Dongki Hong

Synthesis of Ultrathin Si Nanosheets from Natural Clays for Lithium-Ion Battery Anodes

Two-dimensional Si nanosheets have been studied as a promising candidate for lithium-ion battery anode materials. However, Si nanosheets reported so far showed poor cycling performances and required further improvements. In this work, we utilize inexpensive natural clays for preparing high quality Si nanosheets via a one-step simultaneous molten salt-induced exfoliation and chemical reduction process. This approach produces high purity mesoporous Si nanosheets in high yield. As a control experiment, two-step process (pre-exfoliated silicate sheets and subsequent chemical reduction) cannot sustain their original two-dimensional structure. In contrast, one-step method results in a production of 5 nm-thick highly porous Si nanosheets. Carbon-coated Si nanosheet anodes exhibit a high reversible capacity of 865 mAh g(-1) at 1.0 A g(-1) with an outstanding capacity retention of 92.3% after 500 cycles. It also delivers high rate capability, corresponding to a capacity of 60% at 20 A g(-1) compared to that of 2.0 A g(-1). Furthermore, the Si nanosheet electrodes show volume expansion of only 42% after 200 cycles.

Practical considerations of Si-based anodes for lithium-ion battery applications

Using Si-based anodes in Li-ion batteries is one of the most feasible approaches to achieve high energy densities despite their disadvantages, such as low conductivity and massive volume expansion, which cause unstable solid electrolyte interphase layers with mechanical failure. The forefront in research and development to address the above challenges suggests the possibility of fully commercially viable cells using various structural and interfacial modifications. In particular, we present a discussion of each dimension of Si-based anodes in multiple controlled systems, including plain, hollow, porous, and uniquely engineered structures, which are further evaluated based on their anode performances, such as initial reversibility, capacity retention for extended cycles with its efficiency, degree of volume expansion tolerance, and rate capabilities, by several practical standards in half cells. With these practical considerations, multi-dimensional structures with uniform size distributions (micrometers, on average) are strongly desired to satisfy the rigorous requirements for widespread applications. Furthermore, we closely examined several full cells composed of Si-based multicomponent anodes coupled with suitable cathodes based on practical standards to propose future research directions for Si-based anodes to keep pace with the rapidly changing market demands for diverse energy storage systems.

Multifunctional natural agarose as an alternative material for high-performance rechargeable lithium-ion batteries

Agarose, which is one of the natural polysaccharides that is generally extracted from seaweed, has recently attracted great attention as an environmentally-benign building element for a wide variety of applications. Notably, its disaccharide repeating units bearing ether/hydroxyl groups carry unprecedented performance benefits far beyond those accessible with traditional synthetic polymers. Herein, intrigued by these unusual chemical features of agarose, we explore its potential applicability as an alternative electrode binder and also as a carbon source for high-performance rechargeable lithium-ion batteries. The agarose binder enables silicon (Si) active materials to be tightly adhered to copper foil current collectors, thereby providing significant improvement in the electrochemical performance of the resulting Si anode (specific capacity = 2000 mA h g−1 and capacity retention after 200 cycles = 71%). In addition, agarose can be exploited as a cathode binder. An LiMn2O4 cathode containing agarose binder shows an excellent cell performance (initial coulombic efficiency of ∼96.2% and capacity retention after 400 cycles of ∼100%). Through the selective carbonization of Si-dispersed agarose, Si/C (hard carbon) composite active materials are successfully synthesized. Eventually, the Si/C composite anode and the LiMn2O4 cathode mentioned above are assembled to produce a full cell featuring the use of agarose as an alternative green material. Benefiting from the exceptional multifunctionality of agarose, the full cell presents a stable cycling performance (capacity retention after 50 cycles of >87%).

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.

