Anaba A. Anani

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.

Carbon electrode materials for electrochemical cells and method of making same

A method for preparing an amorphous carbon material for use as an electrode, such as the anode of an electrochemical cell. The amorphous carbon is fabricated in a one heating step process from multi-functional organic monomers. Electrodes so fabricated may be incorporated into electrochemical cells (10) as the anode (20) thereof.

Impedance effects of rechargeable batteries on digital communication devices

Abstract Present (and future) wireless communications use (will use) digital electronics to bunch and pack more data into the available communications spectrum. This strategy requires an energy storage device capable of delivering large, pulsed currents which may approach 4 A for 15 msec once every 90 msec. Emerging energy storage technologies, such as nickel-metal hydride and Li-ion cells, have enhanced energy densities but greater internal impedance when compared to the traditional nickel-cadmium cells. Experiments simulating battery discharge in a digital wireless communications application demonstrate the effects of internal impedance on the overall ability of the cells to meet digital device requirements. Results indicate that, currently, nickel-cadmium may be the best available technology for these digital applications.