Low-temperature performance of Li-ion cells with a LiBF<Subscript>4</Subscript>-based electrolyte

The effect of LiBF₄ on the low-temperature performance of a Li-ion cell was studied by using a 1:1:1 (wt) EC/DMC/DEC mixed solvent. The results show that the LiBF₄-based electrolyte has a 2- to 3-fold lower ionic conductivity and shows rather higher freezing temperature compared with a LiPF₆-based electrolyte. Owing to electrolyte freezing, cycling performance of the Li-ion cell using LiBF₄ was significantly decreased when the temperature fell below –20 °C. However, impedance data show that at –20 °C the LiBF₄ cell has lower charge-transfer resistance than the LiPF₆ cell. In spite of the relatively lower conductivity of the LiBF₄-based electrolyte, the cell based on it shows slightly lower polarization and higher capacity in the liquid temperature range (above –20 °C) of the electrolyte. This fact reveals that ionic conductivity of the electrolytes is not a limitation to the low-temperature performance of the Li-ion cell. Therefore, LiBF₄ may be a good salt for the low-temperature electrolyte of a Li-ion cell if a solvent system that is of low freezing temperature, high solubility to LiBF₄, and good compatibility with a graphite anode can be formulated. Electronic Publication     

Alkaline composite film as a separator for rechargeable lithium batteries

We report a new type of separator film for application in rechargeable lithium and lithium-ion batteries. The films are made of mainly alkaline calcium carbonate (CaCO₃) and a small amount of polymer binder. Owing to porosity and capillarity, the composite films show excellent wettability with non-aqueous liquid electrolytes. Typically, the composite films composed of CaCO₃ and Teflon and wetted with 1 M LiPF₆ dissolved in a solvent mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (30:70 wt%) exhibit an ionic conductivity as high as 2.5–4 mS/cm at 20 °C, in a comparable range with that (3.4 mS/cm) of the commercial Celgard membrane. In the batteries, the composite film not only serves as a physical separator but also neutralizes acidic products, such as HF formed by LiPF₆ hydrolysis, as well as those formed by solvent oxidative decomposition. A Li/LiMn₂O₄ test cell was employed to examine the electrochemical compatibility of the composite film. We observed that the composite film cell showed an improved cycling performance since the alkaline CaCO₃ neutralizes the acidic products, which otherwise promote dissolution of the electrode active materials. More importantly, the composite film cell displayed a superior performance on high-rate cycling, which was probably the result of the less resistive interface formed between the electrode and the composite film.     

Physical and chemical effects of ultrasound vibration on polymer melt in extrusion

The physical and chemical effects of ultrasound on polypropylene (PP) melts in extrusion were investigated. By applying ultrasound vibration to the entrance of the die, apparent pressure and viscosity of PP can be obviously decreased under the appropriate ultrasound power. Ultrasound has both physical and chemical effects on the polymer melt. In our study with specific polymer and ultrasound system, we determined that the chemical effect makes up 35–40% of the total effect of ultrasound on the apparent viscosity reduction of PP melts at most of the studied intensities. The physical effect plays a more important role in the ultrasound-applied extrusion than the chemical effect. This chemical effect is an irreversible and permanent change in molecule weight and the molecular-weight distribution due to ultrasound. As the ultrasound intensity increases, the molecular weight of PP reduces and its molecular-weight distribution becomes narrower; the orientation of PP molecules along the flow direction reduces (in melt state) and the crystallinity of PP samples (in solid state) decreases by applying the ultrasound vibration. Ultrasound vibration increases the motion of molecular chains and makes them more disorder; it also affects the relaxation process of polymer melts by shortening the relaxation time of chain segments, leading to weakening the elastic effect and decreasing the extruding swell ratios. All the factors discussed above reduce the non-Newtonian flow characteristics of the polymer melt and result in the viscosity drop of the polymer melt in extrusion.     

Fabrication and evaluation of a polymer Li-ion battery with microporous gel electrolyte

This paper introduces an easy method for the fabrication of polymer Li-ion batteries with microporous gel electrolyte (MGE). The MGE is a multiphase electrolyte, which is composed of liquid electrolyte, gel electrolyte, and polymer matrix. The MGE not only has high ionic conductivity and good adhesion to the electrodes at low temperatures, but also retains good mechanical strength at elevated temperatures. Therefore, the MGE batteries are able to operate over a wide temperature range. During battery fabrication, the MGE is formed in situ by introducing liquid electrolyte into a swellable microporous polymer membrane and then heating or cycling the battery. In this work, the chemical compatibility of MGE with metal lithium during 60 °C storage and with LiMn₂O₄ cathode during cycling was studied. In addition, graphite/MGE/LiMn₂O₄ Li-ion batteries were made and evaluated.     

Lithium ethylene dicarbonate identified as the primary product of chemical and electrochemical reduction of EC in 1.2 M LiPF6/EC:EMC electrolyte

Lithium ethylene dicarbonate ((CH2OCO2Li)2) was chemically synthesized and its Fourier transform infrared (FTIR) spectrum was obtained and compared with that of surface films formed on Ni after cyclic voltammetry (CV) in 1.2 M lithium hexafluorophosphate (LiPF6)/ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7, w/w) electrolyte and on metallic lithium cleaved in-situ in the same electrolyte. By comparison of IR experimental spectra with that of the synthesized compound, we established that the title compound is the predominant surface species in both instances. Detailed analysis of the IR spectrum utilizing quantum chemical (Hartree-Fock) calculations indicates that intermolecular association through O...Li...O interactions is very important in this compound. It is likely that the title compound in the passivation layer has a highly associated structure, but the exact intermolecular conformation could not be established on the basis of analysis of the IR spectrum.     

Phase Diagram of EC–DMC Binary System and Enthalpic Determination of Its Eutectic Composition

We construct with a differential scanning calorimeter (DSC) a phase diagram for the ethylene carbonate (EC)-dimethyl carbonate (DMC) binary system for its liquid-solid phase equilibria. We determine the eutectic composition of the binary system using an enthalpic method that we devised based on the composition dependence of the enthalpy of solidus melting, with highly consistent results. We also discuss the merits and limitations of this enthalpic method. This revised version was published online in July 2006 with corrections to the Cover Date.     

Syntheses and characterization of lithium alkyl mono- and dicarbonates as components of surface films in Li-ion batteries

A homologous series of lithium alkyl mono- and dicarbonate salts was synthesized as model reference compounds for the frequently proposed components constituting the electrolyte/electrode interface in Li-ion batteries. The physicochemical characterization of these reference compounds in the bulk state using thermal analyses and X-ray photoelectron, nuclear magnetic resonance, and Fourier transform infrared spectroscopies establishes a reliable database of comparison for the studies on the surface chemistry of electrodes harvested from Li-ion cells.