Hochun Lee

Electronegativity-induced enhancement of thermal stability by succinonitrile as an additive for Li ion batteries

Succinonitrile (SN, CN–[CH2]2–CN) is evaluated as an additive for improving thermal stability in ethylene carbonate (EC)-based electrolytes for lithium ion batteries. Without any sacrifice of performance such as cyclability and capacity, the introduction of SN into an electrolyte with a graphite anode and LixCoO2 cathode leads to (1) reducing the amount of gas emitted at high temperature, (2) increasing the onset temperature of exothermic reactions and (3) decreasing the amount of exothermal heat. The improvement in the thermal stability is considered to be due to strong complex formation between the surface metal atoms of LixCoO2 and nitrile (–CN) groups of SN, from spectroscopic studies based on photoelectrons induced by X-rays and by considering that the exothermic heat and gas evolution are caused by interfacial reactions between the electrolyte and cathode.

Revival of TE2A; a better chelate for Cu(II) ions than TETA?

A highly effective synthetic route for TE2A was developed and the (64)Cu-labeled TE2A complexes showed higher kinetic inertness and faster clearance than most commonly used TETA analogs.

Co-Use of Cyclohexyl Benzene and Biphenyl for Overcharge Protection of Lithium-Ion Batteries

Cyclohexyl benzene (CHB) is widely used as an electrolyte additive for overcharge protection of lithium-ion batteries. This study reports that CHB and biphenyl (BP) mixtures are much more effective than CHB. On the overcharging tests for graphite-LiCoO 2 cells with a nominal capacity of 760 mAh, CHB and BP mixtures expand the safety region up to 12 V/2 A where CHB alone without BP can never reach. Using the linear sweep voltammetry and the electrochemical quartz crystal microbalance, it was found that the CHB and BP mixture exhibits much bigger oxidation current and forms a larger amount of polymeric film than the numeric sum of each components response. The origin of the synergistic effects between CHB and BP has been speculated based on their different electrochemical characteristics.

Fluoropropane sultone as an SEI-forming additive that outperforms vinylene carbonate

Vinylene carbonate (VC) has been the best performing solid electrolyte interphase (SEI) additive for the current lithium-ion batteries (LIBs). However, it is also true that the current LIB technology is being stagnated by the limit set by VC. This study introduces 3-fluoro-1,3-propane sultone (FPS) as a novel SEI additive to replace VC and another popular SEI additive, 1,3-propane sultone (PS). Both density functional calculations and electrochemical experiments confirm that the presence of an electron withdrawing fluorine group is favourable in terms of anodic stability and SEI forming ability. In the cyclability of LiCoO2/graphite cells over a wide temperature range (25–60 °C), FPS exhibits remarkable enhancement compared with PS, and is even superior to VC. During elevated temperature (90 °C) storage of the cells, VC suffers from severe swelling, whereas FPS causes little thermal degradation. Considering the high anodic stability, the excellent cyclability, and the good thermal stability, FPS is an outstanding SEI additive that can expand the performance boundary of the current LIBs.

Ion and water transports in Prussian blue films investigated with electrochemical quartz crystal microbalance

Water and ion transport in electrochemically prepared Prussian blue (PB, Iron(III) hexacyanoferrate(II)) films has been investigated with the electrochemical quartz crystal microbalance (EQCM) and the electrochemical/electrogravimetric impedance techniques. It is shown that the freshly prepared PB film is highly hydrated and that it undergoes an irreversible mass change during the first cathodic scan. The latter result supports the previously proposed structural reorganization scheme of the PB film from the insoluble form Fe4[Fe(CN)6]3·6H2O to the soluble form MFeFe(CN)6 (M is a monovalent cation). It is also shown that, during the first cathodic scan, a substantial amount of water is excluded from the PB film. After the structural reorganization, ion transport during the redox reaction of the PB film is cation-dominant with a small fraction of accompanying water transport.

Succinonitrile as a corrosion inhibitor of copper current collectors for overdischarge protection of lithium ion batteries.

