Yongku Kang

Electrodeposited 3D porous silicon/copper films with excellent stability and high rate performance for lithium-ion batteries

We report a simple two-step fabrication process of 3D porous Si/copper films by an electrodeposition method using a hydrogen gas bubble template. A 3D porous Si/copper film provides a large surface area, a highly conductive pathway, a short ion diffusion length, and buffer spaces to accommodate the stress during the cycling processes.

Improved cycle efficiency of lithium metal electrodes in Li–O2 batteries by a two-dimensionally ordered nanoporous separator

We demonstrate a facile but very effective approach to improve the cycling efficiency of metallic lithium electrodes by controlling the pore morphology of separators. We employed anodized porous alumina as the model nanoporous separator and demonstrated the improvement of cycle efficiency of lithium electrodes in lithium–oxygen cells.

Facile fabrication of highly flexible graphene paper for high-performance flexible lithium ion battery anode

Freestanding paper-like materials prepared from chemically derived graphene have considerable potential as a carbon-based electrode in high-performance flexible energy storage devices. Herein, a highly flexible graphene paper (GP) assembled from graphene nanoplatelets (GNPs) with the aid of graphene oxides (GOs) is reported for a high-performance, binder- and conducting additive-free anode in lithium-ion batteries (LIBs). In contrast to previous reports on GPs based on a flow-directed assembly of graphene sheets, this GNP/GO paper exhibited a highly wrinkled and disordered morphology. When the GNP/GO paper was applied as a LIB anode, it showed a high specific capacity of 694 mA h g−1 and high rate performance. Furthermore, a pouch-type flexible LIB using the GNP/GO paper also showed a stable cycling behavior and practical performance. This GNP/GO paper electrode prepared using a simple, yet effective assembly of graphene derivatives, is highly promising for the fabrication of flexible energy storage devices.

Electrospun nanofibers with a core–shell structure of silicon nanoparticles and carbon nanotubes in carbon for use as lithium-ion battery anodes

Core–shell structured nanofibers, consisting of silicon nanoparticles and carbon nanotubes encased in carbon (SCNFs), were fabricated for use as an anode material in lithium-ion batteries (LIBs). This entailed first electrospinning of precursor solutions containing a blend of silicon nanoparticles (SiNPs), carbon nanotubes (CNTs), and polyvinylpyrrolidone (PVP) for the core, and polyacrylonitrile (PAN) for the shell. The final SCNF structure was obtained by carbonization at 1000 °C for 1 h under nitrogen; the core–shell structure achieved with varying carbon contents was determined by scanning electron microscopy, transmission electron microscopy, and water contact angle measurements. An evaluation of the electrochemical performance of SCNF-based anodes in LIBs found that a SCNF electrode with 1 wt% CNTs has an initial delithiation capacity as high as 1500 mA h g−1 at C/10 rate and a retained capability of 50% at high rates (10C). Following the 100th cycle at 1C, a capacity of 1000 mA h g−1 and coulombic efficiency of 99% were achieved, the former representing 74.1% of the original capacity (1350 mA h g−1). Thus, not only does the robust carbon shell of SCNFs minimize the effect of volume expansion in the SiNPs, but the CNTs in the core also provide a greater number of conductive pathways, both between SiNPs and to the carbon shell, which assist electrochemical reactions.

Facile synthesis of palladium nanodendrites supported on graphene nanoplatelets: an efficient catalyst for low overpotentials in lithium–oxygen batteries

Although morphology-controlled metal nanocatalysts supported on graphene sheets are promising, highly effective catalysts for various electrochemical reactions, their preparation is still challenging. In this paper, we report a facile method for preparing structures with highly branched Pd nanodendrites (PdNDs) supported on graphene nanoplatelets (GNPs) (PdNDs–GNP) and their application as a cathode catalyst in a nonaqueous Li–O2 battery. PdNDs formed on the GNP sheets via a particle-attachment mechanism had an average size of approximately 14 nm, strongly anchored on the GNP sheets, and were well distributed. Binder-free, flexible PdNDs–GNP/graphene oxide (GO) paper electrodes were fabricated and used in a nonaqueous Li–O2 battery. Because of the high catalytic activity of the PdNDs–GNP structures, the Li–O2 cell using the PdNDs–GNP/GO paper electrode exhibited substantially lower overpotentials both on discharge and charge compared with those of the GNP/GO paper electrode without Pd nanocatalysts and even those of the paper electrode consisting of irregularly shaped Pd nanoparticles supported on GNP (PdNPs–GNP) and GO. We found that Li2O2 formed on the PdNDs–GNP/GO paper electrode had a sheet-like morphology, which decomposed more efficiently than did the large toroidal product formed on the GNP/GO paper electrode. Consequently, the Li–O2 cell using the PdNDs–GNP/GO paper electrode exhibited greatly enhanced cyclability over 30 cycles as compared with that of the GNP/GO paper electrode (15 cycles).

