Showing papers by "T.R. Jow published in 2003"

Low-temperature performance of Li-ion cells with a LiBF 4 -based electrolyte

Abstract: 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.

Alkaline composite film as a separator for rechargeable lithium batteries

Abstract: 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.

Poly(acrylonitrile-methyl methacrylate) as a non-fluorinated binder for the graphite anode of Li-ion batteries

Abstract: Poly(vinylidene fluoride) (PVDF) has been a choice of the binder for both cathode and graphite anode in stateof-the-art Li-ion batteries [1, 2]. However, there is currently a demand to replace PVDF with non-fluorinated binder since at elevated temperatures the fluorinated polymers react with lithiated graphite (LixC6) and metal lithium to form more stable LiF and >C@CF– double bonds. In particular, the reaction of PVDF and metal lithium produces an enthalpy as high as 7180 J (g PVDF) [1]. It has been reported that in the presence of electrolytes, PVDF and lithiated graphite undergo a series of exothermic reactions, including (i) temperature-induced degradation of the solid electrolyte interface (SEI) at 120–140 C, (ii) reactions of lithiated graphite and electrolyte at 210–230 C, and (iii) dehydrofluorination of PVDF initiated by LixC6 at >260 C [1, 2]. Among these reactions, the last one is known to be very exothermic and is believed to be a potential source for the thermal runaway of Li-ion batteries under abuse conditions. Therefore, safety concerns with Li-ion batteries may arise from the use of PVDF in the graphite anode. To replace the rather reactive PVDF, we attempt to evaluate poly(acrylonitrile-methyl methacrylate) (AMMA) as a non-fluorinated binder for the graphite anode of Li-ion batteries. Differential scanning calorimetry (DSC) study has shown that the heat of reaction of the lithiated graphite and electrolyte can be reduced significantly by using AMMA instead of PVDF [3]. That is, total enthalpies for the reactions (as indicated by two exothermic peaks in DSC curves) of the fully lithiated graphite (MAG-10, Hitachi Chemical) and electrolyte (1.2 M LiPF6 3:7 EC/EMC) in the temperature range 280–340 C can be reduced to 1211 J (g AMMA) from 2699 J (g PVDF). In this paper, we will electrochemically evaluate AMMA as a binder of the graphite anode of Li-ion batteries. 2. Experimental details