Na-Ri Kang

Hydrophobic Sponge Structure-Based Triboelectric Nanogenerator

Hydrophobic sponge structure-based triboelectric nanogenerators using an inverse opal structured film for sustainable energy harvesting over a wide range of humid atmosphere have been successfully demonstrated. The output voltage and current density reach a record value of 130 V and 0.10 mA cm(-2) , respectively, giving over 10-fold power enhancement, compared with the flat film-based triboelectric nanogenerator.

Breathing silicon anodes for durable high-power operations

Silicon anode materials have been developed to achieve high capacity lithium ion batteries for operating smart phones and driving electric vehicles for longer time. Serious volume expansion induced by lithiation, which is the main drawback of silicon, has been challenged by multi-faceted approaches. Mechanically rigid and stiff polymers (e.g. alginate and carboxymethyl cellulose) were considered as the good choices of binders for silicon because they grab silicon particles in a tight and rigid way so that pulverization and then break-away of the active mass from electric pathways are suppressed. Contrary to the public wisdom, in this work, we demonstrate that electrochemical performances are secured better by letting silicon electrodes breathe in and out lithium ions with volume change rather than by fixing their dimensions. The breathing electrodes were achieved by using a polysaccharide (pullulan), the conformation of which is modulated from chair to boat during elongation. The conformational transition of pullulan was originated from its α glycosidic linkages while the conventional rigid polysaccharide binders have β linkages.

Dependency of Electrochemical Performances of Silicon Lithium-Ion Batteries on Glycosidic Linkages of Polysaccharide Binders.

Molecular structures of polysaccharide binders determining mechanical properties were correlated to electrochemical performances of silicon anodes for lithium-ion batteries. Glycosidic linkages (α and β) and side chains (-COOH and -OH) were selected and proven as the major factors of the molecular structures. Three different single-component polysaccharides were compared: pectin for α-linkages versus carboxylic methyl cellulose (CMC) for β-linkages from the linkages standpoint, and pectin as a COOH-containing polymer and amylose as its non-COOH counterpart from the side chains standpoint. Pectin was remarkably superior to CMC and amylose in cyclability and rate capability of battery cells based on silicon anodes. The pectin binder allowed volume expansion of silicon electrodes with keeping high porosity during lithiation due to the elastic nature caused by the chair-to-boat conformation in α-linkages of its backbone. Physical integrity of pectin-based electrodes was not challenged during repeated lithiation/delithiation cycles without crack development that was observed in rigid CMC-based electrodes. Covalent bonds formed between carboxylic side chains of pectin and silicon surface oxide prevented active silicon mass from being detached away from electric pathways. However, hydrogen bonds between hydroxyl side chains of amylose and silicon surface oxide were not strong enough to keep the silicon mass electrochemically active after cyclability tests.

Amphiphilic Graft Copolymers as a Versatile Binder for Various Electrodes of High‐Performance Lithium‐Ion Batteries

It is known that grafting one polymer onto another polymer backbone is a powerful strategy capable of combining dual benefits from each parent polymer. Thus amphiphilic graft copolymer precursors (poly(vinylidene difluoride)-graft-poly(tert-butylacrylate) (PVDF-g-PtBA)) have been developed via atom transfer radical polymerization, and demonstrated its outstanding properties as a promising binder for high-performance lithium-ion battery (LIB) by using in situ pyrolytic transformation of PtBA to poly(acrylic acid) segments. In addition to its superior mechanical properties and accommodation capability of volume expansion, the Si anode with PVDF-g-PtBA exhibits the excellent charge and discharge capacities of 2672 and 2958 mAh g(-1) with the capacity retention of 84% after 50 cycles. More meaningfully, the graft copolymer binder shows good operating characteristics in both LiN0.5 M1.5 O4 cathode and neural graphite anode, respectively. By containing such diverse features, a graft copolymer-loaded LiN0.5 M1.5 O4 /Si-NG full cell has been successfully achieved, which delivers energy density as high as 546 Wh kg(-1) with cycle retention of ≈70% after 50 cycles (1 C). For the first time, this work sheds new light on the unique nature of the graft copolymer binders in LIB application, which will provide a practical solution for volume expansion and low efficiency problems, leading to a high-energy-density lithium-ion chemistry.

Wall-thickness-dependent strength of nanotubular ZnO

We fabricate nanotubular ZnO with wall thickness of 45, 92, 123 nm using nanoporous gold (np-Au) with ligament diameter at necks of 1.43 μm as sacrificial template. Through micro-tensile and micro-compressive testing of nanotubular ZnO structures, we find that the exponent m in

Study on ductile mode machining of single-crystal silicon by mechanical machining

\bar{\sigma }\propto {\bar{\rho }}^{m}

Self-similarity in the structure of coarsened nanoporous gold

σ¯∝ρ¯m, where