Jason N. Howard

Characterization of microporous separators for lithium-ion batteries

Several properties including porosity, pore-size distribution, thickness value, electrochemical stability and mechanical properties have to be optimized before a membrane can qualify as a separator for a lithium-ion battery. In this paper we present results of characterization studies carried out on some commercially available lithium-ion battery separators. The relevance of these results to battery performance and safety are also discussed. Porosity values were measured using a simple liquid absorption test and gas permeabilities were measured using a novel pressure drop technique that is similar in principle to the Gurley test. For separators from one particular manufacturer, the trend observed in the pressure drop times was found to be in agreement with the Gurley numbers reported by the separator manufacturer. Shutdown characteristics of the separators were studied by measuring the impedance of batteries containing the separators as a function of temperature. Overcharge tests were also performed to confirm that separator shutdown is indeed a useful mechanism for preventing thermal runaway situations. Polyethylene containing separators, in particular trilayer laminates of polypropylene, polyethylene and polypropylene, appear to have the most attractive properties for preventing thermal runaway in lithium ion cells.

Thermal Stability Studies of Li‐Ion Cells and Components

Thermal stability of fully charged 550 mAh prismatic Li‐ion cells (Sn‐doped carbon) and their components are investigated. Accelerating rate calorimetry (ARC) is used to determine the onset temperature of exothermic chemical reactions that force the cell into thermal runaway. Differential scanning calorimetry (DSC) and thermogravimetry analysis are used to determine the thermal stability of the cells positive electrode (PE) and negative electrode (NE) materials from 35 to 400°C. The cell self‐heating exothermic reactions start at 123°C, and thermal runaway occurs near 167°C. The total exothermic heat generation of the NE and PE materials are 697 and 407 J/g, respectively. Heat generations of the NE and PE materials, washed in diethyl carbonate (DEC) and dried at ≈65°C under vacuum, are significantly lower than unwashed samples. Lithium plating increases the heat generation of the NE material at temperatures near the lithium melting point. Comparison of the heat generation profiles from DSC and ARC tests indicates that thermal runaway of this cell is close to the decomposition temperature range of the unwashed PE material. We conclude that the heat generation from the decomposition of PE material and reaction of that with electrolyte initiates thermal runaway in a Li‐ion cell, under thermally or abusive conditions.

Thermal stability studies of binder materials in anodes for lithium-ion batteries

The negative electrode (NE) for lithium-ion batteries is conventionally made by casting a mixture of various carbon materials with polyvinylidene difluoride (PVDF) onto copper foil. Differential scanning calorimetry and accelerating rate calorimetry were used to evaluate the thermal stability of several lithiated NE materials: synthetic graphite (SFG-44), mesocarbon microbeads (MCMB), lignin-based hard carbon (HC), and mixtures of these materials. The exothermic heat generation of lithiated NEs, in the absence of the electrolyte, is attributed to the reaction of PVDF with lithiated carbon (Li x C 6 ). For all samples here, the total exothermic heat generation increases with an increase in lithiation content. The onset temperature for the thermal reaction of PVDF with SFG-44 or MCMB does not depend on the lithiation content. However, this onset temperature decreases as lithiation increases in HC electrodes. These differences are attributed to structural differences between highly graphitic SFG-44 and MCMB compared with the far less graphitic HC. Total heat generation increases with PVDF binder content. An alternative resin-based binder, phenolformaldehyde phenolic-resin (C 7 H 6 O) n , is proposed. Full or partial substitution of this material for PVDF lowers the exothermic heat of reaction of the binder agent with lithiated NE materials.