Folding Graphene Film Yields High Areal Energy Storage in Lithium-Ion Batteries

We show that a high energy density can be achieved in a practical manner with freestanding electrodes without using conductive carbon, binders, and current collectors. We made and used a folded graphene composite electrode designed for a high areal capacity anode. The traditional thick graphene composite electrode, such as made by filtering graphene oxide to create a thin film and reducing it such as through chemical or thermal methods, has sluggish reaction kinetics. Instead, we have made and tested a thin composite film electrode that was folded several times using a water-assisted method; it provides a continuous electron transport path in the fold regions and introduces more channels between the folded layers, which significantly enhances the electron/ion transport kinetics. A fold electrode consisting of SnO2/graphene with high areal loading of 5 mg cm-2 has a high areal capacity of 4.15 mAh cm-2, well above commercial graphite anodes (2.50-3.50 mAh cm-2), while the thickness is maintained as low as ∼20 μm. The fold electrode shows stable cycling over 500 cycles at 1.70 mA cm-2 and improved rate capability compared to thick electrodes with the same mass loading but without folds. A full cell of fold electrode coupled with LiCoO2 cathode was assembled and delivered an areal capacity of 2.84 mAh cm-2 after 300 cycles. This folding strategy can be extended to other electrode materials and rechargeable batteries.

Cost-effective approach for structural evolution of Si-based multicomponent for Li-ion battery anodes

Silicon-based composite materials have been attracting significant attention because they offer an effective strategy for resolving the drawbacks of Si anodes, such as mechanical failure caused by significant volume changes and the resulting unstable interface. These drawbacks significantly affect the electrochemical properties of Si anodes and thereby inhibit their commercialization. Coupling Si anodes with inactive Al2O3 materials is one possible way of addressing these issues. In this study, we developed Si-based multicomponent anode materials comprising a multiscale porous silicon framework uniformly passivated with thin Al2O3 layers. This was achieved using a cost-effective, simple process of selective etching and wet oxidation. We found that the structural properties depended strongly on the Al residue in the core, which later generated asymmetries in the structure and the effective passivation layers. Our novel materials exhibited an excellent battery performance because of the structural robustness conferred by the Al/Al2O3 core support combined with the mechanical stability of the Al2O3 layers. The outermost protecting layers were also shown to enhance Li-ion diffusion through the Li-ion-conducting layers; this stabilized the solid–electrolyte-interphase layers. The wet oxidized Al–Si alloy (ASWO) exhibited an improved cycling performance with 81.9% capacity retention after 500 cycles at 0.2C in a Li half cell. In addition, a full cell using an ASWO-natural graphite (NG) anode and a lithium cobalt oxide (LCO) cathode exhibited excellent cycling performance with 75.3% capacity retention after 200 cycles at 1C.

Hybridizing germanium anodes with polysaccharide-derived nitrogen-doped carbon for high volumetric capacity of Li-ion batteries

Achieving high volumetric energy Ge anodes leaves behind a big challenge such as a huge volume expansion upon Li-ion uptake. Among various strategies, the introduction of conductive and buffering carbon layers can resolve the typical problems (such as a large volume change and poor electrical conductivity) of alloy-type anodes to some extent. On the other hand, a cost-effective and scalable synthesis method has yet to be revealed. In this study, a highly conductive carbon (ANHC) layer derived from polysaccharide with a high nitrogen-doping level (>10%) effectively mitigates the structural deformation of Ge anodes, which is also independently involved in the reversible redox reaction with an improved electrochemical performance compared to typical graphite anodes. The ANHC/Ge self-assembled by a carbothermal reduction process has remarkable anode performance in a half cell, including a stable cycle life (95% capacity retention after 500 cycles at a rate of 1C) with a high volumetric capacity of >1500 mA h cm−3 and a significant suppression of electrode swelling (<21%). In addition, the full cell consisting of the ANHC/Ge anode and LiCoO2 cathode shows excellent cyclability corresponding to a capacity retention of 73% over 300 cycles at a rate of 1C, which offers ultra-high volumetric energy applicable in various energy storage applications.

Directed Self-Assembly of Asymmetric Block Copolymers in Thin Films Driven by Uniaxially Aligned Topographic Patterns

We present a simple, versatile approach to generate highly ordered nanostructures of block copolymers (BCPs) using rubbed surfaces. A block of poly(tetrafluoroethylene) (PTFE) was dragged across a flat substrate surface above the melting point of PTFE transferring a highly aligned PTFE topographic pattern to the substrate. Si wafer, glass, and polyimide films were used as substrates. Thin films of cylinder-forming asymmetric polystyrene-block-poly(2-vinylpyridine) copolymers (S2VPs) were solvent annealed on the surfaces having the transferred surface pattern to induce their directed self-assembly. Cylinders of P2VP oriented normal to the surface are markedly aligned along the rubbing direction and used as templates to generate extremely uniform arrays of various metallic nanoparticles of gold, silver, and platinum over a large area.