An electrochemically grown three-dimensional porous Si@Ni inverse opal structure for high-performance Li ion battery anodes

We report a facile method for the fabrication of a three-dimensional (3D) porous silicon@nickel (Si@Ni) inverse opal structure for Li ion batteries by using an electrodeposition method and a colloidal crystal as a sacrificial template. The Ni inverse opal structure was fabricated first by electrodeposition of Ni on the pre-formed colloidal crystal template, followed by removal of the template. Finally, the Si@Ni inverse opal structure was obtained by electrodeposition of Si onto the Ni inverse opal structure. The highly porous structure of the electrode containing a conductive and mechanically strong Ni scaffold could sufficiently accommodate volume expansion during the Si–Li alloying. A coin cell using the Si@Ni inverse opal structure as an anode exhibited a high charge capacity of 2548.5 mA h g−1, stable cycling retention, and high rate performance without the need for binders or conducting additives.

Enhanced energy and O2 evolution efficiency using an in situ electrochemically N-doped carbon electrode in non-aqueous Li–O2 batteries

N-doped carbon materials were prepared by in situ electrochemical pre-treatment of a carbon electrode in a deaerated non-aqueous electrolyte containing lithium nitrate. Li–O2 batteries, applying the novel N-doped carbon electrode, show a significantly reduced overpotential and enhanced O2-evolution efficiency.

Facile Synthesis of Composition-Controlled Graphene-Supported PtPd Alloy Nanocatalysts and Their Applications in Methanol Electro-Oxidation and Lithium-Oxygen Batteries

A new and simple approach is reported for the synthesis of uniformly dispersed PtPd alloy nanocatalysts supported on graphene nanoplatelets (GNPs) (PtPd-GNPs) through the introduction of bifunctional materials, which can modify the GNP surface and simultaneously reduce metal ions. With the use of poly(4-styrenesulfonic acid), poly(vinyl pyrrolidone), poly(diallyldimethylammonium chloride), and poly(vinyl alcohol) as bifunctional materials, PtPd-GNPs can be produced through a procedure that is far simpler than previously reported methods. The as-prepared nanocrystals on GNPs clearly exhibit uniform PtPd alloy structures of around 2u2005nm in size, which are strongly anchored and well distributed on the GNP sheets. The Pt/Pd atomic ratio and loading density of the nanocrystals on the GNPs are controlled easily by changing the metal precursor feed ratio and the mass ratio of GNP to the metal precursor, respectively. As a result of the synergism between Pt and Pd, the as-prepared PtPd-GNPs exhibit markedly enhanced electrocatalytic performance during methanol electro-oxidation compared with monometallic Pt-GNP or commercially available Pt/C. Furthermore, the PtPd-GNP nanocatalysts also show greatly enhanced catalytic activity toward the oxygen reduction/evolution reaction in a lithium-oxygen (Li-O2 ) process, resulting in greatly improved cycling stability of a Li-O2 battery.

Synthesis and electrochemical properties of gel polymer electrolyte using poly(2-(dimethylamino)ethyl methacrylate-co-methyl methacrylate) for fabricating lithium ion polymer battery

Random copolymers comprising 2-(dimethylamino)ethyl methacrylate (DMAEMA) and methyl methacrylate (MMA) are synthesized by radical polymerization using 2,2′-azobis(2-methylpropionitrile) (AIBN) as an initiator. Gel polymer electrolytes (GPEs) are prepared by in situ thermal curing using different ratios of siloxane-epoxide cross-linker to poly(DMAEMA-co-MMA) and various contents and types of liquid electrolytes. GPEs offer several advantages such as in situ thermal cross-linking without requiring an additional radical initiator, relatively shorter curing time (~3 h) and lower curing temperature. When the ratio of the siloxane-epoxide cross-linker to poly(DMAEMA-co-MMA) is 1:5, the GPE with 98 wt% liquid electrolyte exhibits the highest ionic conductivity of 8.87×10−3 S/cm at 30 °C. The electrochemical stability window of the GPE is measured to be 5.1 V vs. Li/Li+. A unit cell comprising LiCoO2/GPE/graphite exhibits an initial discharge capacity of 145.6 mAh/g at 0.1 C, and the unit cell has good rate capability and cycling performance.