Hun-Joon Sohn

Prospective materials and applications for Li secondary batteries

Li-ion batteries have been employed successfully in various small electronic devices for the last two decades, and the types of applications are currently expanding to include electric vehicles (EVs), power tools, and large electric power storage units. In order to be implemented in these emerging markets, novel materials for negative and positive electrodes as well as electrolytes need to be developed to achieve high energy density, high power, and safe lithium rechargeable batteries. Here, the trends of the market and development of materials for each application are introduced, and some of next generation Li-ion batteries are discussed.

Quartz (SiO2): a new energy storage anode material for Li-ion batteries

SiO2 is one of the most abundant materials on Earth. It is cost-effective and also environmentally benign when used as an energy material. Although SiO2 was inactive to Li, it was engineered to react directly by a simple process. It exhibited a strong potential as a promising anode for Li-ion batteries.

The Insertion Mechanism of Lithium into Mg2Si Anode Material for Li‐Ion Batteries

The reaction mechanism of lithium insertion into Mg{sub 2}Si was studied using various analytic techniques including electrochemical measurements, X-ray diffraction (XRD), and Auger electron spectroscopy (AES). Electrochemical tests demonstrated that 1 mol Mg{sub 2}Si reacted with 3.9 mol Li from which the initial capacity obtained was approximately 1,370 mAh/g. Ex situ XRD and AES data showed that lithium intercalated into the Mg{sub 2}Si lattice first followed by alloying with Si and Mg. The degradation mechanism of Mg{sub 2}Si during cycling was investigated because the Mg{sub 2}Si materials degraded rapidly within ten cycles. The electrode material disintegrated and Li remained within the active material after ten cycles. The XRD and scanning electron microscope data suggested that the degradation mechanism of Mg{sub 2}Si was due to the volume change during the alloying/dealloying reaction, and the volume expansion/contraction made the Mg{sub 2}Si electrode materials electrically isolated.

Enhanced Lithium‐Ion Transport in PEO‐Based Composite Polymer Electrolytes with Ferroelectric BaTiO3

The ion-conduction properties of a polyethylene oxide (PEO)-based composite polymer electrolyte comprised of PEO, LiClO{sub 4}, and the ferroelectric material BaTiO{sub 3} were studied. The addition of BaTiO{sub 3} resulted in an increase in conductivity over the temperature range 25--115 C. The optimum amount of BaTiO{sub 3} (purity 99.9%, particle size 0.6--1.2 {micro}m) was 1.4 wt %, which is very low in comparison with previously reported composite polymer electrolytes. The ionic conductivity of a composite polymer electrolyte containing 1.4 wt % BaTiO{sub 3} was 1 {times} 10{sup {minus}5} S/cm at 25 C, which is at least one order of magnitude higher than that of the pristine polymer electrolyte (4 {times} 10{sup {minus}7} S/cm). The transport number of the lithium ion in this composite polymer electrolyte was higher than that of the pristine polymer electrolyte. The increase in the conductivity and the lithium-ion transport number is explained on the basis of the spontaneous polarization of the ferroelectric material due to its particular crystal structure. The addition of BaTiO{sub 3} powder greatly enhanced the lithium/electrolyte interface stability.

Electrochemical Characterizations of Germanium and Carbon-Coated Germanium Composite Anode for Lithium-Ion Batteries

The electrochemical reaction mechanism of germanium with lithium at room temperature was investigated. Two distinct voltage plateaus corresponding to the formation of Li 9 Ge 4 and Li 7 Ge 2 were observed, and the final products were identified as a mixture of Li 15 Ge 4 and Li 22 Ge 5 phases. A three-step reaction mechanism of germanium with lithium was suggested. Carbon-coated germanium composites prepared by mechanical milling and pyrolysis were evaluated as anode materials for lithium-ion batteries. The composite materials, Ge-mesocarbon microbeads and Ge-poly(vinyl alcohol), showed good cyclability and retained fairly large capacities of 473 and 579 mAh/g, respectively, up to 50 cycles when cycled within a limited voltage window.

