Sangjin Park

Direct synthesis of self-assembled ferrite/carbon hybrid nanosheets for high performance lithium-ion battery anodes.

Extensive applications of rechargeable lithium-ion batteries (LIBs) to various portable electronic devices and hybrid electric vehicles result in the increasing demand for the development of electrode materials with improved electrochemical performance including high energy, power density, and excellent cyclability, while maintaining low production cost. Here, we present a direct synthesis of ferrite/carbon hybrid nanosheets for high performance lithium-ion battery anodes. Uniform-sized ferrite nanocrystals and carbon materials were synthesized simultaneously through a single heating procedure using metal-oleate complex as the precursors for both ferrite and carbon. 2-D nanostructures were obtained by using sodium sulfate salt powder as a sacrificial template. The 2-D ferrite/carbon nanocomposites exhibited excellent cycling stability and rate performance derived from 2-D nanostructural characteristics. The synthetic procedure is simple, inexpensive, and scalable for mass production, and the highly ordered 2-D structure of these nanocomposites has great potential for many future applications.

Solid-State NMR and Electrochemical Dilatometry Study on Li + Uptake/Extraction Mechanism in SiO Electrode

This work was supported by KOSEF via the Research Center for Energy Conversion and Storage. We are grateful to the Daegu Center at the Korea Basic Science Institute for helpful discussions and NMR measurements. We also acknowledge Dr. R. Kotz and Dr. P. Novak (Paul Scherrer Institute, Switzerland) for their assistance in fabricating the electrochemical dilatometer.

The Role of Metallic Fe and Carbon Matrix in Fe2O3 / Fe / Carbon Nanocomposite for Lithium-Ion Batteries

A comparative study is made on the cycle and rate performance of three Fe 2 O 3 -containing electrodes. The first electrode is made from a commercial nanosized (< 100 nm) Fe 2 O 3 powder, whereas the second is made from a carbon-supported nanosized Fe 2 O 3 (Fe 2 O 3 /carbon composite). The third electrode is prepared by modifying the second, incorporating the metallic Fe into the Fe 2 O 3 particles (Fe 2 0 3 /Fe/carbon composite). Three electrodes show the following performance order in the cycle retention and rate capability: nanosized Fe 2 O 3 < Fe 2 O 3 /carbon composite < Fe 2 O 3 /Fe/carbon composite. The reverse order is, however, found on both the electrode volume change and the evolution of internal resistance. Thus, the best performance observed with the Fe 2 O 3 /Fe/carbon composite electrode has been ascribed to the presence of a carbon support and metallic Fe, both of which seem to play two important roles: the buffering action against the massive volume change in Fe 2 O 3 particles and providing an electronically conductive network within the swollen electrode layer.

Electrochemical Dilatometry Study on Si-Embedded Carbon Nanotube Powder Electrodes

Si-embedded carbon nanotube (Si-C/NT) powders were prepared by dispersing carbon nanotubes (CNTs) and Si in the tetrahydrofuran solution containing poly(vinyl chloride) (PVC) as a dispersion agent, and then carbonizing the PVC. A better cycle performance was observed with the Si-C/NT containing larger void volume. The origin of this feature was addressed by an electrochemical dilatometry study, where it was found that the electrode swelling becomes less significant when the Si-C/NT possesses a larger void space. It is believed that the void space plays a buffering role against the volume expansion of Si, alleviating the breakdown of electrode integrity.

Passivating Ability of Surface Film Derived from Vinylene Carbonate on Tin Negative Electrode

The passivating ability of surface film derived from vinylene carbonate (VC) is addressed on tin (Sn) negative electrode after a comparative study on the thickness, film growth pattern, chemical composition, and mechanical flexibility of the surface films generated from VC-free and VC-added electrolytes. The surface film derived from the former electrolyte is enriched by inorganic fluorinated and carbonate species, and shows a poor passivating ability to cause a continued electrolyte decomposition, film growth and eventual electrode failure. In contrast, organic carbon-oxygen species are dominant in the film derived from the VC-added electrolyte. Even if this film is thinner than the other, it passivates the Sn electrode surface more effectively. As a result, the film growth and electrode polarization are less significant. The superior passivating ability of organic-rich surface film has been ascribed to a uniform coverage and higher mechanical flexibility.

Polymer-derived carbon nanofiber network supported SnO2 nanocrystals: a superior lithium secondary battery material

In the present study, new three-dimensional tin dioxide–carbon (SnO2–C) composites as anode materials for lithium-ion batteries are achieved via a simple hydrothermal route and subsequent calcination process using polypyrrole-based carbon networks as the support and conductive buffering layer. The structure and morphology of the novel composites are characterized by powder X-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction techniques. The resulting carbon networks composed of highly flexible, hollow, and end-opening nanofibers with diameters of approximately 40–70 nm and lengths of several microns are homogenously coated by nano-crystalline SnO2 (ca. 3–5 nm in size). The electrochemical performance of the mentioned SnO2–C (15.1% carbon) is investigated by cyclic voltammetry and discharge–charge cycling on half-cells in the potential range of 0.005–2 V at 25 °C. Galvanostatic cycling shows a stable and high charge capacity (598.3 mA h g−1) at a current density of 100 mA g−1 over 50 cycles with a low capacity fading of about 0.7% per cycle. By increasing the rate after 5 cycles in steps from 50 to 300 mA g−1 up to 30 cycles, a high reversible capacity (657.9 mA h g−1) is retained. Much improved lithium ion storage properties in terms of capacity, rate capability, and cycling stability may benefit from both the buffering action of conductive carbon networks and the size effect of SnO2 nanocrystals.

Metal-Aided Electrochemical Etching of Silicon for Ambient Red to Blue Photoluminescence

Ambient full-color photoluminescence from porous silicon has been realized through electrochemical etching aided by an oxidative metal such as Zn without any postanodizing treatment. The emission can be tuned to any wavelength in the visible range by simply changing anodizing current density from 20 to 100 mA/cm 2 . The luminescence intensity is high enough that all colors are visible to naked eyes. Aging behavior of the red, green, and blue photoluminesence is also investigated over a three-month time span. While the pinning of the peak wavelength occurs with aging at a certain level for all the peaks, the blue emission always gets pinned at around 420 nm whether the initial emission is green or blue.

The Roles of Electrolyte Additives on Low-temperature Performances of Graphite Negative Electrode

표준 전해액에 2중량%의 VC(vinylene carbonate)와 FEC(fluoroethylene carbonate)를 각각 첨가한 전해액으로부터 흑연 음극 표면에 SEI(solid electrolyte interphase) 층을 형성시키고, SEI 특성에 따른 흑연 음극의 저온(

Failure mechanisms of LiNi0.5Mn1.5O4 electrode at elevated temperature

-30^{\circ}C

Si‐Encapsulating Hollow Carbon Electrodes via Electroless Etching for Lithium‐Ion Batteries

) 충방전 특성을 조사하였다. 흑연의 충 방전 용량은 FEC를 첨가한 전해액, 표준 전해액, 그리고 VC를 첨가한 전해액의 순서로 감소하였고, 충 방시 발생하는 과전압은 반대경향을 보이며 증가하였다. 이는 첨가제의 종류에 따라 생성된 SEI 층의 저항과 전하전달저항에 차이가 있음을 설명하는데, 이를 SEI 층의 화학 조성과 두께를 비교하여 확인하였다. 표준 전해액으로부터 생성된 SEI 층은 C-O 성분을 포함하는 고분자 형태의 화합물과 리튬 염의 환원분해로 생성된