual potential change is originated from the gradual change of electron density of Ti3+/Ti4+ redox center. Also, Because SrLTO has half-vacant 8f Wyckoff position, SrLTO is able to store more Li+ than NaLTO, which all of Na atoms fully occupy the 8f Wyckoff position.

Although a large number of reports on 2D flexible and stretchable energy storage devices have emerged within the past decade, 1D structure systems such as wires for higher flexibility, bio-compatibility and wearability have been desired for broader application of electronics. As a result of these requirements, wire-shaped devices among the cutting edge technologies in energy storage field are taking center stage for application to postmodern electronics and various fields such as fashion and culture. In view of mechanics, plane structures in 2D essentially have a unidirectional folding characteristics. On the other hand, wires with 1D structure have an omnidirectional serpentine structure in the other axes. In contrast to 2D planar formats, it is much easier for wire types to be integrated into more sophisticated structures such as woven textiles that can be used in our daily life. Although wire systems have a large structural potential, some fundamental obstacles hinder the use of wire-type energy storage. Electrochemical performances of wires are still underneath those of 2D plane cells. Many researchers have made efforts to overcome these barriers of wire-type energy storage devices. Most of them suggests ways to shape electrodes into cylindrical wires and a choice of material to maximize the performance as high as possible. In this study, however, we did not stick to using superb materials or constructing a cylinder, but theoretically analysed the origin of undefeatable charge-storage property of 2D plane over 1D wire. Based on this analysis we verified that a difference between wire-type and conventional planar supercapacitors induced the fundamental limits of wire-type supercapacitors. This limit is termed “energy lag effect.” With these derived information, we have first proposed the simultaneous incorporation of favorable electrochemical performance of 2D system and high flexibility of 1D system into solely one device. To exclude the identified limits, we designed a supercapacitor thread in unique structure that completely differs from conventional wire-type energy storage systems. The supercapacitor thread was designed as a dual plane-helix structure and proved not to show energy lag effects, experimentally or analytically.

Wearable electrical devices have attracted tremendous attention along with the improvements of electronic technologies. One of the most important prerequisites of such electric systems is an energy storage system with durability against various stresses in daily life (e.g. mechanical impacts or invasion of humidity). In this research, we report on a development of all-solid-state, foldable and washable supercapacitor system. A constructed carbon network was readily assembled into a full-cell by its self-adhering architecture. The as-prepared system operated stably even under the conditions of severe folding and washing without capacity fading, which is attributed to both robust electrical connectivity of the fibrous carbon network and intimate contact between solid electrolyte/electrodes. In addition, a simple modification of electrode greatly enhances the energy density of system. The collected results suggest that this supercapacitor system can be a promising candidate for practically available and wearable energy storage systems with high cost-effectiveness and scalability.

A demand for lithium ion batteries which able to operate under harsh condition (e.g. low temperature) is increasing due to its application for the electric vehicles. However, there are still technical bottlenecks for fabrication of a battery cell operating at low temperature. To solve this problem, various parameters and its effects for the kinetic mechanism during battery charge/discharge should be clarified. Computational approach is exploited to understand the relationship between performance of battery and kinetic parameters. In this study, we choose three independent parameters which affects to capacity and operating potential of the battery: the diffusion coefficient of lithium ion in active material, the lithiation constant at the surface of the active material and the particle radius of the active material. Based on pseudo-2D model, the capacity and the operating potential are calculated by varying diffusion coefficient, rate constant, and particle radius. Because diffusion coefficient and rate constant is dependent on temperature, the performance of battery drastically decline at certain temperature. From this theoretical basis, we calculate the particle radius of active material for maximizing the capacity and the optimal operating voltage at low temperature.

The recent boom in electronics requires high performance and stretchable energy storage systems. As a result, the growth of energy storage system with patterning assembly technologies has been accelerated. In this research, we introduce a novel design in the supercapacitors, which enables us to realize the unlimitedly increased potential window characteristics that are unavailable when conventional schemes are applied. The patterning assembly of superapacitors improves the electrochemical performance of the energy storage by a high surface-to-volume ratio and non-use of binders and separators in their fabrication. Moreover, the technique generates physical advantages, such as folding and stretching of the supercapacitor system. The new supercapacitor system is composed of periodically isolated electrodes and sectionalized electrolytes without restrictions on electrode or electrolyte selections. The isolated electrodes and sectionalized electrolytes are key factors for the extremely packed analogical series circuit system which permits energy and power to be simultaneously increased. This energy storage system also assumes an origami-type that accommodates extremely high stable stretching. We believe that our concept provides noteworthy implications across a number of disciplines toward ultimate energy, power and stretchable devices for next generation of electronics.

Although high surface area and short lithium ion diffusion of nano-structure have dramatically improved the performance as the Li ion battery until now, inevitably accompanying problems such as electrode side reaction or structural instability made one develop for rationally designed architecture. In this research, a rational design of TiO2 architecture is exploited in order to elicit its intrinsic Li storage performance. We proposed the 3D dendritic TiO2 sphere as an idealized architecture which comprised of specialized nanorods. When fabricated into an electrode, the 3D dendritic TiO2 sphere is capable of delivering an outstanding efficiency in conjunction with long cycle charge/discharge stability. This approach, which is based on theoretical studies and experimental validation, provides guidance for tailoring electrode materials for use in Li storage systems.