Electrochemical Impedance Analysis for Lithium Ion Intercalation into Graphitized Carbons

Electrochemical impedance spectroscopy (EIS) was employed to study electrochemical behaviors during intercalation of into graphitized carbon anode. Analysis was carried out on three regions of frequencies mainly below 0.3 V. The first depressed semicircle in the high‐frequency region had two‐dimensional characteristics and did not vary over the entire potential range. The second semicircle in the mid‐frequency region had a potential dependency above 0.3 V. Impedance spectra at the lower frequency region were attributed to the finite diffusion of , and the order of the chemical diffusion coefficient was approximately .

Ferroelectric Materials as a Ceramic Filler in Solid Composite Polyethylene Oxide‐Based Electrolytes

The electrochemical properties of mixed‐phase composite electrolytes based on poly(ethylene oxide) (PEO), lithium salts , and ferroelectric materials have been studied. The ion‐conduction and lithium‐ion transference numbers of the composite polymer electrolytes were enhanced by the addition of these ferroelectric materials as a ceramic filler. The conductivity behavior of the composite electrolyte depended on the combination of lithium salt and the ferroelectric materials. The conductivity enhancement in the PEO‐LiX composite electrolytes with ferroelectric materials was rationalized by correlating the association tendency of anions with lithium cations and the spontaneous polarization of the ferroelectric ceramics due to their particular crystal structure. The combined addition of rutile type and assured long‐term interfacial stability. All the electrolytes studied here showed decomposition potentials higher than 4V vs. .

Characterizations and electrochemical behaviors of disproportionated SiO and its composite for rechargeable Li-ion batteries

Commercially available solid SiO is composed of amorphous Si with silicon suboxides of various valence states. Solid SiO is thermodynamically unstable at all temperatures, which would disproportionate to Si and SiO2. Disproportionation of SiO at several temperatures was characterized with various analytical techniques, such as XRD, XPS, NMR and HRTEM methods, and it was found that the sample heat treated at 1000 °C contained well-developed Si nanocrystals uniformly dispersed within an amorphous SiOx matrix. Using this sample, a nano-Si/SiOx/graphite composite was obtained by a straightforward high-energy mechanical milling technique. The nano-Si/SiOx/graphite composite was tested as an anode material for Li secondary batteries, and showed better electrochemical behaviors than those of pure SiO.

Reaction Mechanism of Tin Phosphide Anode by Mechanochemical Method for Lithium Secondary Batteries

Nanosized Sn 4 P 3 with a layered structure was synthesized by a mechanochemical method, and electrochemical and local structural characteristics of tin phosphide during charge/discharge were studied for its use as an anode material for lithium secondary batteries. As the amount of lithium insertion increased, tin phosphide was converted into lithium phosphides followed by lithiumtin alloy formation, which was confirmed by differential capacity plots and X-ray absorption spectroscopic (XAS) analysis. Based on X-ray diffraction, XAS, and electrochemical data, a three-step reaction mechanism of Sn 4 P 3 with lithium was suggested. Tin phosphide showed a good cyclability and retained a fairly large capacity of 370 mAh/g up to 50 cycles when cycled within a limited voltage window.

SnO2@Co3O4 hollow nano-spheres for a Li-ion battery anode with extraordinary performance

AbstractSnO2@Co3O4 hollow nano-spheres have been prepared using the template-based sol-gel coating technique and their electrochemical performance as an anode for lithium-ion battery (LIB) was investigated. The size of synthesized hollow spheres was about 50 nm with the shell thickness of 7–8 nm. The fabricated SnO2@Co3O4 hollow nano-sphere electrode exhibited an extraordinary reversible capacity (962 mAh·g−1 after 100 cycles at 100 mA·g−1), good cyclability, and high rate capability, which was attributed to the Co-enhanced reversibility of the Li2O reduction reaction during cycling.