Ji Heon Ryu

An amorphous red phosphorus/carbon composite as a promising anode material for sodium ion batteries.

An amorphous red phosphorus/carbon composite is obtained through a facile and simple ball milling process, and its electrochemical performance as an anode material for Na ion batteries is evaluated. The composite shows excellent electrochemical performance including a high specific capacity of 1890 mA h g(-1), negligible capacity fading over 30 cycles, an ideal redox potential (0.4 V vs. Na/Na(+)), and an excellent rate performance, thus making it a promising candidate for Na ion batteries.

Failure Modes of Silicon Powder Negative Electrode in Lithium Secondary Batteries

Si composite negative electrodes for lithium secondary batteries degrade in the dealloying period with an abrupt increase in internal resistance that is caused by a breakdown of conductive network made between Si and carbon particles. This results from a volume contraction of Si particles after expansion in the previous alloying process. Due to the large internal resistance, the dealloying reaction is not completed while Si remains as a lithiated state. The anodic performance is greatly improved either by applying a pressure on the cells or loading a larger amount of conductive carbon in the composite electrodes.

Tin Phosphide as a Promising Anode Material for Na‐Ion Batteries

Sn4 P3 is introduced for the first time as an anode material for Na-ion batteries. Sn4 P3 delivers a high reversible capacity of 718 mA h g(-1), and shows very stable cycle performance with negligible capa-city fading over 100 cycles, which is attributed to the confinement effect of Sn nanocrystallites in the amorphous phosphorus matrix during cycling.

Sn-Carbon Core-Shell Powder for Anode in Lithium Secondary Batteries

bMaterials Laboratroy, Samsung Advanced Institute of Technology, Suwon, 440-600, Korea Spherical Sn-carbon core-shell powder was synthesized through a resorcinol-formaldehyde ~RF! microemulsion polymerization performed in the presence of hydrophobized Sn nanoparticles. The Sn-carbon core-shell structure was found to greatly enhance the cycle life compared to the mixture of Sn and spherical carbon when evaluated as the anode in lithium-ion batteries. A core-shell powder containing 20 wt % Sn showed 69% capacity retention at the 40th cycle when cycled between 0 and 2.0 V ~vs Li/Li + ! at a constant current of 40 mA g ˛1 . The mixture of 20 wt % Sn nanopowder and 80 wt % spherical carbon powder exhibited only 10% capacity retention in the same test condition. It is believed that the improved cyclability achieved with the core-shell powder is largely attributed to the inhibition of aggregation between Sn nanoparticles. The marginal polarization due to an intimate electrical contact made between Sn core and carbon shell is an additional advantageous feature achieved with this electrode.

Surface Film Formation on LiNi0.5Mn1.5O4 Electrode in an Ionic Liquid Solvent at Elevated Temperature

A comparative study is made on the surface film formation on the high-voltage LiNi 0.5 Mn 1.5 O 4 positive electrode at elevated temperature (55°C) in two different electrolytes; LiPF 6 /organic carbonate and LiTFSI/ionic liquid (propylmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide, PMPyr-TFSI). The surface film derived by a decomposition of the former electrolyte is enriched by inorganic fluorinated species, which becomes thicker with cycling to lead a continued electrode polarization and cell failure. In contrast, organic carbon species are dominant in the film derived from the latter electrolyte. This organic-rich film deposited in the earlier period of cycling seems to effectively passivate the positive electrode presumably due to uniform coverage. As a result, the film does not grow with cycling and electrode polarization is not serious to give a stable cycling behavior.

Linear-Sweep Thermammetry Study on Corrosion Behavior of Al Current Collector in Ionic Liquid Solvent

The corrosion behavior of A1 foil as the current collector for lithium-ion batteries is studied by linear-sweep thermammetry. The onset temperature for Al pitting corrosion depends on Li salt that is dissolved in an ionic liquid solvent; lithium bis(trifluo- romethanesulfonyl)imide (LiTFSI) < lithium bis(perfluoroethanesulfonyl)imide (LiBETI) < LiPF 6 < LiBF 4 , With LiBF 4 , no corrosion current is observed until 1 10°C. X-ray photoelectron spectroscopy study reveals that this Al surface is covered by Al-F compound (presumably AlF 3 ). Due to the formation of a highly passivating AlF 3 layer in this electrolyte, the high voltage LiNi 0.5 Mn 1.5 O 4 positive electrode coated on Al foil can be successfully cycled at 65°C without electrode failure.

Comparative Study on Surface Films from Ionic Liquids Containing Saturated and Unsaturated Substituent for LiCoO2

A comparative study is made on the surface films that are deposited on a LiCoO 2 electrode by oxidative decomposition of room-temperature ionic liquids (RTILs) that have either a saturated (propyl) or an unsaturated (allyl) substituent on the pyrrolidinium cation. The surface film deposited from the former RTIL does not so perfectly cover the LiCoO 2 electrode surface that the film deposition continues with cycling, leading to a gradual increase in the electrode polarization and an eventual capacity fading. From the allyl-containing RTIL, however, a uniformly covered surface film is deposited even after a single charging to give better cyclability to the LiCoO 2 electrode. The latter film contains a larger amount of organic carbon species relative to that found in the former. An enhanced film property is also observed by adding vinylene carbonate that has an unsaturated moiety, ensuring that the unsaturated functional groups are responsible for such favorable film properties.

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.

Poly(phenanthrenequinone) as a conductive binder for nano-sized silicon negative electrodes

3,6-Poly(phenanthrenequinone) (PPQ) is synthesized and tested as a conductive binder. The PPQ binder, formulated with nano-sized Si powder without conductive carbon, is n-doped by accepting electrons and Li+ ions to become a mixed conductor in the first charging period. The resulting n-doped PPQ binder remains conductive thereafter within the working potential of Si (0.0–0.5 V vs. Li/Li+). Within the composite electrode, the PPQ binder is uniformly dispersed to effectively convey electrons from the current collector to the Si particles. Namely, the PPQ binder by itself plays the roles of conductive carbon and a polymer binder that are loaded in the conventional composite electrodes. Due to the highly conductive nature, the loading of the PPQ binder can be minimized down to 10 wt%, which is close to that used for practical electrode formulation, with reasonable rate and cycle performances of the nano-Si electrode.

Allylic ionic liquid electrolyte-assisted electrochemical surface passivation of LiCoO2 for advanced, safe lithium-ion batteries

Room-temperature ionic liquid (RTIL) electrolytes have attracted much attention for use in advanced, safe lithium-ion batteries (LIB) owing to their nonvolatility, high conductivity, and great thermal stability. However, LIBs containing RTIL-electrolytes exhibit poor cyclability because electrochemical side reactions cause problematic surface failures of the cathode. Here, we demonstrate that a thin, homogeneous surface film, which is electrochemically generated on LiCoO2 from an RTIL-electrolyte containing an unsaturated substituent on the cation (1-allyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide, AMPip-TFSI), can avert undesired side reactions. The derived surface film comprised of a high amount of organic species from the RTIL cations homogenously covered LiCoO2 with a <25 nm layer and helped suppress unfavorable thermal reactions as well as electrochemical side reactions. The superior performance of the cell containing the AMPip-TFSI electrolyte was further elucidated by surface, electrochemical, and thermal analyses.