Succinonitrile (SN) is investigated as an electrolyte additive for copper corrosion inhibition to provide overdischarge (OD) protection to lithium ion batteries (LIBs). The anodic Cu corrosion, occurring above 3.5 V (vs Li/Li(+)) in conventional LIB electrolytes, is suppressed until a voltage of 4.5 V is reached in the presence of SN. The corrosion inhibition by SN is ascribed to the formation of an SN-induced passive layer, which spontaneously develops on the copper surface during the first anodic scan. The passive layer is composed mainly of Cu(SN)2PF6 units, which is evidenced by Raman spectroscopy and electrochemical quartz crystal microbalance measurements. The effects of the SN additive on OD protection are confirmed by using 750 mAh pouch-type full cells of LiCoO2 and graphite with lithium metal as a reference electrode. Addition of SN completely prevents corrosion of the copper current collector in the full cell configuration, thereby tuning the LIB chemistry to be inherently immune to the OD abuses.

New macrobicyclic chelator for the development of ultrastable 64Cu-radiolabeled bioconjugate.

Ethylene cross-bridged cyclam with two acetate pendant arms, ECB-TE2A, is known to form the most kinetically stable (64)Cu complexes. However, its usefulness as a bifunctional chelator is limited because of its harsh radiolabeling conditions. Herein, we report new cross-bridged cyclam chelator for the development of ultrastable (64)Cu-radiolabeled bioconjugates. Propylene cross-bridged TE2A (PCB-TE2A) was successfully synthesized in an efficient way. The Cu(II) complex of PCB-TE2A exhibited much higher kinetic stability than ECB-TE2A in acid decomplexation studies, and also showed high resistance to reduction-mediated demetalation. Furthermore, the quantitative radiolabeling of PCB-TE2A with (64)Cu was achieved under milder conditions compared to ECB-TE2A. Biodistribution studies strongly indicate that the (64)Cu complexes of PCB-TE2A cleared out rapidly from the body with minimum decomplexation.

Comparison of Voltammetric Responses over the Cathodic Region in LiPF6 and LiBETI with and without HF

This study reports the effects of HF on the voltammetric responses below 3 V (vs. Li/Li + ) in LiPF 6 solutions being widely used in rechargeable lithium-ion batteries (LIBs). During the first cathodic scan on a Pt electrode, two reduction peaks are observed between 3.0 and 1.5 V in LiPF 6 solutions irrespective of the solvents used. The reduction reactions are absent in LiBETI solutions but present after the addition of trace amount of HF. Thus, the reduction reactions in LiPF 6 solutions are due to the HF, an inevitable impurity of LiPF 6 solutions. Based on the results of the rotating disk electrode and the rotating ring disk electrode experiments, two reduction peaks are assigned to the hydrogen underpotential deposition and the hydrogen evolution reaction, respectively. HF reduction reactions forms a surface layer of which the main constituent is LiF. The surface layer suppresses the other reduction reactions otherwise occurring below 3.0 V. For example, the electrodeposition of metallic Mn, which takes place below 1.0 V in LiBETI solutions, is severely hampered in LiPF 6 solutions. The passivity by HF reduction is observed not only for a Pt electrode but also for a glassy carbon electrode.

Mass transport behavior of polypyrrole and poly(N-methylpyrrole) films in acetonitrile solutions

Abstract The mass transport mechanism of polypyrrole (PPy) and poly(N-methylpyrrole) (PMPy) films in an acetonitrile (AN) solution has been investigated with the cyclic electrochemical quartz crystal microbalance (EQCM) technique. Cations as well as anions take part in ion transport during the redox reaction of PPy films, and the break-in process occurs in the first negative scan. On the other hand, anion transport is dominant during the redox reaction of PMPy films, and no break-in process is observed. Solvent transport takes place in the same direction as cation transport in the case of PPy films, whereas it occurs in the opposite direction to anion transport in the case of PMPy films.

Effects of dopant anions and N-substituents on the electrochemical behavior of polypyrrole films in propylene carbonate solution

The electrochemical properties of poly(N-ethylpyrrole) (PEPy) and poly(N-methylpyrrole) (PMPy) films in propylene carbonate (PC) solution, where ion transport is anion-specific, has been investigated using the cyclic electrochemical quartz crystal microbalance (EQCM) technique and electrochemical impedance techniques. The type of dopant anion (PF6− vs. ClO4−) used for the film preparation and the N-substituents (ethyl vs. methyl) have profound effects on the solvent transport behavior, charge capacity, and ionic resistance of the films.